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<?xml version='1.0' encoding='utf-8'?>
<rfc xmlns:xi="http://www.w3.org/2001/XInclude" version="3" category="std" consensus="true" docName="draft-ietf-tram-turnbis-29" indexInclude="true" ipr="trust200902" number="8656" obsoletes="5766, 6156" prepTime="2020-02-21T22:11:30" scripts="Common,Latin" sortRefs="true" submissionType="IETF" symRefs="true" tocDepth="3" tocInclude="true" xml:lang="en">
  <link href="https://datatracker.ietf.org/doc/draft-ietf-tram-turnbis-29" rel="prev"/>
  <link href="https://dx.doi.org/10.17487/rfc8656" rel="alternate"/>
  <link href="urn:issn:2070-1721" rel="alternate"/>
  <front>
    <title abbrev="TURN">Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN)</title>
    <seriesInfo name="RFC" value="8656" stream="IETF"/>
    <author fullname="Tirumaleswar Reddy" initials="T." role="editor" surname="Reddy">
      <organization abbrev="McAfee" showOnFrontPage="true">McAfee, Inc.</organization>
      <address>
        <postal>
          <street>Embassy Golf Link Business Park</street>
          <city>Bangalore</city>
          <region>Karnataka</region>
          <code>560071</code>
          <country>India</country>
        </postal>
        <email>kondtir@gmail.com</email>
      </address>
    </author>
    <author fullname="Alan Johnston" initials="A." role="editor" surname="Johnston">
      <organization showOnFrontPage="true">Villanova University</organization>
      <address>
        <postal>
          <street/>
          <city>Villanova</city>
          <region>PA</region>
          <code/>
          <country>United States of America</country>
        </postal>
        <email>alan.b.johnston@gmail.com</email>
      </address>
    </author>
    <author fullname="Philip Matthews" initials="P." surname="Matthews">
      <organization showOnFrontPage="true">Alcatel-Lucent</organization>
      <address>
        <postal>
          <street>600 March Road</street>
          <city>Ottawa</city>
          <region>Ontario</region>
          <code/>
          <country>Canada</country>
        </postal>
        <email>philip_matthews@magma.ca</email>
      </address>
    </author>
    <author fullname="Jonathan Rosenberg" initials="J." surname="Rosenberg">
      <organization showOnFrontPage="true">jdrosen.net</organization>
      <address>
        <postal>
          <street/>
          <city>Edison</city>
          <region>NJ</region>
          <country>United States of America</country>
        </postal>
        <email>jdrosen@jdrosen.net</email>
        <uri>http://www.jdrosen.net</uri>
      </address>
    </author>
    <date month="02" year="2020"/>
    <area>Transport</area>
    <workgroup>TRAM WG</workgroup>
    <keyword>NAT</keyword>
    <keyword>TURN</keyword>
    <keyword>STUN</keyword>
    <keyword>ICE</keyword>
    <abstract pn="section-abstract">
      <t pn="section-abstract-1">If a host is located behind a NAT, it can be impossible for that host
      to communicate directly with other hosts (peers) in certain
      situations. In these situations, it is necessary for the host to use the
      services of an intermediate node that acts as a communication relay.
      This specification defines a protocol, called "Traversal Using Relays
      around NAT" (TURN), that allows the host to control the operation of the
      relay and to exchange packets with its peers using the relay. TURN
      differs from other relay control protocols in that it allows a client to
      communicate with multiple peers using a single relay address.</t>
      <t pn="section-abstract-2">The TURN protocol was designed to be used as part of the Interactive
      Connectivity Establishment (ICE) approach to NAT traversal,
      though it can also be used without ICE.</t>
      <t pn="section-abstract-3">This document obsoletes RFCs 5766 and 6156.</t>
    </abstract>
    <boilerplate>
      <section anchor="status-of-memo" numbered="false" removeInRFC="false" toc="exclude" pn="section-boilerplate.1">
        <name slugifiedName="name-status-of-this-memo">Status of This Memo</name>
        <t pn="section-boilerplate.1-1">
            This is an Internet Standards Track document.
        </t>
        <t pn="section-boilerplate.1-2">
            This document is a product of the Internet Engineering Task Force
            (IETF).  It represents the consensus of the IETF community.  It has
            received public review and has been approved for publication by
            the Internet Engineering Steering Group (IESG).  Further
            information on Internet Standards is available in Section 2 of 
            RFC 7841.
        </t>
        <t pn="section-boilerplate.1-3">
            Information about the current status of this document, any
            errata, and how to provide feedback on it may be obtained at
            <eref target="https://www.rfc-editor.org/info/rfc8656" brackets="none"/>.
        </t>
      </section>
      <section anchor="copyright" numbered="false" removeInRFC="false" toc="exclude" pn="section-boilerplate.2">
        <name slugifiedName="name-copyright-notice">Copyright Notice</name>
        <t pn="section-boilerplate.2-1">
            Copyright (c) 2020 IETF Trust and the persons identified as the
            document authors. All rights reserved.
        </t>
        <t pn="section-boilerplate.2-2">
            This document is subject to BCP 78 and the IETF Trust's Legal
            Provisions Relating to IETF Documents
            (<eref target="https://trustee.ietf.org/license-info" brackets="none"/>) in effect on the date of
            publication of this document. Please review these documents
            carefully, as they describe your rights and restrictions with
            respect to this document. Code Components extracted from this
            document must include Simplified BSD License text as described in
            Section 4.e of the Trust Legal Provisions and are provided without
            warranty as described in the Simplified BSD License.
        </t>
      </section>
    </boilerplate>
    <toc>
      <section anchor="toc" numbered="false" removeInRFC="false" toc="exclude" pn="section-toc.1">
        <name slugifiedName="name-table-of-contents">Table of Contents</name>
        <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1">
          <li pn="section-toc.1-1.1">
            <t keepWithNext="true" pn="section-toc.1-1.1.1"><xref derivedContent="1" format="counter" sectionFormat="of" target="section-1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-introduction">Introduction</xref></t>
          </li>
          <li pn="section-toc.1-1.2">
            <t keepWithNext="true" pn="section-toc.1-1.2.1"><xref derivedContent="2" format="counter" sectionFormat="of" target="section-2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-terminology">Terminology</xref></t>
          </li>
          <li pn="section-toc.1-1.3">
            <t keepWithNext="true" pn="section-toc.1-1.3.1"><xref derivedContent="3" format="counter" sectionFormat="of" target="section-3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-overview-of-operation">Overview of Operation</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.3.2">
              <li pn="section-toc.1-1.3.2.1">
                <t keepWithNext="true" pn="section-toc.1-1.3.2.1.1"><xref derivedContent="3.1" format="counter" sectionFormat="of" target="section-3.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-transports">Transports</xref></t>
              </li>
              <li pn="section-toc.1-1.3.2.2">
                <t keepWithNext="true" pn="section-toc.1-1.3.2.2.1"><xref derivedContent="3.2" format="counter" sectionFormat="of" target="section-3.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-allocations">Allocations</xref></t>
              </li>
              <li pn="section-toc.1-1.3.2.3">
                <t keepWithNext="true" pn="section-toc.1-1.3.2.3.1"><xref derivedContent="3.3" format="counter" sectionFormat="of" target="section-3.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-permissions">Permissions</xref></t>
              </li>
              <li pn="section-toc.1-1.3.2.4">
                <t keepWithNext="true" pn="section-toc.1-1.3.2.4.1"><xref derivedContent="3.4" format="counter" sectionFormat="of" target="section-3.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-send-mechanism">Send Mechanism</xref></t>
              </li>
              <li pn="section-toc.1-1.3.2.5">
                <t keepWithNext="true" pn="section-toc.1-1.3.2.5.1"><xref derivedContent="3.5" format="counter" sectionFormat="of" target="section-3.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-channels">Channels</xref></t>
              </li>
              <li pn="section-toc.1-1.3.2.6">
                <t keepWithNext="true" pn="section-toc.1-1.3.2.6.1"><xref derivedContent="3.6" format="counter" sectionFormat="of" target="section-3.6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-unprivileged-turn-servers">Unprivileged TURN Servers</xref></t>
              </li>
              <li pn="section-toc.1-1.3.2.7">
                <t keepWithNext="true" pn="section-toc.1-1.3.2.7.1"><xref derivedContent="3.7" format="counter" sectionFormat="of" target="section-3.7"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-avoiding-ip-fragmentation">Avoiding IP Fragmentation</xref></t>
              </li>
              <li pn="section-toc.1-1.3.2.8">
                <t keepWithNext="true" pn="section-toc.1-1.3.2.8.1"><xref derivedContent="3.8" format="counter" sectionFormat="of" target="section-3.8"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-rtp-support">RTP Support</xref></t>
              </li>
              <li pn="section-toc.1-1.3.2.9">
                <t keepWithNext="true" pn="section-toc.1-1.3.2.9.1"><xref derivedContent="3.9" format="counter" sectionFormat="of" target="section-3.9"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-happy-eyeballs-for-turn">Happy Eyeballs for TURN</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.4">
            <t keepWithNext="true" pn="section-toc.1-1.4.1"><xref derivedContent="4" format="counter" sectionFormat="of" target="section-4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-discovery-of-turn-server">Discovery of TURN Server</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.4.2">
              <li pn="section-toc.1-1.4.2.1">
                <t keepWithNext="true" pn="section-toc.1-1.4.2.1.1"><xref derivedContent="4.1" format="counter" sectionFormat="of" target="section-4.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-turn-uri-scheme-semantics">TURN URI Scheme Semantics</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.5">
            <t keepWithNext="true" pn="section-toc.1-1.5.1"><xref derivedContent="5" format="counter" sectionFormat="of" target="section-5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-general-behavior">General Behavior</xref></t>
          </li>
          <li pn="section-toc.1-1.6">
            <t keepWithNext="true" pn="section-toc.1-1.6.1"><xref derivedContent="6" format="counter" sectionFormat="of" target="section-6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-allocations-2">Allocations</xref></t>
          </li>
          <li pn="section-toc.1-1.7">
            <t keepWithNext="true" pn="section-toc.1-1.7.1"><xref derivedContent="7" format="counter" sectionFormat="of" target="section-7"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-creating-an-allocation">Creating an Allocation</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.7.2">
              <li pn="section-toc.1-1.7.2.1">
                <t keepWithNext="true" pn="section-toc.1-1.7.2.1.1"><xref derivedContent="7.1" format="counter" sectionFormat="of" target="section-7.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-sending-an-allocate-request">Sending an Allocate Request</xref></t>
              </li>
              <li pn="section-toc.1-1.7.2.2">
                <t keepWithNext="true" pn="section-toc.1-1.7.2.2.1"><xref derivedContent="7.2" format="counter" sectionFormat="of" target="section-7.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-an-allocate-reque">Receiving an Allocate Request</xref></t>
              </li>
              <li pn="section-toc.1-1.7.2.3">
                <t keepWithNext="true" pn="section-toc.1-1.7.2.3.1"><xref derivedContent="7.3" format="counter" sectionFormat="of" target="section-7.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-an-allocate-succe">Receiving an Allocate Success Response</xref></t>
              </li>
              <li pn="section-toc.1-1.7.2.4">
                <t keepWithNext="true" pn="section-toc.1-1.7.2.4.1"><xref derivedContent="7.4" format="counter" sectionFormat="of" target="section-7.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-an-allocate-error">Receiving an Allocate Error Response</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.8">
            <t keepWithNext="true" pn="section-toc.1-1.8.1"><xref derivedContent="8" format="counter" sectionFormat="of" target="section-8"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-refreshing-an-allocation">Refreshing an Allocation</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.8.2">
              <li pn="section-toc.1-1.8.2.1">
                <t keepWithNext="true" pn="section-toc.1-1.8.2.1.1"><xref derivedContent="8.1" format="counter" sectionFormat="of" target="section-8.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-sending-a-refresh-request">Sending a Refresh Request</xref></t>
              </li>
              <li pn="section-toc.1-1.8.2.2">
                <t keepWithNext="true" pn="section-toc.1-1.8.2.2.1"><xref derivedContent="8.2" format="counter" sectionFormat="of" target="section-8.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-a-refresh-request">Receiving a Refresh Request</xref></t>
              </li>
              <li pn="section-toc.1-1.8.2.3">
                <t keepWithNext="true" pn="section-toc.1-1.8.2.3.1"><xref derivedContent="8.3" format="counter" sectionFormat="of" target="section-8.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-a-refresh-respons">Receiving a Refresh Response</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.9">
            <t keepWithNext="true" pn="section-toc.1-1.9.1"><xref derivedContent="9" format="counter" sectionFormat="of" target="section-9"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-permissions-2">Permissions</xref></t>
          </li>
          <li pn="section-toc.1-1.10">
            <t keepWithNext="true" pn="section-toc.1-1.10.1"><xref derivedContent="10" format="counter" sectionFormat="of" target="section-10"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-createpermission">CreatePermission</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.10.2">
              <li pn="section-toc.1-1.10.2.1">
                <t keepWithNext="true" pn="section-toc.1-1.10.2.1.1"><xref derivedContent="10.1" format="counter" sectionFormat="of" target="section-10.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-forming-a-createpermission-">Forming a CreatePermission Request</xref></t>
              </li>
              <li pn="section-toc.1-1.10.2.2">
                <t keepWithNext="true" pn="section-toc.1-1.10.2.2.1"><xref derivedContent="10.2" format="counter" sectionFormat="of" target="section-10.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-a-createpermissio">Receiving a CreatePermission Request</xref></t>
              </li>
              <li pn="section-toc.1-1.10.2.3">
                <t keepWithNext="true" pn="section-toc.1-1.10.2.3.1"><xref derivedContent="10.3" format="counter" sectionFormat="of" target="section-10.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-a-createpermission">Receiving a CreatePermission Response</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.11">
            <t keepWithNext="true" pn="section-toc.1-1.11.1"><xref derivedContent="11" format="counter" sectionFormat="of" target="section-11"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-send-and-data-methods">Send and Data Methods</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.11.2">
              <li pn="section-toc.1-1.11.2.1">
                <t keepWithNext="true" pn="section-toc.1-1.11.2.1.1"><xref derivedContent="11.1" format="counter" sectionFormat="of" target="section-11.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-forming-a-send-indication">Forming a Send Indication</xref></t>
              </li>
              <li pn="section-toc.1-1.11.2.2">
                <t keepWithNext="true" pn="section-toc.1-1.11.2.2.1"><xref derivedContent="11.2" format="counter" sectionFormat="of" target="section-11.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-a-send-indication">Receiving a Send Indication</xref></t>
              </li>
              <li pn="section-toc.1-1.11.2.3">
                <t keepWithNext="true" pn="section-toc.1-1.11.2.3.1"><xref derivedContent="11.3" format="counter" sectionFormat="of" target="section-11.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-a-udp-datagram">Receiving a UDP Datagram</xref></t>
              </li>
              <li pn="section-toc.1-1.11.2.4">
                <t keepWithNext="true" pn="section-toc.1-1.11.2.4.1"><xref derivedContent="11.4" format="counter" sectionFormat="of" target="section-11.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-a-data-indication">Receiving a Data Indication</xref></t>
              </li>
              <li pn="section-toc.1-1.11.2.5">
                <t keepWithNext="true" pn="section-toc.1-1.11.2.5.1"><xref derivedContent="11.5" format="counter" sectionFormat="of" target="section-11.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-an-icmp-packet">Receiving an ICMP Packet</xref></t>
              </li>
              <li pn="section-toc.1-1.11.2.6">
                <t keepWithNext="true" pn="section-toc.1-1.11.2.6.1"><xref derivedContent="11.6" format="counter" sectionFormat="of" target="section-11.6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-a-data-indication-">Receiving a Data Indication with an ICMP Attribute</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.12">
            <t keepWithNext="true" pn="section-toc.1-1.12.1"><xref derivedContent="12" format="counter" sectionFormat="of" target="section-12"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-channels-2">Channels</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.12.2">
              <li pn="section-toc.1-1.12.2.1">
                <t keepWithNext="true" pn="section-toc.1-1.12.2.1.1"><xref derivedContent="12.1" format="counter" sectionFormat="of" target="section-12.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-sending-a-channelbind-reque">Sending a ChannelBind Request</xref></t>
              </li>
              <li pn="section-toc.1-1.12.2.2">
                <t keepWithNext="true" pn="section-toc.1-1.12.2.2.1"><xref derivedContent="12.2" format="counter" sectionFormat="of" target="section-12.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-a-channelbind-req">Receiving a ChannelBind Request</xref></t>
              </li>
              <li pn="section-toc.1-1.12.2.3">
                <t keepWithNext="true" pn="section-toc.1-1.12.2.3.1"><xref derivedContent="12.3" format="counter" sectionFormat="of" target="section-12.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-a-channelbind-res">Receiving a ChannelBind Response</xref></t>
              </li>
              <li pn="section-toc.1-1.12.2.4">
                <t keepWithNext="true" pn="section-toc.1-1.12.2.4.1"><xref derivedContent="12.4" format="counter" sectionFormat="of" target="section-12.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-the-channeldata-message">The ChannelData Message</xref></t>
              </li>
              <li pn="section-toc.1-1.12.2.5">
                <t keepWithNext="true" pn="section-toc.1-1.12.2.5.1"><xref derivedContent="12.5" format="counter" sectionFormat="of" target="section-12.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-sending-a-channeldata-messa">Sending a ChannelData Message</xref></t>
              </li>
              <li pn="section-toc.1-1.12.2.6">
                <t keepWithNext="true" pn="section-toc.1-1.12.2.6.1"><xref derivedContent="12.6" format="counter" sectionFormat="of" target="section-12.6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-a-channeldata-mes">Receiving a ChannelData Message</xref></t>
              </li>
              <li pn="section-toc.1-1.12.2.7">
                <t keepWithNext="true" pn="section-toc.1-1.12.2.7.1"><xref derivedContent="12.7" format="counter" sectionFormat="of" target="section-12.7"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-relaying-data-from-the-peer">Relaying Data from the Peer</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.13">
            <t keepWithNext="true" pn="section-toc.1-1.13.1"><xref derivedContent="13" format="counter" sectionFormat="of" target="section-13"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-packet-translations">Packet Translations</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.13.2">
              <li pn="section-toc.1-1.13.2.1">
                <t keepWithNext="true" pn="section-toc.1-1.13.2.1.1"><xref derivedContent="13.1" format="counter" sectionFormat="of" target="section-13.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ipv4-to-ipv6-translations">IPv4-to-IPv6 Translations</xref></t>
              </li>
              <li pn="section-toc.1-1.13.2.2">
                <t keepWithNext="true" pn="section-toc.1-1.13.2.2.1"><xref derivedContent="13.2" format="counter" sectionFormat="of" target="section-13.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ipv6-to-ipv6-translations">IPv6-to-IPv6 Translations</xref></t>
              </li>
              <li pn="section-toc.1-1.13.2.3">
                <t keepWithNext="true" pn="section-toc.1-1.13.2.3.1"><xref derivedContent="13.3" format="counter" sectionFormat="of" target="section-13.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ipv6-to-ipv4-translations">IPv6-to-IPv4 Translations</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.14">
            <t keepWithNext="true" pn="section-toc.1-1.14.1"><xref derivedContent="14" format="counter" sectionFormat="of" target="section-14"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-udp-to-udp-relay">UDP-to-UDP Relay</xref></t>
          </li>
          <li pn="section-toc.1-1.15">
            <t keepWithNext="true" pn="section-toc.1-1.15.1"><xref derivedContent="15" format="counter" sectionFormat="of" target="section-15"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-tcp-to-udp-relay">TCP-to-UDP Relay</xref></t>
          </li>
          <li pn="section-toc.1-1.16">
            <t keepWithNext="true" pn="section-toc.1-1.16.1"><xref derivedContent="16" format="counter" sectionFormat="of" target="section-16"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-udp-to-tcp-relay">UDP-to-TCP Relay</xref></t>
          </li>
          <li pn="section-toc.1-1.17">
            <t keepWithNext="true" pn="section-toc.1-1.17.1"><xref derivedContent="17" format="counter" sectionFormat="of" target="section-17"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-stun-methods">STUN Methods</xref></t>
          </li>
          <li pn="section-toc.1-1.18">
            <t keepWithNext="true" pn="section-toc.1-1.18.1"><xref derivedContent="18" format="counter" sectionFormat="of" target="section-18"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-stun-attributes">STUN Attributes</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.18.2">
              <li pn="section-toc.1-1.18.2.1">
                <t keepWithNext="true" pn="section-toc.1-1.18.2.1.1"><xref derivedContent="18.1" format="counter" sectionFormat="of" target="section-18.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-channel-number">CHANNEL-NUMBER</xref></t>
              </li>
              <li pn="section-toc.1-1.18.2.2">
                <t keepWithNext="true" pn="section-toc.1-1.18.2.2.1"><xref derivedContent="18.2" format="counter" sectionFormat="of" target="section-18.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-lifetime">LIFETIME</xref></t>
              </li>
              <li pn="section-toc.1-1.18.2.3">
                <t keepWithNext="true" pn="section-toc.1-1.18.2.3.1"><xref derivedContent="18.3" format="counter" sectionFormat="of" target="section-18.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-xor-peer-address">XOR-PEER-ADDRESS</xref></t>
              </li>
              <li pn="section-toc.1-1.18.2.4">
                <t keepWithNext="true" pn="section-toc.1-1.18.2.4.1"><xref derivedContent="18.4" format="counter" sectionFormat="of" target="section-18.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-data">DATA</xref></t>
              </li>
              <li pn="section-toc.1-1.18.2.5">
                <t keepWithNext="true" pn="section-toc.1-1.18.2.5.1"><xref derivedContent="18.5" format="counter" sectionFormat="of" target="section-18.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-xor-relayed-address">XOR-RELAYED-ADDRESS</xref></t>
              </li>
              <li pn="section-toc.1-1.18.2.6">
                <t keepWithNext="true" pn="section-toc.1-1.18.2.6.1"><xref derivedContent="18.6" format="counter" sectionFormat="of" target="section-18.6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-requested-address-family">REQUESTED-ADDRESS-FAMILY</xref></t>
              </li>
              <li pn="section-toc.1-1.18.2.7">
                <t keepWithNext="true" pn="section-toc.1-1.18.2.7.1"><xref derivedContent="18.7" format="counter" sectionFormat="of" target="section-18.7"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-even-port">EVEN-PORT</xref></t>
              </li>
              <li pn="section-toc.1-1.18.2.8">
                <t keepWithNext="true" pn="section-toc.1-1.18.2.8.1"><xref derivedContent="18.8" format="counter" sectionFormat="of" target="section-18.8"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-requested-transport">REQUESTED-TRANSPORT</xref></t>
              </li>
              <li pn="section-toc.1-1.18.2.9">
                <t keepWithNext="true" pn="section-toc.1-1.18.2.9.1"><xref derivedContent="18.9" format="counter" sectionFormat="of" target="section-18.9"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-dont-fragment">DONT-FRAGMENT</xref></t>
              </li>
              <li pn="section-toc.1-1.18.2.10">
                <t keepWithNext="true" pn="section-toc.1-1.18.2.10.1"><xref derivedContent="18.10" format="counter" sectionFormat="of" target="section-18.10"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-reservation-token">RESERVATION-TOKEN</xref></t>
              </li>
              <li pn="section-toc.1-1.18.2.11">
                <t keepWithNext="true" pn="section-toc.1-1.18.2.11.1"><xref derivedContent="18.11" format="counter" sectionFormat="of" target="section-18.11"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-additional-address-family">ADDITIONAL-ADDRESS-FAMILY</xref></t>
              </li>
              <li pn="section-toc.1-1.18.2.12">
                <t keepWithNext="true" pn="section-toc.1-1.18.2.12.1"><xref derivedContent="18.12" format="counter" sectionFormat="of" target="section-18.12"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-address-error-code">ADDRESS-ERROR-CODE</xref></t>
              </li>
              <li pn="section-toc.1-1.18.2.13">
                <t keepWithNext="true" pn="section-toc.1-1.18.2.13.1"><xref derivedContent="18.13" format="counter" sectionFormat="of" target="section-18.13"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-icmp">ICMP</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.19">
            <t keepWithNext="true" pn="section-toc.1-1.19.1"><xref derivedContent="19" format="counter" sectionFormat="of" target="section-19"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-stun-error-response-codes">STUN Error Response Codes</xref></t>
          </li>
          <li pn="section-toc.1-1.20">
            <t keepWithNext="true" pn="section-toc.1-1.20.1"><xref derivedContent="20" format="counter" sectionFormat="of" target="section-20"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-detailed-example">Detailed Example</xref></t>
          </li>
          <li pn="section-toc.1-1.21">
            <t keepWithNext="true" pn="section-toc.1-1.21.1"><xref derivedContent="21" format="counter" sectionFormat="of" target="section-21"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-security-considerations">Security Considerations</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.21.2">
              <li pn="section-toc.1-1.21.2.1">
                <t keepWithNext="true" pn="section-toc.1-1.21.2.1.1"><xref derivedContent="21.1" format="counter" sectionFormat="of" target="section-21.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-outsider-attacks">Outsider Attacks</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.21.2.1.2">
                  <li pn="section-toc.1-1.21.2.1.2.1">
                    <t keepWithNext="true" pn="section-toc.1-1.21.2.1.2.1.1"><xref derivedContent="21.1.1" format="counter" sectionFormat="of" target="section-21.1.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-obtaining-unauthorized-allo">Obtaining Unauthorized Allocations</xref></t>
                  </li>
                  <li pn="section-toc.1-1.21.2.1.2.2">
                    <t keepWithNext="true" pn="section-toc.1-1.21.2.1.2.2.1"><xref derivedContent="21.1.2" format="counter" sectionFormat="of" target="section-21.1.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-offline-dictionary-attacks">Offline Dictionary Attacks</xref></t>
                  </li>
                  <li pn="section-toc.1-1.21.2.1.2.3">
                    <t keepWithNext="true" pn="section-toc.1-1.21.2.1.2.3.1"><xref derivedContent="21.1.3" format="counter" sectionFormat="of" target="section-21.1.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-faked-refreshes-and-permiss">Faked Refreshes and Permissions</xref></t>
                  </li>
                  <li pn="section-toc.1-1.21.2.1.2.4">
                    <t keepWithNext="true" pn="section-toc.1-1.21.2.1.2.4.1"><xref derivedContent="21.1.4" format="counter" sectionFormat="of" target="section-21.1.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-fake-data">Fake Data</xref></t>
                  </li>
                  <li pn="section-toc.1-1.21.2.1.2.5">
                    <t keepWithNext="true" pn="section-toc.1-1.21.2.1.2.5.1"><xref derivedContent="21.1.5" format="counter" sectionFormat="of" target="section-21.1.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-impersonating-a-server">Impersonating a Server</xref></t>
                  </li>
                  <li pn="section-toc.1-1.21.2.1.2.6">
                    <t keepWithNext="true" pn="section-toc.1-1.21.2.1.2.6.1"><xref derivedContent="21.1.6" format="counter" sectionFormat="of" target="section-21.1.6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-eavesdropping-traffic">Eavesdropping Traffic</xref></t>
                  </li>
                  <li pn="section-toc.1-1.21.2.1.2.7">
                    <t keepWithNext="true" pn="section-toc.1-1.21.2.1.2.7.1"><xref derivedContent="21.1.7" format="counter" sectionFormat="of" target="section-21.1.7"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-turn-loop-attack">TURN Loop Attack</xref></t>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.21.2.2">
                <t keepWithNext="true" pn="section-toc.1-1.21.2.2.1"><xref derivedContent="21.2" format="counter" sectionFormat="of" target="section-21.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-firewall-considerations">Firewall Considerations</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.21.2.2.2">
                  <li pn="section-toc.1-1.21.2.2.2.1">
                    <t keepWithNext="true" pn="section-toc.1-1.21.2.2.2.1.1"><xref derivedContent="21.2.1" format="counter" sectionFormat="of" target="section-21.2.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-faked-permissions">Faked Permissions</xref></t>
                  </li>
                  <li pn="section-toc.1-1.21.2.2.2.2">
                    <t keepWithNext="true" pn="section-toc.1-1.21.2.2.2.2.1"><xref derivedContent="21.2.2" format="counter" sectionFormat="of" target="section-21.2.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-blacklisted-ip-addresses">Blacklisted IP Addresses</xref></t>
                  </li>
                  <li pn="section-toc.1-1.21.2.2.2.3">
                    <t keepWithNext="true" pn="section-toc.1-1.21.2.2.2.3.1"><xref derivedContent="21.2.3" format="counter" sectionFormat="of" target="section-21.2.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-running-servers-on-well-kno">Running Servers on Well-Known Ports</xref></t>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.21.2.3">
                <t keepWithNext="true" pn="section-toc.1-1.21.2.3.1"><xref derivedContent="21.3" format="counter" sectionFormat="of" target="section-21.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-insider-attacks">Insider Attacks</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.21.2.3.2">
                  <li pn="section-toc.1-1.21.2.3.2.1">
                    <t keepWithNext="true" pn="section-toc.1-1.21.2.3.2.1.1"><xref derivedContent="21.3.1" format="counter" sectionFormat="of" target="section-21.3.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-dos-against-turn-server">DoS against TURN Server</xref></t>
                  </li>
                  <li pn="section-toc.1-1.21.2.3.2.2">
                    <t keepWithNext="true" pn="section-toc.1-1.21.2.3.2.2.1"><xref derivedContent="21.3.2" format="counter" sectionFormat="of" target="section-21.3.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-anonymous-relaying-of-malic">Anonymous Relaying of Malicious Traffic</xref></t>
                  </li>
                  <li pn="section-toc.1-1.21.2.3.2.3">
                    <t keepWithNext="true" pn="section-toc.1-1.21.2.3.2.3.1"><xref derivedContent="21.3.3" format="counter" sectionFormat="of" target="section-21.3.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-manipulating-other-allocati">Manipulating Other Allocations</xref></t>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.21.2.4">
                <t keepWithNext="true" pn="section-toc.1-1.21.2.4.1"><xref derivedContent="21.4" format="counter" sectionFormat="of" target="section-21.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-tunnel-amplification-attack">Tunnel Amplification Attack</xref></t>
              </li>
              <li pn="section-toc.1-1.21.2.5">
                <t keepWithNext="true" pn="section-toc.1-1.21.2.5.1"><xref derivedContent="21.5" format="counter" sectionFormat="of" target="section-21.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-other-considerations">Other Considerations</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.22">
            <t keepWithNext="true" pn="section-toc.1-1.22.1"><xref derivedContent="22" format="counter" sectionFormat="of" target="section-22"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-iana-considerations">IANA Considerations</xref></t>
          </li>
          <li pn="section-toc.1-1.23">
            <t keepWithNext="true" pn="section-toc.1-1.23.1"><xref derivedContent="23" format="counter" sectionFormat="of" target="section-23"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-iab-considerations">IAB Considerations</xref></t>
          </li>
          <li pn="section-toc.1-1.24">
            <t keepWithNext="true" pn="section-toc.1-1.24.1"><xref derivedContent="24" format="counter" sectionFormat="of" target="section-24"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-changes-since-rfc-5766">Changes since RFC 5766</xref></t>
          </li>
          <li pn="section-toc.1-1.25">
            <t keepWithNext="true" pn="section-toc.1-1.25.1"><xref derivedContent="25" format="counter" sectionFormat="of" target="section-25"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-updates-to-rfc-6156">Updates to RFC 6156</xref></t>
          </li>
          <li pn="section-toc.1-1.26">
            <t keepWithNext="true" pn="section-toc.1-1.26.1"><xref derivedContent="26" format="counter" sectionFormat="of" target="section-26"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-references">References</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.26.2">
              <li pn="section-toc.1-1.26.2.1">
                <t keepWithNext="true" pn="section-toc.1-1.26.2.1.1"><xref derivedContent="26.1" format="counter" sectionFormat="of" target="section-26.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-normative-references">Normative References</xref></t>
              </li>
              <li pn="section-toc.1-1.26.2.2">
                <t keepWithNext="true" pn="section-toc.1-1.26.2.2.1"><xref derivedContent="26.2" format="counter" sectionFormat="of" target="section-26.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-informative-references">Informative References</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.27">
            <t keepWithNext="true" pn="section-toc.1-1.27.1"><xref derivedContent="" format="none" sectionFormat="of" target="section-appendix.a"/><xref derivedContent="" format="title" sectionFormat="of" target="name-acknowledgements">Acknowledgements</xref></t>
          </li>
          <li pn="section-toc.1-1.28">
            <t keepWithNext="true" pn="section-toc.1-1.28.1"><xref derivedContent="" format="none" sectionFormat="of" target="section-appendix.b"/><xref derivedContent="" format="title" sectionFormat="of" target="name-authors-addresses">Authors' Addresses</xref></t>
          </li>
        </ul>
      </section>
    </toc>
  </front>
  <middle>
    <section numbered="true" toc="include" removeInRFC="false" pn="section-1">
      <name slugifiedName="name-introduction">Introduction</name>
      <t pn="section-1-1">A host behind a NAT may wish to exchange packets with other hosts,
      some of which may also be behind NATs. To do this, the hosts involved
      can use "hole punching" techniques (see <xref target="RFC5128" format="default" sectionFormat="of" derivedContent="RFC5128"/>)
      in an attempt to discover a direct communication path; that is, a
      communication path that goes from one host to another through
      intervening NATs and routers but does not traverse any relays.</t>
      <t pn="section-1-2">As described in <xref target="RFC5128" format="default" sectionFormat="of" derivedContent="RFC5128"/> and <xref target="RFC4787" format="default" sectionFormat="of" derivedContent="RFC4787"/>, hole punching techniques will fail
      if both hosts are behind NATs that are not well behaved. For example, if
      both hosts are behind NATs that have a mapping behavior of
      "address-dependent mapping" or "address- and port-dependent mapping"
      (see <xref target="RFC4787" sectionFormat="of" section="4.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4787#section-4.1" derivedContent="RFC4787"/>), then
      hole punching techniques generally fail.</t>
      <t pn="section-1-3">When a direct communication path cannot be found, it is necessary to
      use the services of an intermediate host that acts as a relay for the
      packets. This relay typically sits in the public Internet and relays
      packets between two hosts that both sit behind NATs.</t>
      <t pn="section-1-4">In many enterprise networks, direct UDP transmissions are not
      permitted between clients on the internal networks and external IP
      addresses. To permit media sessions in such a situation to use UDP and
      avoid forcing them through TCP, an Enterprise Firewall can be configured
      to allow UDP traffic relayed through an Enterprise relay server. WebRTC
      requires support for this scenario (see <xref target="RFC7478" sectionFormat="of" section="2.3.5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7478#section-2.3.5.1" derivedContent="RFC7478"/>). Some users of SIP or WebRTC
      want IP location privacy from the remote peer. In this scenario, the
      client can select a relay server offering IP location privacy and only
      convey the relayed candidates to the peer for ICE connectivity checks
      (see <xref target="I-D.ietf-rtcweb-security" sectionFormat="of" section="4.2.4" format="default" derivedLink="https://tools.ietf.org/html/draft-ietf-rtcweb-security-12#section-4.2.4" derivedContent="SEC-WEBRTC"/>).</t>
      <t pn="section-1-5">This specification defines a protocol, called "TURN", that allows a
      host behind a NAT (called the "TURN client") to request that another host
      (called the "TURN server") act as a relay.

      The client can arrange for the server to relay packets to and from
      certain other hosts (called "peers"), and the client can control aspects
      of how the relaying is done. The client does this by obtaining an IP
      address and port on the server, called the "relayed transport
      address". When a peer sends a packet to the relayed transport address,
      the server relays the transport protocol data from the packet to the
      client. The data encapsulated within a message header that allows the
      client to know the peer from which the transport protocol data was
      relayed by the server.

      If the server receives an ICMP error packet, the server also relays
      certain Layer 3 and 4 header fields from the ICMP header to the
      client. When the client sends a message to the server, the server
      identifies the remote peer from the message header and relays the
      message data to the intended peer.</t>
      <t pn="section-1-6">A client using TURN must have some way to communicate the relayed
      transport address to its peers and to learn each peer's IP address and
      port (more precisely, each peer's server-reflexive transport address;
      see <xref target="sec-overview" format="default" sectionFormat="of" derivedContent="Section 3"/>). How this is done is out of the
      scope of the TURN protocol. One way this might be done is for the client
      and peers to exchange email messages. Another way is for the client and
      its peers to use a special-purpose "introduction" or "rendezvous"
      protocol (see <xref target="RFC5128" format="default" sectionFormat="of" derivedContent="RFC5128"/> for more details).</t>
      <t pn="section-1-7">If TURN is used with ICE <xref target="RFC8445" format="default" sectionFormat="of" derivedContent="RFC8445"/>,
      then the relayed transport address and the IP addresses and ports of the
      peers are included in the ICE candidate information that the rendezvous
      protocol must carry. For example, if TURN and ICE are used as part of a
      multimedia solution using SIP <xref target="RFC3261" format="default" sectionFormat="of" derivedContent="RFC3261"/>,
      then SIP serves the role of the rendezvous protocol, carrying the ICE
      candidate information inside the body of SIP messages <xref target="I-D.ietf-mmusic-ice-sip-sdp" format="default" sectionFormat="of" derivedContent="SDP-ICE"/>. If TURN and ICE are used with some
      other rendezvous protocol, then ICE provides guidance on the services
      the rendezvous protocol must perform.</t>
      <t pn="section-1-8">Though the use of a TURN server to enable communication between two
      hosts behind NATs is very likely to work, it comes at a high cost to the
      provider of the TURN server since the server typically needs a
      high-bandwidth connection to the Internet. As a consequence, it is best
      to use a TURN server only when a direct communication path cannot be
      found. When the client and a peer use ICE to determine the communication
      path, ICE will use hole punching techniques to search for a direct path
      first and only use a TURN server when a direct path cannot be found.</t>
      <t pn="section-1-9">TURN was originally invented to support multimedia sessions signaled
      using SIP. Since SIP supports forking, TURN supports multiple peers per
      relayed transport address; a feature not supported by other approaches
      (e.g., SOCKS <xref target="RFC1928" format="default" sectionFormat="of" derivedContent="RFC1928"/>). However, care has been
      taken to make sure that TURN is suitable for other types of
      applications.</t>
      <t pn="section-1-10">TURN was designed as one piece in the larger ICE approach to NAT
      traversal. Implementors of TURN are urged to investigate ICE and
      seriously consider using it for their application. However, it is
      possible to use TURN without ICE.</t>
      <t pn="section-1-11">TURN is an extension to the Session Traversal Utilities for NAT
      (STUN) protocol <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/>. Most, though
      not all, TURN messages are STUN-formatted messages. A reader of this
      document should be familiar with STUN.</t>
      <t pn="section-1-12">The TURN specification was originally published as <xref target="RFC5766" format="default" sectionFormat="of" derivedContent="RFC5766"/>, which was updated by <xref target="RFC6156" format="default" sectionFormat="of" derivedContent="RFC6156"/> to add IPv6 support. This document supersedes
      and obsoletes both <xref target="RFC5766" format="default" sectionFormat="of" derivedContent="RFC5766"/> and <xref target="RFC6156" format="default" sectionFormat="of" derivedContent="RFC6156"/>.</t>
    </section>
    <section numbered="true" toc="include" removeInRFC="false" pn="section-2">
      <name slugifiedName="name-terminology">Terminology</name>
      <t pn="section-2-1">
    The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
    "<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as
    described in BCP 14 <xref target="RFC2119" format="default" sectionFormat="of" derivedContent="RFC2119"/> <xref target="RFC8174" format="default" sectionFormat="of" derivedContent="RFC8174"/> 
    when, and only when, they appear in all capitals, as shown here.
