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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 8899: Packetization Layer Path MTU Discovery for Datagram Transports</title>
<meta content="Godred Fairhurst" name="author">
<meta content="Tom Jones" name="author">
<meta content="Michael Tüxen" name="author">
<meta content="Irene Rüngeler" name="author">
<meta content="Timo Völker" name="author">
<meta content="
       
   This document specifies Datagram Packetization Layer Path MTU Discovery
   (DPLPMTUD). This is a robust method for Path MTU Discovery
   (PMTUD) for datagram Packetization Layers (PLs).
   It allows a PL, or a datagram
   application that uses a PL, to discover whether a network path can
   support the current size of datagram.  This can be used to detect and
   reduce the message size when a sender encounters a packet black hole.
   It can also probe a network path to discover whether the maximum
   packet size can be increased.  This provides functionality for datagram
   transports that is equivalent to the PLPMTUD
   specification for TCP, specified in RFC 4821, which it updates.
   It also updates the UDP Usage Guidelines to refer to this method
   for use with UDP datagrams and updates SCTP. 
        
   The document provides implementation notes for incorporating
   Datagram PMTUD into IETF datagram transports or applications that use
   datagram transports. 
       
   This specification updates RFC 4960, RFC 4821, RFC 6951, RFC 8085, and RFC
   8261. 
    " name="description">
<meta content="xml2rfc 3.0.0" name="generator">
<meta content="UDP" name="keyword">
<meta content="SCTP" name="keyword">
<meta content="Transport" name="keyword">
<meta content="PMTUD" name="keyword">
<meta content="PLPMTUD" name="keyword">
<meta content="8899" name="rfc.number">
<link href="rfc8899.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
  for now the contents of this file consists first of the initial CSS work as
  provided to the RFC Formatter (xml2rfc) work, followed by itemized and
  commented changes found necssary during the development of the v3
  formatters.

*/

/* fonts */
@import url('https://fonts.googleapis.com/css?family=Noto+Sans'); /* Sans-serif */
@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;
}
@-ms-viewport {
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  zoom: 1.0;
}
/* general and mobile first */
html {
}
body {
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  margin: 1.5em auto;
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.ears {
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}

/* headings */
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dl {
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/* 
dl.nohang > dt {
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*/
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/* links */
a {
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a[href] {
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a[href]:hover {
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figcaption a[href],
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/* XXX probably not this:
a.selfRef:hover {
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}
#identifiers dt {
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#identifiers dd {
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#identifiers .authors .author {
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}
#identifiers .authors .org {
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}

/* The prepared/rendered info at the very bottom of the page */
.docInfo {
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}
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.docInfo .prepared {
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}

/* table of contents */
#toc  {
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}
nav.toc ul {
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}
nav.toc li {
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}
/* references */
.references dt {
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  font-weight: bold;
  min-width: 7em;
}
.references dd {
  margin-left: 8em;
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}

.refInstance {
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.references .ascii {
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}

/* index */
.index ul {
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}
.index ul ul {
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}
.index li {
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}
.indexIndex {
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}
.index a {
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}
/* make the index two-column on all but the smallest screens */
@media (min-width: 600px) {
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/* authors */
address.vcard {
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}

address.vcard .nameRole {
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}
address.vcard .label {
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address.vcard .type {
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.alternative-contact {
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}
hr.addr {
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/* temporary notes */
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}
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.cref {
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}
.crefSource {
  font-style: italic;
}
/* alternative layout for smaller screens */
@media screen and (max-width: 1023px) {
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  #title {
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  h1 {
    font-size: 24px;
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  h2 {
    font-size: 20px;
    margin-top: -18px;  /* provide offset for in-page anchors */
    padding-top: 38px;
  }
  #identifiers dd {
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  #toc {
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    top: 0;
    right: 0;
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    margin: 0;
    background-color: inherit;
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    padding: 4px 0 4px 6px;
    padding-right: 1em;
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    border-left: 1px solid #ddd;
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}

/* alternative layout for wide screens */
@media screen and (min-width: 1024px) {
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    padding-right: 29em;
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  #toc {
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    right: 42px;
    width: 25%;
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}

/* pagination */
@media print {
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  table {
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}

/* This is commented out here, as the string-set: doesn't
   pass W3C validation currently */
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*/

@page :first {
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@page {
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/* Changes introduced to fix issues found during implementation */
/* Make sure links are clickable even if overlapped by following H* */
a {
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/* Separate body from document info even without intervening H1 */
section {
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/* Top align author divs, to avoid names without organization dropping level with org names */
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/* Leave room in document info to show Internet-Draft on one line */
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/* Don't waste quite as much whitespace between label and value in doc info */
#identifiers dd {
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/* Give floating toc a background color (needed when it's a div inside section */
#toc {
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/* Make the collapsed ToC header render white on gray also when it's a link */
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  #toc h2 a:link,
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/* Give the bottom of the ToC some whitespace */
@media screen and (min-width: 1024px) {
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/* Fix the height/width aspect for ascii art*/
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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
   author, and group it closer with the primary author contact information */
.alternative-contact {
  margin: 0.5em 0 0.25em 0;
}
address .non-ascii {
  margin: 0 0 0 2em;
}

/* 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,
   because of a long floating dt label.*/
.references dd {
  overflow: visible;
}

/* Control caption placement */
caption {
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}

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   script don't end up on the other side of the page. */

address.vcard {
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  margin-right: auto;
}

/* For address alignment dependent on LTR or RTL scripts */
address div.left {
  text-align: left;
}
address div.right {
  text-align: right;
}

