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<!DOCTYPE html>
<html lang="en" class="RFC">
<head>
<meta charset="utf-8">
<meta content="Common,Latin" name="scripts">
<meta content="initial-scale=1.0" name="viewport">
<title>RFC 9143: Negotiating Media Multiplexing Using the Session Description Protocol (SDP)</title>
<meta content="Christer Holmberg" name="author">
<meta content="Harald Tveit Alvestrand" name="author">
<meta content="Cullen Jennings" name="author">
<meta content="
       
        This specification defines a new Session Description
        Protocol (SDP) Grouping Framework extension called 'BUNDLE'. 
        The extension can be used with the SDP offer/answer mechanism 
        to negotiate the usage of a single transport (5-tuple) for 
        sending and receiving media described by multiple SDP media descriptions 
        (&quot;m=&quot; sections). Such transport is referred to as a &quot;BUNDLE transport&quot;, 
        and the media is referred to as &quot;bundled media&quot;. The &quot;m=&quot; sections that
        use the BUNDLE transport form a BUNDLE group.
       
       
        This specification defines a new RTP Control Protocol (RTCP) Source
        Description (SDES) item and a new RTP header extension. 
       
        This specification updates RFCs 3264, 5888, and 7941.
       
       
        This specification obsoletes RFC 8843.
       
    " name="description">
<meta content="xml2rfc 3.12.2" name="generator">
<meta content="RTP" name="keyword">
<meta content="SDP" name="keyword">
<meta content="Bundle" name="keyword">
<meta content="Multiplexing" name="keyword">
<meta content="RTCWEB" name="keyword">
<meta content="CLUE" name="keyword">
<meta content="RTCWEB" name="keyword">
<meta content="MMUSIC" name="keyword">
<meta content="AVT" name="keyword">
<meta content="WEB" name="keyword">
<meta content="Browser" name="keyword">
<meta content="9143" name="rfc.number">
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<link href="rfc9143.xml" rel="alternate" type="application/rfc+xml">
<link href="#copyright" rel="license">
<style type="text/css">/*

  NOTE: Changes at the bottom of this file overrides some earlier settings.

  Once the style has stabilized and has been adopted as an official RFC style,
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*/

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/* This is commented out here, as the string-set: doesn't
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@media screen and (max-width: 1023px) {
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/* Give the bottom of the ToC some whitespace */
@media screen and (min-width: 1024px) {
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table td.text-right,
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/* Make the alternative author contact informatio look less like just another
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address .non-ascii {
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table {
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}

/* Avoid reference text that sits in a block with very wide left margin,
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.references dd {
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}

/* Control caption placement */
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address.vcard {
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/* For address alignment dependent on LTR or RTL scripts */
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address div.right {
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/* Provide table alignment support.  We can't use the alignX classes above
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table.right {
 margin-left: auto;
 margin-right: 0;
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table.center {
 margin-left: auto;
 margin-right: auto;
}
table.left {
 margin-left: 0;
 margin-right: auto;
}

/* Give the table caption label the same styling as the figcaption */
caption a[href] {
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}

@media print {
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  /* avoid overwriting the top border line with the ToC header */
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  /* Avoid page breaks inside dl and author address entries */
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/* Tweak the bcp14 keyword presentation */
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  font-weight: bold;
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}
/* Tweak the invisible space above H* in order not to overlay links in text above */
 h2 {
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  padding-top: 31px;
 }
 h3 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  padding-top: 24px;
 }
 h4 {
  margin-top: -18px;  /* provide offset for in-page anchors */
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<table class="ears">
<thead><tr>
<td class="left">RFC 9143</td>
<td class="center">Bundled Media</td>
<td class="right">February 2022</td>
</tr></thead>
<tfoot><tr>
<td class="left">Holmberg, et al.</td>
<td class="center">Standards Track</td>
<td class="right">[Page]</td>
</tr></tfoot>
</table>
<div id="external-metadata" class="document-information"></div>
<div id="internal-metadata" class="document-information">
<dl id="identifiers">
<dt class="label-stream">Stream:</dt>
<dd class="stream">Internet Engineering Task Force (IETF)</dd>
<dt class="label-rfc">RFC:</dt>
<dd class="rfc"><a href="https://www.rfc-editor.org/rfc/rfc9143" class="eref">9143</a></dd>
<dt class="label-obsoletes">Obsoletes:</dt>
<dd class="obsoletes">
<a href="https://www.rfc-editor.org/rfc/rfc8843" class="eref">8843</a> </dd>
<dt class="label-updates">Updates:</dt>
<dd class="updates">
<a href="https://www.rfc-editor.org/rfc/rfc3264" class="eref">3264</a>, <a href="https://www.rfc-editor.org/rfc/rfc5888" class="eref">5888</a>, <a href="https://www.rfc-editor.org/rfc/rfc7941" class="eref">7941</a> </dd>
<dt class="label-category">Category:</dt>
<dd class="category">Standards Track</dd>
<dt class="label-published">Published:</dt>
<dd class="published">
<time datetime="2022-02" class="published">February 2022</time>
    </dd>
<dt class="label-issn">ISSN:</dt>
<dd class="issn">2070-1721</dd>
<dt class="label-authors">Authors:</dt>
<dd class="authors">
<div class="author">
      <div class="author-name">C. Holmberg</div>
<div class="org">Ericsson</div>
</div>
<div class="author">
      <div class="author-name">H. Alvestrand</div>
<div class="org">Google</div>
</div>
<div class="author">
      <div class="author-name">C. Jennings</div>
<div class="org">Cisco</div>
</div>
</dd>
</dl>
</div>
<h1 id="rfcnum">RFC 9143</h1>
<h1 id="title">Negotiating Media Multiplexing Using the Session Description Protocol (SDP)</h1>
<section id="section-abstract">
      <h2 id="abstract"><a href="#abstract" class="selfRef">Abstract</a></h2>
<p id="section-abstract-1">
        This specification defines a new Session Description
        Protocol (SDP) Grouping Framework extension called 'BUNDLE'. 
        The extension can be used with the SDP offer/answer mechanism 
        to negotiate the usage of a single transport (5-tuple) for 
        sending and receiving media described by multiple SDP media descriptions 
        ("m=" sections). Such transport is referred to as a "BUNDLE transport", 
        and the media is referred to as "bundled media". The "m=" sections that
        use the BUNDLE transport form a BUNDLE group.<a href="#section-abstract-1" class="pilcrow">¶</a></p>
<p id="section-abstract-2">
        This specification defines a new RTP Control Protocol (RTCP) Source
        Description (SDES) item and a new RTP header extension.<a href="#section-abstract-2" class="pilcrow">¶</a></p>
<p id="section-abstract-3">
        This specification updates RFCs 3264, 5888, and 7941.<a href="#section-abstract-3" class="pilcrow">¶</a></p>
<p id="section-abstract-4">
        This specification obsoletes RFC 8843.<a href="#section-abstract-4" class="pilcrow">¶</a></p>
</section>
<div id="status-of-memo">
<section id="section-boilerplate.1">
        <h2 id="name-status-of-this-memo">
<a href="#name-status-of-this-memo" class="section-name selfRef">Status of This Memo</a>
        </h2>
<p id="section-boilerplate.1-1">
            This is an Internet Standards Track document.<a href="#section-boilerplate.1-1" class="pilcrow">¶</a></p>
<p id="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.<a href="#section-boilerplate.1-2" class="pilcrow">¶</a></p>
<p id="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
            <span><a href="https://www.rfc-editor.org/info/rfc9143">https://www.rfc-editor.org/info/rfc9143</a></span>.<a href="#section-boilerplate.1-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="copyright">
<section id="section-boilerplate.2">
        <h2 id="name-copyright-notice">
<a href="#name-copyright-notice" class="section-name selfRef">Copyright Notice</a>
        </h2>
<p id="section-boilerplate.2-1">
            Copyright (c) 2022 IETF Trust and the persons identified as the
            document authors. All rights reserved.<a href="#section-boilerplate.2-1" class="pilcrow">¶</a></p>
<p id="section-boilerplate.2-2">
            This document is subject to BCP 78 and the IETF Trust's Legal
            Provisions Relating to IETF Documents
            (<span><a href="https://trustee.ietf.org/license-info">https://trustee.ietf.org/license-info</a></span>) in effect on the date of
            publication of this document. Please review these documents
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            respect to this document. Code Components extracted from this
            document must include Revised BSD License text as described in
            Section 4.e of the Trust Legal Provisions and are provided without
            warranty as described in the Revised BSD License.<a href="#section-boilerplate.2-2" class="pilcrow">¶</a></p>
<p id="section-boilerplate.2-3">
            This document may contain material from IETF Documents or IETF
            Contributions published or made publicly available before November
            10, 2008. The person(s) controlling the copyright in some of this
            material may not have granted the IETF Trust the right to allow
            modifications of such material outside the IETF Standards Process.
            Without obtaining an adequate license from the person(s)
            controlling the copyright in such materials, this document may not
            be modified outside the IETF Standards Process, and derivative
            works of it may not be created outside the IETF Standards Process,
            except to format it for publication as an RFC or to translate it
            into languages other than English.<a href="#section-boilerplate.2-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="toc">
<section id="section-toc.1">
        <a href="#" onclick="scroll(0,0)" class="toplink">▲</a><h2 id="name-table-of-contents">
<a href="#name-table-of-contents" class="section-name selfRef">Table of Contents</a>
        </h2>
<nav class="toc"><ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.1">
            <p id="section-toc.1-1.1.1"><a href="#section-1" class="xref">1</a>.  <a href="#name-introduction" class="xref">Introduction</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.1.2.1">
                <p id="section-toc.1-1.1.2.1.1" class="keepWithNext"><a href="#section-1.1" class="xref">1.1</a>.  <a href="#name-background" class="xref">Background</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.1.2.2">
                <p id="section-toc.1-1.1.2.2.1" class="keepWithNext"><a href="#section-1.2" class="xref">1.2</a>.  <a href="#name-bundle-mechanism" class="xref">BUNDLE Mechanism</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.1.2.3">
                <p id="section-toc.1-1.1.2.3.1" class="keepWithNext"><a href="#section-1.3" class="xref">1.3</a>.  <a href="#name-protocol-extensions" class="xref">Protocol Extensions</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.1.2.4">
                <p id="section-toc.1-1.1.2.4.1"><a href="#section-1.4" class="xref">1.4</a>.  <a href="#name-changes-from-rfc-8843" class="xref">Changes from RFC 8843</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.2">
            <p id="section-toc.1-1.2.1"><a href="#section-2" class="xref">2</a>.  <a href="#name-terminology" class="xref">Terminology</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.3">
            <p id="section-toc.1-1.3.1"><a href="#section-3" class="xref">3</a>.  <a href="#name-conventions" class="xref">Conventions</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.4">
            <p id="section-toc.1-1.4.1"><a href="#section-4" class="xref">4</a>.  <a href="#name-applicability-statement" class="xref">Applicability Statement</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.5">
            <p id="section-toc.1-1.5.1"><a href="#section-5" class="xref">5</a>.  <a href="#name-sdp-grouping-framework-bund" class="xref">SDP Grouping Framework BUNDLE Extension</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.6">
            <p id="section-toc.1-1.6.1"><a href="#section-6" class="xref">6</a>.  <a href="#name-sdp-bundle-only-attribute" class="xref">SDP 'bundle-only' Attribute</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7">
            <p id="section-toc.1-1.7.1"><a href="#section-7" class="xref">7</a>.  <a href="#name-sdp-offer-answer-procedures" class="xref">SDP Offer/Answer Procedures</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.1">
                <p id="section-toc.1-1.7.2.1.1"><a href="#section-7.1" class="xref">7.1</a>.  <a href="#name-generic-sdp-considerations" class="xref">Generic SDP Considerations</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.1.2.1">
                    <p id="section-toc.1-1.7.2.1.2.1.1"><a href="#section-7.1.1" class="xref">7.1.1</a>.  <a href="#name-connection-data-c" class="xref">Connection Data ("c=")</a></p>
</li>
                  <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.1.2.2">
                    <p id="section-toc.1-1.7.2.1.2.2.1"><a href="#section-7.1.2" class="xref">7.1.2</a>.  <a href="#name-bandwidth-b" class="xref">Bandwidth ("b=")</a></p>
</li>
                  <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.1.2.3">
                    <p id="section-toc.1-1.7.2.1.2.3.1"><a href="#section-7.1.3" class="xref">7.1.3</a>.  <a href="#name-attributes-a" class="xref">Attributes ("a=")</a></p>
</li>
                </ul>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.2">
                <p id="section-toc.1-1.7.2.2.1"><a href="#section-7.2" class="xref">7.2</a>.  <a href="#name-generating-the-initial-bund" class="xref">Generating the Initial BUNDLE Offer</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.2.2.1">
                    <p id="section-toc.1-1.7.2.2.2.1.1"><a href="#section-7.2.1" class="xref">7.2.1</a>.  <a href="#name-suggesting-the-offerer-tagg" class="xref">Suggesting the Offerer-Tagged "m=" Section</a></p>
</li>
                  <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.2.2.2">
                    <p id="section-toc.1-1.7.2.2.2.2.1"><a href="#section-7.2.2" class="xref">7.2.2</a>.  <a href="#name-example-initial-bundle-offe" class="xref">Example: Initial BUNDLE Offer</a></p>
</li>
                </ul>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.3">
                <p id="section-toc.1-1.7.2.3.1"><a href="#section-7.3" class="xref">7.3</a>.  <a href="#name-generating-the-sdp-answer" class="xref">Generating the SDP Answer</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.3.2.1">
                    <p id="section-toc.1-1.7.2.3.2.1.1"><a href="#section-7.3.1" class="xref">7.3.1</a>.  <a href="#name-answerer-selection-of-tagge" class="xref">Answerer Selection of Tagged "m=" Sections</a></p>
</li>
                  <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.3.2.2">
                    <p id="section-toc.1-1.7.2.3.2.2.1"><a href="#section-7.3.2" class="xref">7.3.2</a>.  <a href="#name-moving-a-media-description-" class="xref">Moving a Media Description Out of a BUNDLE Group</a></p>
</li>
                  <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.3.2.3">
                    <p id="section-toc.1-1.7.2.3.2.3.1"><a href="#section-7.3.3" class="xref">7.3.3</a>.  <a href="#name-rejecting-a-media-descripti" class="xref">Rejecting a Media Description in a BUNDLE Group</a></p>
</li>
                  <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.3.2.4">
                    <p id="section-toc.1-1.7.2.3.2.4.1"><a href="#section-7.3.4" class="xref">7.3.4</a>.  <a href="#name-example-sdp-answer" class="xref">Example: SDP Answer</a></p>
</li>
                  <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.3.2.5">
                    <p id="section-toc.1-1.7.2.3.2.5.1"><a href="#section-7.3.5" class="xref">7.3.5</a>.  <a href="#name-rfc-8843-considerations" class="xref">RFC 8843 Considerations</a></p>
</li>
                </ul>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.4">
                <p id="section-toc.1-1.7.2.4.1"><a href="#section-7.4" class="xref">7.4</a>.  <a href="#name-offerer-processing-of-the-s" class="xref">Offerer Processing of the SDP Answer</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.4.2.1">
                    <p id="section-toc.1-1.7.2.4.2.1.1"><a href="#section-7.4.1" class="xref">7.4.1</a>.  <a href="#name-rfc-8843-considerations-2" class="xref">RFC 8843 Considerations</a></p>
</li>
                </ul>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.5">
                <p id="section-toc.1-1.7.2.5.1"><a href="#section-7.5" class="xref">7.5</a>.  <a href="#name-modifying-the-session" class="xref">Modifying the Session</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.5.2.1">
                    <p id="section-toc.1-1.7.2.5.2.1.1"><a href="#section-7.5.1" class="xref">7.5.1</a>.  <a href="#name-adding-a-media-description-" class="xref">Adding a Media Description to a BUNDLE Group</a></p>
</li>
                  <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.5.2.2">
                    <p id="section-toc.1-1.7.2.5.2.2.1"><a href="#section-7.5.2" class="xref">7.5.2</a>.  <a href="#name-moving-a-media-description-o" class="xref">Moving a Media Description Out of a BUNDLE Group</a></p>
</li>
                  <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.5.2.3">
                    <p id="section-toc.1-1.7.2.5.2.3.1"><a href="#section-7.5.3" class="xref">7.5.3</a>.  <a href="#name-disabling-a-media-descripti" class="xref">Disabling a Media Description in a BUNDLE Group</a></p>
</li>
                </ul>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.6">
                <p id="section-toc.1-1.7.2.6.1"><a href="#section-7.6" class="xref">7.6</a>.  <a href="#name-3pcc-considerations" class="xref">3PCC Considerations</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.8">
            <p id="section-toc.1-1.8.1"><a href="#section-8" class="xref">8</a>.  <a href="#name-protocol-identification" class="xref">Protocol Identification</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.8.2.1">
                <p id="section-toc.1-1.8.2.1.1"><a href="#section-8.1" class="xref">8.1</a>.  <a href="#name-stun-dtls-and-srtp" class="xref">STUN, DTLS, and SRTP</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.9">
            <p id="section-toc.1-1.9.1"><a href="#section-9" class="xref">9</a>.  <a href="#name-rtp-considerations" class="xref">RTP Considerations</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.9.2.1">
                <p id="section-toc.1-1.9.2.1.1"><a href="#section-9.1" class="xref">9.1</a>.  <a href="#name-single-rtp-session" class="xref">Single RTP Session</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.9.2.1.2.1">
                    <p id="section-toc.1-1.9.2.1.2.1.1"><a href="#section-9.1.1" class="xref">9.1.1</a>.  <a href="#name-payload-type-pt-value-reuse" class="xref">Payload Type (PT) Value Reuse</a></p>
</li>
                </ul>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.9.2.2">
                <p id="section-toc.1-1.9.2.2.1"><a href="#section-9.2" class="xref">9.2</a>.  <a href="#name-associating-rtp-rtcp-stream" class="xref">Associating RTP/RTCP Streams with the Correct SDP Media Description</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.9.2.3">
                <p id="section-toc.1-1.9.2.3.1"><a href="#section-9.3" class="xref">9.3</a>.  <a href="#name-rtp-rtcp-multiplexing" class="xref">RTP/RTCP Multiplexing</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.9.2.3.2.1">
                    <p id="section-toc.1-1.9.2.3.2.1.1"><a href="#section-9.3.1" class="xref">9.3.1</a>.  <a href="#name-sdp-offer-answer-procedures-2" class="xref">SDP Offer/Answer Procedures</a></p>
</li>
                </ul>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.10">
            <p id="section-toc.1-1.10.1"><a href="#section-10" class="xref">10</a>. <a href="#name-ice-considerations" class="xref">ICE Considerations</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.11">
            <p id="section-toc.1-1.11.1"><a href="#section-11" class="xref">11</a>. <a href="#name-dtls-considerations" class="xref">DTLS Considerations</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.12">
            <p id="section-toc.1-1.12.1"><a href="#section-12" class="xref">12</a>. <a href="#name-rtp-header-extensions-consi" class="xref">RTP Header Extensions Consideration</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.13">
            <p id="section-toc.1-1.13.1"><a href="#section-13" class="xref">13</a>. <a href="#name-updates-to-rfc-3264" class="xref">Updates to RFC 3264</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.13.2.1">
                <p id="section-toc.1-1.13.2.1.1"><a href="#section-13.1" class="xref">13.1</a>.  <a href="#name-original-text-from-rfc-3264" class="xref">Original Text from RFC 3264, Section 5.1, Paragraph 2</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.13.2.2">
                <p id="section-toc.1-1.13.2.2.1"><a href="#section-13.2" class="xref">13.2</a>.  <a href="#name-new-text-replacing-rfc-3264" class="xref">New Text Replacing RFC 3264, Section 5.1, Paragraph 2</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.13.2.3">
                <p id="section-toc.1-1.13.2.3.1"><a href="#section-13.3" class="xref">13.3</a>.  <a href="#name-original-text-from-rfc-3264-" class="xref">Original Text from RFC 3264, Section 8.4, Paragraph 6</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.13.2.4">
                <p id="section-toc.1-1.13.2.4.1"><a href="#section-13.4" class="xref">13.4</a>.  <a href="#name-new-text-replacing-rfc-3264-" class="xref">New Text Replacing RFC 3264, Section 8.4, Paragraph 6</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.14">
            <p id="section-toc.1-1.14.1"><a href="#section-14" class="xref">14</a>. <a href="#name-update-to-rfc-5888" class="xref">Update to RFC 5888</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.14.2.1">
                <p id="section-toc.1-1.14.2.1.1"><a href="#section-14.1" class="xref">14.1</a>.  <a href="#name-original-text-from-rfc-5888" class="xref">Original Text from RFC 5888, Section 9.2, Paragraph 3</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.14.2.2">
                <p id="section-toc.1-1.14.2.2.1"><a href="#section-14.2" class="xref">14.2</a>.  <a href="#name-new-text-replacing-rfc-5888" class="xref">New Text Replacing RFC 5888, Section 9.2, Paragraph 3</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.15">
            <p id="section-toc.1-1.15.1"><a href="#section-15" class="xref">15</a>. <a href="#name-rtp-rtcp-extensions-for-ide" class="xref">RTP/RTCP Extensions for identification-tag Transport</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.15.2.1">
                <p id="section-toc.1-1.15.2.1.1"><a href="#section-15.1" class="xref">15.1</a>.  <a href="#name-rtcp-mid-sdes-item" class="xref">RTCP MID SDES Item</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.15.2.2">
                <p id="section-toc.1-1.15.2.2.1"><a href="#section-15.2" class="xref">15.2</a>.  <a href="#name-rtp-sdes-header-extension-f" class="xref">RTP SDES Header Extension for MID</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.16">
            <p id="section-toc.1-1.16.1"><a href="#section-16" class="xref">16</a>. <a href="#name-iana-considerations" class="xref">IANA Considerations</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.16.2.1">
                <p id="section-toc.1-1.16.2.1.1"><a href="#section-16.1" class="xref">16.1</a>.  <a href="#name-sdes-item" class="xref">SDES Item</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.16.2.2">
                <p id="section-toc.1-1.16.2.2.1"><a href="#section-16.2" class="xref">16.2</a>.  <a href="#name-rtp-sdes-header-extension-u" class="xref">RTP SDES Header Extension URI</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.16.2.3">
                <p id="section-toc.1-1.16.2.3.1"><a href="#section-16.3" class="xref">16.3</a>.  <a href="#name-sdp-attribute" class="xref">SDP Attribute</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.16.2.4">
                <p id="section-toc.1-1.16.2.4.1"><a href="#section-16.4" class="xref">16.4</a>.  <a href="#name-sdp-group-semantics" class="xref">SDP Group Semantics</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.17">
            <p id="section-toc.1-1.17.1"><a href="#section-17" class="xref">17</a>. <a href="#name-security-considerations" class="xref">Security Considerations</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.18">
            <p id="section-toc.1-1.18.1"><a href="#section-18" class="xref">18</a>. <a href="#name-examples" class="xref">Examples</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.18.2.1">
                <p id="section-toc.1-1.18.2.1.1"><a href="#section-18.1" class="xref">18.1</a>.  <a href="#name-example-tagged-m-section-se" class="xref">Example: Tagged "m=" Section Selections</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.18.2.2">
                <p id="section-toc.1-1.18.2.2.1"><a href="#section-18.2" class="xref">18.2</a>.  <a href="#name-example-bundle-group-reject" class="xref">Example: BUNDLE Group Rejected</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.18.2.3">
                <p id="section-toc.1-1.18.2.3.1"><a href="#section-18.3" class="xref">18.3</a>.  <a href="#name-example-offerer-adds-a-medi" class="xref">Example: Offerer Adds a Media Description to a BUNDLE Group</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.18.2.4">
                <p id="section-toc.1-1.18.2.4.1"><a href="#section-18.4" class="xref">18.4</a>.  <a href="#name-example-offerer-moves-a-med" class="xref">Example: Offerer Moves a Media Description Out of a BUNDLE Group</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.18.2.5">
                <p id="section-toc.1-1.18.2.5.1"><a href="#section-18.5" class="xref">18.5</a>.  <a href="#name-example-offerer-disables-a-" class="xref">Example: Offerer Disables a Media Description within a BUNDLE Group</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.19">
            <p id="section-toc.1-1.19.1"><a href="#section-19" class="xref">19</a>. <a href="#name-references" class="xref">References</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.19.2.1">
                <p id="section-toc.1-1.19.2.1.1"><a href="#section-19.1" class="xref">19.1</a>.  <a href="#name-normative-references" class="xref">Normative References</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.19.2.2">
                <p id="section-toc.1-1.19.2.2.1"><a href="#section-19.2" class="xref">19.2</a>.  <a href="#name-informative-references" class="xref">Informative References</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.20">
            <p id="section-toc.1-1.20.1"><a href="#appendix-A" class="xref">Appendix A</a>.  <a href="#name-design-considerations" class="xref">Design Considerations</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.20.2.1">
                <p id="section-toc.1-1.20.2.1.1"><a href="#appendix-A.1" class="xref">A.1</a>.  <a href="#name-ua-interoperability" class="xref">UA Interoperability</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.20.2.2">
                <p id="section-toc.1-1.20.2.2.1"><a href="#appendix-A.2" class="xref">A.2</a>.  <a href="#name-usage-of-port-number-value-" class="xref">Usage of Port Number Value Zero</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.20.2.3">
                <p id="section-toc.1-1.20.2.3.1"><a href="#appendix-A.3" class="xref">A.3</a>.  <a href="#name-b2bua-and-proxy-interoperab" class="xref">B2BUA and Proxy Interoperability</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.20.2.3.2.1">
                    <p id="section-toc.1-1.20.2.3.2.1.1"><a href="#appendix-A.3.1" class="xref">A.3.1</a>.  <a href="#name-traffic-policing" class="xref">Traffic Policing</a></p>
</li>
                  <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.20.2.3.2.2">
                    <p id="section-toc.1-1.20.2.3.2.2.1"><a href="#appendix-A.3.2" class="xref">A.3.2</a>.  <a href="#name-bandwidth-allocation" class="xref">Bandwidth Allocation</a></p>
</li>
                </ul>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.20.2.4">
                <p id="section-toc.1-1.20.2.4.1"><a href="#appendix-A.4" class="xref">A.4</a>.  <a href="#name-candidate-gathering" class="xref">Candidate Gathering</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.21">
            <p id="section-toc.1-1.21.1"><a href="#appendix-B" class="xref"></a><a href="#name-acknowledgements" class="xref">Acknowledgements</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.22">
            <p id="section-toc.1-1.22.1"><a href="#appendix-C" class="xref"></a><a href="#name-authors-addresses" class="xref">Authors' Addresses</a></p>
</li>
        </ul>
</nav>
</section>
</div>
<section id="section-1">
      <h2 id="name-introduction">
<a href="#section-1" class="section-number selfRef">1. </a><a href="#name-introduction" class="section-name selfRef">Introduction</a>
      </h2>
<section id="section-1.1">
        <h3 id="name-background">
<a href="#section-1.1" class="section-number selfRef">1.1. </a><a href="#name-background" class="section-name selfRef">Background</a>
        </h3>
<p id="section-1.1-1">
        When the SDP offer/answer mechanism <span>[<a href="#RFC3264" class="xref">RFC3264</a>]</span>  
        is used to negotiate the establishment of multimedia communication sessions, if separate 
        transports (5-tuples) are negotiated for each individual media stream, each transport consumes 
        additional resources (especially when Interactive Connectivity Establishment (ICE)
        <span>[<a href="#RFC8445" class="xref">RFC8445</a>]</span> is used). 
        For this reason, it is attractive to use a single transport for multiple media streams.<a href="#section-1.1-1" class="pilcrow">¶</a></p>
</section>
<section id="section-1.2">
        <h3 id="name-bundle-mechanism">
<a href="#section-1.2" class="section-number selfRef">1.2. </a><a href="#name-bundle-mechanism" class="section-name selfRef">BUNDLE Mechanism</a>
        </h3>
<p id="section-1.2-1">
        This specification defines a way to use a single transport (BUNDLE transport) 
        for sending and receiving media (bundled media) described by multiple SDP media descriptions 
        ("m=" sections). The address:port combination used by an endpoint for sending and receiving 
        bundled media is referred to as the "BUNDLE address:port". The set of SDP attributes that are 
        applied to each "m=" section within a BUNDLE group is referred to as "BUNDLE attributes". 
        The same BUNDLE transport is used for sending and receiving bundled media, which
        means that the symmetric Real-time Transport Protocol (RTP) mechanism <span>[<a href="#RFC4961" class="xref">RFC4961</a>]</span> is always used for RTP-based bundled media.<a href="#section-1.2-1" class="pilcrow">¶</a></p>
<p id="section-1.2-2">
        This specification defines a new SDP Grouping Framework <span>[<a href="#RFC5888" class="xref">RFC5888</a>]</span> extension called 'BUNDLE'. The extension can be used with the Session Description
        Protocol (SDP) offer/answer mechanism <span>[<a href="#RFC3264" class="xref">RFC3264</a>]</span>
        to negotiate which "m=" sections will become part of a BUNDLE group. In addition, the offerer and 
        answerer <span>[<a href="#RFC3264" class="xref">RFC3264</a>]</span> use the BUNDLE extension to negotiate 
        the BUNDLE addresses:ports (offerer BUNDLE address:port and answerer BUNDLE address:port) and the 
        set of BUNDLE attributes (offerer BUNDLE attributes and answerer BUNDLE attributes) that will be applied to
        each "m=" section within the BUNDLE group.<a href="#section-1.2-2" class="pilcrow">¶</a></p>
<p id="section-1.2-3">
        The use of a BUNDLE transport allows the usage of a single set of
        ICE candidates <span>[<a href="#RFC8445" class="xref">RFC8445</a>]</span> for the whole BUNDLE group.<a href="#section-1.2-3" class="pilcrow">¶</a></p>
<p id="section-1.2-4">
        A given BUNDLE address:port <span class="bcp14">MUST</span> only be associated with a single BUNDLE group. If an SDP offer 
        or SDP answer (hereafter referred to as "offer" and "answer") contains multiple BUNDLE groups, the procedures in this specification apply to each 
        group independently. All RTP-based bundled media associated with a given BUNDLE group belong to a single
        RTP session <span>[<a href="#RFC3550" class="xref">RFC3550</a>]</span>.<a href="#section-1.2-4" class="pilcrow">¶</a></p>
<p id="section-1.2-5">
        The BUNDLE extension is backward compatible. Endpoints that do not support the extension
        are expected to generate offers and answers without an SDP 'group:BUNDLE' attribute and
        assign a unique address:port to each "m=" section within an offer and answer, according
        to the procedures in
       <span>[<a href="#RFC3264" class="xref">RFC3264</a>]</span> and <span>[<a href="#RFC4566" class="xref">RFC4566</a>]</span>.<a href="#section-1.2-5" class="pilcrow">¶</a></p>
</section>
<section id="section-1.3">
        <h3 id="name-protocol-extensions">
<a href="#section-1.3" class="section-number selfRef">1.3. </a><a href="#name-protocol-extensions" class="section-name selfRef">Protocol Extensions</a>
        </h3>
<p id="section-1.3-1">
        In addition to defining the new SDP Grouping Framework extension, this specification defines
        the following protocol extensions and makes the following updates to RFCs.  This specification:<a href="#section-1.3-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-1.3-2.1">
          defines a new SDP attribute, 'bundle-only', which can be used to
          request that a specific "m=" section (and the associated media) be used only if kept 
          within a BUNDLE group.<a href="#section-1.3-2.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-1.3-2.2">  
          updates RFC 3264 <span>[<a href="#RFC3264" class="xref">RFC3264</a>]</span> to also allow assigning a zero port value to an "m=" section 
          in cases where the media described by the "m=" section is not disabled or rejected.<a href="#section-1.3-2.2" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-1.3-2.3">
          defines a new RTCP <span>[<a href="#RFC3550" class="xref">RFC3550</a>]</span> SDES item, Media Identification ('MID'), and a new RTP SDES header
          extension that can be used to associate RTP streams with "m=" sections.<a href="#section-1.3-2.3" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-1.3-2.4">
          updates <span>[<a href="#RFC7941" class="xref">RFC7941</a>]</span> by adding an exception,
          for the MID RTP header extension, to the requirement regarding protection 
          of an SDES RTP header extension carrying an SDES item for the MID RTP 
          header extension.<a href="#section-1.3-2.4" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-1.3-2.5">
          updates <span>[<a href="#RFC5888" class="xref">RFC5888</a>]</span> by allowing an SDP 'group'
          attribute to contain an identification-tag that identifies an "m=" section with the port value set to zero.<a href="#section-1.3-2.5" class="pilcrow">¶</a>
</li>
        </ul>
</section>
<div id="sec-changes-8843">
<section id="section-1.4">
        <h3 id="name-changes-from-rfc-8843">
<a href="#section-1.4" class="section-number selfRef">1.4. </a><a href="#name-changes-from-rfc-8843" class="section-name selfRef">Changes from RFC 8843</a>
        </h3>
<p id="section-1.4-1">
  When <span>[<a href="#RFC8843" class="xref">RFC8843</a>]</span> and <span>[<a href="#RFC8829" class="xref">RFC8829</a>]</span>
  were published, an inconsistency between the specifications was identified. The procedures regarding assigning 
  the port value to a bundled "m=" section in an answer (initial or subsequent) and a subsequent offer
  were inconsistent. This specification removes the inconsistency by aligning the port value assignment
  procedure with the procedure in <span>[<a href="#RFC8829" class="xref">RFC8829</a>]</span>.<a href="#section-1.4-1" class="pilcrow">¶</a></p>
<p id="section-1.4-2">
  In addition, this document implements changes from the following errata reports: <span>[<a href="#Err6431" class="xref">Err6431</a>]</span>, <span>[<a href="#Err6437" class="xref">Err6437</a>]</span>.<a href="#section-1.4-2" class="pilcrow">¶</a></p>
</section>
</div>
</section>
<div id="sec-term">
<section id="section-2">
      <h2 id="name-terminology">
<a href="#section-2" class="section-number selfRef">2. </a><a href="#name-terminology" class="section-name selfRef">Terminology</a>
      </h2>
<span class="break"></span><dl class="dlParallel" id="section-2-1">
        <dt id="section-2-1.1">"m=" section:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.2"> SDP bodies contain one or more media descriptions, referred to
      as "m=" sections. Each "m=" section is represented by an SDP "m=" line and zero or more
      SDP attributes associated with the "m=" line. A local address:port combination is
      assigned to each "m=" section.<a href="#section-2-1.2" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.3">5-tuple:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.4">A collection of the following values: source address, source
      port, destination address, destination port, and transport-layer
      protocol.<a href="#section-2-1.4" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.5">Unique address:port:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.6">An address:port combination that is assigned to
      only one "m=" section in an offer or answer.<a href="#section-2-1.6" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.7">Offerer BUNDLE-tag:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.8">The first identification-tag in a given
      SDP 'group:BUNDLE' attribute identification-tag list in an offer.<a href="#section-2-1.8" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.9">Answerer BUNDLE-tag:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.10">The first identification-tag in a given
      SDP 'group:BUNDLE' attribute identification-tag list in an answer.<a href="#section-2-1.10" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.11"> Suggested offerer-tagged "m=" section:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.12"> The bundled "m=" section identified by the offerer BUNDLE-tag in an initial BUNDLE offer,
      before a BUNDLE group has been negotiated.<a href="#section-2-1.12" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.13">Offerer-tagged "m=" section:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.14">The bundled "m=" section identified by the offerer BUNDLE-tag in a subsequent offer.
      The "m=" section contains characteristics (offerer BUNDLE address:port and offerer BUNDLE attributes) that are applied to
      each "m=" section within the BUNDLE group.<a href="#section-2-1.14" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.15">Answerer-tagged "m=" section:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.16">The bundled "m=" section identified by the answerer BUNDLE-tag in an answer 
      (initial BUNDLE answer or subsequent). The "m=" section contains characteristics (answerer BUNDLE address:port and answerer BUNDLE attributes) 
      that are applied to each "m=" section within the BUNDLE group.<a href="#section-2-1.16" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.17">BUNDLE address:port:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.18">An address:port combination that an endpoint uses for sending and receiving 
      bundled media.<a href="#section-2-1.18" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.19">Offerer BUNDLE address:port:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.20">The address:port combination used by the offerer
      for sending and receiving media.<a href="#section-2-1.20" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.21">Answerer BUNDLE address:port:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.22">The address:port combination used by the answerer
      for sending and receiving media.<a href="#section-2-1.22" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.23">BUNDLE attributes:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.24"> IDENTICAL and TRANSPORT multiplexing category SDP 
      attributes. Once a BUNDLE group has been created, the attribute values apply
      to each bundled "m=" section within the BUNDLE group.<a href="#section-2-1.24" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.25">Offerer BUNDLE attributes:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.26">IDENTICAL and TRANSPORT multiplexing category SDP 
      attributes included in the offerer-tagged "m=" section.<a href="#section-2-1.26" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.27">Answerer BUNDLE attributes:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.28">IDENTICAL and TRANSPORT multiplexing category SDP 
      attributes included in the answerer-tagged "m=" section.<a href="#section-2-1.28" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.29">BUNDLE transport:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.30">The transport (5-tuple) used by all media described by the
      "m=" sections within a BUNDLE group.<a href="#section-2-1.30" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.31">BUNDLE group:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.32">A set of bundled "m=" sections, created using an SDP offer/answer
      exchange, that uses a single BUNDLE transport and a single set of BUNDLE attributes
      for sending and receiving all media (bundled media) described by the set of "m=" sections. 
      The same BUNDLE transport is used for sending and receiving bundled media.<a href="#section-2-1.32" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.33">Bundled "m=" section:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.34">An "m=" section, whose identification-tag
      is placed in an SDP 'group:BUNDLE' attribute identification-tag list
      in an offer or answer.<a href="#section-2-1.34" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.35">Bundle-only "m=" section:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.36">A bundled "m=" section that contains an
      SDP 'bundle-only' attribute.<a href="#section-2-1.36" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.37">Bundled media:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.38">All media associated with a given BUNDLE group.<a href="#section-2-1.38" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.39">Initial BUNDLE offer:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.40">The first offer, within an SDP session (e.g., a SIP dialog
      when SIP <span>[<a href="#RFC3261" class="xref">RFC3261</a>]</span> is used to carry SDP), in which
      the offerer indicates that it wants to negotiate a given BUNDLE group.<a href="#section-2-1.40" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.41">Initial BUNDLE answer:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.42">The answer to an initial BUNDLE offer in which the offerer indicates that it wants to negotiate
      a BUNDLE group, and the answerer accepts the creation of the BUNDLE group. The BUNDLE group is 
      created once the answerer sends the initial BUNDLE answer.<a href="#section-2-1.42" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.43">Subsequent offer:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.44">An offer that contains a BUNDLE group that
      has been created as part of a previous offer/answer exchange.<a href="#section-2-1.44" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.45">Subsequent answer:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.46">An answer to a subsequent offer.<a href="#section-2-1.46" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-1.47">Identification-tag:</dt>
        <dd style="margin-left: 1.5em" id="section-2-1.48">A unique token value that is used to identify an
      "m=" section. The SDP 'mid' attribute <span>[<a href="#RFC5888" class="xref">RFC5888</a>]</span> in an "m=" section carries the unique identification-tag 
      assigned to that "m=" section. The session-level SDP 'group' attribute 
      <span>[<a href="#RFC5888" class="xref">RFC5888</a>]</span> carries a list of 
      identification-tags, identifying the "m=" sections associated with that
      particular 'group' attribute.<a href="#section-2-1.48" class="pilcrow">¶</a>
</dd>
      <dd class="break"></dd>
</dl>
</section>
</div>
<section id="section-3">
      <h2 id="name-conventions">
<a href="#section-3" class="section-number selfRef">3. </a><a href="#name-conventions" class="section-name selfRef">Conventions</a>
      </h2>
<p id="section-3-1">
    The key words "<span class="bcp14">MUST</span>", "<span class="bcp14">MUST NOT</span>",
    "<span class="bcp14">REQUIRED</span>", "<span class="bcp14">SHALL</span>", "<span class="bcp14">SHALL NOT</span>",
    "<span class="bcp14">SHOULD</span>", "<span class="bcp14">SHOULD NOT</span>",
    "<span class="bcp14">RECOMMENDED</span>", "<span class="bcp14">NOT RECOMMENDED</span>",
    "<span class="bcp14">MAY</span>", and "<span class="bcp14">OPTIONAL</span>" in this document are to be
    interpreted as described in BCP 14 <span>[<a href="#RFC2119" class="xref">RFC2119</a>]</span> <span>[<a href="#RFC8174" class="xref">RFC8174</a>]</span> when, and only when, they appear in all capitals, as
    shown here.<a href="#section-3-1" class="pilcrow">¶</a></p>
</section>
<section id="section-4">
      <h2 id="name-applicability-statement">
<a href="#section-4" class="section-number selfRef">4. </a><a href="#name-applicability-statement" class="section-name selfRef">Applicability Statement</a>
      </h2>
<p id="section-4-1">
      The mechanism in this specification only applies to SDP <span>[<a href="#RFC4566" class="xref">RFC4566</a>]</span>, when used together with the SDP offer/answer
      mechanism <span>[<a href="#RFC3264" class="xref">RFC3264</a>]</span>.
      Declarative usage of SDP is out of scope of this document and is
      thus undefined.<a href="#section-4-1" class="pilcrow">¶</a></p>
</section>
<div id="sec-group">
<section id="section-5">
      <h2 id="name-sdp-grouping-framework-bund">
<a href="#section-5" class="section-number selfRef">5. </a><a href="#name-sdp-grouping-framework-bund" class="section-name selfRef">SDP Grouping Framework BUNDLE Extension</a>
      </h2>
<p id="section-5-1">
       This section defines a new SDP Grouping Framework <span>[<a href="#RFC5888" class="xref">RFC5888</a>]</span> extension, 'BUNDLE'. The BUNDLE extension can be used with the SDP
       offer/answer mechanism to negotiate a set of "m=" sections that will become part of a BUNDLE 
       group. Within a BUNDLE group, each "m=" section uses a BUNDLE transport for sending and 
       receiving bundled media. Each endpoint uses a single address:port combination for sending and
       receiving the bundled media.<a href="#section-5-1" class="pilcrow">¶</a></p>
<p id="section-5-2">
       The BUNDLE extension is indicated using an SDP 'group' attribute with a semantics value 
       <span>[<a href="#RFC5888" class="xref">RFC5888</a>]</span> of "BUNDLE".
       An identification-tag is assigned to each bundled
       "m=" section, and each identification-tag is listed in the SDP 'group:BUNDLE'
       attribute identification-tag list. Each "m=" section whose identification-tag
       is listed in the identification-tag list is associated with a given
       BUNDLE group.<a href="#section-5-2" class="pilcrow">¶</a></p>
<p id="section-5-3">
       SDP bodies can contain multiple BUNDLE groups. Any given bundled "m="
       section <span class="bcp14">MUST NOT</span> be associated with more than one BUNDLE group at any given
       time.<a href="#section-5-3" class="pilcrow">¶</a></p>
<p id="section-5-4">
       NOTE: The order of the "m=" sections listed in the SDP 'group:BUNDLE' attribute
       identification-tag list does not have to be the same as the order in which
       the "m=" sections occur in the SDP.<a href="#section-5-4" class="pilcrow">¶</a></p>
<p id="section-5-5">
       The multiplexing category <span>[<a href="#RFC8859" class="xref">RFC8859</a>]</span> for the 'group:BUNDLE'
       attribute is 'NORMAL'.<a href="#section-5-5" class="pilcrow">¶</a></p>
<p id="section-5-6">
       <a href="#sec-sdp-oa" class="xref">Section 7</a> defines the
       detailed SDP offer/answer procedures for the BUNDLE extension.<a href="#section-5-6" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-bundle-only">
<section id="section-6">
      <h2 id="name-sdp-bundle-only-attribute">
<a href="#section-6" class="section-number selfRef">6. </a><a href="#name-sdp-bundle-only-attribute" class="section-name selfRef">SDP 'bundle-only' Attribute</a>
      </h2>
<p id="section-6-1">
      This section defines a new SDP media-level attribute <span>[<a href="#RFC4566" class="xref">RFC4566</a>]</span>, 'bundle-only'. 'bundle-only' is a property attribute
      <span>[<a href="#RFC4566" class="xref">RFC4566</a>]</span>; hence, it has no value.<a href="#section-6-1" class="pilcrow">¶</a></p>
<p id="section-6-2">
      In order to ensure that an answerer that does not support the BUNDLE extension always
      rejects a bundled "m=" section in an offer, the offerer can assign a zero port value to the "m="
      section. According to <span>[<a href="#RFC3264" class="xref">RFC3264</a>]</span>, an answerer
      will reject such an "m=" section.
      By including an SDP 'bundle-only' attribute in a bundled "m=" section, the offerer can
      request that the answerer accept the "m=" section only if the answerer supports the BUNDLE
      extension and if the answerer keeps the "m=" section within the associated BUNDLE group.<a href="#section-6-2" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlParallel" id="section-6-3">
        <dt id="section-6-3.1">Name:</dt>
        <dd style="margin-left: 1.5em" id="section-6-3.2">bundle-only<a href="#section-6-3.2" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-6-3.3">Value:</dt>
        <dd style="margin-left: 1.5em" id="section-6-3.4">N/A<a href="#section-6-3.4" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-6-3.5">Usage Level:</dt>
        <dd style="margin-left: 1.5em" id="section-6-3.6">media<a href="#section-6-3.6" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-6-3.7">Charset Dependent:</dt>
        <dd style="margin-left: 1.5em" id="section-6-3.8">no<a href="#section-6-3.8" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-6-3.9">Example:</dt>
        <dd style="margin-left: 1.5em" id="section-6-3.10">a=bundle-only<a href="#section-6-3.10" class="pilcrow">¶</a>
</dd>
      <dd class="break"></dd>
</dl>
<p id="section-6-4">
      The usage of the 'bundle-only' attribute is only defined for a
      bundled "m=" section with a zero port value. Other usage is
      unspecified. If an offerer or answerer receives a 'bundle-only'
      attribute in a non-bundled "m=" section, the offerer or answerer
      <span class="bcp14">MUST</span> discard the attribute.<a href="#section-6-4" class="pilcrow">¶</a></p>
<p id="section-6-5">
      <a href="#sec-sdp-oa" class="xref">Section 7</a> defines the detailed SDP
      offer/answer procedures for the 'bundle-only' attribute.<a href="#section-6-5" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-sdp-oa">
<section id="section-7">
      <h2 id="name-sdp-offer-answer-procedures">
<a href="#section-7" class="section-number selfRef">7. </a><a href="#name-sdp-offer-answer-procedures" class="section-name selfRef">SDP Offer/Answer Procedures</a>
      </h2>
<p id="section-7-1">
        This section describes the SDP offer/answer <span>[<a href="#RFC3264" class="xref">RFC3264</a>]</span> procedures for:<a href="#section-7-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-7-2.1">
            Negotiating a BUNDLE group;<a href="#section-7-2.1" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-7-2.2">
            Suggesting and selecting the tagged "m=" sections (offerer-tagged "m=" section and answerer-tagged "m=" section);<a href="#section-7-2.2" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-7-2.3">
            Adding an "m=" section to a BUNDLE group;<a href="#section-7-2.3" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-7-2.4">
            Moving an "m=" section out of a BUNDLE group; and<a href="#section-7-2.4" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-7-2.5">
            Disabling an "m=" section within a BUNDLE group.<a href="#section-7-2.5" class="pilcrow">¶</a>
</li>
      </ul>
<p id="section-7-3">
        The generic rules and procedures defined in <span>[<a href="#RFC3264" class="xref">RFC3264</a>]</span> and <span>[<a href="#RFC5888" class="xref">RFC5888</a>]</span>
        also apply to the BUNDLE extension. For example, if an offer is rejected
        by the answerer, the previously negotiated addresses:ports, SDP parameters, and characteristics
        (including those associated with a BUNDLE group) apply. Hence, if an offerer
        generates an offer in order to negotiate a BUNDLE group
        and the answerer rejects the offer, the BUNDLE group is not created.<a href="#section-7-3" class="pilcrow">¶</a></p>
<p id="section-7-4">
        The procedures in this section are independent of the media type or
        "m=" line proto value assigned to a bundled "m=" section. <a href="#sec-bundle-only" class="xref">Section 6</a> defines
        additional considerations for the usage of the SDP 'bundle-only' attribute.
        <a href="#sec-rtp" class="xref">Section 9</a> defines additional
        considerations for RTP-based media.
      