      </t>
      <t pn="section-2-2">Readers are expected to be familiar with <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/> and the terms defined there.</t>
      <t pn="section-2-3">The following terms are used in this document:</t>
      <dl newline="true" spacing="normal" pn="section-2-4">
        <dt pn="section-2-4.1">TURN:</dt>
        <dd pn="section-2-4.2">The protocol spoken between a TURN client and a
          TURN server. It is an extension to the STUN protocol <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/>. The protocol allows a client
          to allocate and use a relayed transport address.</dd>
        <dt pn="section-2-4.3">TURN client:</dt>
        <dd pn="section-2-4.4">A STUN client that implements this
          specification.</dd>
        <dt pn="section-2-4.5">TURN server:</dt>
        <dd pn="section-2-4.6">A STUN server that implements this
          specification. It relays data between a TURN client and its
          peer(s).</dd>
        <dt pn="section-2-4.7">Peer:</dt>
        <dd pn="section-2-4.8">A host with which the TURN client wishes to
          communicate. The TURN server relays traffic between the TURN client
          and its peer(s). The peer does not interact with the TURN server
          using the protocol defined in this document; rather, the peer
          receives data sent by the TURN server, and the peer sends data
          towards the TURN server.</dd>
        <dt pn="section-2-4.9">Transport Address:</dt>
        <dd pn="section-2-4.10">The combination of an IP address
          and a port.</dd>
        <dt pn="section-2-4.11">Host Transport Address:</dt>
        <dd pn="section-2-4.12">A transport address on a
          client or a peer.</dd>
        <dt pn="section-2-4.13">Server-Reflexive Transport Address:</dt>
        <dd pn="section-2-4.14">A transport
          address on the "external side" of a NAT. This address is allocated
          by the NAT to correspond to a specific host transport address.</dd>
        <dt pn="section-2-4.15">Relayed Transport Address:</dt>
        <dd pn="section-2-4.16">A transport address on the
          TURN server that is used for relaying packets between the client and
          a peer. A peer sends to this address on the TURN server, and the
          packet is then relayed to the client.</dd>
        <dt pn="section-2-4.17">TURN Server Transport Address:</dt>
        <dd pn="section-2-4.18">A transport address on
          the TURN server that is used for sending TURN messages to the
          server. This is the transport address that the client uses to
          communicate with the server.</dd>
        <dt pn="section-2-4.19">Peer Transport Address:</dt>
        <dd pn="section-2-4.20">The transport address of the
          peer as seen by the server. When the peer is behind a NAT, this is
          the peer's server-reflexive transport address.</dd>
        <dt pn="section-2-4.21">Allocation:</dt>
        <dd pn="section-2-4.22">The relayed transport address granted to a
          client through an Allocate request, along with related state, such
          as permissions and expiration timers.</dd>
        <dt pn="section-2-4.23">5-tuple:</dt>
        <dd pn="section-2-4.24">The combination (client IP address and port, server IP address and
        port, and transport protocol (currently one of UDP, TCP, DTLS/UDP, or
        TLS/TCP)) used to communicate between the client and the server. The
        5-tuple uniquely identifies this communication stream. The 5-tuple
        also uniquely identifies the Allocation on the server.</dd>
        <dt pn="section-2-4.25">Transport Protocol:</dt>
        <dd pn="section-2-4.26">The protocol above IP that carries TURN Requests, Responses, and
        Indications as well as providing identifiable flows using a
        5-tuple. In this specification, UDP and TCP are defined as transport
        protocols; this document also describes the use of UDP and TCP in
        combination with a security layer using DTLS and TLS,
        respectively.</dd>
        <dt pn="section-2-4.27">Channel:</dt>
        <dd pn="section-2-4.28">A channel number and associated peer
          transport address. Once a channel number is bound to a peer's
          transport address, the client and server can use the more
          bandwidth-efficient ChannelData message to exchange data.</dd>
        <dt pn="section-2-4.29">Permission:</dt>
        <dd pn="section-2-4.30">The IP address and transport protocol (but
          not the port) of a peer that is permitted to send traffic to the
          TURN server and have that traffic relayed to the TURN client. The
          TURN server will only forward traffic to its client from peers that
          match an existing permission.</dd>
        <dt pn="section-2-4.31">Realm:</dt>
        <dd pn="section-2-4.32">A string used to describe the server or a
          context within the server. The realm tells the client which username
          and password combination to use to authenticate requests.</dd>
        <dt pn="section-2-4.33">Nonce:</dt>
        <dd pn="section-2-4.34">A string chosen at random by the server and
          included in the server response. To prevent replay attacks, the
          server should change the nonce regularly.</dd>
        <dt pn="section-2-4.35">(D)TLS:</dt>
        <dd pn="section-2-4.36">This term is used for statements that apply to
          both Transport Layer Security <xref target="RFC8446" format="default" sectionFormat="of" derivedContent="RFC8446"/> and
          Datagram Transport Layer Security <xref target="RFC6347" format="default" sectionFormat="of" derivedContent="RFC6347"/>.</dd>
      </dl>
    </section>
    <section anchor="sec-overview" numbered="true" toc="include" removeInRFC="false" pn="section-3">
      <name slugifiedName="name-overview-of-operation">Overview of Operation</name>
      <t pn="section-3-1">This section gives an overview of the operation of TURN. It is
      non-normative.</t>
      <t pn="section-3-2">In a typical configuration, a TURN client is connected to a private
      network <xref target="RFC1918" format="default" sectionFormat="of" derivedContent="RFC1918"/> and, through one or more
      NATs, to the public Internet. On the public Internet is a TURN
      server. Elsewhere in the Internet are one or more peers with which the
      TURN client wishes to communicate. These peers may or may not be behind
      one or more NATs.  The client uses the server as a relay to send packets
      to these peers and to receive packets from these peers.</t>
      <figure anchor="fig-turn-model" align="left" suppress-title="false" pn="figure-1">
        <artwork name="" type="" align="left" alt="" pn="section-3-3.1">
                                    Peer A
                                    Server-Reflexive    +---------+
                                    Transport Address   |         |
                                    192.0.2.150:32102   |         |
                                        |              /|         |
                      TURN              |            / ^|  Peer A |
   Client's           Server            |           /  ||         |
   Host Transport     Transport         |         //   ||         |
   Address            Address           |       //     |+---------+
198.51.100.2:49721  192.0.2.15:3478     |+-+  //     Peer A
           |            |               ||N| /       Host Transport
           |   +-+      |               ||A|/        Address
           |   | |      |               v|T|     203.0.113.2:49582
           |   | |      |               /+-+       
+---------+|   | |      |+---------+   /              +---------+
|         ||   |N|      ||         | //               |         |
| TURN    |v   | |      v| TURN    |/                 |         |
| Client  |----|A|-------| Server  |------------------|  Peer B |
|         |    | |^      |         |^                ^|         |
|         |    |T||      |         ||                ||         |
+---------+    | ||      +---------+|                |+---------+
               | ||                 |                |
               | ||                 |                |
               +-+|                 |                |
                  |                 |                |
                  |                 |                |
         Client's                   |             Peer B
         Server-Reflexive     Relayed             Transport
         Transport Address    Transport Address   Address
         192.0.2.1:7000       192.0.2.15:50000    192.0.2.210:49191</artwork>
      </figure>
      <t pn="section-3-4"><xref target="fig-turn-model" format="default" sectionFormat="of" derivedContent="Figure 1"/> shows a typical deployment. In
      this figure, the TURN client and the TURN server are separated by a NAT,
      with the client on the private side and the server on the public side of
      the NAT. This NAT is assumed to be a "bad" NAT; for example,
      it might have a mapping property of "address-and-port-dependent mapping"
      (see <xref target="RFC4787" format="default" sectionFormat="of" derivedContent="RFC4787"/>).</t>
      <t pn="section-3-5">The client talks to the server from a (IP address, port) combination
      called the client's "host transport address". (The combination of an IP
      address and port is called a "transport address".)</t>
      <t pn="section-3-6">The client sends TURN messages from its host transport address to a
      transport address on the TURN server that is known as the "TURN server
      transport address". The client learns the TURN server transport address
      through some unspecified means (e.g., configuration), and this address
      is typically used by many clients simultaneously.</t>
      <t pn="section-3-7">Since the client is behind a NAT, the server sees packets from the
      client as coming from a transport address on the NAT itself. This
      address is known as the client's "server-reflexive transport
      address"; packets sent by the server to the client's
      server-reflexive transport address will be forwarded by the NAT to the
      client's host transport address.</t>
      <t pn="section-3-8">The client uses TURN commands to create and manipulate an ALLOCATION
      on the server. An allocation is a data structure on the server. This
      data structure contains, amongst other things, the relayed transport
      address for the allocation. The relayed transport address is the
      transport address on the server that peers can use to have the server
      relay data to the client. An allocation is uniquely identified by its
      relayed transport address.</t>
      <t pn="section-3-9">Once an allocation is created, the client can send application data
      to the server along with an indication of to which peer the data is to
      be sent, and the server will relay this data to the intended peer. The
      client sends the application data to the server inside a TURN message;
      at the server, the data is extracted from the TURN message and sent to
      the peer in a UDP datagram. In the reverse direction, a peer can send
      application data in a UDP datagram to the relayed transport address for
      the allocation; the server will then encapsulate this data inside a TURN
      message and send it to the client along with an indication of which peer
      sent the data. Since the TURN message always contains an indication of
      which peer the client is communicating with, the client can use a single
      allocation to communicate with multiple peers.</t>
      <t pn="section-3-10">When the peer is behind a NAT, the client must identify the peer
      using its server-reflexive transport address rather than its host
      transport address. For example, to send application data to Peer A in
      the example above, the client must specify 192.0.2.150:32102 (Peer A's
      server-reflexive transport address) rather than 203.0.113.2:49582 (Peer
      A's host transport address).</t>
      <t pn="section-3-11">Each allocation on the server belongs to a single client and has
      either one or two relayed transport addresses that are used only by that
      allocation. Thus, when a packet arrives at a relayed transport address
      on the server, the server knows for which client the data is
      intended.</t>
      <t pn="section-3-12">The client may have multiple allocations on a server at the same
      time.</t>
      <section anchor="sec-transports" numbered="true" toc="include" removeInRFC="false" pn="section-3.1">
        <name slugifiedName="name-transports">Transports</name>
        <t pn="section-3.1-1">TURN, as defined in this specification, always uses UDP between the
        server and the peer. However, this specification allows the use of any
        one of UDP, TCP, Transport Layer Security (TLS) over TCP, or Datagram
        Transport Layer Security (DTLS) over UDP to carry the TURN messages
        between the client and the server.</t>
        <table align="center" pn="table-1">
          <thead>
            <tr>
              <th align="center" colspan="1" rowspan="1">TURN client to TURN server</th>
              <th align="center" colspan="1" rowspan="1">TURN server to peer</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td align="center" colspan="1" rowspan="1">UDP</td>
              <td align="center" colspan="1" rowspan="1">UDP</td>
            </tr>
            <tr>
              <td align="center" colspan="1" rowspan="1">TCP</td>
              <td align="center" colspan="1" rowspan="1">UDP</td>
            </tr>
            <tr>
              <td align="center" colspan="1" rowspan="1">TLS-over-TCP</td>
              <td align="center" colspan="1" rowspan="1">UDP</td>
            </tr>
            <tr>
              <td align="center" colspan="1" rowspan="1">DTLS-over-UDP</td>
              <td align="center" colspan="1" rowspan="1">UDP</td>
            </tr>
          </tbody>
        </table>
        <t pn="section-3.1-3">If TCP or TLS-over-TCP is used between the client and the server,
        then the server will convert between these transports and UDP
        transport when relaying data to/from the peer.</t>
        <t pn="section-3.1-4">Since this version of TURN only supports UDP between the server and
        the peer, it is expected that most clients will prefer to use UDP
        between the client and the server as well. That being the case, some
        readers may wonder: Why also support TCP and TLS-over-TCP?</t>
        <t pn="section-3.1-5">TURN supports TCP transport between the client and the server
        because some firewalls are configured to block UDP entirely. These
        firewalls block UDP but not TCP, in part because TCP has properties
        that make the intention of the nodes being protected by the firewall
        more obvious to the firewall. For example, TCP has a three-way
        handshake that makes it clearer that the protected node really wishes
        to have that particular connection established, while for UDP, the best
        the firewall can do is guess which flows are desired by using
        filtering rules. Also, TCP has explicit connection teardown; while for
        UDP, the firewall has to use timers to guess when the flow is
        finished.</t>
        <t pn="section-3.1-6">TURN supports TLS-over-TCP transport and DTLS-over-UDP transport
        between the client and the server because (D)TLS provides additional
        security properties not provided by TURN's default digest
        authentication, properties that some clients may wish to take
        advantage of. In particular, (D)TLS provides a way for the client to
        ascertain that it is talking to the correct server and provides for
        confidentiality of TURN control messages. 

If (D)TLS transport is used between the TURN client and the TURN server, refer
to <xref target="RFC8489" sectionFormat="of" section="6.2.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8489#section-6.2.3" derivedContent="RFC8489"/> for more
information about cipher suites, server certificate validation, and
authentication of TURN servers.

The guidance given in <xref target="RFC7525" format="default" sectionFormat="of" derivedContent="RFC7525"/>
          <bcp14>MUST</bcp14> be followed to avoid attacks on (D)TLS. TURN does not
require (D)TLS because the overhead of using (D)TLS is higher than that of
digest authentication; for example, using (D)TLS likely means that most
application data will be doubly encrypted (once by (D)TLS and once to ensure
it is still encrypted in the UDP datagram).</t>
        <t pn="section-3.1-7">There is an extension to TURN for TCP transport between the server
        and the peers <xref target="RFC6062" format="default" sectionFormat="of" derivedContent="RFC6062"/>. For this
        reason, allocations that use UDP between the server and the peers are
        known as "UDP allocations", while allocations that use TCP between the
        server and the peers are known as "TCP allocations". This specification
        describes only UDP allocations.</t>
        <t pn="section-3.1-8">In some applications for TURN, the client may send and receive
        packets other than TURN packets on the host transport address it uses
        to communicate with the server. This can happen, for example, when
        using TURN with ICE. In these cases, the client can distinguish TURN
        packets from other packets by examining the source address of the
        arriving packet; those arriving from the TURN server will be TURN
        packets. The algorithm of demultiplexing packets received from
        multiple protocols on the host transport address is discussed in <xref target="RFC7983" format="default" sectionFormat="of" derivedContent="RFC7983"/>.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-3.2">
        <name slugifiedName="name-allocations">Allocations</name>
        <t pn="section-3.2-1">To create an allocation on the server, the client uses an Allocate
        transaction. The client sends an Allocate request to the server, and
        the server replies with an Allocate success response containing the
        allocated relayed transport address. The client can include attributes
        in the Allocate request that describe the type of allocation it
        desires (e.g., the lifetime of the allocation). Since relaying data
        has security implications, the server requires that the client
        authenticate itself, typically using STUN's long-term credential
        mechanism or the STUN Extension for Third-Party Authorization <xref target="RFC7635" format="default" sectionFormat="of" derivedContent="RFC7635"/>, to show that it is authorized to use the
        server.</t>
        <t pn="section-3.2-2">Once a relayed transport address is allocated, a client must keep
        the allocation alive. To do this, the client periodically sends a
        Refresh request to the server. TURN deliberately uses a different
        method (Refresh rather than Allocate) for refreshes to ensure that the
        client is informed if the allocation vanishes for some reason.</t>
        <t pn="section-3.2-3">The frequency of the Refresh transaction is determined by the
        lifetime of the allocation. The default lifetime of an allocation is
        10 minutes; this value was chosen to be long enough so that
        refreshing is not typically a burden on the client while expiring
        allocations where the client has unexpectedly quit in a timely manner.
        However, the client can request a longer lifetime in the Allocate
        request and may modify its request in a Refresh request, and the
        server always indicates the actual lifetime in the response. The
        client must issue a new Refresh transaction within "lifetime" seconds
        of the previous Allocate or Refresh transaction. Once a client no
        longer wishes to use an allocation, it should delete the allocation
        using a Refresh request with a requested lifetime of zero.</t>
        <t pn="section-3.2-4">Both the server and client keep track of a value known as the
        "5-tuple". At the client, the 5-tuple consists of the client's host
        transport address, the server transport address, and the transport
        protocol used by the client to communicate with the server. At the
        server, the 5-tuple value is the same except that the client's host
        transport address is replaced by the client's server-reflexive
        address since that is the client's address as seen by the server.</t>
        <t pn="section-3.2-5">Both the client and the server remember the 5-tuple used in the
        Allocate request. Subsequent messages between the client and the
        server use the same 5-tuple. In this way, the client and server know
        which allocation is being referred to. If the client wishes to
        allocate a second relayed transport address, it must create a second
        allocation using a different 5-tuple (e.g., by using a different
        client host address or port).</t>
        <aside pn="section-3.2-6">
          <t pn="section-3.2-6.1">NOTE: While the terminology used in this document refers to
            5-tuples, the TURN server can store whatever identifier it likes
            that yields identical results. Specifically, an implementation may
            use a file descriptor in place of a 5-tuple to represent a TCP
            connection.</t>
        </aside>
        <figure anchor="fig-allocate" align="left" suppress-title="false" pn="figure-2">
          <artwork name="" type="" align="left" alt="" pn="section-3.2-7.1">
TURN                                 TURN          Peer         Peer
client                               server         A            B
  |-- Allocate request ---------------&gt;|            |            |
  |   (invalid or missing credentials) |            |            |     
  |                                    |            |            |
  |&lt;--------------- Allocate failure --|            |            |
  |              (401 Unauthenticated) |            |            |
  |                                    |            |            |
  |-- Allocate request ---------------&gt;|            |            |
  |               (valid credentials)  |            |            |
  |                                    |            |            |
  |&lt;---------- Allocate success resp --|            |            |
  |            (192.0.2.15:50000)      |            |            |
  //                                   //           //           //
  |                                    |            |            |
  |-- Refresh request ----------------&gt;|            |            |
  |                                    |            |            |
  |&lt;----------- Refresh success resp --|            |            |
  |                                    |            |            |
</artwork>
        </figure>
        <t pn="section-3.2-8">In <xref target="fig-allocate" format="default" sectionFormat="of" derivedContent="Figure 2"/>, the client sends an
        Allocate request to the server with invalid or missing credentials.
        Since the server requires that all requests be authenticated using
        STUN's long-term credential mechanism, the server rejects the request
        with a 401 (Unauthorized) error code. The client then tries again,
        this time including credentials. This time, the server accepts the
        Allocate request and returns an Allocate success response containing
        (amongst other things) the relayed transport address assigned to the
        allocation. Sometime later, the client decides to refresh the
        allocation; thus, it sends a Refresh request to the server. The refresh
        is accepted and the server replies with a Refresh success
        response.</t>
      </section>
      <section anchor="sec-permission-overview" numbered="true" toc="include" removeInRFC="false" pn="section-3.3">
        <name slugifiedName="name-permissions">Permissions</name>
        <t pn="section-3.3-1">To ease concerns amongst enterprise IT administrators that TURN
        could be used to bypass corporate firewall security, TURN includes the
        notion of permissions. TURN permissions mimic the address-restricted
        filtering mechanism of NATs that comply with <xref target="RFC4787" format="default" sectionFormat="of" derivedContent="RFC4787"/>.</t>
        <t pn="section-3.3-2">An allocation can have zero or more permissions. Each permission
        consists of an IP address and a lifetime. When the server receives a
        UDP datagram on the allocation's relayed transport address, it first
        checks the list of permissions. If the source IP address of the
        datagram matches a permission, the application data is relayed to the
        client; otherwise, the UDP datagram is silently discarded.</t>
        <t pn="section-3.3-3">A permission expires after 5 minutes if it is not refreshed, and
        there is no way to explicitly delete a permission. This behavior was
        selected to match the behavior of a NAT that complies with <xref target="RFC4787" format="default" sectionFormat="of" derivedContent="RFC4787"/>.</t>
        <t pn="section-3.3-4">The client can install or refresh a permission using either a
        CreatePermission request or a ChannelBind request. Using the
        CreatePermission request, multiple permissions can be installed or
        refreshed with a single request; this is important for applications
        that use ICE. For security reasons, permissions can only be installed
        or refreshed by transactions that can be authenticated; thus, Send
        indications and ChannelData messages (which are used to send data to
        peers) do not install or refresh any permissions.</t>
        <t pn="section-3.3-5">Note that permissions are within the context of an allocation, so
        adding or expiring a permission in one allocation does not affect
        other allocations.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-3.4">
        <name slugifiedName="name-send-mechanism">Send Mechanism</name>
        <t pn="section-3.4-1">There are two mechanisms for the client and peers to exchange
        application data using the TURN server. The first mechanism uses the
        Send and Data methods, the second mechanism uses channels. Common to
        both mechanisms is the ability of the client to communicate with
        multiple peers using a single allocated relayed transport address;
        thus, both mechanisms include a means for the client to indicate to
        the server which peer should receive the data and for the server to
        indicate to the client which peer sent the data.</t>
        <t pn="section-3.4-2">The Send mechanism uses Send and Data indications. Send indications
        are used to send application data from the client to the server, while
        Data indications are used to send application data from the server to
        the client.</t>
        <t pn="section-3.4-3">When using the Send mechanism, the client sends a Send indication
        to the TURN server containing (a) an XOR-PEER-ADDRESS attribute
        specifying the (server-reflexive) transport address of the peer and
        (b) a DATA attribute holding the application data. When the TURN
        server receives the Send indication, it extracts the application data
        from the DATA attribute and sends it in a UDP datagram to the peer,
        using the allocated relay address as the source address. Note that
        there is no need to specify the relayed transport address since it is
        implied by the 5-tuple used for the Send indication.</t>
        <t pn="section-3.4-4">In the reverse direction, UDP datagrams arriving at the relayed
        transport address on the TURN server are converted into Data
        indications and sent to the client, with the server-reflexive
        transport address of the peer included in an XOR-PEER-ADDRESS
        attribute and the data itself in a DATA attribute. Since the relayed
        transport address uniquely identified the allocation, the server knows
        which client should receive the data.</t>
        <t pn="section-3.4-5">Some ICMP (Internet Control Message Protocol) packets arriving at
        the relayed transport address on the TURN server may be converted into
        Data indications and sent to the client, with the transport address of
        the peer included in an XOR-PEER-ADDRESS attribute and the ICMP type
        and code in an ICMP attribute. ICMP attribute forwarding always uses
        Data indications containing the XOR-PEER-ADDRESS and ICMP attributes,
        even when using the channel mechanism to forward UDP data.</t>
        <t pn="section-3.4-6">Send and Data indications cannot be authenticated since the
        long-term credential mechanism of STUN does not support authenticating
        indications. This is not as big an issue as it might first appear
        since the client-to-server leg is only half of the total path to the
        peer. Applications that want end-to-end security should encrypt the
        data sent between the client and a peer.</t>
        <t pn="section-3.4-7">Because Send indications are not authenticated, it is possible for
        an attacker to send bogus Send indications to the server, which will
        then relay these to a peer. To partly mitigate this attack, TURN
        requires that the client install a permission towards a peer before
        sending data to it using a Send indication. The technique to fully
        mitigate the attack is discussed in <xref target="fate-data" format="default" sectionFormat="of" derivedContent="Section 21.1.4"/>.</t>
        <figure anchor="fig-send-data" align="left" suppress-title="false" pn="figure-3">
          <artwork name="" type="" align="left" alt="" pn="section-3.4-8.1">
TURN                                TURN           Peer          Peer
client                              server          A             B
  |                                   |             |             |
  |-- CreatePermission req (Peer A) -&gt;|             |             |
  |&lt;- CreatePermission success resp --|             |             |
  |                                   |             |             |
  |--- Send ind (Peer A)-------------&gt;|             |             |
  |                                   |=== data ===&gt;|             |
  |                                   |             |             |
  |                                   |&lt;== data ====|             |
  |&lt;------------- Data ind (Peer A) --|             |             |
  |                                   |             |             |
  |                                   |             |             |
  |--- Send ind (Peer B)-------------&gt;|             |             |
  |                                   | dropped     |             |
  |                                   |             |             |
  |                                   |&lt;== data ==================|
  |                           dropped |             |             |
  |                                   |             |             |
</artwork>
        </figure>
        <t pn="section-3.4-9">In <xref target="fig-send-data" format="default" sectionFormat="of" derivedContent="Figure 3"/>, the client has already
        created an allocation and now wishes to send data to its peers. The
        client first creates a permission by sending the server a
        CreatePermission request specifying Peer A's (server-reflexive) IP
        address in the XOR-PEER-ADDRESS attribute; if this was not done, the
        server would not relay data between the client and the server. The
        client then sends data to Peer A using a Send indication; at the
        server, the application data is extracted and forwarded in a UDP
        datagram to Peer A, using the relayed transport address as the source
        transport address. When a UDP datagram from Peer A is received at the
        relayed transport address, the contents are placed into a Data
        indication and forwarded to the client. Later, the client attempts to
        exchange data with Peer B; however, no permission has been installed
        for Peer B, so the Send indication from the client and the UDP
        datagram from the peer are both dropped by the server.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-3.5">
        <name slugifiedName="name-channels">Channels</name>
        <t pn="section-3.5-1">For some applications (e.g., Voice over IP (VoIP)), the 36 bytes of
        overhead that a Send indication or Data indication adds to the
        application data can substantially increase the bandwidth required
        between the client and the server. To remedy this, TURN offers a
        second way for the client and server to associate data with a specific
        peer.</t>
        <t pn="section-3.5-2">This second way uses an alternate packet format known as the
        "ChannelData message". The ChannelData message does not use the STUN
        header used by other TURN messages, but instead has a 4-byte header
        that includes a number known as a "channel number". Each channel number
        in use is bound to a specific peer; thus, it serves as a shorthand for
        the peer's host transport address.</t>
        <t pn="section-3.5-3">To bind a channel to a peer, the client sends a ChannelBind request
        to the server and includes an unbound channel number and the
        transport address of the peer. Once the channel is bound, the client
        can use a ChannelData message to send the server data destined for the
        peer. Similarly, the server can relay data from that peer towards the
        client using a ChannelData message.</t>
        <t pn="section-3.5-4">Channel bindings last for 10 minutes unless refreshed; this
        lifetime was chosen to be longer than the permission lifetime. Channel
        bindings are refreshed by sending another ChannelBind request
        rebinding the channel to the peer. Like permissions (but unlike
        allocations), there is no way to explicitly delete a channel binding;
        the client must simply wait for it to time out.</t>
        <figure anchor="fig-channels" align="left" suppress-title="false" pn="figure-4">
          <artwork name="" type="" align="left" alt="" pn="section-3.5-5.1">
TURN                                TURN           Peer          Peer
client                              server          A             B
  |                                   |             |             |
  |-- ChannelBind req ---------------&gt;|             |             |
  | (Peer A to 0x4001)                |             |             |
  |                                   |             |             |
  |&lt;---------- ChannelBind succ resp -|             |             |
  |                                   |             |             |
  |-- (0x4001) data -----------------&gt;|             |             |
  |                                   |=== data ===&gt;|             |
  |                                   |             |             |
  |                                   |&lt;== data ====|             |
  |&lt;------------------ (0x4001) data -|             |             |
  |                                   |             |             |
  |--- Send ind (Peer A)-------------&gt;|             |             |
  |                                   |=== data ===&gt;|             |
  |                                   |             |             |
  |                                   |&lt;== data ====|             |
  |&lt;------------------ (0x4001) data -|             |             |
  |                                   |             |             |
</artwork>
        </figure>
        <t pn="section-3.5-6"><xref target="fig-channels" format="default" sectionFormat="of" derivedContent="Figure 4"/> shows the channel mechanism in
        use. The client has already created an allocation and now wishes to
        bind a channel to Peer A. To do this, the client sends a ChannelBind
        request to the server, specifying the transport address of Peer A and
        a channel number (0x4001). After that, the client can send application
        data encapsulated inside ChannelData messages to Peer A: this is shown
        as "(0x4001) data" where 0x4001 is the channel number. When the
        ChannelData message arrives at the server, the server transfers the
        data to a UDP datagram and sends it to Peer A (which is the peer bound
        to channel number 0x4001).</t>
        <t pn="section-3.5-7">In the reverse direction, when Peer A sends a UDP datagram to the
        relayed transport address, this UDP datagram arrives at the server on
        the relayed transport address assigned to the allocation. Since the
        UDP datagram was received from Peer A, which has a channel number
        assigned to it, the server encapsulates the data into a ChannelData
        message when sending the data to the client.</t>
        <t pn="section-3.5-8">Once a channel has been bound, the client is free to intermix
        ChannelData messages and Send indications. In the figure, the client
        later decides to use a Send indication rather than a ChannelData
        message to send additional data to Peer A. The client might decide to
        do this, for example, so it can use the DONT-FRAGMENT attribute (see
        the next section). However, once a channel is bound, the server will
        always use a ChannelData message, as shown in the call flow.</t>
        <t pn="section-3.5-9">Note that ChannelData messages can only be used for peers to which
        the client has bound a channel. In the example above, Peer A has been
        bound to a channel, but Peer B has not, so application data to and
        from Peer B would use the Send mechanism.</t>
      </section>
      <section anchor="unpriv" numbered="true" toc="include" removeInRFC="false" pn="section-3.6">
        <name slugifiedName="name-unprivileged-turn-servers">Unprivileged TURN Servers</name>
        <t pn="section-3.6-1">This version of TURN is designed so that the server can be
        implemented as an application that runs in user space under commonly
        available operating systems without requiring special privileges. This
        design decision was made to make it easy to deploy a TURN server: for
        example, to allow a TURN server to be integrated into a peer-to-peer
        application so that one peer can offer NAT traversal services to
        another peer and to use (D)TLS to secure the TURN connection.</t>
        <t pn="section-3.6-2">This design decision has the following implications for data
        relayed by a TURN server:</t>
        <ul spacing="normal" bare="false" empty="false" pn="section-3.6-3">
          <li pn="section-3.6-3.1">The value of the Diffserv field may not be preserved across the
            server;</li>
          <li pn="section-3.6-3.2">The Time to Live (TTL) field may be reset, rather than
            decremented, across the server;</li>
          <li pn="section-3.6-3.3">The Explicit Congestion Notification (ECN) field may be reset
            by the server;</li>
          <li pn="section-3.6-3.4">There is no end-to-end fragmentation since the packet is
            reassembled at the server.</li>
        </ul>
        <t pn="section-3.6-4">Future work may specify alternate TURN semantics that address
        these limitations.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-3.7">
        <name slugifiedName="name-avoiding-ip-fragmentation">Avoiding IP Fragmentation</name>
        <t pn="section-3.7-1">For reasons described in <xref target="FRAG-HARMFUL" format="default" sectionFormat="of" derivedContent="FRAG-HARMFUL"/>, applications, especially those sending large
        volumes of data, should avoid having their packets fragmented. <xref target="I-D.ietf-intarea-frag-fragile" format="default" sectionFormat="of" derivedContent="FRAG-FRAGILE"/> discusses issues associated
        with IP fragmentation and proposes alternatives to IP
        fragmentation.

	Applications using TCP can, more or less, ignore this
        issue because fragmentation avoidance is now a standard part of TCP,
        but applications using UDP (and, thus, any application using this
        version of TURN) need to avoid IP fragmentation by sending
        sufficiently small messages or by using UDP fragmentation <xref target="I-D.ietf-tsvwg-udp-options" format="default" sectionFormat="of" derivedContent="UDP-OPT"/>. Note that the UDP
        fragmentation option needs to be supported by both endpoints, and at
        the time of writing of this document, UDP fragmentation support is
        under discussion and is not deployed.</t>
        <t pn="section-3.7-2">The application running on the client and the peer can take one of
        two approaches to avoid IP fragmentation until UDP fragmentation
        support is available. The first uses messages that are limited to a
        predetermined fixed maximum, and the second relies on network feedback
        to adapt that maximum.</t>
        <t pn="section-3.7-3">The first approach is to avoid sending large amounts of application
        data in the TURN messages/UDP datagrams exchanged between the client
        and the peer. This is the approach taken by most VoIP 
        applications. In this approach, the application <bcp14>MUST</bcp14>
        assume a Path MTU (PMTU) of 1280 bytes because IPv6 requires that every
        link in the Internet has an MTU of 1280 octets or greater as
        specified in <xref target="RFC8200" format="default" sectionFormat="of" derivedContent="RFC8200"/>. If IPv4
        support on legacy or otherwise unusual networks is a consideration,
        the application <bcp14>MAY</bcp14> assume an effective MTU of 576
        bytes for IPv4 datagrams, as every IPv4 host must be capable of
        receiving a packet with a length equal to 576 bytes as discussed in
        <xref target="RFC0791" format="default" sectionFormat="of" derivedContent="RFC0791"/> and <xref target="RFC1122" format="default" sectionFormat="of" derivedContent="RFC1122"/>.</t>
        <t pn="section-3.7-4">The exact amount of application data that can be included while
        avoiding fragmentation depends on the details of the TURN session
        between the client and the server: whether UDP, TCP, or (D)TLS
        transport is used; whether ChannelData messages or Send/Data
        indications are used; and whether any additional attributes (such as
        the DONT-FRAGMENT attribute) are included. Another factor, which is
        hard to determine, is whether the MTU is reduced somewhere along the
        path for other reasons, such as the use of IP-in-IP tunneling.</t>
        <t pn="section-3.7-5">As a guideline, sending a maximum of 500 bytes of application data
        in a single TURN message (by the client on the client-to-server leg)
        or a UDP datagram (by the peer on the peer-to-server leg) will
        generally avoid IP fragmentation. To further reduce the chance of
        fragmentation, it is recommended that the client use ChannelData
        messages when transferring significant volumes of data since the
        overhead of the ChannelData message is less than Send and Data
        indications.</t>
        <t pn="section-3.7-6">The second approach the client and peer can take to avoid
        fragmentation is to use a path MTU discovery algorithm to determine
        the maximum amount of application data that can be sent without
        fragmentation. The classic path MTU discovery algorithm defined in
        <xref target="RFC1191" format="default" sectionFormat="of" derivedContent="RFC1191"/> may not be able to discover the MTU of
        the transmission path between the client and the peer since:</t>
        <ul empty="false" spacing="normal" bare="false" pn="section-3.7-7">
          <li pn="section-3.7-7.1">A probe packet with a Don't Fragment (DF) bit in the IPv4 header set to test a
            path for a larger MTU can be dropped by routers, or</li>
          <li pn="section-3.7-7.2">ICMP error messages can be dropped by middleboxes.</li>
        </ul>
        <t pn="section-3.7-8">As a result, the client and server need to use a path MTU discovery
        algorithm that does not require ICMP messages. The Packetized Path MTU
        Discovery algorithm defined in <xref target="RFC4821" format="default" sectionFormat="of" derivedContent="RFC4821"/> is one
        such algorithm, and a set of algorithms is defined in <xref target="I-D.ietf-tsvwg-datagram-plpmtud" format="default" sectionFormat="of" derivedContent="MTU-DATAGRAM"/>. </t>
        <t pn="section-3.7-9"><xref target="I-D.ietf-tram-stun-pmtud" format="default" sectionFormat="of" derivedContent="MTU-STUN"/> is an
        implementation of <xref target="RFC4821" format="default" sectionFormat="of" derivedContent="RFC4821"/> that uses STUN to
        discover the path MTU; so it might be a suitable approach to be used
        in conjunction with a TURN server that supports the DONT-FRAGMENT
        attribute. When the client includes the DONT-FRAGMENT attribute in a
        Send indication, this tells the server to set the DF bit in the
        resulting UDP datagram that it sends to the peer. Since some servers
        may be unable to set the DF bit, the client should also include this
        attribute in the Allocate request; any server that does not support
        the DONT-FRAGMENT attribute will indicate this by rejecting the
        Allocate request. If the TURN server carrying out packet translation
        from IPv4-to-IPv6 is unable to access the state of the Don't Fragment (DF)
        bit in the IPv4 header, it <bcp14>MUST</bcp14> reject the Allocate request with
        the DONT-FRAGMENT attribute.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-3.8">
        <name slugifiedName="name-rtp-support">RTP Support</name>
        <t pn="section-3.8-1">One of the envisioned uses of TURN is as a relay for clients and
        peers wishing to exchange real-time data (e.g., voice or video) using
        RTP. To facilitate the use of TURN for this purpose, TURN includes
        some special support for older versions of RTP.</t>
        <t pn="section-3.8-2">Old versions of RTP <xref target="RFC3550" format="default" sectionFormat="of" derivedContent="RFC3550"/> required that
        the RTP stream be on an even port number and the associated RTP
        Control Protocol (RTCP) stream, if present, be on the next highest
        port. To allow clients to work with peers that still require this,
        TURN allows the client to request that the server allocate a relayed
        transport address with an even port number and optionally request
        the server reserve the next-highest port number for a subsequent
        allocation.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-3.9">
        <name slugifiedName="name-happy-eyeballs-for-turn">Happy Eyeballs for TURN</name>
        <t pn="section-3.9-1">If an IPv4 path to reach a TURN server is found, but the TURN
        server's IPv6 path is not working, a dual-stack TURN client can
        experience a significant connection delay compared to an IPv4-only
        TURN client. To overcome these connection setup problems, the TURN
        client needs to query both A and AAAA records for the TURN server
        specified using a domain name and try connecting to the TURN server
        using both IPv6 and IPv4 addresses in a fashion similar to the Happy
        Eyeballs mechanism defined in <xref target="RFC8305" format="default" sectionFormat="of" derivedContent="RFC8305"/>. The TURN
        client performs the following steps based on the transport protocol
        being used to connect to the TURN server.</t>
        <ul spacing="normal" bare="false" empty="false" pn="section-3.9-2">
          <li pn="section-3.9-2.1">For TCP or TLS-over-TCP, the results of the Happy Eyeballs
            procedure <xref target="RFC8305" format="default" sectionFormat="of" derivedContent="RFC8305"/> are used by the TURN
            client for sending its TURN messages to the server.</li>
          <li pn="section-3.9-2.2">For clear text UDP, send TURN Allocate requests to both IP
            address families as discussed in <xref target="RFC8305" format="default" sectionFormat="of" derivedContent="RFC8305"/>
            without authentication information.

