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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 8863: Interactive Connectivity Establishment Patiently Awaiting Connectivity (ICE PAC)</title>
<meta content="Christer Holmberg" name="author">
<meta content="Justin Uberti" name="author">
<meta content="
       
        During the process of establishing peer-to-peer connectivity,
        Interactive Connectivity Establishment (ICE) agents can encounter
        situations where they have no candidate pairs to check, and, as a
        result, conclude that ICE processing has failed. However, because
        additional candidate pairs can be discovered during ICE processing,
        declaring failure at this point may be premature. This document
        discusses when these situations can occur.
       
       
        This document updates RFCs 8445 and 8838 by requiring that
        an ICE agent wait a minimum amount of time before declaring
        ICE failure, even if there are no candidate pairs left to check.
       
    " name="description">
<meta content="xml2rfc 3.5.0" name="generator">
<meta content="ICE" name="keyword">
<meta content="PAC" name="keyword">
<meta content="Candidate" name="keyword">
<meta content="8863" name="rfc.number">
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    WeasyPrint 51
-->
<link href="rfc8863.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,
  this can be consolidated so that style settings occur only in one place, but
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  provided to the RFC Formatter (xml2rfc) work, followed by itemized and
  commented changes found necssary during the development of the v3
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*/

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@import url('https://fonts.googleapis.com/css?family=Noto+Serif'); /* Serif (print) */
@import url('https://fonts.googleapis.com/css?family=Roboto+Mono'); /* Monospace */

@viewport {
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  width: extend-to-zoom;
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@-ms-viewport {
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/* general and mobile first */
html {
}
body {
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*/
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/* links */
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a[href] {
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/* XXX probably not this:
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/* make the index two-column on all but the smallest screens */
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}

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/* pagination */
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/* This is commented out here, as the string-set: doesn't
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/* Changes introduced to fix issues found during implementation */
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/* Separate body from document info even without intervening H1 */
section {
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/* Give the bottom of the ToC some whitespace */
@media screen and (min-width: 1024px) {
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/* Add styling for a link in the ToC that points to the top of the document */
a.toplink {
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}

/* Fix the dl styling to match the RFC 7992 attributes */
dl > dt,
dl.dlParallel > dt {
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}
dl.dlNewline > dt {
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/* Provide styling for table cell text alignment */
table td.text-left,
table th.text-left {
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}
table td.text-center,
table th.text-center {
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}
table td.text-right,
table th.text-right {
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}

/* Make the alternative author contact informatio look less like just another
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.alternative-contact {
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}
address .non-ascii {
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/* With it being possible to set tables with alignment
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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 {
  overflow: visible;
}

/* Control caption placement */
caption {
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}

/* Limit the width of the author address vcard, so names in right-to-left
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address.vcard {
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  margin-right: auto;
}

/* For address alignment dependent on LTR or RTL scripts */
address div.left {
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}
address div.right {
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}