        <a href="#sec-ice" class="xref">Section 10</a> defines additional
        considerations for the usage of the ICE mechanism
        <span>[<a href="#RFC8445" class="xref">RFC8445</a>]</span>.<a href="#section-7-4" class="pilcrow">¶</a></p>
<p id="section-7-5">
        Offers and answers can contain multiple BUNDLE groups. The procedures in this
        section apply independently to a given BUNDLE group.<a href="#section-7-5" class="pilcrow">¶</a></p>
<div id="sec-sdp-cons">
<section id="section-7.1">
        <h3 id="name-generic-sdp-considerations">
<a href="#section-7.1" class="section-number selfRef">7.1. </a><a href="#name-generic-sdp-considerations" class="section-name selfRef">Generic SDP Considerations</a>
        </h3>
<p id="section-7.1-1">
          This section describes generic restrictions associated with the usage of
          SDP parameters within a BUNDLE group. It also describes how to calculate a 
          value for the whole BUNDLE group, when parameter and attribute values have been assigned 
          to each bundled "m=" section.<a href="#section-7.1-1" class="pilcrow">¶</a></p>
<div id="sec-sdp-cons-c">
<section id="section-7.1.1">
          <h4 id="name-connection-data-c">
<a href="#section-7.1.1" class="section-number selfRef">7.1.1. </a><a href="#name-connection-data-c" class="section-name selfRef">Connection Data ("c=")</a>
          </h4>
<p id="section-7.1.1-1">
            The "c=" line nettype value <span>[<a href="#RFC4566" class="xref">RFC4566</a>]</span> associated with a bundled "m=" section <span class="bcp14">MUST</span> be 'IN'.<a href="#section-7.1.1-1" class="pilcrow">¶</a></p>
<p id="section-7.1.1-2">
            The "c=" line addrtype value <span>[<a href="#RFC4566" class="xref">RFC4566</a>]</span> associated with a bundled "m=" section <span class="bcp14">MUST</span> be 'IP4' or
            'IP6'. The same value <span class="bcp14">MUST</span> be associated with each "m=" section.<a href="#section-7.1.1-2" class="pilcrow">¶</a></p>
<p style="margin-left: 1.5em" id="section-7.1.1-3">
            NOTE: Extensions to this specification can specify usage of the BUNDLE
            mechanism for other nettype and addrtype values than the ones listed above.<a href="#section-7.1.1-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-sdp-cons-b">
<section id="section-7.1.2">
          <h4 id="name-bandwidth-b">
<a href="#section-7.1.2" class="section-number selfRef">7.1.2. </a><a href="#name-bandwidth-b" class="section-name selfRef">Bandwidth ("b=")</a>
          </h4>
<p id="section-7.1.2-1">
            An offerer and answerer <span class="bcp14">MUST</span> use the rules and restrictions defined
            in <span>[<a href="#RFC8859" class="xref">RFC8859</a>]</span> for
            associating the SDP bandwidth ("b=") line with bundled "m=" sections.<a href="#section-7.1.2-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-sdp-oa-cat">
<section id="section-7.1.3">
          <h4 id="name-attributes-a">
<a href="#section-7.1.3" class="section-number selfRef">7.1.3. </a><a href="#name-attributes-a" class="section-name selfRef">Attributes ("a=")</a>
          </h4>
<p id="section-7.1.3-1">
            An offerer and answerer <span class="bcp14">MUST</span> include SDP attributes in every bundled "m=" section where applicable,
            following the normal offer/answer procedures for each attribute, with the following exceptions:<a href="#section-7.1.3-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-7.1.3-2.1">
              In the initial BUNDLE offer, the offerer <span class="bcp14">MUST NOT</span>
              include IDENTICAL and TRANSPORT multiplexing category SDP
              attributes (BUNDLE attributes) in bundle-only "m=" sections. The
              offerer <span class="bcp14">MUST</span> include such attributes in all other
              bundled "m=" sections. In the initial BUNDLE offer, each bundled
              "m=" line can contain a different set of BUNDLE attributes and
              attribute values. Once the offerer-tagged "m=" section has been
              selected, the BUNDLE attributes contained in the offerer-tagged
              "m=" section will apply to each bundled "m=" section within the
              BUNDLE group.<a href="#section-7.1.3-2.1" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-7.1.3-2.2">
              In a subsequent offer or in an answer (initial or subsequent),
              the offerer and answerer <span class="bcp14">MUST</span> include IDENTICAL
              and TRANSPORT multiplexing category SDP attributes (BUNDLE
              attributes) only in the tagged "m=" section (offerer-tagged "m="
              section or answerer-tagged "m=" section). The offerer and
              answerer <span class="bcp14">MUST NOT</span> include such attributes in any
              other bundled "m=" section. The BUNDLE attributes contained in
              the tagged "m=" section will apply to each bundled "m=" section
              within the BUNDLE group.<a href="#section-7.1.3-2.2" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-7.1.3-2.3">
              In an offer (initial BUNDLE offer or subsequent) or in an
              answer (initial BUNDLE answer or subsequent), the offerer and
              answerer <span class="bcp14">MUST</span> include SDP attributes from
              categories other than IDENTICAL and TRANSPORT in each bundled
              "m=" section that a given attribute applies to. Each bundled
              "m=" line can contain a different set of such attributes and
              attribute values, as such attributes only apply to the given
              bundled "m=" section in which they are included.<a href="#section-7.1.3-2.3" class="pilcrow">¶</a>
</li>
          </ul>
<p style="margin-left: 1.5em" id="section-7.1.3-3">
            NOTE: A consequence of the rules above is that media-specific
            IDENTICAL and TRANSPORT multiplexing category SDP attributes that
            are applicable only to some of the bundled "m=" sections within
            the BUNDLE group might appear in the tagged "m=" section for which
            they are not applicable. For instance, the tagged "m=" section
            might contain an SDP 'rtcp-mux' attribute even if the tagged "m="
            section does not describe RTP-based media (but another bundled
            "m=" section within the BUNDLE group does describe RTP-based
            media).<a href="#section-7.1.3-3" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="sec-sdp-oa-ino">
<section id="section-7.2">
        <h3 id="name-generating-the-initial-bund">
<a href="#section-7.2" class="section-number selfRef">7.2. </a><a href="#name-generating-the-initial-bund" class="section-name selfRef">Generating the Initial BUNDLE Offer</a>
        </h3>
<p id="section-7.2-1">
          The procedures in this section apply to the first offer within an SDP session (e.g., a SIP
          dialog when SIP <span>[<a href="#RFC3261" class="xref">RFC3261</a>]</span> is used to carry SDP) in which the 
          offerer indicates that it wants to negotiate a given BUNDLE group. This could occur in the 
          initial offer, or in a subsequent offer, of the SDP session.<a href="#section-7.2-1" class="pilcrow">¶</a></p>
<p id="section-7.2-2">
          When an offerer generates an initial BUNDLE offer, in order to negotiate a
          BUNDLE group, it <span class="bcp14">MUST</span>:<a href="#section-7.2-2" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-7.2-3.1">
              Assign a unique address:port to each bundled "m=" section
              following the procedures in <span>[<a href="#RFC3264" class="xref">RFC3264</a>]</span>, excluding any bundle-only "m=" sections (see below);<a href="#section-7.2-3.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.2-3.2">
              Pick a bundled "m=" section as the suggested offerer-tagged "m=" (<a href="#sec-sdp-oa-ino-req" class="xref">Section 7.2.1</a>);<a href="#section-7.2-3.2" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.2-3.3">
              Include SDP attributes in the bundled "m=" sections following the rules 
              in <a href="#sec-sdp-oa-cat" class="xref">Section 7.1.3</a>;<a href="#section-7.2-3.3" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.2-3.4">
              Include an SDP 'group:BUNDLE' attribute in the offer; and<a href="#section-7.2-3.4" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.2-3.5">
              Place the identification-tag of each bundled "m=" section in the
              SDP 'group:BUNDLE' attribute identification-tag list. The offerer BUNDLE-tag
              indicates the suggested offerer-tagged "m=" section.<a href="#section-7.2-3.5" class="pilcrow">¶</a>
</li>
        </ul>
<p style="margin-left: 1.5em" id="section-7.2-4">
          NOTE: When the offerer assigns unique addresses:ports to multiple bundled "m=" sections, the offerer needs to be prepared
          to receive bundled media on each unique address:port until it receives the associated answer and finds out which
          bundled "m=" section (and associated address:port combination) the answerer has selected as the offerer-tagged "m=" section.<a href="#section-7.2-4" class="pilcrow">¶</a></p>
<p id="section-7.2-5">
          If the offerer wants to request that the answerer accept a given bundled "m=" section only if
          the answerer keeps the "m=" section within the negotiated BUNDLE group, the offerer <span class="bcp14">MUST</span>:<a href="#section-7.2-5" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-7.2-6.1">
              Include an SDP 'bundle-only' attribute (<a href="#sec-sdp-oa-ino-req" class="xref">Section 7.2.1</a>) in the "m=" section, and<a href="#section-7.2-6.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.2-6.2">
              Assign a zero port value to the "m=" section.<a href="#section-7.2-6.2" class="pilcrow">¶</a>
</li>
        </ul>
<p style="margin-left: 1.5em" id="section-7.2-7">
          NOTE: If the offerer assigns a zero port value to a bundled "m=" section but does not include an
          SDP 'bundle-only' attribute in the "m=" section, it is an indication that the offerer wants
          to disable the "m=" section (<a href="#sec-sdp-oa-mod-dis" class="xref">Section 7.5.3</a>).<a href="#section-7.2-7" class="pilcrow">¶</a></p>
<p id="section-7.2-8">
          Sections <a href="#sec-sdp-oa-ino-ex" class="xref">7.2.2</a> and <a href="#sec-example-add" class="xref">18.1</a> show
          an example of an initial BUNDLE offer.<a href="#section-7.2-8" class="pilcrow">¶</a></p>
<div id="sec-sdp-oa-ino-req">
<section id="section-7.2.1">
          <h4 id="name-suggesting-the-offerer-tagg">
<a href="#section-7.2.1" class="section-number selfRef">7.2.1. </a><a href="#name-suggesting-the-offerer-tagg" class="section-name selfRef">Suggesting the Offerer-Tagged "m=" Section</a>
          </h4>
<p id="section-7.2.1-1">
          In the initial BUNDLE offer, the bundled "m=" section indicated by the offerer BUNDLE-tag is the suggested offerer-tagged "m=" section. 
          The address:port combination associated with the "m=" section will be used by the offerer for sending and receiving bundled
          media if the answerer selects the "m=" section as the offerer-tagged "m=" section (<a href="#sec-sdp-oa-ans-off" class="xref">Section 7.3.1</a>). In addition, if the answerer selects the "m=" section as the offerer-tagged "m=" section, 
          the BUNDLE attributes included in the "m=" section will be applied to each "m=" section within the negotiated BUNDLE group.<a href="#section-7.2.1-1" class="pilcrow">¶</a></p>
<p id="section-7.2.1-2">
          The offerer <span class="bcp14">MUST NOT</span> suggest a bundle-only "m=" section as the offerer-tagged "m=" section.<a href="#section-7.2.1-2" class="pilcrow">¶</a></p>
<p id="section-7.2.1-3">
          It is <span class="bcp14">RECOMMENDED</span> that the suggested offerer-tagged "m=" section be a bundled "m=" section 
          which the offerer believes is unlikely to be rejected or moved out of the BUNDLE group by the answerer. 
          How such an assumption is made is outside the scope of this document.<a href="#section-7.2.1-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-sdp-oa-ino-ex">
<section id="section-7.2.2">
          <h4 id="name-example-initial-bundle-offe">
<a href="#section-7.2.2" class="section-number selfRef">7.2.2. </a><a href="#name-example-initial-bundle-offe" class="section-name selfRef">Example: Initial BUNDLE Offer</a>
          </h4>
<p id="section-7.2.2-1">
          The following example shows an initial BUNDLE offer. The offer includes two
          "m=" sections in the offer and suggests that both "m=" sections be included in a BUNDLE group. 
          The audio "m=" section is the suggested offerer-tagged "m=" section, indicated by placing the 
          identification-tag associated with the "m=" section (offerer BUNDLE-tag) first in the SDP 'group:BUNDLE' 
          attribute identification-id list.<a href="#section-7.2.2-1" class="pilcrow">¶</a></p>
<p id="section-7.2.2-2" class="keepWithNext">SDP Offer<a href="#section-7.2.2-2" class="pilcrow">¶</a></p>
<div id="section-7.2.2-3">
<pre class="lang-sdp sourcecode">
  v=0
  o=alice 2890844526 2890844526 IN IP6 2001:db8::3
  s=
  c=IN IP6 2001:db8::3
  t=0 0
  a=group:BUNDLE foo bar