	    If the TURN server requires
            authentication, it will send back a 401 unauthenticated response;
            the TURN client will use the first UDP connection on which a 401
            error response is received. If a 401 error response is received
            from both IP address families, then the TURN client can silently
            abandon the UDP connection on the IP address family with lower
            precedence. If the TURN server does not require authentication (as
            described in <xref target="RFC8155" sectionFormat="of" section="9" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8155#section-9" derivedContent="RFC8155"/>), it is
            possible for both Allocate requests to succeed. In this case, the
            TURN client sends a Refresh with a LIFETIME value of zero on the
            allocation using the IP address family with lower precedence to
            delete the allocation.</li>
          <li pn="section-3.9-2.3">For DTLS over UDP, initiate a DTLS handshake to both IP address
          families as discussed in <xref target="RFC8305" format="default" sectionFormat="of" derivedContent="RFC8305"/>,
          and use the first DTLS session that is established. If the DTLS
          session is established on both IP address families, then the client
          sends a DTLS close_notify alert to terminate the DTLS session using
          the IP address family with lower precedence. If the TURN over DTLS
          server has been configured to require a cookie exchange (<xref target="RFC6347" sectionFormat="of" section="4.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc6347#section-4.2" derivedContent="RFC6347"/>) and
          a HelloVerifyRequest is received from the TURN servers on both IP
          address families, then the client can silently abandon the
          connection on the IP address family with lower precedence.</li>
        </ul>
      </section>
    </section>
    <section numbered="true" toc="include" removeInRFC="false" pn="section-4">
      <name slugifiedName="name-discovery-of-turn-server">Discovery of TURN Server</name>
      <t pn="section-4-1">Methods of TURN server discovery, including using anycast, are
      described in <xref target="RFC8155" format="default" sectionFormat="of" derivedContent="RFC8155"/>. If a host with
      multiple interfaces discovers a TURN server in each interface, the
      mechanism described in <xref target="RFC7982" format="default" sectionFormat="of" derivedContent="RFC7982"/> can be
      used by the TURN client to influence the TURN server selection. The
      syntax of the "turn" and "turns" URIs are defined in <xref target="RFC7065" sectionFormat="of" section="3.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7065#section-3.1" derivedContent="RFC7065"/>. DTLS as a transport
      protocol for TURN is defined in <xref target="RFC7350" format="default" sectionFormat="of" derivedContent="RFC7350"/>.</t>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-4.1">
        <name slugifiedName="name-turn-uri-scheme-semantics">TURN URI Scheme Semantics</name>
        <t pn="section-4.1-1">The "turn" and "turns" URI schemes are used to designate a TURN
        server (also known as a "relay") on Internet hosts accessible using the
        TURN protocol. The TURN protocol supports sending messages over UDP,
        TCP, TLS-over-TCP, or DTLS-over-UDP. The "turns" URI scheme <bcp14>MUST</bcp14> be
        used when TURN is run over TLS-over-TCP or in DTLS-over-UDP, and the
        "turn" scheme <bcp14>MUST</bcp14> be used otherwise. The required &lt;host&gt; part
        of the "turn" URI denotes the TURN server host. The &lt;port&gt; part,
        if present, denotes the port on which the TURN server is awaiting
        connection requests. If it is absent, the default port is 3478 for
        both UDP and TCP. The default port for TURN over TLS and TURN over
        DTLS is 5349.</t>
      </section>
    </section>
    <section anchor="sec-general-behavior" numbered="true" toc="include" removeInRFC="false" pn="section-5">
      <name slugifiedName="name-general-behavior">General Behavior</name>
      <t pn="section-5-1">This section contains general TURN processing rules that apply to all
      TURN messages.</t>
      <t pn="section-5-2">TURN is an extension to STUN. All TURN messages, with the exception
      of the ChannelData message, are STUN-formatted messages. All the base
      processing rules described in <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/> apply to STUN-formatted messages.
      This means that all the message-forming and message-processing
      descriptions in this document are implicitly prefixed with the rules of
      <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/>.</t>
      <t pn="section-5-3"><xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/> specifies an
      authentication mechanism called the "long-term credential mechanism". TURN
      servers and clients <bcp14>MUST</bcp14> implement this mechanism, and the
      authentication options are discussed in <xref target="sec-rcv-allocate" format="default" sectionFormat="of" derivedContent="Section 7.2"/>.</t>
      <t pn="section-5-4">Note that the long-term credential mechanism applies only to requests
      and cannot be used to authenticate indications; thus, indications in
      TURN are never authenticated. If the server requires requests to be
      authenticated, then the server's administrator <bcp14>MUST</bcp14> choose a realm value
      that will uniquely identify the username and password combination that
      the client must use, even if the client uses multiple servers under
      different administrations. The server's administrator <bcp14>MAY</bcp14> choose to
      allocate a unique username to each client, or it <bcp14>MAY</bcp14> choose to allocate the
      same username to more than one client (for example, to all clients from
      the same department or company). For each Allocate request, the server
      <bcp14>SHOULD</bcp14> generate a new random nonce when the allocation is first
      attempted following the randomness recommendations in <xref target="RFC4086" format="default" sectionFormat="of" derivedContent="RFC4086"/> and <bcp14>SHOULD</bcp14> expire the nonce at least once every
      hour during the lifetime of the allocation. The server uses the
      mechanism described in <xref target="RFC8489" sectionFormat="of" section="9.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8489#section-9.2" derivedContent="RFC8489"/> to indicate that it supports
      <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/>.</t>
      <t pn="section-5-5">All requests after the initial Allocate must use the same username as
      that used to create the allocation to prevent attackers from hijacking
      the client's allocation.</t>
      <t pn="section-5-6">Specifically, if:
</t>
      <ul bare="false" empty="false" spacing="normal" pn="section-5-7">
        <li pn="section-5-7.1">the server requires the use of the long-term credential mechanism, and;
</li>
        <li pn="section-5-7.2">a non-Allocate request passes authentication under this mechanism, and;
</li>
        <li pn="section-5-7.3">the 5-tuple identifies an existing allocation, but;
</li>
        <li pn="section-5-7.4">the request does not use the same username as used to create the allocation,
</li>
      </ul>
      <t pn="section-5-8"> then the request <bcp14>MUST</bcp14> be rejected with a 441 (Wrong
Credentials) error.</t>
      <t pn="section-5-9">When a TURN message arrives at the server from the client, the server
      uses the 5-tuple in the message to identify the associated allocation.
      For all TURN messages (including ChannelData) EXCEPT an Allocate
      request, if the 5-tuple does not identify an existing allocation, then
      the message <bcp14>MUST</bcp14> either be rejected with a 437 Allocation Mismatch error
      (if it is a request) or be silently ignored (if it is an indication or a
      ChannelData message). A client receiving a 437 error response to a
      request other than Allocate <bcp14>MUST</bcp14> assume the allocation no longer
      exists.</t>
      <t pn="section-5-10"><xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/> defines a number of
      attributes, including the SOFTWARE and FINGERPRINT attributes. The
      client <bcp14>SHOULD</bcp14> include the SOFTWARE attribute in all Allocate and Refresh
      requests and <bcp14>MAY</bcp14> include it in any other requests or indications. The
      server <bcp14>SHOULD</bcp14> include the SOFTWARE attribute in all Allocate and Refresh
      responses (either success or failure) and <bcp14>MAY</bcp14> include it in other
      responses or indications. The client and the server <bcp14>MAY</bcp14> include the
      FINGERPRINT attribute in any STUN-formatted messages defined in this
      document.</t>
      <t pn="section-5-11">TURN does not use the backwards-compatibility mechanism described in
      <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/>.</t>
      <t pn="section-5-12">TURN, as defined in this specification, supports both IPv4 and IPv6.
      IPv6 support in TURN includes IPv4-to-IPv6, IPv6-to-IPv6, and
      IPv6-to-IPv4 relaying. When only a single address type is desired, the
      REQUESTED-ADDRESS-FAMILY attribute is used to explicitly request the
      address type the TURN server will allocate (e.g., an IPv4-only node may
      request the TURN server to allocate an IPv6 address). If both IPv4 and
      IPv6 are desired, the single ADDITIONAL-ADDRESS-FAMILY attribute
      indicates a request to the server to allocate one IPv4 and one IPv6
      relay address in a single Allocate request. This saves local ports on
      the client and reduces the number of messages sent between the client
      and the TURN server.</t>
      <t pn="section-5-13">By default, TURN runs on the same ports as STUN: 3478 for TURN over
      UDP and TCP, and 5349 for TURN over (D)TLS. However, TURN has its own
      set of Service Record (SRV) names: "turn" for UDP and TCP, and "turns"
      for (D)TLS. Either the DNS resolution procedures or the ALTERNATE-SERVER
      procedures, both described in <xref target="sec-create-allocation" format="default" sectionFormat="of" derivedContent="Section 7"/>, can be used to run TURN on a
      different port.</t>
      <t pn="section-5-14">To ensure interoperability, a TURN server <bcp14>MUST</bcp14> support the use of UDP
      transport between the client and the server, and it <bcp14>SHOULD</bcp14> support the use
      of TCP, TLS-over-TCP, and DTLS-over-UDP transports.</t>
      <t pn="section-5-15">When UDP or DTLS-over-UDP transport is used between the client and
      the server, the client will retransmit a request if it does not receive
      a response within a certain timeout period. Because of this, the server
      may receive two (or more) requests with the same 5-tuple and same
      transaction id. STUN requires that the server recognize this case and
      treat the request as idempotent (see <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/>). Some implementations may choose
      to meet this requirement by remembering all received requests and the
      corresponding responses for 40 seconds (<xref target="RFC8489" sectionFormat="of" section="6.3.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8489#section-6.3.1" derivedContent="RFC8489"/>). Other implementations may
      choose to reprocess the request and arrange that such reprocessing
      returns essentially the same response. To aid implementors who choose
      the latter approach (the so-called "stateless stack approach"), this
      specification includes some implementation notes on how this might be
      done. Implementations are free to choose either approach or some
      other approach that gives the same results.</t>
      <t pn="section-5-16">To mitigate either intentional or unintentional denial-of-service
      attacks against the server by clients with valid usernames and
      passwords, it is <bcp14>RECOMMENDED</bcp14> that the server impose limits on both the
      number of allocations active at one time for a given username and on the
      amount of bandwidth those allocations can use. The server should reject
      new allocations that would exceed the limit on the allowed number of
      allocations active at one time with a 486 (Allocation Quota Exceeded)
      (see <xref target="sec-rcv-allocate" format="default" sectionFormat="of" derivedContent="Section 7.2"/>), and since UDP does not
      include a congestion control mechanism, it should discard application
      data traffic that exceeds the bandwidth quota.</t>
    </section>
    <section anchor="sec-allocations" numbered="true" toc="include" removeInRFC="false" pn="section-6">
      <name slugifiedName="name-allocations-2">Allocations</name>
      <t pn="section-6-1">All TURN operations revolve around allocations, and all TURN messages
      are associated with either a single or dual allocation. An allocation
      conceptually consists of the following state data:</t>
      <ul spacing="normal" bare="false" empty="false" pn="section-6-2">
        <li pn="section-6-2.1">the relayed transport address or addresses;</li>
        <li pn="section-6-2.2">the 5-tuple: (client's IP address, client's port, server IP
          address, server port, and transport protocol);</li>
        <li pn="section-6-2.3">the authentication information;</li>
        <li pn="section-6-2.4">the time-to-expiry for each relayed transport address;</li>
        <li pn="section-6-2.5">a list of permissions for each relayed transport address;</li>
        <li pn="section-6-2.6">a list of channel-to-peer bindings for each relayed transport
          address.</li>
      </ul>
      <t pn="section-6-3">The relayed transport address is the transport address
      allocated by the server for communicating with peers, while the 5-tuple
      describes the communication path between the client and the server. On
      the client, the 5-tuple uses the client's host transport address; on the
      server, the 5-tuple uses the client's server-reflexive transport
      address. The relayed transport address <bcp14>MUST</bcp14> be unique across all
      allocations so it can be used to uniquely identify the allocation, and
      an allocation in this context can be either a single or dual
      allocation.</t>
      <t pn="section-6-4">The authentication information (e.g., username, password, realm, and
      nonce) is used to both verify subsequent requests and to compute the
      message integrity of responses. The username, realm, and nonce values
      are initially those used in the authenticated Allocate request that
      creates the allocation, though the server can change the nonce value
      during the lifetime of the allocation using a 438 (Stale Nonce) reply.
      For security reasons, the server <bcp14>MUST NOT</bcp14> store the
      password explicitly and <bcp14>MUST</bcp14> store the key value, which
      is a cryptographic hash over the username, realm, and password (see
      <xref target="RFC8489" sectionFormat="of" section="16.1.3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8489#section-16.1.3" derivedContent="RFC8489"/>).</t>
      <t pn="section-6-5">Note that if the response contains a PASSWORD-ALGORITHMS attribute
      and this attribute contains both MD5 and SHA-256 algorithms, and the
      client also supports both the algorithms, the request <bcp14>MUST</bcp14> contain a
      PASSWORD-ALGORITHM attribute with the SHA-256 algorithm.</t>
      <t pn="section-6-6">The time-to-expiry is the time in seconds left until the allocation
      expires. Each Allocate or Refresh transaction sets this timer, which
      then ticks down towards zero. By default, each Allocate or Refresh
      transaction resets this timer to the default lifetime value of 600
      seconds (10 minutes), but the client can request a different value in
      the Allocate and Refresh request. Allocations can only be refreshed
      using the Refresh request; sending data to a peer does not refresh an
      allocation. When an allocation expires, the state data associated with
      the allocation can be freed.</t>
      <t pn="section-6-7">The list of permissions is described in <xref target="sec-permissions" format="default" sectionFormat="of" derivedContent="Section 9"/> and the list of channels is described
      in <xref target="sec-channels" format="default" sectionFormat="of" derivedContent="Section 12"/>.</t>
    </section>
    <section anchor="sec-create-allocation" numbered="true" toc="include" removeInRFC="false" pn="section-7">
      <name slugifiedName="name-creating-an-allocation">Creating an Allocation</name>
      <t pn="section-7-1">An allocation on the server is created using an Allocate
      transaction.</t>
      <section anchor="sec-send-allocate" numbered="true" toc="include" removeInRFC="false" pn="section-7.1">
        <name slugifiedName="name-sending-an-allocate-request">Sending an Allocate Request</name>
        <t pn="section-7.1-1">The client forms an Allocate request as follows.</t>
        <t pn="section-7.1-2">The client first picks a host transport address. It is <bcp14>RECOMMENDED</bcp14>
        that the client pick a currently unused transport address, typically
        by allowing the underlying OS to pick a currently unused port.</t>
        <t pn="section-7.1-3">The client then picks a transport protocol that the client supports
        to use between the client and the server based on the transport
        protocols supported by the server. Since this specification only
        allows UDP between the server and the peers, it is <bcp14>RECOMMENDED</bcp14> that
        the client pick UDP unless it has a reason to use a different
        transport. One reason to pick a different transport would be that the
        client believes, either through configuration or discovery or by
        experiment, that it is unable to contact any TURN server using UDP.
        See <xref target="sec-transports" format="default" sectionFormat="of" derivedContent="Section 3.1"/> for more discussion.</t>
        <t pn="section-7.1-4">The client also picks a server transport address, which <bcp14>SHOULD</bcp14> be
        done as follows. The client uses one or more procedures described in
        <xref target="RFC8155" format="default" sectionFormat="of" derivedContent="RFC8155"/> to discover a TURN server and uses the
        TURN server resolution mechanism defined in <xref target="RFC5928" format="default" sectionFormat="of" derivedContent="RFC5928"/> and <xref target="RFC7350" format="default" sectionFormat="of" derivedContent="RFC7350"/> to get a
        list of server transport addresses that can be tried to create a TURN
        allocation.</t>
        <t pn="section-7.1-5">The client <bcp14>MUST</bcp14> include a REQUESTED-TRANSPORT attribute in the
        request.

	This attribute specifies the transport protocol between the
        server and the peers (note that this is *not* the transport protocol
        that appears in the 5-tuple). In this specification, the
        REQUESTED-TRANSPORT type is always UDP. This attribute is included to
        allow future extensions to specify other protocols.</t>
        <t pn="section-7.1-6">If the client wishes to obtain a relayed transport address of a
        specific address type, then it includes a REQUESTED-ADDRESS-FAMILY
        attribute in the request. This attribute indicates the specific
        address type the client wishes the TURN server to allocate. Clients
        <bcp14>MUST NOT</bcp14> include more than one REQUESTED-ADDRESS-FAMILY attribute in
        an Allocate request. Clients <bcp14>MUST NOT</bcp14> include a
        REQUESTED-ADDRESS-FAMILY attribute in an Allocate request that
        contains a RESERVATION-TOKEN attribute, for the reason that the server
        uses the previously reserved transport address corresponding to the
        included token and the client cannot obtain a relayed transport
        address of a specific address type.</t>
        <t pn="section-7.1-7">If the client wishes to obtain one IPv6 and one IPv4 relayed
        transport address, then it includes an ADDITIONAL-ADDRESS-FAMILY
        attribute in the request. This attribute specifies that the server
        must allocate both address types. The attribute value in the
        ADDITIONAL-ADDRESS-FAMILY <bcp14>MUST</bcp14> be set to 0x02 (IPv6 address family).
        Clients <bcp14>MUST NOT</bcp14> include REQUESTED-ADDRESS-FAMILY and
        ADDITIONAL-ADDRESS-FAMILY attributes in the same request. Clients <bcp14>MUST NOT</bcp14> include the ADDITIONAL-ADDRESS-FAMILY attribute in an Allocate request
        that contains a RESERVATION-TOKEN attribute.

	Clients <bcp14>MUST NOT</bcp14> include
        the ADDITIONAL-ADDRESS-FAMILY attribute in an Allocate request that
        contains an EVEN-PORT attribute with the R (Reserved) bit set to 1.

	

        The reason behind the restriction is that if the EVEN-PORT attribute with the R bit set to 1 is allowed
        with the ADDITIONAL-ADDRESS-FAMILY attribute, two tokens will have to
        be returned in the success response and changes will be required to the way
        the RESERVATION-TOKEN attribute is handled.</t>
        <t pn="section-7.1-8">If the client wishes the server to initialize the time-to-expiry
        field of the allocation to some value other than the default lifetime,
        then it <bcp14>MAY</bcp14> include a LIFETIME attribute specifying its desired value.
        This is just a hint, and the server may elect to use a different
        value. Note that the server will ignore requests to initialize the
        field to less than the default value.</t>
        <t pn="section-7.1-9">If the client wishes to later use the DONT-FRAGMENT attribute in
        one or more Send indications on this allocation, then the client
        <bcp14>SHOULD</bcp14> include the DONT-FRAGMENT attribute in the Allocate request.
        This allows the client to test whether this attribute is supported by
        the server.</t>
        <t pn="section-7.1-10">If the client requires the port number of the relayed transport
        address to be even, the client includes the EVEN-PORT attribute. If this
        attribute is not included, then the port can be even or odd. By
        setting the R bit in the EVEN-PORT attribute to 1, the client can
        request that the server reserve the next highest port number (on the
        same IP address) for a subsequent allocation. If the R bit is 0, no
        such request is made.</t>
        <t pn="section-7.1-11">The client <bcp14>MAY</bcp14> also include a RESERVATION-TOKEN attribute in the
        request to ask the server to use a previously reserved port for the
        allocation. If the RESERVATION-TOKEN attribute is included, then the
        client <bcp14>MUST</bcp14> omit the EVEN-PORT attribute.</t>
        <t pn="section-7.1-12">Once constructed, the client sends the Allocate request on the
        5-tuple.</t>
      </section>
      <section anchor="sec-rcv-allocate" numbered="true" toc="include" removeInRFC="false" pn="section-7.2">
        <name slugifiedName="name-receiving-an-allocate-reque">Receiving an Allocate Request</name>
        <t pn="section-7.2-1">When the server receives an Allocate request, it performs the
        following checks:</t>
        <ol spacing="normal" type="1" start="1" pn="section-7.2-2">
          <li pn="section-7.2-2.1" derivedCounter="1.">The TURN server provided by the local or access network
          <bcp14>MAY</bcp14> allow an unauthenticated request in order to
          accept Allocation requests from new and/or guest users in the
          network who do not necessarily possess long-term credentials for
          STUN authentication.  The security implications of STUN and making
          STUN authentication optional are discussed in <xref target="RFC8155" format="default" sectionFormat="of" derivedContent="RFC8155"/>. Otherwise, the server <bcp14>MUST</bcp14>
          require that the request be authenticated. If the request is
          authenticated, the authentication <bcp14>MUST</bcp14> be done either
          using the long-term credential mechanism of <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/> or using the STUN Extension for Third-Party
          Authorization <xref target="RFC7635" format="default" sectionFormat="of" derivedContent="RFC7635"/> unless the
          client and server agree to use another mechanism through some
          procedure outside the scope of this document.</li>
          <li pn="section-7.2-2.2" derivedCounter="2.">The server checks if the 5-tuple is currently in use by an
            existing allocation. If yes, the server rejects the request with a
            437 (Allocation Mismatch) error.</li>
          <li pn="section-7.2-2.3" derivedCounter="3.">The server checks if the request contains a REQUESTED-TRANSPORT
            attribute. If the REQUESTED-TRANSPORT attribute is not included or
            is malformed, the server rejects the request with a 400 (Bad
            Request) error. Otherwise, if the attribute is included but
            specifies a protocol that is not supported by the server, the
            server rejects the request with a 442 (Unsupported Transport
            Protocol) error.</li>
          <li pn="section-7.2-2.4" derivedCounter="4.">The request may contain a DONT-FRAGMENT attribute. If it does,
            but the server does not support sending UDP datagrams with the DF
            bit set to 1 (see Sections <xref target="sec-ip-header-fields" format="counter" sectionFormat="of" derivedContent="14"/> and
            <xref target="sec-ip-header-fields-tcp-udp" format="counter" sectionFormat="of" derivedContent="15"/>), then the
            server treats the DONT-FRAGMENT attribute in the Allocate request
            as an unknown comprehension-required attribute.</li>
          <li pn="section-7.2-2.5" derivedCounter="5.">The server checks if the request contains a RESERVATION-TOKEN
            attribute. If yes, and the request also contains an EVEN-PORT or
            REQUESTED-ADDRESS-FAMILY or ADDITIONAL-ADDRESS-FAMILY attribute,
            the server rejects the request with a 400 (Bad Request) error.
            Otherwise, it checks to see if the token is valid (i.e., the token
            is in range and has not expired, and the corresponding relayed
            transport address is still available). If the token is not valid
            for some reason, the server rejects the request with a 508
            (Insufficient Capacity) error.</li>
          <li pn="section-7.2-2.6" derivedCounter="6.">The server checks if the request contains both
            REQUESTED-ADDRESS-FAMILY and ADDITIONAL-ADDRESS-FAMILY attributes.
            If yes, then the server rejects the request with a 400 (Bad
            Request) error.</li>
          <li pn="section-7.2-2.7" derivedCounter="7.">If the server does not support the address family requested by
            the client in REQUESTED-ADDRESS-FAMILY, or if the allocation of the
            requested address family is disabled by local policy, it <bcp14>MUST</bcp14>
            generate an Allocate error response, and it <bcp14>MUST</bcp14> include an
            ERROR-CODE attribute with the 440 (Address Family not Supported)
            response code. If the REQUESTED-ADDRESS-FAMILY attribute is absent
            and the server does not support the IPv4 address family, the server
            <bcp14>MUST</bcp14> include an ERROR-CODE attribute with the 440 (Address Family
            not Supported) response code. If the REQUESTED-ADDRESS-FAMILY
            attribute is absent and the server supports the IPv4 address family,
            the server <bcp14>MUST</bcp14> allocate an IPv4 relayed transport address for the
            TURN client.</li>
          <li pn="section-7.2-2.8" derivedCounter="8.">The server checks if the request contains an EVEN-PORT
            attribute with the R bit set to 1. If yes, and the request also
            contains an ADDITIONAL-ADDRESS-FAMILY attribute, the server
            rejects the request with a 400 (Bad Request) error. Otherwise, the
            server checks if it can satisfy the request (i.e., can allocate a
            relayed transport address as described below). If the server
            cannot satisfy the request, then the server rejects the request
            with a 508 (Insufficient Capacity) error.</li>
          <li pn="section-7.2-2.9" derivedCounter="9.">The server checks if the request contains an
            ADDITIONAL-ADDRESS-FAMILY attribute. If yes, and the attribute
            value is 0x01 (IPv4 address family), then the server rejects the
            request with a 400 (Bad Request) error. Otherwise, the server
            checks if it can allocate relayed transport addresses of both
            address types. If the server cannot satisfy the request, then the
            server rejects the request with a 508 (Insufficient Capacity)
            error. If the server can partially meet the request, i.e., if it
            can only allocate one relayed transport address of a specific
            address type, then it includes ADDRESS-ERROR-CODE attribute in the
            success response to inform the client the reason for partial
            failure of the request. The error code value signaled in the
            ADDRESS-ERROR-CODE attribute could be 440 (Address Family not
            Supported) or 508 (Insufficient Capacity). If the server can fully
            meet the request, then the server allocates one IPv4 and one IPv6
            relay address and returns an Allocate success response containing
            the relayed transport addresses assigned to the dual allocation in
            two XOR-RELAYED-ADDRESS attributes.</li>
          <li pn="section-7.2-2.10" derivedCounter="10.">At any point, the server <bcp14>MAY</bcp14> choose to reject the
          request with a 486 (Allocation Quota Reached) error if it feels the
          client is trying to exceed some locally defined allocation
          quota. The server is free to define this allocation quota any way it
          wishes, but it <bcp14>SHOULD</bcp14> define it based on the username
          used to authenticate the request and not on the client's transport
          address.</li>
          <li pn="section-7.2-2.11" derivedCounter="11.">Also, at any point, the server <bcp14>MAY</bcp14> choose to reject the request
            with a 300 (Try Alternate) error if it wishes to redirect the
            client to a different server. The use of this error code and
            attribute follows the specification in <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/>.</li>
        </ol>
        <t pn="section-7.2-3">If all the checks pass, the server creates the allocation. The
        5-tuple is set to the 5-tuple from the Allocate request, while the
        list of permissions and the list of channels are initially empty.</t>
        <t pn="section-7.2-4">The server chooses a relayed transport address for the allocation
        as follows:</t>
        <ul spacing="normal" bare="false" empty="false" pn="section-7.2-5">
          <li pn="section-7.2-5.1">If the request contains a RESERVATION-TOKEN attribute, the
            server uses the previously reserved transport address
            corresponding to the included token (if it is still available).
            Note that the reservation is a server-wide reservation and is not
            specific to a particular allocation since the Allocate request
            containing the RESERVATION-TOKEN uses a different 5-tuple than the
            Allocate request that made the reservation. The 5-tuple for the
            Allocate request containing the RESERVATION-TOKEN attribute can be
            any allowed 5-tuple; it can use a different client IP address and
            port, a different transport protocol, and even a different server IP
            address and port (provided, of course, that the server IP address
            and port are ones on which the server is listening for TURN
            requests).</li>
          <li pn="section-7.2-5.2">If the request contains an EVEN-PORT attribute with the R bit
            set to 0, then the server allocates a relayed transport address
            with an even port number.</li>
          <li pn="section-7.2-5.3">If the request contains an EVEN-PORT attribute with the R bit
            set to 1, then the server looks for a pair of port numbers N and
            N+1 on the same IP address, where N is even. Port N is used in the
            current allocation, while the relayed transport address with port
            N+1 is assigned a token and reserved for a future allocation. The
            server <bcp14>MUST</bcp14> hold this reservation for at least 30 seconds and <bcp14>MAY</bcp14>
            choose to hold longer (e.g., until the allocation with port N
            expires). The server then includes the token in a
            RESERVATION-TOKEN attribute in the success response.</li>
          <li pn="section-7.2-5.4">Otherwise, the server allocates any available relayed transport
            address.</li>
        </ul>
        <t pn="section-7.2-6">In all cases, the server <bcp14>SHOULD</bcp14> only allocate ports from the range
        49152 - 65535 (the Dynamic and/or Private Port range <xref target="PORT-NUMBERS" format="default" sectionFormat="of" derivedContent="PORT-NUMBERS"/>), unless the TURN server application
        knows, through some means not specified here, that other applications
        running on the same host as the TURN server application will not be
        impacted by allocating ports outside this range. This condition can
        often be satisfied by running the TURN server application on a
        dedicated machine and/or by arranging that any other applications on
        the machine allocate ports before the TURN server application starts.
        In any case, the TURN server <bcp14>SHOULD NOT</bcp14> allocate ports in the range 0
        - 1023 (the Well-Known Port range) to discourage clients from using
        TURN to run standard services.</t>
        <aside pn="section-7.2-7">
          <t pn="section-7.2-7.1">NOTE: The use of randomized port assignments to avoid certain
            types of attacks is described in <xref target="RFC6056" format="default" sectionFormat="of" derivedContent="RFC6056"/>.
            It is <bcp14>RECOMMENDED</bcp14> that a TURN server implement a randomized port
            assignment algorithm from <xref target="RFC6056" format="default" sectionFormat="of" derivedContent="RFC6056"/>. This is
            especially applicable to servers that choose to pre-allocate a
            number of ports from the underlying OS and then later assign them
            to allocations; for example, a server may choose this technique to
            implement the EVEN-PORT attribute.</t>
        </aside>
        <t pn="section-7.2-8">The server determines the initial value of the time-to-expiry field
        as follows. If the request contains a LIFETIME attribute, then the
        server computes the minimum of the client's proposed lifetime and the
        server's maximum allowed lifetime. If this computed value is greater
        than the default lifetime, then the server uses the computed lifetime
        as the initial value of the time-to-expiry field. Otherwise, the
        server uses the default lifetime. It is <bcp14>RECOMMENDED</bcp14> that the server
        use a maximum allowed lifetime value of no more than 3600 seconds (1
        hour). Servers that implement allocation quotas or charge users for
        allocations in some way may wish to use a smaller maximum allowed
        lifetime (perhaps as small as the default lifetime) to more quickly
        remove orphaned allocations (that is, allocations where the
        corresponding client has crashed or terminated, or the client
        connection has been lost for some reason). Also, note that the time-
        to-expiry is recomputed with each successful Refresh request, and thus,
        the value computed here applies only until the first refresh.</t>
        <t pn="section-7.2-9">Once the allocation is created, the server replies with a success
        response. The success response contains:</t>
        <ul spacing="normal" bare="false" empty="false" pn="section-7.2-10">
          <li pn="section-7.2-10.1">An XOR-RELAYED-ADDRESS attribute containing the relayed
            transport address or two XOR-RELAYED-ADDRESS attributes containing
            the relayed transport addresses.</li>
          <li pn="section-7.2-10.2">A LIFETIME attribute containing the current value of the
            time-to-expiry timer.</li>
          <li pn="section-7.2-10.3">A RESERVATION-TOKEN attribute (if a second relayed transport
            address was reserved).</li>
          <li pn="section-7.2-10.4">An XOR-MAPPED-ADDRESS attribute containing the client's IP
            address and port (from the 5-tuple).</li>
        </ul>
        <aside pn="section-7.2-11">
          <t pn="section-7.2-11.1">NOTE: The XOR-MAPPED-ADDRESS attribute is included in the
            response as a convenience to the client. TURN itself does not make
            use of this value, but clients running ICE can often need this
            value and can thus avoid having to do an extra Binding transaction
            with some STUN server to learn it.</t>
        </aside>
        <t pn="section-7.2-12">The response (either success or error) is sent back to the client
        on the 5-tuple.</t>
        <aside pn="section-7.2-13">
          <t pn="section-7.2-13.1">NOTE: When the Allocate request is sent over UDP, <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/> requires that the server
            handle the possible retransmissions of the request so that
            retransmissions do not cause multiple allocations to be created.
            Implementations may achieve this using the so-called "stateless
            stack approach" as follows. To detect retransmissions when the
            original request was successful in creating an allocation, the
            server can store the transaction id that created the request with
            the allocation data and compare it with incoming Allocate requests
            on the same 5-tuple. Once such a request is detected, the server
            can stop parsing the request and immediately generate a success
            response. When building this response, the value of the LIFETIME
            attribute can be taken from the time-to-expiry field in the
            allocate state data, even though this value may differ slightly
            from the LIFETIME value originally returned. In addition, the
            server may need to store an indication of any reservation token
            returned in the original response so that this may be returned in
            any retransmitted responses.</t>
          <t pn="section-7.2-13.2">For the case where the original request was unsuccessful in
            creating an allocation, the server may choose to do nothing
            special. Note, however, that there is a rare case where the server
            rejects the original request but accepts the retransmitted request
            (because conditions have changed in the brief intervening time
            period). If the client receives the first failure response, it
            will ignore the second (success) response and believe that an
            allocation was not created.

	    An allocation created in this manner
            will eventually time out since the client will not refresh it.
            Furthermore, if the client later retries with the same 5-tuple but
            a different transaction id, it will receive a 437 (Allocation
            Mismatch) error response, which will cause it to retry with a different 5-tuple.
            The server may use a smaller maximum lifetime value to minimize
            the lifetime of allocations "orphaned" in this manner.</t>
        </aside>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-7.3">
        <name slugifiedName="name-receiving-an-allocate-succe">Receiving an Allocate Success Response</name>
        <t pn="section-7.3-1">If the client receives an Allocate success response, then it <bcp14>MUST</bcp14>
        check that the mapped address and the relayed transport address or
        addresses are part of an address family or families that the client
        understands and is prepared to handle. If these addresses are not part
        of an address family or families that the client is prepared to
        handle, then the client <bcp14>MUST</bcp14> delete the allocation (<xref target="sec-refreshing-allocation" format="default" sectionFormat="of" derivedContent="Section 8"/>) and <bcp14>MUST NOT</bcp14> attempt to
        create another allocation on that server until it believes the
        mismatch has been fixed.</t>
        <t pn="section-7.3-2">Otherwise, the client creates its own copy of the allocation data
        structure to track what is happening on the server. In particular, the
        client needs to remember the actual lifetime received back from the
        server, rather than the value sent to the server in the request. The
        client must also remember the 5-tuple used for the request and the
        username and password it used to authenticate the request to ensure
        that it reuses them for subsequent messages. The client also needs to
        track the channels and permissions it establishes on the server.</t>
        <t pn="section-7.3-3">If the client receives an Allocate success response but with an
        ADDRESS-ERROR-CODE attribute in the response and the error code value
        signaled in the ADDRESS-ERROR-CODE attribute is 440 (Address Family
        not Supported), the client <bcp14>MUST NOT</bcp14> retry its request
        for the rejected address type. If the client receives an
        ADDRESS-ERROR-CODE attribute in the response and the error code value
        signaled in the ADDRESS-ERROR-CODE attribute is 508 (Insufficient
        Capacity), the client <bcp14>SHOULD</bcp14> wait at least 1 minute
        before trying to request any more allocations on this server for the
        rejected address type.</t>
        <t pn="section-7.3-4">The client will probably wish to send the relayed transport address
        to peers (using some method not specified here) so the peers can
        communicate with it. The client may also wish to use the
        server-reflexive address it receives in the XOR-MAPPED-ADDRESS
        attribute in its ICE processing.</t>
      </section>
      <section anchor="sec-allocate-error-response" numbered="true" toc="include" removeInRFC="false" pn="section-7.4">
        <name slugifiedName="name-receiving-an-allocate-error">Receiving an Allocate Error Response</name>
        <t pn="section-7.4-1">If the client receives an Allocate error response, then the
        processing depends on the actual error code returned:</t>
        <dl newline="true" spacing="normal" pn="section-7.4-2">
          <dt pn="section-7.4-2.1">408 (Request timed out):</dt>
          <dd pn="section-7.4-2.2">There is either a problem with the
          server or a problem reaching the server with the chosen
          transport. The client considers the current transaction as having
          failed but <bcp14>MAY</bcp14> choose to retry the Allocate request
          using a different transport (e.g., TCP instead of UDP).</dd>
          <dt pn="section-7.4-2.3">300 (Try Alternate):</dt>
          <dd pn="section-7.4-2.4">The server would like the client to use
            the server specified in the ALTERNATE-SERVER attribute instead.
            The client considers the current transaction as having failed, but it
            <bcp14>SHOULD</bcp14> try the Allocate request with the alternate server before
            trying any other servers (e.g., other servers discovered using the
            DNS resolution procedures). When trying the Allocate request with
            the alternate server, the client follows the ALTERNATE-SERVER
            procedures specified in <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/>.</dd>
          <dt pn="section-7.4-2.5">400 (Bad Request):</dt>
          <dd pn="section-7.4-2.6">The server believes the client's request is
            malformed for some reason. The client considers the current
            transaction as having failed. The client <bcp14>MAY</bcp14> notify the user or
            operator and <bcp14>SHOULD NOT</bcp14> retry the request with this server until
            it believes the problem has been fixed.</dd>
          <dt pn="section-7.4-2.7">401 (Unauthorized):</dt>
          <dd pn="section-7.4-2.8"> If the client has followed the procedures
            of the long-term credential mechanism and still gets this error,
            then the server is not accepting the client's credentials. In this
            case, the client considers the current transaction as having
            failed and <bcp14>SHOULD</bcp14> notify the user or operator. The client <bcp14>SHOULD NOT</bcp14> send any further requests to this server until it believes the
            problem has been fixed.</dd>
          <dt pn="section-7.4-2.9">403 (Forbidden):</dt>
          <dd pn="section-7.4-2.10">The request is valid, but the server is
            refusing to perform it, likely due to administrative restrictions.
            The client considers the current transaction as having failed. The
            client <bcp14>MAY</bcp14> notify the user or operator and <bcp14>SHOULD NOT</bcp14> retry the
            same request with this server until it believes the problem has
            been fixed.</dd>
          <dt pn="section-7.4-2.11">420 (Unknown Attribute):</dt>
          <dd pn="section-7.4-2.12">If the client included a DONT-FRAGMENT
            attribute in the request and the server rejected the request with
            a 420 error code and listed the DONT-FRAGMENT attribute in the
            UNKNOWN-ATTRIBUTES attribute in the error response, then the
            client now knows that the server does not support the
            DONT-FRAGMENT attribute. The client considers the current
            transaction as having failed but <bcp14>MAY</bcp14> choose to retry the Allocate
            request without the DONT-FRAGMENT attribute.</dd>
          <dt pn="section-7.4-2.13">437 (Allocation Mismatch):</dt>
          <dd pn="section-7.4-2.14">This indicates that the client has
            picked a 5-tuple that the server sees as already in use. One way
            this could happen is if an intervening NAT assigned a mapped
            transport address that was used by another client that recently
            crashed. The client considers the current transaction as having
            failed. The client <bcp14>SHOULD</bcp14> pick another client transport address
            and retry the Allocate request (using a different transaction id).
            The client <bcp14>SHOULD</bcp14> try three different client transport addresses
            before giving up on this server. Once the client gives up on the
            server, it <bcp14>SHOULD NOT</bcp14> try to create another allocation on the
            server for 2 minutes.</dd>
          <dt pn="section-7.4-2.15">438 (Stale Nonce):</dt>
          <dd pn="section-7.4-2.16">See the procedures for the long-term
            credential mechanism <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/>.</dd>
          <dt pn="section-7.4-2.17">440 (Address Family not Supported):</dt>
          <dd pn="section-7.4-2.18">The server does not support
            the address family requested by the client. If the client receives
            an Allocate error response with the 440 (Address Family not
	    Supported) error code, the client <bcp14>MUST NOT</bcp14> retry the request.</dd>
          <dt pn="section-7.4-2.19">441 (Wrong Credentials):</dt>
          <dd pn="section-7.4-2.20">The client should not receive this
            error in response to an Allocate request. The client <bcp14>MAY</bcp14> notify the
            user or operator and <bcp14>SHOULD NOT</bcp14> retry the same request with this
            server until it believes the problem has been fixed.</dd>
          <dt pn="section-7.4-2.21">442 (Unsupported Transport Address):</dt>
          <dd pn="section-7.4-2.22">The client should not
            receive this error in response to a request for a UDP allocation.