/* Provide table alignment support.  We can't use the alignX classes above
   since they do unwanted things with caption and other styling. */
table.right {
 margin-left: auto;
 margin-right: 0;
}
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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    display: none;
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  /* avoid overwriting the top border line with the ToC header */
  #toc {
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  /* Avoid page breaks inside dl and author address entries */
  .vcard {
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}
/* Tweak the bcp14 keyword presentation */
.bcp14 {
  font-variant: small-caps;
  font-weight: bold;
  font-size: 0.9em;
}
/* Tweak the invisible space above H* in order not to overlay links in text above */
 h2 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  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 */
  padding-top: 24px;
 }
/* Float artwork pilcrow to the right */
@media screen {
  .artwork a.pilcrow {
    display: block;
    line-height: 0.7;
    margin-top: 0.15em;
  }
}
/* Make pilcrows on dd visible */
@media screen {
  dd:hover > a.pilcrow {
    visibility: visible;
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}
/* Make the placement of figcaption match that of a table's caption
   by removing the figure's added bottom margin */
.alignLeft.art-text,
.alignCenter.art-text,
.alignRight.art-text {
   margin-bottom: 0;
}
.alignLeft,
.alignCenter,
.alignRight {
  margin: 1em 0 0 0;
}
/* In print, the pilcrow won't show on hover, so prevent it from taking up space,
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@media print {
  a.pilcrow {
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<link href="https://dx.doi.org/10.17487/rfc8863" rel="alternate">
  <link href="urn:issn:2070-1721" rel="alternate">
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<table class="ears">
<thead><tr>
<td class="left">RFC 8863</td>
<td class="center">ICE PAC</td>
<td class="right">January 2021</td>
</tr></thead>
<tfoot><tr>
<td class="left">Holmberg &amp; Uberti</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/rfc8863" class="eref">8863</a></dd>
<dt class="label-updates">Updates:</dt>
<dd class="updates">
<a href="https://www.rfc-editor.org/rfc/rfc8445" class="eref">8445</a>, <a href="https://www.rfc-editor.org/rfc/rfc8838" class="eref">8838</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="2021-01" class="published">January 2021</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">J. Uberti</div>
<div class="org">Google</div>
</div>
</dd>
</dl>
</div>
<h1 id="rfcnum">RFC 8863</h1>
<h1 id="title">Interactive Connectivity Establishment Patiently Awaiting Connectivity (ICE PAC)</h1>
<section id="section-abstract">
      <h2 id="abstract"><a href="#abstract" class="selfRef">Abstract</a></h2>
<p id="section-abstract-1">
        During the process of establishing peer-to-peer connectivity,
        Interactive Connectivity Establishment (ICE) agents can encounter
        situations where they have no candidate pairs to check, and, as a
        result, conclude that ICE processing has failed. However, because
        additional candidate pairs can be discovered during ICE processing,
        declaring failure at this point may be premature. This document
        discusses when these situations can occur.<a href="#section-abstract-1" class="pilcrow">¶</a></p>
<p id="section-abstract-2">
        This document updates RFCs 8445 and 8838 by requiring that
        an ICE agent wait a minimum amount of time before declaring
        ICE failure, even if there are no candidate pairs left to check.<a href="#section-abstract-2" 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/rfc8863">https://www.rfc-editor.org/info/rfc8863</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) 2021 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
            carefully, as they describe your rights and restrictions with
            respect to this document. Code Components extracted from this
            document must include Simplified BSD License text as described in
            Section 4.e of the Trust Legal Provisions and are provided without
            warranty as described in the Simplified BSD License.<a href="#section-boilerplate.2-2" 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="toc compact ulEmpty">
<li class="toc compact ulEmpty" id="section-toc.1-1.1">
            <p id="section-toc.1-1.1.1" class="keepWithNext"><a href="#section-1" class="xref">1</a>.  <a href="#name-introduction" class="xref">Introduction</a><a href="#section-toc.1-1.1.1" class="pilcrow">¶</a></p>
</li>
          <li class="toc compact ulEmpty" id="section-toc.1-1.2">
            <p id="section-toc.1-1.2.1" class="keepWithNext"><a href="#section-2" class="xref">2</a>.  <a href="#name-conventions" class="xref">Conventions</a><a href="#section-toc.1-1.2.1" class="pilcrow">¶</a></p>
</li>
          <li class="toc compact 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-relevant-scenarios" class="xref">Relevant Scenarios</a><a href="#section-toc.1-1.3.1" class="pilcrow">¶</a></p>