  m=audio 10000 RTP/AVP 0 8 97
  b=AS:200
  a=mid:foo
  a=rtcp-mux
  a=rtpmap:0 PCMU/8000
  a=rtpmap:8 PCMA/8000
  a=rtpmap:97 iLBC/8000
  a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

  m=video 10002 RTP/AVP 31 32
  b=AS:1000
  a=mid:bar
  a=rtcp-mux
  a=rtpmap:31 H261/90000
  a=rtpmap:32 MPV/90000
  a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
</pre><a href="#section-7.2.2-3" class="pilcrow">¶</a>
</div>
<p id="section-7.2.2-4">


          The following example shows an initial BUNDLE offer. The offer includes two
          "m=" sections in the offer and suggests that both "m=" sections are included in a BUNDLE group. 
          The offerer includes an SDP 'bundle-only' attribute in the video "m=" section to request that the
          answerer accept the "m=" section only if the answerer supports the BUNDLE extension and if the
          answerer keeps the "m=" section within the associated BUNDLE group.
          The audio "m=" section is the suggested offerer-tagged "m=" section, indicated by placing the 
          identification-tag associated with the "m=" section (offerer BUNDLE-tag) first in the SDP 'group:BUNDLE' 
          attribute identification-id list.<a href="#section-7.2.2-4" class="pilcrow">¶</a></p>
<p id="section-7.2.2-5" class="keepWithNext">SDP Offer<a href="#section-7.2.2-5" class="pilcrow">¶</a></p>
<div id="section-7.2.2-6">
<pre class="lang-sdp sourcecode">
  v=0
  o=alice 2890844526 2890844526 IN IP6 2001:db8::3
  s=
  c=IN IP6 2001:db8::3
  t=0 0
  a=group:BUNDLE foo bar

  m=audio 10000 RTP/AVP 0 8 97
  b=AS:200
  a=mid:foo
  a=rtcp-mux
  a=rtpmap:0 PCMU/8000
  a=rtpmap:8 PCMA/8000
  a=rtpmap:97 iLBC/8000
  a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

  m=video 0 RTP/AVP 31 32
  b=AS:1000
  a=mid:bar
  a=bundle-only
  a=rtpmap:31 H261/90000
  a=rtpmap:32 MPV/90000
  a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
</pre><a href="#section-7.2.2-6" class="pilcrow">¶</a>
</div>
</section>
</div>
</section>
</div>
<div id="sec-sdp-oa-ans">
<section id="section-7.3">
        <h3 id="name-generating-the-sdp-answer">
<a href="#section-7.3" class="section-number selfRef">7.3. </a><a href="#name-generating-the-sdp-answer" class="section-name selfRef">Generating the SDP Answer</a>
        </h3>
<p id="section-7.3-1">
          When an answerer generates an answer (initial BUNDLE answer or subsequent) that contains a BUNDLE group, the following general
          SDP Grouping Framework restrictions, defined in <span>[<a href="#RFC5888" class="xref">RFC5888</a>]</span>, also apply to the BUNDLE group:<a href="#section-7.3-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-7.3-2.1">
              The answerer is only allowed to include a BUNDLE group in an
              initial BUNDLE answer if the offerer requested the BUNDLE group
              to be created in the corresponding initial BUNDLE offer;<a href="#section-7.3-2.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.3-2.2">
              The answerer is only allowed to include a BUNDLE group in a
              subsequent answer if the corresponding subsequent offer contains
              a previously negotiated BUNDLE group;<a href="#section-7.3-2.2" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.3-2.3">
              The answerer is only allowed to include a bundled "m=" section
              in an answer if the "m=" section was indicated as bundled in the
              corresponding offer; and<a href="#section-7.3-2.3" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.3-2.4">
              The answerer is only allowed to include a bundled "m=" section
              in the same BUNDLE group as the bundled "m=" line in the
              corresponding offer.<a href="#section-7.3-2.4" class="pilcrow">¶</a>
</li>
        </ul>
<p id="section-7.3-3">
          In addition, when an answerer generates an answer (initial BUNDLE
          answer or subsequent) that contains a BUNDLE group, the answerer
          <span class="bcp14">MUST</span>:<a href="#section-7.3-3" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-7.3-4.1">
              In case of an initial BUNDLE answer, select the offerer-tagged
              "m=" section using the procedures in <a href="#sec-sdp-oa-ans-off" class="xref">Section 7.3.1</a>. In case of a
              subsequent answer, the offerer-tagged "m=" section is indicated
              in the corresponding subsequent offer and <span class="bcp14">MUST NOT</span> be changed by the answerer;<a href="#section-7.3-4.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.3-4.2">
              Select the answerer-tagged "m=" section (<a href="#sec-sdp-oa-ans-off" class="xref">Section 7.3.1</a>);<a href="#section-7.3-4.2" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.3-4.3">
             Assign the answerer BUNDLE address:port to the answerer-tagged
             "m=" section and to every other bundled "m=" section within the BUNDLE
             group;<a href="#section-7.3-4.3" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.3-4.4">
              Include SDP attributes in the bundled "m=" sections following the rules 
              in <a href="#sec-sdp-oa-cat" class="xref">Section 7.1.3</a>;<a href="#section-7.3-4.4" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.3-4.5">
              Include an SDP 'group:BUNDLE' attribute in the answer; and<a href="#section-7.3-4.5" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.3-4.6">
              Place the identification-tag of each bundled "m=" section in the
              SDP 'group:BUNDLE' attribute identification-tag list. The answerer BUNDLE-tag
              indicates the answerer-tagged "m=" section (<a href="#sec-sdp-oa-ans-off" class="xref">Section 7.3.1</a>).<a href="#section-7.3-4.6" class="pilcrow">¶</a>
</li>
        </ul>
<p id="section-7.3-5">
          If the answerer does not want to keep an "m=" section within a BUNDLE group, it <span class="bcp14">MUST</span>:<a href="#section-7.3-5" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-7.3-6.1">
              Move the "m=" section out of the BUNDLE group (<a href="#sec-sdp-oa-ans-mov" class="xref">Section 7.3.2</a>); or<a href="#section-7.3-6.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.3-6.2">
              Reject the "m=" section (<a href="#sec-sdp-oa-ans-rej" class="xref">Section 7.3.3</a>).<a href="#section-7.3-6.2" class="pilcrow">¶</a>
</li>
        </ul>
<p id="section-7.3-7">
          The answerer can modify the answerer BUNDLE address:port, add and remove SDP attributes, or modify
          SDP attribute values in a subsequent answer. Changes to the answerer BUNDLE address:port 
          and the answerer BUNDLE attributes will be applied to each bundled "m=" section within the BUNDLE group.<a href="#section-7.3-7" class="pilcrow">¶</a></p>
<p style="margin-left: 1.5em" id="section-7.3-8">
          NOTE: If a bundled "m=" section in an offer contains a zero port value, but the "m=" section 
          does not contain an SDP 'bundle-only' attribute, it is an indication that the offerer wants 
          to disable the "m=" section (<a href="#sec-sdp-oa-mod-dis" class="xref">Section 7.5.3</a>).<a href="#section-7.3-8" class="pilcrow">¶</a></p>
<div id="sec-sdp-oa-ans-off">
<section id="section-7.3.1">
          <h4 id="name-answerer-selection-of-tagge">
<a href="#section-7.3.1" class="section-number selfRef">7.3.1. </a><a href="#name-answerer-selection-of-tagge" class="section-name selfRef">Answerer Selection of Tagged "m=" Sections</a>
          </h4>
<p id="section-7.3.1-1">
          When selecting the offerer-tagged "m=" section, the answerer <span class="bcp14">MUST</span> 
          first check whether the "m=" section fulfills the following criteria 
          (<a href="#sec-sdp-oa-ino-req" class="xref">Section 7.2.1</a>):<a href="#section-7.3.1-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-7.3.1-2.1">
              The answerer will not move the "m=" section out of the BUNDLE group
              (<a href="#sec-sdp-oa-ans-mov" class="xref">Section 7.3.2</a>);<a href="#section-7.3.1-2.1" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-7.3.1-2.2">
              The answerer will not reject the "m=" section (<a href="#sec-sdp-oa-ans-rej" class="xref">Section 7.3.3</a>); and<a href="#section-7.3.1-2.2" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-7.3.1-2.3">
              The "m=" section does not contain a zero port value.<a href="#section-7.3.1-2.3" class="pilcrow">¶</a>
</li>
          </ul>
<p id="section-7.3.1-3">
          If all of the criteria above are fulfilled, the answerer <span class="bcp14">MUST</span> select
          the "m=" section as the offerer-tagged "m=" section and <span class="bcp14">MUST</span> also mark
          the corresponding "m=" section in the answer as the answerer-tagged "m=" section.
          In the answer, the answerer BUNDLE-tag indicates the answerer-tagged "m=" section.<a href="#section-7.3.1-3" class="pilcrow">¶</a></p>
<p id="section-7.3.1-4">
          If one or more of the criteria are not fulfilled, the answerer <span class="bcp14">MUST</span> pick the next
          identification-tag in the identification-tag list in the offer and perform the same criteria
          check for the "m=" section indicated by that identification-tag. If there are no
          more identification-tags in the identification-tag list, the answerer <span class="bcp14">MUST NOT</span>
          create the BUNDLE group. In addition, unless the answerer rejects the whole offer,
          the answerer <span class="bcp14">MUST</span> apply the answerer procedures for moving an "m=" section out of a
          BUNDLE group (<a href="#sec-sdp-oa-ans-mov" class="xref">Section 7.3.2</a>) or
          rejecting an "m=" section within a BUNDLE group (<a href="#sec-sdp-oa-ans-rej" class="xref">Section 7.3.3</a>) to every bundled "m=" section in the offer when 
          creating the answer.<a href="#section-7.3.1-4" class="pilcrow">¶</a></p>
<p id="section-7.3.1-5">
          <a href="#sec-example-add" class="xref">Section 18.1</a> shows an
          example of an offerer BUNDLE address:port selection.<a href="#section-7.3.1-5" class="pilcrow">¶</a></p>
<p id="section-7.3.1-6">
          Sections <a href="#sec-sdp-oa-ans-ex" class="xref">7.3.4</a> and 
          <a href="#sec-example-add" class="xref">18.1</a> show an example of an
          answerer-tagged "m=" section selection.<a href="#section-7.3.1-6" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-sdp-oa-ans-mov">
<section id="section-7.3.2">
          <h4 id="name-moving-a-media-description-">
<a href="#section-7.3.2" class="section-number selfRef">7.3.2. </a><a href="#name-moving-a-media-description-" class="section-name selfRef">Moving a Media Description Out of a BUNDLE Group</a>
          </h4>
<p id="section-7.3.2-1">
          When an answerer generates the answer, the answerer <span class="bcp14">MUST</span> first check the following criteria if it wants to move a bundled "m=" section out of
          the negotiated BUNDLE group:<a href="#section-7.3.2-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-7.3.2-2.1">
              In the corresponding offer, the "m=" section is within a previously negotiated BUNDLE group, and<a href="#section-7.3.2-2.1" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-7.3.2-2.2">
              In the corresponding offer, the "m=" section contains an SDP 'bundle-only' attribute.<a href="#section-7.3.2-2.2" class="pilcrow">¶</a>
</li>
          </ul>
<p id="section-7.3.2-3">
          If either criterion above is fulfilled, the answerer cannot move the "m=" section out of 
          the BUNDLE group in the answer. The answerer can reject the whole offer, reject each
          bundled "m=" section within the BUNDLE group (<a href="#sec-sdp-oa-ans-rej" class="xref">Section 7.3.3</a>), 
          or keep the "m=" section within the BUNDLE group in the answer and later create an offer where 
          the "m=" section is moved out of the BUNDLE group (<a href="#sec-sdp-oa-mod-mov" class="xref">Section 7.5.2</a>).<a href="#section-7.3.2-3" class="pilcrow">¶</a></p>
<p style="margin-left: 1.5em" id="section-7.3.2-4">
          NOTE: One consequence of the rules above is that, once a BUNDLE group has been negotiated, a bundled "m=" section
          cannot be moved out of the BUNDLE group in an answer. Instead, an offer is needed.<a href="#section-7.3.2-4" class="pilcrow">¶</a></p>
<p id="section-7.3.2-5">
          When the answerer generates an answer in which it moves a bundled "m=" section out
          of a BUNDLE group, the answerer:<a href="#section-7.3.2-5" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-7.3.2-6.1">
              <span class="bcp14">MUST</span> assign a unique address:port to the "m=" section;<a href="#section-7.3.2-6.1" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-7.3.2-6.2">
              <span class="bcp14">MUST</span> include any applicable SDP attribute in the "m=" section using the normal
            offer/answer procedures for each attribute;<a href="#section-7.3.2-6.2" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-7.3.2-6.3">
              <span class="bcp14">MUST NOT</span> place the identification-tag associated with the "m=" section in
            the SDP 'group:BUNDLE' attribute identification-tag list associated with
            the BUNDLE group; and<a href="#section-7.3.2-6.3" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-7.3.2-6.4">
              <span class="bcp14">MUST NOT</span> include an SDP 'bundle-only' attribute to the "m=" section.<a href="#section-7.3.2-6.4" class="pilcrow">¶</a>
</li>
          </ul>
<p id="section-7.3.2-7">
          Because an answerer is not allowed to move an "m=" section from one BUNDLE group to another within an answer
          (<a href="#sec-sdp-oa-ans" class="xref">Section 7.3</a>), if
          the answerer wants to move an "m=" section from one BUNDLE group to another, it <span class="bcp14">MUST</span> first move the
          "m=" section out of the current BUNDLE group and then generate an offer where the "m=" section is
          added to another BUNDLE group (<a href="#sec-sdp-oa-mod-add" class="xref">Section 7.5.1</a>).<a href="#section-7.3.2-7" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-sdp-oa-ans-rej">
<section id="section-7.3.3">
          <h4 id="name-rejecting-a-media-descripti">
<a href="#section-7.3.3" class="section-number selfRef">7.3.3. </a><a href="#name-rejecting-a-media-descripti" class="section-name selfRef">Rejecting a Media Description in a BUNDLE Group</a>
          </h4>
<p id="section-7.3.3-1">
          When an answerer wants to reject a bundled "m=" section in an answer, it <span class="bcp14">MUST</span> first check 
          the following criterion:<a href="#section-7.3.3-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-7.3.3-2.1">
            In the corresponding offer (subsequent), the "m=" section is the offerer-tagged "m=" section.<a href="#section-7.3.3-2.1" class="pilcrow">¶</a>
</li>
          </ul>
<p id="section-7.3.3-3">
          If the criterion above is fulfilled, the answerer cannot reject the "m=" section in 
          the answer. The answerer can reject the whole offer, reject each bundled "m=" section 
          within the BUNDLE group, or keep the "m=" section within the BUNDLE group in the answer and later create
          an offer where the "m=" section is disabled within the BUNDLE group (<a href="#sec-sdp-oa-mod-dis" class="xref">Section 7.5.3</a>).<a href="#section-7.3.3-3" class="pilcrow">¶</a></p>
<p id="section-7.3.3-4">
          When an answerer generates an answer in which it rejects a bundled "m=" section, 
          the answerer:<a href="#section-7.3.3-4" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-7.3.3-5.1">
              <span class="bcp14">MUST</span> assign a zero port value to the "m=" section, according to the procedures in 
            <span>[<a href="#RFC3264" class="xref">RFC3264</a>]</span>;<a href="#section-7.3.3-5.1" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-7.3.3-5.2">
              <span class="bcp14">MUST NOT</span> place the identification-tag associated with the "m=" section in
            the SDP 'group:BUNDLE' attribute identification-tag list associated with
            the BUNDLE group; and<a href="#section-7.3.3-5.2" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-7.3.3-5.3">
              <span class="bcp14">MUST NOT</span> include an SDP 'bundle-only' attribute in the "m=" section.<a href="#section-7.3.3-5.3" class="pilcrow">¶</a>
</li>
          </ul>
</section>
</div>
<div id="sec-sdp-oa-ans-ex">
<section id="section-7.3.4">
          <h4 id="name-example-sdp-answer">
<a href="#section-7.3.4" class="section-number selfRef">7.3.4. </a><a href="#name-example-sdp-answer" class="section-name selfRef">Example: SDP Answer</a>
          </h4>
<p id="section-7.3.4-1">
          The example below shows an answer based on the corresponding offer in
          <a href="#sec-sdp-oa-ino-ex" class="xref">Section 7.2.2</a>.
          The answerer accepts both bundled "m=" sections within the created 
          BUNDLE group. The audio "m=" section is the answerer-tagged "m=" section, indicated
          by placing the identification-tag associated with the "m=" section
          (answerer BUNDLE-tag) first in the SDP 'group:BUNDLE' attribute
          identification-id list.<a href="#section-7.3.4-1" class="pilcrow">¶</a></p>
<p id="section-7.3.4-2" class="keepWithNext">SDP Answer<a href="#section-7.3.4-2" class="pilcrow">¶</a></p>
<div id="section-7.3.4-3">
<pre class="lang-sdp sourcecode">
  v=0
  o=bob 2808844564 2808844564 IN IP6 2001:db8::1
  s=
  c=IN IP6 2001:db8::1
  t=0 0
  a=group:BUNDLE foo bar

  m=audio 20000 RTP/AVP 0
  b=AS:200
  a=mid:foo
  a=rtcp-mux
  a=rtpmap:0 PCMU/8000
  a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

  m=video 20000 RTP/AVP 32
  b=AS:1000
  a=mid:bar
  a=rtpmap:32 MPV/90000
  a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
</pre><a href="#section-7.3.4-3" class="pilcrow">¶</a>
</div>
</section>
</div>
<div id="sec-sdp-oa-ans-8843">
<section id="section-7.3.5">
          <h4 id="name-rfc-8843-considerations">
<a href="#section-7.3.5" class="section-number selfRef">7.3.5. </a><a href="#name-rfc-8843-considerations" class="section-name selfRef">RFC 8843 Considerations</a>
          </h4>
<p id="section-7.3.5-1">
            In <span>[<a href="#RFC8843" class="xref">RFC8843</a>]</span>, instead of assigning the offerer BUNDLE address:port to
            each "m=" section within the BUNDLE group when modifying the session
            (<a href="#sec-sdp-oa-mod" class="xref">Section 7.5</a>), the offerer only assigned the offerer BUNDLE
            address:port to the offerer-tagged "m=" section.  For every other "m=" section
            within the BUNDLE group, the offerer included an SDP 'bundle-only' attribute in,
            and assigned a zero port value to, the "m=" section. The way an answerer compliant
            with this specification processes such offer is considered an implementation issue
            (e.g., based on whether the answerer needs to be backward compatible with offerers compliant with <span>[<a href="#RFC8843" class="xref">RFC8843</a>]</span>) and is outside the scope of this specification. The example below
            shows such an SDP Offer:<a href="#section-7.3.5-1" class="pilcrow">¶</a></p>
<p id="section-7.3.5-2" class="keepWithNext">SDP Offer<a href="#section-7.3.5-2" class="pilcrow">¶</a></p>
<div id="section-7.3.5-3">
<pre class="lang-sdp sourcecode">
  v=0
  o=alice 2890844526 2890844526 IN IP6 2001:db8::3
  s=
  c=IN IP6 2001:db8::3
  t=0 0
  a=group:BUNDLE foo bar

  m=audio 10000 RTP/AVP 0 8 97
  b=AS:200
  a=mid:foo
  a=rtcp-mux
  a=rtpmap:0 PCMU/8000
  a=rtpmap:8 PCMA/8000
  a=rtpmap:97 iLBC/8000
  a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

  m=video 0 RTP/AVP 31 32
  b=AS:1000
  a=mid:bar
  a=bundle-only
  a=rtpmap:31 H261/90000
  a=rtpmap:32 MPV/90000
  a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
</pre><a href="#section-7.3.5-3" class="pilcrow">¶</a>
</div>
</section>
</div>
</section>
</div>
<div id="sec-sdp-oa-off-ans">
<section id="section-7.4">
        <h3 id="name-offerer-processing-of-the-s">
<a href="#section-7.4" class="section-number selfRef">7.4. </a><a href="#name-offerer-processing-of-the-s" class="section-name selfRef">Offerer Processing of the SDP Answer</a>
        </h3>
<p id="section-7.4-1">
        When an offerer receives an answer, if the answer contains a BUNDLE group, the offerer
        <span class="bcp14">MUST</span> check that any bundled "m=" section in the answer was indicated as bundled in the
        corresponding offer (for the same BUNDLE group). If there is no mismatch, the offerer <span class="bcp14">MUST</span> apply the properties (BUNDLE address:port,
        BUNDLE attributes, etc.) of the offerer-tagged "m=" section (selected by the answerer; see 
        <a href="#sec-sdp-oa-ans-off" class="xref">Section 7.3.1</a>) to each bundled "m=" section
        within the BUNDLE group.<a href="#section-7.4-1" class="pilcrow">¶</a></p>
<p style="margin-left: 1.5em" id="section-7.4-2">
        NOTE: As the answerer might reject one or more bundled "m=" sections in an initial BUNDLE offer 
        or move a bundled "m=" section out of a BUNDLE group, a given bundled "m=" section in the 
        offer might not be indicated as bundled in the corresponding answer.<a href="#section-7.4-2" class="pilcrow">¶</a></p>
<p id="section-7.4-3">
        If the answer does not contain a BUNDLE group, the offerer <span class="bcp14">MUST</span> process the answer
        as a normal answer.<a href="#section-7.4-3" class="pilcrow">¶</a></p>
<div id="sec-sdp-oa-off-8843">
<section id="section-7.4.1">
          <h4 id="name-rfc-8843-considerations-2">
<a href="#section-7.4.1" class="section-number selfRef">7.4.1. </a><a href="#name-rfc-8843-considerations-2" class="section-name selfRef">RFC 8843 Considerations</a>
          </h4>
<p id="section-7.4.1-1">
            In <span>[<a href="#RFC8843" class="xref">RFC8843</a>]</span>, instead of assigning the answerer BUNDLE address:port to
            each "m=" section within the BUNDLE group when generating the
            SDP Answer (<a href="#sec-sdp-oa-ans" class="xref">Section 7.3</a>), the answerer only assigned the
            answerer BUNDLE address:port to the answerer-tagged "m=" section.
            For every other "m=" section within the BUNDLE group, the answerer
            included an SDP 'bundle-only' attribute in, and assigned a zero port
            value to, the "m=" section. The way an offerer compliant
            with this specification processes such an SDP Answer is considered an implementation issue
            (e.g., based on whether the answerer needs to be backward compatible with offerers compliant with <span>[<a href="#RFC8843" class="xref">RFC8843</a>]</span>) and is outside the scope of this specification. The example below
            shows such an SDP Answer:<a href="#section-7.4.1-1" class="pilcrow">¶</a></p>
<p id="section-7.4.1-2" class="keepWithNext">SDP Answer<a href="#section-7.4.1-2" class="pilcrow">¶</a></p>
<div id="section-7.4.1-3">
<pre class="lang-sdp sourcecode">
  v=0
  o=bob 2808844564 2808844564 IN IP6 2001:db8::1
  s=
  c=IN IP6 2001:db8::1
  t=0 0
  a=group:BUNDLE foo bar