            The client <bcp14>MAY</bcp14> notify the user or operator and <bcp14>SHOULD NOT</bcp14>
            reattempt the request with this server until it believes the
            problem has been fixed.</dd>
          <dt pn="section-7.4-2.23">486 (Allocation Quota Reached):</dt>
          <dd pn="section-7.4-2.24">The server is currently unable
            to create any more allocations with this username. The client
            considers the current transaction as having failed. The client
            <bcp14>SHOULD</bcp14> wait at least 1 minute before trying to create any more
            allocations on the server.</dd>
          <dt pn="section-7.4-2.25">508 (Insufficient Capacity):</dt>
          <dd pn="section-7.4-2.26">

	    The server has no more relayed transport addresses available or has
	    none with the requested properties, or the one that was reserved
	    is no longer available.  The client considers the current
	    operation as having failed. If the client is using either the
	    EVEN-PORT or the RESERVATION-TOKEN attribute, then the client
	    <bcp14>MAY</bcp14> choose to remove or modify this attribute and
	    try again immediately. Otherwise, the client <bcp14>SHOULD</bcp14>
	    wait at least 1 minute before trying to create any more
	    allocations on this server.</dd>
        </dl>
        <t pn="section-7.4-3">Note that the error code values 486 and 508 indicate to a
        eavesdropper that several other users are using the server at this
        time, similar to that of the HTTP error response code 503, but
	it does
        not reveal any information about the users using the TURN server.</t>
        <t pn="section-7.4-4">An unknown error response <bcp14>MUST</bcp14> be handled as described in <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/>.</t>
      </section>
    </section>
    <section anchor="sec-refreshing-allocation" numbered="true" toc="include" removeInRFC="false" pn="section-8">
      <name slugifiedName="name-refreshing-an-allocation">Refreshing an Allocation</name>
      <t pn="section-8-1">A Refresh transaction can be used to either (a) refresh an existing
      allocation and update its time-to-expiry or (b) delete an existing
      allocation.</t>
      <t pn="section-8-2">If a client wishes to continue using an allocation, then the client
      <bcp14>MUST</bcp14> refresh it before it expires. It is suggested that the client
      refresh the allocation roughly 1 minute before it expires. If a client
      no longer wishes to use an allocation, then it <bcp14>SHOULD</bcp14> explicitly delete
      the allocation. A client <bcp14>MAY</bcp14> refresh an allocation at any time for other
      reasons.</t>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-8.1">
        <name slugifiedName="name-sending-a-refresh-request">Sending a Refresh Request</name>
        <t pn="section-8.1-1">If the client wishes to immediately delete an existing allocation,
        it includes a LIFETIME attribute with a value of zero. All other forms of
        the request refresh the allocation.</t>
        <t pn="section-8.1-2">When refreshing a dual allocation, the client includes
        a REQUESTED-ADDRESS-FAMILY attribute indicating the address family type
        that should be refreshed. If no REQUESTED-ADDRESS-FAMILY attribute is included,
        then the request should be treated as applying to all current
        allocations. The client <bcp14>MUST</bcp14> only include a family type it previously
        allocated and has not yet deleted. This process can also be used to
        delete an allocation of a specific address type by setting the
        lifetime of that Refresh request to zero. Deleting a single allocation
        destroys any permissions or channels associated with that particular
        allocation; it <bcp14>MUST NOT</bcp14> affect any permissions or channels associated
        with allocations for the other address family.</t>
        <t pn="section-8.1-3">The Refresh transaction updates the time-to-expiry timer of an
        allocation. If the client wishes the server to set the time-to-expiry
        timer to something other than the default lifetime, it includes a
        LIFETIME attribute with the requested value. The server then computes
        a new time-to-expiry value in the same way as it does for an Allocate
        transaction, with the exception that a requested lifetime of zero causes
        the server to immediately delete the allocation.</t>
      </section>
      <section anchor="sec-rcv-refresh" numbered="true" toc="include" removeInRFC="false" pn="section-8.2">
        <name slugifiedName="name-receiving-a-refresh-request">Receiving a Refresh Request</name>
        <t pn="section-8.2-1">When the server receives a Refresh request, it processes the
        request as per <xref target="sec-general-behavior" format="default" sectionFormat="of" derivedContent="Section 5"/> plus the
        specific rules mentioned here.</t>
        <t pn="section-8.2-2">If the server receives a Refresh Request with a
        REQUESTED-ADDRESS-FAMILY attribute and the attribute value does not
        match the address family of the allocation, the server <bcp14>MUST</bcp14> reply with
        a 443 (Peer Address Family Mismatch) Refresh error response.</t>
        <t pn="section-8.2-3">The server computes a value called the "desired lifetime" as
        follows: if the request contains a LIFETIME attribute and the
        attribute value is zero, then the "desired lifetime" is zero. Otherwise, if
        the request contains a LIFETIME attribute, then the server computes
        the minimum of the client's requested lifetime and the server's
        maximum allowed lifetime. If this computed value is greater than the
        default lifetime, then the "desired lifetime" is the computed value.
        Otherwise, the "desired lifetime" is the default lifetime.</t>
        <t pn="section-8.2-4">Subsequent processing depends on the "desired lifetime" value:</t>
        <ul spacing="normal" bare="false" empty="false" pn="section-8.2-5">
          <li pn="section-8.2-5.1">If the "desired lifetime" is zero, then the request succeeds and
            the allocation is deleted.</li>
          <li pn="section-8.2-5.2">If the "desired lifetime" is non-zero, then the request
            succeeds and the allocation's time-to-expiry is set to the
            "desired lifetime".</li>
        </ul>
        <t pn="section-8.2-6">If the request succeeds, then the server sends a success
        response containing:</t>
        <ul spacing="normal" bare="false" empty="false" pn="section-8.2-7">
          <li pn="section-8.2-7.1">A LIFETIME attribute containing the current value of the
            time-to-expiry timer.</li>
        </ul>
        <aside pn="section-8.2-8">
          <t pn="section-8.2-8.1">NOTE: A server need not do anything special to implement
            idempotency of Refresh requests over UDP using the "stateless
            stack approach". Retransmitted Refresh requests with a non-zero
            "desired lifetime" will simply refresh the allocation. A
            retransmitted Refresh request with a zero "desired lifetime" will
            cause a 437 (Allocation Mismatch) response if the allocation has
            already been deleted, but the client will treat this as equivalent
            to a success response (see below).</t>
        </aside>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-8.3">
        <name slugifiedName="name-receiving-a-refresh-respons">Receiving a Refresh Response</name>
        <t pn="section-8.3-1">If the client receives a success response to its Refresh request
        with a non-zero lifetime, it updates its copy of the allocation data
        structure with the time-to-expiry value contained in the response. If
        the client receives a 437 (Allocation Mismatch) error response to its
        request to refresh the allocation, it should consider the allocation
        no longer exists. If the client receives a 438 (Stale Nonce) error to
        its request to refresh the allocation, it should reattempt the request
        with the new nonce value.</t>
        <t pn="section-8.3-2">If the client receives a 437 (Allocation Mismatch) error response
        to a request to delete the allocation, then the allocation no longer
        exists and it should consider its request as having effectively
        succeeded.</t>
      </section>
    </section>
    <section anchor="sec-permissions" numbered="true" toc="include" removeInRFC="false" pn="section-9">
      <name slugifiedName="name-permissions-2">Permissions</name>
      <t pn="section-9-1">For each allocation, the server keeps a list of zero or more
      permissions. Each permission consists of an IP address and an associated
      time-to-expiry. While a permission exists, all peers using the IP
      address in the permission are allowed to send data to the client. The
      time-to-expiry is the number of seconds until the permission expires.
      Within the context of an allocation, a permission is uniquely identified
      by its associated IP address.</t>
      <t pn="section-9-2">By sending either CreatePermission requests or ChannelBind requests,
      the client can cause the server to install or refresh a permission for a
      given IP address. This causes one of two things to happen:</t>
      <ul spacing="normal" bare="false" empty="false" pn="section-9-3">
        <li pn="section-9-3.1">If no permission for that IP address exists, then a permission is
          created with the given IP address and a time-to-expiry equal to
          Permission Lifetime.</li>
        <li pn="section-9-3.2">If a permission for that IP address already exists, then the
          time-to-expiry for that permission is reset to Permission
          Lifetime.</li>
      </ul>
      <t pn="section-9-4">The Permission Lifetime <bcp14>MUST</bcp14> be 300 seconds (= 5 minutes).</t>
      <t pn="section-9-5">Each permission's time-to-expiry decreases down once per second
      until it reaches zero, at which point, the permission expires and is
      deleted.</t>
      <t pn="section-9-6">CreatePermission and ChannelBind requests may be freely intermixed on
      a permission. A given permission may be initially installed and/or
      refreshed with a CreatePermission request and then later refreshed with
      a ChannelBind request, or vice versa.</t>
      <t pn="section-9-7">When a UDP datagram arrives at the relayed transport address for the
      allocation, the server extracts the source IP address from the IP
      header. The server then compares this address with the IP address
      associated with each permission in the list of permissions for the
      allocation. Note that only addresses are compared and port numbers are
      not considered. If no match is found, relaying is not permitted and the
      server silently discards the UDP datagram. If an exact match is found,
      the permission check is considered to have succeeded and the server
      continues to process the UDP datagram as specified elsewhere (<xref target="sec-sending-data-indication" format="default" sectionFormat="of" derivedContent="Section 11.3"/>).</t>
      <t pn="section-9-8">The permissions for one allocation are totally unrelated to the
      permissions for a different allocation. If an allocation expires, all
      its permissions expire with it.</t>
      <aside pn="section-9-9">
        <t pn="section-9-9.1">NOTE: Though TURN permissions expire after 5 minutes, many NATs
          deployed at the time of publication expire their UDP bindings
          considerably faster. Thus, an application using TURN will probably
          wish to send some sort of keep-alive traffic at a much faster rate.
          Applications using ICE should follow the keep-alive guidelines of
          ICE <xref target="RFC8445" format="default" sectionFormat="of" derivedContent="RFC8445"/>, and applications not using ICE
          are advised to do something similar.</t>
      </aside>
    </section>
    <section numbered="true" toc="include" removeInRFC="false" pn="section-10">
      <name slugifiedName="name-createpermission">CreatePermission</name>
      <t pn="section-10-1">TURN supports two ways for the client to install or refresh
      permissions on the server. This section describes one way: the
      CreatePermission request.</t>
      <t pn="section-10-2">A CreatePermission request may be used in conjunction with either the
      Send mechanism in <xref target="sec-sendanddata" format="default" sectionFormat="of" derivedContent="Section 11"/> or the Channel
      mechanism in <xref target="sec-channels" format="default" sectionFormat="of" derivedContent="Section 12"/>.</t>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-10.1">
        <name slugifiedName="name-forming-a-createpermission-">Forming a CreatePermission Request</name>
        <t pn="section-10.1-1">The client who wishes to install or refresh one or more permissions
        can send a CreatePermission request to the server.</t>
        <t pn="section-10.1-2">When forming a CreatePermission request, the client <bcp14>MUST</bcp14> include at
        least one XOR-PEER-ADDRESS attribute and <bcp14>MAY</bcp14> include more than one
        such attribute. The IP address portion of each XOR-PEER-ADDRESS
        attribute contains the IP address for which a permission should be
        installed or refreshed. The port portion of each XOR-PEER-ADDRESS
        attribute will be ignored and can be any arbitrary value. The various
        XOR-PEER-ADDRESS attributes <bcp14>MAY</bcp14> appear in any order. The client <bcp14>MUST</bcp14>
        only include XOR-PEER-ADDRESS attributes with addresses of the same
        address family as that of the relayed transport address for the
        allocation. For dual allocations obtained using the
        ADDITIONAL-ADDRESS-FAMILY attribute, the client <bcp14>MAY</bcp14> include
        XOR-PEER-ADDRESS attributes with addresses of IPv4 and IPv6 address
        families.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-10.2">
        <name slugifiedName="name-receiving-a-createpermissio">Receiving a CreatePermission Request</name>
        <t pn="section-10.2-1">When the server receives the CreatePermission request, it processes
        as per <xref target="sec-general-behavior" format="default" sectionFormat="of" derivedContent="Section 5"/> plus the specific
        rules mentioned here.</t>
        <t pn="section-10.2-2">The message is checked for validity. The CreatePermission request
        <bcp14>MUST</bcp14> contain at least one XOR-PEER-ADDRESS attribute and <bcp14>MAY</bcp14> contain
        multiple such attributes. If no such attribute exists, or if any of
        these attributes are invalid, then a 400 (Bad Request) error is
        returned. If the request is valid, but the server is unable to satisfy
        the request due to some capacity limit or similar, then a 508
        (Insufficient Capacity) error is returned.</t>
        <t pn="section-10.2-3">If an XOR-PEER-ADDRESS attribute contains an address of an address
        family that is not the same as that of a relayed transport address for
        the allocation, the server <bcp14>MUST</bcp14> generate an error response with the
        443 (Peer Address Family Mismatch) response code.</t>
        <t pn="section-10.2-4">The server <bcp14>MAY</bcp14> impose restrictions on the IP address allowed in the
        XOR-PEER-ADDRESS attribute; if a value is not allowed, the server
        rejects the request with a 403 (Forbidden) error.</t>
        <t pn="section-10.2-5">If the message is valid and the server is capable of carrying out
        the request, then the server installs or refreshes a permission for
        the IP address contained in each XOR-PEER-ADDRESS attribute as
        described in <xref target="sec-permissions" format="default" sectionFormat="of" derivedContent="Section 9"/>. The port portion
        of each attribute is ignored and may be any arbitrary value.</t>
        <t pn="section-10.2-6">The server then responds with a CreatePermission success response.
        There are no mandatory attributes in the success response.</t>
        <aside pn="section-10.2-7">
          <t pn="section-10.2-7.1">NOTE: A server need not do anything special to implement
            idempotency of CreatePermission requests over UDP using the
            "stateless stack approach". Retransmitted CreatePermission
            requests will simply refresh the permissions.</t>
        </aside>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-10.3">
        <name slugifiedName="name-receiving-a-createpermission">Receiving a CreatePermission Response</name>
        <t pn="section-10.3-1">If the client receives a valid CreatePermission success response,
        then the client updates its data structures to indicate that the
        permissions have been installed or refreshed.</t>
      </section>
    </section>
    <section anchor="sec-sendanddata" numbered="true" toc="include" removeInRFC="false" pn="section-11">
      <name slugifiedName="name-send-and-data-methods">Send and Data Methods</name>
      <t pn="section-11-1">TURN supports two mechanisms for sending and receiving data from
      peers. This section describes the use of the Send and Data mechanisms,
      while <xref target="sec-channels" format="default" sectionFormat="of" derivedContent="Section 12"/> describes the use of the
      Channel mechanism.</t>
      <section anchor="sec-forming-indication" numbered="true" toc="include" removeInRFC="false" pn="section-11.1">
        <name slugifiedName="name-forming-a-send-indication">Forming a Send Indication</name>
        <t pn="section-11.1-1">The client can use a Send indication to pass data to the server for
        relaying to a peer. A client may use a Send indication even if a
        channel is bound to that peer. However, the client <bcp14>MUST</bcp14> ensure that
        there is a permission installed for the IP address of the peer to
        which the Send indication is being sent; this prevents a third party
        from using a TURN server to send data to arbitrary destinations.</t>
        <t pn="section-11.1-2">When forming a Send indication, the client <bcp14>MUST</bcp14> include an
        XOR-PEER-ADDRESS attribute and a DATA attribute. The XOR-PEER-ADDRESS
        attribute contains the transport address of the peer to which the data
        is to be sent, and the DATA attribute contains the actual application
        data to be sent to the peer.</t>
        <t pn="section-11.1-3">The client <bcp14>MAY</bcp14> include a DONT-FRAGMENT attribute in the Send
        indication if it wishes the server to set the DF bit on the UDP
        datagram sent to the peer.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-11.2">
        <name slugifiedName="name-receiving-a-send-indication">Receiving a Send Indication</name>
        <t pn="section-11.2-1">When the server receives a Send indication, it processes as per
        <xref target="sec-general-behavior" format="default" sectionFormat="of" derivedContent="Section 5"/> plus the specific rules
        mentioned here.</t>
        <t pn="section-11.2-2">The message is first checked for validity. The Send indication <bcp14>MUST</bcp14>
        contain both an XOR-PEER-ADDRESS attribute and a DATA attribute. If
        one of these attributes is missing or invalid, then the message is
        discarded. Note that the DATA attribute is allowed to contain zero
        bytes of data.</t>
        <t pn="section-11.2-3">The Send indication may also contain the DONT-FRAGMENT attribute.
        If the server is unable to set the DF bit on outgoing UDP datagrams
        when this attribute is present, then the server acts as if the
        DONT-FRAGMENT attribute is an unknown comprehension-required attribute
        (and thus the Send indication is discarded).</t>
        <t pn="section-11.2-4">The server also checks that there is a permission installed for the
        IP address contained in the XOR-PEER-ADDRESS attribute. If no such
        permission exists, the message is discarded. Note that a Send
        indication never causes the server to refresh the permission.</t>
        <t pn="section-11.2-5">The server <bcp14>MAY</bcp14> impose restrictions on the IP address and port
        values allowed in the XOR-PEER-ADDRESS attribute; if a value is not
        allowed, the server silently discards the Send indication.</t>
        <t pn="section-11.2-6">If everything is OK, then the server forms a UDP datagram as
        follows:</t>
        <ul spacing="normal" bare="false" empty="false" pn="section-11.2-7">
          <li pn="section-11.2-7.1">the source transport address is the relayed transport address
            of the allocation, where the allocation is determined by the
            5-tuple on which the Send indication arrived;</li>
          <li pn="section-11.2-7.2">the destination transport address is taken from the
            XOR-PEER-ADDRESS attribute;</li>
          <li pn="section-11.2-7.3">the data following the UDP header is the contents of the value
            field of the DATA attribute.</li>
        </ul>
        <t pn="section-11.2-8">The handling of the DONT-FRAGMENT attribute (if present), is
        described in Sections <xref target="sec-ip-header-fields" format="counter" sectionFormat="of" derivedContent="14"/> and <xref target="sec-ip-header-fields-tcp-udp" format="counter" sectionFormat="of" derivedContent="15"/>.</t>
        <t pn="section-11.2-9">The resulting UDP datagram is then sent to the peer.</t>
      </section>
      <section anchor="sec-sending-data-indication" numbered="true" toc="include" removeInRFC="false" pn="section-11.3">
        <name slugifiedName="name-receiving-a-udp-datagram">Receiving a UDP Datagram</name>
        <t pn="section-11.3-1">When the server receives a UDP datagram at a currently allocated
        relayed transport address, the server looks up the allocation
        associated with the relayed transport address. The server then checks
        to see whether the set of permissions for the allocation allow the
        relaying of the UDP datagram as described in <xref target="sec-permissions" format="default" sectionFormat="of" derivedContent="Section 9"/>.</t>
        <t pn="section-11.3-2">If relaying is permitted, then the server checks if there is a
        channel bound to the peer that sent the UDP datagram (see <xref target="sec-channels" format="default" sectionFormat="of" derivedContent="Section 12"/>). If a channel is bound, then processing
        proceeds as described in <xref target="sec-channel-relaying" format="default" sectionFormat="of" derivedContent="Section 12.7"/>.</t>
        <t pn="section-11.3-3">If relaying is permitted but no channel is bound to the peer, then
        the server forms and sends a Data indication. The Data indication <bcp14>MUST</bcp14>
        contain both an XOR-PEER-ADDRESS and a DATA attribute. The DATA
        attribute is set to the value of the "data octets" field
        from the datagram, and the XOR-PEER-ADDRESS attribute is set to the
        source transport address of the received UDP datagram. The Data
        indication is then sent on the 5-tuple associated with the
        allocation.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-11.4">
        <name slugifiedName="name-receiving-a-data-indication">Receiving a Data Indication</name>
        <t pn="section-11.4-1">When the client receives a Data indication, it checks that the Data
        indication contains an XOR-PEER-ADDRESS attribute and discards the
        indication if it does not. The client <bcp14>SHOULD</bcp14> also check that the
        XOR-PEER-ADDRESS attribute value contains an IP address with which the
        client believes there is an active permission and discard the Data
        indication otherwise.</t>
        <aside pn="section-11.4-2">
          <t pn="section-11.4-2.1">NOTE: The latter check protects the client against an attacker
            who somehow manages to trick the server into installing
            permissions not desired by the client.</t>
        </aside>
        <t pn="section-11.4-3">If the XOR-PEER-ADDRESS is present and valid, the client checks
        that the Data indication contains either a DATA attribute or an ICMP
        attribute and discards the indication if it does not. Note that a DATA
        attribute is allowed to contain zero bytes of data. Processing of Data
        indications with an ICMP attribute is described in <xref target="receive-senderror" format="default" sectionFormat="of" derivedContent="Section 11.6"/>.</t>
        <t pn="section-11.4-4">If the Data indication passes the above checks, the client delivers
        the data octets inside the DATA attribute to the application, along
        with an indication that they were received from the peer whose
        transport address is given by the XOR-PEER-ADDRESS attribute.</t>
      </section>
      <section anchor="sec-sending-senderror-indication" numbered="true" toc="include" removeInRFC="false" pn="section-11.5">
        <name slugifiedName="name-receiving-an-icmp-packet">Receiving an ICMP Packet</name>
        <t pn="section-11.5-1">When the server receives an ICMP packet, the server verifies that
        the type is either 3 or 11 for an ICMPv4 <xref target="RFC0792" format="default" sectionFormat="of" derivedContent="RFC0792"/> packet or either 1, 2, or 3 for an ICMPv6
        <xref target="RFC4443" format="default" sectionFormat="of" derivedContent="RFC4443"/> packet. It also verifies that the IP
        packet in the ICMP packet payload contains a UDP header. If either of
        these conditions fail, then the ICMP packet is silently dropped. If a
        UDP header is present, the server extracts the source and destination
        IP address and UDP port information.</t>
        <t pn="section-11.5-2">The server looks up the allocation whose relayed transport address
        corresponds to the encapsulated packet's source IP address and UDP
        port. If no such allocation exists, the packet is silently dropped.
        The server then checks to see whether the set of permissions for the
        allocation allows the relaying of the ICMP packet. For ICMP packets,
        the source IP address <bcp14>MUST NOT</bcp14> be checked against the permissions list
        as it would be for UDP packets. Instead, the server extracts the
        destination IP address from the encapsulated IP header. The server
        then compares this address with the IP address associated with each
        permission in the list of permissions for the allocation. If no match
        is found, relaying is not permitted and the server silently discards
        the ICMP packet. Note that only addresses are compared and port
        numbers are not considered.</t>
        <t pn="section-11.5-3">If relaying is permitted, then the server forms and sends a Data
        indication. The Data indication <bcp14>MUST</bcp14> contain both an XOR-PEER-ADDRESS
        and an ICMP attribute. The ICMP attribute is set to the value of the
        type and code fields from the ICMP packet. The IP address portion of
        XOR-PEER-ADDRESS attribute is set to the destination IP address in the
        encapsulated IP header. At the time of writing of this specification,
        Socket APIs on some operating systems do not deliver the destination
        port in the encapsulated UDP header to applications without superuser
        privileges. If destination port in the encapsulated UDP header is
        available to the server, then the port portion of the XOR-PEER-ADDRESS
        attribute is set to the destination port; otherwise, the port portion is
        set to zero. The Data indication is then sent on the 5-tuple associated
        with the allocation.</t>
        <aside pn="section-11.5-4">
          <t pn="section-11.5-4.1">Implementation Note: New ICMP types or codes can be defined in
        future specifications. If the server receives an ICMP error packet,
        and the new type or code field can help the client to make use of the
        ICMP error notification and generate feedback to the application
        layer, the server sends the Data indication with an ICMP attribute
        conveying the new ICMP type or code.</t>
        </aside>
      </section>
      <section anchor="receive-senderror" numbered="true" toc="include" removeInRFC="false" pn="section-11.6">
        <name slugifiedName="name-receiving-a-data-indication-">Receiving a Data Indication with an ICMP Attribute</name>
        <t pn="section-11.6-1">When the client receives a Data indication with an ICMP attribute,
        it checks that the Data indication contains an XOR-PEER-ADDRESS
        attribute and discards the indication if it does not. The client
        <bcp14>SHOULD</bcp14> also check that the XOR-PEER-ADDRESS attribute value contains
        an IP address with an active permission and discard the Data
        indication otherwise.</t>
        <t pn="section-11.6-2">If the Data indication passes the above checks, the client signals
        the application of the error condition along with an indication that
        it was received from the peer whose transport address is given by the
        XOR-PEER-ADDRESS attribute. The application can make sense of the
        meaning of the type and code values in the ICMP attribute by using the
        family field in the XOR-PEER-ADDRESS attribute.</t>
      </section>
    </section>
    <section anchor="sec-channels" numbered="true" toc="include" removeInRFC="false" pn="section-12">
      <name slugifiedName="name-channels-2">Channels</name>
      <t pn="section-12-1">Channels provide a way for the client and server to send application
      data using ChannelData messages, which have less overhead than Send and
      Data indications.</t>
      <t pn="section-12-2">The ChannelData message (see <xref target="sec-channeldata-msg" format="default" sectionFormat="of" derivedContent="Section 12.4"/>) starts with a two-byte field that
      carries the channel number. The values of this field are allocated as
      follows:</t>
      <table anchor="channels" align="center" pn="table-2">
        <tbody>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x0000 through 0x3FFF:</td>
            <td align="left" colspan="1" rowspan="1">These values can never be used for channel numbers.</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x4000 through 0x4FFF:</td>
            <td align="left" colspan="1" rowspan="1">These values are the allowed channel numbers (4096 possible values).</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x5000 through 0xFFFF:</td>
            <td align="left" colspan="1" rowspan="1">Reserved (For DTLS-SRTP multiplexing collision avoidance, see <xref target="RFC7983" format="default" sectionFormat="of" derivedContent="RFC7983"/>).</td>
          </tr>
        </tbody>
      </table>
      <t pn="section-12-4">Note that the channel number range is not backwards compatible with
      <xref target="RFC5766" format="default" sectionFormat="of" derivedContent="RFC5766"/>, which could prevent a client
      compliant with RFC 5766 from establishing channel bindings with a
      TURN server that complies with this specification.</t>
      <t pn="section-12-5">According to <xref target="RFC7983" format="default" sectionFormat="of" derivedContent="RFC7983"/>, ChannelData messages can
      be distinguished from other multiplexed protocols by examining the first
      byte of the message:</t>
      <table anchor="fig-demultiplexing" align="center" pn="table-3">
        <tbody>
          <tr>
            <td align="left" colspan="1" rowspan="1">[0..3]</td>
            <td align="center" colspan="1" rowspan="1">STUN</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">[16..19]</td>
            <td align="center" colspan="1" rowspan="1">ZRTP</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">[20..63]</td>
            <td align="center" colspan="1" rowspan="1">DTLS</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">[64..79]</td>
            <td align="center" colspan="1" rowspan="1">TURN Channel</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">[128..191]</td>
            <td align="center" colspan="1" rowspan="1">RTP/RTCP</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">Others</td>
            <td align="center" colspan="1" rowspan="1">Reserved; <bcp14>MUST</bcp14> be dropped and an alert
      <bcp14>MAY</bcp14> be logged</td>
          </tr>
        </tbody>
      </table>
      <t pn="section-12-7">Reserved values may be used in the future by other protocols. When
      the client uses channel binding, it <bcp14>MUST</bcp14> comply with the demultiplexing
      scheme discussed above.</t>
      <t pn="section-12-8">Channel bindings are always initiated by the client. The client can
      bind a channel to a peer at any time during the lifetime of the
      allocation. The client may bind a channel to a peer before exchanging
      data with it or after exchanging data with it (using Send and Data
      indications) for some time, or may choose never to bind a channel to it.
      The client can also bind channels to some peers while not binding
      channels to other peers.</t>
      <t pn="section-12-9">Channel bindings are specific to an allocation so that the use of a
      channel number or peer transport address in a channel binding in one
      allocation has no impact on their use in a different allocation. If an
      allocation expires, all its channel bindings expire with it.</t>
      <t pn="section-12-10">A channel binding consists of:</t>
      <ul spacing="normal" bare="false" empty="false" pn="section-12-11">
        <li pn="section-12-11.1">a channel number;</li>
        <li pn="section-12-11.2">a transport address (of the peer); and</li>
        <li pn="section-12-11.3">A time-to-expiry timer.</li>
      </ul>
      <t pn="section-12-12">Within the context of an allocation, a channel binding is
      uniquely identified either by the channel number or by the peer's
      transport address. Thus, the same channel cannot be bound to two
      different transport addresses, nor can the same transport address be
      bound to two different channels.</t>
      <t pn="section-12-13">A channel binding lasts for 10 minutes unless refreshed. Refreshing
      the binding (by the server receiving a ChannelBind request rebinding the
      channel to the same peer) resets the time-to-expiry timer back to 10
      minutes.</t>
      <t pn="section-12-14">When the channel binding expires, the channel becomes unbound. Once
      unbound, the channel number can be bound to a different transport
      address, and the transport address can be bound to a different channel
      number. To prevent race conditions, the client <bcp14>MUST</bcp14> wait 5 minutes after
      the channel binding expires before attempting to bind the channel number
      to a different transport address or the transport address to a different
      channel number.</t>
      <t pn="section-12-15">When binding a channel to a peer, the client <bcp14>SHOULD</bcp14> be prepared to
      receive ChannelData messages on the channel from the server as soon as
      it has sent the ChannelBind request. Over UDP, it is possible for the
      client to receive ChannelData messages from the server before it
      receives a ChannelBind success response.</t>
      <t pn="section-12-16">In the other direction, the client <bcp14>MAY</bcp14> elect to send ChannelData
      messages before receiving the ChannelBind success response. Doing so,
      however, runs the risk of having the ChannelData messages dropped by the
      server if the ChannelBind request does not succeed for some reason
      (e.g., packet lost if the request is sent over UDP or the server being
      unable to fulfill the request). A client that wishes to be safe should
      either queue the data or use Send indications until the channel binding
      is confirmed.</t>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-12.1">
        <name slugifiedName="name-sending-a-channelbind-reque">Sending a ChannelBind Request</name>
        <t pn="section-12.1-1">A channel binding is created or refreshed using a ChannelBind
        transaction. A ChannelBind transaction also creates or refreshes a
        permission towards the peer (see <xref target="sec-permissions" format="default" sectionFormat="of" derivedContent="Section 9"/>).</t>
        <t pn="section-12.1-2">To initiate the ChannelBind transaction, the client forms a
        ChannelBind request. The channel to be bound is specified in a
        CHANNEL-NUMBER attribute, and the peer's transport address is
        specified in an XOR-PEER-ADDRESS attribute. <xref target="sec-receiving-ChannelBind" format="default" sectionFormat="of" derivedContent="Section 12.2"/> describes the restrictions
        on these attributes. The client <bcp14>MUST</bcp14> only include an XOR-PEER-ADDRESS
        attribute with an address of the same address family as that of a
        relayed transport address for the allocation.</t>
        <t pn="section-12.1-3">Rebinding a channel to the same transport address that it is
        already bound to provides a way to refresh a channel binding and the
        corresponding permission without sending data to the peer. Note,
        however, that permissions need to be refreshed more frequently than
        channels.</t>
      </section>
      <section anchor="sec-receiving-ChannelBind" numbered="true" toc="include" removeInRFC="false" pn="section-12.2">
        <name slugifiedName="name-receiving-a-channelbind-req">Receiving a ChannelBind Request</name>
        <t pn="section-12.2-1">When the server receives a ChannelBind request, it processes as per
        <xref target="sec-general-behavior" format="default" sectionFormat="of" derivedContent="Section 5"/> plus the specific rules
        mentioned here.</t>
        <t pn="section-12.2-2">The server checks the following:</t>
        <ul spacing="normal" bare="false" empty="false" pn="section-12.2-3">
          <li pn="section-12.2-3.1">The request contains both a CHANNEL-NUMBER and an
            XOR-PEER-ADDRESS attribute;</li>
          <li pn="section-12.2-3.2">The channel number is in the range 0x4000 through 0x4FFF
            (inclusive);</li>
          <li pn="section-12.2-3.3">The channel number is not currently bound to a different
            transport address (same transport address is OK);</li>
          <li pn="section-12.2-3.4">The transport address is not currently bound to a different
            channel number.</li>
        </ul>
        <t pn="section-12.2-4">If any of these tests fail, the server replies with a 400 (Bad
        Request) error. If the XOR-PEER-ADDRESS attribute contains an address
        of an address family that is not the same as that of a relayed
        transport address for the allocation, the server <bcp14>MUST</bcp14> generate an
        error response with the 443 (Peer Address Family Mismatch) response
        code.</t>
        <t pn="section-12.2-5">The server <bcp14>MAY</bcp14> impose restrictions on the IP address and port
        values allowed in the XOR-PEER-ADDRESS attribute; if a value is not
        allowed, the server rejects the request with a 403 (Forbidden)
        error.</t>
        <t pn="section-12.2-6">If the request is valid, but the server is unable to fulfill the
        request due to some capacity limit or similar, the server replies with
        a 508 (Insufficient Capacity) error.</t>
        <t pn="section-12.2-7">Otherwise, the server replies with a ChannelBind success response.
        There are no required attributes in a successful ChannelBind
        response.</t>
        <t pn="section-12.2-8">If the server can satisfy the request, then the server creates or
        refreshes the channel binding using the channel number in the
        CHANNEL-NUMBER attribute and the transport address in the
        XOR-PEER-ADDRESS attribute. The server also installs or refreshes a
        permission for the IP address in the XOR-PEER-ADDRESS attribute as
        described in <xref target="sec-permissions" format="default" sectionFormat="of" derivedContent="Section 9"/>.</t>
        <aside pn="section-12.2-9">
          <t pn="section-12.2-9.1">NOTE: A server need not do anything special to implement
            idempotency of ChannelBind requests over UDP using the "stateless
            stack approach". Retransmitted ChannelBind requests will simply
            refresh the channel binding and the corresponding permission.
            Furthermore, the client must wait 5 minutes before binding a
            previously bound channel number or peer address to a different
            channel, eliminating the possibility that the transaction would
            initially fail but succeed on a retransmission.</t>
        </aside>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-12.3">
        <name slugifiedName="name-receiving-a-channelbind-res">Receiving a ChannelBind Response</name>
        <t pn="section-12.3-1">When the client receives a ChannelBind success response, it updates
        its data structures to record that the channel binding is now active.