<ul class="toc compact ulEmpty">
<li class="toc compact ulEmpty" id="section-toc.1-1.3.2.1">
                <p id="section-toc.1-1.3.2.1.1" class="keepWithNext"><a href="#section-3.1" class="xref">3.1</a>.  <a href="#name-no-candidates-from-peer" class="xref">No Candidates from Peer</a><a href="#section-toc.1-1.3.2.1.1" class="pilcrow">¶</a></p>
</li>
              <li class="toc compact ulEmpty" id="section-toc.1-1.3.2.2">
                <p id="section-toc.1-1.3.2.2.1"><a href="#section-3.2" class="xref">3.2</a>.  <a href="#name-all-candidates-discarded" class="xref">All Candidates Discarded</a><a href="#section-toc.1-1.3.2.2.1" class="pilcrow">¶</a></p>
</li>
              <li class="toc compact ulEmpty" id="section-toc.1-1.3.2.3">
                <p id="section-toc.1-1.3.2.3.1"><a href="#section-3.3" class="xref">3.3</a>.  <a href="#name-immediate-candidate-pair-fa" class="xref">Immediate Candidate Pair Failure</a><a href="#section-toc.1-1.3.2.3.1" class="pilcrow">¶</a></p>
</li>
            </ul>
</li>
          <li class="toc compact 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-update-to-rfc-8445" class="xref">Update to RFC 8445</a><a href="#section-toc.1-1.4.1" class="pilcrow">¶</a></p>
</li>
          <li class="toc compact 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-update-to-rfc-8838" class="xref">Update to RFC 8838</a><a href="#section-toc.1-1.5.1" class="pilcrow">¶</a></p>
</li>
          <li class="toc compact 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-security-considerations" class="xref">Security Considerations</a><a href="#section-toc.1-1.6.1" class="pilcrow">¶</a></p>
</li>
          <li class="toc compact 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-iana-considerations" class="xref">IANA Considerations</a><a href="#section-toc.1-1.7.1" class="pilcrow">¶</a></p>
</li>
          <li class="toc compact 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-normative-references" class="xref">Normative References</a><a href="#section-toc.1-1.8.1" class="pilcrow">¶</a></p>
</li>
          <li class="toc compact ulEmpty" id="section-toc.1-1.9">
            <p id="section-toc.1-1.9.1"><a href="#section-appendix.a" class="xref"></a><a href="#name-acknowledgements" class="xref">Acknowledgements</a><a href="#section-toc.1-1.9.1" class="pilcrow">¶</a></p>
</li>
          <li class="toc compact ulEmpty" id="section-toc.1-1.10">
            <p id="section-toc.1-1.10.1"><a href="#section-appendix.b" class="xref"></a><a href="#name-authors-addresses" class="xref">Authors' Addresses</a><a href="#section-toc.1-1.10.1" class="pilcrow">¶</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>
<p id="section-1-1">
        <span>[<a href="#RFC8445" class="xref">RFC8445</a>]</span> describes a protocol, Interactive Connectivity Establishment (ICE),
        for Network Address Translator (NAT) traversal for UDP-based communication.<a href="#section-1-1" class="pilcrow">¶</a></p>
<p id="section-1-2">
        When using ICE, endpoints will typically exchange ICE candidates,
        form a list of candidate pairs, and then test each candidate pair to see
        if connectivity can be established. If the test for a given pair fails,
        it is marked accordingly, and if all pairs have failed, the overall
        ICE process typically is considered to have failed.<a href="#section-1-2" class="pilcrow">¶</a></p>
<p id="section-1-3">
        During the process of connectivity checks, additional candidates may
        be created as a result of successful inbound checks from the remote
        peer. Such candidates are referred to as peer-reflexive candidates;
        once discovered, these candidates will be used to form new candidate pairs, which will
        be tested like any other. However, there is an inherent problem
        here; if, before learning about any peer-reflexive candidates, an
        endpoint runs out of candidate pairs to check, either because it has
        none or it considers them all to have failed, it will prematurely
        declare failure and terminate ICE processing. This problem can
        occur in many common situations.<a href="#section-1-3" class="pilcrow">¶</a></p>
<p id="section-1-4">
        This specification updates <span>[<a href="#RFC8445" class="xref">RFC8445</a>]</span>
        and <span>[<a href="#RFC8838" class="xref">RFC8838</a>]</span> by simply
        requiring that an ICE agent wait a minimum amount of time before
        declaring ICE failure, even if there are no candidate pairs to check
        or all candidate pairs have failed. This delay provides enough time
        for the discovery of peer-reflexive candidates, which may eventually
        lead to ICE processing completing successfully.<a href="#section-1-4" class="pilcrow">¶</a></p>
</section>
<section id="section-2">
      <h2 id="name-conventions">
<a href="#section-2" class="section-number selfRef">2. </a><a href="#name-conventions" class="section-name selfRef">Conventions</a>
      </h2>
<p id="section-2-1">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-2-1" class="pilcrow">¶</a></p>
</section>