/* 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 */
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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 {
  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 {
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    display: block;
    line-height: 0.7;
    margin-top: 0.15em;
  }
}
/* Make pilcrows on dd visible */
@media screen {
  dd:hover > a.pilcrow {
    visibility: visible;
  }
}
/* 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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<link href="rfc-local.css" rel="stylesheet" type="text/css">
<link href="https://dx.doi.org/10.17487/rfc8899" rel="alternate">
  <link href="urn:issn:2070-1721" rel="alternate">
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<table class="ears">
<thead><tr>
<td class="left">RFC 8899</td>
<td class="center">DPLPMTUD</td>
<td class="right">September 2020</td>
</tr></thead>
<tfoot><tr>
<td class="left">Fairhurst, 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/rfc8899" class="eref">8899</a></dd>
<dt class="label-updates">Updates:</dt>
<dd class="updates">
<a href="https://www.rfc-editor.org/rfc/rfc4821" class="eref">4821</a>, <a href="https://www.rfc-editor.org/rfc/rfc4960" class="eref">4960</a>, <a href="https://www.rfc-editor.org/rfc/rfc6951" class="eref">6951</a>, <a href="https://www.rfc-editor.org/rfc/rfc8085" class="eref">8085</a>, <a href="https://www.rfc-editor.org/rfc/rfc8261" class="eref">8261</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="2020-09" class="published">September 2020</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">G. Fairhurst</div>
<div class="org">University of Aberdeen</div>
</div>
<div class="author">
      <div class="author-name">T. Jones</div>
<div class="org">University of Aberdeen</div>
</div>
<div class="author">
      <div class="author-name">M. Tüxen</div>
<div class="org">Münster University of Applied Sciences</div>
</div>
<div class="author">
      <div class="author-name">I. Rüngeler</div>
<div class="org">Münster University of Applied Sciences</div>
</div>
<div class="author">
      <div class="author-name">T. Völker</div>
<div class="org">Münster University of Applied Sciences</div>
</div>
</dd>
</dl>
</div>
<h1 id="rfcnum">RFC 8899</h1>
<h1 id="title">Packetization Layer Path MTU Discovery for Datagram Transports</h1>
<section id="section-abstract">
      <h2 id="abstract"><a href="#abstract" class="selfRef">Abstract</a></h2>
<p id="section-abstract-1">
   This document specifies Datagram Packetization Layer Path MTU Discovery
   (DPLPMTUD). This is a robust method for Path MTU Discovery
   (PMTUD) for datagram Packetization Layers (PLs).
   It allows a PL, or a datagram
   application that uses a PL, to discover whether a network path can
   support the current size of datagram.  This can be used to detect and
   reduce the message size when a sender encounters a packet black hole.
   It can also probe a network path to discover whether the maximum
   packet size can be increased.  This provides functionality for datagram
   transports that is equivalent to the PLPMTUD
   specification for TCP, specified in RFC 4821, which it updates.
   It also updates the UDP Usage Guidelines to refer to this method
   for use with UDP datagrams and updates SCTP.<a href="#section-abstract-1" class="pilcrow">¶</a></p>
<p id="section-abstract-2"> 
   The document provides implementation notes for incorporating
   Datagram PMTUD into IETF datagram transports or applications that use
   datagram transports.<a href="#section-abstract-2" class="pilcrow">¶</a></p>
<p id="section-abstract-3">
   This specification updates RFC 4960, RFC 4821, RFC 6951, RFC 8085, and RFC
   8261.<a href="#section-abstract-3" 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/rfc8899">https://www.rfc-editor.org/info/rfc8899</a></span>.<a href="#section-boilerplate.1-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="copyright">
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<a href="#name-copyright-notice" class="section-name selfRef">Copyright Notice</a>
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            Copyright (c) 2020 IETF Trust and the persons identified as the
            document authors. All rights reserved.<a href="#section-boilerplate.2-1" class="pilcrow">¶</a></p>
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            This document is subject to BCP 78 and the IETF Trust's Legal
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            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="ulEmpty compact toc">
<li class="ulEmpty compact toc" 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><a href="#section-toc.1-1.1.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" 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-classical-path-mtu-discover" class="xref">Classical Path MTU Discovery</a><a href="#section-toc.1-1.1.2.1.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" 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-packetization-layer-path-mt" class="xref">Packetization Layer Path MTU Discovery</a><a href="#section-toc.1-1.1.2.2.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" 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-path-mtu-discovery-for-data" class="xref">Path MTU Discovery for Datagram Services</a><a href="#section-toc.1-1.1.2.3.1" class="pilcrow">¶</a></p>
</li>
            </ul>
</li>
          <li class="ulEmpty compact toc" 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><a href="#section-toc.1-1.2.1" class="pilcrow">¶</a></p>
</li>
          <li class="ulEmpty compact toc" 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-features-required-to-provid" class="xref">Features Required to Provide Datagram PLPMTUD</a><a href="#section-toc.1-1.3.1" class="pilcrow">¶</a></p>
</li>
          <li class="ulEmpty compact toc" 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-dplpmtud-mechanisms" class="xref">DPLPMTUD Mechanisms</a><a href="#section-toc.1-1.4.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" id="section-toc.1-1.4.2.1">
                <p id="section-toc.1-1.4.2.1.1"><a href="#section-4.1" class="xref">4.1</a>.  <a href="#name-plpmtu-probe-packets" class="xref">PLPMTU Probe Packets</a><a href="#section-toc.1-1.4.2.1.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.4.2.2">
                <p id="section-toc.1-1.4.2.2.1"><a href="#section-4.2" class="xref">4.2</a>.  <a href="#name-confirmation-of-probed-pack" class="xref">Confirmation of Probed Packet Size</a><a href="#section-toc.1-1.4.2.2.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.4.2.3">
                <p id="section-toc.1-1.4.2.3.1"><a href="#section-4.3" class="xref">4.3</a>.  <a href="#name-black-hole-detection-and-re" class="xref">Black Hole Detection and Reducing the PLPMTU</a><a href="#section-toc.1-1.4.2.3.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.4.2.4">
                <p id="section-toc.1-1.4.2.4.1"><a href="#section-4.4" class="xref">4.4</a>.  <a href="#name-the-maximum-packet-size-mps" class="xref">The Maximum Packet Size (MPS)</a><a href="#section-toc.1-1.4.2.4.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.4.2.5">
                <p id="section-toc.1-1.4.2.5.1"><a href="#section-4.5" class="xref">4.5</a>.  <a href="#name-disabling-the-effect-of-pmt" class="xref">Disabling the Effect of PMTUD</a><a href="#section-toc.1-1.4.2.5.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.4.2.6">
                <p id="section-toc.1-1.4.2.6.1"><a href="#section-4.6" class="xref">4.6</a>.  <a href="#name-response-to-ptb-messages" class="xref">Response to PTB Messages</a><a href="#section-toc.1-1.4.2.6.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" id="section-toc.1-1.4.2.6.2.1">
                    <p id="section-toc.1-1.4.2.6.2.1.1"><a href="#section-4.6.1" class="xref">4.6.1</a>.  <a href="#name-validation-of-ptb-messages" class="xref">Validation of PTB Messages</a><a href="#section-toc.1-1.4.2.6.2.1.1" class="pilcrow">¶</a></p>
</li>
                  <li class="ulEmpty compact toc" id="section-toc.1-1.4.2.6.2.2">
                    <p id="section-toc.1-1.4.2.6.2.2.1"><a href="#section-4.6.2" class="xref">4.6.2</a>.  <a href="#name-use-of-ptb-messages" class="xref">Use of PTB Messages</a><a href="#section-toc.1-1.4.2.6.2.2.1" class="pilcrow">¶</a></p>
</li>
                </ul>
</li>
            </ul>
</li>
          <li class="ulEmpty compact toc" 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-datagram-packetization-laye" class="xref">Datagram Packetization Layer PMTUD</a><a href="#section-toc.1-1.5.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" id="section-toc.1-1.5.2.1">
                <p id="section-toc.1-1.5.2.1.1"><a href="#section-5.1" class="xref">5.1</a>.  <a href="#name-dplpmtud-components" class="xref">DPLPMTUD Components</a><a href="#section-toc.1-1.5.2.1.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" id="section-toc.1-1.5.2.1.2.1">
                    <p id="section-toc.1-1.5.2.1.2.1.1"><a href="#section-5.1.1" class="xref">5.1.1</a>.  <a href="#name-timers" class="xref">Timers</a><a href="#section-toc.1-1.5.2.1.2.1.1" class="pilcrow">¶</a></p>
</li>
                  <li class="ulEmpty compact toc" id="section-toc.1-1.5.2.1.2.2">
                    <p id="section-toc.1-1.5.2.1.2.2.1"><a href="#section-5.1.2" class="xref">5.1.2</a>.  <a href="#name-constants" class="xref">Constants</a><a href="#section-toc.1-1.5.2.1.2.2.1" class="pilcrow">¶</a></p>
</li>
                  <li class="ulEmpty compact toc" id="section-toc.1-1.5.2.1.2.3">
                    <p id="section-toc.1-1.5.2.1.2.3.1"><a href="#section-5.1.3" class="xref">5.1.3</a>.  <a href="#name-variables" class="xref">Variables</a><a href="#section-toc.1-1.5.2.1.2.3.1" class="pilcrow">¶</a></p>
</li>
                  <li class="ulEmpty compact toc" id="section-toc.1-1.5.2.1.2.4">
                    <p id="section-toc.1-1.5.2.1.2.4.1"><a href="#section-5.1.4" class="xref">5.1.4</a>.  <a href="#name-overview-of-dplpmtud-phases" class="xref">Overview of DPLPMTUD Phases</a><a href="#section-toc.1-1.5.2.1.2.4.1" class="pilcrow">¶</a></p>
</li>
                </ul>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.5.2.2">
                <p id="section-toc.1-1.5.2.2.1"><a href="#section-5.2" class="xref">5.2</a>.  <a href="#name-state-machine" class="xref">State Machine</a><a href="#section-toc.1-1.5.2.2.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.5.2.3">
                <p id="section-toc.1-1.5.2.3.1"><a href="#section-5.3" class="xref">5.3</a>.  <a href="#name-search-to-increase-the-plpm" class="xref">Search to Increase the PLPMTU</a><a href="#section-toc.1-1.5.2.3.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" id="section-toc.1-1.5.2.3.2.1">
                    <p id="section-toc.1-1.5.2.3.2.1.1"><a href="#section-5.3.1" class="xref">5.3.1</a>.  <a href="#name-probing-for-a-larger-plpmtu" class="xref">Probing for a Larger PLPMTU</a><a href="#section-toc.1-1.5.2.3.2.1.1" class="pilcrow">¶</a></p>
</li>
                  <li class="ulEmpty compact toc" id="section-toc.1-1.5.2.3.2.2">
                    <p id="section-toc.1-1.5.2.3.2.2.1"><a href="#section-5.3.2" class="xref">5.3.2</a>.  <a href="#name-selection-of-probe-sizes" class="xref">Selection of Probe Sizes</a><a href="#section-toc.1-1.5.2.3.2.2.1" class="pilcrow">¶</a></p>
</li>
                  <li class="ulEmpty compact toc" id="section-toc.1-1.5.2.3.2.3">
                    <p id="section-toc.1-1.5.2.3.2.3.1"><a href="#section-5.3.3" class="xref">5.3.3</a>.  <a href="#name-resilience-to-inconsistent-" class="xref">Resilience to Inconsistent Path Information</a><a href="#section-toc.1-1.5.2.3.2.3.1" class="pilcrow">¶</a></p>
</li>
                </ul>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.5.2.4">
                <p id="section-toc.1-1.5.2.4.1"><a href="#section-5.4" class="xref">5.4</a>.  <a href="#name-robustness-to-inconsistent-" class="xref">Robustness to Inconsistent Paths</a><a href="#section-toc.1-1.5.2.4.1" class="pilcrow">¶</a></p>
</li>
            </ul>
</li>
          <li class="ulEmpty compact toc" 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-specification-of-protocol-s" class="xref">Specification of Protocol-Specific Methods</a><a href="#section-toc.1-1.6.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" id="section-toc.1-1.6.2.1">
                <p id="section-toc.1-1.6.2.1.1"><a href="#section-6.1" class="xref">6.1</a>.  <a href="#name-application-support-for-dpl" class="xref">Application Support for DPLPMTUD with UDP or UDP-Lite</a><a href="#section-toc.1-1.6.2.1.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" id="section-toc.1-1.6.2.1.2.1">
                    <p id="section-toc.1-1.6.2.1.2.1.1"><a href="#section-6.1.1" class="xref">6.1.1</a>.  <a href="#name-application-request" class="xref">Application Request</a><a href="#section-toc.1-1.6.2.1.2.1.1" class="pilcrow">¶</a></p>
</li>
                  <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.1.2.2">
                    <p id="section-toc.1-1.6.2.1.2.2.1"><a href="#section-6.1.2" class="xref">6.1.2</a>.  <a href="#name-application-response" class="xref">Application Response</a><a href="#section-toc.1-1.6.2.1.2.2.1" class="pilcrow">¶</a></p>
</li>
                  <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.1.2.3">
                    <p id="section-toc.1-1.6.2.1.2.3.1"><a href="#section-6.1.3" class="xref">6.1.3</a>.  <a href="#name-sending-application-probe-p" class="xref">Sending Application Probe Packets</a><a href="#section-toc.1-1.6.2.1.2.3.1" class="pilcrow">¶</a></p>
</li>
                  <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.1.2.4">
                    <p id="section-toc.1-1.6.2.1.2.4.1"><a href="#section-6.1.4" class="xref">6.1.4</a>.  <a href="#name-initial-connectivity" class="xref">Initial Connectivity</a><a href="#section-toc.1-1.6.2.1.2.4.1" class="pilcrow">¶</a></p>
</li>
                  <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.1.2.5">
                    <p id="section-toc.1-1.6.2.1.2.5.1"><a href="#section-6.1.5" class="xref">6.1.5</a>.  <a href="#name-validating-the-path" class="xref">Validating the Path</a><a href="#section-toc.1-1.6.2.1.2.5.1" class="pilcrow">¶</a></p>
</li>
                  <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.1.2.6">
                    <p id="section-toc.1-1.6.2.1.2.6.1"><a href="#section-6.1.6" class="xref">6.1.6</a>.  <a href="#name-handling-of-ptb-messages" class="xref">Handling of PTB Messages</a><a href="#section-toc.1-1.6.2.1.2.6.1" class="pilcrow">¶</a></p>
</li>
                </ul>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2">
                <p id="section-toc.1-1.6.2.2.1"><a href="#section-6.2" class="xref">6.2</a>.  <a href="#name-dplpmtud-for-sctp" class="xref">DPLPMTUD for SCTP</a><a href="#section-toc.1-1.6.2.2.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2.2.1">
                    <p id="section-toc.1-1.6.2.2.2.1.1"><a href="#section-6.2.1" class="xref">6.2.1</a>.  <a href="#name-sctp-ipv4-and-sctp-ipv6" class="xref">SCTP/IPv4 and SCTP/IPv6</a><a href="#section-toc.1-1.6.2.2.2.1.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2.2.1.2.1">
                        <p id="section-toc.1-1.6.2.2.2.1.2.1.1"><a href="#section-6.2.1.1" class="xref">6.2.1.1</a>.  <a href="#name-initial-connectivity-2" class="xref">Initial Connectivity</a><a href="#section-toc.1-1.6.2.2.2.1.2.1.1" class="pilcrow">¶</a></p>
</li>
                      <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2.2.1.2.2">
                        <p id="section-toc.1-1.6.2.2.2.1.2.2.1"><a href="#section-6.2.1.2" class="xref">6.2.1.2</a>.  <a href="#name-sending-sctp-probe-packets" class="xref">Sending SCTP Probe Packets</a><a href="#section-toc.1-1.6.2.2.2.1.2.2.1" class="pilcrow">¶</a></p>
</li>
                      <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2.2.1.2.3">
                        <p id="section-toc.1-1.6.2.2.2.1.2.3.1"><a href="#section-6.2.1.3" class="xref">6.2.1.3</a>.  <a href="#name-validating-the-path-with-sc" class="xref">Validating the Path with SCTP</a><a href="#section-toc.1-1.6.2.2.2.1.2.3.1" class="pilcrow">¶</a></p>
</li>
                      <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2.2.1.2.4">
                        <p id="section-toc.1-1.6.2.2.2.1.2.4.1"><a href="#section-6.2.1.4" class="xref">6.2.1.4</a>.  <a href="#name-ptb-message-handling-by-sct" class="xref">PTB Message Handling by SCTP</a><a href="#section-toc.1-1.6.2.2.2.1.2.4.1" class="pilcrow">¶</a></p>
</li>
                    </ul>
</li>
                  <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2.2.2">
                    <p id="section-toc.1-1.6.2.2.2.2.1"><a href="#section-6.2.2" class="xref">6.2.2</a>.  <a href="#name-dplpmtud-for-sctp-udp" class="xref">DPLPMTUD for SCTP/UDP</a><a href="#section-toc.1-1.6.2.2.2.2.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2.2.2.2.1">
                        <p id="section-toc.1-1.6.2.2.2.2.2.1.1"><a href="#section-6.2.2.1" class="xref">6.2.2.1</a>.  <a href="#name-initial-connectivity-3" class="xref">Initial Connectivity</a><a href="#section-toc.1-1.6.2.2.2.2.2.1.1" class="pilcrow">¶</a></p>
</li>
                      <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2.2.2.2.2">
                        <p id="section-toc.1-1.6.2.2.2.2.2.2.1"><a href="#section-6.2.2.2" class="xref">6.2.2.2</a>.  <a href="#name-sending-sctp-udp-probe-pack" class="xref">Sending SCTP/UDP Probe Packets</a><a href="#section-toc.1-1.6.2.2.2.2.2.2.1" class="pilcrow">¶</a></p>
</li>
                      <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2.2.2.2.3">
                        <p id="section-toc.1-1.6.2.2.2.2.2.3.1"><a href="#section-6.2.2.3" class="xref">6.2.2.3</a>.  <a href="#name-validating-the-path-with-sct" class="xref">Validating the Path with SCTP/UDP</a><a href="#section-toc.1-1.6.2.2.2.2.2.3.1" class="pilcrow">¶</a></p>
</li>
                      <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2.2.2.2.4">
                        <p id="section-toc.1-1.6.2.2.2.2.2.4.1"><a href="#section-6.2.2.4" class="xref">6.2.2.4</a>.  <a href="#name-handling-of-ptb-messages-by" class="xref">Handling of PTB Messages by SCTP/UDP</a><a href="#section-toc.1-1.6.2.2.2.2.2.4.1" class="pilcrow">¶</a></p>
</li>
                    </ul>
</li>
                  <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2.2.3">
                    <p id="section-toc.1-1.6.2.2.2.3.1"><a href="#section-6.2.3" class="xref">6.2.3</a>.  <a href="#name-dplpmtud-for-sctp-dtls" class="xref">DPLPMTUD for SCTP/DTLS</a><a href="#section-toc.1-1.6.2.2.2.3.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2.2.3.2.1">
                        <p id="section-toc.1-1.6.2.2.2.3.2.1.1"><a href="#section-6.2.3.1" class="xref">6.2.3.1</a>.  <a href="#name-initial-connectivity-4" class="xref">Initial Connectivity</a><a href="#section-toc.1-1.6.2.2.2.3.2.1.1" class="pilcrow">¶</a></p>
</li>
                      <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2.2.3.2.2">
                        <p id="section-toc.1-1.6.2.2.2.3.2.2.1"><a href="#section-6.2.3.2" class="xref">6.2.3.2</a>.  <a href="#name-sending-sctp-dtls-probe-pac" class="xref">Sending SCTP/DTLS Probe Packets</a><a href="#section-toc.1-1.6.2.2.2.3.2.2.1" class="pilcrow">¶</a></p>
</li>
                      <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2.2.3.2.3">
                        <p id="section-toc.1-1.6.2.2.2.3.2.3.1"><a href="#section-6.2.3.3" class="xref">6.2.3.3</a>.  <a href="#name-validating-the-path-with-sctp" class="xref">Validating the Path with SCTP/DTLS</a><a href="#section-toc.1-1.6.2.2.2.3.2.3.1" class="pilcrow">¶</a></p>
</li>
                      <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2.2.3.2.4">
                        <p id="section-toc.1-1.6.2.2.2.3.2.4.1"><a href="#section-6.2.3.4" class="xref">6.2.3.4</a>.  <a href="#name-handling-of-ptb-messages-by-" class="xref">Handling of PTB Messages by SCTP/DTLS</a><a href="#section-toc.1-1.6.2.2.2.3.2.4.1" class="pilcrow">¶</a></p>
</li>
                    </ul>
</li>
                </ul>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.3">
                <p id="section-toc.1-1.6.2.3.1"><a href="#section-6.3" class="xref">6.3</a>.  <a href="#name-dplpmtud-for-quic" class="xref">DPLPMTUD for QUIC</a><a href="#section-toc.1-1.6.2.3.1" class="pilcrow">¶</a></p>
</li>
            </ul>
</li>
          <li class="ulEmpty compact toc" 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="ulEmpty compact toc" 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-security-considerations" class="xref">Security Considerations</a><a href="#section-toc.1-1.8.1" class="pilcrow">¶</a></p>
</li>
          <li class="ulEmpty compact toc" 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-references" class="xref">References</a><a href="#section-toc.1-1.9.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" 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-normative-references" class="xref">Normative References</a><a href="#section-toc.1-1.9.2.1.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" 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-informative-references" class="xref">Informative References</a><a href="#section-toc.1-1.9.2.2.1" class="pilcrow">¶</a></p>
</li>
            </ul>
</li>
          <li class="ulEmpty compact toc" id="section-toc.1-1.10">
            <p id="section-toc.1-1.10.1"><a href="#section-appendix.a" class="xref"></a><a href="#name-acknowledgments" class="xref">Acknowledgments</a><a href="#section-toc.1-1.10.1" class="pilcrow">¶</a></p>
</li>
          <li class="ulEmpty compact toc" id="section-toc.1-1.11">
            <p id="section-toc.1-1.11.1"><a href="#section-appendix.b" class="xref"></a><a href="#name-authors-addresses" class="xref">Authors' Addresses</a><a href="#section-toc.1-1.11.1" 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">The IETF has specified datagram transport using UDP, 
      Stream Control Transmission Protocol (SCTP), and  
      Datagram Congestion Control Protocol (DCCP),
      as well as protocols layered on top of these transports (e.g., SCTP/UDP,
      DCCP/UDP, QUIC/UDP) and direct datagram transport over the IP network
      layer. This document describes a robust method for Path MTU Discovery
      (PMTUD) that can be used with these transport protocols (or the
      applications that use their transport service) to discover an
      appropriate size of packet to use across an Internet path.<a href="#section-1-1" class="pilcrow">¶</a></p>
<div id="Classic-PMTUD">
<section id="section-1.1">
        <h3 id="name-classical-path-mtu-discover">
<a href="#section-1.1" class="section-number selfRef">1.1. </a><a href="#name-classical-path-mtu-discover" class="section-name selfRef">Classical Path MTU Discovery</a>
        </h3>
<p id="section-1.1-1">Classical Path Maximum Transmission Unit Discovery (PMTUD) can be
        used with any transport that is able to process ICMP Packet Too Big
        (PTB) messages (e.g., <span>[<a href="#RFC1191" class="xref">RFC1191</a>]</span> and <span>[<a href="#RFC8201" class="xref">RFC8201</a>]</span>). In this document, the term PTB message is
        applied to both IPv4 ICMP Unreachable messages (Type 3) that carry the
        error Fragmentation Needed (Type 3, Code 4) <span>[<a href="#RFC0792" class="xref">RFC0792</a>]</span> and ICMPv6 Packet Too Big messages (Type 2)
        <span>[<a href="#RFC4443" class="xref">RFC4443</a>]</span>. When a sender receives a PTB message,
        it reduces the effective MTU to the value reported as the link MTU in
        the PTB message. Classical PMTUD specifies a method of periodically increasing 
        the packet size in an attempt to discover an increase in the 
        supported PMTU.
        The packets sent with a size larger than the current effective PMTU
        are known as probe packets.<a href="#section-1.1-1" class="pilcrow">¶</a></p>
<p id="section-1.1-2">Packets not intended as probe packets are either fragmented to the
        current effective PMTU, or the attempt to send fails with an error
        code. Applications can be provided with a primitive to let them
        read the Maximum Packet Size (MPS), which is derived from the current effective
        PMTU.<a href="#section-1.1-2" class="pilcrow">¶</a></p>
<p id="section-1.1-3">Classical PMTUD is subject to protocol failures. One failure arises
 when traffic using a packet size larger than the actual PMTU is
 black-holed (all datagrams larger than the actual PMTU are discarded).
 This could arise when the PTB messages are not sent back to the
 sender for some reason (for example, see <span>[<a href="#RFC2923" class="xref">RFC2923</a>]</span>).<a href="#section-1.1-3" class="pilcrow">¶</a></p>
<p id="section-1.1-4">Examples of where PTB messages are not delivered include the following:<a href="#section-1.1-4" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-1.1-5.1">The generation of ICMP messages is usually rate limited. This
            could result in no PTB messages being generated to the sender (see
            <span><a href="https://www.rfc-editor.org/rfc/rfc4443#section-2.4" class="relref">Section 2.4</a> of [<a href="#RFC4443" class="xref">RFC4443</a>]</span>).<a href="#section-1.1-5.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-1.1-5.2">ICMP messages can be filtered by middleboxes, including
            firewalls <span>[<a href="#RFC4890" class="xref">RFC4890</a>]</span>. A firewall
            could be configured with a policy to block incoming ICMP messages,
            which would prevent reception of PTB messages by a sending
            endpoint behind this firewall.<a href="#section-1.1-5.2" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-1.1-5.3">When the router issuing the ICMP message drops a tunneled
            packet, the resulting ICMP message is directed to the tunnel
            ingress. This tunnel endpoint is responsible for forwarding the
            ICMP message, processing the quoted packet within the
            payload field to remove the effect of the tunnel and returning a
            correctly formatted ICMP message to the sender <span>[<a href="#I-D.ietf-intarea-tunnels" class="xref">TUNNELS</a>]</span>. Failure to do this
            prevents the PTB message from reaching the original sender.<a href="#section-1.1-5.3" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-1.1-5.4">Asymmetry in forwarding can result in there being no return
            route to the original sender, which would prevent an ICMP message
            from being delivered to the sender. This issue can also arise when
            either policy-based or Equal-Cost Multipath (ECMP) routing
            is used or when a middlebox acts as an application load balancer. 
   An example of which is an ECMP router choosing a path toward the server 
   based on the bytes in the IP payload. In this case, if a packet 
   sent by the server encounters a problem after the ECMP router, then 
   the ECMP router needs to direct any resulting ICMP message toward 
   the original sender.<a href="#section-1.1-5.4" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-1.1-5.5">There are additional cases where the next-hop destination fails
            to receive a packet because of its size. This could be due to
            misconfiguration of the layer 2 path between nodes, for instance
            the MTU configured in a layer 2 switch, or misconfiguration of the
            Maximum Receive Unit (MRU). If a packet is dropped by the link,
            this will not cause a PTB message to be sent to the original