        It also updates its data structures to record that the corresponding
        permission has been installed or refreshed.</t>
        <t pn="section-12.3-2">If the client receives a ChannelBind failure response that
        indicates that the channel information is out of sync between the
        client and the server (e.g., an unexpected 400 "Bad Request"
        response), then it is <bcp14>RECOMMENDED</bcp14> that the client immediately delete
        the allocation and start afresh with a new allocation.</t>
      </section>
      <section anchor="sec-channeldata-msg" numbered="true" toc="include" removeInRFC="false" pn="section-12.4">
        <name slugifiedName="name-the-channeldata-message">The ChannelData Message</name>
        <t pn="section-12.4-1">The ChannelData message is used to carry application data between
        the client and the server. It has the following format:</t>
        <figure align="left" suppress-title="false" pn="figure-5">
          <artwork name="" type="" align="left" alt="" pn="section-12.4-2.1">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Channel Number        |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
/                       Application Data                        /
/                                                               /
|                                                               |
|                               +-------------------------------+
|                               |
+-------------------------------+</artwork>
        </figure>
        <t pn="section-12.4-3">The Channel Number field specifies the number of the channel on
        which the data is traveling, and thus, the address of the peer that is
        sending or is to receive the data.</t>
        <t pn="section-12.4-4">The Length field specifies the length in bytes of the application
        data field (i.e., it does not include the size of the ChannelData
        header). Note that 0 is a valid length.</t>
        <t pn="section-12.4-5">The Application Data field carries the data the client is trying to
        send to the peer, or that the peer is sending to the client.</t>
      </section>
      <section anchor="sec-sending-channeldata-msg" numbered="true" toc="include" removeInRFC="false" pn="section-12.5">
        <name slugifiedName="name-sending-a-channeldata-messa">Sending a ChannelData Message</name>
        <t pn="section-12.5-1">Once a client has bound a channel to a peer, then when the client
        has data to send to that peer, it may use either a ChannelData message
        or a Send indication; that is, the client is not obligated to use the
        channel when it exists and may freely intermix the two message types
        when sending data to the peer. The server, on the other hand, <bcp14>MUST</bcp14> use
        the ChannelData message if a channel has been bound to the peer. The
        server uses a Data indication to signal the XOR-PEER-ADDRESS and ICMP
        attributes to the client even if a channel has been bound to the
        peer.</t>
        <t pn="section-12.5-2">The fields of the ChannelData message are filled in as described in
        <xref target="sec-channeldata-msg" format="default" sectionFormat="of" derivedContent="Section 12.4"/>.</t>
        <t pn="section-12.5-3">Over TCP and TLS-over-TCP, the ChannelData message <bcp14>MUST</bcp14> be padded
        to a multiple of four bytes in order to ensure the alignment of
        subsequent messages. The padding is not reflected in the length field
        of the ChannelData message, so the actual size of a ChannelData
        message (including padding) is (4 + Length) rounded up to the nearest
        multiple of 4 (see <xref target="RFC8489" section="14" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8489#section-14" derivedContent="RFC8489"/>). Over UDP, the padding is not
        required but <bcp14>MAY</bcp14> be included.</t>
        <t pn="section-12.5-4">The ChannelData message is then sent on the 5-tuple associated with
        the allocation.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-12.6">
        <name slugifiedName="name-receiving-a-channeldata-mes">Receiving a ChannelData Message</name>
        <t pn="section-12.6-1">The receiver of the ChannelData message uses the first byte to
        distinguish it from other multiplexed protocols as described in <xref target="fig-demultiplexing" format="default" sectionFormat="of" derivedContent="Table 3"/>. If the message uses a
        value in the reserved range (0x5000 through 0xFFFF), then the message
        is silently discarded.</t>
        <t pn="section-12.6-2">If the ChannelData message is received in a UDP datagram, and if
        the UDP datagram is too short to contain the claimed length of the
        ChannelData message (i.e., the UDP header length field value is less
        than the ChannelData header length field value + 4 + 8), then the
        message is silently discarded.</t>
        <t pn="section-12.6-3">If the ChannelData message is received over TCP or over
        TLS-over-TCP, then the actual length of the ChannelData message is as
        described in <xref target="sec-sending-channeldata-msg" format="default" sectionFormat="of" derivedContent="Section 12.5"/>.</t>
        <t pn="section-12.6-4">If the ChannelData message is received on a channel that is not
        bound to any peer, then the message is silently discarded.</t>
        <t pn="section-12.6-5">On the client, it is <bcp14>RECOMMENDED</bcp14> that the client discard the
        ChannelData message if the client believes there is no active
        permission towards the peer. On the server, the receipt of a
        ChannelData message <bcp14>MUST NOT</bcp14> refresh either the channel binding or the
        permission towards the peer.</t>
        <t pn="section-12.6-6">On the server, if no errors are detected, the server relays the
        application data to the peer by forming a UDP datagram as
        follows:</t>
        <ul spacing="normal" bare="false" empty="false" pn="section-12.6-7">
          <li pn="section-12.6-7.1">the source transport address is the relayed transport address
            of the allocation, where the allocation is determined by the
            5-tuple on which the ChannelData message arrived;</li>
          <li pn="section-12.6-7.2">the destination transport address is the transport address to
            which the channel is bound;</li>
          <li pn="section-12.6-7.3">the data following the UDP header is the contents of the data
            field of the ChannelData message.</li>
        </ul>
        <t pn="section-12.6-8">The resulting UDP datagram is then sent to the peer. Note
        that if the Length field in the ChannelData message is 0, then there
        will be no data in the UDP datagram, but the UDP datagram is still
        formed and sent (<xref target="RFC6263" sectionFormat="of" section="4.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc6263#section-4.1" derivedContent="RFC6263"/>).</t>
      </section>
      <section anchor="sec-channel-relaying" numbered="true" toc="include" removeInRFC="false" pn="section-12.7">
        <name slugifiedName="name-relaying-data-from-the-peer">Relaying Data from the Peer</name>
        <t pn="section-12.7-1">When the server receives a UDP datagram on the relayed transport
        address associated with an allocation, the server processes it as
        described in <xref target="sec-sending-data-indication" format="default" sectionFormat="of" derivedContent="Section 11.3"/>. If
        that section indicates that a ChannelData message should be sent
        (because there is a channel bound to the peer that sent to the UDP
        datagram), then the server forms and sends a ChannelData message as
        described in <xref target="sec-sending-channeldata-msg" format="default" sectionFormat="of" derivedContent="Section 12.5"/>.</t>
        <t pn="section-12.7-2">When the server receives an ICMP packet, the server processes it as
        described in <xref target="sec-sending-senderror-indication" format="default" sectionFormat="of" derivedContent="Section 11.5"/>.</t>
      </section>
    </section>
    <section numbered="true" toc="include" removeInRFC="false" pn="section-13">
      <name slugifiedName="name-packet-translations">Packet Translations</name>
      <t pn="section-13-1">This section addresses IPv4-to-IPv6, IPv6-to-IPv4, and IPv6-to-IPv6
      translations. Requirements for translation of the IP addresses and port
      numbers of the packets are described above. The following sections
      specify how to translate other header fields.</t>
      <t pn="section-13-2">As discussed in <xref target="unpriv" format="default" sectionFormat="of" derivedContent="Section 3.6"/>, translations in TURN
      are designed so that a TURN server can be implemented as an application
      that runs in user space under commonly available operating systems and
      that does not require special privileges. The translations specified in
      the following sections follow this principle.</t>
      <t pn="section-13-3">The descriptions below have two parts: a preferred behavior and an
      alternate behavior. The server <bcp14>SHOULD</bcp14> implement the preferred behavior,
      but if that is not possible for a particular field, the server <bcp14>MUST</bcp14>
      implement the alternate behavior and <bcp14>MUST NOT</bcp14> do anything else for the
      reasons detailed in <xref target="RFC7915" format="default" sectionFormat="of" derivedContent="RFC7915"/>. The TURN server
      solely relies on the DF bit in the IPv4 header and the Fragment header
      in the IPv6 header to handle fragmentation using the approach described in
      <xref target="RFC7915" format="default" sectionFormat="of" derivedContent="RFC7915"/> and does not rely on the DONT-FRAGMENT
      attribute; ignoring the DONT-FRAGMENT attribute is only applicable for UDP-to-UDP
      relay and not for TCP-to-UDP relay.</t>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-13.1">
        <name slugifiedName="name-ipv4-to-ipv6-translations">IPv4-to-IPv6 Translations</name>
        <t pn="section-13.1-1">Time to Live (TTL) field</t>
        <ul empty="true" spacing="normal" bare="false" pn="section-13.1-2">
          <li pn="section-13.1-2.1">Preferred Behavior: As specified in <xref target="RFC7915" sectionFormat="of" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7915#section-4" derivedContent="RFC7915"/>.</li>
          <li pn="section-13.1-2.2">Alternate Behavior: Set the outgoing value to the default for
            outgoing packets.</li>
        </ul>
        <t pn="section-13.1-3">Traffic Class</t>
        <ul empty="true" spacing="normal" bare="false" pn="section-13.1-4">
          <li pn="section-13.1-4.1">Preferred behavior: As specified in <xref target="RFC7915" sectionFormat="of" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7915#section-4" derivedContent="RFC7915"/>.</li>
          <li pn="section-13.1-4.2">Alternate behavior: The TURN server sets the Traffic Class to
            the default value for outgoing packets.</li>
        </ul>
        <t pn="section-13.1-5">Flow Label</t>
        <ul empty="true" spacing="normal" bare="false" pn="section-13.1-6">
          <li pn="section-13.1-6.1">Preferred behavior: The TURN server can use the 5-tuple of
            relayed transport address, peer transport address, and UDP protocol
            number to identify each flow and to generate and set the flow
            label value in the IPv6 packet as discussed in <xref target="RFC6437" sectionFormat="of" section="3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc6437#section-3" derivedContent="RFC6437"/>. If the TURN server is incapable of
            generating the flow label value from the IPv6 packet's 5-tuple, it
            sets the Flow label to zero.</li>
          <li pn="section-13.1-6.2">Alternate behavior: The alternate behavior is the same as the
            preferred behavior for a TURN server that does not support flow
            labels.</li>
        </ul>
        <t pn="section-13.1-7">Hop Limit</t>
        <ul empty="true" spacing="normal" bare="false" pn="section-13.1-8">
          <li pn="section-13.1-8.1">Preferred behavior: As specified in <xref target="RFC7915" sectionFormat="of" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7915#section-4" derivedContent="RFC7915"/>.</li>
          <li pn="section-13.1-8.2">Alternate behavior: The TURN server sets the Hop Limit to the
            default value for outgoing packets.</li>
        </ul>
        <t pn="section-13.1-9">Fragmentation</t>
        <ul empty="true" spacing="normal" bare="false" pn="section-13.1-10">
          <li pn="section-13.1-10.1">Preferred behavior: As specified in <xref target="RFC7915" sectionFormat="of" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7915#section-4" derivedContent="RFC7915"/>.</li>
          <li pn="section-13.1-10.2">Alternate behavior: The TURN server assembles incoming
            fragments. The TURN server follows its default behavior to send
            outgoing packets.</li>
          <li pn="section-13.1-10.3">For both preferred and alternate behavior, the DONT-FRAGMENT
            attribute <bcp14>MUST</bcp14> be ignored by the server.</li>
        </ul>
        <t pn="section-13.1-11">Extension Headers</t>
        <ul empty="true" spacing="normal" bare="false" pn="section-13.1-12">
          <li pn="section-13.1-12.1">Preferred behavior: The outgoing packet uses the system
            defaults for IPv6 extension headers, with the exception of the
            Fragment header as described above.</li>
          <li pn="section-13.1-12.2">Alternate behavior: Same as preferred.</li>
        </ul>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-13.2">
        <name slugifiedName="name-ipv6-to-ipv6-translations">IPv6-to-IPv6 Translations</name>
        <t pn="section-13.2-1">Flow Label</t>
        <t pn="section-13.2-2">NOTE: The TURN server should consider that it is handling two different
        IPv6 flows. Therefore, the Flow label <xref target="RFC6437" format="default" sectionFormat="of" derivedContent="RFC6437"/>
          <bcp14>SHOULD NOT</bcp14> be copied as part of the translation.
</t>
        <ul empty="true" spacing="normal" bare="false" pn="section-13.2-3">
          <li pn="section-13.2-3.1">Preferred behavior: The TURN server can use the 5-tuple of relayed
  transport address, peer transport address, and UDP protocol number to
  identify each flow and to generate and set the flow label value in the IPv6
  packet as discussed in <xref target="RFC6437" sectionFormat="of" section="3" format="default" derivedLink="https://rfc-editor.org/rfc/rfc6437#section-3" derivedContent="RFC6437"/>. If the TURN server is incapable of generating the flow
  label value from the IPv6 packet's 5-tuple, it sets the Flow label to
  zero.</li>
          <li pn="section-13.2-3.2">Alternate behavior: The alternate behavior is the same as the
            preferred behavior for a TURN server that does not support flow
            labels.</li>
        </ul>
        <t pn="section-13.2-4">Hop Limit</t>
        <ul empty="true" spacing="normal" bare="false" pn="section-13.2-5">
          <li pn="section-13.2-5.1">Preferred behavior: The TURN server acts as a regular router
            with respect to decrementing the Hop Limit and generating an
            ICMPv6 error if it reaches zero.</li>
          <li pn="section-13.2-5.2">Alternate behavior: The TURN server sets the Hop Limit to the
            default value for outgoing packets.</li>
        </ul>
        <t pn="section-13.2-6">Fragmentation</t>
        <ul empty="true" spacing="normal" bare="false" pn="section-13.2-7">
          <li pn="section-13.2-7.1">Preferred behavior: If the incoming packet did not include a
            Fragment header and the outgoing packet size does not exceed the
            outgoing link's MTU, the TURN server sends the outgoing packet
            without a Fragment header.</li>
          <li pn="section-13.2-7.2">If the incoming packet did not include a Fragment header and
            the outgoing packet size exceeds the outgoing link's MTU, the TURN
            server drops the outgoing packet and sends an ICMP message of type
            2 code 0 ("Packet too big") to the sender of the incoming packet.
            If the ICMPv6 packet ("Packet too big") is being sent to the peer,
            the TURN server <bcp14>SHOULD</bcp14> reduce the MTU reported in the ICMP message
            by 48 bytes to allow room for the overhead of a Data
            indication.</li>
          <li pn="section-13.2-7.3">If the incoming packet included a Fragment header and the
            outgoing packet size (with a Fragment header included) does not
            exceed the outgoing link's MTU, the TURN server sends the outgoing
            packet with a Fragment header. The TURN server sets the fields of
            the Fragment header as appropriate for a packet originating from
            the server.</li>
          <li pn="section-13.2-7.4">If the incoming packet included a Fragment header and the
            outgoing packet size exceeds the outgoing link's MTU, the TURN
            server <bcp14>MUST</bcp14> fragment the outgoing packet into fragments of no more
            than 1280 bytes. The TURN server sets the fields of the Fragment
            header as appropriate for a packet originating from the
            server.</li>
          <li pn="section-13.2-7.5">Alternate behavior: The TURN server assembles incoming
            fragments. The TURN server follows its default behavior to send
            outgoing packets.</li>
          <li pn="section-13.2-7.6">For both preferred and alternate behavior, the DONT-FRAGMENT
            attribute <bcp14>MUST</bcp14> be ignored by the server.</li>
        </ul>
        <t pn="section-13.2-8">Extension Headers</t>
        <ul empty="true" spacing="normal" bare="false" pn="section-13.2-9">
          <li pn="section-13.2-9.1">Preferred behavior: The outgoing packet uses the system
            defaults for IPv6 extension headers, with the exception of the
            Fragment header as described above.</li>
          <li pn="section-13.2-9.2">Alternate behavior: Same as preferred.</li>
        </ul>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-13.3">
        <name slugifiedName="name-ipv6-to-ipv4-translations">IPv6-to-IPv4 Translations</name>
        <t pn="section-13.3-1">Type of Service and Precedence</t>
        <ul empty="true" spacing="normal" bare="false" pn="section-13.3-2">
          <li pn="section-13.3-2.1">Preferred behavior: As specified in <xref target="RFC7915" sectionFormat="of" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7915#section-5" derivedContent="RFC7915"/>.</li>
          <li pn="section-13.3-2.2">Alternate behavior: The TURN server sets the Type of Service
            and Precedence to the default value for outgoing packets.</li>
        </ul>
        <t pn="section-13.3-3">Time to Live</t>
        <ul empty="true" spacing="normal" bare="false" pn="section-13.3-4">
          <li pn="section-13.3-4.1">Preferred behavior: As specified in <xref target="RFC7915" sectionFormat="of" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7915#section-5" derivedContent="RFC7915"/>.</li>
          <li pn="section-13.3-4.2">Alternate behavior: The TURN server sets the Time to Live to
            the default value for outgoing packets.</li>
        </ul>
        <t pn="section-13.3-5">Fragmentation</t>
        <ul empty="true" spacing="normal" bare="false" pn="section-13.3-6">
          <li pn="section-13.3-6.1">Preferred behavior: As specified in <xref target="RFC7915" sectionFormat="of" section="5" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7915#section-5" derivedContent="RFC7915"/>. Additionally, when the outgoing packet's
            size exceeds the outgoing link's MTU, the TURN server needs to
            generate an ICMP error (ICMPv6 "Packet too big") reporting the MTU
            size. If the ICMPv4 packet (Destination Unreachable (Type 3) with
            Code 4) is being sent to the peer, the TURN server <bcp14>SHOULD</bcp14> reduce
            the MTU reported in the ICMP message by 48 bytes to allow room for
            the overhead of a Data indication.</li>
          <li pn="section-13.3-6.2">Alternate behavior: The TURN server assembles incoming
            fragments. The TURN server follows its default behavior to send
            outgoing packets.</li>
          <li pn="section-13.3-6.3">For both preferred and alternate behavior, the DONT-FRAGMENT
            attribute <bcp14>MUST</bcp14> be ignored by the server.</li>
        </ul>
      </section>
    </section>
    <section anchor="sec-ip-header-fields" numbered="true" toc="include" removeInRFC="false" pn="section-14">
      <name slugifiedName="name-udp-to-udp-relay">UDP-to-UDP Relay</name>
      <t pn="section-14-1">This section describes how the server sets various fields in the IP
      header for UDP-to-UDP relay from the client to the peer or vice versa.
      The descriptions in this section apply (a) when the server sends a UDP
      datagram to the peer or (b) when the server sends a Data indication or
      ChannelData message to the client over UDP transport. The descriptions
      in this section do not apply to TURN messages sent over TCP or TLS
      transport from the server to the client.</t>
      <t pn="section-14-2">The descriptions below have two parts: a preferred behavior and an
      alternate behavior. The server <bcp14>SHOULD</bcp14> implement the preferred behavior,
      but if that is not possible for a particular field, then it <bcp14>SHOULD</bcp14>
      implement the alternative behavior.</t>
      <t pn="section-14-3">Differentiated Services Code Point (DSCP) field <xref target="RFC2474" format="default" sectionFormat="of" derivedContent="RFC2474"/></t>
      <ul empty="true" spacing="normal" bare="false" pn="section-14-4">
        <li pn="section-14-4.1">Preferred Behavior: Set the outgoing value to the incoming value
          unless the server includes a differentiated services classifier and
          marker <xref target="RFC2474" format="default" sectionFormat="of" derivedContent="RFC2474"/>.</li>
        <li pn="section-14-4.2">Alternate Behavior: Set the outgoing value to a fixed value,
          which by default is Best Effort unless configured otherwise.</li>
        <li pn="section-14-4.3">In both cases, if the server is immediately adjacent to a
          differentiated services classifier and marker, then DSCP <bcp14>MAY</bcp14> be set
          to any arbitrary value in the direction towards the classifier.</li>
      </ul>
      <t pn="section-14-5">Explicit Congestion Notification (ECN) field <xref target="RFC3168" format="default" sectionFormat="of" derivedContent="RFC3168"/></t>
      <ul empty="true" spacing="normal" bare="false" pn="section-14-6">
        <li pn="section-14-6.1">Preferred Behavior: Set the outgoing value to the incoming value.
          The server may perform Active Queue Management, in which case it
          <bcp14>SHOULD</bcp14> behave as an ECN-aware router <xref target="RFC3168" format="default" sectionFormat="of" derivedContent="RFC3168"/>
          and can mark traffic with Congestion Experienced (CE) instead of
          dropping the packet. The use of ECT(1) is subject to experimental
          usage <xref target="RFC8311" format="default" sectionFormat="of" derivedContent="RFC8311"/>.</li>
        <li pn="section-14-6.2">Alternate Behavior: Set the outgoing value to Not-ECT
          (=0b00).</li>
      </ul>
      <t pn="section-14-7">IPv4 Fragmentation fields (applicable only for IPv4-to-IPv4
      relay)</t>
      <ul empty="true" spacing="normal" bare="false" pn="section-14-8">
        <li pn="section-14-8.1">Preferred Behavior: When the server sends a packet to a peer in
          response to a Send indication containing the DONT-FRAGMENT
          attribute, then set the outgoing UDP packet to not fragment. In all
          other cases, when sending an outgoing packet containing application
          data (e.g., Data indication, a ChannelData message, or the DONT-FRAGMENT
          attribute not included in the Send indication), copy the DF bit from
          the DF bit of the incoming packet that contained the application
          data.</li>
        <li pn="section-14-8.2">Set the other fragmentation fields (Identification, More
          Fragments, Fragment Offset) as appropriate for a packet originating
          from the server.</li>
        <li pn="section-14-8.3">Alternate Behavior: As described in the Preferred Behavior,
          except always assume the incoming DF bit is 0.</li>
        <li pn="section-14-8.4">In both the Preferred and Alternate Behaviors, the resulting
          packet may be too large for the outgoing link. If this is the case,
          then the normal fragmentation rules apply <xref target="RFC1122" format="default" sectionFormat="of" derivedContent="RFC1122"/>.</li>
      </ul>
      <t pn="section-14-9">IPv4 Options</t>
      <ul empty="true" spacing="normal" bare="false" pn="section-14-10">
        <li pn="section-14-10.1">Preferred Behavior: The outgoing packet uses the system defaults
          for IPv4 options.</li>
        <li pn="section-14-10.2">Alternate Behavior: Same as preferred.</li>
      </ul>
    </section>
    <section anchor="sec-ip-header-fields-tcp-udp" numbered="true" toc="include" removeInRFC="false" pn="section-15">
      <name slugifiedName="name-tcp-to-udp-relay">TCP-to-UDP Relay</name>
      <t pn="section-15-1">This section describes how the server sets various fields in the IP
      header for TCP-to-UDP relay from the client to the peer. The
      descriptions in this section apply when the server sends a UDP datagram
      to the peer. Note that the server does not perform per-packet
      translation for TCP-to-UDP relaying.</t>
      <t pn="section-15-2">Multipath TCP <xref target="I-D.ietf-mptcp-rfc6824bis" format="default" sectionFormat="of" derivedContent="TCP-EXT"/> is not supported by this version of TURN because TCP
      multipath is not used by either SIP or WebRTC protocols <xref target="RFC7478" format="default" sectionFormat="of" derivedContent="RFC7478"/> for media and non-media data. TCP
      connection between the TURN client and server can use the TCP
      Authentication Option (TCP-AO) <xref target="RFC5925" format="default" sectionFormat="of" derivedContent="RFC5925"/>, but UDP does not provide a similar type of
      authentication, though it might be added in the future <xref target="I-D.ietf-tsvwg-udp-options" format="default" sectionFormat="of" derivedContent="UDP-OPT"/>. Even if both
      TCP-AO and UDP authentication would be used between TURN client and
      server, it would not change the end-to-end security properties of the
      application payload being relayed. Therefore, applications using TURN
      will need to secure their application data end to end appropriately,
      e.g., Secure Real-time Transport Protocol (SRTP) for RTP
      applications. Note that the TCP-AO option obsoletes the TCP MD5
      option.</t>
      <t pn="section-15-3">Unlike UDP, TCP without the TCP Fast Open extension <xref target="RFC7413" format="default" sectionFormat="of" derivedContent="RFC7413"/> does not support 0-RTT session
      resumption. The TCP user timeout <xref target="RFC5482" format="default" sectionFormat="of" derivedContent="RFC5482"/> equivalent for application data relayed by the TURN
      is the use of RTP control protocol (RTCP). As a reminder, RTCP is a
      fundamental and integral part of RTP.</t>
      <t pn="section-15-4">The descriptions below have two parts: a preferred behavior and an
      alternate behavior. The server <bcp14>SHOULD</bcp14> implement the preferred behavior,
      but if that is not possible for a particular field, then it <bcp14>SHOULD</bcp14>
      implement the alternative behavior.</t>
      <t pn="section-15-5">For the UDP datagram sent to the peer based on a Send Indication or
      ChannelData message arriving at the TURN server over a TCP Transport,
      the server sets various fields in the IP header as follows:</t>
      <t pn="section-15-6">Differentiated Services Code Point (DSCP) field <xref target="RFC2474" format="default" sectionFormat="of" derivedContent="RFC2474"/></t>
      <ul empty="true" spacing="normal" bare="false" pn="section-15-7">
        <li pn="section-15-7.1">Preferred Behavior: The TCP connection can only use a single DSCP,
          so inter-flow differentiation is not possible; see
          <xref target="RFC7657" sectionFormat="of" section="5.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7657#section-5.1" derivedContent="RFC7657"/>. The server sets the
          outgoing value to the DSCP used by the TCP connection,
          unless the server includes a differentiated services classifier and
          marker <xref target="RFC2474" format="default" sectionFormat="of" derivedContent="RFC2474"/>.</li>
        <li pn="section-15-7.2">Alternate Behavior: Set the outgoing value to a fixed value,
          which by default is Best Effort unless configured otherwise.</li>
        <li pn="section-15-7.3">In both cases, if the server is immediately adjacent to a
          differentiated services classifier and marker, then DSCP <bcp14>MAY</bcp14> be set
          to any arbitrary value in the direction towards the classifier.</li>
      </ul>
      <t pn="section-15-8">Explicit Congestion Notification (ECN) field <xref target="RFC3168" format="default" sectionFormat="of" derivedContent="RFC3168"/></t>
      <ul empty="true" spacing="normal" bare="false" pn="section-15-9">
        <li pn="section-15-9.1">Preferred Behavior: No mechanism is defined to indicate what ECN
          value should be used for the outgoing UDP datagrams of an
          allocation; therefore, set the outgoing value to Not-ECT (=0b00).</li>
        <li pn="section-15-9.2">Alternate Behavior: Same as preferred.</li>
      </ul>
      <t pn="section-15-10">IPv4 Fragmentation fields (applicable only for IPv4-to-IPv4
      relay)</t>
      <ul empty="true" spacing="normal" bare="false" pn="section-15-11">
        <li pn="section-15-11.1">Preferred Behavior: When the server sends a packet to a peer in
          response to a Send indication containing the DONT-FRAGMENT
          attribute, set the outgoing UDP packet to not fragment. In all
          other cases, when sending an outgoing UDP packet containing
          application data (e.g., Data indication, ChannelData message, or
          DONT-FRAGMENT attribute not included in the Send indication), set
          the DF bit in the outgoing IP header to 0.</li>
        <li pn="section-15-11.2">Alternate Behavior: Same as preferred.</li>
      </ul>
      <t pn="section-15-12">IPv6 Fragmentation fields</t>
      <ul empty="true" spacing="normal" bare="false" pn="section-15-13">
        <li pn="section-15-13.1">Preferred Behavior: If the TCP traffic arrives over IPv6, the
          server relies on the presence of the DONT-FRAGMENT attribute in the send
          indication to set the outgoing UDP packet to not fragment.</li>
        <li pn="section-15-13.2">Alternate Behavior: Same as preferred.</li>
      </ul>
      <t pn="section-15-14">IPv4 Options</t>
      <ul empty="true" spacing="normal" bare="false" pn="section-15-15">
        <li pn="section-15-15.1">Preferred Behavior: The outgoing packet uses the system defaults
          for IPv4 options.</li>
        <li pn="section-15-15.2">Alternate Behavior: Same as preferred.</li>
      </ul>
    </section>
    <section anchor="UDP-to-TCP" numbered="true" toc="include" removeInRFC="false" pn="section-16">
      <name slugifiedName="name-udp-to-tcp-relay">UDP-to-TCP Relay</name>
      <t pn="section-16-1">This section describes how the server sets various fields in the IP
      header for UDP-to-TCP relay from the peer to the client. The
      descriptions in this section apply when the server sends a Data
      indication or ChannelData message to the client over TCP or TLS
      transport. Note that the server does not perform per-packet translation
      for UDP-to-TCP relaying.</t>
      <t pn="section-16-2">The descriptions below have two parts: a preferred behavior and an
      alternate behavior. The server <bcp14>SHOULD</bcp14> implement the preferred behavior,
      but if that is not possible for a particular field, then it <bcp14>SHOULD</bcp14>
      implement the alternative behavior.</t>
      <t pn="section-16-3">The TURN server sets IP header fields in the TCP packets on a
      per-connection basis for the TCP connection as follows:</t>
      <t pn="section-16-4">Differentiated Services Code Point (DSCP) field <xref target="RFC2474" format="default" sectionFormat="of" derivedContent="RFC2474"/></t>
      <ul empty="true" spacing="normal" bare="false" pn="section-16-5">
        <li pn="section-16-5.1">Preferred Behavior: Ignore the incoming DSCP value. When TCP is
          used between the client and the server, a single DSCP should be used
          for all traffic on that TCP connection. Note, TURN/ICE occurs before
          application data is exchanged.</li>
        <li pn="section-16-5.2">Alternate Behavior: Same as preferred.</li>
      </ul>
      <t pn="section-16-6">Explicit Congestion Notification (ECN) field <xref target="RFC3168" format="default" sectionFormat="of" derivedContent="RFC3168"/></t>
      <ul empty="true" spacing="normal" bare="false" pn="section-16-7">
        <li pn="section-16-7.1">Preferred Behavior: Ignore; ECN signals are dropped in the TURN
          server for the incoming UDP datagrams from the peer.</li>
        <li pn="section-16-7.2">Alternate Behavior: Same as preferred.</li>
      </ul>
      <t pn="section-16-8">Fragmentation </t>
      <ul empty="true" spacing="normal" bare="false" pn="section-16-9">
        <li pn="section-16-9.1">Preferred Behavior: Any fragmented packets are reassembled in the
          server and then forwarded to the client over the TCP connection.
          ICMP messages resulting from the UDP datagrams sent to the peer are
          processed by the server as described in <xref target="sec-sending-senderror-indication" format="default" sectionFormat="of" derivedContent="Section 11.5"/> and forwarded to
          the client using TURN's mechanism for relevant ICMP types and
          codes.</li>
        <li pn="section-16-9.2">Alternate Behavior: Same as preferred.</li>
      </ul>
      <t pn="section-16-10">Extension Headers</t>
      <ul empty="true" spacing="normal" bare="false" pn="section-16-11">
        <li pn="section-16-11.1">Preferred behavior: The outgoing packet uses the system defaults
          for IPv6 extension headers.</li>
        <li pn="section-16-11.2">Alternate behavior: Same as preferred.</li>
      </ul>
      <t pn="section-16-12">IPv4 Options</t>
      <ul empty="true" spacing="normal" bare="false" pn="section-16-13">
        <li pn="section-16-13.1">Preferred Behavior: The outgoing packet uses the system defaults
          for IPv4 options.</li>
        <li pn="section-16-13.2">Alternate Behavior: Same as preferred.</li>
      </ul>
    </section>
    <section anchor="sec-stun-methods" numbered="true" toc="include" removeInRFC="false" pn="section-17">
      <name slugifiedName="name-stun-methods">STUN Methods</name>
      <t pn="section-17-1">This section lists the code points for the STUN methods defined in
      this specification. See elsewhere in this document for the semantics of
      these methods.</t>
      <table anchor="stun-methods" align="center" pn="table-4">
        <tbody>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x003</td>
            <td align="left" colspan="1" rowspan="1">Allocate</td>
            <td align="left" colspan="1" rowspan="1">(only request/response semantics defined)</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x004</td>
            <td align="left" colspan="1" rowspan="1">Refresh</td>
            <td align="left" colspan="1" rowspan="1">(only request/response semantics defined)</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x006</td>
            <td align="left" colspan="1" rowspan="1">Send</td>
            <td align="left" colspan="1" rowspan="1">(only indication semantics defined)</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x007</td>
            <td align="left" colspan="1" rowspan="1">Data</td>
            <td align="left" colspan="1" rowspan="1">(only indication semantics defined)</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x008</td>
            <td align="left" colspan="1" rowspan="1">CreatePermission</td>
            <td align="left" colspan="1" rowspan="1">(only request/response semantics defined)</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x009</td>
            <td align="left" colspan="1" rowspan="1">ChannelBind</td>
            <td align="left" colspan="1" rowspan="1">(only request/response semantics defined)</td>
          </tr>
        </tbody>
      </table>
    </section>
    <section anchor="sec-stun-attributes" numbered="true" toc="include" removeInRFC="false" pn="section-18">
      <name slugifiedName="name-stun-attributes">STUN Attributes</name>
      <t pn="section-18-1">This STUN extension defines the following
        attributes:</t>
      <table anchor="stun-attributes" align="center" pn="table-5">
        <tbody>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x000C</td>
            <td align="left" colspan="1" rowspan="1">CHANNEL-NUMBER</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x000D</td>
            <td align="left" colspan="1" rowspan="1">LIFETIME</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x0010</td>
            <td align="left" colspan="1" rowspan="1">Reserved (was BANDWIDTH)</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x0012</td>
            <td align="left" colspan="1" rowspan="1">XOR-PEER-ADDRESS</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x0013</td>
            <td align="left" colspan="1" rowspan="1">DATA</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x0016</td>
            <td align="left" colspan="1" rowspan="1">XOR-RELAYED-ADDRESS</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x0017</td>
            <td align="left" colspan="1" rowspan="1">REQUESTED-ADDRESS-FAMILY</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x0018</td>
            <td align="left" colspan="1" rowspan="1">EVEN-PORT</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x0019</td>
            <td align="left" colspan="1" rowspan="1">REQUESTED-TRANSPORT</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x001A</td>
            <td align="left" colspan="1" rowspan="1">DONT-FRAGMENT</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x0021</td>
            <td align="left" colspan="1" rowspan="1">Reserved (was TIMER-VAL)</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x0022</td>
            <td align="left" colspan="1" rowspan="1">RESERVATION-TOKEN</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x8000</td>
            <td align="left" colspan="1" rowspan="1">ADDITIONAL-ADDRESS-FAMILY</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x8001</td>
            <td align="left" colspan="1" rowspan="1">ADDRESS-ERROR-CODE</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x8004</td>
            <td align="left" colspan="1" rowspan="1">ICMP</td>
          </tr>
        </tbody>
      </table>
      <t pn="section-18-3">Some of these attributes have lengths that are not multiples of 4. By
      the rules of STUN, any attribute whose length is not a multiple of 4
      bytes <bcp14>MUST</bcp14> be immediately followed by 1 to 3 padding bytes to ensure the
      next attribute (if any) would start on a 4-byte boundary (see <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/>).</t>
      <section anchor="channelnums" numbered="true" toc="include" removeInRFC="false" pn="section-18.1">
        <name slugifiedName="name-channel-number">CHANNEL-NUMBER</name>
        <t pn="section-18.1-1">The CHANNEL-NUMBER attribute contains the number of the channel.
        The value portion of this attribute is 4 bytes long and consists of a
        16-bit unsigned integer followed by a two-octet RFFU (Reserved For
        Future Use) field, which <bcp14>MUST</bcp14> be set to 0 on transmission and <bcp14>MUST</bcp14> be
        ignored on reception.</t>
        <figure align="left" suppress-title="false" pn="figure-6">
          <artwork name="" type="" align="left" alt="" pn="section-18.1-2.1">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Channel Number         |         RFFU = 0              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        </figure>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-18.2">
        <name slugifiedName="name-lifetime">LIFETIME</name>
        <t pn="section-18.2-1">The LIFETIME attribute represents the duration for which the server
        will maintain an allocation in the absence of a refresh. The TURN
        client can include the LIFETIME attribute with the desired lifetime in
        Allocate and Refresh requests. The value portion of this attribute is
        4 bytes long and consists of a 32-bit unsigned integral value
        representing the number of seconds remaining until expiration.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-18.3">
        <name slugifiedName="name-xor-peer-address">XOR-PEER-ADDRESS</name>
        <t pn="section-18.3-1">The XOR-PEER-ADDRESS attribute specifies the address and port of the peer as
        seen from the TURN server. (For example, the peer's server-reflexive
        transport address if the peer is behind a NAT.) It is encoded in the
        same way as the XOR-MAPPED-ADDRESS attribute <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/>.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-18.4">
        <name slugifiedName="name-data">DATA</name>
        <t pn="section-18.4-1">The DATA attribute is present in all Send indications. If the ICMP
        attribute is not present in a Data indication, it contains a DATA
        attribute. The value portion of this attribute is variable length and
        consists of the application data (that is, the data that would
        immediately follow the UDP header if the data was sent directly
        between the client and the peer). The application data is equivalent
        to the "UDP user data" and does not include the "surplus area" defined
        in <xref target="I-D.ietf-tsvwg-udp-options" sectionFormat="of" section="4" format="default" derivedLink="https://tools.ietf.org/html/draft-ietf-tsvwg-udp-options-08#section-4" derivedContent="UDP-OPT"/>. If
        the length of this attribute is not a multiple of 4, then padding must
        be added after this attribute.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-18.5">
        <name slugifiedName="name-xor-relayed-address">XOR-RELAYED-ADDRESS</name>
        <t pn="section-18.5-1">The XOR-RELAYED-ADDRESS attribute is present in Allocate responses. It
        specifies the address and port that the server allocated to the
        client. It is encoded in the same way as the XOR-MAPPED-ADDRESS
	attribute <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/>.</t>
      </section>
      <section anchor="sec-requested-address-family" numbered="true" toc="include" removeInRFC="false" pn="section-18.6">
        <name slugifiedName="name-requested-address-family">REQUESTED-ADDRESS-FAMILY</name>
        <t pn="section-18.6-1">This attribute is used in Allocate and Refresh requests to specify
        the address type requested by the client. The value of this attribute
        is 4 bytes with the following format:</t>
        <figure align="left" suppress-title="false" pn="figure-7">
          <artwork name="" type="" align="left" alt="" pn="section-18.6-2.1">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Family    |            Reserved                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        </figure>
        <dl newline="false" spacing="normal" pn="section-18.6-3">
          <dt pn="section-18.6-3.1">Family:</dt>
          <dd pn="section-18.6-3.2">There are two values defined for this field and specified in
          <xref target="RFC8489" section="14.1" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8489#section-14.1" derivedContent="RFC8489"/>: 0x01 for
          IPv4 addresses and 0x02 for IPv6 addresses.</dd>
          <dt pn="section-18.6-3.3">Reserved:</dt>
          <dd pn="section-18.6-3.4">At this point, the 24 bits in the Reserved
            field <bcp14>MUST</bcp14> be set to zero by the client and <bcp14>MUST</bcp14> be ignored by the
            server.</dd>
        </dl>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-18.7">
        <name slugifiedName="name-even-port">EVEN-PORT</name>
        <t pn="section-18.7-1">This attribute allows the client to request that the port in the
        relayed transport address be even and (optionally) that the server
        reserve the next-higher port number. The value portion of this
        attribute is 1 byte long. Its format is:</t>
        <figure align="left" suppress-title="false" pn="figure-8">
          <artwork name="" type="" align="left" alt="" pn="section-18.7-2.1">
   0               
   0 1 2 3 4 5 6 7 
  +-+-+-+-+-+-+-+-+
  |R|    RFFU     |
  +-+-+-+-+-+-+-+-+</artwork>
        </figure>
        <t pn="section-18.7-3">The value contains a single 1-bit flag:</t>
        <dl newline="false" spacing="normal" pn="section-18.7-4">
          <dt pn="section-18.7-4.1">R:</dt>
          <dd pn="section-18.7-4.2">If 1, the server is requested to reserve the
            next-higher port number (on the same IP address) for a subsequent
            allocation. If 0, no such reservation is requested.</dd>
          <dt pn="section-18.7-4.3">RFFU:</dt>
          <dd pn="section-18.7-4.4">Reserved For Future Use.</dd>
        </dl>
        <t pn="section-18.7-5">The RFFU field must be set to zero on transmission and
        ignored on reception.</t>
        <t pn="section-18.7-6">Since the length of this attribute is not a multiple of 4, padding
        must immediately follow this attribute.</t>
      </section>
      <section anchor="sec-requested-transport" numbered="true" toc="include" removeInRFC="false" pn="section-18.8">
        <name slugifiedName="name-requested-transport">REQUESTED-TRANSPORT</name>
        <t pn="section-18.8-1">This attribute is used by the client to request a specific
        transport protocol for the allocated transport address. The value of
        this attribute is 4 bytes with the following format:</t>
        <figure align="left" suppress-title="false" pn="figure-9">
          <artwork name="" type="" align="left" alt="" pn="section-18.8-2.1">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Protocol   |                    RFFU                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        </figure>
        <t pn="section-18.8-3">The Protocol field specifies the desired protocol. The code points
        used in this field are taken from those allowed in the Protocol field
        in the IPv4 header and the NextHeader field in the IPv6 header <xref target="PROTOCOL-NUMBERS" format="default" sectionFormat="of" derivedContent="PROTOCOL-NUMBERS"/>. This specification only
        allows the use of code point 17 (User Datagram Protocol).</t>
        <t pn="section-18.8-4">The RFFU field <bcp14>MUST</bcp14> be set to zero on transmission and <bcp14>MUST</bcp14> be
        ignored on reception. It is reserved for future uses.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-18.9">
        <name slugifiedName="name-dont-fragment">DONT-FRAGMENT</name>
        <t pn="section-18.9-1">This attribute is used by the client to request that the server set
        the DF (Don't Fragment) bit in the IP header when relaying the
        application data onward to the peer and for determining the server
        capability in Allocate requests. This attribute has no value part, and
        thus, the attribute length field is 0.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-18.10">
        <name slugifiedName="name-reservation-token">RESERVATION-TOKEN</name>
        <t pn="section-18.10-1">The RESERVATION-TOKEN attribute contains a token that uniquely
        identifies a relayed transport address being held in reserve by the
        server. The server includes this attribute in a success response to
        tell the client about the token, and the client includes this
        attribute in a subsequent Allocate request to request the server use
        that relayed transport address for the allocation.</t>
        <t pn="section-18.10-2">The attribute value is 8 bytes and contains the token value.</t>
      </section>
      <section anchor="sec-additional-address-family" numbered="true" toc="include" removeInRFC="false" pn="section-18.11">
        <name slugifiedName="name-additional-address-family">ADDITIONAL-ADDRESS-FAMILY</name>
        <t pn="section-18.11-1">This attribute is used by clients to request the allocation of an
        IPv4 and IPv6 address type from a server. It is encoded in the same
        way as the REQUESTED-ADDRESS-FAMILY attribute; see <xref target="sec-requested-address-family" format="default" sectionFormat="of" derivedContent="Section 18.6"/>. The
        ADDITIONAL-ADDRESS-FAMILY attribute <bcp14>MAY</bcp14> be present in
        the Allocate request. The attribute value of 0x02 (IPv6 address) is
        the only valid value in Allocate request.</t>
      </section>
      <section anchor="sec-address-error-code" numbered="true" toc="include" removeInRFC="false" pn="section-18.12">
        <name slugifiedName="name-address-error-code">ADDRESS-ERROR-CODE</name>
        <t pn="section-18.12-1">This attribute is used by servers to signal the reason for not
        allocating the requested address family. The value portion of this
        attribute is variable length with the following format:</t>
        <figure align="left" suppress-title="false" pn="figure-10">
          <artwork name="" type="" align="left" alt="" pn="section-18.12-2.1">
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Family       |    Reserved             |Class|     Number    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Reason Phrase (variable)                                ..