<section id="section-3">
      <h2 id="name-relevant-scenarios">
<a href="#section-3" class="section-number selfRef">3. </a><a href="#name-relevant-scenarios" class="section-name selfRef">Relevant Scenarios</a>
      </h2>
<p id="section-3-1">
          As noted above, the core problem this specification attempts to
          address is the situation where even after local gathering and remote
          candidate signaling have completed, the ICE agent immediately ends up
          with no valid pairs and no candidate pairs left to check, resulting in
          a premature ICE failure. This failure is premature because not
          enough time has elapsed to allow for discovery of peer-reflexive
          candidates from inbound connectivity checks; if discovered, these
          candidates are very likely to result in a valid pair.<a href="#section-3-1" class="pilcrow">¶</a></p>
<p id="section-3-2">
          In most ICE scenarios, the lengthy timeouts for connectivity check transactions,
          typically tens of seconds, will prevent this problem from occurring. However, there
          are certain specific cases where this problem will frequently occur.<a href="#section-3-2" class="pilcrow">¶</a></p>
<section id="section-3.1">
        <h3 id="name-no-candidates-from-peer">
<a href="#section-3.1" class="section-number selfRef">3.1. </a><a href="#name-no-candidates-from-peer" class="section-name selfRef">No Candidates from Peer</a>
        </h3>
<p id="section-3.1-1">
            Per <span>[<a href="#RFC8838" class="xref">RFC8838</a>]</span>, an ICE agent can provide zero candidates of
            its own. If the agent somehow knows that the remote endpoint is
            directly reachable, gathering local candidates is unnecessary and
            will only cause delays; the peer agent can discover the
            appropriate local candidate via connectivity checks.<a href="#section-3.1-1" class="pilcrow">¶</a></p>
<p id="section-3.1-2">
            However, following the procedures from
            <span>[<a href="#RFC8445" class="xref">RFC8445</a>]</span> strictly will result in immediate
            ICE failure, since the checklist at the peer agent will be
            empty.<a href="#section-3.1-2" class="pilcrow">¶</a></p>
</section>
<section id="section-3.2">
        <h3 id="name-all-candidates-discarded">
<a href="#section-3.2" class="section-number selfRef">3.2. </a><a href="#name-all-candidates-discarded" class="section-name selfRef">All Candidates Discarded</a>
        </h3>
<p id="section-3.2-1">
            Even if the ICE agent provides candidates, they may be discarded
            by the peer agent if it does not know what to do with them.
            For example, candidates may use an address family that the peer
            agent does not support (e.g., a host candidate with an IPv6
            address in a NAT64 scenario) or that may not be usable for some other
            reason.<a href="#section-3.2-1" class="pilcrow">¶</a></p>
<p id="section-3.2-2">
            In these scenarios, when the candidates are discarded, the
            checklist at the peer agent will once again be empty, leading
            to immediate ICE failure.<a href="#section-3.2-2" class="pilcrow">¶</a></p>
</section>
<section id="section-3.3">
        <h3 id="name-immediate-candidate-pair-fa">
<a href="#section-3.3" class="section-number selfRef">3.3. </a><a href="#name-immediate-candidate-pair-fa" class="section-name selfRef">Immediate Candidate Pair Failure</a>
        </h3>
<p id="section-3.3-1">
            <span><a href="https://www.rfc-editor.org/rfc/rfc8445#section-7.2.5.2" class="relref">Section 7.2.5.2</a> of [<a href="#RFC8445" class="xref">RFC8445</a>]</span>
            describes several
            situations in which a candidate pair will be considered to have
            failed, well before the connectivity check transaction timeout.<a href="#section-3.3-1" class="pilcrow">¶</a></p>
<p id="section-3.3-2">
            As a result, even if the ICE agent provides usable candidates,
            the pairs created by the peer agent may fail immediately when
            checked, e.g., a check to a non-routable address that receives an
            immediate ICMP error.<a href="#section-3.3-2" class="pilcrow">¶</a></p>
<p id="section-3.3-3">
            In this situation, the checklist at the peer agent may contain
            only failed pairs, resulting in immediate ICE failure.<a href="#section-3.3-3" class="pilcrow">¶</a></p>
</section>
</section>
<section id="section-4">
      <h2 id="name-update-to-rfc-8445">
<a href="#section-4" class="section-number selfRef">4. </a><a href="#name-update-to-rfc-8445" class="section-name selfRef">Update to RFC 8445</a>
      </h2>
<p id="section-4-1">
        In order to avoid the problem raised by this document, the ICE agent
        needs to wait enough time to allow peer-reflexive candidates to be
        discovered. Accordingly, when a full ICE implementation begins its
        ICE processing, as described in <span>[<a href="#RFC8445" class="xref">RFC8445</a>], <a href="https://www.rfc-editor.org/rfc/rfc8445#section-6.1" class="relref">Section 6.1</a></span>, it <span class="bcp14">MUST</span> set a