            sender.<a href="#section-1.1-5.5" class="pilcrow">¶</a>
</li>
        </ul>
<p id="section-1.1-6">Another failure could result if a node that is not on the network
        path sends a PTB message that attempts to force a sender to change the
        effective PMTU <span>[<a href="#RFC8201" class="xref">RFC8201</a>]</span>. A sender can protect
        itself from reacting to such messages by utilizing the quoted packet
        within a PTB message payload to validate that the received PTB message
        was generated in response to a packet that had actually originated
        from the sender. However, there are situations where a sender would be
        unable to provide this validation. Examples where the validation of the
        PTB message is not possible include the following:<a href="#section-1.1-6" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-1.1-7.1">
            <p id="section-1.1-7.1.1">When a router issuing the ICMP message implements RFC 792 <span>[<a href="#RFC0792" class="xref">RFC0792</a>]</span>, it is only required to include the first
            64 bits of the IP payload of the packet within the quoted payload.
            There could be insufficient bytes remaining for the sender to
            interpret the quoted transport information.<a href="#section-1.1-7.1.1" class="pilcrow">¶</a></p>
<p id="section-1.1-7.1.2"> Note: The
            recommendation in RFC 1812 <span>[<a href="#RFC1812" class="xref">RFC1812</a>]</span> is that
            IPv4 routers return a quoted packet with as much of the original
            datagram as possible without the length of the ICMP datagram
            exceeding 576 bytes. IPv6 routers include as much of the invoking
            packet as possible without the ICMPv6 packet exceeding 1280 bytes
            <span>[<a href="#RFC4443" class="xref">RFC4443</a>]</span>.<a href="#section-1.1-7.1.2" class="pilcrow">¶</a></p>
</li>
          <li class="normal" id="section-1.1-7.2">The use of tunnels and/or encryption can reduce the size of the quoted
            packet returned to the original source address, increasing the
            risk that there could be insufficient bytes remaining for the
            sender to interpret the quoted transport information.<a href="#section-1.1-7.2" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-1.1-7.3">Even when the PTB message includes sufficient bytes of the
            quoted packet, the network layer could lack sufficient context to
            validate the message because validation depends on information
            about the active transport flows at an endpoint node (e.g., the
            socket/address pairs being used and other protocol header
            information).<a href="#section-1.1-7.3" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-1.1-7.4">When a packet is encapsulated/tunneled over an encrypted
            transport, the tunnel/encapsulation ingress might have
            insufficient context, or computational power, to reconstruct the
            transport header that would be needed to perform validation.<a href="#section-1.1-7.4" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-1.1-7.5"> When an ICMP message is generated by a router in a network
            segment that has inserted a header into a packet, the quoted packet
            could contain additional protocol header information that was not
            included in the original sent packet and that the PL sender does
            not process or may not know how to process. This could disrupt the
            ability of the sender to validate this PTB message.<a href="#section-1.1-7.5" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-1.1-7.6"> A Network Address Translation (NAT) device that translates a
            packet header ought to also translate ICMP messages and update
            the ICMP-quoted packet <span>[<a href="#RFC5508" class="xref">RFC5508</a>]</span> in
            that message. If this is not correctly translated,
            then the sender would not be able to associate the message
            with the PL that originated the packet, and hence this
            ICMP message cannot be validated.<a href="#section-1.1-7.6" class="pilcrow">¶</a>
</li>
        </ul>
</section>
</div>
<section id="section-1.2">
        <h3 id="name-packetization-layer-path-mt">
<a href="#section-1.2" class="section-number selfRef">1.2. </a><a href="#name-packetization-layer-path-mt" class="section-name selfRef">Packetization Layer Path MTU Discovery</a>
        </h3>
<p id="section-1.2-1">The term Packetization Layer (PL) has been introduced to describe
          the layer that is responsible for placing data blocks into the payload
          of IP packets and selecting an appropriate MPS. This function is often
          performed by a transport protocol (e.g., DCCP, RTP, SCTP, QUIC)
          but can also be performed by other encapsulation methods working
          above the transport layer.<a href="#section-1.2-1" class="pilcrow">¶</a></p>
<p id="section-1.2-2">In contrast to PMTUD, Packetization Layer Path MTU Discovery
        (PLPMTUD) <span>[<a href="#RFC4821" class="xref">RFC4821</a>]</span> introduces a method
        that does not rely upon reception
        and validation of PTB messages. It is therefore more robust than
        Classical PMTUD. This has become the recommended approach for
        implementing discovery of the PMTU <span>[<a href="#BCP145" class="xref">BCP145</a>]</span>.<a href="#section-1.2-2" class="pilcrow">¶</a></p>
<p id="section-1.2-3">This document updates <span>[<a href="#RFC4821" class="xref">RFC4821</a>]</span> to specify the PLPMTUD method for
        datagram PLs and also updates <span>[<a href="#BCP145" class="xref">BCP145</a>]</span> to refer to the method
        specified in this document for use
        with UDP datagrams instead of the method in <span>[<a href="#RFC4821" class="xref">RFC4821</a>]</span>.<a href="#section-1.2-3" class="pilcrow">¶</a></p>
<p id="section-1.2-4">It uses a general strategy in which the PL sends probe packets to
        search for the largest size of unfragmented datagram that can be sent
        over a network path. Probe packets are sent to explore using a larger
        packet size. If a probe packet is successfully delivered (as determined
        by the PL), then the PLPMTU is raised to the size of the successful
        probe. If a black hole is detected (e.g., where packets of size PLPMTU
        are consistently not received), the method reduces the PLPMTU.<a href="#section-1.2-4" class="pilcrow">¶</a></p>
<p id="section-1.2-5">Datagram PLPMTUD introduces flexibility in implementation.
        At one extreme, it can be configured to only perform
        black hole detection and recovery with increased robustness compared to
        Classical PMTUD. At the other extreme, all PTB processing can
        be disabled, and PLPMTUD replaces Classical PMTUD.<a href="#section-1.2-5" class="pilcrow">¶</a></p>
<p id="section-1.2-6">PLPMTUD can also include additional consistency checks without
        increasing the risk that data is lost when probing to discover the
        Path MTU. For example, information available at the PL, or higher
        layers, enables received PTB messages to be validated before being
        utilized.<a href="#section-1.2-6" class="pilcrow">¶</a></p>
</section>
<section id="section-1.3">
        <h3 id="name-path-mtu-discovery-for-data">
<a href="#section-1.3" class="section-number selfRef">1.3. </a><a href="#name-path-mtu-discovery-for-data" class="section-name selfRef">Path MTU Discovery for Datagram Services</a>
        </h3>
<p id="section-1.3-1"><a href="#Spec" class="xref">Section 5</a> of this document presents a set of
        algorithms for datagram protocols to discover the largest size of
        unfragmented datagram that can be sent over a network path. The method
        relies upon features of the PL described in <a href="#Requirements" class="xref">Section 3</a> and applies to transport protocols
        operating over IPv4 and IPv6. It does not require cooperation from the
        lower layers, although it can utilize PTB messages when these received
        messages are made available to the PL.<a href="#section-1.3-1" class="pilcrow">¶</a></p>
<p id="section-1.3-2">The message size guidelines in Section  
        <a href="https://www.rfc-editor.org/rfc/rfc8085#section-3.2" class="relref">3.2</a> 
        of the UDP Usage Guidelines <span>[<a href="#BCP145" class="xref">BCP145</a>]</span> state that "an application <span class="bcp14">SHOULD</span>
        either use the Path MTU information provided by the IP layer or
        implement Path MTU Discovery (PMTUD)" but do not provide a mechanism
        for discovering the largest size of unfragmented datagram that can be
        used on a network path. The present document updates RFC 8085 to
        specify this method in place of PLPMTUD <span>[<a href="#RFC4821" class="xref">RFC4821</a>]</span>
        and provides a mechanism for sharing the discovered largest size as the
        MPS (see <a href="#mps" class="xref">Section 4.4</a>).<a href="#section-1.3-2" class="pilcrow">¶</a></p>
<p id="section-1.3-3"><span><a href="https://www.rfc-editor.org/rfc/rfc4821#section-10.2" class="relref">Section 10.2</a> of [<a href="#RFC4821" class="xref">RFC4821</a>]</span> recommended a PLPMTUD
 probing method for the Stream Control Transport Protocol (SCTP). SCTP
 utilizes probe packets consisting of a minimal-sized HEARTBEAT chunk
 bundled with a PAD chunk as defined in <span>[<a href="#RFC4820" class="xref">RFC4820</a>]</span>.
 However, RFC 4821 did not provide a complete specification. The present
 document replaces that description by providing a complete
 specification.<a href="#section-1.3-3" class="pilcrow">¶</a></p>
<p id="section-1.3-4">The Datagram Congestion Control Protocol (DCCP) <span>[<a href="#RFC4340" class="xref">RFC4340</a>]</span> requires implementations to support Classical
        PMTUD and states that a DCCP sender "<span class="bcp14">MUST</span> maintain the MPS allowed for
        each active DCCP session". It also defines the current congestion
        control MPS (CCMPS) supported by a network path. This recommends use
        of PMTUD and suggests use of control packets (DCCP-Sync) as path
        probe packets because they do not risk application data loss. The
        method defined in this specification can be used with DCCP.<a href="#section-1.3-4" class="pilcrow">¶</a></p>
<p id="section-1.3-5"><a href="#dplpmtud-mechanisms" class="xref">Section 4</a> and <a href="#Spec" class="xref">Section 5</a> define the protocol mechanisms and specification
        for Datagram Packetization Layer Path MTU Discovery (DPLPMTUD).<a href="#section-1.3-5" class="pilcrow">¶</a></p>
<p id="section-1.3-6"><a href="#protocol_specific_methods" class="xref">Section 6</a> specifies the
        method for datagram transports and provides information to enable the
        implementation of PLPMTUD with other datagram transports and
        applications that use datagram transports.<a href="#section-1.3-6" class="pilcrow">¶</a></p>
<p id="section-1.3-7"><a href="#protocol_specific_methods" class="xref">Section 6</a>
        also provides recommendations for SCTP endpoints,
        updating <span>[<a href="#RFC4960" class="xref">RFC4960</a>]</span>, <span>[<a href="#RFC6951" class="xref">RFC6951</a>]</span>, and <span>[<a href="#RFC8261" class="xref">RFC8261</a>]</span> to use the
        method specified in this document instead of the method in <span>[<a href="#RFC4821" class="xref">RFC4821</a>]</span>.<a href="#section-1.3-7" class="pilcrow">¶</a></p>
</section>
</section>
<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>
<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>
<p id="section-2-2">The following terminology is defined. Relevant terms are directly
      copied from <span>[<a href="#RFC4821" class="xref">RFC4821</a>]</span>, and the definitions in <span>[<a href="#RFC1122" class="xref">RFC1122</a>]</span> apply.<a href="#section-2-2" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlParallel" id="section-2-3">
        <dt id="section-2-3.1">Acknowledged PL:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.2"> A PL that includes a mechanism that can confirm successful
          delivery of datagrams to the remote PL endpoint (e.g., SCTP).
          Typically, the PL receiver returns acknowledgments corresponding to
          the received datagrams, which can be utilized to detect black-holing
          of packets (c.f., Unacknowledged PL).<a href="#section-2-3.2" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.3">Actual PMTU:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.4">The actual PMTU is the PMTU of a network path between a sender
          PL and a destination PL, which the DPLPMTUD algorithm seeks to
          determine.<a href="#section-2-3.4" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.5">Black Hole:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.6">
          <p id="section-2-3.6.1">A black hole is encountered when a sender is unaware that
            packets are not being delivered to the destination endpoint. Two
            types of black hole are relevant to DPLPMTUD:<a href="#section-2-3.6.1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-2-3.6.2.1">Packets encounter a packet black hole when packets are not
              delivered to the destination endpoint (e.g., when the sender
              transmits packets of a particular size with a previously known
              effective PMTU, and they are discarded by the network).<a href="#section-2-3.6.2.1" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-2-3.6.2.2">
              An ICMP black hole is encountered when the sender is unaware
              that packets are not delivered to the destination endpoint
              because PTB messages are not received by the originating PL
              sender.<a href="#section-2-3.6.2.2" class="pilcrow">¶</a>
</li>
          </ul>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.7">Classical Path MTU Discovery:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.8">Classical PMTUD is a process described in <span>[<a href="#RFC1191" class="xref">RFC1191</a>]</span> and <span>[<a href="#RFC8201" class="xref">RFC8201</a>]</span> in which nodes
          rely on PTB messages to learn the largest size of unfragmented
          packet that can be used across a network path.<a href="#section-2-3.8" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.9">Datagram:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.10">A datagram is a transport-layer protocol data unit, transmitted
          in the payload of an IP packet.<a href="#section-2-3.10" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.11">DPLPMTUD:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.12">Datagram Packetization Layer Path MTU Discovery (DPLPMTUD),
PLPMTUD performed using a datagram transport protocol.<a href="#section-2-3.12" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.13">Effective PMTU:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.14">The effective PMTU is the current estimated value for PMTU that is
          used by a PMTUD. This is equivalent to the PLPMTU derived by PLPMTUD
          plus the size of any headers added below the PL, including the IP
          layer headers.<a href="#section-2-3.14" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.15">EMTU_S:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.16">The effective MTU for sending (EMTU_S) is defined in <span>[<a href="#RFC1122" class="xref">RFC1122</a>]</span> as "the maximum IP datagram size that may be
          sent, for a particular combination of IP source and destination
          addresses...".<a href="#section-2-3.16" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.17">EMTU_R:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.18">The effective MTU for receiving (EMTU_R) is designated in <span>[<a href="#RFC1122" class="xref">RFC1122</a>]</span> as "the largest datagram size that can be
          reassembled".<a href="#section-2-3.18" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.19">Link:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.20">A link is a communication facility or medium over which nodes can
          communicate at the link layer, i.e., a layer below the IP layer.
          Examples are Ethernet LANs and Internet (or higher) layer
          tunnels.<a href="#section-2-3.20" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.21">Link MTU:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.22">
          <p id="section-2-3.22.1">The link Maximum Transmission Unit (MTU) is the size in bytes of
          the largest IP packet, including the IP header and payload, that can
          be transmitted over a link. Note that this could more properly be
          called the IP MTU, to be consistent with how other standards
          organizations use the acronym. This includes the IP header but
          excludes link layer headers and other framing that is not part of IP
          or the IP payload. Other standards organizations generally define
          the link MTU to include the link layer headers. This specification
          continues the requirement in <span>[<a href="#RFC4821" class="xref">RFC4821</a>]</span>
          that states,
          "All links <span class="bcp14">MUST</span> enforce their MTU: links that might 
          non-deterministically deliver packets that are larger than their rated
          MTU <span class="bcp14">MUST</span> consistently discard such packets."<a href="#section-2-3.22.1" class="pilcrow">¶</a></p>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.23">MAX_PLPMTU:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.24">The MAX_PLPMTU is the largest size of PLPMTU that DPLPMTUD will
   attempt to use (see the constants defined in <a href="#Constants" class="xref">Section 5.1.2</a>).<a href="#section-2-3.24" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.25">MIN_PLPMTU:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.26">The MIN_PLPMTU is the smallest size of PLPMTU that DPLPMTUD will
   attempt to use (see the constants defined in <a href="#Constants" class="xref">Section 5.1.2</a>).<a href="#section-2-3.26" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.27">MPS:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.28">The Maximum Packet Size (MPS) is the largest size of
          application data block that can be sent across a network path by a PL
          using a single datagram (see <a href="#mps" class="xref">Section 4.4</a>).<a href="#section-2-3.28" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.29">MSL:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.30">The Maximum Segment Lifetime (MSL) is the maximum delay a packet is
          expected to experience across a path, taken as 2 minutes <span>[<a href="#BCP145" class="xref">BCP145</a>]</span>.<a href="#section-2-3.30" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.31">Packet:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.32">A packet is the IP header(s) and any extension headers/options
   plus the IP payload.<a href="#section-2-3.32" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.33">Packetization Layer (PL):</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.34">The PL is a layer of the network stack that places data into
          packets and performs transport protocol functions. Examples of a PL
          include TCP, SCTP, SCTP over UDP, SCTP over DTLS, or QUIC.<a href="#section-2-3.34" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.35">Path:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.36">The path is the set of links and routers traversed by a packet
          between a source node and a destination node by a particular
          flow.<a href="#section-2-3.36" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.37">Path MTU (PMTU):</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.38">The Path MTU (PMTU) is the minimum of the link MTU of all the
          links forming a network path between a source node and a destination
          node, as used by PMTUD.<a href="#section-2-3.38" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.39">PTB:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.40"> In this document, the term PTB message is applied to both IPv4
   ICMP Unreachable messages (Type 3) that carry the error Fragmentation
   Needed (Type 3, Code 4) <span>[<a href="#RFC0792" class="xref">RFC0792</a>]</span> and ICMPv6
   Packet Too Big messages (Type 2) <span>[<a href="#RFC4443" class="xref">RFC4443</a>]</span>.<a href="#section-2-3.40" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.41">PTB_SIZE:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.42">The PTB_SIZE is a value reported in a validated PTB message that
          indicates next-hop link MTU of a router along the path.<a href="#section-2-3.42" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.43">PL_PTB_SIZE:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.44">The size reported in a validated PTB message, reduced by the size
   of all headers added by layers below the PL.<a href="#section-2-3.44" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.45">PLPMTU:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.46">The Packetization Layer PMTU is an estimate of the largest size
          of PL datagram that can be sent by a path,  controlled by PLPMTUD.<a href="#section-2-3.46" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.47">PLPMTUD:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.48">Packetization Layer Path MTU Discovery (PLPMTUD), the method
          described in this document for datagram PLs, which is an extension
          to Classical PMTU Discovery.<a href="#section-2-3.48" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.49">Probe packet:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.50">A probe packet is a datagram sent with a purposely chosen size
          (typically the current PLPMTU or larger) to detect if packets of
          this size can be successfully sent end-to-end across the network
          path.<a href="#section-2-3.50" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-2-3.51">Unacknowledged PL:</dt>
        <dd style="margin-left: 1.5em" id="section-2-3.52">A PL that does not itself provide a mechanism to confirm delivery
          of datagrams to the remote PL endpoint (e.g., UDP), and therefore
          requires DPLPMTUD to provide a mechanism to detect black-holing of
          packets (c.f., Acknowledged PL).<a href="#section-2-3.52" class="pilcrow">¶</a>
</dd>
      <dd class="break"></dd>
</dl>
</section>
<div id="Requirements">
<section id="section-3">
      <h2 id="name-features-required-to-provid">
<a href="#section-3" class="section-number selfRef">3. </a><a href="#name-features-required-to-provid" class="section-name selfRef">Features Required to Provide Datagram PLPMTUD</a>
      </h2>
<p id="section-3-1">The principles expressed in <span>[<a href="#RFC4821" class="xref">RFC4821</a>]</span> apply to
      the use of the technique with any PL.
      TCP PLPMTUD has been defined using standard TCP protocol mechanisms.
      Unlike TCP, a datagram PL requires additional mechanisms and
      considerations to implement PLPMTUD.<a href="#section-3-1" class="pilcrow">¶</a></p>
<p id="section-3-2">The requirements for datagram PLPMTUD are:<a href="#section-3-2" class="pilcrow">¶</a></p>
<ol start="1" type="1" class="normal type-1" id="section-3-3">
        <li id="section-3-3.1"> Managing the PLPMTU: For datagram PLs, the PLPMTU is managed by
          DPLPMTUD. A PL <span class="bcp14">MUST NOT</span> send a datagram (other than a probe packet)
          with a size at the PL that is larger than the current
          PLPMTU.<a href="#section-3-3.1" class="pilcrow">¶</a>
</li>
        <li id="section-3-3.2"> Probe packets: The network interface below the PL is <span class="bcp14">REQUIRED</span> to
          provide a way to transmit a probe packet that is larger than the
          PLPMTU. In IPv4, a probe packet <span class="bcp14">MUST</span> be sent with the Don't
          Fragment (DF) bit set in the IP header and without network layer
          endpoint fragmentation. In IPv6, a probe packet is always sent
          without source fragmentation (as specified in 
          <span><a href="https://www.rfc-editor.org/rfc/rfc8201#section-5.4" class="relref">Section 5.4</a> of [<a href="#RFC8201" class="xref">RFC8201</a>]</span>).<a href="#section-3-3.2" class="pilcrow">¶</a>
</li>
        <li id="section-3-3.3">Reception feedback: The destination PL endpoint is <span class="bcp14">REQUIRED</span> to
          provide a feedback method that indicates to the DPLPMTUD sender when
          a probe packet has been received by the destination PL endpoint.
          <a href="#protocol_specific_methods" class="xref">Section 6</a> provides examples of
          how a PL can provide this acknowledgment of received probe packets.<a href="#section-3-3.3" class="pilcrow">¶</a>
</li>
        <li id="section-3-3.4">Probe loss recovery: It is <span class="bcp14">RECOMMENDED</span> to use probe packets that
          do not carry any user data that would require retransmission if lost.
          Most datagram transports permit this. If a probe packet contains user
          data requiring retransmission in case of loss, the PL (or layers
          above) is <span class="bcp14">REQUIRED</span> to arrange any retransmission and/or repair of any
          resulting loss. The PL is <span class="bcp14">REQUIRED</span> to be robust in the case where
          probe packets are lost due to other reasons (including link
          transmission error, congestion).<a href="#section-3-3.4" class="pilcrow">¶</a>
</li>
        <li id="section-3-3.5">PMTU parameters: A DPLPMTUD sender is <span class="bcp14">RECOMMENDED</span> to utilize
   information about the maximum size of packet that can be transmitted
   by the sender on the local link (e.g., the local link MTU). A PL
   sender <span class="bcp14">MAY</span> utilize similar information about the maximum size of
   network-layer packet that a receiver can accept when this is supplied
   (note this could be less than EMTU_R). This avoids implementations
   trying to send probe packets that cannot be transferred by the local
   link. Too high of a value could reduce the efficiency of the search
   algorithm. Some applications also have a maximum transport protocol
   data unit (PDU) size, in which case there is no benefit from probing
   for a size larger than this (unless a transport allows multiplexing
   multiple applications' PDUs into the same datagram).<a href="#section-3-3.5" class="pilcrow">¶</a>
</li>
        <li id="section-3-3.6">Processing PTB messages: A DPLPMTUD sender <span class="bcp14">MAY</span> optionally utilize
          PTB messages received from the network layer to help identify when a
          network path does not support the current size of probe packet. Any
          received PTB message <span class="bcp14">MUST</span> be validated before it is used to update
          the PLPMTU discovery information <span>[<a href="#RFC8201" class="xref">RFC8201</a>]</span>.
          This validation confirms that the PTB message was sent in response to
          a packet originated by the sender and needs to be performed before
          the PLPMTU discovery method reacts to the PTB message. A PTB message
          <span class="bcp14">MUST NOT</span> be used to increase the PLPMTU <span>[<a href="#RFC8201" class="xref">RFC8201</a>]</span> 
          but could trigger a probe to test for a larger PLPMTU. 