  m=audio 20000 RTP/AVP 0
  b=AS:200
  a=mid:foo
  a=rtcp-mux
  a=rtpmap:0 PCMU/8000
  a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

  m=video 0 RTP/AVP 32
  b=AS:1000
  a=mid:bar
  a=bundle-only
  a=rtpmap:32 MPV/90000
  a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
</pre><a href="#section-7.4.1-3" class="pilcrow">¶</a>
</div>
</section>
</div>
</section>
</div>
<div id="sec-sdp-oa-mod">
<section id="section-7.5">
        <h3 id="name-modifying-the-session">
<a href="#section-7.5" class="section-number selfRef">7.5. </a><a href="#name-modifying-the-session" class="section-name selfRef">Modifying the Session</a>
        </h3>
<p id="section-7.5-1">
          When a BUNDLE group has been previously negotiated and an offerer generates a subsequent
          offer, the offerer <span class="bcp14">MUST</span>:<a href="#section-7.5-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-7.5-2.1">
              Pick one bundled "m=" section as the offerer-tagged "m=" section. The offerer
              can pick either the "m=" section that was previously selected by the answerer
              as the offerer-tagged "m=" section or another bundled "m=" section within the
              BUNDLE group;<a href="#section-7.5-2.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.5-2.2">
              Assign a BUNDLE address:port (previously negotiated or newly
              suggested) to the offerer-tagged "m=" section and to every other
              bundled "m=" section within the BUNDLE group;<a href="#section-7.5-2.2" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.5-2.3">
              Include SDP attributes in the bundled "m=" sections following the rules in 
              <a href="#sec-sdp-oa-cat" class="xref">Section 7.1.3</a>;<a href="#section-7.5-2.3" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.5-2.4">
              Include an SDP 'group:BUNDLE' attribute in the offer; and<a href="#section-7.5-2.4" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.5-2.5">
              Place the identification-tag of each bundled "m=" section in the
              SDP 'group:BUNDLE' attribute identification-tag list. The offerer BUNDLE-tag
              indicates the offerer-tagged "m=" section.<a href="#section-7.5-2.5" class="pilcrow">¶</a>
</li>
        </ul>
<p id="section-7.5-3">  
          The offerer <span class="bcp14">MUST NOT</span> pick a given bundled "m=" section as the offerer-tagged "m=" section if:<a href="#section-7.5-3" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-7.5-4.1">
              The offerer wants to move the "m=" section out of the BUNDLE group
              (<a href="#sec-sdp-oa-mod-mov" class="xref">Section 7.5.2</a>), or<a href="#section-7.5-4.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-7.5-4.2">
              The offerer wants to disable the "m=" section (<a href="#sec-sdp-oa-mod-dis" class="xref">Section 7.5.3</a>).<a href="#section-7.5-4.2" class="pilcrow">¶</a>
</li>
        </ul>
<p id="section-7.5-5">
          The offerer can modify the offerer BUNDLE address:port, add and remove SDP attributes, or modify
          SDP attribute values in the subsequent offer. Changes to the offerer BUNDLE address:port and the
          offerer BUNDLE attributes will (if the offer is accepted by the answerer) be applied to each 
          bundled "m=" section within the BUNDLE group.<a href="#section-7.5-5" class="pilcrow">¶</a></p>
<div id="sec-sdp-oa-mod-add">
<section id="section-7.5.1">
          <h4 id="name-adding-a-media-description-">
<a href="#section-7.5.1" class="section-number selfRef">7.5.1. </a><a href="#name-adding-a-media-description-" class="section-name selfRef">Adding a Media Description to a BUNDLE Group</a>
          </h4>
<p id="section-7.5.1-1">
          When an offerer generates a subsequent offer in which it wants to add a bundled "m=" section to
          a previously negotiated BUNDLE group, the offerer follows the procedures in <a href="#sec-sdp-oa-mod" class="xref">Section 7.5</a>. The offerer picks either the added "m=" section or an "m=" section
          previously added to the BUNDLE group as the offerer-tagged "m=" section.<a href="#section-7.5.1-1" class="pilcrow">¶</a></p>
<p style="margin-left: 1.5em" id="section-7.5.1-2">
          NOTE: As described in <a href="#sec-sdp-oa-ans-mov" class="xref">Section 7.3.2</a>, the answerer cannot move the added "m=" section 
          out of the BUNDLE group in its answer. If the answerer wants to move the "m=" section out of the BUNDLE group, it will have to first accept 
          it into the BUNDLE group in the answer and then send a subsequent offer where the "m=" section is moved out of the BUNDLE group
          (<a href="#sec-sdp-oa-mod-mov" class="xref">Section 7.5.2</a>).<a href="#section-7.5.1-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-sdp-oa-mod-mov">
<section id="section-7.5.2">
          <h4 id="name-moving-a-media-description-o">
<a href="#section-7.5.2" class="section-number selfRef">7.5.2. </a><a href="#name-moving-a-media-description-o" class="section-name selfRef">Moving a Media Description Out of a BUNDLE Group</a>
          </h4>
<p id="section-7.5.2-1">
          When an offerer generates a subsequent offer in which it wants to remove a bundled "m=" section from
          a BUNDLE group, the offerer:<a href="#section-7.5.2-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-7.5.2-2.1">
              <span class="bcp14">MUST</span> assign a unique address:port to the "m=" section;<a href="#section-7.5.2-2.1" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-7.5.2-2.2">
              <span class="bcp14">MUST</span> include SDP attributes in the "m=" section following the
            normal offer/answer rules for each attribute;<a href="#section-7.5.2-2.2" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-7.5.2-2.3">
              <span class="bcp14">MUST NOT</span> place the identification-tag associated with the "m=" section in
            the SDP 'group:BUNDLE' attribute identification-tag list associated with
            the BUNDLE group; and<a href="#section-7.5.2-2.3" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-7.5.2-2.4">
              <span class="bcp14">MUST NOT</span> assign an SDP 'bundle-only' attribute to the "m=" section.<a href="#section-7.5.2-2.4" class="pilcrow">¶</a>
</li>
          </ul>
<p id="section-7.5.2-3">
          For the other bundled "m=" sections within the BUNDLE group, the offerer follows the procedures 
          in <a href="#sec-sdp-oa-mod" class="xref">Section 7.5</a>.<a href="#section-7.5.2-3" class="pilcrow">¶</a></p>
<p id="section-7.5.2-4">
          An offerer <span class="bcp14">MUST NOT</span> move an "m=" section from one BUNDLE group to another within a 
          single offer. If the offerer wants to move an "m=" section from one BUNDLE group to 
          another, it <span class="bcp14">MUST</span> first move the BUNDLE group out of the current BUNDLE group and then 
          generate a second offer where the "m=" section is added to another BUNDLE group 
          (<a href="#sec-sdp-oa-mod-add" class="xref">Section 7.5.1</a>).<a href="#section-7.5.2-4" class="pilcrow">¶</a></p>
<p id="section-7.5.2-5">
          <a href="#sec-example-off-mov" class="xref">Section 18.4</a>
          shows an example of an offer for moving an "m=" section out of a BUNDLE group.<a href="#section-7.5.2-5" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-sdp-oa-mod-dis">
<section id="section-7.5.3">
          <h4 id="name-disabling-a-media-descripti">
<a href="#section-7.5.3" class="section-number selfRef">7.5.3. </a><a href="#name-disabling-a-media-descripti" class="section-name selfRef">Disabling a Media Description in a BUNDLE Group</a>
          </h4>
<p id="section-7.5.3-1">
          When an offerer generates a subsequent offer in which it wants to disable a bundled "m=" section from
          a BUNDLE group, the offerer:<a href="#section-7.5.3-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-7.5.3-2.1">
              <span class="bcp14">MUST</span> assign a zero port value to the "m=" section, following the procedures 
            in <span>[<a href="#RFC4566" class="xref">RFC4566</a>]</span>;<a href="#section-7.5.3-2.1" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-7.5.3-2.2">
              <span class="bcp14">MUST NOT</span> place the identification-tag associated with the "m=" section in
            the SDP 'group:BUNDLE' attribute identification-tag list associated with
            the BUNDLE group; and<a href="#section-7.5.3-2.2" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-7.5.3-2.3">
              <span class="bcp14">MUST NOT</span> assign an SDP 'bundle-only' attribute to the "m=" section.<a href="#section-7.5.3-2.3" class="pilcrow">¶</a>
</li>
          </ul>
<p id="section-7.5.3-3">
          For the other bundled "m=" sections within the BUNDLE group, the offerer follows the procedures 
          in <a href="#sec-sdp-oa-mod" class="xref">Section 7.5</a>.<a href="#section-7.5.3-3" class="pilcrow">¶</a></p>
<p id="section-7.5.3-4">
          <a href="#sec-example-off-dis" class="xref">Section 18.5</a>
          shows an example of an offer and answer for disabling an "m=" section within a
          BUNDLE group.<a href="#section-7.5.3-4" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="sec-sdp-oa-sip">
<section id="section-7.6">
        <h3 id="name-3pcc-considerations">
<a href="#section-7.6" class="section-number selfRef">7.6. </a><a href="#name-3pcc-considerations" class="section-name selfRef">3PCC Considerations</a>
        </h3>
<p id="section-7.6-1">
  In some third-party call control (3PCC) scenarios, a new session will be
   established between an endpoint that is currently part of an ongoing
   session and an endpoint that is not currently part of an ongoing
   session.  In this situation, the endpoint that is not part of a
   session, while expecting an initial offer, can receive an SDP offer
   created as a subsequent offer.  The text below describes how this can
   occur with the Session Initiation Protocol (SIP) <span>[<a href="#RFC3261" class="xref">RFC3261</a>]</span>.<a href="#section-7.6-1" class="pilcrow">¶</a></p>
<p id="section-7.6-2">
          SIP <span>[<a href="#RFC3261" class="xref">RFC3261</a>]</span> allows 
          a User Agent Client (UAC) to send a re-INVITE request without an SDP body (sometimes referred to as an "empty re-INVITE").
          In such cases, the User Agent Server (UAS) will include an SDP Offer in the associated 200 (OK) response; when the UAS is a part of an
   ongoing SIP session, this offer will be a subsequent offer.  This
   offer will be received by the 3PCC controller (UAC) and then
   forwarded to another User Agent (UA).  When that UA is not part of an
   ongoing SIP session, as noted above, it will process the offer as an
   initial SDP offer.<a href="#section-7.6-2" class="pilcrow">¶</a></p>
<p id="section-7.6-3">
   When the BUNDLE mechanism is used, an initial BUNDLE offer is
   constructed using different rules than subsequent BUNDLE offers, and
   it cannot be assumed that a UA is able to correctly process a
   subsequent BUNDLE offer as an initial BUNDLE offer.  Therefore, the
   3PCC controller <span class="bcp14">SHOULD</span> take action to mitigate this problem,
   e.g., rewrite the subsequent BUNDLE offer into a valid initial BUNDLE
   offer (<a href="#sec-sdp-oa-ino" class="xref">Section 7.2</a>), before it forwards the BUNDLE offer to a UA.<a href="#section-7.6-3" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="sec-protocol-id">
<section id="section-8">
      <h2 id="name-protocol-identification">
<a href="#section-8" class="section-number selfRef">8. </a><a href="#name-protocol-identification" class="section-name selfRef">Protocol Identification</a>
      </h2>
<p id="section-8-1">
      Each "m=" section within a BUNDLE group <span class="bcp14">MUST</span> use the same transport-
      layer protocol. If bundled "m=" sections use different upper-layer protocols 
      on top of the transport-layer protocol, there <span class="bcp14">MUST</span> exist a publicly
      available specification that describes how a mechanism associates 
      received data with the correct protocol for this particular protocol combination.<a href="#section-8-1" class="pilcrow">¶</a></p>
<p id="section-8-2">
      In addition, if received data can be associated with more than
      one bundled "m=" section, there <span class="bcp14">MUST</span> exist a publicly available
      specification that describes a mechanism for associating the
      received data with the correct "m=" section.<a href="#section-8-2" class="pilcrow">¶</a></p>
<p id="section-8-3">
      This document describes a mechanism to identify the
      protocol of received data among the Session Traversal Utilities for NAT (STUN), Datagram Transport Layer Security (DTLS), and the Secure Real-time Transport Protocol (SRTP)
      (in any combination) when UDP is used as a transport-layer protocol,
      but it does not describe how to identify different protocols transported on
      DTLS. 
      While the mechanism is generally applicable to other protocols and
      transport-layer protocols, any such use requires further specification 
      that encompasses how to multiplex multiple protocols on a given transport-layer protocol
      and how to associate received data with the correct protocols.<a href="#section-8-3" class="pilcrow">¶</a></p>
<div id="sec-packets-id-sds">
<section id="section-8.1">
        <h3 id="name-stun-dtls-and-srtp">
<a href="#section-8.1" class="section-number selfRef">8.1. </a><a href="#name-stun-dtls-and-srtp" class="section-name selfRef">STUN, DTLS, and SRTP</a>
        </h3>
<p id="section-8.1-1">
        <span><a href="https://www.rfc-editor.org/rfc/rfc5764#section-5.1.2" class="relref">Section 5.1.2</a> of [<a href="#RFC5764" class="xref">RFC5764</a>]</span> describes a
        mechanism to identify the protocol of a received packet among the STUN, DTLS, and 
        SRTP protocols (in any combination).
        If an offer or answer includes a bundled "m=" section that represents these protocols, the offerer
        or answerer <span class="bcp14">MUST</span> support the mechanism described in <span>[<a href="#RFC5764" class="xref">RFC5764</a>]</span>, and no explicit negotiation is required in order to indicate support
        and usage of the mechanism.<a href="#section-8.1-1" class="pilcrow">¶</a></p>
<p id="section-8.1-2">
        <span>[<a href="#RFC5764" class="xref">RFC5764</a>]</span> does not describe how to identify
        different protocols transported on DTLS, only how to identify the DTLS protocol itself. If
        multiple protocols are transported on DTLS, there <span class="bcp14">MUST</span> exist a specification describing a
        mechanism for identifying each individual protocol. In addition, if a received DTLS packet
        can be associated with more than one "m=" section, there <span class="bcp14">MUST</span> exist a specification that
        describes a mechanism for associating the received DTLS packets with the correct "m=" section.<a href="#section-8.1-2" class="pilcrow">¶</a></p>
<p id="section-8.1-3">
        <a href="#sec-rtp-pt" class="xref">Section 9.2</a> describes how to
        associate the packets in a received SRTP stream with the correct "m=" section.<a href="#section-8.1-3" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="sec-rtp">
<section id="section-9">
      <h2 id="name-rtp-considerations">
<a href="#section-9" class="section-number selfRef">9. </a><a href="#name-rtp-considerations" class="section-name selfRef">RTP Considerations</a>
      </h2>
<div id="sec-rtp-sessions">
<section id="section-9.1">
        <h3 id="name-single-rtp-session">
<a href="#section-9.1" class="section-number selfRef">9.1. </a><a href="#name-single-rtp-session" class="section-name selfRef">Single RTP Session</a>
        </h3>
<p id="section-9.1-1">
            All RTP-based media within a single BUNDLE group belong to a
            single RTP session <span>[<a href="#RFC3550" class="xref">RFC3550</a>]</span>.<a href="#section-9.1-1" class="pilcrow">¶</a></p>
<p id="section-9.1-2">
            Since a single BUNDLE transport is used for sending and receiving bundled media,
            the symmetric RTP mechanism <span>[<a href="#RFC4961" class="xref">RFC4961</a>]</span>
          <span class="bcp14">MUST</span> be used for RTP-based bundled media.<a href="#section-9.1-2" class="pilcrow">¶</a></p>
<p id="section-9.1-3">
            Since a single RTP session is used for each BUNDLE group, all
            "m=" sections representing RTP-based media within a BUNDLE group will
            share a single synchronization source (SSRC) numbering space <span>[<a href="#RFC3550" class="xref">RFC3550</a>]</span>.<a href="#section-9.1-3" class="pilcrow">¶</a></p>
<p id="section-9.1-4">
            The following rules and restrictions apply for a single RTP
            session:<a href="#section-9.1-4" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-9.1-5.1">
                A specific payload type value can be used in multiple bundled "m=" sections
                only if each codec associated with the payload type number shares an identical
                codec configuration (<a href="#sec-rtp-sessions-pt" class="xref">Section 9.1.1</a>).<a href="#section-9.1-5.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.1-5.2">
                The proto value in each bundled RTP-based "m=" section <span class="bcp14">MUST</span> be identical
                (e.g., RTP/AVPF).<a href="#section-9.1-5.2" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.1-5.3">
                The RTP MID header extension <span class="bcp14">MUST</span> be enabled by including
                an SDP 'extmap' attribute <span>[<a href="#RFC8285" class="xref">RFC8285</a>]</span>,
                with a 'urn:ietf:params:rtp- hdrext:sdes:mid' URI value defined in this specification in each
                bundled RTP-based "m=" section in every offer and answer.<a href="#section-9.1-5.3" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.1-5.4">
                A given SSRC <span class="bcp14">MUST NOT</span> transmit RTP packets using payload types that
                originate from different bundled "m=" sections.<a href="#section-9.1-5.4" class="pilcrow">¶</a>
</li>
        </ul>
<p style="margin-left: 1.5em" id="section-9.1-6">
            NOTE: The last bullet above is to avoid sending multiple media types
            from the same SSRC. If transmission of multiple media types is done
            with time overlap, RTP and RTCP fail to function. Even if done in the proper sequence, this causes RTP timestamp rate switching issues
            <span>[<a href="#RFC7160" class="xref">RFC7160</a>]</span>. However,
            once an SSRC has left the RTP session (by sending an RTCP BYE packet),
            that SSRC can be reused by another source (possibly associated
            with a different bundled "m=" section) after a delay of 5 RTCP reporting intervals
            (the delay is to ensure the SSRC has timed out in case the RTCP BYE
            packet was lost <span>[<a href="#RFC3550" class="xref">RFC3550</a>]</span>).<a href="#section-9.1-6" class="pilcrow">¶</a></p>
<p id="section-9.1-7">
            <span>[<a href="#RFC7657" class="xref">RFC7657</a>]</span> defines Differentiated Services 
            (Diffserv) considerations for RTP-based bundled media sent using a mixture of Diffserv Codepoints.<a href="#section-9.1-7" class="pilcrow">¶</a></p>
<div id="sec-rtp-sessions-pt">
<section id="section-9.1.1">
          <h4 id="name-payload-type-pt-value-reuse">
<a href="#section-9.1.1" class="section-number selfRef">9.1.1. </a><a href="#name-payload-type-pt-value-reuse" class="section-name selfRef">Payload Type (PT) Value Reuse</a>
          </h4>
<p id="section-9.1.1-1">
            Multiple bundled "m=" sections might describe RTP-based media. As all RTP-based
            media associated with a BUNDLE group belong to the same RTP session, in order
            for a given payload type value to be used inside more than one bundled "m=" section,
            all codecs associated with the payload type number <span class="bcp14">MUST</span> share an identical codec
            configuration. This means that the codecs <span class="bcp14">MUST</span> share the same media type,
            encoding name, clock rate, and any parameter that can affect the codec configuration
            and packetization. <span>[<a href="#RFC8859" class="xref">RFC8859</a>]</span> lists SDP attributes whose attribute
            values are required to be identical for all codecs that use the
     same payload type value.<a href="#section-9.1.1-1" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="sec-rtp-pt">
<section id="section-9.2">
        <h3 id="name-associating-rtp-rtcp-stream">
<a href="#section-9.2" class="section-number selfRef">9.2. </a><a href="#name-associating-rtp-rtcp-stream" class="section-name selfRef">Associating RTP/RTCP Streams with the Correct SDP Media Description</a>
        </h3>
<p id="section-9.2-1">
            As described in <span>[<a href="#RFC3550" class="xref">RFC3550</a>]</span>, RTP packets are associated with RTP
            streams <span>[<a href="#RFC7656" class="xref">RFC7656</a>]</span>. Each RTP stream is identified by an SSRC
            value, and each RTP packet includes an SSRC field that is
            used to associate the packet with the correct RTP stream.
            RTCP packets also use SSRCs to identify which RTP streams the
            packet relates to. However, an RTCP packet can contain multiple SSRC
            fields in the course of providing feedback or reports on different RTP
            streams; therefore, they can be associated with multiple such streams.<a href="#section-9.2-1" class="pilcrow">¶</a></p>
<p id="section-9.2-2">
            In order to be able to process received RTP/RTCP packets
            correctly, it <span class="bcp14">MUST</span> be possible to associate an RTP stream with
            the correct "m=" section, as the "m=" section and SDP attributes
            associated with the "m=" section contain information needed to
            process the packets.<a href="#section-9.2-2" class="pilcrow">¶</a></p>
<p id="section-9.2-3">
            As all RTP streams associated with a BUNDLE group use the
            same transport for sending and receiving RTP/RTCP
            packets, the local address:port combination part of the transport
            cannot be used to associate an RTP stream with the correct "m=" section. 
            In addition, multiple RTP streams might be associated with the same "m="
            section.<a href="#section-9.2-3" class="pilcrow">¶</a></p>
<p id="section-9.2-4">
            An offerer and answerer can inform each other which SSRC
            values they will use for an RTP stream by using the SDP 'ssrc'
            attribute <span>[<a href="#RFC5576" class="xref">RFC5576</a>]</span>.
            However, an offerer will not know which SSRC values the
            answerer will use until the offerer has received the answer
            providing that information. Due to this, before the offerer has
            received the answer, the offerer will not be able to associate
            an RTP stream with the correct "m=" section using the SSRC value
            associated with the RTP stream. In addition, the offerer and
            answerer may start using new SSRC values mid-session, without
            informing each other about using the SDP 'ssrc' attribute.<a href="#section-9.2-4" class="pilcrow">¶</a></p>
<p id="section-9.2-5">
            In order for an offerer and answerer to always be able to
            associate an RTP stream with the correct "m=" section, the offerer
            and answerer using the BUNDLE extension <span class="bcp14">MUST</span> support the
            mechanism defined in <a href="#sec-receiver-id" class="xref">Section 15</a>, 
            where the offerer and answerer insert the identification-tag associated 
            with an "m=" section (provided by the remote peer) into RTP and RTCP 
            packets associated with a BUNDLE group.<a href="#section-9.2-5" class="pilcrow">¶</a></p>
<p id="section-9.2-6">
             When using this mechanism, the mapping from an SSRC to an
             identification-tag is carried in RTP header extensions or RTCP SDES
             packets, as specified in <a href="#sec-receiver-id" class="xref">Section 15</a>. 
             Since a compound RTCP packet can contain multiple RTCP SDES packets 
             and each RTCP SDES packet can contain multiple chunks, a single RTCP 
             packet can contain several mappings of SSRC to identification-tag. The 
             offerer and answerer maintain tables used for routing that are updated 
             each time an RTP/RTCP packet contains new information that affects how 
             packets are to be routed.<a href="#section-9.2-6" class="pilcrow">¶</a></p>
<p id="section-9.2-7">
            However, some legacy implementations may not include this identification-tag
            in their RTP and RTCP traffic when using the BUNDLE mechanism and
            instead use a mechanism based on the payload type to associate RTP streams
            with SDP "m=" sections.  In this situation, each "m=" section needs to
            use unique payload type values in order for the payload type to be a
            reliable indicator of the relevant "m=" section for the RTP stream. If an
            implementation fails to ensure unique payload type values, it will be 
            impossible to associate the RTP stream using that payload type value
            to a particular "m=" section. 
            Note that when using the payload type to associate RTP streams with
            "m=" sections, an RTP stream, identified by its SSRC, will be mapped
            to an "m=" section when the first packet of that RTP stream is
            received, and the mapping will not be changed even if the payload
            type used by that RTP stream changes.  In other words, the SSRC
            cannot "move" to a different "m=" section simply by changing the
            payload type.<a href="#section-9.2-7" class="pilcrow">¶</a></p>
<p id="section-9.2-8">
             Applications can implement RTP stacks in different
             ways. The algorithm below details one way that RTP streams can be
             associated with "m=" sections, but it is not meant to be prescriptive
             about exactly how an RTP stack needs to be implemented.
             Applications <span class="bcp14">MAY</span> use any algorithm that achieves equivalent results to
             those described in the algorithm below.<a href="#section-9.2-8" class="pilcrow">¶</a></p>
<p id="section-9.2-9">
             To prepare to associate RTP streams with the correct
             "m=" section, the following steps <span class="bcp14">MUST</span> be followed for each BUNDLE group:<a href="#section-9.2-9" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-9.2-10.1">
                 Construct a table mapping a MID to an "m=" section for each "m="
                 section in this BUNDLE group. Note that an "m=" section may only
                 have one MID.<a href="#section-9.2-10.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.2-10.2">
                 Construct a table mapping SSRCs of incoming RTP streams to an "m=" section for
                 each "m=" section in this BUNDLE group and for each SSRC
                 configured for receiving in that "m=" section.<a href="#section-9.2-10.2" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.2-10.3">
                 Construct a table mapping the SSRC of each outgoing RTP stream 
                 to an "m=" section for each "m=" section in this BUNDLE group and for each SSRC
                 configured for sending in that "m=" section.<a href="#section-9.2-10.3" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.2-10.4">
                 Construct a table mapping a payload type to an "m=" section for
                 each "m=" section in the BUNDLE group and for each payload type
                 configured for receiving in that "m=" section. If any payload
                 type is configured for receiving in more than one "m=" section
                 in the BUNDLE group, do not include it in the table, as it
                 cannot be used to uniquely identify an "m=" section.<a href="#section-9.2-10.4" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.2-10.5">
                 Note that for each of these tables, there can only be one
                 mapping for any given key (MID, SSRC, or PT). In other
                 words, the tables are not multimaps.<a href="#section-9.2-10.5" class="pilcrow">¶</a>
</li>
        </ul>
<p id="section-9.2-11">
             As "m=" sections are added or removed from the BUNDLE groups or
             their configurations are changed, the tables above <span class="bcp14">MUST</span> also be
             updated.<a href="#section-9.2-11" class="pilcrow">¶</a></p>
<p id="section-9.2-12">
             When an RTP packet is received, it <span class="bcp14">MUST</span> be delivered to the RTP
             stream corresponding to its SSRC. That RTP stream <span class="bcp14">MUST</span> then be
             associated with the correct "m=" section within a BUNDLE group for
             additional processing, according to the following steps:<a href="#section-9.2-12" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-9.2-13.1">
                 If the MID associated with the RTP stream is not in the
                 table mapping a MID to an "m=" section, then the RTP stream is not
                 decoded, and the payload data is discarded.<a href="#section-9.2-13.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.2-13.2">
                 If the packet has a MID and the packet's extended sequence number
                 is greater than that of the last MID update, as discussed in
                 <span>[<a href="#RFC7941" class="xref">RFC7941</a>], <a href="https://www.rfc-editor.org/rfc/rfc7941#section-4.2.6" class="relref">Section 4.2.6</a></span>, update the MID associated
                 with the RTP stream to match the MID carried in the RTP packet and then
                 update the mapping tables to include an entry that maps the SSRC of
                 that RTP stream to the "m=" section for that MID.<a href="#section-9.2-13.2" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.2-13.3">
                 If the SSRC of the RTP stream is in the incoming SSRC
                 mapping table, check that the payload type used by the RTP
                 stream matches a payload type included in the matching
                 "m=" section. If so, associate the RTP stream with that
                 "m=" section. Otherwise, the RTP stream is not decoded, and the
                 payload data is discarded.<a href="#section-9.2-13.3" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.2-13.4">
                 If the payload type used by the RTP stream is in the
                 payload type table, update the incoming SSRC mapping table
                 to include an entry that maps the RTP stream's SSRC to the
                 "m=" section for that payload type. Associate the RTP stream
                 with the corresponding "m=" section.<a href="#section-9.2-13.4" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.2-13.5">
                 Otherwise, mark the RTP stream as "not for decoding" and
                 discard the payload.<a href="#section-9.2-13.5" class="pilcrow">¶</a>
</li>
        </ul>
<p id="section-9.2-14">
           If the RTP packet contains one or more contributing source (CSRC)
           identifiers, then each CSRC is looked up in the incoming SSRC table,
           and a copy of the RTP packet is associated with the corresponding
           "m=" section for additional processing.<a href="#section-9.2-14" class="pilcrow">¶</a></p>
<p id="section-9.2-15">
              For each RTCP packet received (including each RTCP
              packet that is part of a compound RTCP packet), the
              packet is processed as usual by the RTP layer, then
              associated with the appropriate "m=" sections and
              processed for the RTP streams represented by those "m="
              sections.  This routing is type dependent, as each kind
              of RTCP packet has its own mechanism for associating it
              with the relevant RTP streams.<a href="#section-9.2-15" class="pilcrow">¶</a></p>
<p id="section-9.2-16">
              RTCP packets that cannot be associated with an
              appropriate "m=" section <span class="bcp14">MUST</span> still be
              processed as usual by the RTP layer, which updates the
              metadata associated with the corresponding RTP
              streams. This situation can occur with certain
              multiparty RTP topologies or when RTCP packets are sent
              containing a subset of the SDES information.<a href="#section-9.2-16" class="pilcrow">¶</a></p>
<p id="section-9.2-17">
              Additional rules for processing various types of RTCP packets are
              explained below.<a href="#section-9.2-17" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-9.2-18.1">
                If the RTCP packet is of type SDES, for each chunk in the packet
                whose SSRC is found in the incoming SSRC table, deliver a copy
                of the SDES packet to the "m=" section associated with that SSRC.
                In addition, for any SDES MID items contained in these chunks,
                if the MID is found in the table mapping a MID to an "m=" section,
                update the incoming SSRC table to include an entry that
                maps the RTP stream associated with the chunk's SSRC to the "m=" section 
                associated with that MID, unless the packet is older than the packet 
                that most recently updated the mapping for this SSRC, as discussed in
                <span>[<a href="#RFC7941" class="xref">RFC7941</a>], <a href="https://www.rfc-editor.org/rfc/rfc7941#section-4.2.6" class="relref">Section 4.2.6</a></span>.<a href="#section-9.2-18.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.2-18.2">
                Note that if an SDES packet is received as part of a
                compound RTCP packet, the SSRC to "m=" section mapping
                might not exist until the SDES packet is handled
                (e.g., in the case where RTCP for a source is received
                before any RTP packets).