        timer, henceforth known as the "PAC timer" (Patiently Awaiting Connectivity), to 
        ensure that ICE will run for a minimum amount of time before determining
        failure.<a href="#section-4-1" class="pilcrow">¶</a></p>
<p id="section-4-2">
        Specifically, the ICE agent will start its timer once it believes
        ICE connectivity checks are starting. This occurs when the agent has
        sent the values needed to perform connectivity checks
        (e.g., the Username Fragment and Password denoted in
        <span>[<a href="#RFC8445" class="xref">RFC8445</a>], <a href="https://www.rfc-editor.org/rfc/rfc8445#section-5.3" class="relref">Section 5.3</a></span>)
        and has received some indication that the remote side is
        ready to start connectivity checks, typically via receipt of the values
        mentioned above. Note that the agent will start the timer even if it
        has not sent or received any ICE candidates.<a href="#section-4-2" class="pilcrow">¶</a></p>
<p id="section-4-3">
        The <span class="bcp14">RECOMMENDED</span> duration for the PAC timer is equal to the agent's
        connectivity check transaction timeout, including all retransmissions.
        When using default values for retransmission timeout (RTO) and Rc, this amounts to 39.5 seconds,
        as explained in <span>[<a href="#RFC5389" class="xref">RFC5389</a>], <a href="https://www.rfc-editor.org/rfc/rfc5389#section-7.2.1" class="relref">Section 7.2.1</a></span>.
        This timeout value is chosen to roughly coincide with the maximum
        possible duration of ICE connectivity checks from the remote peer,
        which, if successful, could create peer-reflexive candidates. Because
        the ICE agent doesn't know the exact number of candidate pairs and pacing
        interval in use by the remote side, this timeout value is simply a
        guess, albeit an educated one. Regardless, for this particular problem,
        the desired benefits will be realized as long as the agent waits
        some reasonable amount of time, and, as usual, the application is in
        the best position to determine what is reasonable for its scenario.<a href="#section-4-3" class="pilcrow">¶</a></p>
<p id="section-4-4">
        While the timer is still running, the ICE agent <span class="bcp14">MUST NOT</span> update a checklist state
        from Running to Failed, even if there are no pairs left in the checklist to check.
        As a result, the ICE agent will not remove any data streams or set the state of the
        ICE session to Failed as long as the timer is running.<a href="#section-4-4" class="pilcrow">¶</a></p>
<p id="section-4-5">
        When the timer period eventually elapses, the ICE agent <span class="bcp14">MUST</span> resume typical
        ICE processing, including setting the state of any checklists to Failed if they
        have no pairs left to check and handling any consequences as indicated
        in <span>[<a href="#RFC8445" class="xref">RFC8445</a>], <a href="https://www.rfc-editor.org/rfc/rfc8445#section-8.1.2" class="relref">Section 8.1.2</a></span>.
        Naturally, if there are no such checklists, no action is necessary.<a href="#section-4-5" class="pilcrow">¶</a></p>
<p id="section-4-6">
        One consequence of this behavior is that in cases where ICE should fail,
        e.g., where both sides provide candidates with unsupported address families,
        ICE will no longer fail immediately -- it will only fail when the
        PAC timer expires. However, because most ICE scenarios
        require an extended period of time to determine failure, the
        fact that some specific scenarios no longer fail quickly should have
        minimal application impact, if any.<a href="#section-4-6" class="pilcrow">¶</a></p>
<p id="section-4-7">
        Note also that the PAC timer is potentially relevant to the ICE nomination
        procedure described in <span>[<a href="#RFC8445" class="xref">RFC8445</a>], <a href="https://www.rfc-editor.org/rfc/rfc8445#section-8.1.1" class="relref">Section 8.1.1</a></span>. That
        specification does not define a minimum duration for ICE processing
        prior to nomination of a candidate pair, but in order to select the
        best candidate pair, ICE needs to run for enough time in order to allow
        peer-reflexive candidates to be discovered and checked, as noted above.
        Accordingly, the controlling ICE agent <span class="bcp14">SHOULD</span> wait a sufficient amount
        of time before nominating candidate pairs, and it <span class="bcp14">MAY</span> use the PAC timer
        to do so. As always, the controlling ICE agent retains
        full discretion and <span class="bcp14">MAY</span> decide, based on its own criteria, to nominate
        pairs prior to the PAC timer period elapsing.<a href="#section-4-7" class="pilcrow">¶</a></p>
</section>
<section id="section-5">
      <h2 id="name-update-to-rfc-8838">
<a href="#section-5" class="section-number selfRef">5. </a><a href="#name-update-to-rfc-8838" class="section-name selfRef">Update to RFC 8838</a>