   A valid PTB_SIZE is converted to a PL_PTB_SIZE before it is to be
   used in the DPLPMTUD state machine. A PL_PTB_SIZE that is greater
   than that currently probed <span class="bcp14">SHOULD</span> be ignored. (This PTB message ought
   to be discarded without further processing but could be utilized as
   an input that enables a resilience mode).<a href="#section-3-3.6" class="pilcrow">¶</a>
</li>
        <li id="section-3-3.7">Probing and congestion control: A PL <span class="bcp14">MAY</span> use a congestion
   controller to decide when to send a probe packet. If transmission of
   probe packets is limited by the congestion controller, this could
   result in transmission of probe packets being delayed or suspended
   during congestion. When the transmission of probe packets is not
   controlled by the congestion controller, the interval between probe
   packets <span class="bcp14">MUST</span> be at least one RTT.
      
          Loss of a probe packet <span class="bcp14">SHOULD NOT</span> be treated as an indication of
          congestion and <span class="bcp14">SHOULD NOT</span> trigger a congestion control reaction <span>[<a href="#RFC4821" class="xref">RFC4821</a>]</span>
          because this could result in unnecessary reduction of the sending rate.
      
          An update to the PLPMTU (or MPS) <span class="bcp14">MUST NOT</span> increase the congestion
          window measured in bytes <span>[<a href="#RFC4821" class="xref">RFC4821</a>]</span>. Therefore,
          an increase in the packet size does not cause an increase in the data
          rate in bytes per second.

          A PL that maintains the congestion window in terms of a limit to
          the number of outstanding fixed-size packets <span class="bcp14">SHOULD</span> adapt this limit
          to compensate for the size of the actual packets.