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        </figure>
        <dl newline="false" spacing="normal" pn="section-18.12-3">
          <dt pn="section-18.12-3.1">Family:</dt>
          <dd pn="section-18.12-3.2">There are two values defined for this field and specified in
          <xref target="RFC8489" sectionFormat="of" section="14.1" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8489#section-14.1" derivedContent="RFC8489"/>: 0x01 for
          IPv4 addresses and 0x02 for IPv6 addresses.</dd>
          <dt pn="section-18.12-3.3">Reserved:</dt>
          <dd pn="section-18.12-3.4">At this point, the 13 bits in the Reserved
            field <bcp14>MUST</bcp14> be set to zero by the server and <bcp14>MUST</bcp14> be ignored by the
            client.</dd>
          <dt pn="section-18.12-3.5">Class:</dt>
          <dd pn="section-18.12-3.6">The Class represents the hundreds digit of
            the error code and is defined in <xref target="RFC8489" sectionFormat="of" section="14.8" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8489#section-14.8" derivedContent="RFC8489"/>.</dd>
          <dt pn="section-18.12-3.7">Number:</dt>
          <dd pn="section-18.12-3.8">This 8-bit field contains the reason the server
            cannot allocate one of the requested address types. The error code
            values could be either 440 (Address Family not Supported) or 508
            (Insufficient Capacity). The number representation is defined in
            <xref target="RFC8489" sectionFormat="of" section="14.8" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8489#section-14.8" derivedContent="RFC8489"/>.</dd>
          <dt pn="section-18.12-3.9">Reason Phrase:</dt>
          <dd pn="section-18.12-3.10">The recommended reason phrases for
            error codes 440 and 508 are explained in <xref target="sec-stun-errors" format="default" sectionFormat="of" derivedContent="Section 19"/>. The reason phrase <bcp14>MUST</bcp14> be a
            UTF-8 <xref target="RFC3629" format="default" sectionFormat="of" derivedContent="RFC3629"/> encoded sequence of less than
            128 characters (which can be as long as 509 bytes when encoding
            them or 763 bytes when decoding them).</dd>
        </dl>
      </section>
      <section anchor="icmp" numbered="true" toc="include" removeInRFC="false" pn="section-18.13">
        <name slugifiedName="name-icmp">ICMP</name>
        <t pn="section-18.13-1">This attribute is used by servers to signal the reason a UDP
        packet was dropped. The following is the format of the ICMP
        attribute.</t>
        <figure align="left" suppress-title="false" pn="figure-11">
          <artwork name="" type="" align="left" alt="" pn="section-18.13-2.1">
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Reserved                     |  ICMP Type  |  ICMP Code      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Error Data                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        </figure>
        <dl newline="false" spacing="normal" pn="section-18.13-3">
          <dt pn="section-18.13-3.1">Reserved:</dt>
          <dd pn="section-18.13-3.2">This field <bcp14>MUST</bcp14> be set to 0 when sent and
            <bcp14>MUST</bcp14> be ignored when received.</dd>
          <dt pn="section-18.13-3.3">ICMP Type:</dt>
          <dd pn="section-18.13-3.4">The field contains the value of the ICMP
            type. Its interpretation depends on whether the ICMP was received
            over IPv4 or IPv6.</dd>
          <dt pn="section-18.13-3.5">ICMP Code:</dt>
          <dd pn="section-18.13-3.6">The field contains the value of the ICMP
            code. Its interpretation depends on whether the ICMP was received
            over IPv4 or IPv6.</dd>
          <dt pn="section-18.13-3.7">Error Data:</dt>
          <dd pn="section-18.13-3.8">This field size is 4 bytes long. If the ICMPv6 type is 2
          ("Packet too big" message) or ICMPv4 type is 3 (Destination
          Unreachable) and Code is 4 (fragmentation needed and DF set), the
          Error Data field will be set to the Maximum Transmission Unit of the
          next-hop link (<xref target="RFC4443" sectionFormat="of" section="3.2" format="default" derivedLink="https://rfc-editor.org/rfc/rfc4443#section-3.2" derivedContent="RFC4443"/> and <xref target="RFC1191" sectionFormat="of" section="4" format="default" derivedLink="https://rfc-editor.org/rfc/rfc1191#section-4" derivedContent="RFC1191"/>). For other ICMPv6 types and ICMPv4 types and
          codes, the Error Data field <bcp14>MUST</bcp14> be set to zero.</dd>
        </dl>
      </section>
    </section>
    <section anchor="sec-stun-errors" numbered="true" toc="include" removeInRFC="false" pn="section-19">
      <name slugifiedName="name-stun-error-response-codes">STUN Error Response Codes</name>
      <t pn="section-19-1">This document defines the following error response codes:</t>
      <dl newline="true" spacing="normal" pn="section-19-2">
        <dt pn="section-19-2.1">403 (Forbidden):</dt>
        <dd pn="section-19-2.2">The request was valid but cannot be
          performed due to administrative or similar restrictions.</dd>
        <dt pn="section-19-2.3">437 (Allocation Mismatch):</dt>
        <dd pn="section-19-2.4">A request was received by
          the server that requires an allocation to be in place, but no
          allocation exists, or a request was received that requires no
          allocation, but an allocation exists.</dd>
        <dt pn="section-19-2.5">440 (Address Family not Supported):</dt>
        <dd pn="section-19-2.6">The server does
          not support the address family requested by the client.</dd>
        <dt pn="section-19-2.7">441 (Wrong Credentials):</dt>
        <dd pn="section-19-2.8">(Wrong Credentials): The credentials in the
          (non-Allocate) request do not match those used to create the
          allocation.</dd>
        <dt pn="section-19-2.9">442 (Unsupported Transport Protocol):</dt>
        <dd pn="section-19-2.10">The Allocate
          request asked the server to use a transport protocol between the
          server and the peer that the server does not support. NOTE: This
          does NOT refer to the transport protocol used in the 5-tuple.</dd>
        <dt pn="section-19-2.11">443 (Peer Address Family Mismatch):</dt>
        <dd pn="section-19-2.12">A peer address is
          part of a different address family than that of the relayed
          transport address of the allocation.</dd>
        <dt pn="section-19-2.13">486 (Allocation Quota Reached):</dt>
        <dd pn="section-19-2.14">No more allocations
          using this username can be created at the present time.</dd>
        <dt pn="section-19-2.15">508 (Insufficient Capacity):</dt>
        <dd pn="section-19-2.16">The server is unable to
          carry out the request due to some capacity limit being reached. In
          an Allocate response, this could be due to the server having no more
          relayed transport addresses available at that time, having none with
          the requested properties, or the one that corresponds to the
          specified reservation token is not available.</dd>
      </dl>
    </section>
    <section numbered="true" toc="include" removeInRFC="false" pn="section-20">
      <name slugifiedName="name-detailed-example">Detailed Example</name>
      <t pn="section-20-1">This section gives an example of the use of TURN, showing in detail
      the contents of the messages exchanged. The example uses the network
      diagram shown in the Overview (<xref target="fig-turn-model" format="default" sectionFormat="of" derivedContent="Figure 1"/>).</t>
      <t pn="section-20-2">For each message, the attributes included in the message and their
      values are shown. For convenience, values are shown in a human-readable
      format rather than showing the actual octets; for example,
      "XOR-RELAYED-ADDRESS=192.0.2.15:9000" shows that the XOR-RELAYED-ADDRESS
      attribute is included with an address of 192.0.2.15 and a port of 9000;
      here, the address and port are shown before the xor-ing is done. For
      attributes with string-like values (e.g., SOFTWARE="Example client,
      version 1.03" and NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"), the value
      of the attribute is shown in quotes for readability, but these quotes do
      not appear in the actual value.</t>
      <figure align="left" suppress-title="false" pn="figure-12">
        <artwork name="" type="" align="left" alt="" pn="section-20-3.1">
TURN                                 TURN          Peer         Peer
client                               server         A            B
  |                                    |            |            |
  |--- Allocate request --------------&gt;|            |            |
  |    Transaction-Id=0xA56250D3F17ABE679422DE85    |            |
  |    SOFTWARE="Example client, version 1.03"      |            |
  |    LIFETIME=3600 (1 hour)          |            |            |
  |    REQUESTED-TRANSPORT=17 (UDP)    |            |            | 
  |    DONT-FRAGMENT                   |            |            |
  |                                    |            |            |
  |&lt;-- Allocate error response --------|            |            |
  |    Transaction-Id=0xA56250D3F17ABE679422DE85    |            |
  |    SOFTWARE="Example server, version 1.17"      |            |
  |    ERROR-CODE=401 (Unauthorized)   |            |            |
  |    REALM="example.com"             |            |            |
  |    NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"     |            |
  |    PASSWORD-ALGORITHMS=MD5 and SHA256           |            |
  |                                    |            |            |
  |--- Allocate request --------------&gt;|            |            |
  |    Transaction-Id=0xC271E932AD7446A32C234492    |            |
  |    SOFTWARE="Example client 1.03"  |            |            |
  |    LIFETIME=3600 (1 hour)          |            |            |
  |    REQUESTED-TRANSPORT=17 (UDP)    |            |            |
  |    DONT-FRAGMENT                   |            |            |
  |    USERNAME="George"               |            |            |
  |    REALM="example.com"             |            |            |
  |    NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"     |            |
  |    PASSWORD-ALGORITHMS=MD5 and SHA256           |            |
  |    PASSWORD-ALGORITHM=SHA256       |            |            |    
  |    MESSAGE-INTEGRITY=...           |            |            |
  |    MESSAGE-INTEGRITY-SHA256=...    |            |            |
  |                                    |            |            |
  |&lt;-- Allocate success response ------|            |            |
  |    Transaction-Id=0xC271E932AD7446A32C234492    |            |
  |    SOFTWARE="Example server, version 1.17"      |            |
  |    LIFETIME=1200 (20 minutes)      |            |            |
  |    XOR-RELAYED-ADDRESS=192.0.2.15:50000         |            |
  |    XOR-MAPPED-ADDRESS=192.0.2.1:7000            |            |
  |    MESSAGE-INTEGRITY-SHA256=...    |            |            |
</artwork>
      </figure>
      <t pn="section-20-4">The client begins by selecting a host transport address to use for
      the TURN session; in this example, the client has selected
      198.51.100.2:49721 as shown in <xref target="fig-turn-model" format="default" sectionFormat="of" derivedContent="Figure 1"/>.
      The client then sends an Allocate request to the server at the server
      transport address. The client randomly selects a 96-bit transaction id
      of 0xA56250D3F17ABE679422DE85 for this transaction; this is encoded in
      the transaction id field in the fixed header. The client includes a
      SOFTWARE attribute that gives information about the client's software;
      here, the value is "Example client, version 1.03" to indicate that this
      is version 1.03 of something called the "Example client". The client
      includes the LIFETIME attribute because it wishes the allocation to have
      a longer lifetime than the default of 10 minutes; the value of this
      attribute is 3600 seconds, which corresponds to 1 hour. The client must
      always include a REQUESTED-TRANSPORT attribute in an Allocate request,
      and the only value allowed by this specification is 17, which indicates
      UDP transport between the server and the peers. The client also includes
      the DONT-FRAGMENT attribute because it wishes to use the DONT-FRAGMENT
      attribute later in Send indications; this attribute consists of only an
      attribute header; there is no value part. We assume the client has not
      recently interacted with the server; thus, the client does not include
      the USERNAME, USERHASH, REALM, NONCE, PASSWORD-ALGORITHMS,
      PASSWORD-ALGORITHM, MESSAGE-INTEGRITY, or MESSAGE-INTEGRITY-SHA256
      attribute. Finally, note that the order of attributes in a message is
      arbitrary (except for the MESSAGE-INTEGRITY, MESSAGE-INTEGRITY-SHA256
      and FINGERPRINT attributes), and the client could have used a different
      order.</t>
      <t pn="section-20-5">Servers require any request to be authenticated. Thus, when the
      server receives the initial Allocate request, it rejects the request
      because the request does not contain the authentication attributes.
      Following the procedures of the long-term credential mechanism of STUN
      <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/>, the server includes an
      ERROR-CODE attribute with a value of 401 (Unauthorized), a REALM
      attribute that specifies the authentication realm used by the server (in
      this case, the server's domain "example.com"), and a nonce value in a
      NONCE attribute. The NONCE attribute starts with the "nonce cookie" with
      the STUN Security Feature "Password algorithm" bit set to 1. The server
      includes a PASSWORD-ALGORITHMS attribute that specifies the list of
      algorithms that the server can use to derive the long-term password. If
      the server sets the STUN Security Feature "Username anonymity" bit to 1,
      then the client uses the USERHASH attribute instead of the USERNAME
      attribute in the Allocate request to anonymize the username. The server
      also includes a SOFTWARE attribute that gives information about the
      server's software.</t>
      <t pn="section-20-6">The client, upon receipt of the 401 error, reattempts the Allocate
      request, this time including the authentication attributes. The client
      selects a new transaction id and then populates the new Allocate
      request with the same attributes as before. The client includes a
      USERNAME attribute and uses the realm value received from the server to
      help it determine which value to use; here, the client is configured to
      use the username "George" for the realm "example.com". The client
      includes the PASSWORD-ALGORITHM attribute indicating the algorithm that
      the server must use to derive the long-term password. The client also
      includes the REALM, PASSWORD-ALGORITHMS, and NONCE attributes, which are
      just copied from the 401 error response. Finally, the client includes
      MESSAGE-INTEGRITY-SHA256 attribute as the last attributes in the
      message whose value is Hashed Message Authentication Code - Secure Hash
      Algorithm 2 (HMAC-SHA2) hash over the contents of the message (shown as
      just "..." above); this HMAC-SHA2 computation includes a password value.
      Thus, an attacker cannot compute the message integrity value without
      somehow knowing the secret password.</t>
      <t pn="section-20-7">The server, upon receipt of the authenticated Allocate request,
      checks that everything is OK, then creates an allocation. The server
      replies with an Allocate success response. The server includes a
      LIFETIME attribute giving the lifetime of the allocation; here, the
      server has reduced the client's requested 1-hour lifetime to just 20
      minutes because this particular server doesn't allow lifetimes longer
      than 20 minutes. The server includes an XOR-RELAYED-ADDRESS attribute
      whose value is the relayed transport address of the allocation. The
      server includes an XOR-MAPPED-ADDRESS attribute whose value is the
      server-reflexive address of the client; this value is not used otherwise
      in TURN but is returned as a convenience to the client. The server
      includes a MESSAGE-INTEGRITY-SHA256 attribute to authenticate the
      response and to ensure its integrity; note that the response does not
      contain the USERNAME, REALM, and NONCE attributes. The server also
      includes a SOFTWARE attribute.</t>
      <figure align="left" suppress-title="false" pn="figure-13">
        <artwork name="" type="" align="left" alt="" pn="section-20-8.1">
TURN                                 TURN          Peer         Peer
client                               server         A            B
  |--- CreatePermission request ------&gt;|            |            |
  |    Transaction-Id=0xE5913A8F460956CA277D3319    |            |
  |    XOR-PEER-ADDRESS=192.0.2.150:0  |            |            |
  |    USERNAME="George"               |            |            |
  |    REALM="example.com"             |            |            |
  |    NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"     |            |
  |    PASSWORD-ALGORITHMS=MD5 and SHA256           |            |
  |    PASSWORD-ALGORITHM=SHA256       |            |            |  
  |    MESSAGE-INTEGRITY-SHA256=...    |            |            |
  |                                    |            |            |
  |&lt;-- CreatePermission success resp.--|            |            |
  |    Transaction-Id=0xE5913A8F460956CA277D3319    |            |
  |    MESSAGE-INTEGRITY-SHA256=...    |            |            |
</artwork>
      </figure>
      <t pn="section-20-9">The client then creates a permission towards Peer A in preparation
      for sending it some application data. This is done through a
      CreatePermission request. The XOR-PEER-ADDRESS attribute contains the IP
      address for which a permission is established (the IP address of peer
      A); note that the port number in the attribute is ignored when used in a
      CreatePermission request, and here it has been set to 0; also, note how
      the client uses Peer A's server-reflexive IP address and not its
      (private) host address. The client uses the same username, realm, and
      nonce values as in the previous request on the allocation. Though it is
      allowed to do so, the client has chosen not to include a SOFTWARE
      attribute in this request.</t>
      <t pn="section-20-10">The server receives the CreatePermission request, creates the
      corresponding permission, and then replies with a CreatePermission
      success response. Like the client, the server chooses not to include the
      SOFTWARE attribute in its reply. Again, note how success responses
      contain a MESSAGE-INTEGRITY-SHA256 attribute (assuming the server uses
      the long-term credential mechanism) but no USERNAME, REALM, and NONCE
      attributes.</t>
      <figure align="left" suppress-title="false" pn="figure-14">
        <artwork name="" type="" align="left" alt="" pn="section-20-11.1">
TURN                                 TURN          Peer         Peer
client                               server         A            B
  |--- Send indication ---------------&gt;|            |            |
  |    Transaction-Id=0x1278E9ACA2711637EF7D3328    |            |
  |    XOR-PEER-ADDRESS=192.0.2.150:32102           |            |
  |    DONT-FRAGMENT                   |            |            |
  |    DATA=...                        |            |            |
  |                                    |- UDP dgm -&gt;|            |
  |                                    | data=...   |            |
  |                                    |            |            |
  |                                    |&lt;- UDP dgm -|            |
  |                                    |  data=...  |            |
  |&lt;-- Data indication ----------------|            |            |
  |    Transaction-Id=0x8231AE8F9242DA9FF287FEFF    |            |
  |    XOR-PEER-ADDRESS=192.0.2.150:32102           |            |
  |    DATA=...                        |            |            |
</artwork>
      </figure>
      <t pn="section-20-12">The client now sends application data to Peer A using a Send
      indication. Peer A's server-reflexive transport address is specified in
      the XOR-PEER-ADDRESS attribute, and the application data (shown here as
      just "...") is specified in the DATA attribute. The client is doing a
      form of path MTU discovery at the application layer and, thus, specifies
      (by including the DONT-FRAGMENT attribute) that the server should set
      the DF bit in the UDP datagram to send to the peer. Indications cannot
      be authenticated using the long-term credential mechanism of STUN, so no
      MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute is included in
      the message. An application wishing to ensure that its data is not
      altered or forged must integrity-protect its data at the application
      level.</t>
      <t pn="section-20-13">Upon receipt of the Send indication, the server extracts the
      application data and sends it in a UDP datagram to Peer A, with the
      relayed transport address as the source transport address of the
      datagram and with the DF bit set as requested. Note that had the
      client not previously established a permission for Peer A's
      server-reflexive IP address, the server would have silently
      discarded the Send indication instead.</t>
      <t pn="section-20-14">Peer A then replies with its own UDP datagram containing application
      data. The datagram is sent to the relayed transport address on the
      server. When this arrives, the server creates a Data indication
      containing the source of the UDP datagram in the XOR-PEER-ADDRESS
      attribute, and the data from the UDP datagram in the DATA attribute. The
      resulting Data indication is then sent to the client.</t>
      <figure align="left" suppress-title="false" pn="figure-15">
        <artwork name="" type="" align="left" alt="" pn="section-20-15.1">
TURN                                 TURN          Peer          Peer
client                               server         A             B
  |--- ChannelBind request -----------&gt;|            |             |
  |    Transaction-Id=0x6490D3BC175AFF3D84513212    |             |
  |    CHANNEL-NUMBER=0x4000           |            |             |
  |    XOR-PEER-ADDRESS=192.0.2.210:49191           |             |
  |    USERNAME="George"               |            |             |
  |    REALM="example.com"             |            |             |
  |    NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"     |             |
  |    PASSWORD-ALGORITHMS=MD5 and SHA256           |             |
  |    PASSWORD-ALGORITHM=SHA256       |            |             |  
  |    MESSAGE-INTEGRITY-SHA256=...    |            |             |
  |                                    |            |             |
  |&lt;-- ChannelBind success response ---|            |             |
  |    Transaction-Id=0x6490D3BC175AFF3D84513212    |             |
  |    MESSAGE-INTEGRITY-SHA256=...    |            |             |
</artwork>
      </figure>
      <t pn="section-20-16">The client now binds a channel to Peer B, specifying a free channel
      number (0x4000) in the CHANNEL-NUMBER attribute, and Peer B's transport
      address in the XOR-PEER-ADDRESS attribute. As before, the client reuses
      the username, realm, and nonce from its last request in the message.</t>
      <t pn="section-20-17">Upon receipt of the request, the server binds the channel number to
      the peer, installs a permission for Peer B's IP address, and then
      replies with a ChannelBind success response.</t>
      <figure align="left" suppress-title="false" pn="figure-16">
        <artwork name="" type="" align="left" alt="" pn="section-20-18.1">
TURN                                TURN           Peer          Peer
client                              server          A             B
  |--- ChannelData ------------------&gt;|             |             |
  |    Channel-number=0x4000          |--- UDP datagram ---------&gt;|
  |    Data=...                       |    Data=...               |
  |                                   |             |             |
  |                                   |&lt;-- UDP datagram ----------|
  |                                   |    Data=... |             |
  |&lt;-- ChannelData -------------------|             |             |
  |    Channel-number=0x4000          |             |             |
  |    Data=...                       |             |             |
</artwork>
      </figure>
      <t pn="section-20-19">The client now sends a ChannelData message to the server with data
      destined for Peer B. The ChannelData message is not a STUN message;
      thus, it has no transaction id. Instead, it has only three fields: a channel
      number, data, and data length; here, the channel number field is 0x4000
      (the channel the client just bound to Peer B). When the server receives
      the ChannelData message, it checks that the channel is currently bound
      (which it is) and then sends the data onward to Peer B in a UDP
      datagram, using the relayed transport address as the source transport
      address, and 192.0.2.210:49191 (the value of the XOR-PEER-ADDRESS
      attribute in the ChannelBind request) as the destination transport
      address.</t>
      <t pn="section-20-20">Later, Peer B sends a UDP datagram back to the relayed transport
      address. This causes the server to send a ChannelData message to the
      client containing the data from the UDP datagram. The server knows to
      which client to send the ChannelData message because of the relayed
      transport address at which the UDP datagram arrived, and it knows to use
      channel 0x4000 because this is the channel bound to 192.0.2.210:49191.
      Note that if there had not been any channel number bound to that
      address, the server would have used a Data indication instead.</t>
      <figure align="left" suppress-title="false" pn="figure-17">
        <artwork name="" type="" align="left" alt="" pn="section-20-21.1">
TURN                                 TURN          Peer         Peer
client                               server         A            B
  |--- ChannelBind request -----------&gt;|            |            |
  |    Transaction-Id=0xE5913A8F46091637EF7D3328    |            |
  |    CHANNEL-NUMBER=0x4000           |            |            |
  |    XOR-PEER-ADDRESS=192.0.2.210:49191           |            |
  |    USERNAME="George"               |            |            |
  |    REALM="example.com"             |            |            |
  |    NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"     |            |
  |    PASSWORD-ALGORITHMS=MD5 and SHA256           |            |
  |    PASSWORD-ALGORITHM=SHA256       |            |            |  
  |    MESSAGE-INTEGRITY-SHA256=...    |            |            |
  |                                    |            |            |
  |&lt;-- ChannelBind success response ---|            |            |
  |    Transaction-Id=0xE5913A8F46091637EF7D3328    |            |
  |    MESSAGE-INTEGRITY-SHA256=...    |            |            |
</artwork>
      </figure>
      <t pn="section-20-22">The channel binding lasts for 10 minutes unless refreshed. The TURN
      client refreshes the binding by sending a ChannelBind request rebinding
      the channel to the same peer (Peer B's IP address). The server processes
      the ChannelBind request, rebinds the channel to the same peer, and resets
      the time-to-expiry timer back to 10 minutes.</t>
      <figure align="left" suppress-title="false" pn="figure-18">
        <artwork name="" type="" align="left" alt="" pn="section-20-23.1">
TURN                                 TURN          Peer         Peer
client                               server         A            B
  |--- Refresh request ---------------&gt;|            |            |
  |    Transaction-Id=0x0864B3C27ADE9354B4312414    |            |
  |    SOFTWARE="Example client 1.03"  |            |            |
  |    USERNAME="George"               |            |            |
  |    REALM="example.com"             |            |            |
  |    NONCE="oobMatJos2gAAAadl7W7PeDU4hKE72jda"    |            |
  |    PASSWORD-ALGORITHMS=MD5 and SHA256           |            |
  |    PASSWORD-ALGORITHM=SHA256       |            |            |  
  |    MESSAGE-INTEGRITY-SHA256=...    |            |            |
  |                                    |            |            |
  |&lt;-- Refresh error response ---------|            |            |
  |    Transaction-Id=0x0864B3C27ADE9354B4312414    |            |
  |    SOFTWARE="Example server, version 1.17"      |            |
  |    ERROR-CODE=438 (Stale Nonce)    |            |            |
  |    REALM="example.com"             |            |            |
  |    NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"     |            |
  |    PASSWORD-ALGORITHMS=MD5 and SHA256           |            |
  |                                    |            |            |
  |--- Refresh request ---------------&gt;|            |            |
  |    Transaction-Id=0x427BD3E625A85FC731DC4191    |            |
  |    SOFTWARE="Example client 1.03"  |            |            |
  |    USERNAME="George"               |            |            |
  |    REALM="example.com"             |            |            |
  |    NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"     |            |
  |    PASSWORD-ALGORITHMS=MD5 and SHA256           |            |
  |    PASSWORD-ALGORITHM=SHA256       |            |            | 
  |    MESSAGE-INTEGRITY-SHA256=...    |            |            |
  |                                    |            |            |
  |&lt;-- Refresh success response -------|            |            |
  |    Transaction-Id=0x427BD3E625A85FC731DC4191    |            |
  |    SOFTWARE="Example server, version 1.17"      |            |
  |    LIFETIME=600 (10 minutes)       |            |            |
  |    MESSAGE-INTEGRITY=...           |            |            |
</artwork>
      </figure>
      <t pn="section-20-24">Sometime before the 20-minute lifetime is up, the client refreshes
      the allocation. This is done using a Refresh request. As before, the
      client includes the latest username, realm, and nonce values in the
      request. The client also includes the SOFTWARE attribute, following the
      recommended practice of always including this attribute in Allocate and
      Refresh messages. When the server receives the Refresh request, it
      notices that the nonce value has expired and so replies with a 438 (Stale
      Nonce) error given a new nonce value. The client then reattempts the
      request, this time with the new nonce value. This second attempt is
      accepted, and the server replies with a success response. Note that the
      client did not include a LIFETIME attribute in the request, so the
      server refreshes the allocation for the default lifetime of 10 minutes
      (as can be seen by the LIFETIME attribute in the success response).</t>
    </section>
    <section anchor="sec-security" numbered="true" toc="include" removeInRFC="false" pn="section-21">
      <name slugifiedName="name-security-considerations">Security Considerations</name>
      <t pn="section-21-1">This section considers attacks that are possible in a TURN
      deployment and discusses how they are mitigated by mechanisms in the
      protocol or recommended practices in the implementation.</t>
      <t pn="section-21-2">Most of the attacks on TURN are mitigated by the server requiring
      requests be authenticated. Thus, this specification requires the use of
      authentication. The mandatory-to-implement mechanism is the long- term
      credential mechanism of STUN. Other authentication mechanisms of equal
      or stronger security properties may be used. However, it is important to
      ensure that they can be invoked in an interoperable way.</t>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-21.1">
        <name slugifiedName="name-outsider-attacks">Outsider Attacks</name>
        <t pn="section-21.1-1">Outsider attacks are ones where the attacker has no credentials in
        the system and is attempting to disrupt the service seen by the
        client or the server.</t>
        <section numbered="true" toc="include" removeInRFC="false" pn="section-21.1.1">
          <name slugifiedName="name-obtaining-unauthorized-allo">Obtaining Unauthorized Allocations</name>
          <t pn="section-21.1.1-1">An attacker might wish to obtain allocations on a TURN server for
          any number of nefarious purposes. A TURN server provides a mechanism
          for sending and receiving packets while cloaking the actual IP
          address of the client. This makes TURN servers an attractive target
          for attackers who wish to use it to mask their true identity.</t>
          <t pn="section-21.1.1-2">An attacker might also wish to simply utilize the services of a
          TURN server without paying for them. Since TURN services require
          resources from the provider, it is anticipated that their usage will
          come with a cost.</t>
          <t pn="section-21.1.1-3">These attacks are prevented using the long-term credential
          mechanism, which allows the TURN server to determine the identity of
          the requestor and whether the requestor is allowed to obtain the
          allocation.</t>
        </section>
        <section numbered="true" toc="include" removeInRFC="false" pn="section-21.1.2">
          <name slugifiedName="name-offline-dictionary-attacks">Offline Dictionary Attacks</name>
          <t pn="section-21.1.2-1">The long-term credential mechanism used by TURN is subject to
          offline dictionary attacks. An attacker that is capable of
          eavesdropping on a message exchange between a client and server can
          determine the password by trying a number of candidate passwords and
          seeing if one of them is correct. This attack works when the
          passwords are low entropy such as a word from the dictionary. This
          attack can be mitigated by using strong passwords with large
          entropy. In situations where even stronger mitigation is required,
          (D)TLS transport between the client and the server can be used.</t>
        </section>
        <section numbered="true" toc="include" removeInRFC="false" pn="section-21.1.3">
          <name slugifiedName="name-faked-refreshes-and-permiss">Faked Refreshes and Permissions</name>
          <t pn="section-21.1.3-1">An attacker might wish to attack an active allocation by sending
          it a Refresh request with an immediate expiration in order to
          delete it and disrupt service to the client. This is prevented by
          authentication of refreshes. Similarly, an attacker wishing to send
          CreatePermission requests to create permissions to undesirable
          destinations is prevented from doing so through authentication. The
          motivations for such an attack are described in <xref target="sec-firewall" format="default" sectionFormat="of" derivedContent="Section 21.2"/>.</t>
        </section>
        <section anchor="fate-data" numbered="true" toc="include" removeInRFC="false" pn="section-21.1.4">
          <name slugifiedName="name-fake-data">Fake Data</name>
          <t pn="section-21.1.4-1">An attacker might wish to send data to the client or the peer as
          if they came from the peer or client, respectively. To do that, the
          attacker can send the client a faked Data indication or ChannelData
          message, or send the TURN server a faked Send indication or
          ChannelData message.</t>
          <t pn="section-21.1.4-2">Since indications and ChannelData messages are not authenticated,
          this attack is not prevented by TURN. However, this attack is
          generally present in IP-based communications and is not
          substantially worsened by TURN. Consider a normal, non-TURN IP
          session between hosts A and B. An attacker can send packets to B as
          if they came from A by sending packets towards B with a spoofed IP
          address of A. This attack requires the attacker to know the IP
          addresses of A and B. With TURN, an attacker wishing to send packets
          towards a client using a Data indication needs to know its IP
          address (and port), the IP address and port of the TURN server, and
          the IP address and port of the peer (for inclusion in the
          XOR-PEER-ADDRESS attribute). To send a fake ChannelData message to a
          client, an attacker needs to know the IP address and port of the
          client, the IP address and port of the TURN server, and the channel
          number. This particular combination is mildly more guessable than in
          the non-TURN case.</t>
          <t pn="section-21.1.4-3">These attacks are more properly mitigated by application-layer
          authentication techniques. In the case of real-time traffic, usage
          of SRTP <xref target="RFC3711" format="default" sectionFormat="of" derivedContent="RFC3711"/> prevents these attacks.</t>
          <t pn="section-21.1.4-4">In some situations, the TURN server may be situated in the
          network such that it is able to send to hosts to which the client
          cannot directly send. This can happen, for example, if the server is
          located behind a firewall that allows packets from outside the
          firewall to be delivered to the server, but not to other hosts
          behind the firewall. In these situations, an attacker could send the
          server a Send indication with an XOR-PEER-ADDRESS attribute
          containing the transport address of one of the other hosts behind
          the firewall. If the server was to allow relaying of traffic to
          arbitrary peers, then this would provide a way for the attacker to
          attack arbitrary hosts behind the firewall.</t>
          <t pn="section-21.1.4-5">To mitigate this attack, TURN requires that the client establish
          a permission to a host before sending it data. Thus, an attacker can
          only attack hosts with which the client is already communicating
          unless the attacker is able to create authenticated requests.
          Furthermore, the server administrator may configure the server to
          restrict the range of IP addresses and ports to which it will relay
          data. To provide even greater security, the server administrator can
          require that the client use (D)TLS for all communication between the
          client and the server.</t>
        </section>
        <section numbered="true" toc="include" removeInRFC="false" pn="section-21.1.5">
          <name slugifiedName="name-impersonating-a-server">Impersonating a Server</name>
          <t pn="section-21.1.5-1">When a client learns a relayed address from a TURN server, it
          uses that relayed address in application protocols to receive
          traffic. Therefore, an attacker wishing to intercept or redirect
          that traffic might try to impersonate a TURN server and provide the
          client with a faked relayed address.</t>
          <t pn="section-21.1.5-2">This attack is prevented through the long-term credential
          mechanism, which provides message integrity for responses in
          addition to verifying that they came from the server. Furthermore,
          an attacker cannot replay old server responses as the transaction id
          in the STUN header prevents this. Replay attacks are further
          thwarted through frequent changes to the nonce value.</t>
        </section>
        <section numbered="true" toc="include" removeInRFC="false" pn="section-21.1.6">
          <name slugifiedName="name-eavesdropping-traffic">Eavesdropping Traffic</name>
          <t pn="section-21.1.6-1">If the TURN client and server use the STUN Extension for
          Third-Party Authorization <xref target="RFC7635" format="default" sectionFormat="of" derivedContent="RFC7635"/> (for
          example, it is used in WebRTC), the username does not reveal the real
          user's identity; the USERNAME attribute carries an ephemeral and
          unique key identifier. If the TURN client and server use the STUN
          long-term credential mechanism and the username reveals the real
          user's identity, the client <bcp14>MUST</bcp14> either use the USERHASH attribute
          instead of the USERNAME attribute to anonymize the username or use
          (D)TLS transport between the client and the server.</t>
          <t pn="section-21.1.6-2">If the TURN client and server use the STUN long-term credential
          mechanism, and realm information is privacy sensitive, TURN can be
          run over (D)TLS. As a reminder, STUN Extension for Third-Party
          Authorization does not use realm.</t>
          <t pn="section-21.1.6-3">The SOFTWARE attribute can reveal the specific software version
          of the TURN client and server to the eavesdropper, and it might possibly
          allow attacks against vulnerable software that is known to contain
          security vulnerabilities. If the software version is known to
          contain security vulnerabilities, TURN <bcp14>SHOULD</bcp14> be run over (D)TLS to
          prevent leaking the SOFTWARE attribute in clear text. If zero-day
          vulnerabilities are detected in the software version, the endpoint
          policy can be modified to mandate the use of (D)TLS until the patch
          is in place to fix the flaw.</t>
          <t pn="section-21.1.6-4">TURN concerns itself primarily with authentication and message
          integrity. Confidentiality is only a secondary concern as TURN
          control messages do not include information that is particularly
          sensitive with the exception of USERNAME, REALM, and SOFTWARE. The
          primary protocol content of the messages is the IP address of the
          peer. If it is important to prevent an eavesdropper on a TURN
          connection from learning this, TURN can be run over (D)TLS.</t>
          <t pn="section-21.1.6-5">Confidentiality for the application data relayed by TURN is best
          provided by the application protocol itself since running TURN over
          (D)TLS does not protect application data between the server and the
          peer. If confidentiality of application data is important, then the
          application should encrypt or otherwise protect its data. For
          example, for real-time media, confidentiality can be provided by
          using SRTP.</t>
        </section>
        <section numbered="true" toc="include" removeInRFC="false" pn="section-21.1.7">
          <name slugifiedName="name-turn-loop-attack">TURN Loop Attack</name>
          <t pn="section-21.1.7-1">An attacker might attempt to cause data packets to loop
          indefinitely between two TURN servers. The attack goes as follows:
          first, the attacker sends an Allocate request to server A using the
          source address of server B. Server A will send its response to
          server B, and for the attack to succeed, the attacker must have the
          ability to either view or guess the contents of this response so
          that the attacker can learn the allocated relayed transport address.
          The attacker then sends an Allocate request to server B using the
          source address of server A. Again, the attacker must be able to view
          or guess the contents of the response so it can learn the
          allocated relayed transport address. Using the same spoofed source
          address technique, the attacker then binds a channel number on
          server A to the relayed transport address on server B and similarly
          binds the same channel number on server B to the relayed transport
          address on server A. Finally, the attacker sends a ChannelData
          message to server A.</t>
          <t pn="section-21.1.7-2">The result is a data packet that loops from the relayed transport
          address on server A to the relayed transport address on server B,
          then from server B's transport address to server A's transport
          address, and then around the loop again.</t>
          <t pn="section-21.1.7-3">This attack is mitigated as follows: by requiring all requests to
          be authenticated and/or by randomizing the port number allocated for
          the relayed transport address, the server forces the attacker to
          either intercept or view responses sent to a third party (in this
          case, the other server) so that the attacker can authenticate the
          requests and learn the relayed transport address. Without one of
          these two measures, an attacker can guess the contents of the
          responses without needing to see them, which makes the attack much
          easier to perform. Furthermore, by requiring authenticated requests,
          the server forces the attacker to have credentials acceptable to the
          server, which turns this from an outsider attack into an insider
          attack and allows the attack to be traced back to the client
          initiating it.</t>
          <t pn="section-21.1.7-4">The attack can be further mitigated by imposing a per-username
          limit on the bandwidth used to relay data by allocations owned by
          that username to limit the impact of this attack on other
          allocations. More mitigation can be achieved by decrementing the TTL
          when relaying data packets (if the underlying OS allows this).</t>
        </section>
      </section>
      <section anchor="sec-firewall" numbered="true" toc="include" removeInRFC="false" pn="section-21.2">
        <name slugifiedName="name-firewall-considerations">Firewall Considerations</name>
        <t pn="section-21.2-1">A key security consideration of TURN is that TURN should not weaken
        the protections afforded by firewalls deployed between a client and a
        TURN server. It is anticipated that TURN servers will often be present
        on the public Internet, and clients may often be inside enterprise
        networks with corporate firewalls. If TURN servers provide a
        "backdoor" for reaching into the enterprise, TURN will be blocked by
        these firewalls.</t>
        <t pn="section-21.2-2">TURN servers therefore emulate the behavior of NAT devices that
        implement address-dependent filtering <xref target="RFC4787" format="default" sectionFormat="of" derivedContent="RFC4787"/>,
        a property common in many firewalls as well. When a NAT or firewall
        implements this behavior, packets from an outside IP address are only
        allowed to be sent to an internal IP address and port if the internal
        IP address and port had recently sent a packet to that outside IP
        address. TURN servers introduce the concept of permissions, which
        provide exactly this same behavior on the TURN server. An attacker
        cannot send a packet to a TURN server and expect it to be relayed
        towards the client, unless the client has tried to contact the
        attacker first.</t>
        <t pn="section-21.2-3">It is important to note that some firewalls have policies that are
        even more restrictive than address-dependent filtering. Firewalls can
        also be configured with address- and port-dependent filtering, or they
        can be configured to disallow inbound traffic entirely. In these
        cases, if a client is allowed to connect the TURN server,
        communications to the client will be less restrictive than what the
        firewall would normally allow.</t>
        <section numbered="true" toc="include" removeInRFC="false" pn="section-21.2.1">
          <name slugifiedName="name-faked-permissions">Faked Permissions</name>
          <t pn="section-21.2.1-1">In firewalls and NAT devices, permissions are granted implicitly
          through the traversal of a packet from the inside of the network
          towards the outside peer. Thus, a permission cannot, by definition,
          be created by any entity except one inside the firewall or NAT. With
          TURN, this restriction no longer holds. Since the TURN server sits
          outside the firewall, an attacker outside the firewall can now send
          a message to the TURN server and try to create a permission for
          itself.</t>
          <t pn="section-21.2.1-2">This attack is prevented because all messages that create
          permissions (i.e., ChannelBind and CreatePermission) are
          authenticated.</t>
        </section>
        <section numbered="true" toc="include" removeInRFC="false" pn="section-21.2.2">
          <name slugifiedName="name-blacklisted-ip-addresses">Blacklisted IP Addresses</name>
          <t pn="section-21.2.2-1">Many firewalls can be configured with blacklists that prevent a
          client behind the firewall from sending packets to, or receiving
          packets from, ranges of blacklisted IP addresses. This is
          accomplished by inspecting the source and destination addresses of
          packets entering and exiting the firewall, respectively.</t>
          <t pn="section-21.2.2-2">This feature is also present in TURN since TURN servers are
          allowed to arbitrarily restrict the range of addresses of peers that
          they will relay to.</t>
        </section>
        <section numbered="true" toc="include" removeInRFC="false" pn="section-21.2.3">
          <name slugifiedName="name-running-servers-on-well-kno">Running Servers on Well-Known Ports</name>
          <t pn="section-21.2.3-1">A malicious client behind a firewall might try to connect to a
          TURN server and obtain an allocation that it then uses to run a
          server. For example, a client might try to run a DNS server or FTP
          server.</t>
          <t pn="section-21.2.3-2">This is not possible in TURN. A TURN server will never accept
          traffic from a peer for which the client has not installed a
          permission. Thus, peers cannot just connect to the allocated port in
          order to obtain the service.</t>
        </section>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-21.3">
        <name slugifiedName="name-insider-attacks">Insider Attacks</name>
        <t pn="section-21.3-1">In insider attacks, a client has legitimate credentials but defies
        the trust relationship that goes with those credentials. These attacks
        cannot be prevented by cryptographic means but need to be considered
        in the design of the protocol.</t>
        <section numbered="true" toc="include" removeInRFC="false" pn="section-21.3.1">
          <name slugifiedName="name-dos-against-turn-server">DoS against TURN Server</name>
          <t pn="section-21.3.1-1">A client wishing to disrupt service to other clients might obtain
          an allocation and then flood it with traffic in an attempt to swamp
          the server and prevent it from servicing other legitimate clients.