      </h2>
<p id="section-5-1">
        Trickle ICE <span>[<a href="#RFC8838" class="xref">RFC8838</a>]</span>
        considers a similar problem, namely whether an ICE agent should allow
        a checklist to enter the Failed state if more candidates might
        still be provided by the remote peer. The solution, specified in
        <span>[<a href="#RFC8838" class="xref">RFC8838</a>], <a href="https://www.rfc-editor.org/rfc/rfc8838#section-8" class="relref">Section 8</a></span>, is to
        wait until an end-of-candidates indication has been received
        before determining ICE failure.<a href="#section-5-1" class="pilcrow">¶</a></p>
<p id="section-5-2">
        However, for the same reasons described above,
        the ICE agent may discover peer-reflexive candidates after it has
        received the end-of-candidates indication, and so the solution
        proposed by this document <span class="bcp14">MUST</span> still be used even when
        the ICE agent is using Trickle ICE.<a href="#section-5-2" class="pilcrow">¶</a></p>
<p id="section-5-3">
        Note also that sending an end-of-candidates indication is only a
        <span class="bcp14">SHOULD</span>-strength requirement, which means that ICE agents will need
        to implement a backup mechanism to decide when all candidates
        have been received, typically a timer. Accordingly, ICE agents
        <span class="bcp14">MAY</span> use the PAC timer to also serve as an end-of-candidates fallback.<a href="#section-5-3" class="pilcrow">¶</a></p>
</section>
<div id="section.sec">
<section id="section-6">
      <h2 id="name-security-considerations">
<a href="#section-6" class="section-number selfRef">6. </a><a href="#name-security-considerations" class="section-name selfRef">Security Considerations</a>
      </h2>
<p id="section-6-1">
        The security considerations for ICE are defined in <span>[<a href="#RFC8445" class="xref">RFC8445</a>]</span>.
        This specification only recommends that ICE agents wait for a certain
        period of time before they declare
        ICE failure; it does not introduce new security considerations.<a href="#section-6-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="section.iana">
<section id="section-7">
      <h2 id="name-iana-considerations">
<a href="#section-7" class="section-number selfRef">7. </a><a href="#name-iana-considerations" class="section-name selfRef">IANA Considerations</a>
      </h2>
<p id="section-7-1">
        This document has no IANA actions.<a href="#section-7-1" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-8">
      <h2 id="name-normative-references">
<a href="#section-8" class="section-number selfRef">8. </a><a href="#name-normative-references" class="section-name selfRef">Normative References</a>
      </h2>
<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="RFC5389">[RFC5389]</dt>
      <dd>
<span class="refAuthor">Rosenberg, J.</span><span class="refAuthor">, Mahy, R.</span><span class="refAuthor">, Matthews, P.</span><span class="refAuthor">, and D. Wing</span>, <span class="refTitle">"Session Traversal Utilities for NAT (STUN)"</span>, <span class="seriesInfo">RFC 5389</span>, <span class="seriesInfo">DOI 10.17487/RFC5389</span>, <time datetime="2008-10" class="refDate">October 2008</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc5389">https://www.rfc-editor.org/info/rfc5389</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="RFC8445">[RFC8445]</dt>
      <dd>
<span class="refAuthor">Keranen, A.</span><span class="refAuthor">, Holmberg, C.</span><span class="refAuthor">, and 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="RFC8838">[RFC8838]</dt>
    <dd>
<span class="refAuthor">Ivov, E.</span><span class="refAuthor">, Uberti, J.</span><span class="refAuthor">, and 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>
</dl>
</section>
<div id="sec-acks">
<section id="section-appendix.a">
      <h2 id="name-acknowledgements">
<a href="#name-acknowledgements" class="section-name selfRef">Acknowledgements</a>
      </h2>
<p id="section-appendix.a-1">
        <span class="contact-name">Roman Shpount</span>, <span class="contact-name">Nils Ohlmeier</span>, and <span class="contact-name">Peter Thatcher</span> provided lots of useful input and
        comments.<a href="#section-appendix.a-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="authors-addresses">
<section id="section-appendix.b">
      <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">Justin Uberti</span></div>
<div dir="auto" class="left"><span class="org">Google</span></div>
<div dir="auto" class="left"><span class="street-address">747 6th St W</span></div>
<div dir="auto" class="left">
<span class="locality">Kirkland</span>, <span class="region">WA</span> <span class="postal-code">98033</span>
</div>
<div dir="auto" class="left"><span class="country-name">United States of America</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:justin@uberti.name" class="email">justin@uberti.name</a>
</div>
</address>
</section>
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