   The transmission of probe packets can interact with the operation
   of a PL that performs burst mitigation or pacing, and the PL could need
   transmission of probe packets to be regulated by these methods.<a href="#section-3-3.7" class="pilcrow">¶</a>
</li>
        <li id="section-3-3.8">Probing and flow control:  Flow control at the PL concerns the
   end-to-end flow of data using the PL service. Flow control <span class="bcp14">SHOULD NOT</span> apply to DPLPMTU when probe packets use a design that does not
   carry user data to the remote application.<a href="#section-3-3.8" class="pilcrow">¶</a>
</li>
        <li id="section-3-3.9">Shared PLPMTU state: The PMTU value
          calculated from the PLPMTU <span class="bcp14">MAY</span> also be stored with the
          corresponding entry associated with the destination in the IP
          layer cache and used by other PL instances. The specification of
          PLPMTUD <span>[<a href="#RFC4821" class="xref">RFC4821</a>]</span> states, "If PLPMTUD updates
          the MTU for a particular path, all Packetization Layer sessions that
          share the path representation (as described in Section
          <a href="https://www.rfc-editor.org/rfc/rfc4821#section-5.2" class="relref">5.2</a>)
          <span class="bcp14">SHOULD</span> be notified to make use of the new
          MTU". Such methods <span class="bcp14">MUST</span> be robust to the wide variety of underlying
          network forwarding behaviors. 
          <span><a href="https://www.rfc-editor.org/rfc/rfc8201#section-5.2" class="relref">Section 5.2</a> of [<a href="#RFC8201" class="xref">RFC8201</a>]</span> 
          provides guidance on the caching of PMTU
          information and also the relation to IPv6 flow labels.<a href="#section-3-3.9" class="pilcrow">¶</a>
</li>
      </ol>
<p id="section-3-4">In addition, the following principles are stated for design of a
      DPLPMTUD method:<a href="#section-3-4" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-3-5.1">A PL <span class="bcp14">MAY</span> be designed to segment data blocks larger than the MPS
          into multiple datagrams. However, not all datagram PLs support
          segmentation of data blocks. It is <span class="bcp14">RECOMMENDED</span> that methods avoid
          forcing an application to use an arbitrary small MPS for transmission
          while the method is searching for the currently supported PLPMTU. A
          reduced MPS can adversely impact the performance of an
          application.<a href="#section-3-5.1" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-3-5.2"> To assist applications in choosing a suitable data block size, the
          PL is <span class="bcp14">RECOMMENDED</span> to provide a primitive that
          returns the MPS derived from the PLPMTU to the
          higher layer using the PL. The value of the MPS can change following
          a change in the path or loss of probe packets.<a href="#section-3-5.2" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-3-5.3">Path validation: It is <span class="bcp14">RECOMMENDED</span> that methods are robust to
          path changes that could have occurred since the path characteristics
          were last confirmed and to the possibility of inconsistent path
          information being received.<a href="#section-3-5.3" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-3-5.4">Datagram reordering: A method is <span class="bcp14">REQUIRED</span> to be robust to the
          possibility that a flow encounters reordering or that the traffic
          (including probe packets) is divided over more than one network
          path.<a href="#section-3-5.4" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-3-5.5">Datagram delay and duplication: The feedback
          mechanism is <span class="bcp14">REQUIRED</span> to be robust to the possibility that packets
          could be significantly delayed or duplicated along a network path.<a href="#section-3-5.5" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-3-5.6">When to probe: It is <span class="bcp14">RECOMMENDED</span> that methods determine whether
          the path has changed since it last measured the path. This can help
          determine when to probe the path again.<a href="#section-3-5.6" class="pilcrow">¶</a>
</li>
      </ul>
</section>
</div>
<div id="dplpmtud-mechanisms">
<section id="section-4">
      <h2 id="name-dplpmtud-mechanisms">
<a href="#section-4" class="section-number selfRef">4. </a><a href="#name-dplpmtud-mechanisms" class="section-name selfRef">DPLPMTUD Mechanisms</a>
      </h2>
<p id="section-4-1">This section lists the protocol mechanisms used in this
      specification.<a href="#section-4-1" class="pilcrow">¶</a></p>
<div id="Probe">
<section id="section-4.1">
        <h3 id="name-plpmtu-probe-packets">
<a href="#section-4.1" class="section-number selfRef">4.1. </a><a href="#name-plpmtu-probe-packets" class="section-name selfRef">PLPMTU Probe Packets</a>
        </h3>
<p id="section-4.1-1">The DPLPMTUD method relies upon the PL sender being able to generate
        probe packets with a specific size. TCP is able to generate these probe
        packets by choosing to appropriately segment data being sent <span>[<a href="#RFC4821" class="xref">RFC4821</a>]</span>. In contrast, a datagram PL that constructs a
        probe packet has to either request an application to send a data block
        that is larger than that generated by an application, or to utilize
        padding functions to extend a datagram beyond the size of the
        application data block. Protocols that permit exchange of control
        messages (without an application data block) can generate a probe
        packet by extending a control message with padding data. The total size
        of a probe packet includes all headers and padding added to the payload
        data being sent (e.g., including protocol option fields,
        security-related fields such as an Authenticated Encryption with
        Associated Data (AEAD) tag, and TLS record layer padding).<a href="#section-4.1-1" class="pilcrow">¶</a></p>
<p id="section-4.1-2">A receiver is <span class="bcp14">REQUIRED</span> to be able to distinguish an in-band data
        block from any added padding. This is needed to ensure that any added
        padding is not passed on to an application at the receiver.<a href="#section-4.1-2" class="pilcrow">¶</a></p>
<p id="section-4.1-3">This results in three possible ways that a sender can create a
        probe packet:<a href="#section-4.1-3" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlParallel" id="section-4.1-4">
          <dt id="section-4.1-4.1">Probing using padding data:</dt>
          <dd style="margin-left: 1.5em" id="section-4.1-4.2">A probe packet that contains only control information together
            with any padding, which is needed to inflate to the size
            of the probe packet. Since these probe packets do not
            carry an application-supplied data block, they do not typically
            require retransmission, although they do still consume network
            capacity and incur endpoint processing.<a href="#section-4.1-4.2" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-4.1-4.3">Probing using application data and padding data:</dt>
          <dd style="margin-left: 1.5em" id="section-4.1-4.4">A probe packet that contains a data block supplied by an
            application that is combined with padding to inflate the length of
            the datagram to the size of the probe packet.<a href="#section-4.1-4.4" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-4.1-4.5">Probing using application data:</dt>
          <dd style="margin-left: 1.5em" id="section-4.1-4.6">A probe packet that contains a data block supplied by an
            application that matches the size of the probe packet.
            This method requests the application to issue a data block of the
            desired probe size.<a href="#section-4.1-4.6" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
</dl>
<p id="section-4.1-5">A PL that uses a probe packet carrying application data and that needs
 protection from the loss of this probe packet could perform
 transport-layer retransmission/repair of the data block (e.g., by
 retransmitting after loss is detected or by duplicating the data block
 in a datagram without the padding data). This retransmitted data block
 might possibly need to be sent using a smaller PLPMTU, which could
 force the PL to use a smaller packet size to traverse the end-to-end
 path. (This could utilize endpoint network-layer fragmentation or a PL
 that can resegment the data block into multiple datagrams).<a href="#section-4.1-5" class="pilcrow">¶</a></p>
<p id="section-4.1-6">DPLPMTUD <span class="bcp14">MAY</span> choose to use only one of these methods to simplify
        the implementation.<a href="#section-4.1-6" class="pilcrow">¶</a></p>
<p id="section-4.1-7">Probe messages sent by a PL <span class="bcp14">MUST</span> contain enough information to
        uniquely identify the probe within the Maximum Segment Lifetime (e.g.,
        including a unique identifier from the PL or the DPLPMTUD
        implementation), while being robust to reordering and replay of probe
        response and PTB messages.<a href="#section-4.1-7" class="pilcrow">¶</a></p>
</section>
</div>
<div id="Valid">
<section id="section-4.2">
        <h3 id="name-confirmation-of-probed-pack">
<a href="#section-4.2" class="section-number selfRef">4.2. </a><a href="#name-confirmation-of-probed-pack" class="section-name selfRef">Confirmation of Probed Packet Size</a>
        </h3>
<p id="section-4.2-1">The PL needs a method to determine (confirm) when probe packets
        have been successfully received end-to-end across a network path.<a href="#section-4.2-1" class="pilcrow">¶</a></p>
<p id="section-4.2-2">Transport protocols can include end-to-end methods that detect and
 report reception of specific datagrams that they send (e.g., DCCP,
 SCTP, and QUIC provide keep-alive/heartbeat features). When supported,
 this mechanism <span class="bcp14">MAY</span> also be used by DPLPMTUD to acknowledge reception of
 a probe packet.<a href="#section-4.2-2" class="pilcrow">¶</a></p>
<p id="section-4.2-3">A PL that does not acknowledge data reception (e.g., UDP and
        UDP-Lite) is unable itself to detect when the packets that it sends
        are discarded because their size is greater than the actual PMTU.
        These PLs need to rely on an application protocol to detect
        this loss.<a href="#section-4.2-3" class="pilcrow">¶</a></p>
<p id="section-4.2-4"><a href="#protocol_specific_methods" class="xref">Section 6</a> specifies this
        function for a set of IETF-specified protocols.<a href="#section-4.2-4" class="pilcrow">¶</a></p>
</section>
</div>
<div id="mechanism-bhd">
<section id="section-4.3">
        <h3 id="name-black-hole-detection-and-re">
<a href="#section-4.3" class="section-number selfRef">4.3. </a><a href="#name-black-hole-detection-and-re" class="section-name selfRef">Black Hole Detection and Reducing the PLPMTU</a>
        </h3>
<p id="section-4.3-1"> The description that follows uses the set of constants defined in
        <a href="#Constants" class="xref">Section 5.1.2</a> and variables defined in <a href="#Variables" class="xref">Section 5.1.3</a>.<a href="#section-4.3-1" class="pilcrow">¶</a></p>
<p id="section-4.3-2">Black hole detection is
        triggered by an indication that the network path could be unable to
        support the current PLPMTU size.<a href="#section-4.3-2" class="pilcrow">¶</a></p>
<p id="section-4.3-3">There are three indicators that can be used to detect black holes:<a href="#section-4.3-3" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-4.3-4.1">A validated PTB message can be received that indicates a PL_PTB_SIZE
        less than the current PLPMTU. A DPLPMTUD method <span class="bcp14">MUST NOT</span> rely solely on
        this method.<a href="#section-4.3-4.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-4.3-4.2">A PL can use the DPLPMTUD probing mechanism to periodically
            generate probe packets of the size of the current PLPMTU (e.g.,
            using the CONFIRMATION_TIMER, <a href="#Timers" class="xref">Section 5.1.1</a>). A
            timer tracks whether acknowledgments are received. Successive loss
            of probes is an indication that the current path no longer
            supports the PLPMTU (e.g., when the number of probe packets sent
            without receiving an acknowledgment, PROBE_COUNT, becomes greater
            than MAX_PROBES).<a href="#section-4.3-4.2" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-4.3-4.3">A PL can utilize an event that indicates the network path no
            longer sustains the sender's PLPMTU size. This could use a
            mechanism implemented within the PL to detect excessive loss of
            data sent with a specific packet size and then conclude that this
            excessive loss could be a result of an invalid PLPMTU (as in
            PLPMTUD for TCP <span>[<a href="#RFC4821" class="xref">RFC4821</a>]</span>).<a href="#section-4.3-4.3" class="pilcrow">¶</a>
</li>
        </ul>
<p id="section-4.3-5">The three methods can result in different transmission patterns for
        packet probes and are expected to result in different responsiveness
        following a change in the actual PMTU.<a href="#section-4.3-5" class="pilcrow">¶</a></p>
<p id="section-4.3-6">A PL <span class="bcp14">MAY</span> inhibit sending probe packets when no application data has
        been sent since the previous probe packet. A PL that resumes sending
        user data <span class="bcp14">MAY</span> continue PLPMTU discovery for each path. This allows it
        to use an up-to-date PLPMTU. However, this could result in additional
        packets being sent.<a href="#section-4.3-6" class="pilcrow">¶</a></p>
<p id="section-4.3-7">When the method detects that the current PLPMTU is not supported,
        DPLPMTUD sets a lower PLPMTU and a lower MPS. The PL then
        confirms that the new PLPMTU can be successfully used across the path.
        A probe packet could need to be smaller than the size of the data
        block generated by the application.<a href="#section-4.3-7" class="pilcrow">¶</a></p>
</section>
</div>
<div id="mps">
<section id="section-4.4">
        <h3 id="name-the-maximum-packet-size-mps">
<a href="#section-4.4" class="section-number selfRef">4.4. </a><a href="#name-the-maximum-packet-size-mps" class="section-name selfRef">The Maximum Packet Size (MPS)</a>
        </h3>
<p id="section-4.4-1"> The result of probing determines a usable PLPMTU, which is used to
        set the MPS used by the application. The MPS is smaller than the
        PLPMTU because it is reduced by the size of PL headers (including the
        overhead of security-related fields such as an AEAD tag and TLS record
        layer padding). The relationship between the MPS and the PLPMTUD is
        illustrated in <a href="#fig-mps-relationship" class="xref">Figure 1</a>.<a href="#section-4.4-1" class="pilcrow">¶</a></p>
<span id="name-relationship-between-mps-an"></span><div id="fig-mps-relationship">
<figure id="figure-1">
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                  <text text-anchor="middle" font-family="monospace" x="48" y="4" fill="black" font-size="1em">d</text>
                  <text text-anchor="middle" font-family="monospace" x="32" y="20" fill="black" font-size="1em">a</text>
                  <text text-anchor="middle" font-family="monospace" x="192" y="20" fill="black" font-size="1em">P</text>
                  <text text-anchor="middle" font-family="monospace" x="200" y="20" fill="black" font-size="1em">S</text>
                  <text text-anchor="middle" font-family="monospace" x="136" y="84" fill="black" font-size="1em">|</text>
                  <text text-anchor="middle" font-family="monospace" x="240" y="84" fill="black" font-size="1em">t</text>
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                  <text text-anchor="middle" font-family="monospace" x="0" y="4" fill="black" font-size="1em">A</text>
                  <text text-anchor="middle" font-family="monospace" x="72" y="4" fill="black" font-size="1em">i</text>
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                  <text text-anchor="middle" font-family="monospace" x="16" y="20" fill="black" font-size="1em">h</text>
                </g>
              </svg><a href="#section-4.4-2.1.1" class="pilcrow">¶</a>
</div>
</div>
<figcaption><a href="#figure-1" class="selfRef">Figure 1</a>:
<a href="#name-relationship-between-mps-an" class="selfRef">Relationship between MPS and PLPMTU</a>
          </figcaption></figure>
</div>
<p id="section-4.4-3">A PL is unable to send a packet (other than a probe packet) with a
        size larger than the current PLPMTU at the network layer. To avoid this,
        a PL <span class="bcp14">MAY</span> be designed to segment data blocks larger than the MPS into
        multiple datagrams.<a href="#section-4.4-3" class="pilcrow">¶</a></p>
<p id="section-4.4-4"> DPLPMTUD seeks to avoid IP fragmentation. An attempt to send a data
        block larger than the MPS will therefore fail if a PL is unable to
        segment data. To determine the largest data block that can be sent, a
        PL <span class="bcp14">SHOULD</span> provide applications with a primitive that returns the MPS,
        derived from the current PLPMTU.<a href="#section-4.4-4" class="pilcrow">¶</a></p>
<p id="section-4.4-5"> If DPLPMTUD results in a change to the MPS, the application needs
 to adapt to the new MPS. A particular case can arise when packets have
 been sent with a size less than the MPS and the PLPMTU was subsequently
 reduced. If these packets are lost, the PL <span class="bcp14">MAY</span> segment the data using
 the new MPS. If a PL is unable to resegment a previously sent datagram
 (e.g., <span>[<a href="#RFC4960" class="xref">RFC4960</a>]</span>), then the sender either discards
 the datagram or could perform retransmission using network-layer
 fragmentation to form multiple IP packets not larger than the PLPMTU.
 For IPv4, the use of endpoint fragmentation by the sender is preferred
 over clearing the DF bit in the IPv4 header. Operational experience
 reveals that IP fragmentation can reduce the reliability of Internet
 communication <span>[<a href="#RFC8900" class="xref">RFC8900</a>]</span>,
 which may reduce the probability of successful retransmission.<a href="#section-4.4-5" class="pilcrow">¶</a></p>
</section>
</div>
<div id="outgoingpmtu">
<section id="section-4.5">
        <h3 id="name-disabling-the-effect-of-pmt">
<a href="#section-4.5" class="section-number selfRef">4.5. </a><a href="#name-disabling-the-effect-of-pmt" class="section-name selfRef">Disabling the Effect of PMTUD</a>
        </h3>
<p id="section-4.5-1">A PL implementing this specification <span class="bcp14">MUST</span> suspend network layer
          processing of outgoing packets that enforces a PMTU <span>[<a href="#RFC1191" class="xref">RFC1191</a>]</span><span>[<a href="#RFC8201" class="xref">RFC8201</a>]</span> for each flow
          utilizing DPLPMTUD and instead use DPLPMTUD to control the size of
          packets that are sent by a flow. This removes the need for the
          network layer to drop or to fragment sent packets that have a size
          greater than the PMTU.<a href="#section-4.5-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="mechanism-ptb">
<section id="section-4.6">
        <h3 id="name-response-to-ptb-messages">
<a href="#section-4.6" class="section-number selfRef">4.6. </a><a href="#name-response-to-ptb-messages" class="section-name selfRef">Response to PTB Messages</a>
        </h3>
<p id="section-4.6-1">This method requires the DPLPMTUD sender to validate any received
        PTB message before using the PTB information. The response to a PTB
        message depends on the PL_PTB_SIZE calculated from the PTB_SIZE in the
        PTB message, the state of the PLPMTUD state machine, and the IP
        protocol being used.<a href="#section-4.6-1" class="pilcrow">¶</a></p>
<p id="section-4.6-2"><a href="#PTB" class="xref">Section 4.6.1</a> describes validation for both IPv4
        ICMP Unreachable messages (Type 3) and ICMPv6 Packet Too Big messages,
        both of which are referred to as PTB messages in this document.<a href="#section-4.6-2" class="pilcrow">¶</a></p>
<div id="PTB">
<section id="section-4.6.1">
          <h4 id="name-validation-of-ptb-messages">
<a href="#section-4.6.1" class="section-number selfRef">4.6.1. </a><a href="#name-validation-of-ptb-messages" class="section-name selfRef">Validation of PTB Messages</a>
          </h4>
<p id="section-4.6.1-1">This section specifies utilization and validation of PTB messages.<a href="#section-4.6.1-1" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-4.6.1-2.1">A simple implementation <span class="bcp14">MAY</span> ignore received PTB messages, and
              in this case, the PLPMTU is not updated when a PTB message is
              received.<a href="#section-4.6.1-2.1" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-4.6.1-2.2">A PL that supports PTB messages <span class="bcp14">MUST</span> validate these messages
              before they are further processed.<a href="#section-4.6.1-2.2" class="pilcrow">¶</a>
</li>
          </ul>
<p id="section-4.6.1-3">A PL that receives a PTB message from a router or middlebox
   performs ICMP validation (see 
          <span><a href="https://www.rfc-editor.org/rfc/rfc8201#section-4" class="relref">Section 4</a> of [<a href="#RFC8201" class="xref">RFC8201</a>]</span> and 
          <span><a href="https://www.rfc-editor.org/rfc/rfc8085#section-5.2" class="relref">Section 5.2</a> of [<a href="#BCP145" class="xref">BCP145</a>]</span>).
          Because DPLPMTUD operates at the PL, the
   PL needs to check that each received PTB message is received in
   response to a packet transmitted by the endpoint PL performing
   DPLPMTUD.<a href="#section-4.6.1-3" class="pilcrow">¶</a></p>
<p id="section-4.6.1-4">The PL <span class="bcp14">MUST</span> check the protocol information in the quoted packet
          carried in an ICMP PTB message payload to validate the message
          originated from the sending node. This validation includes
          determining that the combination of the IP addresses, the protocol, the
   source port, and destination port match those returned in the
          quoted packet -- this is also necessary for the PTB message to be
          passed to the corresponding PL.<a href="#section-4.6.1-4" class="pilcrow">¶</a></p>
<p id="section-4.6.1-5">The validation <span class="bcp14">SHOULD</span> utilize information that is not simple
          for an off-path attacker to determine <span>[<a href="#BCP145" class="xref">BCP145</a>]</span>. For example, it could check the value of a
          protocol header field known only to the two PL endpoints. A datagram
          application that uses well-known source and destination ports ought
          to also rely on other information to complete this validation.<a href="#section-4.6.1-5" class="pilcrow">¶</a></p>
<p id="section-4.6.1-6">These checks are intended to provide protection from packets that
          originate from a node that is not on the network path. A PTB message
          that does not complete the validation <span class="bcp14">MUST NOT</span> be further utilized by
          the DPLPMTUD method, as discussed in the Security Considerations
          section (<a href="#Security" class="xref">Section 8</a>).<a href="#section-4.6.1-6" class="pilcrow">¶</a></p>
<p id="section-4.6.1-7"><a href="#validPTB_SIZE" class="xref">Section 4.6.2</a> describes this processing of
   PTB messages.<a href="#section-4.6.1-7" class="pilcrow">¶</a></p>
</section>
</div>
<div id="validPTB_SIZE">
<section id="section-4.6.2">
          <h4 id="name-use-of-ptb-messages">
<a href="#section-4.6.2" class="section-number selfRef">4.6.2. </a><a href="#name-use-of-ptb-messages" class="section-name selfRef">Use of PTB Messages</a>
          </h4>
<p id="section-4.6.2-1">PTB messages that have been validated <span class="bcp14">MAY</span> be utilized by the
   DPLPMTUD algorithm but <span class="bcp14">MUST NOT</span> be used directly to set the PLPMTU.<a href="#section-4.6.2-1" class="pilcrow">¶</a></p>
<p id="section-4.6.2-2">Before using the size reported in the PTB message, it must first be
   converted to a PL_PTB_SIZE. The PL_PTB_SIZE is smaller than the
   PTB_SIZE because it is reduced by headers below the PL, including any
   IP options or extensions added to the PL packet.<a href="#section-4.6.2-2" class="pilcrow">¶</a></p>
<p id="section-4.6.2-3">A method that utilizes these PTB messages can improve the speed at
   which the algorithm detects an appropriate PLPMTU by triggering an
   immediate probe for the PL_PTB_SIZE (resulting in a network-layer
   packet of size PTB_SIZE), compared to one that relies solely on
   probing using a timer-based search algorithm.<a href="#section-4.6.2-3" class="pilcrow">¶</a></p>
<p id="section-4.6.2-4">A set of checks are intended to provide protection from a router
   that reports an unexpected PTB_SIZE. The PL also needs to check that
   the indicated PL_PTB_SIZE is less than the size used by probe packets
   and at least the minimum size accepted.<a href="#section-4.6.2-4" class="pilcrow">¶</a></p>
<p id="section-4.6.2-5"> This section provides a summary of how PTB messages can be
   utilized, using the set of constants defined in <a href="#Constants" class="xref">Section 5.1.2</a>. 
          This processing depends on the PL_PTB_SIZE and the current value of a set of variables:<a href="#section-4.6.2-5" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlNewline" id="section-4.6.2-6">
            <dt id="section-4.6.2-6.1">PL_PTB_SIZE &lt; MIN_PLPMTU</dt>
            <dd style="margin-left: 1.5em" id="section-4.6.2-6.2">
              <ul class="normal">
<li class="normal" id="section-4.6.2-6.2.1.1">Invalid PL_PTB_SIZE, see <a href="#PTB" class="xref">Section 4.6.1</a>.<a href="#section-4.6.2-6.2.1.1" class="pilcrow">¶</a>
</li>
                <li class="normal" id="section-4.6.2-6.2.1.2">PTB message ought to be discarded without further
                  processing (i.e., PLPMTU is not modified).<a href="#section-4.6.2-6.2.1.2" class="pilcrow">¶</a>
</li>
                <li class="normal" id="section-4.6.2-6.2.1.3">The information could be utilized as an input that
                  triggers the enabling of a resilience mode (see <a href="#Resilience" class="xref">Section 5.3.3</a>).<a href="#section-4.6.2-6.2.1.3" class="pilcrow">¶</a>
</li>
              </ul>
</dd>
            <dd class="break"></dd>
<dt id="section-4.6.2-6.3">MIN_PLPMTU &lt; PL_PTB_SIZE &lt; BASE_PLPMTU</dt>
            <dd style="margin-left: 1.5em" id="section-4.6.2-6.4">
              <ul class="normal">
<li class="normal" id="section-4.6.2-6.4.1.1">A robust PL <span class="bcp14">MAY</span> enter an error state (see <a href="#States" class="xref">Section 5.2</a>) for an IPv4 path when the PL_PTB_SIZE
                  reported in the PTB message is larger than or equal to 68
                  bytes <span>[<a href="#RFC0791" class="xref">RFC0791</a>]</span> and when this is less
                  than the BASE_PLPMTU.<a href="#section-4.6.2-6.4.1.1" class="pilcrow">¶</a>
</li>
                <li class="normal" id="section-4.6.2-6.4.1.2">A robust PL <span class="bcp14">MAY</span> enter an error state (see <a href="#States" class="xref">Section 5.2</a>) for an IPv6 path when the PL_PTB_SIZE
                  reported in the PTB message is larger than or equal to 1280
                  bytes <span>[<a href="#RFC8200" class="xref">RFC8200</a>]</span> and when this is less