          This is mitigated by the recommendation that the server limit the
          amount of bandwidth it will relay for a given username. This won't
          prevent a client from sending a large amount of traffic, but it
          allows the server to immediately discard traffic in excess.</t>
          <t pn="section-21.3.1-2">Since each allocation uses a port number on the IP address of the
          TURN server, the number of allocations on a server is finite. An
          attacker might attempt to consume all of them by requesting a large
          number of allocations. This is prevented by the recommendation that
          the server impose a limit on the number of allocations active at a
          time for a given username.</t>
        </section>
        <section numbered="true" toc="include" removeInRFC="false" pn="section-21.3.2">
          <name slugifiedName="name-anonymous-relaying-of-malic">Anonymous Relaying of Malicious Traffic</name>
          <t pn="section-21.3.2-1">TURN servers provide a degree of anonymization. A client can send
          data to peers without revealing its own IP address. TURN servers may
          therefore become attractive vehicles for attackers to launch attacks
          against targets without fear of detection. Indeed, it is possible
          for a client to chain together multiple TURN servers such that any
          number of relays can be used before a target receives a packet.</t>
          <t pn="section-21.3.2-2">Administrators who are worried about this attack can maintain
          logs that capture the actual source IP and port of the client and
          perhaps even every permission that client installs. This will allow
          for forensic tracing to determine the original source should it be
          discovered that an attack is being relayed through a TURN
          server.</t>
        </section>
        <section numbered="true" toc="include" removeInRFC="false" pn="section-21.3.3">
          <name slugifiedName="name-manipulating-other-allocati">Manipulating Other Allocations</name>
          <t pn="section-21.3.3-1">An attacker might attempt to disrupt service to other users of
          the TURN server by sending Refresh requests or CreatePermission
          requests that (through source address spoofing) appear to be coming
          from another user of the TURN server. TURN prevents this by
          requiring that the credentials used in CreatePermission, Refresh,
          and ChannelBind messages match those used to create the initial
          allocation. Thus, the fake requests from the attacker will be
          rejected.</t>
        </section>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-21.4">
        <name slugifiedName="name-tunnel-amplification-attack">Tunnel Amplification Attack</name>
        <t pn="section-21.4-1">An attacker might attempt to cause data packets to loop numerous
        times between a TURN server and a tunnel between IPv4 and IPv6. The
        attack goes as follows:</t>
        <t pn="section-21.4-2">Suppose an attacker knows that a tunnel endpoint will forward
        encapsulated packets from a given IPv6 address (this doesn't
        necessarily need to be the tunnel endpoint's address). Suppose he then
        spoofs two packets from this address: </t>
        <ol spacing="normal" type="1" start="1" pn="section-21.4-3">
          <li pn="section-21.4-3.1" derivedCounter="1.">An Allocate request asking for a v4 address, and</li>
          <li pn="section-21.4-3.2" derivedCounter="2.">A ChannelBind request establishing a channel to the IPv4
            address of the tunnel endpoint.</li>
        </ol>
        <t pn="section-21.4-4">Then, he has set up an amplification attack: </t>
        <ul spacing="normal" bare="false" empty="false" pn="section-21.4-5">
          <li pn="section-21.4-5.1">The TURN server will re-encapsulate IPv6 UDP data in v4 and
            send it to the tunnel endpoint.</li>
          <li pn="section-21.4-5.2">The tunnel endpoint will de-encapsulate packets from the v4
            interface and send them to v6.</li>
        </ul>
        <t pn="section-21.4-6">So, if the attacker sends a packet of the following form:</t>
        <figure align="left" suppress-title="false" pn="figure-19">
          <artwork name="" type="" align="left" alt="" pn="section-21.4-7.1">
  IPv6: src=2001:DB8:1::1 dst=2001:DB8::2
  UDP:  &lt;ports&gt;
  TURN: &lt;channel id&gt;
  IPv6: src=2001:DB8:1::1 dst=2001:DB8::2
  UDP:  &lt;ports&gt;
  TURN: &lt;channel id&gt;
  IPv6: src=2001:DB8:1::1 dst=2001:DB8::2
  UDP:  &lt;ports&gt;
  TURN: &lt;channel id&gt;
  ...      </artwork>
        </figure>
        <t pn="section-21.4-8">then the TURN server and the tunnel endpoint will send it
        back and forth until the last TURN header is consumed, at which point
        the TURN server will send an empty packet that the tunnel endpoint
        will drop.</t>
        <t pn="section-21.4-9">The amplification potential here is limited by the MTU, so it's not
        huge: IPv6+UDP+TURN takes 334 bytes, so a four-to-one amplification
        out of a 1500-byte packet is possible. But, the attacker could still
        increase traffic volume by sending multiple packets or by establishing
        multiple channels spoofed from different addresses behind the same
        tunnel endpoint.</t>
        <t pn="section-21.4-10">The attack is mitigated as follows. It is <bcp14>RECOMMENDED</bcp14> that TURN
        servers not accept allocation or channel-binding requests from
        addresses known to be tunneled, and that they not forward data to such
        addresses. In particular, a TURN server <bcp14>MUST NOT</bcp14> accept Teredo or 6to4
        addresses in these requests.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-21.5">
        <name slugifiedName="name-other-considerations">Other Considerations</name>
        <t pn="section-21.5-1">Any relay addresses learned through an Allocate request will not
        operate properly with IPsec Authentication Header (AH) <xref target="RFC4302" format="default" sectionFormat="of" derivedContent="RFC4302"/> in transport or tunnel
        mode. However, tunnel-mode IPsec Encapsulating Security Payload (ESP)
        <xref target="RFC4303" format="default" sectionFormat="of" derivedContent="RFC4303"/> should still operate.</t>
      </section>
    </section>
    <section numbered="true" toc="include" removeInRFC="false" pn="section-22">
      <name slugifiedName="name-iana-considerations">IANA Considerations</name>
      <t pn="section-22-1">The code points for the STUN methods defined in this specification are
      listed in <xref target="sec-stun-methods" format="default" sectionFormat="of" derivedContent="Section 17"/>. IANA has
      updated the references from <xref target="RFC5766" format="default" sectionFormat="of" derivedContent="RFC5766"/> to
      this document (for the STUN methods listed in <xref target="sec-stun-methods" format="default" sectionFormat="of" derivedContent="Section 17"/>).</t>
      <t pn="section-22-2">The code points for the STUN attributes defined in this specification
      are listed in <xref target="sec-stun-attributes" format="default" sectionFormat="of" derivedContent="Section 18"/>. IANA has
      updated the references from <xref target="RFC5766" format="default" sectionFormat="of" derivedContent="RFC5766"/> to
      this document (for the STUN attributes CHANNEL-NUMBER, LIFETIME, Reserved
      (was BANDWIDTH), XOR-PEER-ADDRESS, DATA, XOR-RELAYED-ADDRESS,
      REQUESTED-ADDRESS-FAMILY, EVEN-PORT, REQUESTED-TRANSPORT, DONT-FRAGMENT,
      Reserved (was TIMER-VAL), and RESERVATION-TOKEN listed in <xref target="sec-stun-attributes" format="default" sectionFormat="of" derivedContent="Section 18"/>).</t>
      <t pn="section-22-3">The code points for the STUN error codes defined in this specification
      are listed in <xref target="sec-stun-errors" format="default" sectionFormat="of" derivedContent="Section 19"/>. IANA has
      updated the references from <xref target="RFC5766" format="default" sectionFormat="of" derivedContent="RFC5766"/>
      and <xref target="RFC6156" format="default" sectionFormat="of" derivedContent="RFC6156"/> to this document (for the STUN error codes listed in
      <xref target="sec-stun-errors" format="default" sectionFormat="of" derivedContent="Section 19"/>).</t>
      <t pn="section-22-4">IANA has updated the references to <xref target="RFC5766" format="default" sectionFormat="of" derivedContent="RFC5766"/> to this document for the SRV service name of "turn" for TURN over UDP
      or TCP and the service name of "turns" for TURN over (D)TLS.</t>
      <t pn="section-22-5">IANA has created a registry for TURN channel numbers (the "Traversal
      Using Relays around NAT (TURN) Channel Numbers" registry), initially
      populated as follows:</t>
      <table anchor="turn-channel-numbers" align="center" pn="table-6">
        <tbody>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x0000 through 0x3FFF:</td>
            <td align="left" colspan="1" rowspan="1">Reserved and not available for use since they conflict with the STUN
      header.</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x4000 through 0x4FFF:</td>
            <td align="left" colspan="1" rowspan="1">A TURN implementation is free to use channel numbers in this range.</td>
          </tr>
          <tr>
            <td align="left" colspan="1" rowspan="1">0x5000 through 0xFFFF:</td>
            <td align="left" colspan="1" rowspan="1">Reserved (For DTLS-SRTP multiplexing collision avoidance, see <xref target="RFC7983" format="default" sectionFormat="of" derivedContent="RFC7983"/>)</td>
          </tr>
        </tbody>
      </table>
      <t pn="section-22-7">Any change to this registry must be made through an IETF
      Standards Action.</t>
    </section>
    <section numbered="true" toc="include" removeInRFC="false" pn="section-23">
      <name slugifiedName="name-iab-considerations">IAB Considerations</name>
      <t pn="section-23-1">The IAB has studied the problem of Unilateral Self-Address Fixing
      (UNSAF), which is the general process by which a client attempts to
      determine its address in another realm on the other side of a NAT
      through a collaborative protocol reflection mechanism <xref target="RFC3424" format="default" sectionFormat="of" derivedContent="RFC3424"/>. The TURN extension is an example of
      a protocol that performs this type of function. The IAB has mandated
      that any protocols developed for this purpose document a specific set of
      considerations. These considerations and the responses for TURN are
      documented in this section.</t>
      <t pn="section-23-2">Consideration 1: Precise definition of a specific, limited-scope
      problem that is to be solved with the UNSAF proposal. A short-term fix
      should not be generalized to solve other problems. Such generalizations
      lead to the prolonged dependence on and usage of the supposed short-term
      fix, meaning that it is no longer accurate to call it
      "short-term".</t>
      <t pn="section-23-3">Response: TURN is a protocol for communication between a relay (=
      TURN server) and its client. The protocol allows a client that is behind
      a NAT to obtain and use a public IP address on the relay. As a
      convenience to the client, TURN also allows the client to determine its
      server-reflexive transport address.</t>
      <t pn="section-23-4">Consideration 2: Description of an exit strategy/transition plan. The
      better short-term fixes are the ones that will naturally see less and
      less use as the appropriate technology is deployed.</t>
      <t pn="section-23-5">Response: TURN will no longer be needed once there are no longer any
      NATs. Unfortunately, as of the date of publication of this document, it
      no longer seems very likely that NATs will go away any time soon.
      However, the need for TURN will also decrease as the number of NATs with
      the mapping property of Endpoint-Independent Mapping <xref target="RFC4787" format="default" sectionFormat="of" derivedContent="RFC4787"/> increases.</t>
      <t pn="section-23-6">Consideration 3: Discussion of specific issues that may render
      systems more "brittle". For example, approaches that involve using data
      at multiple network layers create more dependencies, increase debugging
      challenges, and make it harder to transition.</t>
      <t pn="section-23-7">Response: TURN is "brittle" in that it requires the NAT bindings
      between the client and the server to be maintained unchanged for the
      lifetime of the allocation. This is typically done using keep-alives. If
      this is not done, then the client will lose its allocation and can no
      longer exchange data with its peers.</t>
      <t pn="section-23-8">Consideration 4: Identify requirements for longer-term, sound
      technical solutions; contribute to the process of finding the right
      longer-term solution.</t>
      <t pn="section-23-9">Response: The need for TURN will be reduced once NATs implement the
      recommendations for NAT UDP behavior documented in <xref target="RFC4787" format="default" sectionFormat="of" derivedContent="RFC4787"/>. Applications are also strongly
      urged to use ICE <xref target="RFC8445" format="default" sectionFormat="of" derivedContent="RFC8445"/> to
      communicate with peers; though ICE uses TURN, it does so only as a last
      resort, and it uses it in a controlled manner.</t>
      <t pn="section-23-10">Consideration 5: Discussion of the impact of the noted practical
      issues with existing deployed NATs and experience reports.</t>
      <t pn="section-23-11">Response: Some NATs deployed today exhibit a mapping behavior other
      than Endpoint-Independent mapping. These NATs are difficult to work
      with, as they make it difficult or impossible for protocols like ICE to
      use server-reflexive transport addresses on those NATs. A client behind
      such a NAT is often forced to use a relay protocol like TURN because
      "UDP hole punching" techniques <xref target="RFC5128" format="default" sectionFormat="of" derivedContent="RFC5128"/> do not
      work.</t>
    </section>
    <section numbered="true" toc="include" removeInRFC="false" pn="section-24">
      <name slugifiedName="name-changes-since-rfc-5766">Changes since RFC 5766</name>
      <t pn="section-24-1">This section lists the major changes in the TURN protocol from the
      original <xref target="RFC5766" format="default" sectionFormat="of" derivedContent="RFC5766"/> specification.</t>
      <ul spacing="normal" bare="false" empty="false" pn="section-24-2">
        <li pn="section-24-2.1">IPv6 support.</li>
        <li pn="section-24-2.2">REQUESTED-ADDRESS-FAMILY attribute.</li>
        <li pn="section-24-2.3">Description of the tunnel amplification attack.</li>
        <li pn="section-24-2.4">DTLS support.</li>
        <li pn="section-24-2.5">Add support for receiving ICMP packets.</li>
        <li pn="section-24-2.6">Updates PMTUD.</li>
        <li pn="section-24-2.7">Discovery of TURN server.</li>
        <li pn="section-24-2.8">TURN URI Scheme Semantics.</li>
        <li pn="section-24-2.9">Happy Eyeballs for TURN.</li>
        <li pn="section-24-2.10">Align with the changes in STUN <xref target="RFC8489" format="default" sectionFormat="of" derivedContent="RFC8489"/>.</li>
      </ul>
    </section>
    <section numbered="true" toc="include" removeInRFC="false" pn="section-25">
      <name slugifiedName="name-updates-to-rfc-6156">Updates to RFC 6156</name>
      <t pn="section-25-1">This section lists the major updates to <xref target="RFC6156" format="default" sectionFormat="of" derivedContent="RFC6156"/> in this specification.</t>
      <ul spacing="normal" bare="false" empty="false" pn="section-25-2">
        <li pn="section-25-2.1">ADDITIONAL-ADDRESS-FAMILY and ADDRESS-ERROR-CODE attributes.</li>
        <li pn="section-25-2.2">440 (Address Family not Supported) and 443 (Peer Address Family
          Mismatch) responses.</li>
        <li pn="section-25-2.3">More details on packet translation.</li>
        <li pn="section-25-2.4">TCP-to-UDP and UDP-to-TCP relaying.</li>
      </ul>
    </section>
  </middle>
  <back>
    <displayreference target="I-D.ietf-tram-stun-pmtud" to="MTU-STUN"/>
    <displayreference target="I-D.ietf-mmusic-ice-sip-sdp" to="SDP-ICE"/>
    <displayreference target="I-D.ietf-tsvwg-udp-options" to="UDP-OPT"/>
    <displayreference target="I-D.ietf-intarea-frag-fragile" to="FRAG-FRAGILE"/>
    <displayreference target="I-D.ietf-rtcweb-security" to="SEC-WEBRTC"/>
    <displayreference target="I-D.ietf-tsvwg-datagram-plpmtud" to="MTU-DATAGRAM"/>
    <displayreference target="I-D.ietf-mptcp-rfc6824bis" to="TCP-EXT"/>
    <references pn="section-26">
      <name slugifiedName="name-references">References</name>
      <references pn="section-26.1">
        <name slugifiedName="name-normative-references">Normative References</name>
        <reference anchor="PROTOCOL-NUMBERS" target="https://www.iana.org/assignments/protocol-numbers" quoteTitle="true" derivedAnchor="PROTOCOL-NUMBERS">
          <front>
            <title>Protocol Numbers</title>
            <author>
              <organization showOnFrontPage="true">IANA</organization>
            </author>
            <date/>
          </front>
        </reference>
        <reference anchor="RFC0792" target="https://www.rfc-editor.org/info/rfc792" quoteTitle="true" derivedAnchor="RFC0792">
          <front>
            <title>Internet Control Message Protocol</title>
            <author initials="J." surname="Postel" fullname="J. Postel">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="1981" month="September"/>
          </front>
          <seriesInfo name="STD" value="5"/>
          <seriesInfo name="RFC" value="792"/>
          <seriesInfo name="DOI" value="10.17487/RFC0792"/>
        </reference>
        <reference anchor="RFC1122" target="https://www.rfc-editor.org/info/rfc1122" quoteTitle="true" derivedAnchor="RFC1122">
          <front>
            <title>Requirements for Internet Hosts - Communication Layers</title>
            <author initials="R." surname="Braden" fullname="R. Braden" role="editor">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="1989" month="October"/>
            <abstract>
              <t>This RFC is an official specification for the Internet community.  It incorporates by reference, amends, corrects, and supplements the primary protocol standards documents relating to hosts.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="STD" value="3"/>
          <seriesInfo name="RFC" value="1122"/>
          <seriesInfo name="DOI" value="10.17487/RFC1122"/>
        </reference>
        <reference anchor="RFC2119" target="https://www.rfc-editor.org/info/rfc2119" quoteTitle="true" derivedAnchor="RFC2119">
          <front>
            <title>Key words for use in RFCs to Indicate Requirement Levels</title>
            <author initials="S." surname="Bradner" fullname="S. Bradner">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="1997" month="March"/>
            <abstract>
              <t>In many standards track documents several words are used to signify the requirements in the specification.  These words are often capitalized. This document defines these words as they should be interpreted in IETF documents.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="14"/>
          <seriesInfo name="RFC" value="2119"/>
          <seriesInfo name="DOI" value="10.17487/RFC2119"/>
        </reference>
        <reference anchor="RFC2474" target="https://www.rfc-editor.org/info/rfc2474" quoteTitle="true" derivedAnchor="RFC2474">
          <front>
            <title>Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers</title>
            <author initials="K." surname="Nichols" fullname="K. Nichols">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S." surname="Blake" fullname="S. Blake">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="F." surname="Baker" fullname="F. Baker">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="D." surname="Black" fullname="D. Black">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="1998" month="December"/>
            <abstract>
              <t>This document defines the IP header field, called the DS (for differentiated services) field.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="2474"/>
          <seriesInfo name="DOI" value="10.17487/RFC2474"/>
        </reference>
        <reference anchor="RFC3168" target="https://www.rfc-editor.org/info/rfc3168" quoteTitle="true" derivedAnchor="RFC3168">
          <front>
            <title>The Addition of Explicit Congestion Notification (ECN) to IP</title>
            <author initials="K." surname="Ramakrishnan" fullname="K. Ramakrishnan">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S." surname="Floyd" fullname="S. Floyd">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="D." surname="Black" fullname="D. Black">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2001" month="September"/>
            <abstract>
              <t>This memo specifies the incorporation of ECN (Explicit Congestion Notification) to TCP and IP, including ECN's use of two bits in the IP header.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="3168"/>
          <seriesInfo name="DOI" value="10.17487/RFC3168"/>
        </reference>
        <reference anchor="RFC3629" target="https://www.rfc-editor.org/info/rfc3629" quoteTitle="true" derivedAnchor="RFC3629">
          <front>
            <title>UTF-8, a transformation format of ISO 10646</title>
            <author initials="F." surname="Yergeau" fullname="F. Yergeau">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2003" month="November"/>
            <abstract>
              <t>ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems.  The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo.  UTF-8 has the characteristic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values.  This memo obsoletes and replaces RFC 2279.</t>
            </abstract>
          </front>
          <seriesInfo name="STD" value="63"/>
          <seriesInfo name="RFC" value="3629"/>
          <seriesInfo name="DOI" value="10.17487/RFC3629"/>
        </reference>
        <reference anchor="RFC4443" target="https://www.rfc-editor.org/info/rfc4443" quoteTitle="true" derivedAnchor="RFC4443">
          <front>
            <title>Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification</title>
            <author initials="A." surname="Conta" fullname="A. Conta">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S." surname="Deering" fullname="S. Deering">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="M." surname="Gupta" fullname="M. Gupta" role="editor">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2006" month="March"/>
            <abstract>
              <t>This document describes the format of a set of control messages used in ICMPv6 (Internet Control Message Protocol).  ICMPv6 is the Internet Control Message Protocol for Internet Protocol version 6 (IPv6).  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="STD" value="89"/>
          <seriesInfo name="RFC" value="4443"/>
          <seriesInfo name="DOI" value="10.17487/RFC4443"/>
        </reference>
        <reference anchor="RFC6347" target="https://www.rfc-editor.org/info/rfc6347" quoteTitle="true" derivedAnchor="RFC6347">
          <front>
            <title>Datagram Transport Layer Security Version 1.2</title>
            <author initials="E." surname="Rescorla" fullname="E. Rescorla">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="N." surname="Modadugu" fullname="N. Modadugu">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2012" month="January"/>
            <abstract>
              <t>This document specifies version 1.2 of the Datagram Transport Layer Security (DTLS) protocol.  The DTLS protocol provides communications privacy for datagram protocols.  The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery.  The DTLS protocol is based on the Transport Layer Security (TLS) protocol and provides equivalent security guarantees.  Datagram semantics of the underlying transport are preserved by the DTLS protocol.  This document updates DTLS 1.0 to work with TLS version 1.2.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="6347"/>
          <seriesInfo name="DOI" value="10.17487/RFC6347"/>
        </reference>
        <reference anchor="RFC6437" target="https://www.rfc-editor.org/info/rfc6437" quoteTitle="true" derivedAnchor="RFC6437">
          <front>
            <title>IPv6 Flow Label Specification</title>
            <author initials="S." surname="Amante" fullname="S. Amante">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="B." surname="Carpenter" fullname="B. Carpenter">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S." surname="Jiang" fullname="S. Jiang">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="J." surname="Rajahalme" fullname="J. Rajahalme">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2011" month="November"/>
            <abstract>
              <t>This document specifies the IPv6 Flow Label field and the minimum requirements for IPv6 nodes labeling flows, IPv6 nodes forwarding labeled packets, and flow state establishment methods.  Even when mentioned as examples of possible uses of the flow labeling, more detailed requirements for specific use cases are out of the scope for this document.</t>
              <t>The usage of the Flow Label field enables efficient IPv6 flow classification based only on IPv6 main header fields in fixed positions.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="6437"/>
          <seriesInfo name="DOI" value="10.17487/RFC6437"/>
        </reference>
        <reference anchor="RFC7065" target="https://www.rfc-editor.org/info/rfc7065" quoteTitle="true" derivedAnchor="RFC7065">
          <front>
            <title>Traversal Using Relays around NAT (TURN) Uniform Resource Identifiers</title>
            <author initials="M." surname="Petit-Huguenin" fullname="M. Petit-Huguenin">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S." surname="Nandakumar" fullname="S. Nandakumar">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="G." surname="Salgueiro" fullname="G. Salgueiro">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="P." surname="Jones" fullname="P. Jones">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2013" month="November"/>
            <abstract>
              <t>This document specifies the syntax of Uniform Resource Identifier (URI) schemes for the Traversal Using Relays around NAT (TURN) protocol.  It defines two URI schemes to provision the TURN Resolution Mechanism (RFC 5928).</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7065"/>
          <seriesInfo name="DOI" value="10.17487/RFC7065"/>
        </reference>
        <reference anchor="RFC7350" target="https://www.rfc-editor.org/info/rfc7350" quoteTitle="true" derivedAnchor="RFC7350">
          <front>
            <title>Datagram Transport Layer Security (DTLS) as Transport for Session Traversal Utilities for NAT (STUN)</title>
            <author initials="M." surname="Petit-Huguenin" fullname="M. Petit-Huguenin">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="G." surname="Salgueiro" fullname="G. Salgueiro">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2014" month="August"/>
            <abstract>
              <t>This document specifies the usage of Datagram Transport Layer Security (DTLS) as a transport protocol for Session Traversal Utilities for NAT (STUN).  It provides guidance on when and how to use DTLS with the currently standardized STUN usages.  It also specifies modifications to the STUN and Traversal Using Relay NAT (TURN) URIs and to the TURN resolution mechanism to facilitate the resolution of STUN and TURN URIs into the IP address and port of STUN and TURN servers supporting DTLS as a transport protocol.  This document updates RFCs 5389 and 5928.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7350"/>
          <seriesInfo name="DOI" value="10.17487/RFC7350"/>
        </reference>
        <reference anchor="RFC7525" target="https://www.rfc-editor.org/info/rfc7525" quoteTitle="true" derivedAnchor="RFC7525">
          <front>
            <title>Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)</title>
            <author initials="Y." surname="Sheffer" fullname="Y. Sheffer">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="R." surname="Holz" fullname="R. Holz">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="P." surname="Saint-Andre" fullname="P. Saint-Andre">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2015" month="May"/>
            <abstract>
              <t>Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) are widely used to protect data exchanged over application protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP.  Over the last few years, several serious attacks on TLS have emerged, including attacks on its most commonly used cipher suites and their modes of operation.  This document provides recommendations for improving the security of deployed services that use TLS and DTLS. The recommendations are applicable to the majority of use cases.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="195"/>
          <seriesInfo name="RFC" value="7525"/>
          <seriesInfo name="DOI" value="10.17487/RFC7525"/>
        </reference>
        <reference anchor="RFC7915" target="https://www.rfc-editor.org/info/rfc7915" quoteTitle="true" derivedAnchor="RFC7915">
          <front>
            <title>IP/ICMP Translation Algorithm</title>
            <author initials="C." surname="Bao" fullname="C. Bao">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="X." surname="Li" fullname="X. Li">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="F." surname="Baker" fullname="F. Baker">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="T." surname="Anderson" fullname="T. Anderson">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="F." surname="Gont" fullname="F. Gont">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2016" month="June"/>
            <abstract>
              <t>This document describes the Stateless IP/ICMP Translation Algorithm (SIIT), which translates between IPv4 and IPv6 packet headers (including ICMP headers).  This document obsoletes RFC 6145.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7915"/>
          <seriesInfo name="DOI" value="10.17487/RFC7915"/>
        </reference>
        <reference anchor="RFC7982" target="https://www.rfc-editor.org/info/rfc7982" quoteTitle="true" derivedAnchor="RFC7982">
          <front>
            <title>Measurement of Round-Trip Time and Fractional Loss Using Session Traversal Utilities for NAT (STUN)</title>
            <author initials="P." surname="Martinsen" fullname="P. Martinsen">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="T." surname="Reddy" fullname="T. Reddy">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="D." surname="Wing" fullname="D. Wing">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="V." surname="Singh" fullname="V. Singh">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2016" month="September"/>
            <abstract>
              <t>A host with multiple interfaces needs to choose the best interface for communication.  Oftentimes, this decision is based on a static configuration and does not consider the path characteristics, which may affect the user experience.</t>
              <t>This document describes a mechanism for an endpoint to measure the path characteristics fractional loss and RTT using Session Traversal Utilities for NAT (STUN) messages.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7982"/>
          <seriesInfo name="DOI" value="10.17487/RFC7982"/>
        </reference>
        <reference anchor="RFC8174" target="https://www.rfc-editor.org/info/rfc8174" quoteTitle="true" derivedAnchor="RFC8174">
          <front>
            <title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</title>
            <author initials="B." surname="Leiba" fullname="B. Leiba">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2017" month="May"/>
            <abstract>
              <t>RFC 2119 specifies common key words that may be used in protocol  specifications.  This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the  defined special meanings.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="14"/>
          <seriesInfo name="RFC" value="8174"/>
          <seriesInfo name="DOI" value="10.17487/RFC8174"/>
        </reference>
        <reference anchor="RFC8200" target="https://www.rfc-editor.org/info/rfc8200" quoteTitle="true" derivedAnchor="RFC8200">
          <front>
            <title>Internet Protocol, Version 6 (IPv6) Specification</title>
            <author initials="S." surname="Deering" fullname="S. Deering">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="R." surname="Hinden" fullname="R. Hinden">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2017" month="July"/>
            <abstract>
              <t>This document specifies version 6 of the Internet Protocol (IPv6). It obsoletes RFC 2460.</t>
            </abstract>
          </front>
          <seriesInfo name="STD" value="86"/>
          <seriesInfo name="RFC" value="8200"/>
          <seriesInfo name="DOI" value="10.17487/RFC8200"/>
        </reference>
        <reference anchor="RFC8305" target="https://www.rfc-editor.org/info/rfc8305" quoteTitle="true" derivedAnchor="RFC8305">
          <front>
            <title>Happy Eyeballs Version 2: Better Connectivity Using Concurrency</title>
            <author initials="D." surname="Schinazi" fullname="D. Schinazi">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="T." surname="Pauly" fullname="T. Pauly">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2017" month="December"/>
            <abstract>
              <t>Many communication protocols operating over the modern Internet use hostnames.  These often resolve to multiple IP addresses, each of which may have different performance and connectivity characteristics.  Since specific addresses or address families (IPv4 or IPv6) may be blocked, broken, or sub-optimal on a network, clients that attempt multiple connections in parallel have a chance of establishing a connection more quickly.  This document specifies requirements for algorithms that reduce this user-visible delay and provides an example algorithm, referred to as "Happy Eyeballs".  This document obsoletes the original algorithm description in RFC 6555.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8305"/>
          <seriesInfo name="DOI" value="10.17487/RFC8305"/>
        </reference>
        <reference anchor="RFC8446" target="https://www.rfc-editor.org/info/rfc8446" quoteTitle="true" derivedAnchor="RFC8446">
          <front>
            <title>The Transport Layer Security (TLS) Protocol Version 1.3</title>
            <author initials="E." surname="Rescorla" fullname="E. Rescorla">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2018" month="August"/>
            <abstract>
              <t>This document specifies version 1.3 of the Transport Layer Security (TLS) protocol.  TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery.</t>
              <t>This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961.  This document also specifies new requirements for TLS 1.2 implementations.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8446"/>
          <seriesInfo name="DOI" value="10.17487/RFC8446"/>
        </reference>
        <reference anchor="RFC8489" target="https://www.rfc-editor.org/info/rfc8489" quoteTitle="true" derivedAnchor="RFC8489">
          <front>
            <title>Session Traversal Utilities for NAT (STUN)</title>
            <seriesInfo name="RFC" value="8489"/>
            <seriesInfo name="DOI" value="10.17487/RFC8489"/>
            <author initials="M" surname="Petit-Huguenin" fullname="Marc Petit-Huguenin">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="G" surname="Salgueiro" fullname="Gonzalo Salgueiro">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="J" surname="Rosenberg" fullname="Jonathan Rosenberg">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="D" surname="Wing" fullname="Dan Wing">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="R" surname="Mahy" fullname="Rohan Mahy">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="P" surname="Matthews" fullname="Philip Matthews">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="February" year="2020"/>
          </front>
        </reference>
      </references>
      <references pn="section-26.2">
        <name slugifiedName="name-informative-references">Informative References</name>
        <reference anchor="I-D.ietf-intarea-frag-fragile" quoteTitle="true" target="https://tools.ietf.org/html/draft-ietf-intarea-frag-fragile-17" derivedAnchor="FRAG-FRAGILE">
          <front>
            <title>IP Fragmentation Considered Fragile</title>
            <author initials="R" surname="Bonica" fullname="Ron Bonica">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="F" surname="Baker" fullname="Fred Baker">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="G" surname="Huston" fullname="Geoff Huston">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="R" surname="Hinden" fullname="Robert Hinden">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="O" surname="Troan" fullname="Ole Troan">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="F" surname="Gont" fullname="Fernando Gont">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="September" day="30" year="2019"/>
            <abstract>
              <t>This document describes IP fragmentation and explains how it introduces fragility to Internet communication.  This document also proposes alternatives to IP fragmentation and provides recommendations for developers and network operators.</t>
            </abstract>
          </front>
          <seriesInfo name="Internet-Draft" value="draft-ietf-intarea-frag-fragile-17"/>
          <format type="TXT" target="http://www.ietf.org/internet-drafts/draft-ietf-intarea-frag-fragile-17.txt"/>
          <refcontent>Work in Progress</refcontent>
        </reference>
        <reference anchor="FRAG-HARMFUL" target="https://www.hpl.hp.com/techreports/Compaq-DEC/WRL-87-3.pdf" quoteTitle="true" derivedAnchor="FRAG-HARMFUL">
          <front>
            <title>Fragmentation Considered Harmful</title>
            <author fullname="Kent" initials="C." surname="Kent">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="Mogul" initials="J." surname="Mogul">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="December" year="1987"/>
          </front>
        </reference>
        <reference anchor="I-D.ietf-tsvwg-datagram-plpmtud" quoteTitle="true" target="https://tools.ietf.org/html/draft-ietf-tsvwg-datagram-plpmtud-14" derivedAnchor="MTU-DATAGRAM">
          <front>
            <title>Packetization Layer Path MTU Discovery for Datagram Transports</title>
            <author initials="G" surname="Fairhurst" fullname="Gorry Fairhurst">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="T" surname="Jones" fullname="Tom Jones">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="M" surname="Tuexen" fullname="Michael Tuexen">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="I" surname="Ruengeler" fullname="Irene Ruengeler">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="T" surname="Voelker" fullname="Timo Voelker">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="February" day="12" year="2020"/>
            <abstract>
              <t>This document describes a robust method for Path MTU Discovery (PMTUD) for datagram Packetization Layers (PLs).  It describes an extension to RFC 1191 and RFC 8201, which specifies ICMP-based Path MTU Discovery for IPv4 and IPv6.  The method allows a PL, or a datagram application that uses a PL, to discover whether a network path can support the current size of datagram.  This can be used to detect and reduce the message size when a sender encounters a packet black hole (where packets are discarded).  The method can probe a network path with progressively larger packets to discover whether the maximum packet size can be increased.  This allows a sender to determine an appropriate packet size, providing functionality for datagram transports that is equivalent to the Packetization Layer PMTUD specification for TCP, specified in RFC 4821.  The document updates RFC 4821 to specify the method for datagram PLs, and updates RFC 8085 as the method to use in place of RFC 4821 with UDP datagrams.  Section 7.3 of RFC4960 recommends an endpoint apply the techniques in RFC4821 on a per-destination-address basis. RFC4960 is updated to recommend that SCTP uses the method specified in this document instead of the method in RFC4821.  The document also provides implementation notes for incorporating Datagram PMTUD into IETF datagram transports or applications that use datagram transports.  When published, this specification updates RFC 4821 and RFC 8085.</t>
            </abstract>
          </front>
          <seriesInfo name="Internet-Draft" value="draft-ietf-tsvwg-datagram-plpmtud-14"/>
          <format type="TXT" target="http://www.ietf.org/internet-drafts/draft-ietf-tsvwg-datagram-plpmtud-14.txt"/>
          <refcontent>Work in Progress</refcontent>
        </reference>
        <reference anchor="I-D.ietf-tram-stun-pmtud" quoteTitle="true" target="https://tools.ietf.org/html/draft-ietf-tram-stun-pmtud-15" derivedAnchor="MTU-STUN">
          <front>
            <title>Packetization Layer Path MTU Discovery (PLMTUD) For UDP Transports Using Session Traversal Utilities for NAT (STUN)</title>
            <author initials="M" surname="Petit-Huguenin" fullname="Marc Petit-Huguenin">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="G" surname="Salgueiro" fullname="Gonzalo Salgueiro">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="F" surname="Garrido" fullname="Felipe Garrido">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="December" day="17" year="2019"/>
            <abstract>
              <t>The datagram exchanged between two Internet endpoints have to go through a series of physical and virtual links that may have different limits on the upper size of the datagram they can transmit without fragmentation.  Because fragmentation is considered harmful, most transports and protocols are designed with a mechanism that permits dynamic measurement of the maximum size of a datagram.  This mechanism is called Packetization Layer Path MTU Discovery (PLPMTUD). But the UDP transport and some of the protocols that use UDP were designed without that feature.  The Session Traversal Utilities for NAT (STUN) Usage described in this document permits retrofitting an existing UDP-based protocol with such a feature.  Similarly, a new UDP-based protocol could simply reuse the mechanism described in this document.</t>
            </abstract>
          </front>
          <seriesInfo name="Internet-Draft" value="draft-ietf-tram-stun-pmtud-15"/>
          <format type="TXT" target="http://www.ietf.org/internet-drafts/draft-ietf-tram-stun-pmtud-15.txt"/>
          <refcontent>Work in Progress</refcontent>
        </reference>
        <reference anchor="PORT-NUMBERS" target="https://www.iana.org/assignments/port-numbers" quoteTitle="true" derivedAnchor="PORT-NUMBERS">
          <front>
            <title>Service Name and Transport Protocol Port Number Registry</title>
            <author>
              <organization showOnFrontPage="true">IANA</organization>
            </author>
            <date/>
          </front>
        </reference>
        <reference anchor="RFC0791" target="https://www.rfc-editor.org/info/rfc791" quoteTitle="true" derivedAnchor="RFC0791">
          <front>
            <title>Internet Protocol</title>
            <author initials="J." surname="Postel" fullname="J. Postel">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="1981" month="September"/>
          </front>
          <seriesInfo name="STD" value="5"/>
          <seriesInfo name="RFC" value="791"/>
          <seriesInfo name="DOI" value="10.17487/RFC0791"/>
        </reference>
        <reference anchor="RFC1191" target="https://www.rfc-editor.org/info/rfc1191" quoteTitle="true" derivedAnchor="RFC1191">
          <front>
            <title>Path MTU discovery</title>
            <author initials="J.C." surname="Mogul" fullname="J.C. Mogul">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S.E." surname="Deering" fullname="S.E. Deering">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="1990" month="November"/>
            <abstract>
              <t>This memo describes a technique for dynamically discovering the maximum transmission unit (MTU) of an arbitrary internet path.  It specifies a small change to the way routers generate one type of ICMP message.  For a path that passes through a router that has not been so changed, this technique might not discover the correct Path MTU, but it will always choose a Path MTU as accurate as, and in many cases more accurate than, the Path MTU that would be chosen by current practice.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="1191"/>
          <seriesInfo name="DOI" value="10.17487/RFC1191"/>
        </reference>
        <reference anchor="RFC1918" target="https://www.rfc-editor.org/info/rfc1918" quoteTitle="true" derivedAnchor="RFC1918">
          <front>
            <title>Address Allocation for Private Internets</title>
            <author initials="Y." surname="Rekhter" fullname="Y. Rekhter">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="B." surname="Moskowitz" fullname="B. Moskowitz">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="D." surname="Karrenberg" fullname="D. Karrenberg">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="G. J." surname="de Groot" fullname="G. J. de Groot">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="E." surname="Lear" fullname="E. Lear">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="1996" month="February"/>
            <abstract>
              <t>This document describes address allocation for private internets.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="5"/>
          <seriesInfo name="RFC" value="1918"/>
          <seriesInfo name="DOI" value="10.17487/RFC1918"/>