 Therefore, it can be
                beneficial for an implementation to delay RTCP packet
                routing, such that it either prioritizes processing of
                the SDES item to generate or update the mapping or
                buffers the RTCP information that needs to be routed
                until the SDES item(s) has been processed.

 If the
                implementation is unable to follow this
                recommendation, the consequence could be that some
                RTCP information from this particular RTCP compound
                packet is not provided to higher layers.  The impact
                from this is likely minor when this information
                relates to a future incoming RTP stream.<a href="#section-9.2-18.2" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.2-18.3">
                If the RTCP packet is of type BYE, it indicates that the RTP streams
                referenced in the packet are ending. Therefore, for each SSRC
                indicated in the packet that is found in the incoming SSRC table,
                first deliver a copy of the BYE packet to the "m=" section associated
                with that SSRC, and then remove the entry for that SSRC from the
                incoming SSRC table after an appropriate delay to
                account for "straggler packets", as specified in <span>[<a href="#RFC3550" class="xref">RFC3550</a>], <a href="https://www.rfc-editor.org/rfc/rfc3550#section-6.2.1" class="relref">Section 6.2.1</a></span>.<a href="#section-9.2-18.3" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.2-18.4">
                If the RTCP packet is of type sender report (SR) or receiver report (RR), for each report block in
                the report whose "SSRC of source" is found in the outgoing
                SSRC table, deliver a copy of the SR or RR packet to the "m=" section
                associated with that SSRC. In addition, if the packet is of type
                SR and the sender SSRC for the packet is found in the
                incoming SSRC table, deliver a copy of the SR packet to the "m=" section
                associated with that SSRC.<a href="#section-9.2-18.4" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.2-18.5">
                If the implementation supports the RTCP Extended Report (XR) and the packet is of
                type XR, as defined in <span>[<a href="#RFC3611" class="xref">RFC3611</a>]</span>,
                for each report block in the report whose "SSRC of source"
                is found in the outgoing SSRC table, deliver a copy of the
                XR packet to the "m=" section associated with that SSRC.
                In addition, if the sender SSRC for the packet is found in the
                incoming SSRC table, deliver a copy of the XR packet to the "m=" section
                associated with that SSRC.<a href="#section-9.2-18.5" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.2-18.6">
                If the RTCP packet is a feedback message of type RTPFB (transport-layer FB
   message) or PSFB (payload-specific FB message),
                as defined in <span>[<a href="#RFC4585" class="xref">RFC4585</a>]</span>, it will contain a
                media source SSRC, and this SSRC is used for routing certain
                subtypes of feedback messages. However, several subtypes of
                PSFB and RTPFB messages include a target SSRC(s) in a section called
                Feedback Control Information (FCI). For these messages,
                the target SSRC(s) is used for routing.<a href="#section-9.2-18.6" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.2-18.7">
            <p id="section-9.2-18.7.1">
                If the RTCP packet is a feedback packet that does not include
                target SSRCs in its FCI section, and the media source SSRC is
                found in the outgoing SSRC table, deliver the
                feedback packet to the "m=" section associated with that SSRC.
                RTPFB and PSFB types that are handled in this way include:<a href="#section-9.2-18.7.1" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlParallel" id="section-9.2-18.7.2">
              <dt id="section-9.2-18.7.2.1">Generic NACK:</dt>
              <dd style="margin-left: 1.5em" id="section-9.2-18.7.2.2">(PT=RTPFB, FMT=1) <span>[<a href="#RFC4585" class="xref">RFC4585</a>]</span><a href="#section-9.2-18.7.2.2" class="pilcrow">¶</a>
</dd>
              <dd class="break"></dd>
<dt id="section-9.2-18.7.2.3">Picture Loss Indication (PLI):</dt>
              <dd style="margin-left: 1.5em" id="section-9.2-18.7.2.4">(PT=PSFB, FMT=1) <span>[<a href="#RFC4585" class="xref">RFC4585</a>]</span><a href="#section-9.2-18.7.2.4" class="pilcrow">¶</a>
</dd>
              <dd class="break"></dd>
<dt id="section-9.2-18.7.2.5">Slice Loss Indication (SLI):</dt>
              <dd style="margin-left: 1.5em" id="section-9.2-18.7.2.6"> (PT=PSFB, FMT=2) <span>[<a href="#RFC4585" class="xref">RFC4585</a>]</span><a href="#section-9.2-18.7.2.6" class="pilcrow">¶</a>
</dd>
              <dd class="break"></dd>
<dt id="section-9.2-18.7.2.7">Reference Picture Selection Indication (RPSI):</dt>
              <dd style="margin-left: 1.5em" id="section-9.2-18.7.2.8">(PT=PSFB, FMT=3) <span>[<a href="#RFC4585" class="xref">RFC4585</a>]</span><a href="#section-9.2-18.7.2.8" class="pilcrow">¶</a>
</dd>
            <dd class="break"></dd>
</dl>
</li>
        </ul>
<ul class="normal">
<li class="normal" id="section-9.2-19.1">
                If the RTCP packet is a feedback message that does include a target
                SSRC(s) in its FCI section, it can either be a request or a
                notification. Requests reference an RTP stream that is being
                sent by the message recipient, whereas notifications are responses
                to an earlier request and therefore reference an RTP stream that
                is being received by the message recipient.<a href="#section-9.2-19.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-9.2-19.2">
            <p id="section-9.2-19.2.1">
                If the RTCP packet is a feedback request that includes a target SSRC(s),
                for each target SSRC that is found in the outgoing SSRC table,
                deliver a copy of the RTCP packet to the "m=" section associated with
                that SSRC. PSFB and RTPFB types that are handled in this way include:<a href="#section-9.2-19.2.1" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlParallel" id="section-9.2-19.2.2">
              <dt id="section-9.2-19.2.2.1">Full Intra Request (FIR):</dt>
              <dd style="margin-left: 1.5em" id="section-9.2-19.2.2.2">(PT=PSFB, FMT=4) <span>[<a href="#RFC5104" class="xref">RFC5104</a>]</span><a href="#section-9.2-19.2.2.2" class="pilcrow">¶</a>
</dd>
              <dd class="break"></dd>
<dt id="section-9.2-19.2.2.3">Temporal-Spatial Trade-off Request (TSTR):</dt>
              <dd style="margin-left: 1.5em" id="section-9.2-19.2.2.4">(PT=PSFB, FMT=5) <span>[<a href="#RFC5104" class="xref">RFC5104</a>]</span><a href="#section-9.2-19.2.2.4" class="pilcrow">¶</a>
</dd>
              <dd class="break"></dd>
<dt id="section-9.2-19.2.2.5">H.271 Video Back Channel Message (VBCM):</dt>
              <dd style="margin-left: 1.5em" id="section-9.2-19.2.2.6">(PT=PSFB, FMT=7) <span>[<a href="#RFC5104" class="xref">RFC5104</a>]</span><a href="#section-9.2-19.2.2.6" class="pilcrow">¶</a>
</dd>
              <dd class="break"></dd>
<dt id="section-9.2-19.2.2.7">Temporary Maximum Media Stream Bit Rate Request (TMMBR):</dt>
              <dd style="margin-left: 1.5em" id="section-9.2-19.2.2.8">(PT=RTPFB, FMT=3) <span>[<a href="#RFC5104" class="xref">RFC5104</a>]</span><a href="#section-9.2-19.2.2.8" class="pilcrow">¶</a>
</dd>
              <dd class="break"></dd>
<dt id="section-9.2-19.2.2.9">Layer Refresh Request (LRR):</dt>
              <dd style="margin-left: 1.5em" id="section-9.2-19.2.2.10">(PT=PSFB, FMT=10) <span>[<a href="#I-D.ietf-avtext-lrr" class="xref">LLR-RTCP</a>]</span>.<a href="#section-9.2-19.2.2.10" class="pilcrow">¶</a>
</dd>
            <dd class="break"></dd>
</dl>
</li>
        </ul>
<ul class="normal">
<li class="normal" id="section-9.2-20.1">
            <p id="section-9.2-20.1.1">
                If the RTCP packet is a feedback notification that includes a target SSRC(s),
                for each target SSRC that is found in the incoming SSRC table,
                deliver a copy of the RTCP packet to the "m=" section associated with
                the RTP stream with a matching SSRC. PSFB and RTPFB types that are handled in this way include:<a href="#section-9.2-20.1.1" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlParallel" id="section-9.2-20.1.2">
              <dt id="section-9.2-20.1.2.1">Temporal-Spatial Trade-off Notification (TSTN):</dt>
              <dd style="margin-left: 1.5em" id="section-9.2-20.1.2.2">(PT=PSFB, FMT=6) <span>[<a href="#RFC5104" class="xref">RFC5104</a>]</span>. This message
                        is a notification in response to a prior TSTR.<a href="#section-9.2-20.1.2.2" class="pilcrow">¶</a>
</dd>
              <dd class="break"></dd>
<dt id="section-9.2-20.1.2.3">Temporary Maximum Media Stream Bit Rate Notification (TMMBN):</dt>
              <dd style="margin-left: 1.5em" id="section-9.2-20.1.2.4">(PT=RTPFB, FMT=4) <span>[<a href="#RFC5104" class="xref">RFC5104</a>]</span>. This message is a 
                        notification in response to a prior TMMBR, but it can also be sent 
                        unsolicited.<a href="#section-9.2-20.1.2.4" class="pilcrow">¶</a>
</dd>
            <dd class="break"></dd>
</dl>
</li>
        </ul>
<ul class="normal ulEmpty">
<li class="normal ulEmpty" id="section-9.2-21.1">
              If the RTCP packet is of type APP, then it is handled in an application-specific
              manner. If the application does not recognize the APP packet,
              then it <span class="bcp14">MUST</span> be discarded.<a href="#section-9.2-21.1" class="pilcrow">¶</a>
</li>
        </ul>
</section>
</div>
<div id="sec-rtprtcp-mux">
<section id="section-9.3">
        <h3 id="name-rtp-rtcp-multiplexing">
<a href="#section-9.3" class="section-number selfRef">9.3. </a><a href="#name-rtp-rtcp-multiplexing" class="section-name selfRef">RTP/RTCP Multiplexing</a>
        </h3>
<p id="section-9.3-1">
          Within a BUNDLE group, the offerer and answerer <span class="bcp14">MUST</span> enable
          RTP/RTCP multiplexing <span>[<a href="#RFC5761" class="xref">RFC5761</a>]</span> for the RTP-based bundled media (i.e., the
          same transport will be used for both RTP packets and RTCP packets). 
          In addition, the offerer and answerer <span class="bcp14">MUST</span> support the 
          SDP 'rtcp-mux-only' attribute <span>[<a href="#RFC8858" class="xref">RFC8858</a>]</span>.<a href="#section-9.3-1" class="pilcrow">¶</a></p>
<div id="sec-rtprtcp-mux-oa">
<section id="section-9.3.1">
          <h4 id="name-sdp-offer-answer-procedures-2">
<a href="#section-9.3.1" class="section-number selfRef">9.3.1. </a><a href="#name-sdp-offer-answer-procedures-2" class="section-name selfRef">SDP Offer/Answer Procedures</a>
          </h4>
<p id="section-9.3.1-1">
            This section describes how an offerer and answerer use the
            SDP 'rtcp-mux' <span>[<a href="#RFC5761" class="xref">RFC5761</a>]</span> and SDP 'rtcp-mux-only' attributes
            <span>[<a href="#RFC8858" class="xref">RFC8858</a>]</span>
            to negotiate usage of RTP/RTCP multiplexing for RTP-based bundled media.<a href="#section-9.3.1-1" class="pilcrow">¶</a></p>
<p id="section-9.3.1-2">
            RTP/RTCP multiplexing only applies to RTP-based media. However, as described in
            <a href="#sec-sdp-oa-cat" class="xref">Section 7.1.3</a>, within an offer
            or answer, the SDP 'rtcp-mux' and SDP 'rtcp-mux-only' attributes might be included in
            a bundled "m=" section for non-RTP-based media (if such an "m=" section is the offerer-tagged 
            "m=" section or answerer-tagged "m=" section).<a href="#section-9.3.1-2" class="pilcrow">¶</a></p>
<div id="sec-rtprtcp-mux-oa-ino">
<section id="section-9.3.1.1">
            <h5 id="name-generating-the-initial-bundl">
<a href="#section-9.3.1.1" class="section-number selfRef">9.3.1.1. </a><a href="#name-generating-the-initial-bundl" class="section-name selfRef">Generating the Initial BUNDLE Offer</a>
            </h5>
<p id="section-9.3.1.1-1">
            When an offerer generates an initial BUNDLE offer, if the offer contains
            one or more bundled "m=" sections for RTP-based  media (or if there is a chance that "m=" sections
            for RTP-based media will later be added to the BUNDLE group), the offerer <span class="bcp14">MUST</span>
            include an SDP 'rtcp-mux' attribute <span>[<a href="#RFC5761" class="xref">RFC5761</a>]</span> in each 
            bundled "m=" section (excluding any bundle-only "m=" sections). In addition, the offerer <span class="bcp14">MAY</span> include an
            SDP 'rtcp-mux-only' attribute <span>[<a href="#RFC8858" class="xref">RFC8858</a>]</span> 
            in one or more bundled "m=" sections for RTP-based media.<a href="#section-9.3.1.1-1" class="pilcrow">¶</a></p>
<p style="margin-left: 1.5em" id="section-9.3.1.1-2">
            NOTE: Whether the offerer includes the SDP 'rtcp-mux-only' attribute
            depends on whether the offerer supports fallback to usage of a separate
            port for RTCP in case the answerer moves one or more "m=" sections for RTP-based media 
            out of the BUNDLE group in the answer.<a href="#section-9.3.1.1-2" class="pilcrow">¶</a></p>
<p style="margin-left: 1.5em" id="section-9.3.1.1-3">
            NOTE: If the offerer includes an SDP 'rtcp-mux' attribute in the bundled "m=" sections but does not include an SDP 'rtcp-mux-only' attribute,
            the offerer can also include an SDP 'rtcp' attribute <span>[<a href="#RFC3605" class="xref">RFC3605</a>]</span> in one or more RTP-based bundled "m=" sections in order 
            to provide a fallback port for RTCP, as described in <span>[<a href="#RFC5761" class="xref">RFC5761</a>]</span>. However, the fallback port will only be applied to "m=" sections for RTP-based
            media that are moved out of the BUNDLE group by the answerer.<a href="#section-9.3.1.1-3" class="pilcrow">¶</a></p>
<p id="section-9.3.1.1-4">
            In the initial BUNDLE offer, the address:port combination for RTCP <span class="bcp14">MUST</span> be unique in each
            bundled "m=" section for RTP-based media (excluding a bundle-only "m=" section), similar to RTP.<a href="#section-9.3.1.1-4" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-rtprtcp-mux-oa-ans">
<section id="section-9.3.1.2">
            <h5 id="name-generating-the-sdp-answer-2">
<a href="#section-9.3.1.2" class="section-number selfRef">9.3.1.2. </a><a href="#name-generating-the-sdp-answer-2" class="section-name selfRef">Generating the SDP Answer</a>
            </h5>
<p id="section-9.3.1.2-1">
            When an answerer generates an answer, if the answerer supports RTP-based media
            and if a bundled "m=" section in the corresponding offer contained an SDP 'rtcp-mux' attribute,
            the answerer <span class="bcp14">MUST</span> enable usage of RTP/RTCP multiplexing, even if there currently
            are no bundled "m=" sections for RTP-based media within the BUNDLE group. The answerer <span class="bcp14">MUST</span> include
            an SDP 'rtcp-mux' attribute in the answerer-tagged "m=" section, following the procedures for 
            BUNDLE attributes (<a href="#sec-sdp-oa-cat" class="xref">Section 7.1.3</a>). 
            In addition, if the "m=" section that is selected as the offerer-tagged "m=" section contained
            an SDP 'rtcp-mux-only' attribute, the answerer <span class="bcp14">MUST</span> include an SDP 'rtcp-mux-only' attribute 
            in the answerer-tagged "m=" section.<a href="#section-9.3.1.2-1" class="pilcrow">¶</a></p>
<p id="section-9.3.1.2-2">
            In an initial BUNDLE offer, if the suggested offerer-tagged "m=" section contained an
            SDP 'rtcp-mux-only' attribute, the "m=" section was for RTP-based media. If the
            answerer does not accept the "m=" section in the created BUNDLE group and moves the
            "m=" section out of the BUNDLE group (<a href="#sec-sdp-oa-ans-mov" class="xref">Section 7.3.2</a>),
            the answerer <span class="bcp14">MUST</span> include the attribute in the moved "m=" section and enable RTP/RTCP
            multiplexing for the media associated with the "m=" section. If the answerer rejects the "m=" section 
            (<a href="#sec-sdp-oa-ans-rej" class="xref">Section 7.3.3</a>),
            the answerer <span class="bcp14">MUST NOT</span> include the attribute.<a href="#section-9.3.1.2-2" class="pilcrow">¶</a></p>
<p id="section-9.3.1.2-3">
            The answerer <span class="bcp14">MUST NOT</span> include an SDP 'rtcp' attribute in any bundled "m=" section
            in the answer. The answerer will use the port value of the offerer-tagged 
            "m=" section sending RTP and RTCP packets associated with RTP-based bundled media
            towards the offerer.<a href="#section-9.3.1.2-3" class="pilcrow">¶</a></p>
<p id="section-9.3.1.2-4">
            If the usage of RTP/RTCP multiplexing within a BUNDLE group has been
            negotiated in a previous offer/answer exchange, the answerer <span class="bcp14">MUST</span>
            include an SDP 'rtcp-mux' attribute in the answerer-tagged "m=" section. 
            It is not possible to disable RTP/RTCP multiplexing within a BUNDLE group.<a href="#section-9.3.1.2-4" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-rtprtcp-mux-oa-pra">
<section id="section-9.3.1.3">
            <h5 id="name-offerer-processing-of-the-sd">
<a href="#section-9.3.1.3" class="section-number selfRef">9.3.1.3. </a><a href="#name-offerer-processing-of-the-sd" class="section-name selfRef">Offerer Processing of the SDP Answer</a>
            </h5>
<p id="section-9.3.1.3-1">
            When an offerer receives an answer, if the answerer has accepted
            the usage of RTP/RTCP multiplexing (<a href="#sec-rtprtcp-mux-oa-ans" class="xref">Section 9.3.1.2</a>),
            the answerer follows the procedures for RTP/RTCP multiplexing defined
            in <span>[<a href="#RFC5761" class="xref">RFC5761</a>]</span>. The
            offerer will use the port value of the answerer-tagged "m=" section
            for sending RTP and RTCP packets associated with
            RTP-based bundled media towards the answerer.<a href="#section-9.3.1.3-1" class="pilcrow">¶</a></p>
<p style="margin-left: 1.5em" id="section-9.3.1.3-2">
            NOTE: It is considered a protocol error if the answerer has not
            accepted the usage of RTP/RTCP multiplexing for RTP-based "m=" sections
            that the answerer included in the BUNDLE group.<a href="#section-9.3.1.3-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-rtprtcp-mux-oa-mod">
<section id="section-9.3.1.4">
            <h5 id="name-modifying-the-session-2">
<a href="#section-9.3.1.4" class="section-number selfRef">9.3.1.4. </a><a href="#name-modifying-the-session-2" class="section-name selfRef">Modifying the Session</a>
            </h5>
<p id="section-9.3.1.4-1">
            When an offerer generates a subsequent offer, the offerer <span class="bcp14">MUST</span> include
            an SDP 'rtcp-mux' attribute in the offerer-tagged "m=" section, following 
            the procedures for IDENTICAL multiplexing category attributes in 
            <a href="#sec-sdp-oa-cat" class="xref">Section 7.1.3</a>.<a href="#section-9.3.1.4-1" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
</section>
</div>
</section>
</div>
<div id="sec-ice">
<section id="section-10">
      <h2 id="name-ice-considerations">
<a href="#section-10" class="section-number selfRef">10. </a><a href="#name-ice-considerations" class="section-name selfRef">ICE Considerations</a>
      </h2>
<p id="section-10-1">
        This section describes how to use the BUNDLE grouping extension together
        with the ICE mechanism <span>[<a href="#RFC8445" class="xref">RFC8445</a>]</span>.<a href="#section-10-1" class="pilcrow">¶</a></p>
<p id="section-10-2">
        The generic procedures for negotiating the usage of ICE using SDP, defined
        in <span>[<a href="#RFC8839" class="xref">RFC8839</a>]</span>, also apply to the usage of ICE
        with BUNDLE, with the following exceptions:<a href="#section-10-2" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-10-3.1">
            When the BUNDLE transport has been established, ICE connectivity checks and keepalives 
            only need to be performed for the BUNDLE transport, instead of per individual bundled "m=" section 
            within the BUNDLE group.<a href="#section-10-3.1" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-10-3.2">
            The generic SDP attribute offer/answer considerations (<a href="#sec-sdp-oa-cat" class="xref">Section 7.1.3</a>) also apply to ICE-related 
            attributes. Therefore, when an offerer sends an initial BUNDLE offer (in order to negotiate a BUNDLE group), the offerer
            includes ICE-related media-level attributes in each bundled "m=" section (excluding any bundle-only "m=" sections),
            and each "m=" section <span class="bcp14">MUST</span> contain unique ICE properties. When an answerer generates an answer (initial BUNDLE answer or subsequent) 
            that contains a BUNDLE group and when an offerer sends a subsequent offer that contains a BUNDLE group, ICE-related media-level 
            attributes are only included in the tagged "m=" section (suggested offerer-tagged "m=" section or answerer-tagged "m=" section), and
            the ICE properties are applied to each bundled "m=" section within the BUNDLE group.<a href="#section-10-3.2" class="pilcrow">¶</a>
</li>
      </ul>
<p style="margin-left: 1.5em" id="section-10-4">
        NOTE: Most ICE-related media-level SDP attributes belong to the TRANSPORT multiplexing category
        <span>[<a href="#RFC8859" class="xref">RFC8859</a>]</span>, and the generic SDP attribute offer/answer
        considerations for the TRANSPORT multiplexing category apply to the attributes. However, in the case of
        ICE-related attributes, the same considerations also apply to ICE-related media-level attributes that
        belong to other multiplexing categories.<a href="#section-10-4" class="pilcrow">¶</a></p>
<p style="margin-left: 1.5em" id="section-10-5">
        NOTE: The following ICE-related media-level SDP attributes are defined in
        <span>[<a href="#RFC8839" class="xref">RFC8839</a>]</span>: 'candidate', 'remote-candidates', 'ice-mismatch',
        'ice-ufrag', 'ice-pwd', and 'ice-pacing'.<a href="#section-10-5" class="pilcrow">¶</a></p>
<p id="section-10-6">
        Initially, before ICE has produced selected candidate pairs that will be used for media, there might
        be multiple transports established (if multiple candidate pairs are tested). Once ICE has selected
        candidate pairs, they form the BUNDLE transport.<a href="#section-10-6" class="pilcrow">¶</a></p>
<p id="section-10-7">
        Support and usage of the ICE mechanism together with the BUNDLE extension
        is <span class="bcp14">OPTIONAL</span>, and the procedures in this section only apply when the
        ICE mechanism is used. Note that applications might mandate usage
        of the ICE mechanism even if the BUNDLE extension is not used.<a href="#section-10-7" class="pilcrow">¶</a></p>
<p style="margin-left: 1.5em" id="section-10-8">
        NOTE: If the Trickle ICE mechanism <span>[<a href="#RFC8840" class="xref">RFC8840</a>]</span>
        is used, an offerer and answerer might assign a port value of '9' and an IPv4 
        address of '0.0.0.0' (or, the IPv6 equivalent '::') to multiple bundled "m=" sections
        in the initial BUNDLE offer. The offerer and answerer will follow the normal procedures
        for generating the offers and answers, including picking a bundled "m=" section as the
        suggested offerer-tagged "m=" section, selecting the tagged "m=" sections, etc. The only
        difference is that media cannot be sent until one or more candidates have been provided.
        Once a BUNDLE group has been negotiated, trickled candidates associated with a bundled
        "m=" section will be applied to all bundled "m=" sections within the BUNDLE group.<a href="#section-10-8" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-dtls">
<section id="section-11">
      <h2 id="name-dtls-considerations">
<a href="#section-11" class="section-number selfRef">11. </a><a href="#name-dtls-considerations" class="section-name selfRef">DTLS Considerations</a>
      </h2>
<p id="section-11-1">
        One or more media streams within a BUNDLE group might use
        the DTLS protocol
        <span>[<a href="#RFC6347" class="xref">RFC6347</a>]</span>
        in order to encrypt the data or negotiate encryption keys
        if another encryption mechanism is used to encrypt media.<a href="#section-11-1" class="pilcrow">¶</a></p>
<p id="section-11-2">
        When DTLS is used within a BUNDLE group, the following rules
        apply:<a href="#section-11-2" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-11-3.1">
          There can only be one DTLS association
          <span>[<a href="#RFC6347" class="xref">RFC6347</a>]</span>
          associated with the BUNDLE group;<a href="#section-11-3.1" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-11-3.2">
          Each usage of the DTLS association within the BUNDLE
          group <span class="bcp14">MUST</span> use the same mechanism for determining
          which endpoints (the offerer or answerer) become
          DTLS client and DTLS server;<a href="#section-11-3.2" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-11-3.3">