                  than the BASE_PLPMTU.<a href="#section-4.6.2-6.4.1.2" class="pilcrow">¶</a>
</li>
              </ul>
</dd>
            <dd class="break"></dd>
<dt id="section-4.6.2-6.5">BASE_PLPMTU &lt;= PL_PTB_SIZE &lt; PLPMTU</dt>
            <dd style="margin-left: 1.5em" id="section-4.6.2-6.6">
              <ul class="normal">
<li class="normal" id="section-4.6.2-6.6.1.1">This could be an indication of a black hole. The PLPMTU
                  <span class="bcp14">SHOULD</span> be set to BASE_PLPMTU (the PLPMTU is reduced to the
                  BASE_PLPMTU to avoid unnecessary packet loss when a black hole
                  is encountered).<a href="#section-4.6.2-6.6.1.1" class="pilcrow">¶</a>
</li>
                <li class="normal" id="section-4.6.2-6.6.1.2"> The PL ought to start a search to quickly discover the
                  new PLPMTU. The PL_PTB_SIZE reported in the PTB message can be
                  used to initialize a search algorithm.<a href="#section-4.6.2-6.6.1.2" class="pilcrow">¶</a>
</li>
              </ul>
</dd>
            <dd class="break"></dd>
<dt id="section-4.6.2-6.7">PLPMTU &lt; PL_PTB_SIZE &lt; PROBED_SIZE</dt>
            <dd style="margin-left: 1.5em" id="section-4.6.2-6.8">
              <ul class="normal">
<li class="normal" id="section-4.6.2-6.8.1.1">The PLPMTU continues to be valid, but the  size of a
                  packet used to search (PROBED_SIZE) was larger than the
                  actual PMTU.<a href="#section-4.6.2-6.8.1.1" class="pilcrow">¶</a>
</li>
                <li class="normal" id="section-4.6.2-6.8.1.2">The PLPMTU is not updated.<a href="#section-4.6.2-6.8.1.2" class="pilcrow">¶</a>
</li>
                <li class="normal" id="section-4.6.2-6.8.1.3">The PL can use the reported PL_PTB_SIZE from the PTB
                  message as the next search point when it resumes the search
                  algorithm.<a href="#section-4.6.2-6.8.1.3" class="pilcrow">¶</a>
</li>
              </ul>
</dd>
            <dd class="break"></dd>
<dt id="section-4.6.2-6.9">PL_PTB_SIZE &gt;= PROBED_SIZE</dt>
            <dd style="margin-left: 1.5em" id="section-4.6.2-6.10">
              <ul class="normal">
<li class="normal" id="section-4.6.2-6.10.1.1">Inconsistent network signal.<a href="#section-4.6.2-6.10.1.1" class="pilcrow">¶</a>
</li>
                <li class="normal" id="section-4.6.2-6.10.1.2">PTB message ought to be discarded without further
                  processing (i.e., PLPMTU is not modified).<a href="#section-4.6.2-6.10.1.2" class="pilcrow">¶</a>
</li>
                <li class="normal" id="section-4.6.2-6.10.1.3">The information could be utilized as an input to trigger
                  the enabling of a resilience mode.<a href="#section-4.6.2-6.10.1.3" class="pilcrow">¶</a>
</li>
              </ul>
</dd>
          <dd class="break"></dd>
</dl>
</section>
</div>
</section>
</div>
</section>
</div>
<div id="Spec">
<section id="section-5">
      <h2 id="name-datagram-packetization-laye">
<a href="#section-5" class="section-number selfRef">5. </a><a href="#name-datagram-packetization-laye" class="section-name selfRef">Datagram Packetization Layer PMTUD</a>
      </h2>
<p id="section-5-1">This section specifies Datagram PLPMTUD (DPLPMTUD). The method can be
      introduced at various points (as indicated with * in <a href="#fig-plpmtudimplement" class="xref">Figure 2</a>)
      in the IP protocol stack to discover the PLPMTU so that an application
      can utilize an appropriate MPS for the current network path.<a href="#section-5-1" class="pilcrow">¶</a></p>
<p id="section-5-2">DPLPMTUD <span class="bcp14">SHOULD</span> only be performed at one layer between a pair of
      endpoints. Therefore, an upper PL or application should avoid using
      DPLPMTUD when this is already enabled in a lower layer. A PL <span class="bcp14">MUST</span> adjust
      the MPS indicated by DPLPMTUD to account for any additional overhead
      introduced by the PL.<a href="#section-5-2" class="pilcrow">¶</a></p>
<span id="name-examples-where-dplpmtud-can"></span><div id="fig-plpmtudimplement">
<figure id="figure-2">
        <div id="section-5-3.1">
          <div class="artwork art-svg alignLeft" id="section-5-3.1.1">
<svg xmlns="http://www.w3.org/2000/svg" class="diagram" version="1.1" viewBox="0 0 600.0 281.0">
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              </g>
            </svg><a href="#section-5-3.1.1" class="pilcrow">¶</a>
</div>
</div>
<figcaption><a href="#figure-2" class="selfRef">Figure 2</a>:
<a href="#name-examples-where-dplpmtud-can" class="selfRef">Examples Where DPLPMTUD Can Be Implemented</a>
        </figcaption></figure>
</div>
<p id="section-5-4">The central idea of DPLPMTUD is probing by a sender. Probe packets
      are sent to find the maximum size of user message that can be
      completely transferred across the network path from the sender to the
      destination.<a href="#section-5-4" class="pilcrow">¶</a></p>
<p id="section-5-5">The following sections identify the components needed for
      implementation, provide an overview of the phases of operation, and
      specify the state machine and search algorithm.<a href="#section-5-5" class="pilcrow">¶</a></p>
<div id="dplpmtud-components">
<section id="section-5.1">
        <h3 id="name-dplpmtud-components">
<a href="#section-5.1" class="section-number selfRef">5.1. </a><a href="#name-dplpmtud-components" class="section-name selfRef">DPLPMTUD Components</a>
        </h3>
<p id="section-5.1-1">This section describes the timers, constants, and variables of
        DPLPMTUD.<a href="#section-5.1-1" class="pilcrow">¶</a></p>
<div id="Timers">
<section id="section-5.1.1">
          <h4 id="name-timers">
<a href="#section-5.1.1" class="section-number selfRef">5.1.1. </a><a href="#name-timers" class="section-name selfRef">Timers</a>
          </h4>
<p id="section-5.1.1-1">The method utilizes up to three timers:<a href="#section-5.1.1-1" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlParallel" id="section-5.1.1-2">
            <dt id="section-5.1.1-2.1">PROBE_TIMER:</dt>
            <dd style="margin-left: 1.5em" id="section-5.1.1-2.2">
              <p id="section-5.1.1-2.2.1">The PROBE_TIMER is configured to expire after a period
                longer than the maximum time to receive an acknowledgment to a
                probe packet. This value <span class="bcp14">MUST NOT</span> be smaller than 1 second
                and <span class="bcp14">SHOULD</span> be larger than 15 seconds. Guidance on the selection of
                the timer value is provided in Section 
                <a href="https://www.rfc-editor.org/rfc/rfc8085#section-3.1.1" class="relref">3.1.1</a> 
                of the UDP Usage Guidelines <span>[<a href="#BCP145" class="xref">BCP145</a>]</span>.<a href="#section-5.1.1-2.2.1" class="pilcrow">¶</a></p>
</dd>
            <dd class="break"></dd>
<dt id="section-5.1.1-2.3">PMTU_RAISE_TIMER:</dt>
            <dd style="margin-left: 1.5em" id="section-5.1.1-2.4">
              <p id="section-5.1.1-2.4.1">The PMTU_RAISE_TIMER is configured to the period a sender
                will continue to use the current PLPMTU, after which it
                reenters the Search Phase. This timer has a period of 600
                seconds, as recommended by PLPMTUD <span>[<a href="#RFC4821" class="xref">RFC4821</a>]</span>.<a href="#section-5.1.1-2.4.1" class="pilcrow">¶</a></p>
<p id="section-5.1.1-2.4.2">DPLPMTUD <span class="bcp14">MAY</span> inhibit sending probe packets when no
                application data has been sent since the previous probe
                packet. A PL preferring to use an up-to-date PMTU once user
                data is sent again can choose to continue PMTU discovery for
                each path. However, this will result in sending additional
                packets.<a href="#section-5.1.1-2.4.2" class="pilcrow">¶</a></p>
</dd>
            <dd class="break"></dd>
<dt id="section-5.1.1-2.5">CONFIRMATION_TIMER:</dt>
            <dd style="margin-left: 1.5em" id="section-5.1.1-2.6">
              <p id="section-5.1.1-2.6.1">When an acknowledged PL is used, this timer <span class="bcp14">MUST NOT</span> be
                used. For other PLs, the CONFIRMATION_TIMER is configured to
                the period a PL sender waits before confirming the current
                PLPMTU is still supported. This is less than the
                PMTU_RAISE_TIMER and used to decrease the PLPMTU (e.g., when a
                black hole is encountered). Confirmation needs to be frequent
                enough when data is flowing that the sending PL does not black
                hole extensive amounts of traffic. Guidance on selection of
                the timer value are provided in Section
                <a href="https://www.rfc-editor.org/rfc/rfc8085#section-3.1.1" class="relref">3.1.1</a> 
                of the UDP Usage Guidelines <span>[<a href="#BCP145" class="xref">BCP145</a>]</span>.<a href="#section-5.1.1-2.6.1" class="pilcrow">¶</a></p>
<p id="section-5.1.1-2.6.2">DPLPMTUD <span class="bcp14">MAY</span> inhibit sending probe packets when no
                application data has been sent since the previous probe
                packet. A PL preferring to use an up-to-date PMTU once user
                data is sent again, can choose to continue PMTU discovery for
                each path. However, this could result in sending additional
                packets.<a href="#section-5.1.1-2.6.2" class="pilcrow">¶</a></p>
</dd>
          <dd class="break"></dd>
</dl>
<p id="section-5.1.1-3">DPLPMTUD specifies various timers; however, an implementation could
   choose to realize these timer functions using a single timer.<a href="#section-5.1.1-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="Constants">
<section id="section-5.1.2">
          <h4 id="name-constants">
<a href="#section-5.1.2" class="section-number selfRef">5.1.2. </a><a href="#name-constants" class="section-name selfRef">Constants</a>
          </h4>
<p id="section-5.1.2-1">The following constants are defined:<a href="#section-5.1.2-1" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlParallel" id="section-5.1.2-2">
            <dt id="section-5.1.2-2.1">MAX_PROBES:</dt>
            <dd style="margin-left: 1.5em" id="section-5.1.2-2.2">The MAX_PROBES is the maximum value of the PROBE_COUNT
              counter (see <a href="#Variables" class="xref">Section 5.1.3</a>). MAX_PROBES represents
              the limit for the number of consecutive probe attempts of any
              size. Search algorithms benefit from a MAX_PROBES value greater
              than 1 because this can provide robustness to isolated packet
              loss. The default value of MAX_PROBES is 3.<a href="#section-5.1.2-2.2" class="pilcrow">¶</a>
</dd>
            <dd class="break"></dd>
<dt id="section-5.1.2-2.3">MIN_PLPMTU:</dt>
            <dd style="margin-left: 1.5em" id="section-5.1.2-2.4">The MIN_PLPMTU is the smallest size of PLPMTU that DPLPMTUD
       will attempt to use.  An endpoint could need to configure the
       MIN_PLPMTU to provide space for extension headers and other
       encapsulations at layers below the PL. This value can be
       interface and path dependent.  For IPv6, this size is greater
       than or equal to the size at the PL that results in an 1280-byte
       IPv6 packet, as specified in <span>[<a href="#RFC8200" class="xref">RFC8200</a>]</span>. For
       IPv4, this size is greater than or equal to the size at the PL
       that results in an 68-byte IPv4 packet. Note: An IPv4 router is
       required to be able to forward a datagram of 68 bytes without
       further fragmentation. This is the combined size of an IPv4
       header and the minimum fragment size of 8 bytes. In addition,
       receivers are required to be able to reassemble fragmented
       datagrams at least up to 576 bytes, as stated in 
       <span><a href="https://www.rfc-editor.org/rfc/rfc1122#section-3.3.3" class="relref">Section 3.3.3</a> of [<a href="#RFC1122" class="xref">RFC1122</a>]</span>.<a href="#section-5.1.2-2.4" class="pilcrow">¶</a>
</dd>
            <dd class="break"></dd>
<dt id="section-5.1.2-2.5">MAX_PLPMTU:</dt>
            <dd style="margin-left: 1.5em" id="section-5.1.2-2.6">The MAX_PLPMTU is the largest size of PLPMTU. This has to be
              less than or equal to the maximum size of the PL packet that can
              be sent on the outgoing interface (constrained by the local
              interface MTU). When known, this also ought to be less than the
              maximum size of PL packet that can be received by the remote
              endpoint (constrained by EMTU_R). It can be limited by the design
              or configuration of the PL being used. An application, or PL,
              <span class="bcp14">MAY</span> choose a smaller MAX_PLPMTU when there is no need to send
              packets larger than a specific size.<a href="#section-5.1.2-2.6" class="pilcrow">¶</a>
</dd>
            <dd class="break"></dd>
<dt id="section-5.1.2-2.7">BASE_PLPMTU:</dt>
            <dd style="margin-left: 1.5em" id="section-5.1.2-2.8">The BASE_PLPMTU is a configured size expected to work for
       most paths. The size is equal to or larger than the MIN_PLPMTU
       and smaller than the MAX_PLPMTU. For most PLs, a suitable
       BASE_PLPMTU will be larger than 1200 bytes. When using IPv4,
       there is no currently equivalent size specified, and a default
       BASE_PLPMTU of 1200 bytes is <span class="bcp14">RECOMMENDED</span>.<a href="#section-5.1.2-2.8" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
</dl>
</section>
</div>
<div id="Variables">
<section id="section-5.1.3">
          <h4 id="name-variables">
<a href="#section-5.1.3" class="section-number selfRef">5.1.3. </a><a href="#name-variables" class="section-name selfRef">Variables</a>
          </h4>
<p id="section-5.1.3-1">This method utilizes a set of variables:<a href="#section-5.1.3-1" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlParallel" id="section-5.1.3-2">
            <dt id="section-5.1.3-2.1">PROBED_SIZE:</dt>
            <dd style="margin-left: 1.5em" id="section-5.1.3-2.2">The PROBED_SIZE is the size of the current probe packet as
       determined at the PL. This is a tentative value for the PLPMTU,
       which is awaiting confirmation by an acknowledgment.<a href="#section-5.1.3-2.2" class="pilcrow">¶</a>
</dd>
            <dd class="break"></dd>
<dt id="section-5.1.3-2.3">PROBE_COUNT:</dt>
            <dd style="margin-left: 1.5em" id="section-5.1.3-2.4">The PROBE_COUNT is a count of the number of successive
              unsuccessful probe packets that have been sent. Each time a probe
              packet is acknowledged, the value is set to zero. (Some probe
              loss is expected while searching, therefore loss of a single
              probe is not an indication of a PMTU problem.)<a href="#section-5.1.3-2.4" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
</dl>
<p id="section-5.1.3-3"><a href="#fig-mps" class="xref">Figure 3</a> illustrates the relationship between the packet
          size constants and variables at a point of time when the DPLPMTUD
          algorithm performs path probing to increase the size of the PLPMTU.
          A probe packet has been sent of size PROBED_SIZE. Once this is
          acknowledged, the PLPMTU will raise to PROBED_SIZE, allowing the
          DPLPMTUD algorithm to further increase PROBED_SIZE toward sending a probe
          with the size of the actual PMTU.<a href="#section-5.1.3-3" class="pilcrow">¶</a></p>
<span id="name-relationships-between-packe"></span><div id="fig-mps">
<figure id="figure-3">
            <div id="section-5.1.3-4.1">
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                  </g>
                </svg><a href="#section-5.1.3-4.1.1" class="pilcrow">¶</a>
</div>
</div>
<figcaption><a href="#figure-3" class="selfRef">Figure 3</a>:
<a href="#name-relationships-between-packe" class="selfRef">Relationships between Packet Size Constants and Variables</a>
            </figcaption></figure>
</div>
</section>
</div>
<div id="phases">
<section id="section-5.1.4">
          <h4 id="name-overview-of-dplpmtud-phases">
<a href="#section-5.1.4" class="section-number selfRef">5.1.4. </a><a href="#name-overview-of-dplpmtud-phases" class="section-name selfRef">Overview of DPLPMTUD Phases</a>
          </h4>
<p id="section-5.1.4-1">This section provides a high-level, informative view of the
          DPLPMTUD method, by describing the movement of the method through
          several phases of operation. More detail is available in the state
          machine, <a href="#States" class="xref">Section 5.2</a>.<a href="#section-5.1.4-1" class="pilcrow">¶</a></p>
<span id="name-dplpmtud-phases"></span><div id="fig-phases">
<figure id="figure-4">
            <div id="section-5.1.4-2.1">
              <div class="artwork art-svg alignLeft" id="section-5.1.4-2.1.1">
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                </svg><a href="#section-5.1.4-2.1.1" class="pilcrow">¶</a>
</div>
</div>
<figcaption><a href="#figure-4" class="selfRef">Figure 4</a>:
<a href="#name-dplpmtud-phases" class="selfRef">DPLPMTUD Phases</a>
            </figcaption></figure>
</div>
<span class="break"></span><dl class="dlParallel" id="section-5.1.4-3">
            <dt id="section-5.1.4-3.1">Base:</dt>
            <dd style="margin-left: 1.5em" id="section-5.1.4-3.2">
              <p id="section-5.1.4-3.2.1">The Base Phase confirms connectivity to the remote peer
 using packets of the BASE_PLPMTU. The confirmation of
 connectivity is implicit for a connection-oriented PL (where it
 can be performed in a PL connection handshake). A
 connectionless PL sends a probe packet and uses acknowledgment
 of this probe packet to confirm that the remote peer is
 reachable.<a href="#section-5.1.4-3.2.1" class="pilcrow">¶</a></p>
<p id="section-5.1.4-3.2.2">The sender also confirms that BASE_PLPMTU is supported across
                the network path. This may be achieved by using a PL mechanism
                (e.g., using a handshake packet of size BASE_PLPMTU) or by
                sending a probe packet of size BASE_PLPMTU and confirming that
                this is received.<a href="#section-5.1.4-3.2.2" class="pilcrow">¶</a></p>
<p id="section-5.1.4-3.2.3">A probe packet of size BASE_PLPMTU can be sent immediately
                on the initial entry to the Base Phase (following a
                connectivity check). A PL that does not wish to support a path
                with a PLPMTU less than BASE_PLPMTU can simplify the phase into
                a single step by performing the connectivity checks with a
                probe of the BASE_PLPMTU size.<a href="#section-5.1.4-3.2.3" class="pilcrow">¶</a></p>
<p id="section-5.1.4-3.2.4">Once confirmed, DPLPMTUD enters the Search Phase. If the
                Base Phase fails to confirm the BASE_PLPMTU, DPLPMTUD enters
                the Error Phase.<a href="#section-5.1.4-3.2.4" class="pilcrow">¶</a></p>
</dd>
            <dd class="break"></dd>
<dt id="section-5.1.4-3.3">Search:</dt>
            <dd style="margin-left: 1.5em" id="section-5.1.4-3.4">
              <p id="section-5.1.4-3.4.1">The Search Phase utilizes a search algorithm to send probe
                packets to seek to increase the PLPMTU. The algorithm
                concludes when it has found a suitable PLPMTU by entering the
                Search Complete Phase.<a href="#section-5.1.4-3.4.1" class="pilcrow">¶</a></p>
<p id="section-5.1.4-3.4.2">A PL could respond to PTB messages using the PTB to advance
                or terminate the search, see <a href="#mechanism-ptb" class="xref">Section 4.6</a>.<a href="#section-5.1.4-3.4.2" class="pilcrow">¶</a></p>
</dd>
            <dd class="break"></dd>
<dt id="section-5.1.4-3.5">Search Complete:</dt>
            <dd style="margin-left: 1.5em" id="section-5.1.4-3.6">
              <p id="section-5.1.4-3.6.1">The Search Complete Phase is entered when the PLPMTU is
                supported across the network path. A PL can use a
                CONFIRMATION_TIMER to periodically repeat a probe packet for
                the current PLPMTU size. If the sender is unable to confirm
                reachability (e.g., if the CONFIRMATION_TIMER expires) or the
                PL signals a lack of reachability, a black hole has been
                detected and DPLPMTUD enters the Base Phase.<a href="#section-5.1.4-3.6.1" class="pilcrow">¶</a></p>
<p id="section-5.1.4-3.6.2">The PMTU_RAISE_TIMER is used to periodically resume the
                Search Phase to discover if the PLPMTU can be raised. Black
                hole detection causes the sender to enter the Base Phase.<a href="#section-5.1.4-3.6.2" class="pilcrow">¶</a></p>
</dd>
            <dd class="break"></dd>
<dt id="section-5.1.4-3.7">Error:</dt>
            <dd style="margin-left: 1.5em" id="section-5.1.4-3.8">
              <p id="section-5.1.4-3.8.1">The Error Phase is entered when there is conflicting or
                invalid PLPMTU information for the path (e.g., a failure to
                support the BASE_PLPMTU) that causes DPLPMTUD to be unable to
                progress, and the PLPMTU is lowered.<a href="#section-5.1.4-3.8.1" class="pilcrow">¶</a></p>
<p id="section-5.1.4-3.8.2">DPLPMTUD remains in the Error Phase until a consistent view
                of the path can be discovered and it has also been confirmed
                that the path supports the BASE_PLPMTU (or DPLPMTUD is
                suspended).<a href="#section-5.1.4-3.8.2" class="pilcrow">¶</a></p>
</dd>
          <dd class="break"></dd>
</dl>
<p id="section-5.1.4-4"> A method that only reduces the PLPMTU to a suitable size would be
          sufficient to ensure reliable operation but can be very inefficient
          when the actual PMTU changes or when the method (for whatever reason)
          makes a suboptimal choice for the PLPMTU.<a href="#section-5.1.4-4" class="pilcrow">¶</a></p>
<p id="section-5.1.4-5">A full implementation of DPLPMTUD provides an algorithm enabling
          the DPLPMTUD sender to increase the PLPMTU following a change in the
          characteristics of the path, such as when a link is reconfigured
          with a larger MTU, or when there is a change in the set of links
          traversed by an end-to-end flow (e.g., after a routing or path
          failover decision).<a href="#section-5.1.4-5" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="States">
<section id="section-5.2">
        <h3 id="name-state-machine">
<a href="#section-5.2" class="section-number selfRef">5.2. </a><a href="#name-state-machine" class="section-name selfRef">State Machine</a>
        </h3>
<p id="section-5.2-1">A state machine for DPLPMTUD is depicted in <a href="#fig-states" class="xref">Figure 5</a>. If multipath or multihoming is supported,
        a state machine is needed for each path.<a href="#section-5.2-1" class="pilcrow">¶</a></p>
<p id="section-5.2-2">Note: Not all changes are shown to simplify the
          diagram.<a href="#section-5.2-2" class="pilcrow">¶</a></p>
<span id="name-state-machine-for-datagram-"></span><div id="fig-states">
<figure id="figure-5">
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                  <text text-anchor="middle" font-family="monospace" x="64" y="436" fill="black" font-size="1em">C</text>
                  <text text-anchor="middle" font-family="monospace" x="512" y="436" fill="black" font-size="1em">G</text>
                  <text text-anchor="middle" font-family="monospace" x="400" y="596" fill="black" font-size="1em">P</text>
                  <text text-anchor="middle" font-family="monospace" x="104" y="116" fill="black" font-size="1em">i</text>
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                  <text text-anchor="middle" font-family="monospace" x="72" y="260" fill="black" font-size="1em">B</text>
                  <text text-anchor="middle" font-family="monospace" x="200" y="260" fill="black" font-size="1em">t</text>
                  <text text-anchor="middle" font-family="monospace" x="128" y="564" fill="black" font-size="1em">E</text>
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                  <text text-anchor="middle" font-family="monospace" x="144" y="532" fill="black" font-size="1em">I</text>
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                </g>
              </svg><a href="#section-5.2-3.1.1" class="pilcrow">¶</a>
</div>
</div>
<figcaption><a href="#figure-5" class="selfRef">Figure 5</a>:
<a href="#name-state-machine-for-datagram-" class="selfRef">State Machine for Datagram PLPMTUD</a>
          </figcaption></figure>
</div>
<p id="section-5.2-4"></p>
<p id="section-5.2-5">The following states are defined:<a href="#section-5.2-5" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlParallel" id="section-5.2-6">
          <dt id="section-5.2-6.1">DISABLED:</dt>
          <dd style="margin-left: 1.5em" id="section-5.2-6.2">The DISABLED state is the initial state before probing has
            started. It is also entered from any other state, when the PL
            indicates loss of connectivity. This state is left once the PL
            indicates connectivity to the remote PL. When transitioning to the
            BASE state, a probe packet of size BASE_PLPMTU can be sent
            immediately.<a href="#section-5.2-6.2" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
<dt id="section-5.2-6.3">BASE:</dt>
          <dd style="margin-left: 1.5em" id="section-5.2-6.4">
            <p id="section-5.2-6.4.1">The BASE state is used to confirm that the BASE_PLPMTU size is
              supported by the network path and is designed to allow an
              application to continue working when there are transient
              reductions in the actual PMTU. It also seeks to avoid long
              periods when a sender searching for a larger PLPMTU is unaware
              that packets are not being delivered due to a packet or ICMP
              black hole.<a href="#section-5.2-6.4.1" class="pilcrow">¶</a></p>
<p id="section-5.2-6.4.2">On entry, the PROBED_SIZE is set to the BASE_PLPMTU size, and
              the PROBE_COUNT is set to zero.<a href="#section-5.2-6.4.2" class="pilcrow">¶</a></p>
<p id="section-5.2-6.4.3">Each time a probe packet is sent, the PROBE_TIMER is started.
              The state is exited when the probe packet is acknowledged, and
              the PL sender enters the SEARCHING state.<a href="#section-5.2-6.4.3" class="pilcrow">¶</a></p>
<p id="section-5.2-6.4.4">The state is also left when the PROBE_COUNT reaches
              MAX_PROBES or a received PTB message is validated. This causes
              the PL sender to enter the ERROR state.<a href="#section-5.2-6.4.4" class="pilcrow">¶</a></p>
</dd>
          <dd class="break"></dd>
<dt id="section-5.2-6.5">SEARCHING:</dt>
          <dd style="margin-left: 1.5em" id="section-5.2-6.6">
            <p id="section-5.2-6.6.1">The SEARCHING state is the main probing state. This state is
              entered when probing for the BASE_PLPMTU completes.<a href="#section-5.2-6.6.1" class="pilcrow">¶</a></p>
<p id="section-5.2-6.6.2">Each time a probe packet is acknowledged, the PROBE_COUNT is
              set to zero, the PLPMTU is set to the PROBED_SIZE, and then the
              PROBED_SIZE is increased using the search algorithm (as described
              in <a href="#searchincrease" class="xref">Section 5.3</a>).<a href="#section-5.2-6.6.2" class="pilcrow">¶</a></p>
<p id="section-5.2-6.6.3">When a probe packet is sent and not acknowledged within the
              period of the PROBE_TIMER, the PROBE_COUNT is incremented, and
              a new probe packet is transmitted.<a href="#section-5.2-6.6.3" class="pilcrow">¶</a></p>