        </reference>
        <reference anchor="RFC1928" target="https://www.rfc-editor.org/info/rfc1928" quoteTitle="true" derivedAnchor="RFC1928">
          <front>
            <title>SOCKS Protocol Version 5</title>
            <author initials="M." surname="Leech" fullname="M. Leech">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="M." surname="Ganis" fullname="M. Ganis">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="Y." surname="Lee" fullname="Y. Lee">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="R." surname="Kuris" fullname="R. Kuris">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="D." surname="Koblas" fullname="D. Koblas">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="L." surname="Jones" fullname="L. Jones">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="1996" month="March"/>
            <abstract>
              <t>This memo describes a protocol that is an evolution of the previous version of the protocol, version 4 [1]. This new protocol stems from active discussions and prototype implementations.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="1928"/>
          <seriesInfo name="DOI" value="10.17487/RFC1928"/>
        </reference>
        <reference anchor="RFC3261" target="https://www.rfc-editor.org/info/rfc3261" quoteTitle="true" derivedAnchor="RFC3261">
          <front>
            <title>SIP: Session Initiation Protocol</title>
            <author initials="J." surname="Rosenberg" fullname="J. Rosenberg">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="H." surname="Schulzrinne" fullname="H. Schulzrinne">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="G." surname="Camarillo" fullname="G. Camarillo">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="A." surname="Johnston" fullname="A. Johnston">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="J." surname="Peterson" fullname="J. Peterson">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="R." surname="Sparks" fullname="R. Sparks">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="M." surname="Handley" fullname="M. Handley">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="E." surname="Schooler" fullname="E. Schooler">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2002" month="June"/>
            <abstract>
              <t>This document describes Session Initiation Protocol (SIP), an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants.  These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="3261"/>
          <seriesInfo name="DOI" value="10.17487/RFC3261"/>
        </reference>
        <reference anchor="RFC3424" target="https://www.rfc-editor.org/info/rfc3424" quoteTitle="true" derivedAnchor="RFC3424">
          <front>
            <title>IAB Considerations for UNilateral Self-Address Fixing (UNSAF) Across Network Address Translation</title>
            <author initials="L." surname="Daigle" fullname="L. Daigle" role="editor">
              <organization showOnFrontPage="true"/>
            </author>
            <author>
              <organization showOnFrontPage="true">IAB</organization>
            </author>
            <date year="2002" month="November"/>
            <abstract>
              <t>As a result of the nature of Network Address Translation (NAT) Middleboxes, communicating endpoints that are separated by one or more NATs do not know how to refer to themselves using addresses that are valid in the addressing realms of their (current and future) peers. Various proposals have been made for "UNilateral Self-Address Fixing (UNSAF)" processes.  These are processes whereby some originating endpoint attempts to determine or fix the address (and port) by which it is known to another endpoint - e.g., to be able to use address data in the protocol exchange, or to advertise a public address from which it will receive connections. This document outlines the reasons for which these proposals can be considered at best as short term fixes to specific problems and the specific issues to be carefully evaluated before creating an UNSAF proposal.  This memo provides information for the Internet community.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="3424"/>
          <seriesInfo name="DOI" value="10.17487/RFC3424"/>
        </reference>
        <reference anchor="RFC3550" target="https://www.rfc-editor.org/info/rfc3550" quoteTitle="true" derivedAnchor="RFC3550">
          <front>
            <title>RTP: A Transport Protocol for Real-Time Applications</title>
            <author initials="H." surname="Schulzrinne" fullname="H. Schulzrinne">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S." surname="Casner" fullname="S. Casner">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="R." surname="Frederick" fullname="R. Frederick">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="V." surname="Jacobson" fullname="V. Jacobson">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2003" month="July"/>
            <abstract>
              <t>This memorandum describes RTP, the real-time transport protocol.  RTP provides end-to-end network transport functions suitable for applications transmitting real-time data, such as audio, video or simulation data, over multicast or unicast network services.  RTP does not address resource reservation and does not guarantee quality-of- service for real-time services.  The data transport is augmented by a control protocol (RTCP) to allow monitoring of the data delivery in a manner scalable to large multicast networks, and to provide minimal control and identification functionality.  RTP and RTCP are designed to be independent of the underlying transport and network layers.  The protocol supports the use of RTP-level translators and mixers. Most of the text in this memorandum is identical to RFC 1889 which it obsoletes.  There are no changes in the packet formats on the wire, only changes to the rules and algorithms governing how the protocol is used. The biggest change is an enhancement to the scalable timer algorithm for calculating when to send RTCP packets in order to minimize transmission in excess of the intended rate when many participants join a session simultaneously.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="STD" value="64"/>
          <seriesInfo name="RFC" value="3550"/>
          <seriesInfo name="DOI" value="10.17487/RFC3550"/>
        </reference>
        <reference anchor="RFC3711" target="https://www.rfc-editor.org/info/rfc3711" quoteTitle="true" derivedAnchor="RFC3711">
          <front>
            <title>The Secure Real-time Transport Protocol (SRTP)</title>
            <author initials="M." surname="Baugher" fullname="M. Baugher">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="D." surname="McGrew" fullname="D. McGrew">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="M." surname="Naslund" fullname="M. Naslund">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="E." surname="Carrara" fullname="E. Carrara">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="K." surname="Norrman" fullname="K. Norrman">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2004" month="March"/>
            <abstract>
              <t>This document describes the Secure Real-time Transport Protocol (SRTP), a profile of the Real-time Transport Protocol (RTP), which can provide confidentiality, message authentication, and replay protection to the RTP traffic and to the control traffic for RTP, the Real-time Transport Control Protocol (RTCP).   [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="3711"/>
          <seriesInfo name="DOI" value="10.17487/RFC3711"/>
        </reference>
        <reference anchor="RFC4086" target="https://www.rfc-editor.org/info/rfc4086" quoteTitle="true" derivedAnchor="RFC4086">
          <front>
            <title>Randomness Requirements for Security</title>
            <author initials="D." surname="Eastlake 3rd" fullname="D. Eastlake 3rd">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="J." surname="Schiller" fullname="J. Schiller">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S." surname="Crocker" fullname="S. Crocker">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2005" month="June"/>
            <abstract>
              <t>Security systems are built on strong cryptographic algorithms that foil pattern analysis attempts.  However, the security of these systems is dependent on generating secret quantities for passwords, cryptographic keys, and similar quantities.  The use of pseudo-random processes to generate secret quantities can result in pseudo-security. A sophisticated attacker may find it easier to reproduce the environment that produced the secret quantities and to search the resulting small set of possibilities than to locate the quantities in the whole of the potential number space.</t>
              <t>Choosing random quantities to foil a resourceful and motivated adversary is surprisingly difficult.  This document points out many pitfalls in using poor entropy sources or traditional pseudo-random number generation techniques for generating such quantities.  It recommends the use of truly random hardware techniques and shows that the existing hardware on many systems can be used for this purpose. It provides suggestions to ameliorate the problem when a hardware solution is not available, and it gives examples of how large such quantities need to be for some applications.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="106"/>
          <seriesInfo name="RFC" value="4086"/>
          <seriesInfo name="DOI" value="10.17487/RFC4086"/>
        </reference>
        <reference anchor="RFC4302" target="https://www.rfc-editor.org/info/rfc4302" quoteTitle="true" derivedAnchor="RFC4302">
          <front>
            <title>IP Authentication Header</title>
            <author initials="S." surname="Kent" fullname="S. Kent">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2005" month="December"/>
            <abstract>
              <t>This document describes an updated version of the IP Authentication Header (AH), which is designed to provide authentication services in IPv4 and IPv6.  This document obsoletes RFC 2402 (November 1998).  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="4302"/>
          <seriesInfo name="DOI" value="10.17487/RFC4302"/>
        </reference>
        <reference anchor="RFC4303" target="https://www.rfc-editor.org/info/rfc4303" quoteTitle="true" derivedAnchor="RFC4303">
          <front>
            <title>IP Encapsulating Security Payload (ESP)</title>
            <author initials="S." surname="Kent" fullname="S. Kent">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2005" month="December"/>
            <abstract>
              <t>This document describes an updated version of the Encapsulating Security Payload (ESP) protocol, which is designed to provide a mix of security services in IPv4 and IPv6.  ESP is used to provide confidentiality, data origin authentication, connectionless integrity, an anti-replay service (a form of partial sequence integrity), and limited traffic flow confidentiality.  This document obsoletes RFC 2406 (November 1998).  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="4303"/>
          <seriesInfo name="DOI" value="10.17487/RFC4303"/>
        </reference>
        <reference anchor="RFC4787" target="https://www.rfc-editor.org/info/rfc4787" quoteTitle="true" derivedAnchor="RFC4787">
          <front>
            <title>Network Address Translation (NAT) Behavioral Requirements for Unicast UDP</title>
            <author initials="F." surname="Audet" fullname="F. Audet" role="editor">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C." surname="Jennings" fullname="C. Jennings">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2007" month="January"/>
            <abstract>
              <t>This document defines basic terminology for describing different types of Network Address Translation (NAT) behavior when handling Unicast UDP and also defines a set of requirements that would allow many applications, such as multimedia communications or online gaming, to work consistently.  Developing NATs that meet this set of requirements will greatly increase the likelihood that these applications will function properly.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="127"/>
          <seriesInfo name="RFC" value="4787"/>
          <seriesInfo name="DOI" value="10.17487/RFC4787"/>
        </reference>
        <reference anchor="RFC4821" target="https://www.rfc-editor.org/info/rfc4821" quoteTitle="true" derivedAnchor="RFC4821">
          <front>
            <title>Packetization Layer Path MTU Discovery</title>
            <author initials="M." surname="Mathis" fullname="M. Mathis">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="J." surname="Heffner" fullname="J. Heffner">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2007" month="March"/>
            <abstract>
              <t>This document describes a robust method for Path MTU Discovery (PMTUD) that relies on TCP or some other Packetization Layer to probe an Internet path with progressively larger packets.  This method is described as an extension to RFC 1191 and RFC 1981, which specify ICMP-based Path MTU Discovery for IP versions 4 and 6, respectively.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="4821"/>
          <seriesInfo name="DOI" value="10.17487/RFC4821"/>
        </reference>
        <reference anchor="RFC5128" target="https://www.rfc-editor.org/info/rfc5128" quoteTitle="true" derivedAnchor="RFC5128">
          <front>
            <title>State of Peer-to-Peer (P2P) Communication across Network Address Translators (NATs)</title>
            <author initials="P." surname="Srisuresh" fullname="P. Srisuresh">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="B." surname="Ford" fullname="B. Ford">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="D." surname="Kegel" fullname="D. Kegel">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2008" month="March"/>
            <abstract>
              <t>This memo documents the various methods known to be in use by applications to establish direct communication in the presence of Network Address Translators (NATs) at the current time.  Although this memo is intended to be mainly descriptive, the Security Considerations section makes some purely advisory recommendations about how to deal with security vulnerabilities the applications could inadvertently create when using the methods described.  This memo covers NAT traversal approaches used by both TCP- and UDP-based applications.  This memo is not an endorsement of the methods described, but merely an attempt to capture them in a document.  This memo provides information for the Internet community.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5128"/>
          <seriesInfo name="DOI" value="10.17487/RFC5128"/>
        </reference>
        <reference anchor="RFC5482" target="https://www.rfc-editor.org/info/rfc5482" quoteTitle="true" derivedAnchor="RFC5482">
          <front>
            <title>TCP User Timeout Option</title>
            <author initials="L." surname="Eggert" fullname="L. Eggert">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="F." surname="Gont" fullname="F. Gont">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2009" month="March"/>
            <abstract>
              <t>The TCP user timeout controls how long transmitted data may remain unacknowledged before a connection is forcefully closed.  It is a local, per-connection parameter.  This document specifies a new TCP option -- the TCP User Timeout Option -- that allows one end of a TCP connection to advertise its current user timeout value.  This information provides advice to the other end of the TCP connection to adapt its user timeout accordingly.  Increasing the user timeouts on both ends of a TCP connection allows it to survive extended periods without end-to-end connectivity.  Decreasing the user timeouts allows busy servers to explicitly notify their clients that they will maintain the connection state only for a short time without connectivity.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5482"/>
          <seriesInfo name="DOI" value="10.17487/RFC5482"/>
        </reference>
        <reference anchor="RFC5766" target="https://www.rfc-editor.org/info/rfc5766" quoteTitle="true" derivedAnchor="RFC5766">
          <front>
            <title>Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN)</title>
            <author initials="R." surname="Mahy" fullname="R. Mahy">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="P." surname="Matthews" fullname="P. Matthews">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="J." surname="Rosenberg" fullname="J. Rosenberg">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2010" month="April"/>
            <abstract>
              <t>If a host is located behind a NAT, then in certain situations it can be impossible for that host to communicate directly with other hosts (peers).  In these situations, it is necessary for the host to use the services of an intermediate node that acts as a communication relay.  This specification defines a protocol, called TURN (Traversal Using Relays around NAT), that allows the host to control the operation of the relay and to exchange packets with its peers using the relay.  TURN differs from some other relay control protocols in that it allows a client to communicate with multiple peers using a single relay address.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5766"/>
          <seriesInfo name="DOI" value="10.17487/RFC5766"/>
        </reference>
        <reference anchor="RFC5925" target="https://www.rfc-editor.org/info/rfc5925" quoteTitle="true" derivedAnchor="RFC5925">
          <front>
            <title>The TCP Authentication Option</title>
            <author initials="J." surname="Touch" fullname="J. Touch">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="A." surname="Mankin" fullname="A. Mankin">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="R." surname="Bonica" fullname="R. Bonica">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2010" month="June"/>
            <abstract>
              <t>This document specifies the TCP Authentication Option (TCP-AO), which obsoletes the TCP MD5 Signature option of RFC 2385 (TCP MD5).  TCP-AO specifies the use of stronger Message Authentication Codes (MACs), protects against replays even for long-lived TCP connections, and provides more details on the association of security with TCP connections than TCP MD5.  TCP-AO is compatible with either a static Master Key Tuple (MKT) configuration or an external, out-of-band MKT management mechanism; in either case, TCP-AO also protects connections when using the same MKT across repeated instances of a connection, using traffic keys derived from the MKT, and coordinates MKT changes between endpoints.  The result is intended to support current infrastructure uses of TCP MD5, such as to protect long-lived connections (as used, e.g., in BGP and LDP), and to support a larger set of MACs with minimal other system and operational changes.  TCP-AO uses a different option identifier than TCP MD5, even though TCP-AO and TCP MD5 are never permitted to be used simultaneously.  TCP-AO supports IPv6, and is fully compatible with the proposed requirements for the replacement of TCP MD5.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5925"/>
          <seriesInfo name="DOI" value="10.17487/RFC5925"/>
        </reference>
        <reference anchor="RFC5928" target="https://www.rfc-editor.org/info/rfc5928" quoteTitle="true" derivedAnchor="RFC5928">
          <front>
            <title>Traversal Using Relays around NAT (TURN) Resolution Mechanism</title>
            <author initials="M." surname="Petit-Huguenin" fullname="M. Petit-Huguenin">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2010" month="August"/>
            <abstract>
              <t>This document defines a resolution mechanism to generate a list of server transport addresses that can be tried to create a Traversal Using Relays around NAT (TURN) allocation.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5928"/>
          <seriesInfo name="DOI" value="10.17487/RFC5928"/>
        </reference>
        <reference anchor="RFC6056" target="https://www.rfc-editor.org/info/rfc6056" quoteTitle="true" derivedAnchor="RFC6056">
          <front>
            <title>Recommendations for Transport-Protocol Port Randomization</title>
            <author initials="M." surname="Larsen" fullname="M. Larsen">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="F." surname="Gont" fullname="F. Gont">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2011" month="January"/>
            <abstract>
              <t>During the last few years, awareness has been raised about a number of "blind" attacks that can be performed against the Transmission Control Protocol (TCP) and similar protocols.  The consequences of these attacks range from throughput reduction to broken connections or data corruption.  These attacks rely on the attacker's ability to guess or know the five-tuple (Protocol, Source Address, Destination Address, Source Port, Destination Port) that identifies the transport protocol instance to be attacked.  This document describes a number of simple and efficient methods for the selection of the client port number, such that the possibility of an attacker guessing the exact value is reduced.  While this is not a replacement for cryptographic methods for protecting the transport-protocol instance, the aforementioned port selection algorithms provide improved security with very little effort and without any key management overhead.  The algorithms described in this document are local policies that may be incrementally deployed and that do not violate the specifications of any of the transport protocols that may benefit from them, such as TCP, UDP, UDP-lite, Stream Control Transmission Protocol (SCTP), Datagram Congestion Control Protocol (DCCP), and RTP (provided that the RTP application explicitly signals the RTP and RTCP port numbers).  This memo documents an Internet Best Current Practice.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="156"/>
          <seriesInfo name="RFC" value="6056"/>
          <seriesInfo name="DOI" value="10.17487/RFC6056"/>
        </reference>
        <reference anchor="RFC6062" target="https://www.rfc-editor.org/info/rfc6062" quoteTitle="true" derivedAnchor="RFC6062">
          <front>
            <title>Traversal Using Relays around NAT (TURN) Extensions for TCP Allocations</title>
            <author initials="S." surname="Perreault" fullname="S. Perreault" role="editor">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="J." surname="Rosenberg" fullname="J. Rosenberg">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2010" month="November"/>
            <abstract>
              <t>This specification defines an extension of Traversal Using Relays around NAT (TURN), a relay protocol for Network Address Translator (NAT) traversal.  This extension allows a TURN client to request TCP allocations, and defines new requests and indications for the TURN server to open and accept TCP connections with the client\'s peers. TURN and this extension both purposefully restrict the ways in which the relayed address can be used.  In particular, it prevents users from running general-purpose servers from ports obtained from the TURN server.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="6062"/>
          <seriesInfo name="DOI" value="10.17487/RFC6062"/>
        </reference>
        <reference anchor="RFC6156" target="https://www.rfc-editor.org/info/rfc6156" quoteTitle="true" derivedAnchor="RFC6156">
          <front>
            <title>Traversal Using Relays around NAT (TURN) Extension for IPv6</title>
            <author initials="G." surname="Camarillo" fullname="G. Camarillo">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="O." surname="Novo" fullname="O. Novo">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S." surname="Perreault" fullname="S. Perreault" role="editor">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2011" month="April"/>
            <abstract>
              <t>This document adds IPv6 support to Traversal Using Relays around NAT (TURN).  IPv6 support in TURN includes IPv4-to-IPv6, IPv6-to-IPv6, and IPv6-to-IPv4 relaying.  This document defines the REQUESTED- ADDRESS-FAMILY attribute for TURN.  The REQUESTED-ADDRESS-FAMILY attribute allows a client to explicitly request the address type the TURN server will allocate (e.g., an IPv4-only node may request the TURN server to allocate an IPv6 address).  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="6156"/>
          <seriesInfo name="DOI" value="10.17487/RFC6156"/>
        </reference>
        <reference anchor="RFC6263" target="https://www.rfc-editor.org/info/rfc6263" quoteTitle="true" derivedAnchor="RFC6263">
          <front>
            <title>Application Mechanism for Keeping Alive the NAT Mappings Associated with RTP / RTP Control Protocol (RTCP) Flows</title>
            <author initials="X." surname="Marjou" fullname="X. Marjou">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="A." surname="Sollaud" fullname="A. Sollaud">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2011" month="June"/>
            <abstract>
              <t>This document lists the different mechanisms that enable applications using the Real-time Transport Protocol (RTP) and the RTP Control Protocol (RTCP) to keep their RTP Network Address Translator (NAT) mappings alive.  It also makes a recommendation for a preferred mechanism.  This document is not applicable to Interactive Connectivity Establishment (ICE) agents.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="6263"/>
          <seriesInfo name="DOI" value="10.17487/RFC6263"/>
        </reference>
        <reference anchor="RFC7413" target="https://www.rfc-editor.org/info/rfc7413" quoteTitle="true" derivedAnchor="RFC7413">
          <front>
            <title>TCP Fast Open</title>
            <author initials="Y." surname="Cheng" fullname="Y. Cheng">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="J." surname="Chu" fullname="J. Chu">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S." surname="Radhakrishnan" fullname="S. Radhakrishnan">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="A." surname="Jain" fullname="A. Jain">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2014" month="December"/>
            <abstract>
              <t>This document describes an experimental TCP mechanism called TCP Fast Open (TFO).  TFO allows data to be carried in the SYN and SYN-ACK packets and consumed by the receiving end during the initial connection handshake, and saves up to one full round-trip time (RTT) compared to the standard TCP, which requires a three-way handshake (3WHS) to complete before data can be exchanged.  However, TFO deviates from the standard TCP semantics, since the data in the SYN could be replayed to an application in some rare circumstances.  Applications should not use TFO unless they can tolerate this issue, as detailed in the Applicability section.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7413"/>
          <seriesInfo name="DOI" value="10.17487/RFC7413"/>
        </reference>
        <reference anchor="RFC7478" target="https://www.rfc-editor.org/info/rfc7478" quoteTitle="true" derivedAnchor="RFC7478">
          <front>
            <title>Web Real-Time Communication Use Cases and Requirements</title>
            <author initials="C." surname="Holmberg" fullname="C. Holmberg">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S." surname="Hakansson" fullname="S. Hakansson">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="G." surname="Eriksson" fullname="G. Eriksson">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2015" month="March"/>
            <abstract>
              <t>This document describes web-based real-time communication use cases. Requirements on the browser functionality are derived from the use cases.</t>
              <t>This document was developed in an initial phase of the work with rather minor updates at later stages.  It has not really served as a tool in deciding features or scope for the WG's efforts so far.  It is being published to record the early conclusions of the WG.  It will not be used as a set of rigid guidelines that specifications and implementations will be held to in the future.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7478"/>
          <seriesInfo name="DOI" value="10.17487/RFC7478"/>
        </reference>
        <reference anchor="RFC7635" target="https://www.rfc-editor.org/info/rfc7635" quoteTitle="true" derivedAnchor="RFC7635">
          <front>
            <title>Session Traversal Utilities for NAT (STUN) Extension for Third-Party Authorization</title>
            <author initials="T." surname="Reddy" fullname="T. Reddy">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="P." surname="Patil" fullname="P. Patil">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="R." surname="Ravindranath" fullname="R. Ravindranath">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="J." surname="Uberti" fullname="J. Uberti">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2015" month="August"/>
            <abstract>
              <t>This document proposes the use of OAuth 2.0 to obtain and validate ephemeral tokens that can be used for Session Traversal Utilities for NAT (STUN) authentication.  The usage of ephemeral tokens ensures that access to a STUN server can be controlled even if the tokens are compromised.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7635"/>
          <seriesInfo name="DOI" value="10.17487/RFC7635"/>
        </reference>
        <reference anchor="RFC7657" target="https://www.rfc-editor.org/info/rfc7657" quoteTitle="true" derivedAnchor="RFC7657">
          <front>
            <title>Differentiated Services (Diffserv) and Real-Time Communication</title>
            <author initials="D." surname="Black" fullname="D. Black" role="editor">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="P." surname="Jones" fullname="P. Jones">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2015" month="November"/>
            <abstract>
              <t>This memo describes the interaction between Differentiated Services (Diffserv) network quality-of-service (QoS) functionality and real- time network communication, including communication based on the Real-time Transport Protocol (RTP).  Diffserv is based on network nodes applying different forwarding treatments to packets whose IP headers are marked with different Diffserv Codepoints (DSCPs). WebRTC applications, as well as some conferencing applications, have begun using the Session Description Protocol (SDP) bundle negotiation mechanism to send multiple traffic streams with different QoS requirements using the same network 5-tuple.  The results of using multiple DSCPs to obtain different QoS treatments within a single network 5-tuple have transport protocol interactions, particularly with congestion control functionality (e.g., reordering).  In addition, DSCP markings may be changed or removed between the traffic source and destination.  This memo covers the implications of these Diffserv aspects for real-time network communication, including WebRTC.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7657"/>
          <seriesInfo name="DOI" value="10.17487/RFC7657"/>
        </reference>
        <reference anchor="RFC7983" target="https://www.rfc-editor.org/info/rfc7983" quoteTitle="true" derivedAnchor="RFC7983">
          <front>
            <title>Multiplexing Scheme Updates for Secure Real-time Transport Protocol (SRTP) Extension for Datagram Transport Layer Security (DTLS)</title>
            <author initials="M." surname="Petit-Huguenin" fullname="M. Petit-Huguenin">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="G." surname="Salgueiro" fullname="G. Salgueiro">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2016" month="September"/>
            <abstract>
              <t>This document defines how Datagram Transport Layer Security (DTLS), Real-time Transport Protocol (RTP), RTP Control Protocol (RTCP), Session Traversal Utilities for NAT (STUN), Traversal Using Relays around NAT (TURN), and ZRTP packets are multiplexed on a single receiving socket.  It overrides the guidance from RFC 5764 ("SRTP                Extension for DTLS"), which suffered from four issues described and fixed in this document.</t>
              <t>This document updates RFC 5764.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7983"/>
          <seriesInfo name="DOI" value="10.17487/RFC7983"/>
        </reference>
        <reference anchor="RFC8155" target="https://www.rfc-editor.org/info/rfc8155" quoteTitle="true" derivedAnchor="RFC8155">
          <front>
            <title>Traversal Using Relays around NAT (TURN) Server Auto Discovery</title>
            <author initials="P." surname="Patil" fullname="P. Patil">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="T." surname="Reddy" fullname="T. Reddy">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="D." surname="Wing" fullname="D. Wing">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2017" month="April"/>
            <abstract>
              <t>Current Traversal Using Relays around NAT (TURN) server discovery mechanisms are relatively static and limited to explicit configuration.  These are usually under the administrative control of the application or TURN service provider, and not the enterprise, ISP, or the network in which the client is located.  Enterprises and ISPs wishing to provide their own TURN servers need auto-discovery mechanisms that a TURN client could use with minimal or no configuration.  This document describes three such mechanisms for TURN server discovery.</t>
              <t>This document updates RFC 5766 to relax the requirement for mutual authentication in certain cases.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8155"/>
          <seriesInfo name="DOI" value="10.17487/RFC8155"/>
        </reference>
        <reference anchor="RFC8311" target="https://www.rfc-editor.org/info/rfc8311" quoteTitle="true" derivedAnchor="RFC8311">
          <front>
            <title>Relaxing Restrictions on Explicit Congestion Notification (ECN) Experimentation</title>
            <author initials="D." surname="Black" fullname="D. Black">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2018" month="January"/>
            <abstract>
              <t>This memo updates RFC 3168, which specifies Explicit Congestion Notification (ECN) as an alternative to packet drops for indicating network congestion to endpoints.  It relaxes restrictions in RFC 3168 that hinder experimentation towards benefits beyond just removal of loss.  This memo summarizes the anticipated areas of experimentation and updates RFC 3168 to enable experimentation in these areas.  An Experimental RFC in the IETF document stream is required to take advantage of any of these enabling updates.  In addition, this memo makes related updates to the ECN specifications for RTP in RFC 6679 and for the Datagram Congestion Control Protocol (DCCP) in RFCs 4341, 4342, and 5622.  This memo also records the conclusion of the ECN nonce experiment in RFC 3540 and provides the rationale for reclassification of RFC 3540 from Experimental to Historic; this reclassification enables new experimental use of the ECT(1) codepoint.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8311"/>
          <seriesInfo name="DOI" value="10.17487/RFC8311"/>
        </reference>
        <reference anchor="RFC8445" target="https://www.rfc-editor.org/info/rfc8445" quoteTitle="true" derivedAnchor="RFC8445">
          <front>
            <title>Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal</title>
            <author initials="A." surname="Keranen" fullname="A. Keranen">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C." surname="Holmberg" fullname="C. Holmberg">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="J." surname="Rosenberg" fullname="J. Rosenberg">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2018" month="July"/>
            <abstract>
              <t>This document describes a protocol for Network Address Translator (NAT) traversal for UDP-based communication.  This protocol is called Interactive Connectivity Establishment (ICE).  ICE makes use of the Session Traversal Utilities for NAT (STUN) protocol and its extension, Traversal Using Relay NAT (TURN).</t>
              <t>This document obsoletes RFC 5245.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8445"/>
          <seriesInfo name="DOI" value="10.17487/RFC8445"/>
        </reference>
        <reference anchor="I-D.ietf-mmusic-ice-sip-sdp" quoteTitle="true" target="https://tools.ietf.org/html/draft-ietf-mmusic-ice-sip-sdp-39" derivedAnchor="SDP-ICE">
          <front>
            <title>Session Description Protocol (SDP) Offer/Answer procedures for Interactive Connectivity Establishment (ICE)</title>
            <author initials="M" surname="Petit-Huguenin" fullname="Marc Petit-Huguenin">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S" surname="Nandakumar" fullname="Suhas Nandakumar">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C" surname="Holmberg" fullname="Christer Holmberg">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="A" surname="Keranen" fullname="Ari Keranen">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="R" surname="Shpount" fullname="Roman Shpount">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="August" day="13" year="2019"/>
            <abstract>
              <t>This document describes Session Description Protocol (SDP) Offer/ Answer procedures for carrying out Interactive Connectivity Establishment (ICE) between the agents.  This document obsoletes RFC 5245.</t>
            </abstract>
          </front>
          <seriesInfo name="Internet-Draft" value="draft-ietf-mmusic-ice-sip-sdp-39"/>
          <format type="TXT" target="http://www.ietf.org/internet-drafts/draft-ietf-mmusic-ice-sip-sdp-39.txt"/>
          <refcontent>Work in Progress</refcontent>
        </reference>
        <reference anchor="I-D.ietf-rtcweb-security" quoteTitle="true" target="https://tools.ietf.org/html/draft-ietf-rtcweb-security-12" derivedAnchor="SEC-WEBRTC">
          <front>
            <title>Security Considerations for WebRTC</title>
            <author initials="E" surname="Rescorla" fullname="Eric Rescorla">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="July" day="5" year="2019"/>
            <abstract>
              <t>WebRTC is a protocol suite for use with real-time applications that can be deployed in browsers - "real time communication on the Web". This document defines the WebRTC threat model and analyzes the security threats of WebRTC in that model.</t>
            </abstract>
          </front>
          <seriesInfo name="Internet-Draft" value="draft-ietf-rtcweb-security-12"/>
          <format type="TXT" target="http://www.ietf.org/internet-drafts/draft-ietf-rtcweb-security-12.txt"/>
          <refcontent>Work in Progress</refcontent>
        </reference>
        <reference anchor="I-D.ietf-mptcp-rfc6824bis" quoteTitle="true" target="https://tools.ietf.org/html/draft-ietf-mptcp-rfc6824bis-18" derivedAnchor="TCP-EXT">
          <front>
            <title>TCP Extensions for Multipath Operation with Multiple Addresses</title>
            <author initials="A" surname="Ford" fullname="Alan Ford">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C" surname="Raiciu" fullname="Costin Raiciu">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="M" surname="Handley" fullname="Mark Handley">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="O" surname="Bonaventure" fullname="Olivier Bonaventure">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C" surname="Paasch" fullname="Christoph Paasch">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="June" day="8" year="2019"/>
            <abstract>
              <t>TCP/IP communication is currently restricted to a single path per connection, yet multiple paths often exist between peers.  The simultaneous use of these multiple paths for a TCP/IP session would improve resource usage within the network and, thus, improve user experience through higher throughput and improved resilience to network failure.  Multipath TCP provides the ability to simultaneously use multiple paths between peers.  This document presents a set of extensions to traditional TCP to support multipath operation.  The protocol offers the same type of service to applications as TCP (i.e., reliable bytestream), and it provides the components necessary to establish and use multiple TCP flows across potentially disjoint paths.  This document specifies v1 of Multipath TCP, obsoleting v0 as specified in RFC6824, through clarifications and modifications primarily driven by deployment experience.</t>
            </abstract>
          </front>
          <seriesInfo name="Internet-Draft" value="draft-ietf-mptcp-rfc6824bis-18"/>
          <format type="TXT" target="http://www.ietf.org/internet-drafts/draft-ietf-mptcp-rfc6824bis-18.txt"/>
          <refcontent>Work in Progress</refcontent>
        </reference>
        <reference anchor="I-D.ietf-tsvwg-udp-options" quoteTitle="true" target="https://tools.ietf.org/html/draft-ietf-tsvwg-udp-options-08" derivedAnchor="UDP-OPT">
          <front>
            <title>Transport Options for UDP</title>
            <author initials="J" surname="Touch" fullname="Joseph Touch">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="September" day="12" year="2019"/>
            <abstract>
              <t>Transport protocols are extended through the use of transport header options. This document extends UDP by indicating the location, syntax, and semantics for UDP transport layer options.</t>
            </abstract>
          </front>
          <seriesInfo name="Internet-Draft" value="draft-ietf-tsvwg-udp-options-08"/>
          <format type="TXT" target="http://www.ietf.org/internet-drafts/draft-ietf-tsvwg-udp-options-08.txt"/>
          <refcontent>Work in Progress</refcontent>
        </reference>
      </references>
    </references>
    <section numbered="false" toc="include" removeInRFC="false" pn="section-appendix.a">
      <name slugifiedName="name-acknowledgements">Acknowledgements</name>
      <t pn="section-appendix.a-1">Most of the text in this note comes from the original TURN
      specification, <xref target="RFC5766" format="default" sectionFormat="of" derivedContent="RFC5766"/>. The authors would like to
      thank <contact fullname="Rohan Mahy"/>, coauthor of the original TURN specification, and everyone
      who had contributed to that document. The authors would also like to
      acknowledge that this document inherits material from <xref target="RFC6156" format="default" sectionFormat="of" derivedContent="RFC6156"/>.</t>
      <t pn="section-appendix.a-2">Thanks to <contact fullname="Justin Uberti"/>, <contact fullname="Pal       Martinsen"/>, <contact fullname="Oleg Moskalenko"/>, <contact fullname="Aijun Wang"/>, and <contact fullname="Simon Perreault"/> for
      their help on the ADDITIONAL-ADDRESS-FAMILY mechanism. The authors would
      like to thank <contact fullname="Gonzalo Salgueiro"/>, <contact fullname="Simon Perreault"/>, <contact fullname="Jonathan Lennox"/>,
      <contact fullname="Brandon Williams"/>, <contact fullname="Karl       Stahl"/>, <contact fullname="Noriyuki Torii"/>, <contact fullname="Nils       Ohlmeier"/>, <contact fullname="Dan Wing"/>, <contact fullname="Vijay       Gurbani"/>, <contact fullname="Joseph Touch"/>, <contact fullname="Justin Uberti"/>, <contact fullname="Christopher Wood"/>,
      <contact fullname="Roman Danyliw"/>, <contact fullname="Eric Vyncke"/>,
      <contact fullname="Adam Roach"/>, <contact fullname="Suresh Krishnan"/>,
      <contact fullname="Mirja Kuehlewind"/>, <contact fullname="Benjamin       Kaduk"/>, and <contact fullname="Oleg Moskalenko"/> for comments and
      review. The authors would like to thank <contact fullname="Marc Petit-Huguenin"/> for his
      contributions to the text.</t>
      <t pn="section-appendix.a-3">Special thanks to <contact fullname="Magnus Westerlund"/> for the detailed AD review.</t>
    </section>
    <section anchor="authors-addresses" numbered="false" removeInRFC="false" toc="include" pn="section-appendix.b">
      <name slugifiedName="name-authors-addresses">Authors' Addresses</name>
      <author fullname="Tirumaleswar Reddy" initials="T." role="editor" surname="Reddy">
        <organization abbrev="McAfee" showOnFrontPage="true">McAfee, Inc.</organization>
        <address>
          <postal>
            <street>Embassy Golf Link Business Park</street>
            <city>Bangalore</city>
            <region>Karnataka</region>
            <code>560071</code>
            <country>India</country>
          </postal>
          <email>kondtir@gmail.com</email>
        </address>
      </author>
      <author fullname="Alan Johnston" initials="A." role="editor" surname="Johnston">
        <organization showOnFrontPage="true">Villanova University</organization>
        <address>
          <postal>
            <street/>
            <city>Villanova</city>
            <region>PA</region>
            <code/>
            <country>United States of America</country>
          </postal>
          <email>alan.b.johnston@gmail.com</email>
        </address>
      </author>
      <author fullname="Philip Matthews" initials="P." surname="Matthews">
        <organization showOnFrontPage="true">Alcatel-Lucent</organization>
        <address>
          <postal>
            <street>600 March Road</street>
            <city>Ottawa</city>
            <region>Ontario</region>
            <code/>
            <country>Canada</country>
          </postal>
          <email>philip_matthews@magma.ca</email>
        </address>
      </author>
      <author fullname="Jonathan Rosenberg" initials="J." surname="Rosenberg">
        <organization showOnFrontPage="true">jdrosen.net</organization>
        <address>
          <postal>
            <street/>
            <city>Edison</city>
            <region>NJ</region>
            <country>United States of America</country>
          </postal>
          <email>jdrosen@jdrosen.net</email>
          <uri>http://www.jdrosen.net</uri>
        </address>
      </author>
    </section>
  </back>
</rfc>