   
          Each usage of the DTLS association within the BUNDLE
          group <span class="bcp14">MUST</span> use the same mechanism for determining
          whether an offer or answer will trigger the
          establishment of a new DTLS association or if 
          an existing DTLS association will be used instead; and<a href="#section-11-3.3" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-11-3.4">
          If the DTLS client supports DTLS-SRTP,
          it <span class="bcp14">MUST</span> include the 'use_srtp' extension
          in the DTLS ClientHello message
          <span>[<a href="#RFC5764" class="xref">RFC5764</a>]</span>.
          The client <span class="bcp14">MUST</span> include the extension even if the usage
          of DTLS-SRTP is not negotiated as part of the
          multimedia session (e.g., the SIP session <span>[<a href="#RFC3261" class="xref">RFC3261</a>]</span>).<a href="#section-11-3.4" class="pilcrow">¶</a>
</li>
      </ul>
<p style="margin-left: 1.5em" id="section-11-4">
        NOTE: The inclusion of the 'use_srtp' extension during the initial
        DTLS handshake ensures that a DTLS renegotiation will not be required
        in order to include the extension in case DTLS-SRTP encrypted media
        is added to the BUNDLE group later during the multimedia session.<a href="#section-11-4" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-extmap">
<section id="section-12">
      <h2 id="name-rtp-header-extensions-consi">
<a href="#section-12" class="section-number selfRef">12. </a><a href="#name-rtp-header-extensions-consi" class="section-name selfRef">RTP Header Extensions Consideration</a>
      </h2>
<p id="section-12-1">When RTP header extensions <span>[<a href="#RFC8285" class="xref">RFC8285</a>]</span> are used in the context of this
       specification, the identifier used for a given extension <span class="bcp14">MUST</span> identify the same
       extension across all the bundled media descriptions.<a href="#section-12-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-3264">
<section id="section-13">
      <h2 id="name-updates-to-rfc-3264">
<a href="#section-13" class="section-number selfRef">13. </a><a href="#name-updates-to-rfc-3264" class="section-name selfRef">Updates to RFC 3264</a>
      </h2>
<p id="section-13-1">
        This section updates <span>[<a href="#RFC3264" class="xref">RFC3264</a>]</span> in order to allow extensions to define the usage of 
        a zero port value in offers and answers for purposes other than removing or disabling 
        media streams. The following sections are being updated:<a href="#section-13-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-13-2.1">"Unicast Streams"; see <span><a href="https://www.rfc-editor.org/rfc/rfc3264#section-5.1" class="relref">Section 5.1</a> of [<a href="#RFC3264" class="xref">RFC3264</a>]</span>.<a href="#section-13-2.1" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-13-2.2">"Putting a Unicast Media Stream on Hold"; see <span><a href="https://www.rfc-editor.org/rfc/rfc3264#section-8.4" class="relref">Section 8.4</a> of [<a href="#RFC3264" class="xref">RFC3264</a>]</span>.<a href="#section-13-2.2" class="pilcrow">¶</a>
</li>
      </ul>
<div id="sec-3264-old-5_1">
<section id="section-13.1">
        <h3 id="name-original-text-from-rfc-3264">
<a href="#section-13.1" class="section-number selfRef">13.1. </a><a href="#name-original-text-from-rfc-3264" class="section-name selfRef">Original Text from RFC 3264, Section 5.1, Paragraph 2</a>
        </h3>
<blockquote id="section-13.1-1">
        For recvonly and sendrecv streams, the port number and address in the
        offer indicate where the offerer would like to receive the media
        stream. For sendonly RTP streams, the address and port number
        indirectly indicate where the offerer wants to receive RTCP reports.
        Unless there is an explicit indication otherwise, reports are sent to
        the port number one higher than the number indicated.  The IP address
        and port present in the offer indicate nothing about the source IP
        address and source port of RTP and RTCP packets that will be sent by
        the offerer. A port number of zero in the offer indicates that the
        stream is offered but <span class="bcp14">MUST NOT</span> be used.  This has no useful semantics
        in an initial offer, but is allowed for reasons of completeness,
        since the answer can contain a zero port indicating a rejected stream
        (Section <a href="https://www.rfc-editor.org/rfc/rfc3264#section-6" class="relref">6</a>). Furthermore, existing streams can be terminated by
        setting the port to zero (Section <a href="https://www.rfc-editor.org/rfc/rfc3264#section-8" class="relref">8</a>). In general, a port number of
        zero indicates that the media stream is not wanted.<a href="#section-13.1-1" class="pilcrow">¶</a>
</blockquote>
</section>
</div>
<div id="sec-3264-new-5_1">
<section id="section-13.2">
        <h3 id="name-new-text-replacing-rfc-3264">
<a href="#section-13.2" class="section-number selfRef">13.2. </a><a href="#name-new-text-replacing-rfc-3264" class="section-name selfRef">New Text Replacing RFC 3264, Section 5.1, Paragraph 2</a>
        </h3>
<blockquote id="section-13.2-1">
          <p id="section-13.2-1.1">
        For recvonly and sendrecv streams, the port number and address in the
        offer indicate where the offerer would like to receive the media
        stream. For sendonly RTP streams, the address and port number
        indirectly indicate where the offerer wants to receive RTCP reports.
        Unless there is an explicit indication otherwise, reports are sent to
        the port number one higher than the number indicated. The IP address
        and port present in the offer indicate nothing about the source IP
        address and source port of the RTP and RTCP packets that will be sent by
        the offerer. By default, a port number of zero in the offer indicates that the
        stream is offered but <span class="bcp14">MUST NOT</span> be used, but an extension mechanism
        might specify different semantics for the usage of a zero port value.
        Furthermore, existing streams can be terminated by setting the port to
        zero (Section <a href="https://www.rfc-editor.org/rfc/rfc3264#section-8" class="relref">8</a>). In general, a port number of zero by default indicates
        that the media stream is not wanted.<a href="#section-13.2-1.1" class="pilcrow">¶</a></p>
</blockquote>
</section>
</div>
<div id="sec-3264-old-8_4">
<section id="section-13.3">
        <h3 id="name-original-text-from-rfc-3264-">
<a href="#section-13.3" class="section-number selfRef">13.3. </a><a href="#name-original-text-from-rfc-3264-" class="section-name selfRef">Original Text from RFC 3264, Section 8.4, Paragraph 6</a>
        </h3>
<blockquote id="section-13.3-1">RFC 2543 [10] specified that placing a user on hold was accomplished
   by setting the connection address to 0.0.0.0.  Its usage for putting
   a call on hold is no longer recommended, since it doesn't allow for
   RTCP to be used with held streams, doesn't work with IPv6, and breaks
   with connection oriented media.  However, it can be useful in an
   initial offer when the offerer knows it wants to use a particular set
   of media streams and formats, but doesn't know the addresses and
   ports at the time of the offer.  Of course, when used, the port
   number <span class="bcp14">MUST NOT</span> be zero, which would specify that the stream has been
   disabled.  An agent <span class="bcp14">MUST</span> be capable of receiving SDP with a
   connection address of 0.0.0.0, in which case it means that neither
   RTP nor RTCP should be sent to the peer.<a href="#section-13.3-1" class="pilcrow">¶</a>
</blockquote>
</section>
</div>
<div id="sec-3264-new-8_4">
<section id="section-13.4">
        <h3 id="name-new-text-replacing-rfc-3264-">
<a href="#section-13.4" class="section-number selfRef">13.4. </a><a href="#name-new-text-replacing-rfc-3264-" class="section-name selfRef">New Text Replacing RFC 3264, Section 8.4, Paragraph 6</a>
        </h3>
<blockquote id="section-13.4-1">
          <p id="section-13.4-1.1"> RFC 2543 <span>[<a href="#RFC2543" class="xref">RFC2543</a>]</span> specifies
        that placing a user on hold was accomplished by setting the
        connection address to 0.0.0.0.  Its usage for putting a call
        on hold is no longer recommended, since it doesn't allow for
        RTCP to be used with held streams, doesn't work with IPv6, and
        breaks with connection oriented media. However, it can be
        useful in an initial offer when the offerer knows it wants to
        use a particular set of media streams and formats, but doesn't
        know the addresses and ports at the time of the offer. Of
        course, when used, the port number <span class="bcp14">MUST NOT</span> be
        zero, if it would specify that the stream has been
        disabled. However, an extension mechanism might specify
        different semantics of the zero port number usage. An agent
        <span class="bcp14">MUST</span> be capable of receiving SDP with a
        connection address of 0.0.0.0, in which case it means that
        neither RTP nor RTCP is to be sent to the peer.<a href="#section-13.4-1.1" class="pilcrow">¶</a></p>
</blockquote>
</section>
</div>
</section>
</div>
<div id="sec-5888">
<section id="section-14">
      <h2 id="name-update-to-rfc-5888">
<a href="#section-14" class="section-number selfRef">14. </a><a href="#name-update-to-rfc-5888" class="section-name selfRef">Update to RFC 5888</a>
      </h2>
<p id="section-14-1">
        This section updates RFC 5888 <span>[<a href="#RFC5888" class="xref">RFC5888</a>]</span> 
        in order for extensions to allow an SDP 'group' attribute containing
        an identification-tag that identifies an "m=" section with the port set to zero.
        "Group Value in Answers" (<span><a href="https://www.rfc-editor.org/rfc/rfc5888#section-9.2" class="relref">Section 9.2</a> of [<a href="#RFC5888" class="xref">RFC5888</a>]</span>) 
        is updated.<a href="#section-14-1" class="pilcrow">¶</a></p>
<div id="sec-5888-old-9_2">
<section id="section-14.1">
        <h3 id="name-original-text-from-rfc-5888">
<a href="#section-14.1" class="section-number selfRef">14.1. </a><a href="#name-original-text-from-rfc-5888" class="section-name selfRef">Original Text from RFC 5888, Section 9.2, Paragraph 3</a>
        </h3>
<blockquote id="section-14.1-1">SIP entities refuse media streams by setting the port to zero in the corresponding
        "m" line. "a=group" lines <span class="bcp14">MUST NOT</span> contain identification-tags that correspond to 
        "m" lines with the port set to zero.<a href="#section-14.1-1" class="pilcrow">¶</a>
</blockquote>
</section>
</div>
<div id="sec-5888-new-9_2">
<section id="section-14.2">
        <h3 id="name-new-text-replacing-rfc-5888">
<a href="#section-14.2" class="section-number selfRef">14.2. </a><a href="#name-new-text-replacing-rfc-5888" class="section-name selfRef">New Text Replacing RFC 5888, Section 9.2, Paragraph 3</a>
        </h3>
<blockquote id="section-14.2-1">
          <p id="section-14.2-1.1"> SIP entities refuse media streams by setting
        the port to zero in the corresponding "m" line. "a=group"
        lines <span class="bcp14">MUST NOT</span> contain identification-tags that
        correspond to "m" lines with the port set to zero, but an
        extension mechanism might specify different semantics for
        including identification-tags that correspond to such "m="
        lines.<a href="#section-14.2-1.1" class="pilcrow">¶</a></p>
</blockquote>
</section>
</div>
</section>
</div>
<div id="sec-receiver-id">
<section id="section-15">
      <h2 id="name-rtp-rtcp-extensions-for-ide">
<a href="#section-15" class="section-number selfRef">15. </a><a href="#name-rtp-rtcp-extensions-for-ide" class="section-name selfRef">RTP/RTCP Extensions for identification-tag Transport</a>
      </h2>
<p id="section-15-1">
        Offerers and answerers <span>[<a href="#RFC3264" class="xref">RFC3264</a>]</span> can associate
        identification-tags with "m=" sections within offers and
        answers using the procedures in <span>[<a href="#RFC5888" class="xref">RFC5888</a>]</span>. Each identification-tag uniquely
        represents an "m=" section.<a href="#section-15-1" class="pilcrow">¶</a></p>
<p id="section-15-2">
        This section defines a new RTCP SDES item <span>[<a href="#RFC3550" class="xref">RFC3550</a>]</span>, 'MID', which is used to carry identification-tags within RTCP
        SDES packets. This section also defines a new RTP SDES header extension
        <span>[<a href="#RFC7941" class="xref">RFC7941</a>]</span>, which
        is used to carry the 'MID' RTCP SDES item in RTP packets.<a href="#section-15-2" class="pilcrow">¶</a></p>
<p id="section-15-3">
        The SDES item and RTP SDES header extension make it possible for a receiver to associate
        each RTP stream with a specific "m=" section with which the receiver has
        associated an identification-tag, even if those "m=" sections are part of the same RTP session.
        The RTP SDES header extension also ensures that the media recipient gets the identification-tag
        upon receipt of the first decodable media and is able to associate the media with the
        correct application.<a href="#section-15-3" class="pilcrow">¶</a></p>
<p id="section-15-4">
        A media recipient informs the media sender about the identification-tag
        associated with an "m=" section through the use of a 'mid' attribute
        <span>[<a href="#RFC5888" class="xref">RFC5888</a>]</span>. The media sender then
        inserts the identification-tag in RTCP and RTP packets sent to the media recipient.<a href="#section-15-4" class="pilcrow">¶</a></p>
<p style="margin-left: 1.5em" id="section-15-5">
        NOTE: The text above defines how identification-tags are carried in offers
        and answers. The usage of other signaling protocols for carrying identification-tags
        is not prevented, but the usage of such protocols is outside the scope of this document.<a href="#section-15-5" class="pilcrow">¶</a></p>
<p id="section-15-6">
        <span>[<a href="#RFC3550" class="xref">RFC3550</a>]</span> defines general procedures
        regarding the RTCP transmission interval. The RTCP MID SDES item <span class="bcp14">SHOULD</span> be sent in
        the first few RTCP packets after joining the session and <span class="bcp14">SHOULD</span> be sent regularly
        thereafter. The exact number of RTCP packets in which this SDES item is sent is
        intentionally not specified here, as it will depend on the expected packet-loss
        rate, the RTCP reporting interval, and the allowable overhead.<a href="#section-15-6" class="pilcrow">¶</a></p>
<p id="section-15-7">
        The RTP SDES header extension for carrying the 'MID' RTCP SDES <span class="bcp14">SHOULD</span> be included
        in some RTP packets at the start of the session and whenever the SSRC changes. It might
        also be useful to include the header extension in RTP packets that comprise access points in the media
        (e.g., with video I-frames). The exact number of RTP packets in which this header
        extension is sent is intentionally not specified here, as it will depend on expected
        packet-loss rate and loss patterns, the overhead the application can tolerate, and
        the importance of immediate receipt of the identification-tag.<a href="#section-15-7" class="pilcrow">¶</a></p>
<p id="section-15-8">
        For robustness, endpoints need to be prepared for situations where the
        reception of the identification-tag is delayed and <span class="bcp14">SHOULD NOT</span> terminate sessions
        in such cases, as the identification-tag is likely to arrive soon.<a href="#section-15-8" class="pilcrow">¶</a></p>
<div id="sec-receiver-id-sdes-item">
<section id="section-15.1">
        <h3 id="name-rtcp-mid-sdes-item">
<a href="#section-15.1" class="section-number selfRef">15.1. </a><a href="#name-rtcp-mid-sdes-item" class="section-name selfRef">RTCP MID SDES Item</a>
        </h3>
<div class="alignLeft art-text artwork" id="section-15.1-1">
<pre>
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      MID=15   |     length    | identification-tag          ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</pre><a href="#section-15.1-1" class="pilcrow">¶</a>
</div>
<p id="section-15.1-2">
          The identification-tag payload is UTF-8 encoded <span>[<a href="#RFC3629" class="xref">RFC3629</a>]</span>, as in SDP.<a href="#section-15.1-2" class="pilcrow">¶</a></p>
<p id="section-15.1-3">
          The identification-tag is not zero terminated.<a href="#section-15.1-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-receiver-id-rtp-he">
<section id="section-15.2">
        <h3 id="name-rtp-sdes-header-extension-f">
<a href="#section-15.2" class="section-number selfRef">15.2. </a><a href="#name-rtp-sdes-header-extension-f" class="section-name selfRef">RTP SDES Header Extension for MID</a>
        </h3>
<p id="section-15.2-1">
          The payload, containing the identification-tag, of the RTP SDES header extension element
          can be encoded using either the 1-byte or the 2-byte header <span>[<a href="#RFC7941" class="xref">RFC7941</a>]</span>. The identification-tag payload is UTF-8
          encoded, as in SDP.<a href="#section-15.2-1" class="pilcrow">¶</a></p>
<p id="section-15.2-2">
          The identification-tag is not zero terminated. Note that the set of header extensions
          included in the packet needs to be padded to the next 32-bit boundary using zero
          bytes <span>[<a href="#RFC8285" class="xref">RFC8285</a>]</span>.<a href="#section-15.2-2" class="pilcrow">¶</a></p>
<p id="section-15.2-3">
          As the identification-tag is included in an RTCP SDES item, an RTP SDES header
          extension, or both, there needs to be some consideration about the packet expansion
          caused by the identification-tag. To avoid Maximum Transmission Unit (MTU) issues
          for the RTP packets, the header extension's size needs to be taken into account when
          encoding the media.<a href="#section-15.2-3" class="pilcrow">¶</a></p>
<p id="section-15.2-4">
          It is recommended that the identification-tag be kept short. Due to the properties of
          the RTP header extension mechanism, when using the 1-byte header, a tag that is 1-3 bytes
          will result in a minimal number of 32-bit words used for the RTP SDES header extension,
          in case no other header extensions are included at the same time. Note: do take into
          account that some single characters when UTF-8 encoded will result in multiple octets.
          The identification-tag <span class="bcp14">MUST NOT</span> contain any user information, and applications <span class="bcp14">SHALL</span> avoid 
          generating the identification-tag using a pattern that enables user or application 
          identification.<a href="#section-15.2-4" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="sec-receiver-id-iana">
<section id="section-16">
      <h2 id="name-iana-considerations">
<a href="#section-16" class="section-number selfRef">16. </a><a href="#name-iana-considerations" class="section-name selfRef">IANA Considerations</a>
      </h2>
<p id="section-16-1">NOTE: Apart from the references, the IANA considerations in this section are identical to those in <span>[<a href="#RFC8843" class="xref">RFC8843</a>]</span>.<a href="#section-16-1" class="pilcrow">¶</a></p>
<div id="sec-receiver-id-iana-sdes-item">
<section id="section-16.1">
        <h3 id="name-sdes-item">
<a href="#section-16.1" class="section-number selfRef">16.1. </a><a href="#name-sdes-item" class="section-name selfRef">SDES Item</a>
        </h3>
<p id="section-16.1-1">
        This document updates the MID SDES entry in the "RTP SDES Item
        Types" registry as follows:<a href="#section-16.1-1" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlCompact dlParallel" id="section-16.1-2">
          <dt id="section-16.1-2.1">Value:</dt>
          <dd style="margin-left: 1.5em" id="section-16.1-2.2">15<a href="#section-16.1-2.2" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-16.1-2.3">Abbrev.:</dt>
          <dd style="margin-left: 1.5em" id="section-16.1-2.4">MID<a href="#section-16.1-2.4" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-16.1-2.5">Name:</dt>
          <dd style="margin-left: 1.5em" id="section-16.1-2.6">Media Identification<a href="#section-16.1-2.6" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-16.1-2.7">Reference:</dt>
          <dd style="margin-left: 1.5em" id="section-16.1-2.8">RFC 9143<a href="#section-16.1-2.8" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
</dl>
</section>
</div>
<div id="sec-receiver-id-iana-rtp-uri">
<section id="section-16.2">
        <h3 id="name-rtp-sdes-header-extension-u">
<a href="#section-16.2" class="section-number selfRef">16.2. </a><a href="#name-rtp-sdes-header-extension-u" class="section-name selfRef">RTP SDES Header Extension URI</a>
        </h3>
<p id="section-16.2-1">
        This document updates the extension URI in the "RTP SDES
        Compact Header Extensions" subregistry of the "RTP Compact
        Header Extensions" sub-registry, according to the
        following data:<a href="#section-16.2-1" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlCompact dlParallel" id="section-16.2-2">
          <dt id="section-16.2-2.1">Extension URI:</dt>
          <dd style="margin-left: 1.5em" id="section-16.2-2.2">urn:ietf:params:rtp-hdrext:sdes:mid<a href="#section-16.2-2.2" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-16.2-2.3">Description:</dt>
          <dd style="margin-left: 1.5em" id="section-16.2-2.4">Media identification<a href="#section-16.2-2.4" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-16.2-2.5">Contact:</dt>
          <dd style="margin-left: 1.5em" id="section-16.2-2.6">IESG (iesg@ietf.org)<a href="#section-16.2-2.6" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-16.2-2.7">Reference:</dt>
          <dd style="margin-left: 1.5em" id="section-16.2-2.8">RFC 9143<a href="#section-16.2-2.8" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
</dl>
<p id="section-16.2-3">
  The SDES item does not reveal privacy information about the users.
  It is simply used to associate RTP-based media with the correct SDP
  media description ("m=" section) in the SDP used to negotiate the 
  media.<a href="#section-16.2-3" class="pilcrow">¶</a></p>
<p id="section-16.2-4"> The purpose of the extension is for the offerer to be able to
  associate received multiplexed RTP-based media before the offerer
  receives the associated answer.<a href="#section-16.2-4" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-receiver-id-iana-sdp-attribute">
<section id="section-16.3">
        <h3 id="name-sdp-attribute">
<a href="#section-16.3" class="section-number selfRef">16.3. </a><a href="#name-sdp-attribute" class="section-name selfRef">SDP Attribute</a>
        </h3>
<p id="section-16.3-1">
        This document updates the SDP media-level attribute,
        'bundle-only', in the "attribute-name (formerly 'att-field')" subregistry of the "Session Description Protocol (SDP) Parameters" registry according to the following data:<a href="#section-16.3-1" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlCompact dlParallel" id="section-16.3-2">
          <dt id="section-16.3-2.1">Attribute name:</dt>
          <dd style="margin-left: 1.5em" id="section-16.3-2.2">bundle-only<a href="#section-16.3-2.2" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-16.3-2.3">Type of attribute:</dt>
          <dd style="margin-left: 1.5em" id="section-16.3-2.4">media<a href="#section-16.3-2.4" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-16.3-2.5">Subject to charset:</dt>
          <dd style="margin-left: 1.5em" id="section-16.3-2.6">No<a href="#section-16.3-2.6" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-16.3-2.7">Purpose:</dt>
          <dd style="margin-left: 1.5em" id="section-16.3-2.8">Request a media description to be accepted
                      in the answer only if kept within a BUNDLE
                      group by the answerer.<a href="#section-16.3-2.8" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-16.3-2.9">Appropriate values:</dt>
          <dd style="margin-left: 1.5em" id="section-16.3-2.10">N/A<a href="#section-16.3-2.10" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-16.3-2.11">Contact name:</dt>
          <dd style="margin-left: 1.5em" id="section-16.3-2.12">IESG<a href="#section-16.3-2.12" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-16.3-2.13">Contact e-mail:</dt>
          <dd style="margin-left: 1.5em" id="section-16.3-2.14">iesg@ietf.org<a href="#section-16.3-2.14" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-16.3-2.15">Reference:</dt>
          <dd style="margin-left: 1.5em" id="section-16.3-2.16">RFC 9143<a href="#section-16.3-2.16" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-16.3-2.17">Mux category:</dt>
          <dd style="margin-left: 1.5em" id="section-16.3-2.18">NORMAL<a href="#section-16.3-2.18" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
</dl>
</section>
</div>
<div id="sec-receiver-id-iana-sdp-group-semantics">
<section id="section-16.4">
        <h3 id="name-sdp-group-semantics">
<a href="#section-16.4" class="section-number selfRef">16.4. </a><a href="#name-sdp-group-semantics" class="section-name selfRef">SDP Group Semantics</a>
        </h3>
<p id="section-16.4-1">
        This document updates the following semantics in the
        "Semantics for the 'group' SDP Attribute" subregistry (under the
        "Session Description Protocol (SDP) Parameters" registry):<a href="#section-16.4-1" class="pilcrow">¶</a></p>
<span id="name-update-to-sdp-group-semanti"></span><div id="Table_1">
<table class="left" id="table-1">
          <caption>
<a href="#table-1" class="selfRef">Table 1</a>:
<a href="#name-update-to-sdp-group-semanti" class="selfRef">Update to SDP Group Semantics</a>
          </caption>
<thead>
            <tr>
              <th class="text-left" rowspan="1" colspan="1">Semantics</th>
              <th class="text-left" rowspan="1" colspan="1">Token</th>
              <th class="text-left" rowspan="1" colspan="1">Mux Category</th>
              <th class="text-left" rowspan="1" colspan="1">Reference</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td class="text-left" rowspan="1" colspan="1">Media bundling</td>
              <td class="text-left" rowspan="1" colspan="1">BUNDLE</td>
              <td class="text-left" rowspan="1" colspan="1">NORMAL</td>
              <td class="text-left" rowspan="1" colspan="1">RFC 9143</td>
            </tr>
          </tbody>
        </table>
</div>
</section>
</div>
</section>
</div>
<div id="sec-security">
<section id="section-17">
      <h2 id="name-security-considerations">
<a href="#section-17" class="section-number selfRef">17. </a><a href="#name-security-considerations" class="section-name selfRef">Security Considerations</a>
      </h2>
<p id="section-17-1">The security considerations defined in <span>[<a href="#RFC3264" class="xref">RFC3264</a>]</span> and <span>[<a href="#RFC5888" class="xref">RFC5888</a>]</span> apply to the BUNDLE extension. BUNDLE
      does not change which information, e.g., RTP streams, flows over
      the network, except for the usage of the MID SDES item as
      discussed below.
      Primarily, it changes which addresses and ports, and
      thus in which (RTP) sessions, the information flows to. This
      affects the security contexts being used and can cause previously
      separated information flows to share the same security context. This has very
      little impact on the performance of the security mechanism of the RTP
      sessions. In cases where one would have applied different security
      policies on the different RTP streams being bundled or where the
      parties having access to the security contexts would have differed
      between the RTP streams, additional analysis of the implications is
      needed before selecting to apply BUNDLE.<a href="#section-17-1" class="pilcrow">¶</a></p>
<p id="section-17-2">The identification-tag, independent of transport, RTCP SDES packet, or
      RTP header extension, can expose the value to parties beyond the
      signaling chain. Therefore, the identification-tag values <span class="bcp14">MUST</span> be
      generated in a fashion that does not leak user information, e.g.,
      randomly or using a per-bundle group counter, and <span class="bcp14">SHOULD</span> be 3 bytes or
      fewer to allow them to efficiently fit into the MID RTP header
      extension. Note that if implementations use different methods for
      generating identification-tags, this could enable fingerprinting of the
      implementation, making it vulnerable to targeted attacks. The
      identification-tag is exposed on the RTP stream level when included in
      the RTP header extensions; however, what it reveals of the RTP media
      stream structure of the endpoint and application was already possible to
      deduce from the RTP streams without the MID SDES header extensions. As
      the identification-tag is also used to route the media stream to the
      right application functionality, it is important that the value
      received is the one intended by the sender; thus, integrity and the
      authenticity of the source are important to prevent denial of service on
      the application. Existing SRTP configurations and other security
       mechanisms protecting the whole RTP/RTCP packets will provide the
      necessary protection.<a href="#section-17-2" class="pilcrow">¶</a></p>
<p id="section-17-3">When the BUNDLE extension is used, the set of configurations of the
   security mechanism used in all the bundled media descriptions will need to
   be compatible so that they can be used simultaneously, at least
   per direction or endpoint. When using SRTP, this will be the case, at least
   for the IETF-defined key-management solutions due to their SDP attributes
   ("a=crypto", "a=fingerprint", "a=mikey") and their classification in <span>[<a href="#RFC8859" class="xref">RFC8859</a>]</span>.<a href="#section-17-3" class="pilcrow">¶</a></p>
<p id="section-17-4">The security considerations of "RTP Header
   Extension for the RTP Control Protocol (RTCP) Source Description Items"
   <span>[<a href="#RFC7941" class="xref">RFC7941</a>]</span> require that when RTCP is confidentiality protected, any SDES
   RTP header extension carrying an SDES item, such as the MID RTP header
      extension, is also protected using commensurate strength algorithms.
      However, assuming the above requirements and recommendations are
      followed, there are no known significant security risks with leaving the
      MID RTP header extension without confidentiality protection. 
      Therefore, this specification updates <span>[<a href="#RFC7941" class="xref">RFC7941</a>]</span> by adding the exception that this requirement 
      <span class="bcp14">MAY</span> be ignored for the MID RTP header extension. Security mechanisms for RTP/RTCP are 
      discussed in "Options for Securing RTP Sessions" <span>[<a href="#RFC7201" class="xref">RFC7201</a>]</span>; 
      for example, SRTP <span>[<a href="#RFC3711" class="xref">RFC3711</a>]</span> can provide the necessary security 
      functions of ensuring the integrity and source authenticity.<a href="#section-17-4" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec-example-alt1">
<section id="section-18">
      <h2 id="name-examples">
<a href="#section-18" class="section-number selfRef">18. </a><a href="#name-examples" class="section-name selfRef">Examples</a>
      </h2>
<div id="sec-example-add">
<section id="section-18.1">
        <h3 id="name-example-tagged-m-section-se">
<a href="#section-18.1" class="section-number selfRef">18.1. </a><a href="#name-example-tagged-m-section-se" class="section-name selfRef">Example: Tagged "m=" Section Selections</a>
        </h3>
<p id="section-18.1-1">
        The example below shows:<a href="#section-18.1-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-18.1-2.1">An initial BUNDLE offer, in which the offerer wants to
          negotiate a BUNDLE group and indicates the audio "m="
          section as the suggested offerer-tagged "m=" section.<a href="#section-18.1-2.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-18.1-2.2">An initial BUNDLE answer, in which the answerer accepts
          the creation of the BUNDLE group, selects the audio "m="
          section in the offer as the offerer-tagged "m=" section,
          selects the audio "m=" section in the answer as the
          answerer-tagged "m=" section, and assigns the answerer BUNDLE
          address:port to that "m=" section.<a href="#section-18.1-2.2" class="pilcrow">¶</a>
</li>
        </ul>
<p id="section-18.1-3" class="keepWithNext">SDP Offer (1)<a href="#section-18.1-3" class="pilcrow">¶</a></p>
<div id="section-18.1-4">
<pre class="lang-sdp sourcecode">
    v=0
    o=alice 2890844526 2890844526 IN IP6 2001:db8::3
    s=
    c=IN IP6 2001:db8::3
    t=0 0
    a=group:BUNDLE foo bar