<p id="section-5.2-6.6.4"> The state is exited to enter SEARCH_COMPLETE when the
              PROBE_COUNT reaches MAX_PROBES, a validated PTB is received that
              corresponds to the last successfully probed size (PL_PTB_SIZE =
              PLPMTU), or a probe of size MAX_PLPMTU is acknowledged (PLPMTU =
              MAX_PLPMTU).<a href="#section-5.2-6.6.4" class="pilcrow">¶</a></p>
<p id="section-5.2-6.6.5"> When a black hole is detected in the SEARCHING state, this causes
              the PL sender to enter the BASE state.<a href="#section-5.2-6.6.5" class="pilcrow">¶</a></p>
</dd>
          <dd class="break"></dd>
<dt id="section-5.2-6.7">SEARCH_COMPLETE:</dt>
          <dd style="margin-left: 1.5em" id="section-5.2-6.8">
            <p id="section-5.2-6.8.1"> The SEARCH_COMPLETE state indicates that a search has
              completed. This is the normal maintenance state, where the PL is
              not probing to update the PLPMTU. DPLPMTUD remains in this state
              until either the PMTU_RAISE_TIMER expires or a black hole is
              detected.<a href="#section-5.2-6.8.1" class="pilcrow">¶</a></p>
<p id="section-5.2-6.8.2">When DPLPMTUD uses an unacknowledged PL and is in the
              SEARCH_COMPLETE state, a CONFIRMATION_TIMER periodically resets
              the PROBE_COUNT and schedules a probe packet with the size of the
              PLPMTU. If MAX_PROBES successive PLPMTUD-sized probes fail to be
              acknowledged, the method enters the BASE state. When used with an
              acknowledged PL (e.g., SCTP), DPLPMTUD <span class="bcp14">SHOULD NOT</span> continue to
              generate PLPMTU probes in this state.<a href="#section-5.2-6.8.2" class="pilcrow">¶</a></p>
</dd>
          <dd class="break"></dd>
<dt id="section-5.2-6.9">ERROR:</dt>
          <dd style="margin-left: 1.5em" id="section-5.2-6.10">
            <p id="section-5.2-6.10.1">The ERROR state represents the case where either the network
              path is not known to support a PLPMTU of at least the BASE_PLPMTU
              size or when there is contradictory information about the network
              path that would otherwise result in excessive variation in the
              MPS signaled to the higher layer. The state implements a method
              to mitigate oscillation in the state-event engine. It signals a
              conservative value of the MPS to the higher layer by the PL. The
              state is exited when packet probes no longer detect the error.
              The PL sender then enters the SEARCHING state.<a href="#section-5.2-6.10.1" class="pilcrow">¶</a></p>
<p id="section-5.2-6.10.2">Implementations are permitted to enable endpoint
              fragmentation if the DPLPMTUD is unable to validate MIN_PLPMTU
              within PROBE_COUNT probes. If DPLPMTUD is unable to validate
              MIN_PLPMTU, the implementation will transition to the DISABLED
              state.<a href="#section-5.2-6.10.2" class="pilcrow">¶</a></p>
<p id="section-5.2-6.10.3">Note: MIN_PLPMTU could be identical to BASE_PLPMTU,
              simplifying the actions in this state.<a href="#section-5.2-6.10.3" class="pilcrow">¶</a></p>
</dd>
        <dd class="break"></dd>
</dl>
</section>
</div>
<div id="searchincrease">
<section id="section-5.3">
        <h3 id="name-search-to-increase-the-plpm">
<a href="#section-5.3" class="section-number selfRef">5.3. </a><a href="#name-search-to-increase-the-plpm" class="section-name selfRef">Search to Increase the PLPMTU</a>
        </h3>
<p id="section-5.3-1">This section describes the algorithms used by DPLPMTUD to search
        for a larger PLPMTU.<a href="#section-5.3-1" class="pilcrow">¶</a></p>
<div id="Increase">
<section id="section-5.3.1">
          <h4 id="name-probing-for-a-larger-plpmtu">
<a href="#section-5.3.1" class="section-number selfRef">5.3.1. </a><a href="#name-probing-for-a-larger-plpmtu" class="section-name selfRef">Probing for a Larger PLPMTU</a>
          </h4>
<p id="section-5.3.1-1">Implementations use a search algorithm across the search range to
          determine whether a larger PLPMTU can be supported across a network
          path.<a href="#section-5.3.1-1" class="pilcrow">¶</a></p>
<p id="section-5.3.1-2">The method discovers the search range by confirming the minimum
   PLPMTU and then using the probe method to select a PROBED_SIZE less
   than or equal to MAX_PLPMTU. MAX_PLPMTU is the minimum of the local
   MTU and EMTU_R (when this is learned from the remote endpoint). The
   MAX_PLPMTU <span class="bcp14">MAY</span> be reduced by an application that sets a maximum to
   the size of datagrams it will send.<a href="#section-5.3.1-2" class="pilcrow">¶</a></p>
<p id="section-5.3.1-3"> The PROBE_COUNT is initialized to zero when the first probe with
   a size greater than or equal to PLPMTU is sent. Each probe packet
   successfully sent to the remote peer is confirmed by acknowledgment
   at the PL (see <a href="#Probe" class="xref">Section 4.1</a>).<a href="#section-5.3.1-3" class="pilcrow">¶</a></p>
<p id="section-5.3.1-4">Each time a probe packet is sent to the destination, the
          PROBE_TIMER is started. The timer is canceled when the PL receives
          acknowledgment that the probe packet has been successfully sent
          across the path (<a href="#Probe" class="xref">Section 4.1</a>). This confirms that the
          PROBED_SIZE is supported, and the PROBED_SIZE value is then assigned
          to the PLPMTU. The search algorithm can continue to send subsequent
          probe packets of an increasing size.<a href="#section-5.3.1-4" class="pilcrow">¶</a></p>
<p id="section-5.3.1-5">If the timer expires before a probe packet is acknowledged, the
          probe has failed to confirm the PROBED_SIZE. Each time the
          PROBE_TIMER expires, the PROBE_COUNT is incremented, the PROBE_TIMER
          is reinitialized, and a new probe of the same size or any other size
          (determined by the search algorithm) can be sent. The maximum number
          of consecutive failed probes is configured (MAX_PROBES). If the
          value of the PROBE_COUNT reaches MAX_PROBES, probing will stop, and
          the PL sender enters the SEARCH_COMPLETE state.<a href="#section-5.3.1-5" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-5.3.2">
          <h4 id="name-selection-of-probe-sizes">
<a href="#section-5.3.2" class="section-number selfRef">5.3.2. </a><a href="#name-selection-of-probe-sizes" class="section-name selfRef">Selection of Probe Sizes</a>
          </h4>
<p id="section-5.3.2-1">The search algorithm determines a minimum useful gain in
          PLPMTU. It would not be constructive for a PL sender to attempt to
          probe for all sizes. This would incur unnecessary load on the path.
          Implementations <span class="bcp14">SHOULD</span> select the set of probe packet
          sizes to maximize the gain in PLPMTU from each search step.<a href="#section-5.3.2-1" class="pilcrow">¶</a></p>
<p id="section-5.3.2-2">Implementations could optimize the search procedure by selecting
          step sizes from a table of common PMTU sizes. When selecting the
          appropriate next size to search, an implementer ought to also
          consider that there can be common sizes of MPS that applications
          seek to use, and there could be common sizes of MTU used within the
          network.<a href="#section-5.3.2-2" class="pilcrow">¶</a></p>
</section>
<div id="Resilience">
<section id="section-5.3.3">
          <h4 id="name-resilience-to-inconsistent-">
<a href="#section-5.3.3" class="section-number selfRef">5.3.3. </a><a href="#name-resilience-to-inconsistent-" class="section-name selfRef">Resilience to Inconsistent Path Information</a>
          </h4>
<p id="section-5.3.3-1">A decision to increase the PLPMTU needs to be resilient to the
          possibility that information learned about the network path is
          inconsistent. A path is inconsistent when, for example, probe
          packets are lost due to other reasons (i.e., not packet size) or due
          to frequent path changes. Frequent path changes could occur by
          unexpected "flapping" -- where some packets from a flow pass along
          one path, but other packets follow a different path with different
          properties.<a href="#section-5.3.3-1" class="pilcrow">¶</a></p>
<p id="section-5.3.3-2">A PL sender is able to detect inconsistency either from the sequence of
          PLPMTU probes that are acknowledged or from the sequence of PTB messages that it
          receives. When inconsistent path information is detected, a PL
          sender could use an alternate search mode that clamps the offered
          MPS to a smaller value for a period of time. This avoids unnecessary
          loss of packets.<a href="#section-5.3.3-2" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="robustness">
<section id="section-5.4">
        <h3 id="name-robustness-to-inconsistent-">
<a href="#section-5.4" class="section-number selfRef">5.4. </a><a href="#name-robustness-to-inconsistent-" class="section-name selfRef">Robustness to Inconsistent Paths</a>
        </h3>
<p id="section-5.4-1">Some paths could be unable to sustain packets of the BASE_PLPMTU
        size. The Error State could be implemented to provide robustness to
        such paths. This allows fallback to a smaller than desired PLPMTU
        rather than suffer connectivity failure. This could utilize methods
        such as endpoint IP fragmentation to enable the PL sender to
        communicate using packets smaller than the BASE_PLPMTU.<a href="#section-5.4-1" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="protocol_specific_methods">
<section id="section-6">
      <h2 id="name-specification-of-protocol-s">
<a href="#section-6" class="section-number selfRef">6. </a><a href="#name-specification-of-protocol-s" class="section-name selfRef">Specification of Protocol-Specific Methods</a>
      </h2>
<p id="section-6-1">DPLPMTUD requires protocol-specific details to be specified for each
      PL that is used.<a href="#section-6-1" class="pilcrow">¶</a></p>
<p id="section-6-2">The first subsection provides guidance on how to implement the
      DPLPMTUD method as a part of an application using UDP or UDP-Lite. The
      guidance also applies to other datagram services that do not include a
      specific transport protocol (such as a tunnel encapsulation). The
      following subsections describe how DPLPMTUD can be implemented as a part
      of the transport service, allowing applications using the service to
      benefit from discovery of the PLPMTU without themselves needing to
      implement this method when using SCTP and QUIC.<a href="#section-6-2" class="pilcrow">¶</a></p>
<section id="section-6.1">
        <h3 id="name-application-support-for-dpl">
<a href="#section-6.1" class="section-number selfRef">6.1. </a><a href="#name-application-support-for-dpl" class="section-name selfRef">Application Support for DPLPMTUD with UDP or UDP-Lite</a>
        </h3>
<p id="section-6.1-1">The current specifications of UDP <span>[<a href="#RFC0768" class="xref">RFC0768</a>]</span>
        and UDP-Lite <span>[<a href="#RFC3828" class="xref">RFC3828</a>]</span> do not define a method in
        the RFC series that supports PLPMTUD. In particular, the UDP transport
        does not provide the transport features needed to implement
        datagram PLPMTUD.<a href="#section-6.1-1" class="pilcrow">¶</a></p>
<p id="section-6.1-2">The DPLPMTUD method can be implemented as a part of an application
        built directly or indirectly on UDP or UDP-Lite but relies on
        higher-layer protocol features to implement the method <span>[<a href="#BCP145" class="xref">BCP145</a>]</span>.<a href="#section-6.1-2" class="pilcrow">¶</a></p>
<p id="section-6.1-3">Some primitives used by DPLPMTUD might not be available via the
        Datagram API (e.g., the ability to access the PLPMTU from the IP-layer
        cache or to interpret received PTB messages).<a href="#section-6.1-3" class="pilcrow">¶</a></p>
<p id="section-6.1-4"> In addition, it is recommended that PMTU discovery is not performed
 by multiple protocol layers. An application <span class="bcp14">SHOULD</span> avoid using
 DPLPMTUD when the underlying transport system provides this capability.
 A common method for managing the PLPMTU has benefits, both in the
 ability to share state between different processes and in opportunities to
 coordinate probing for different PL instances.<a href="#section-6.1-4" class="pilcrow">¶</a></p>
<div id="UDP-REQ">
<section id="section-6.1.1">
          <h4 id="name-application-request">
<a href="#section-6.1.1" class="section-number selfRef">6.1.1. </a><a href="#name-application-request" class="section-name selfRef">Application Request</a>
          </h4>
<p id="section-6.1.1-1">An application needs an application-layer protocol mechanism
          (such as a message acknowledgment method) that solicits a response
          from a destination endpoint. The method <span class="bcp14">SHOULD</span> allow the sender to
          check the value returned in the response to provide additional
          protection from off-path insertion of data <span>[<a href="#BCP145" class="xref">BCP145</a>]</span>. Suitable methods include a parameter known
          only to the two endpoints, such as a session ID or initialized
          sequence number.<a href="#section-6.1.1-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="UDP-Probe">
<section id="section-6.1.2">
          <h4 id="name-application-response">
<a href="#section-6.1.2" class="section-number selfRef">6.1.2. </a><a href="#name-application-response" class="section-name selfRef">Application Response</a>
          </h4>
<p id="section-6.1.2-1">An application needs an application-layer protocol mechanism to
          communicate the response from the destination endpoint. This
          response could indicate successful reception of the probe across the
          path but could also indicate that some (or all packets) have failed
          to reach the destination.<a href="#section-6.1.2-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="UDP-probing">
<section id="section-6.1.3">
          <h4 id="name-sending-application-probe-p">
<a href="#section-6.1.3" class="section-number selfRef">6.1.3. </a><a href="#name-sending-application-probe-p" class="section-name selfRef">Sending Application Probe Packets</a>
          </h4>
<p id="section-6.1.3-1">A probe packet can carry an application data block, but the
          successful transmission of this data is at risk when used for
          probing. Some applications might prefer to use a probe packet that
          does not carry an application data block to avoid disruption of data
          transfer.<a href="#section-6.1.3-1" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-6.1.4">
          <h4 id="name-initial-connectivity">
<a href="#section-6.1.4" class="section-number selfRef">6.1.4. </a><a href="#name-initial-connectivity" class="section-name selfRef">Initial Connectivity</a>
          </h4>
<p id="section-6.1.4-1">An application that does not have other higher-layer information
          confirming connectivity with the remote peer <span class="bcp14">SHOULD</span> implement a
          connectivity mechanism using acknowledged probe packets before
          entering the BASE state.<a href="#section-6.1.4-1" class="pilcrow">¶</a></p>
</section>
<section id="section-6.1.5">
          <h4 id="name-validating-the-path">
<a href="#section-6.1.5" class="section-number selfRef">6.1.5. </a><a href="#name-validating-the-path" class="section-name selfRef">Validating the Path</a>
          </h4>
<p id="section-6.1.5-1">An application that does not have other higher-layer information
          confirming correct delivery of datagrams <span class="bcp14">SHOULD</span> implement the
          CONFIRMATION_TIMER to periodically send probe packets while in the
          SEARCH_COMPLETE state.<a href="#section-6.1.5-1" class="pilcrow">¶</a></p>
</section>
<div id="udpopt_ptb_handling">
<section id="section-6.1.6">
          <h4 id="name-handling-of-ptb-messages">
<a href="#section-6.1.6" class="section-number selfRef">6.1.6. </a><a href="#name-handling-of-ptb-messages" class="section-name selfRef">Handling of PTB Messages</a>
          </h4>
<p id="section-6.1.6-1">An application that is able and wishes to receive PTB messages
          <span class="bcp14">MUST</span> perform ICMP validation as specified in 
          <span><a href="https://www.rfc-editor.org/rfc/rfc8085#section-5.2" class="relref">Section 5.2</a> of [<a href="#BCP145" class="xref">BCP145</a>]</span>. This requires that the application checks
          each received PTB message to validate that it was is received in
          response to transmitted traffic and that the reported PL_PTB_SIZE is
          less than the current probed size (see <a href="#validPTB_SIZE" class="xref">Section 4.6.2</a>). 
          A validated PTB message <span class="bcp14">MAY</span> be used
          as input to the DPLPMTUD algorithm but <span class="bcp14">MUST NOT</span> be used directly to
          set the PLPMTU.<a href="#section-6.1.6-1" class="pilcrow">¶</a></p>
</section>
</div>
</section>
<section id="section-6.2">
        <h3 id="name-dplpmtud-for-sctp">
<a href="#section-6.2" class="section-number selfRef">6.2. </a><a href="#name-dplpmtud-for-sctp" class="section-name selfRef">DPLPMTUD for SCTP</a>
        </h3>
<p id="section-6.2-1"><span><a href="https://www.rfc-editor.org/rfc/rfc4821#section-10.2" class="relref">Section 10.2</a> of [<a href="#RFC4821" class="xref">RFC4821</a>]</span> 
        specifies a recommended PLPMTUD probing method for SCTP, and  
        <span><a href="https://www.rfc-editor.org/rfc/rfc4960#section-7.3" class="relref">Section 7.3</a> of [<a href="#RFC4960" class="xref">RFC4960</a>]</span> 
        recommends an endpoint apply the
        techniques in RFC 4821 on a per-destination-address basis. The
        specification for DPLPMTUD continues the practice of using the PL to
        discover the PMTU but updates RFC4960 with a recommendation to use
        the method specified in this document: The <span class="bcp14">RECOMMENDED</span> method for
        generating probes is to add a chunk consisting only of padding to an
        SCTP message. The PAD chunk defined in <span>[<a href="#RFC4820" class="xref">RFC4820</a>]</span>
          <span class="bcp14">SHOULD</span> be attached to a minimum-length HEARTBEAT (HB) chunk to build a
        probe packet. This enables probing without affecting the transfer of
        user messages and without being limited by congestion control or flow
        control. This is preferred to using DATA chunks (with padding as
        required) as path probes.<a href="#section-6.2-1" class="pilcrow">¶</a></p>
<p id="section-6.2-2"><span><a href="https://www.rfc-editor.org/rfc/rfc4960#section-6.9" class="relref">Section 6.9</a> of [<a href="#RFC4960" class="xref">RFC4960</a>]</span> 
        describes dividing the
    user messages into DATA chunks sent by the PL when using SCTP. This notes
    that once an SCTP message has been sent, it cannot be resegmented. 
    <span>[<a href="#RFC4960" class="xref">RFC4960</a>]</span> describes the method for retransmitting DATA
   chunks when the MPS has been reduced, and <span><a href="https://www.rfc-editor.org/rfc/rfc4960#section-6.9" class="relref">Section 6.9</a> of [<a href="#RFC4960" class="xref">RFC4960</a>]</span>
   describes use of IP fragmentation for this case. 
    This is unchanged by this document.<a href="#section-6.2-2" class="pilcrow">¶</a></p>
<div id="sctp_over_ip">
<section id="section-6.2.1">
          <h4 id="name-sctp-ipv4-and-sctp-ipv6">
<a href="#section-6.2.1" class="section-number selfRef">6.2.1. </a><a href="#name-sctp-ipv4-and-sctp-ipv6" class="section-name selfRef">SCTP/IPv4 and SCTP/IPv6</a>
          </h4>
<section id="section-6.2.1.1">
            <h5 id="name-initial-connectivity-2">
<a href="#section-6.2.1.1" class="section-number selfRef">6.2.1.1. </a><a href="#name-initial-connectivity-2" class="section-name selfRef">Initial Connectivity</a>
            </h5>
<p id="section-6.2.1.1-1">The base protocol is specified in <span>[<a href="#RFC4960" class="xref">RFC4960</a>]</span>. This provides an acknowledged PL. A
            sender can therefore enter the BASE state as soon as connectivity
            has been confirmed.<a href="#section-6.2.1.1-1" class="pilcrow">¶</a></p>
</section>
<div id="sctp_over_ip_probing">
<section id="section-6.2.1.2">
            <h5 id="name-sending-sctp-probe-packets">
<a href="#section-6.2.1.2" class="section-number selfRef">6.2.1.2. </a><a href="#name-sending-sctp-probe-packets" class="section-name selfRef">Sending SCTP Probe Packets</a>
            </h5>
<p id="section-6.2.1.2-1">Probe packets consist of an SCTP common header followed by a
            HEARTBEAT chunk and a PAD chunk. The PAD chunk is used to control
            the length of the probe packet. The HEARTBEAT chunk is used to
            trigger the sending of a HEARTBEAT ACK chunk. The reception of the
            HEARTBEAT ACK chunk acknowledges reception of a successful
            probe. A successful probe updates the association and path
            counters, but an unsuccessful probe is discounted (assumed
            to be a result of choosing too large a PLPMTU).<a href="#section-6.2.1.2-1" class="pilcrow">¶</a></p>
<p id="section-6.2.1.2-2">The SCTP sender needs to be able to determine the total size of
     a probe packet. The HEARTBEAT chunk could carry a Heartbeat
     Information parameter that includes, besides the information
     suggested in <span>[<a href="#RFC4960" class="xref">RFC4960</a>]</span>, the probe size to help
     an implementation associate a HEARTBEAT ACK with the size of probe
     that was sent. The sender could also use other methods, such as
     sending a nonce and verifying the information returned also
     contains the corresponding nonce. The length of the PAD chunk is
     computed by reducing the probing size by the size of the SCTP
     common header and the HEARTBEAT chunk. The payload of the PAD
     chunk contains arbitrary data. When transmitted at the IP layer,
     the PMTU size also includes the IPv4 or IPv6 header(s).<a href="#section-6.2.1.2-2" class="pilcrow">¶</a></p>
<p id="section-6.2.1.2-3"> Probing can start directly after the PL handshake; this can be
     done before data is sent. Assuming this behavior (i.e., the PMTU
     is smaller than or equal to the interface MTU), this process will
     take several round-trip time periods, dependent on the number of
     DPLPMTUD probes sent. The Heartbeat timer can be used to implement
     the PROBE_TIMER.<a href="#section-6.2.1.2-3" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-6.2.1.3">
            <h5 id="name-validating-the-path-with-sc">
<a href="#section-6.2.1.3" class="section-number selfRef">6.2.1.3. </a><a href="#name-validating-the-path-with-sc" class="section-name selfRef">Validating the Path with SCTP</a>
            </h5>
<p id="section-6.2.1.3-1">Since SCTP provides an acknowledged PL, a sender <span class="bcp14">MUST NOT</span>
            implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE
            state.<a href="#section-6.2.1.3-1" class="pilcrow">¶</a></p>
</section>
<div id="sctp_over_ip_ptb_handling">
<section id="section-6.2.1.4">
            <h5 id="name-ptb-message-handling-by-sct">
<a href="#section-6.2.1.4" class="section-number selfRef">6.2.1.4. </a><a href="#name-ptb-message-handling-by-sct" class="section-name selfRef">PTB Message Handling by SCTP</a>
            </h5>
<p id="section-6.2.1.4-1">Normal ICMP validation <span class="bcp14">MUST</span> be performed as specified in
            <span><a href="https://www.rfc-editor.org/rfc/rfc4960#appendix-C" class="relref">Appendix C</a> of [<a href="#RFC4960" class="xref">RFC4960</a>]</span>. 
            This requires that the first 8 bytes of the SCTP common header are quoted in the
            payload of the PTB message, which can be the case for ICMPv4 and
            is normally the case for ICMPv6.<a href="#section-6.2.1.4-1" class="pilcrow">¶</a></p>
<p id="section-6.2.1.4-2">When a PTB message has been validated, the PL_PTB_SIZE
            calculated from the PTB_SIZE reported in the PTB message <span class="bcp14">SHOULD</span> be
            used with the DPLPMTUD algorithm, provided that the reported
            PL_PTB_SIZE is less than the current probe size (see <a href="#mechanism-ptb" class="xref">Section 4.6</a>).<a href="#section-6.2.1.4-2" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<section id="section-6.2.2">
          <h4 id="name-dplpmtud-for-sctp-udp">
<a href="#section-6.2.2" class="section-number selfRef">6.2.2. </a><a href="#name-dplpmtud-for-sctp-udp" class="section-name selfRef">DPLPMTUD for SCTP/UDP</a>
          </h4>
<p id="section-6.2.2-1">The UDP encapsulation of SCTP is specified in <span>[<a href="#RFC6951" class="xref">RFC6951</a>]</span>.<a href="#section-6.2.2-1" class="pilcrow">¶</a></p>
<p id="section-6.2.2-2">This specification updates the reference to RFC 4821 in Section
          <a href="https://www.rfc-editor.org/rfc/rfc6951#section-5.6" class="relref">5.6</a> 
          of RFC 6951 to refer to this document (RFC 8899). RFC 6951 is updated
          by the addition of the following sentence at the end of
          Section <a href="https://www.rfc-editor.org/rfc/rfc6951#section-5.6" class="relref">5.6</a>:<a href="#section-6.2.2-2" class="pilcrow">¶</a></p>
<blockquote id="section-6.2.2-3">
            <p id="section-6.2.2-3.1">The <span class="bcp14">RECOMMENDED</span> method for determining the MTU of the
             path is specified in RFC 8899.<a href="#section-6.2.2-3.1" class="pilcrow">¶</a></p>
</blockquote>
<section id="section-6.2.2.1">
            <h5 id="name-initial-connectivity-3">
<a href="#section-6.2.2.1" class="section-number selfRef">6.2.2.1. </a><a href="#name-initial-connectivity-3" class="section-name selfRef">Initial Connectivity</a>
            </h5>
<p id="section-6.2.2.1-1">A sender can enter the BASE state as soon as SCTP connectivity
            has been confirmed.<a href="#section-6.2.2.1-1" class="pilcrow">¶</a></p>
</section>
<div id="sctp_over_udp_probing">
<section id="section-6.2.2.2">
            <h5 id="name-sending-sctp-udp-probe-pack">
<a href="#section-6.2.2.2" class="section-number selfRef">6.2.2.2. </a><a href="#name-sending-sctp-udp-probe-pack" class="section-name selfRef">Sending SCTP/UDP Probe Packets</a>
            </h5>
<p id="section-6.2.2.2-1">Packet probing can be performed as specified in <a href="#sctp_over_ip_probing" class="xref">Section 6.2.1.2</a>. The size of the probe
     packet includes the 8 bytes of UDP header. This has to be
     considered when filling the probe packet with the PAD chunk.<a href="#section-6.2.2.2-1" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-6.2.2.3">
            <h5 id="name-validating-the-path-with-sct">
<a href="#section-6.2.2.3" class="section-number selfRef">6.2.2.3. </a><a href="#name-validating-the-path-with-sct" class="section-name selfRef">Validating the Path with SCTP/UDP</a>
            </h5>
<p id="section-6.2.2.3-1"> SCTP provides an acknowledged PL; therefore, a sender does not
     implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE
     state.<a href="#section-6.2.2.3-1" class="pilcrow">¶</a></p>
</section>
<div id="sctp_over_udp_ptb_handling">
<section id="section-6.2.2.4">
            <h5 id="name-handling-of-ptb-messages-by">
<a href="#section-6.2.2.4" class="section-number selfRef">6.2.2.4. </a><a href="#name-handling-of-ptb-messages-by" class="section-name selfRef">Handling of PTB Messages by SCTP/UDP</a>
            </h5>
<p id="section-6.2.2.4-1">ICMP validation <span class="bcp14">MUST</span> be performed for PTB messages as specified in 
            <span><a href="https://www.rfc-editor.org/rfc/rfc4960#appendix-C" class="relref">Appendix C</a> of [<a href="#RFC4960" class="xref">RFC4960</a>]</span>. 
            This requires that
            the first 8 bytes of the SCTP common header are contained in the
            PTB message, which can be the case for ICMPv4 (but note the UDP
            header also consumes a part of the quoted packet header) and is
            normally the case for ICMPv6. When the validation is completed, the
            PL_PTB_SIZE calculated from the PTB_SIZE in the PTB message <span class="bcp14">SHOULD</span>
            be used with the DPLPMTUD providing that the reported PL_PTB_SIZE
            is less than the current probe size.<a href="#section-6.2.2.4-1" class="pilcrow">¶</a></p>
</section>
</div>
</section>
<section id="section-6.2.3">
          <h4 id="name-dplpmtud-for-sctp-dtls">
<a href="#section-6.2.3" class="section-number selfRef">6.2.3. </a><a href="#name-dplpmtud-for-sctp-dtls" class="section-name selfRef">DPLPMTUD for SCTP/DTLS</a>
          </h4>
<p id="section-6.2.3-1">The Datagram Transport Layer Security (DTLS) encapsulation of SCTP
          is specified in <span>[<a href="#RFC8261" class="xref">RFC8261</a>]</span>. This is used for
          data channels in WebRTC implementations. This specification updates
          the reference to RFC 4821 in Section 
          <a href="https://www.rfc-editor.org/rfc/rfc8261#section-5" class="relref">5</a>
          of RFC 8261 to refer to this document (RFC 8899).<a href="#section-6.2.3-1" class="pilcrow">¶</a></p>
<section id="section-6.2.3.1">
            <h5 id="name-initial-connectivity-4">
<a href="#section-6.2.3.1" class="section-number selfRef">6.2.3.1. </a><a href="#name-initial-connectivity-4" class="section-name selfRef">Initial Connectivity</a>
            </h5>
<p id="section-6.2.3.1-1">A sender can enter the BASE state as soon as SCTP connectivity
            has been confirmed.<a href="#section-6.2.3.1-1" class="pilcrow">¶</a></p>
</section>
<div id="sctp_over_dtls_probing">
<section id="section-6.2.3.2">
            <h5 id="name-sending-sctp-dtls-probe-pac">
<a href="#section-6.2.3.2" class="section-number selfRef">6.2.3.2. </a><a href="#name-sending-sctp-dtls-probe-pac" class="section-name selfRef">Sending SCTP/DTLS Probe Packets</a>
            </h5>
<p id="section-6.2.3.2-1">Packet probing can be done as specified in <a href="#sctp_over_ip_probing" class="xref">Section 6.2.1.2</a>. The maximum payload is
     reduced by the size of the DTLS headers, which has to be considered
     when filling the PAD chunk. The size of the probe packet includes
     the DTLS PL headers. This has to be considered when filling the
     probe packet with the PAD chunk.<a href="#section-6.2.3.2-1" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-6.2.3.3">
            <h5 id="name-validating-the-path-with-sctp">
<a href="#section-6.2.3.3" class="section-number selfRef">6.2.3.3. </a><a href="#name-validating-the-path-with-sctp" class="section-name selfRef">Validating the Path with SCTP/DTLS</a>
            </h5>
<p id="section-6.2.3.3-1">Since SCTP provides an acknowledged PL, a sender <span class="bcp14">MUST NOT</span>
            implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE
            state.<a href="#section-6.2.3.3-1" class="pilcrow">¶</a></p>
</section>
<div id="sctp_over_dtls_ptb_handling">
<section id="section-6.2.3.4">
            <h5 id="name-handling-of-ptb-messages-by-">
<a href="#section-6.2.3.4" class="section-number selfRef">6.2.3.4. </a><a href="#name-handling-of-ptb-messages-by-" class="section-name selfRef">Handling of PTB Messages by SCTP/DTLS</a>
            </h5>
<p id="section-6.2.3.4-1"><span>[<a href="#RFC4960" class="xref">RFC4960</a>]</span> does not specify a way to
     validate SCTP/DTLS ICMP message payload and neither does this
     document. This can prevent processing of PTB messages at the
     PL.<a href="#section-6.2.3.4-1" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</section>
<section id="section-6.3">
        <h3 id="name-dplpmtud-for-quic">
<a href="#section-6.3" class="section-number selfRef">6.3. </a><a href="#name-dplpmtud-for-quic" class="section-name selfRef">DPLPMTUD for QUIC</a>
        </h3>
<p id="section-6.3-1">QUIC <span>[<a href="#I-D.ietf-quic-transport" class="xref">QUIC</a>]</span> is a UDP-based
 PL that provides reception feedback. The UDP payload includes a QUIC
 packet header, a protected payload, and any authentication fields. It
 supports padding and packet coalescence that can be used to construct
 probe packets.  From the perspective of DPLPMTUD, QUIC can function as
 an acknowledged PL. <span>[<a href="#I-D.ietf-quic-transport" class="xref">QUIC</a>]</span>
 describes the method for using DPLPMTUD with QUIC packets.<a href="#section-6.3-1" class="pilcrow">¶</a></p>
</section>
</section>
</div>
<div id="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>
<div id="Security">
<section id="section-8">
      <h2 id="name-security-considerations">
<a href="#section-8" class="section-number selfRef">8. </a><a href="#name-security-considerations" class="section-name selfRef">Security Considerations</a>
      </h2>
<p id="section-8-1">The security considerations for the use of UDP and SCTP are provided
      in the referenced RFCs.<a href="#section-8-1" class="pilcrow">¶</a></p>
<p id="section-8-2"> To avoid excessive load, the interval between individual probe packets
      <span class="bcp14">MUST</span> be at least one RTT, and the interval between rounds of probing is
      determined by the PMTU_RAISE_TIMER.<a href="#section-8-2" class="pilcrow">¶</a></p>
<p id="section-8-3">A PL sender needs to ensure that the method used to confirm reception
      of probe packets protects from off-path attackers injecting
      packets into the path. This protection is provided in IETF-defined
      protocols (e.g., TCP, SCTP) using a randomly initialized sequence
      number. A description of one way to do this when using UDP is provided
      in Section  
      <a href="https://www.rfc-editor.org/rfc/rfc8085#section-5.1" class="relref">5.1</a>
      of <span>[<a href="#BCP145" class="xref">BCP145</a>]</span>).<a href="#section-8-3" class="pilcrow">¶</a></p>
<p id="section-8-4">There are cases where ICMP Packet Too Big (PTB) messages are not
      delivered due to policy, configuration, or equipment design 
      (see <a href="#Classic-PMTUD" class="xref">Section 1.1</a>). This method therefore does not rely
      upon PTB messages being received but is able to utilize these when they
      are received by the sender. PTB messages could potentially be used to
      cause a node to inappropriately reduce the PLPMTU. A node supporting
      DPLPMTUD <span class="bcp14">MUST</span> therefore appropriately validate the payload of PTB
      messages to ensure these are received in response to transmitted traffic
      (i.e., a reported error condition that corresponds to a datagram
      actually sent by the path layer, see <a href="#PTB" class="xref">Section 4.6.1</a>).<a href="#section-8-4" class="pilcrow">¶</a></p>
<p id="section-8-5">An on-path attacker able to create a PTB message could forge PTB
      messages that include a valid quoted IP packet. Such an attack could be
      used to drive down the PLPMTU. An on-path device could similarly force a
      reduction of the PLPMTU by implementing a policy that drops packets
      larger than a configured size. There are two ways this method can be
      mitigated against such attacks: first, by ensuring that a PL sender never
      reduces the PLPMTU below the base size solely in response to receiving a
      PTB message. This is achieved by first entering the BASE state when such
      a message is received. Second, the design does not require processing of
      PTB messages; a PL sender could therefore suspend processing of PTB
      messages (e.g., in a robustness mode after detecting that subsequent
      probes actually confirm that a size larger than the PTB_SIZE is supported
      by a path).<a href="#section-8-5" class="pilcrow">¶</a></p>
<p id="section-8-6">Parsing the quoted packet inside a PTB message can
      introduce additional per-packet processing at the PL sender.
      This processing <span class="bcp14">SHOULD</span> be limited to avoid a 
      denial-of-service attack when arbitrary headers are included. Rate-limiting
      the processing could result in PTB messages not being
      received by a PL; however, the DPLPMTUD method is robust to such
      loss.<a href="#section-8-6" class="pilcrow">¶</a></p>
<p id="section-8-7">The successful processing of an ICMP message can trigger a probe
      when the reported PTB size is valid, but this does not
      directly update the PLPMTU for the path. This prevents a message
      attempting to black hole data by indicating a size larger than
      supported by the path.<a href="#section-8-7" class="pilcrow">¶</a></p>
<p id="section-8-8">It is possible that the information about a path is not stable. This
      could be a result of forwarding across more than one path that has a
      different actual PMTU or a single path presents a varying PMTU. The
      design of a PLPMTUD implementation <span class="bcp14">SHOULD</span> consider how to mitigate the
      effects of varying path information. One possible mitigation is to
      provide robustness (see <a href="#robustness" class="xref">Section 5.4</a>) in the method
      that avoids oscillation in the MPS.<a href="#section-8-8" class="pilcrow">¶</a></p>
<p id="section-8-9">
      DPLPMTUD methods can introduce padding data to inflate the length
      of the datagram to the total size required for a probe packet. The
      total size of a probe packet includes all headers and padding added
      to the payload data being sent (e.g., including security-related
      fields such as an AEAD tag and TLS record layer padding). The value
      of the padding data does not influence the DPLPMTUD search algorithm,
      and therefore needs to be set consistent with the policy of the PL.<a href="#section-8-9" class="pilcrow">¶</a></p>
<p id="section-8-10"> If a PL can make use of cryptographic confidentiality or
      data-integrity mechanisms, then the design ought to avoid adding anything
      (e.g., padding) to DPLPMTUD probe packets that is not also protected by
      those cryptographic mechanisms.<a href="#section-8-10" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-9">
      <h2 id="name-references">
<a href="#section-9" class="section-number selfRef">9. </a><a href="#name-references" class="section-name selfRef">References</a>
      </h2>
<section id="section-9.1">
        <h3 id="name-normative-references">
<a href="#section-9.1" class="section-number selfRef">9.1. </a><a href="#name-normative-references" class="section-name selfRef">Normative References</a>
        </h3>
<dl class="references">
<dt id="BCP145">[BCP145]</dt>
        <dd>
<span class="refAuthor">Eggert, L.</span><span class="refAuthor">, Fairhurst, G.</span><span class="refAuthor">, and G. Shepherd</span>, <span class="refTitle">"UDP Usage Guidelines"</span>, <span class="seriesInfo">BCP 145</span>, <span class="seriesInfo">RFC 8085</span>, <time datetime="2017-03" class="refDate">March 2017</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/bcp145">https://www.rfc-editor.org/info/bcp145</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC0768">[RFC0768]</dt>
        <dd>
<span class="refAuthor">Postel, J.</span>, <span class="refTitle">"User Datagram Protocol"</span>, <span class="seriesInfo">STD 6</span>, <span class="seriesInfo">RFC 768</span>, <span class="seriesInfo">DOI 10.17487/RFC0768</span>, <time datetime="1980-08" class="refDate">August 1980</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc768">https://www.rfc-editor.org/info/rfc768</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC0791">[RFC0791]</dt>
        <dd>
<span class="refAuthor">Postel, J.</span>, <span class="refTitle">"Internet Protocol"</span>, <span class="seriesInfo">STD 5</span>, <span class="seriesInfo">RFC 791</span>, <span class="seriesInfo">DOI 10.17487/RFC0791</span>, <time datetime="1981-09" class="refDate">September 1981</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc791">https://www.rfc-editor.org/info/rfc791</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC1191">[RFC1191]</dt>
        <dd>
<span class="refAuthor">Mogul, J.</span><span class="refAuthor"> and S. Deering</span>, <span class="refTitle">"Path MTU discovery"</span>, <span class="seriesInfo">RFC 1191</span>, <span class="seriesInfo">DOI 10.17487/RFC1191</span>, <time datetime="1990-11" class="refDate">November 1990</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc1191">https://www.rfc-editor.org/info/rfc1191</a>&gt;</span>. </dd>
<dd class="break"></dd>
<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="RFC3828">[RFC3828]</dt>
        <dd>
<span class="refAuthor">Larzon, L-A.</span><span class="refAuthor">, Degermark, M.</span><span class="refAuthor">, Pink, S.</span><span class="refAuthor">, Jonsson, L-E., Ed.</span><span class="refAuthor">, and G. Fairhurst, Ed.</span>, <span class="refTitle">"The Lightweight User Datagram Protocol (UDP-Lite)"</span>, <span class="seriesInfo">RFC 3828</span>, <span class="seriesInfo">DOI 10.17487/RFC3828</span>, <time datetime="2004-07" class="refDate">July 2004</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc3828">https://www.rfc-editor.org/info/rfc3828</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC4820">[RFC4820]</dt>
        <dd>
<span class="refAuthor">Tuexen, M.</span><span class="refAuthor">, Stewart, R.</span><span class="refAuthor">, and P. Lei</span>, <span class="refTitle">"Padding Chunk and Parameter for the Stream Control Transmission Protocol (SCTP)"</span>, <span class="seriesInfo">RFC 4820</span>, <span class="seriesInfo">DOI 10.17487/RFC4820</span>, <time datetime="2007-03" class="refDate">March 2007</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4820">https://www.rfc-editor.org/info/rfc4820</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC4960">[RFC4960]</dt>
        <dd>
<span class="refAuthor">Stewart, R., Ed.</span>, <span class="refTitle">"Stream Control Transmission Protocol"</span>, <span class="seriesInfo">RFC 4960</span>, <span class="seriesInfo">DOI 10.17487/RFC4960</span>, <time datetime="2007-09" class="refDate">September 2007</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4960">https://www.rfc-editor.org/info/rfc4960</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC6951">[RFC6951]</dt>
        <dd>
<span class="refAuthor">Tuexen, M.</span><span class="refAuthor"> and R. Stewart</span>, <span class="refTitle">"UDP Encapsulation of Stream Control Transmission Protocol (SCTP) Packets for End-Host to End-Host Communication"</span>, <span class="seriesInfo">RFC 6951</span>, <span class="seriesInfo">DOI 10.17487/RFC6951</span>, <time datetime="2013-05" class="refDate">May 2013</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6951">https://www.rfc-editor.org/info/rfc6951</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="RFC8200">[RFC8200]</dt>
        <dd>
<span class="refAuthor">Deering, S.</span><span class="refAuthor"> and R. Hinden</span>, <span class="refTitle">"Internet Protocol, Version 6 (IPv6) Specification"</span>, <span class="seriesInfo">STD 86</span>, <span class="seriesInfo">RFC 8200</span>, <span class="seriesInfo">DOI 10.17487/RFC8200</span>, <time datetime="2017-07" class="refDate">July 2017</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8200">https://www.rfc-editor.org/info/rfc8200</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8201">[RFC8201]</dt>
        <dd>
<span class="refAuthor">McCann, J.</span><span class="refAuthor">, Deering, S.</span><span class="refAuthor">, Mogul, J.</span><span class="refAuthor">, and R. Hinden, Ed.</span>, <span class="refTitle">"Path MTU Discovery for IP version 6"</span>, <span class="seriesInfo">STD 87</span>, <span class="seriesInfo">RFC 8201</span>, <span class="seriesInfo">DOI 10.17487/RFC8201</span>, <time datetime="2017-07" class="refDate">July 2017</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8201">https://www.rfc-editor.org/info/rfc8201</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8261">[RFC8261]</dt>
      <dd>
<span class="refAuthor">Tuexen, M.</span><span class="refAuthor">, Stewart, R.</span><span class="refAuthor">, Jesup, R.</span><span class="refAuthor">, and S. Loreto</span>, <span class="refTitle">"Datagram Transport Layer Security (DTLS) Encapsulation of SCTP Packets"</span>, <span class="seriesInfo">RFC 8261</span>, <span class="seriesInfo">DOI 10.17487/RFC8261</span>, <time datetime="2017-11" class="refDate">November 2017</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8261">https://www.rfc-editor.org/info/rfc8261</a>&gt;</span>. </dd>
<dd class="break"></dd>
</dl>
</section>
<section id="section-9.2">
        <h3 id="name-informative-references">
<a href="#section-9.2" class="section-number selfRef">9.2. </a><a href="#name-informative-references" class="section-name selfRef">Informative References</a>
        </h3>
<dl class="references">
<dt id="I-D.ietf-quic-transport">[QUIC]</dt>
        <dd>
<span class="refAuthor">Iyengar, J., Ed.</span><span class="refAuthor"> and M. Thomson, Ed.</span>, <span class="refTitle">"QUIC: A UDP-Based Multiplexed and Secure Transport"</span>, <span class="refContent">Work in Progress</span>, <span class="seriesInfo">Internet-Draft, draft-ietf-quic-transport-29</span>, <time datetime="2020-06-10" class="refDate">10 June 2020</time>, <span>&lt;<a href="https://tools.ietf.org/html/draft-ietf-quic-transport-29">https://tools.ietf.org/html/draft-ietf-quic-transport-29</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC0792">[RFC0792]</dt>
        <dd>
<span class="refAuthor">Postel, J.</span>, <span class="refTitle">"Internet Control Message Protocol"</span>, <span class="seriesInfo">STD 5</span>, <span class="seriesInfo">RFC 792</span>, <span class="seriesInfo">DOI 10.17487/RFC0792</span>, <time datetime="1981-09" class="refDate">September 1981</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc792">https://www.rfc-editor.org/info/rfc792</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC1122">[RFC1122]</dt>
        <dd>
<span class="refAuthor">Braden, R., Ed.</span>, <span class="refTitle">"Requirements for Internet Hosts - Communication Layers"</span>, <span class="seriesInfo">STD 3</span>, <span class="seriesInfo">RFC 1122</span>, <span class="seriesInfo">DOI 10.17487/RFC1122</span>, <time datetime="1989-10" class="refDate">October 1989</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc1122">https://www.rfc-editor.org/info/rfc1122</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC1812">[RFC1812]</dt>
        <dd>
<span class="refAuthor">Baker, F., Ed.</span>, <span class="refTitle">"Requirements for IP Version 4 Routers"</span>, <span class="seriesInfo">RFC 1812</span>, <span class="seriesInfo">DOI 10.17487/RFC1812</span>, <time datetime="1995-06" class="refDate">June 1995</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc1812">https://www.rfc-editor.org/info/rfc1812</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC2923">[RFC2923]</dt>
        <dd>
<span class="refAuthor">Lahey, K.</span>, <span class="refTitle">"TCP Problems with Path MTU Discovery"</span>, <span class="seriesInfo">RFC 2923</span>, <span class="seriesInfo">DOI 10.17487/RFC2923</span>, <time datetime="2000-09" class="refDate">September 2000</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc2923">https://www.rfc-editor.org/info/rfc2923</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC4340">[RFC4340]</dt>
        <dd>
<span class="refAuthor">Kohler, E.</span><span class="refAuthor">, Handley, M.</span><span class="refAuthor">, and S. Floyd</span>, <span class="refTitle">"Datagram Congestion Control Protocol (DCCP)"</span>, <span class="seriesInfo">RFC 4340</span>, <span class="seriesInfo">DOI 10.17487/RFC4340</span>, <time datetime="2006-03" class="refDate">March 2006</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4340">https://www.rfc-editor.org/info/rfc4340</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC4443">[RFC4443]</dt>
        <dd>
<span class="refAuthor">Conta, A.</span><span class="refAuthor">, Deering, S.</span><span class="refAuthor">, and M. Gupta, Ed.</span>, <span class="refTitle">"Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification"</span>, <span class="seriesInfo">STD 89</span>, <span class="seriesInfo">RFC 4443</span>, <span class="seriesInfo">DOI 10.17487/RFC4443</span>, <time datetime="2006-03" class="refDate">March 2006</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4443">https://www.rfc-editor.org/info/rfc4443</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC4821">[RFC4821]</dt>
        <dd>
<span class="refAuthor">Mathis, M.</span><span class="refAuthor"> and J. Heffner</span>, <span class="refTitle">"Packetization Layer Path MTU Discovery"</span>, <span class="seriesInfo">RFC 4821</span>, <span class="seriesInfo">DOI 10.17487/RFC4821</span>, <time datetime="2007-03" class="refDate">March 2007</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4821">https://www.rfc-editor.org/info/rfc4821</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC4890">[RFC4890]</dt>
        <dd>
<span class="refAuthor">Davies, E.</span><span class="refAuthor"> and J. Mohacsi</span>, <span class="refTitle">"Recommendations for Filtering ICMPv6 Messages in Firewalls"</span>, <span class="seriesInfo">RFC 4890</span>, <span class="seriesInfo">DOI 10.17487/RFC4890</span>, <time datetime="2007-05" class="refDate">May 2007</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4890">https://www.rfc-editor.org/info/rfc4890</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC5508">[RFC5508]</dt>
        <dd>
<span class="refAuthor">Srisuresh, P.</span><span class="refAuthor">, Ford, B.</span><span class="refAuthor">, Sivakumar, S.</span><span class="refAuthor">, and S. Guha</span>, <span class="refTitle">"NAT Behavioral Requirements for ICMP"</span>, <span class="seriesInfo">BCP 148</span>, <span class="seriesInfo">RFC 5508</span>, <span class="seriesInfo">DOI 10.17487/RFC5508</span>, <time datetime="2009-04" class="refDate">April 2009</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc5508">https://www.rfc-editor.org/info/rfc5508</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8900">[RFC8900]</dt>
        <dd>
<span class="refAuthor">Bonica, R.</span><span class="refAuthor">, Baker, F.</span><span class="refAuthor">, Huston, G.</span><span class="refAuthor">, Hinden, R.</span><span class="refAuthor">, Troan, O.</span><span class="refAuthor">, and F. Gont</span>, <span class="refTitle">"IP Fragmentation Considered Fragile"</span>, <span class="seriesInfo">RFC 8900</span>, <span class="seriesInfo">BCP 230</span>, <time datetime="2020-09" class="refDate">September 2020</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8900">https://www.rfc-editor.org/info/rfc8900</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="I-D.ietf-intarea-tunnels">[TUNNELS]</dt>
      <dd>
<span class="refAuthor">Touch, J.</span><span class="refAuthor"> and M. Townsley</span>, <span class="refTitle">"IP Tunnels in the Internet Architecture"</span>, <span class="refContent">Work in Progress</span>, <span class="seriesInfo">Internet-Draft, draft-ietf-intarea-tunnels-10</span>, <time datetime="2019-09-12" class="refDate">12 September 2019</time>, <span>&lt;<a href="https://tools.ietf.org/html/draft-ietf-intarea-tunnels-10">https://tools.ietf.org/html/draft-ietf-intarea-tunnels-10</a>&gt;</span>. </dd>
<dd class="break"></dd>
</dl>
</section>
</section>
<div id="Acknowledgments">
<section id="section-appendix.a">
      <h2 id="name-acknowledgments">
<a href="#name-acknowledgments" class="section-name selfRef">Acknowledgments</a>
      </h2>
<p id="section-appendix.a-1">This work was partially funded by the European Union Horizon 2020
      Research and Innovation Programme under grant agreement No. 644334,
"A New, Evolutive API and Transport-Layer Architecture for the Internet"
      (NEAT). The views expressed are solely those of the author(s).<a href="#section-appendix.a-1" class="pilcrow">¶</a></p>
<p id="section-appendix.a-2">
      Thanks to all who have commented or contributed, the TSVWG and QUIC
      working groups, and <span class="contact-name">Mathew Calder</span> and 
      <span class="contact-name">Julius Flohr</span> for providing early
      implementations.<a href="#section-appendix.a-2" 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">Godred Fairhurst</span></div>
<div dir="auto" class="left"><span class="org">University of Aberdeen</span></div>
<div dir="auto" class="left"><span class="extended-address">School of Engineering</span></div>
<div dir="auto" class="left"><span class="street-address">Fraser Noble Building</span></div>
<div dir="auto" class="left"><span class="locality">Aberdeen</span></div>
<div dir="auto" class="left"><span class="postal-code">AB24 3UE</span></div>
<div dir="auto" class="left"><span class="country-name">United Kingdom</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:gorry@erg.abdn.ac.uk" class="email">gorry@erg.abdn.ac.uk</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Tom Jones</span></div>
<div dir="auto" class="left"><span class="org">University of Aberdeen</span></div>
<div dir="auto" class="left"><span class="extended-address">School of Engineering</span></div>
<div dir="auto" class="left"><span class="street-address">Fraser Noble Building</span></div>
<div dir="auto" class="left"><span class="locality">Aberdeen</span></div>
<div dir="auto" class="left"><span class="postal-code">AB24 3UE</span></div>
<div dir="auto" class="left"><span class="country-name">United Kingdom</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:tom@erg.abdn.ac.uk" class="email">tom@erg.abdn.ac.uk</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Michael Tüxen</span></div>
<div dir="auto" class="left"><span class="org">Münster University of Applied Sciences</span></div>
<div dir="auto" class="left"><span class="street-address">Stegerwaldstrasse 39</span></div>
<div dir="auto" class="left">
<span class="postal-code">48565</span> <span class="locality">Steinfurt</span>
</div>
<div dir="auto" class="left"><span class="country-name">Germany</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:tuexen@fh-muenster.de" class="email">tuexen@fh-muenster.de</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Irene Rüngeler</span></div>
<div dir="auto" class="left"><span class="org">Münster University of Applied Sciences</span></div>
<div dir="auto" class="left"><span class="street-address">Stegerwaldstrasse 39</span></div>
<div dir="auto" class="left">
<span class="postal-code">48565</span> <span class="locality">Steinfurt</span>
</div>
<div dir="auto" class="left"><span class="country-name">Germany</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:i.ruengeler@fh-muenster.de" class="email">i.ruengeler@fh-muenster.de</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Timo Völker</span></div>
<div dir="auto" class="left"><span class="org">Münster University of Applied Sciences</span></div>
<div dir="auto" class="left"><span class="street-address">Stegerwaldstrasse 39</span></div>
<div dir="auto" class="left">
<span class="postal-code">48565</span> <span class="locality">Steinfurt</span>
</div>
<div dir="auto" class="left"><span class="country-name">Germany</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:timo.voelker@fh-muenster.de" class="email">timo.voelker@fh-muenster.de</a>
</div>
</address>
</section>
</div>
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