    m=audio 10000 RTP/AVP 0 8 97
    b=AS:200
    a=mid:foo
    a=rtcp-mux
    a=rtpmap:0 PCMU/8000
    a=rtpmap:8 PCMA/8000
    a=rtpmap:97 iLBC/8000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

    m=video 10002 RTP/AVP 31 32
    b=AS:1000
    a=mid:bar
    a=rtcp-mux
    a=rtpmap:31 H261/90000
    a=rtpmap:32 MPV/90000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
</pre><a href="#section-18.1-4" class="pilcrow">¶</a>
</div>
<p id="section-18.1-5" class="keepWithNext">SDP Answer (2)<a href="#section-18.1-5" class="pilcrow">¶</a></p>
<div id="section-18.1-6">
<pre class="lang-sdp sourcecode">
    v=0
    o=bob 2808844564 2808844564 IN IP6 2001:db8::1
    s=
    c=IN IP6 2001:db8::1
    t=0 0
    a=group:BUNDLE foo bar

    m=audio 20000 RTP/AVP 0
    b=AS:200
    a=mid:foo
    a=rtcp-mux
    a=rtpmap:0 PCMU/8000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

    m=video 20000 RTP/AVP 32
    b=AS:1000
    a=mid:bar
    a=rtpmap:32 MPV/90000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
</pre><a href="#section-18.1-6" class="pilcrow">¶</a>
</div>
</section>
</div>
<div id="sec-example-bunrej">
<section id="section-18.2">
        <h3 id="name-example-bundle-group-reject">
<a href="#section-18.2" class="section-number selfRef">18.2. </a><a href="#name-example-bundle-group-reject" class="section-name selfRef">Example: BUNDLE Group Rejected</a>
        </h3>
<p id="section-18.2-1">
        The example below shows:<a href="#section-18.2-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-18.2-2.1">An initial BUNDLE offer, in which the offerer wants to
          negotiate a BUNDLE group and indicates the audio "m=" section
          as the suggested offerer-tagged "m=" section.<a href="#section-18.2-2.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-18.2-2.2">An initial BUNDLE answer, in which the answerer rejects
          the creation of the BUNDLE group, generates a normal answer,
          and assigns a unique address:port to each "m=" section in
          the answer.<a href="#section-18.2-2.2" class="pilcrow">¶</a>
</li>
        </ul>
<p id="section-18.2-3" class="keepWithNext">SDP Offer (1)<a href="#section-18.2-3" class="pilcrow">¶</a></p>
<div id="section-18.2-4">
<pre class="lang-sdp sourcecode">
    v=0
    o=alice 2890844526 2890844526 IN IP6 2001:db8::3
    s=
    c=IN IP6 2001:db8::3
    t=0 0
    a=group:BUNDLE foo bar

    m=audio 10000 RTP/AVP 0 8 97
    b=AS:200
    a=mid:foo
    a=rtcp-mux
    a=rtpmap:0 PCMU/8000
    a=rtpmap:8 PCMA/8000
    a=rtpmap:97 iLBC/8000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

    m=video 10002 RTP/AVP 31 32
    b=AS:1000
    a=mid:bar
    a=rtcp-mux
    a=rtpmap:31 H261/90000
    a=rtpmap:32 MPV/90000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
</pre><a href="#section-18.2-4" class="pilcrow">¶</a>
</div>
<p id="section-18.2-5" class="keepWithNext">SDP Answer (2)<a href="#section-18.2-5" class="pilcrow">¶</a></p>
<div id="section-18.2-6">
<pre class="lang-sdp sourcecode">
    v=0
    o=bob 2808844564 2808844564 IN IP6 2001:db8::1
    s=
    c=IN IP6 2001:db8::1
    t=0 0

    m=audio 20000 RTP/AVP 0
    b=AS:200
    a=rtcp-mux
    a=rtpmap:0 PCMU/8000

    m=video 30000 RTP/AVP 32
    b=AS:1000
    a=rtcp-mux
    a=rtpmap:32 MPV/90000
</pre><a href="#section-18.2-6" class="pilcrow">¶</a>
</div>
</section>
</div>
<div id="sec-example-off-add">
<section id="section-18.3">
        <h3 id="name-example-offerer-adds-a-medi">
<a href="#section-18.3" class="section-number selfRef">18.3. </a><a href="#name-example-offerer-adds-a-medi" class="section-name selfRef">Example: Offerer Adds a Media Description to a BUNDLE Group</a>
        </h3>
<p id="section-18.3-1">
        The example below shows:<a href="#section-18.3-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-18.3-2.1">A subsequent offer, in which the offerer adds a new bundled "m=" section (video), indicated by the "zen"
          identification-tag, to a previously negotiated BUNDLE group; indicates the new "m=" section as the
          offerer-tagged "m=" section; and assigns the offerer BUNDLE address:port to that "m=" section.<a href="#section-18.3-2.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-18.3-2.2">A subsequent answer, in which the answerer indicates the new video "m=" section in the answer as the answerer-tagged "m=" section
          and assigns the answerer BUNDLE address:port to that "m=" section.<a href="#section-18.3-2.2" class="pilcrow">¶</a>
</li>
        </ul>
<p id="section-18.3-3" class="keepWithNext">SDP Offer (1)<a href="#section-18.3-3" class="pilcrow">¶</a></p>
<div id="section-18.3-4">
<pre class="lang-sdp sourcecode">
    v=0
    o=alice 2890844526 2890844526 IN IP6 2001:db8::3
    s=
    c=IN IP6 2001:db8::3
    t=0 0
    a=group:BUNDLE zen foo bar

    m=audio 10000 RTP/AVP 0 8 97
    b=AS:200
    a=mid:foo
    a=rtpmap:0 PCMU/8000
    a=rtpmap:8 PCMA/8000
    a=rtpmap:97 iLBC/8000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

    m=video 10000 RTP/AVP 31 32
    b=AS:1000
    a=mid:bar
    a=rtpmap:31 H261/90000
    a=rtpmap:32 MPV/90000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

    m=video 10000 RTP/AVP 66
    b=AS:1000
    a=mid:zen
    a=rtcp-mux
    a=rtpmap:66 H261/90000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
</pre><a href="#section-18.3-4" class="pilcrow">¶</a>
</div>
<p id="section-18.3-5" class="keepWithNext">SDP Answer (2)<a href="#section-18.3-5" class="pilcrow">¶</a></p>
<div id="section-18.3-6">
<pre class="lang-sdp sourcecode">
    v=0
    o=bob 2808844564 2808844564 IN IP6 2001:db8::1
    s=
    c=IN IP6 2001:db8::1
    t=0 0
    a=group:BUNDLE zen foo bar

    m=audio 20000 RTP/AVP 0
    b=AS:200
    a=mid:foo
    a=rtpmap:0 PCMU/8000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

    m=video 20000 RTP/AVP 32
    b=AS:1000
    a=mid:bar
    a=rtpmap:32 MPV/90000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

    m=video 20000 RTP/AVP 66
    b=AS:1000
    a=mid:zen
    a=rtcp-mux
    a=rtpmap:66 H261/90000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
</pre><a href="#section-18.3-6" class="pilcrow">¶</a>
</div>
</section>
</div>
<div id="sec-example-off-mov">
<section id="section-18.4">
        <h3 id="name-example-offerer-moves-a-med">
<a href="#section-18.4" class="section-number selfRef">18.4. </a><a href="#name-example-offerer-moves-a-med" class="section-name selfRef">Example: Offerer Moves a Media Description Out of a BUNDLE Group</a>
        </h3>
<p id="section-18.4-1">
        The example below shows:<a href="#section-18.4-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-18.4-2.1">A subsequent offer, in which the offerer removes an "m=" section (video), indicated by the "zen"
          identification-tag, from a previously negotiated BUNDLE group; indicates one of the bundled "m=" sections (audio) 
          remaining in the BUNDLE group as the offerer-tagged "m=" section; and assigns the offerer BUNDLE address:port to that "m=" section.<a href="#section-18.4-2.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-18.4-2.2">A subsequent answer, in which the answerer removes the "m=" section from the BUNDLE group, indicates the audio "m=" section 
          in the answer as the answerer-tagged "m=" section, and assigns the answerer BUNDLE address:port to that "m=" section.<a href="#section-18.4-2.2" class="pilcrow">¶</a>
</li>
        </ul>
<p id="section-18.4-3" class="keepWithNext">SDP Offer (1)<a href="#section-18.4-3" class="pilcrow">¶</a></p>
<div id="section-18.4-4">
<pre class="lang-sdp sourcecode">
    v=0
    o=alice 2890844526 2890844526 IN IP6 2001:db8::3
    s=
    c=IN IP6 2001:db8::3
    t=0 0
    a=group:BUNDLE foo bar

    m=audio 10000 RTP/AVP 0 8 97
    b=AS:200
    a=mid:foo
    a=rtcp-mux
    a=rtpmap:0 PCMU/8000
    a=rtpmap:8 PCMA/8000
    a=rtpmap:97 iLBC/8000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

    m=video 10000 RTP/AVP 31 32
    b=AS:1000
    a=mid:bar
    a=rtpmap:31 H261/90000
    a=rtpmap:32 MPV/90000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

    m=video 50000 RTP/AVP 66
    b=AS:1000
    a=mid:zen
    a=rtcp-mux
    a=rtpmap:66 H261/90000
</pre><a href="#section-18.4-4" class="pilcrow">¶</a>
</div>
<p id="section-18.4-5" class="keepWithNext">SDP Answer (2)<a href="#section-18.4-5" class="pilcrow">¶</a></p>
<div id="section-18.4-6">
<pre class="lang-sdp sourcecode">
    v=0
    o=bob 2808844564 2808844564 IN IP6 2001:db8::1
    s=
    c=IN IP6 2001:db8::1
    t=0 0
    a=group:BUNDLE foo bar

    m=audio 20000 RTP/AVP 0
    b=AS:200
    a=mid:foo
    a=rtcp-mux
    a=rtpmap:0 PCMU/8000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

    m=video 20000 RTP/AVP 32
    b=AS:1000
    a=mid:bar
    a=rtpmap:32 MPV/90000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

    m=video 60000 RTP/AVP 66
    b=AS:1000
    a=mid:zen
    a=rtcp-mux
    a=rtpmap:66 H261/90000
</pre><a href="#section-18.4-6" class="pilcrow">¶</a>
</div>
</section>
</div>
<div id="sec-example-off-dis">
<section id="section-18.5">
        <h3 id="name-example-offerer-disables-a-">
<a href="#section-18.5" class="section-number selfRef">18.5. </a><a href="#name-example-offerer-disables-a-" class="section-name selfRef">Example: Offerer Disables a Media Description within a BUNDLE Group</a>
        </h3>
<p id="section-18.5-1">
        The example below shows:<a href="#section-18.5-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-18.5-2.1">A subsequent offer, in which the offerer disables (by assigning a zero port value) an "m=" section (video), indicated by the "zen"
          identification-tag, from a previously negotiated BUNDLE group; indicates one of the bundled "m=" sections (audio) 
          remaining active in the BUNDLE group as the offerer-tagged "m=" section; and assigns the offerer BUNDLE address:port to that "m=" section.<a href="#section-18.5-2.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-18.5-2.2">A subsequent answer, in which the answerer disables the "m=" section, indicates the audio "m=" section 
          in the answer as the answerer-tagged "m=" section, and assigns the answerer BUNDLE address:port to that "m=" section.<a href="#section-18.5-2.2" class="pilcrow">¶</a>
</li>
        </ul>
<p id="section-18.5-3" class="keepWithNext">SDP Offer (1)<a href="#section-18.5-3" class="pilcrow">¶</a></p>
<div id="section-18.5-4">
<pre class="lang-sdp sourcecode">
    v=0
    o=alice 2890844526 2890844526 IN IP6 2001:db8::3
    s=
    t=0 0
    a=group:BUNDLE foo bar

    m=audio 10000 RTP/AVP 0 8 97
    c=IN IP6 2001:db8::3
    b=AS:200
    a=mid:foo
    a=rtcp-mux
    a=rtpmap:0 PCMU/8000
    a=rtpmap:8 PCMA/8000
    a=rtpmap:97 iLBC/8000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

    m=video 10000 RTP/AVP 31 32
    c=IN IP6 2001:db8::3
    b=AS:1000
    a=mid:bar
    a=rtpmap:31 H261/90000
    a=rtpmap:32 MPV/90000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

    m=video 0 RTP/AVP 66
    a=mid:zen
    a=rtpmap:66 H261/90000
</pre><a href="#section-18.5-4" class="pilcrow">¶</a>
</div>
<p id="section-18.5-5" class="keepWithNext">SDP Answer (2)<a href="#section-18.5-5" class="pilcrow">¶</a></p>
<div id="section-18.5-6">
<pre class="lang-sdp sourcecode">
    v=0
    o=bob 2808844564 2808844564 IN IP6 2001:db8::1
    s=
    t=0 0
    a=group:BUNDLE foo bar

    m=audio 20000 RTP/AVP 0
    c=IN IP6 2001:db8::1
    b=AS:200
    a=mid:foo
    a=rtcp-mux
    a=rtpmap:0 PCMU/8000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

    m=video 20000 RTP/AVP 32
    c=IN IP6 2001:db8::1
    b=AS:1000
    a=mid:bar
    a=rtpmap:32 MPV/90000
    a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid

    m=video 0 RTP/AVP 66
    a=mid:zen
    a=rtpmap:66 H261/90000
</pre><a href="#section-18.5-6" class="pilcrow">¶</a>
</div>
</section>
</div>
</section>
</div>
<section id="section-19">
      <h2 id="name-references">
<a href="#section-19" class="section-number selfRef">19. </a><a href="#name-references" class="section-name selfRef">References</a>
      </h2>
<section id="section-19.1">
        <h3 id="name-normative-references">
<a href="#section-19.1" class="section-number selfRef">19.1. </a><a href="#name-normative-references" class="section-name selfRef">Normative References</a>
        </h3>
<dl class="references">
<dt id="RFC2119">[RFC2119]</dt>
        <dd>
<span class="refAuthor">Bradner, S.</span>, <span class="refTitle">"Key words for use in RFCs to Indicate Requirement Levels"</span>, <span class="seriesInfo">BCP 14</span>, <span class="seriesInfo">RFC 2119</span>, <span class="seriesInfo">DOI 10.17487/RFC2119</span>, <time datetime="1997-03" class="refDate">March 1997</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc2119">https://www.rfc-editor.org/info/rfc2119</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC3264">[RFC3264]</dt>
        <dd>
<span class="refAuthor">Rosenberg, J.</span> and <span class="refAuthor">H. Schulzrinne</span>, <span class="refTitle">"An Offer/Answer Model with Session Description Protocol (SDP)"</span>, <span class="seriesInfo">RFC 3264</span>, <span class="seriesInfo">DOI 10.17487/RFC3264</span>, <time datetime="2002-06" class="refDate">June 2002</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc3264">https://www.rfc-editor.org/info/rfc3264</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC3550">[RFC3550]</dt>
        <dd>
<span class="refAuthor">Schulzrinne, H.</span>, <span class="refAuthor">Casner, S.</span>, <span class="refAuthor">Frederick, R.</span>, and <span class="refAuthor">V. Jacobson</span>, <span class="refTitle">"RTP: A Transport Protocol for Real-Time Applications"</span>, <span class="seriesInfo">STD 64</span>, <span class="seriesInfo">RFC 3550</span>, <span class="seriesInfo">DOI 10.17487/RFC3550</span>, <time datetime="2003-07" class="refDate">July 2003</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc3550">https://www.rfc-editor.org/info/rfc3550</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC3605">[RFC3605]</dt>
        <dd>
<span class="refAuthor">Huitema, C.</span>, <span class="refTitle">"Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)"</span>, <span class="seriesInfo">RFC 3605</span>, <span class="seriesInfo">DOI 10.17487/RFC3605</span>, <time datetime="2003-10" class="refDate">October 2003</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc3605">https://www.rfc-editor.org/info/rfc3605</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC3629">[RFC3629]</dt>
        <dd>
<span class="refAuthor">Yergeau, F.</span>, <span class="refTitle">"UTF-8, a transformation format of ISO 10646"</span>, <span class="seriesInfo">STD 63</span>, <span class="seriesInfo">RFC 3629</span>, <span class="seriesInfo">DOI 10.17487/RFC3629</span>, <time datetime="2003-11" class="refDate">November 2003</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc3629">https://www.rfc-editor.org/info/rfc3629</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC3711">[RFC3711]</dt>
        <dd>
<span class="refAuthor">Baugher, M.</span>, <span class="refAuthor">McGrew, D.</span>, <span class="refAuthor">Naslund, M.</span>, <span class="refAuthor">Carrara, E.</span>, and <span class="refAuthor">K. Norrman</span>, <span class="refTitle">"The Secure Real-time Transport Protocol (SRTP)"</span>, <span class="seriesInfo">RFC 3711</span>, <span class="seriesInfo">DOI 10.17487/RFC3711</span>, <time datetime="2004-03" class="refDate">March 2004</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc3711">https://www.rfc-editor.org/info/rfc3711</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC4566">[RFC4566]</dt>
        <dd>
<span class="refAuthor">Handley, M.</span>, <span class="refAuthor">Jacobson, V.</span>, and <span class="refAuthor">C. Perkins</span>, <span class="refTitle">"SDP: Session Description Protocol"</span>, <span class="seriesInfo">RFC 4566</span>, <span class="seriesInfo">DOI 10.17487/RFC4566</span>, <time datetime="2006-07" class="refDate">July 2006</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4566">https://www.rfc-editor.org/info/rfc4566</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC4961">[RFC4961]</dt>
        <dd>
<span class="refAuthor">Wing, D.</span>, <span class="refTitle">"Symmetric RTP / RTP Control Protocol (RTCP)"</span>, <span class="seriesInfo">BCP 131</span>, <span class="seriesInfo">RFC 4961</span>, <span class="seriesInfo">DOI 10.17487/RFC4961</span>, <time datetime="2007-07" class="refDate">July 2007</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4961">https://www.rfc-editor.org/info/rfc4961</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC5761">[RFC5761]</dt>
        <dd>
<span class="refAuthor">Perkins, C.</span> and <span class="refAuthor">M. Westerlund</span>, <span class="refTitle">"Multiplexing RTP Data and Control Packets on a Single Port"</span>, <span class="seriesInfo">RFC 5761</span>, <span class="seriesInfo">DOI 10.17487/RFC5761</span>, <time datetime="2010-04" class="refDate">April 2010</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc5761">https://www.rfc-editor.org/info/rfc5761</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC5764">[RFC5764]</dt>
        <dd>
<span class="refAuthor">McGrew, D.</span> and <span class="refAuthor">E. Rescorla</span>, <span class="refTitle">"Datagram Transport Layer Security (DTLS) Extension to Establish Keys for the Secure Real-time Transport Protocol (SRTP)"</span>, <span class="seriesInfo">RFC 5764</span>, <span class="seriesInfo">DOI 10.17487/RFC5764</span>, <time datetime="2010-05" class="refDate">May 2010</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc5764">https://www.rfc-editor.org/info/rfc5764</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC5888">[RFC5888]</dt>
        <dd>
<span class="refAuthor">Camarillo, G.</span> and <span class="refAuthor">H. Schulzrinne</span>, <span class="refTitle">"The Session Description Protocol (SDP) Grouping Framework"</span>, <span class="seriesInfo">RFC 5888</span>, <span class="seriesInfo">DOI 10.17487/RFC5888</span>, <time datetime="2010-06" class="refDate">June 2010</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc5888">https://www.rfc-editor.org/info/rfc5888</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC6347">[RFC6347]</dt>
        <dd>
<span class="refAuthor">Rescorla, E.</span> and <span class="refAuthor">N. Modadugu</span>, <span class="refTitle">"Datagram Transport Layer Security Version 1.2"</span>, <span class="seriesInfo">RFC 6347</span>, <span class="seriesInfo">DOI 10.17487/RFC6347</span>, <time datetime="2012-01" class="refDate">January 2012</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6347">https://www.rfc-editor.org/info/rfc6347</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7941">[RFC7941]</dt>
        <dd>
<span class="refAuthor">Westerlund, M.</span>, <span class="refAuthor">Burman, B.</span>, <span class="refAuthor">Even, R.</span>, and <span class="refAuthor">M. Zanaty</span>, <span class="refTitle">"RTP Header Extension for the RTP Control Protocol (RTCP) Source Description Items"</span>, <span class="seriesInfo">RFC 7941</span>, <span class="seriesInfo">DOI 10.17487/RFC7941</span>, <time datetime="2016-08" class="refDate">August 2016</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7941">https://www.rfc-editor.org/info/rfc7941</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8174">[RFC8174]</dt>
        <dd>
<span class="refAuthor">Leiba, B.</span>, <span class="refTitle">"Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"</span>, <span class="seriesInfo">BCP 14</span>, <span class="seriesInfo">RFC 8174</span>, <span class="seriesInfo">DOI 10.17487/RFC8174</span>, <time datetime="2017-05" class="refDate">May 2017</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8174">https://www.rfc-editor.org/info/rfc8174</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8285">[RFC8285]</dt>
        <dd>
<span class="refAuthor">Singer, D.</span>, <span class="refAuthor">Desineni, H.</span>, and <span class="refAuthor">R. Even, Ed.</span>, <span class="refTitle">"A General Mechanism for RTP Header Extensions"</span>, <span class="seriesInfo">RFC 8285</span>, <span class="seriesInfo">DOI 10.17487/RFC8285</span>, <time datetime="2017-10" class="refDate">October 2017</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8285">https://www.rfc-editor.org/info/rfc8285</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8445">[RFC8445]</dt>
        <dd>
<span class="refAuthor">Keranen, A.</span>, <span class="refAuthor">Holmberg, C.</span>, and <span class="refAuthor">J. Rosenberg</span>, <span class="refTitle">"Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal"</span>, <span class="seriesInfo">RFC 8445</span>, <span class="seriesInfo">DOI 10.17487/RFC8445</span>, <time datetime="2018-07" class="refDate">July 2018</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8445">https://www.rfc-editor.org/info/rfc8445</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8839">[RFC8839]</dt>
        <dd>
<span class="refAuthor">Petit-Huguenin, M.</span>, <span class="refAuthor">Nandakumar, S.</span>, <span class="refAuthor">Holmberg, C.</span>, <span class="refAuthor">Keränen, A.</span>, and <span class="refAuthor">R. Shpount</span>, <span class="refTitle">"Session Description Protocol (SDP) Offer/Answer Procedures for Interactive Connectivity Establishment (ICE)"</span>, <span class="seriesInfo">RFC 8839</span>, <span class="seriesInfo">DOI 10.17487/RFC8839</span>, <time datetime="2021-01" class="refDate">January 2021</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8839">https://www.rfc-editor.org/info/rfc8839</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8840">[RFC8840]</dt>
        <dd>
<span class="refAuthor">Ivov, E.</span>, <span class="refAuthor">Stach, T.</span>, <span class="refAuthor">Marocco, E.</span>, and <span class="refAuthor">C. Holmberg</span>, <span class="refTitle">"A Session Initiation Protocol (SIP) Usage for Incremental Provisioning of Candidates for the Interactive Connectivity Establishment (Trickle ICE)"</span>, <span class="seriesInfo">RFC 8840</span>, <span class="seriesInfo">DOI 10.17487/RFC8840</span>, <time datetime="2021-01" class="refDate">January 2021</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8840">https://www.rfc-editor.org/info/rfc8840</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8858">[RFC8858]</dt>
        <dd>
<span class="refAuthor">Holmberg, C.</span>, <span class="refTitle">"Indicating Exclusive Support of RTP and RTP Control Protocol (RTCP) Multiplexing Using the Session Description Protocol (SDP)"</span>, <span class="seriesInfo">RFC 8858</span>, <span class="seriesInfo">DOI 10.17487/RFC8858</span>, <time datetime="2021-01" class="refDate">January 2021</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8858">https://www.rfc-editor.org/info/rfc8858</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8859">[RFC8859]</dt>
      <dd>
<span class="refAuthor">Nandakumar, S.</span>, <span class="refTitle">"A Framework for Session Description Protocol (SDP) Attributes When Multiplexing"</span>, <span class="seriesInfo">RFC 8859</span>, <span class="seriesInfo">DOI 10.17487/RFC8859</span>, <time datetime="2021-01" class="refDate">January 2021</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8859">https://www.rfc-editor.org/info/rfc8859</a>&gt;</span>. </dd>
<dd class="break"></dd>
</dl>
</section>
<section id="section-19.2">
        <h3 id="name-informative-references">
<a href="#section-19.2" class="section-number selfRef">19.2. </a><a href="#name-informative-references" class="section-name selfRef">Informative References</a>
        </h3>
<dl class="references">
<dt id="Err6431">[Err6431]</dt>
        <dd>
<span class="refAuthor">RFC Errata</span>, <span class="refTitle">Erratum ID 6431</span>, <span class="refContent">RFC 8843</span>, <span>&lt;<a href="https://www.rfc-editor.org/errata/eid6431">https://www.rfc-editor.org/errata/eid6431</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="Err6437">[Err6437]</dt>
        <dd>
<span class="refAuthor">RFC Errata</span>, <span class="refTitle">Erratum ID 6437</span>, <span class="refContent">RFC 8843</span>, <span>&lt;<a href="https://www.rfc-editor.org/errata/eid6437">https://www.rfc-editor.org/errata/eid6437</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="I-D.ietf-avtext-lrr">[LLR-RTCP]</dt>
        <dd>
<span class="refAuthor">Lennox, J.</span>, <span class="refAuthor">Hong, D.</span>, <span class="refAuthor">Uberti, J.</span>, <span class="refAuthor">Holmer, S.</span>, and <span class="refAuthor">M. Flodman</span>, <span class="refTitle">"The Layer Refresh Request (LRR) RTCP Feedback Message"</span>, <span class="refContent">Work in Progress</span>, <span class="seriesInfo">Internet-Draft, draft-ietf-avtext-lrr-07</span>, <time datetime="2017-07-02" class="refDate">2 July 2017</time>, <span>&lt;<a href="https://datatracker.ietf.org/doc/html/draft-ietf-avtext-lrr-07">https://datatracker.ietf.org/doc/html/draft-ietf-avtext-lrr-07</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC2543">[RFC2543]</dt>
        <dd>
<span class="refAuthor">Handley, M.</span>, <span class="refAuthor">Schulzrinne, H.</span>, <span class="refAuthor">Schooler, E.</span>, and <span class="refAuthor">J. Rosenberg</span>, <span class="refTitle">"SIP: Session Initiation Protocol"</span>, <span class="seriesInfo">RFC 2543</span>, <span class="seriesInfo">DOI 10.17487/RFC2543</span>, <time datetime="1999-03" class="refDate">March 1999</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc2543">https://www.rfc-editor.org/info/rfc2543</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC3261">[RFC3261]</dt>
        <dd>
<span class="refAuthor">Rosenberg, J.</span>, <span class="refAuthor">Schulzrinne, H.</span>, <span class="refAuthor">Camarillo, G.</span>, <span class="refAuthor">Johnston, A.</span>, <span class="refAuthor">Peterson, J.</span>, <span class="refAuthor">Sparks, R.</span>, <span class="refAuthor">Handley, M.</span>, and <span class="refAuthor">E. Schooler</span>, <span class="refTitle">"SIP: Session Initiation Protocol"</span>, <span class="seriesInfo">RFC 3261</span>, <span class="seriesInfo">DOI 10.17487/RFC3261</span>, <time datetime="2002-06" class="refDate">June 2002</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc3261">https://www.rfc-editor.org/info/rfc3261</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC3611">[RFC3611]</dt>
        <dd>
<span class="refAuthor">Friedman, T., Ed.</span>, <span class="refAuthor">Caceres, R., Ed.</span>, and <span class="refAuthor">A. Clark, Ed.</span>, <span class="refTitle">"RTP Control Protocol Extended Reports (RTCP XR)"</span>, <span class="seriesInfo">RFC 3611</span>, <span class="seriesInfo">DOI 10.17487/RFC3611</span>, <time datetime="2003-11" class="refDate">November 2003</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc3611">https://www.rfc-editor.org/info/rfc3611</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC4585">[RFC4585]</dt>
        <dd>
<span class="refAuthor">Ott, J.</span>, <span class="refAuthor">Wenger, S.</span>, <span class="refAuthor">Sato, N.</span>, <span class="refAuthor">Burmeister, C.</span>, and <span class="refAuthor">J. Rey</span>, <span class="refTitle">"Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)"</span>, <span class="seriesInfo">RFC 4585</span>, <span class="seriesInfo">DOI 10.17487/RFC4585</span>, <time datetime="2006-07" class="refDate">July 2006</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4585">https://www.rfc-editor.org/info/rfc4585</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC5104">[RFC5104]</dt>
        <dd>
<span class="refAuthor">Wenger, S.</span>, <span class="refAuthor">Chandra, U.</span>, <span class="refAuthor">Westerlund, M.</span>, and <span class="refAuthor">B. Burman</span>, <span class="refTitle">"Codec Control Messages in the RTP Audio-Visual Profile with Feedback (AVPF)"</span>, <span class="seriesInfo">RFC 5104</span>, <span class="seriesInfo">DOI 10.17487/RFC5104</span>, <time datetime="2008-02" class="refDate">February 2008</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc5104">https://www.rfc-editor.org/info/rfc5104</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC5576">[RFC5576]</dt>
        <dd>
<span class="refAuthor">Lennox, J.</span>, <span class="refAuthor">Ott, J.</span>, and <span class="refAuthor">T. Schierl</span>, <span class="refTitle">"Source-Specific Media Attributes in the Session Description Protocol (SDP)"</span>, <span class="seriesInfo">RFC 5576</span>, <span class="seriesInfo">DOI 10.17487/RFC5576</span>, <time datetime="2009-06" class="refDate">June 2009</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc5576">https://www.rfc-editor.org/info/rfc5576</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7160">[RFC7160]</dt>
        <dd>
<span class="refAuthor">Petit-Huguenin, M.</span> and <span class="refAuthor">G. Zorn, Ed.</span>, <span class="refTitle">"Support for Multiple Clock Rates in an RTP Session"</span>, <span class="seriesInfo">RFC 7160</span>, <span class="seriesInfo">DOI 10.17487/RFC7160</span>, <time datetime="2014-04" class="refDate">April 2014</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7160">https://www.rfc-editor.org/info/rfc7160</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7201">[RFC7201]</dt>
        <dd>
<span class="refAuthor">Westerlund, M.</span> and <span class="refAuthor">C. Perkins</span>, <span class="refTitle">"Options for Securing RTP Sessions"</span>, <span class="seriesInfo">RFC 7201</span>, <span class="seriesInfo">DOI 10.17487/RFC7201</span>, <time datetime="2014-04" class="refDate">April 2014</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7201">https://www.rfc-editor.org/info/rfc7201</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7656">[RFC7656]</dt>
        <dd>
<span class="refAuthor">Lennox, J.</span>, <span class="refAuthor">Gross, K.</span>, <span class="refAuthor">Nandakumar, S.</span>, <span class="refAuthor">Salgueiro, G.</span>, and <span class="refAuthor">B. Burman, Ed.</span>, <span class="refTitle">"A Taxonomy of Semantics and Mechanisms for Real-Time Transport Protocol (RTP) Sources"</span>, <span class="seriesInfo">RFC 7656</span>, <span class="seriesInfo">DOI 10.17487/RFC7656</span>, <time datetime="2015-11" class="refDate">November 2015</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7656">https://www.rfc-editor.org/info/rfc7656</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7657">[RFC7657]</dt>
        <dd>
<span class="refAuthor">Black, D., Ed.</span> and <span class="refAuthor">P. Jones</span>, <span class="refTitle">"Differentiated Services (Diffserv) and Real-Time Communication"</span>, <span class="seriesInfo">RFC 7657</span>, <span class="seriesInfo">DOI 10.17487/RFC7657</span>, <time datetime="2015-11" class="refDate">November 2015</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7657">https://www.rfc-editor.org/info/rfc7657</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8829">[RFC8829]</dt>
        <dd>
<span class="refAuthor">Uberti, J.</span>, <span class="refAuthor">Jennings, C.</span>, and <span class="refAuthor">E. Rescorla, Ed.</span>, <span class="refTitle">"JavaScript Session Establishment Protocol (JSEP)"</span>, <span class="seriesInfo">RFC 8829</span>, <span class="seriesInfo">DOI 10.17487/RFC8829</span>, <time datetime="2021-01" class="refDate">January 2021</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8829">https://www.rfc-editor.org/info/rfc8829</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8838">[RFC8838]</dt>
        <dd>
<span class="refAuthor">Ivov, E.</span>, <span class="refAuthor">Uberti, J.</span>, and <span class="refAuthor">P. Saint-Andre</span>, <span class="refTitle">"Trickle ICE: Incremental Provisioning of Candidates for the Interactive Connectivity Establishment (ICE) Protocol"</span>, <span class="seriesInfo">RFC 8838</span>, <span class="seriesInfo">DOI 10.17487/RFC8838</span>, <time datetime="2021-01" class="refDate">January 2021</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8838">https://www.rfc-editor.org/info/rfc8838</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8843">[RFC8843]</dt>
      <dd>
<span class="refAuthor">Holmberg, C.</span>, <span class="refAuthor">Alvestrand, H.</span>, and <span class="refAuthor">C. Jennings</span>, <span class="refTitle">"Negotiating Media Multiplexing Using the Session Description Protocol (SDP)"</span>, <span class="seriesInfo">RFC 8843</span>, <span class="seriesInfo">DOI 10.17487/RFC8843</span>, <time datetime="2021-01" class="refDate">January 2021</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8843">https://www.rfc-editor.org/info/rfc8843</a>&gt;</span>. </dd>
<dd class="break"></dd>
</dl>
</section>
</section>
<div id="appendix">
<section id="appendix-A">
      <h2 id="name-design-considerations">
<a href="#appendix-A" class="section-number selfRef">Appendix A. </a><a href="#name-design-considerations" class="section-name selfRef">Design Considerations</a>
      </h2>
<p id="appendix-A-1">
        One of the main issues regarding the BUNDLE grouping
        extensions has been whether, in offers and answers, the same
        port value can be inserted in "m=" lines associated with a
        BUNDLE group, as the purpose of the extension is to negotiate
        the usage of a single transport for media specified by the
        "m=" sections. Issues with both approaches, discussed in <a href="#appendix" class="xref">Appendix A</a>, have been raised. The outcome was to
        specify a mechanism that uses offers with both different and
        identical port values.<a href="#appendix-A-1" class="pilcrow">¶</a></p>
<p id="appendix-A-2">
        Below are the primary issues that have been considered when defining the "BUNDLE"
        grouping extension:<a href="#appendix-A-2" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="olPercent" id="appendix-A-3">
        <dt>1)</dt>
<dd id="appendix-A-3.1">Interoperability with existing User Agents (UAs).<a href="#appendix-A-3.1" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt>2)</dt>
<dd id="appendix-A-3.2">Interoperability with intermediary Back-to-Back User Agent (B2BUA) and proxy entities.<a href="#appendix-A-3.2" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt>3)</dt>
<dd id="appendix-A-3.3">The number of ICE candidates and the time to gather them.<a href="#appendix-A-3.3" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt>4)</dt>
<dd id="appendix-A-3.4">Different error scenarios and when they occur.<a href="#appendix-A-3.4" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt>5)</dt>
<dd id="appendix-A-3.5">SDP offer/answer impacts, including usage of port number value zero.<a href="#appendix-A-3.5" class="pilcrow">¶</a>
</dd>
      <dd class="break"></dd>
</dl>
<section id="appendix-A.1">
        <h3 id="name-ua-interoperability">
<a href="#appendix-A.1" class="section-number selfRef">A.1. </a><a href="#name-ua-interoperability" class="section-name selfRef">UA Interoperability</a>
        </h3>
<p id="appendix-A.1-1">
        Consider the following SDP offer/answer exchange, where Alice sends an offer to Bob:<a href="#appendix-A.1-1" class="pilcrow">¶</a></p>
<p id="appendix-A.1-2" class="keepWithNext">SDP Offer<a href="#appendix-A.1-2" class="pilcrow">¶</a></p>
<div id="appendix-A.1-3">
<pre class="lang-sdp sourcecode">
    v=0
    o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
    s=
    c=IN IP4 atlanta.example.com
    t=0 0

    m=audio 10000 RTP/AVP 97
    a=rtpmap:97 iLBC/8000
    m=video 10002 RTP/AVP 97
    a=rtpmap:97 H261/90000
</pre><a href="#appendix-A.1-3" class="pilcrow">¶</a>
</div>
<p id="appendix-A.1-4" class="keepWithNext">SDP Answer<a href="#appendix-A.1-4" class="pilcrow">¶</a></p>
<div id="appendix-A.1-5">
<pre class="lang-sdp sourcecode">
    v=0
    o=bob 2808844564 2808844564 IN IP4 biloxi.example.com
    s=
    c=IN IP4 biloxi.example.com
    t=0 0

    m=audio 20000 RTP/AVP 97
    a=rtpmap:97 iLBC/8000
    m=video 20002 RTP/AVP 97
    a=rtpmap:97 H261/90000
</pre><a href="#appendix-A.1-5" class="pilcrow">¶</a>
</div>
<p id="appendix-A.1-6">
        <span>[<a href="#RFC4961" class="xref">RFC4961</a>]</span> specifies a way of doing symmetric RTP, but that is a later
        extension to RTP, and Bob cannot assume that Alice supports <span>[<a href="#RFC4961" class="xref">RFC4961</a>]</span>. This
        means that Alice may be sending RTP from a different port than 10000 or
        10002 -- some implementations simply send the RTP from an ephemeral
        port. When Bob's endpoint receives an RTP packet, the only way that Bob
        knows if the packet is to be passed to the video or audio codec is by looking at
        the port it was received on.
        This prompted some SDP implementations to use a port number as an index to find the 
        correct "m=" line in the SDP, since each "m"= section contains a different port number. As a result, some
        implementations that do support symmetric RTP and ICE still use an SDP data
        structure where SDP with "m=" sections with the same port such as:<a href="#appendix-A.1-6" class="pilcrow">¶</a></p>
<p id="appendix-A.1-7" class="keepWithNext">SDP Offer<a href="#appendix-A.1-7" class="pilcrow">¶</a></p>
<div id="appendix-A.1-8">
<pre class="lang-sdp sourcecode">
    v=0
    o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
    s=
    c=IN IP4 atlanta.example.com
    t=0 0

    m=audio 10000 RTP/AVP 97
    a=rtpmap:97 iLBC/8000
    m=video 10000 RTP/AVP 98
    a=rtpmap:98 H261/90000
</pre><a href="#appendix-A.1-8" class="pilcrow">¶</a>
</div>
<p id="appendix-A.1-9">
        will result in the second "m=" section being considered an SDP error
        because it has the same port as the first line.<a href="#appendix-A.1-9" class="pilcrow">¶</a></p>
</section>
<section id="appendix-A.2">
        <h3 id="name-usage-of-port-number-value-">
<a href="#appendix-A.2" class="section-number selfRef">A.2. </a><a href="#name-usage-of-port-number-value-" class="section-name selfRef">Usage of Port Number Value Zero</a>
        </h3>
<p id="appendix-A.2-1">
        In an offer or answer, the media specified by an "m=" section can be
        disabled/rejected by setting the port number value to zero. This is different
        from, e.g., using the SDP direction attributes, where RTCP traffic will
        continue even if the SDP 'inactive' attribute is indicated for the
        associated "m=" section.<a href="#appendix-A.2-1" class="pilcrow">¶</a></p>
<p id="appendix-A.2-2">
        If each "m=" section associated with a BUNDLE group were to contain different
        port values and one of those port values were used for a BUNDLE address:port
        associated with the BUNDLE group, problems would occur if an endpoint wants to
        disable/reject the "m=" section associated with that port by setting the port
        value to zero. After that, no "m=" section would contain the port value that
        is used for the BUNDLE address:port. In addition, it is unclear what would happen
        to the ICE candidates associated with the "m=" section, as they are also used for
        the BUNDLE address:port.<a href="#appendix-A.2-2" class="pilcrow">¶</a></p>
</section>
<section id="appendix-A.3">
        <h3 id="name-b2bua-and-proxy-interoperab">
<a href="#appendix-A.3" class="section-number selfRef">A.3. </a><a href="#name-b2bua-and-proxy-interoperab" class="section-name selfRef">B2BUA and Proxy Interoperability</a>
        </h3>
<p id="appendix-A.3-1">
        Some back-to-back user agents may be configured in a mode where if
        the incoming call leg contains an SDP attribute the B2BUA does not
        understand, the B2BUA still generates that SDP attribute in the Offer
        for the outgoing call leg. Consider a B2BUA that did not understand
        the SDP 'rtcp' attribute, defined in <span>[<a href="#RFC3605" class="xref">RFC3605</a>]</span>, yet acted this way.
        Further, assume that the B2BUA was configured to tear down any call
        where it did not see any RTCP for 5 minutes. In this case, if the B2BUA
        received an Offer like:<a href="#appendix-A.3-1" class="pilcrow">¶</a></p>
<p id="appendix-A.3-2" class="keepWithNext">SDP Offer<a href="#appendix-A.3-2" class="pilcrow">¶</a></p>
<div id="appendix-A.3-3">
<pre class="lang-sdp sourcecode">
    v=0
    o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
    s=
    c=IN IP4 atlanta.example.com
    t=0 0

    m=audio 49170 RTP/AVP 0
    a=rtcp:53020
</pre><a href="#appendix-A.3-3" class="pilcrow">¶</a>
</div>
<p id="appendix-A.3-4">
      it would be looking for RTCP on port 49171 but would not see any
      because the RTCP would be on port 53020, and after five minutes, it would
      tear down the call. Similarly, a B2BUA that did not understand BUNDLE yet
      put it in its offer may be looking for media on the wrong port and
      tear down the call. It is worth noting that a B2BUA that generated an
      Offer with capabilities it does not understand is not compliant with the
      specifications.<a href="#appendix-A.3-4" class="pilcrow">¶</a></p>
<section id="appendix-A.3.1">
          <h4 id="name-traffic-policing">
<a href="#appendix-A.3.1" class="section-number selfRef">A.3.1. </a><a href="#name-traffic-policing" class="section-name selfRef">Traffic Policing</a>
          </h4>
<p id="appendix-A.3.1-1">
          Sometimes intermediaries do not act as B2BUAs, in the sense that
          they don't modify SDP bodies nor do they terminate SIP dialogs.
          However, they may still use SDP information (e.g., IP address and
          port) in order to control traffic gating functions and to set
          traffic policing rules. There might be rules that will trigger
          a session to be terminated in case media is not sent or received
          on the ports retrieved from the SDP. This typically occurs once the
          session is already established and ongoing.<a href="#appendix-A.3.1-1" class="pilcrow">¶</a></p>
</section>
<section id="appendix-A.3.2">
          <h4 id="name-bandwidth-allocation">
<a href="#appendix-A.3.2" class="section-number selfRef">A.3.2. </a><a href="#name-bandwidth-allocation" class="section-name selfRef">Bandwidth Allocation</a>
          </h4>
<p id="appendix-A.3.2-1">
          Sometimes, intermediaries do not act as B2BUAs, in the sense that
          they don't modify SDP bodies nor do they terminate SIP dialogs.
          However, they may still use SDP information (e.g., codecs and
          media types) in order to control bandwidth allocation functions.
          The bandwidth allocation is done per "m=" section, which means that
          it might not be enough if media specified by all "m=" sections
          try to use that bandwidth. That may simply lead to either a bad
          user experience or termination of the call.<a href="#appendix-A.3.2-1" class="pilcrow">¶</a></p>
</section>
</section>
<section id="appendix-A.4">
        <h3 id="name-candidate-gathering">
<a href="#appendix-A.4" class="section-number selfRef">A.4. </a><a href="#name-candidate-gathering" class="section-name selfRef">Candidate Gathering</a>
        </h3>
<p id="appendix-A.4-1">
        When using ICE, a candidate needs to be gathered for each port. This
        takes approximately 20 ms extra for each extra "m=" section due to the NAT
        pacing requirements.


 All of this gathering can be overlapped with other
        things while, e.g., a web page is loading to minimize the impact. If the client
        only wants to generate Traversal Using Relays around NAT (TURN) or STUN ICE candidates for one of the "m="
        lines and then use Trickle ICE <span>[<a href="#RFC8838" class="xref">RFC8838</a>]</span>
        to get the non-host ICE candidates for the rest of the "m=" sections, it <span class="bcp14">MAY</span> do
        that and will not need any additional gathering time.<a href="#appendix-A.4-1" class="pilcrow">¶</a></p>
<p id="appendix-A.4-2">
        Some people have suggested a TURN extension to get a bunch of TURN
        allocations at once. This would only provide a single STUN result, so in
        cases where the other end did not support BUNDLE, it may cause more use of
        the TURN server, but it would be quick in the cases where both sides
        supported BUNDLE and would fall back to a successful call in the other
        cases.<a href="#appendix-A.4-2" class="pilcrow">¶</a></p>
</section>
</section>
</div>
<div id="sec-acks">
<section id="appendix-B">
      <h2 id="name-acknowledgements">
<a href="#name-acknowledgements" class="section-name selfRef">Acknowledgements</a>
      </h2>
<p id="appendix-B-1">
      The usage of the SDP grouping extension for negotiating bundled media is
      based on similar alternatives proposed by <span class="contact-name">Harald       Alvestrand</span> and <span class="contact-name">Cullen Jennings</span>. The BUNDLE
      extension described in this document is based on the different
      alternative proposals, and text (e.g., SDP examples) has been borrowed
      (and, in some cases, modified) from those alternative proposals.<a href="#appendix-B-1" class="pilcrow">¶</a></p>
<p id="appendix-B-2">
      The SDP examples are also modified versions from the ones in the Alvestrand
      proposal.<a href="#appendix-B-2" class="pilcrow">¶</a></p>
<p id="appendix-B-3">
      Thanks to <span class="contact-name">Paul Kyzivat</span>, <span class="contact-name">Martin Thomson</span>, <span class="contact-name">Flemming Andreasen</span>, <span class="contact-name">Thomas Stach</span>, <span class="contact-name">Ari Keränen</span>, <span class="contact-name">Adam Roach</span>, <span class="contact-name">Christian Groves</span>,
      <span class="contact-name">Roman Shpount</span>, <span class="contact-name">Suhas       Nandakumar</span>, <span class="contact-name">Nils Ohlmeier</span>, <span class="contact-name">Jens Guballa</span>, <span class="contact-name">Raju Makaraju</span>, <span class="contact-name">Justin Uberti</span>, <span class="contact-name">Taylor Brandstetter</span>,
      <span class="contact-name">Byron Campen</span>, and <span class="contact-name">Eric       Rescorla</span> for reading the text and providing useful feedback.<a href="#appendix-B-3" class="pilcrow">¶</a></p>
<p id="appendix-B-4">
 Thanks to <span class="contact-name">Bernard Aboba</span>, <span class="contact-name">Peter  Thatcher</span>, <span class="contact-name">Justin Uberti</span>, and <span class="contact-name">Magnus Westerlund</span> for providing the text for the section on
 RTP/RTCP stream association.<a href="#appendix-B-4" class="pilcrow">¶</a></p>
<p id="appendix-B-5">
      Thanks to <span class="contact-name">Magnus Westerlund</span>, <span class="contact-name">Colin Perkins</span>, and <span class="contact-name">Jonathan Lennox</span>
      for providing help and text on the RTP/RTCP procedures.<a href="#appendix-B-5" class="pilcrow">¶</a></p>
<p id="appendix-B-6">
       Thanks to <span class="contact-name">Charlie Kaufman</span> for performing the Sec-Dir review.<a href="#appendix-B-6" class="pilcrow">¶</a></p>
<p id="appendix-B-7">
       Thanks to <span class="contact-name">Linda Dunbar</span> for performing the Gen-ART review.<a href="#appendix-B-7" class="pilcrow">¶</a></p>
<p id="appendix-B-8">
      Thanks to Spotify for providing music for the countless hours of
      document editing.<a href="#appendix-B-8" class="pilcrow">¶</a></p>
</section>
</div>
<div id="authors-addresses">
<section id="appendix-C">
      <h2 id="name-authors-addresses">
<a href="#name-authors-addresses" class="section-name selfRef">Authors' Addresses</a>
      </h2>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Christer Holmberg</span></div>
<div dir="auto" class="left"><span class="org">Ericsson</span></div>
<div dir="auto" class="left"><span class="street-address">Hirsalantie 11</span></div>
<div dir="auto" class="left">FI-<span class="postal-code">02420</span> <span class="locality">Jorvas</span>
</div>
<div dir="auto" class="left"><span class="country-name">Finland</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:christer.holmberg@ericsson.com" class="email">christer.holmberg@ericsson.com</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Harald Tveit Alvestrand</span></div>
<div dir="auto" class="left"><span class="org">Google</span></div>
<div dir="auto" class="left"><span class="street-address">Kungsbron 2</span></div>
<div dir="auto" class="left">SE-<span class="postal-code">11122</span> <span class="locality">Stockholm</span>
</div>
<div dir="auto" class="left"><span class="country-name">Sweden</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:harald@alvestrand.no" class="email">harald@alvestrand.no</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Cullen Jennings</span></div>
<div dir="auto" class="left"><span class="org">Cisco</span></div>
<div dir="auto" class="left"><span class="extended-address">Suite 350</span></div>
<div dir="auto" class="left"><span class="street-address">400 3rd Avenue SW</span></div>
<div dir="auto" class="left">
<span class="locality">Calgary</span> <span class="region">AB</span> <span class="postal-code">T2P 4H2</span>
</div>
<div dir="auto" class="left"><span class="country-name">Canada</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:fluffy@iii.ca" class="email">fluffy@iii.ca</a>
</div>
</address>
</section>
</div>
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