File: rfc8999.html

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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 8999: Version-Independent Properties of QUIC</title>
<meta content="Martin Thomson" name="author">
<meta content="
       This document defines the properties of the QUIC transport protocol that are
common to all versions of the protocol. 
    " name="description">
<meta content="xml2rfc 3.8.0" name="generator">
<meta content="crypto" name="keyword">
<meta content="next generation" name="keyword">
<meta content="protocol" name="keyword">
<meta content="secure" name="keyword">
<meta content="transport" name="keyword">
<meta content="UDP" name="keyword">
<meta content="invariants" name="keyword">
<meta content="8999" name="rfc.number">
<!-- Generator version information:
  xml2rfc 3.8.0
    Python 3.6.10
    appdirs 1.4.4
    ConfigArgParse 1.2.3
    google-i18n-address 2.3.5
    html5lib 1.0.1
    intervaltree 3.0.2
    Jinja2 2.11.2
    kitchen 1.2.6
    lxml 4.4.2
    pycairo 1.19.0
    pycountry 19.8.18
    pyflakes 2.1.1
    PyYAML 5.3.1
    requests 2.22.0
    setuptools 40.6.2
    six 1.14.0
    WeasyPrint 51
-->
<link href="rfc8999.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 {
  zoom: 1.0;
  width: extend-to-zoom;
}
@-ms-viewport {
  width: extend-to-zoom;
  zoom: 1.0;
}
/* general and mobile first */
html {
}
body {
  max-width: 90%;
  margin: 1.5em auto;
  color: #222;
  background-color: #fff;
  font-size: 14px;
  font-family: 'Noto Sans', Arial, Helvetica, sans-serif;
  line-height: 1.6;
  scroll-behavior: smooth;
}
.ears {
  display: none;
}

/* headings */
#title, h1, h2, h3, h4, h5, h6 {
  margin: 1em 0 0.5em;
  font-weight: bold;
  line-height: 1.3;
}
#title {
  clear: both;
  border-bottom: 1px solid #ddd;
  margin: 0 0 0.5em 0;
  padding: 1em 0 0.5em;
}
.author {
  padding-bottom: 4px;
}
h1 {
  font-size: 26px;
  margin: 1em 0;
}
h2 {
  font-size: 22px;
  margin-top: -20px;  /* provide offset for in-page anchors */
  padding-top: 33px;
}
h3 {
  font-size: 18px;
  margin-top: -36px;  /* provide offset for in-page anchors */
  padding-top: 42px;
}
h4 {
  font-size: 16px;
  margin-top: -36px;  /* provide offset for in-page anchors */
  padding-top: 42px;
}
h5, h6 {
  font-size: 14px;
}
#n-copyright-notice {
  border-bottom: 1px solid #ddd;
  padding-bottom: 1em;
  margin-bottom: 1em;
}
/* general structure */
p {
  padding: 0;
  margin: 0 0 1em 0;
  text-align: left;
}
div, span {
  position: relative;
}
div {
  margin: 0;
}
.alignRight.art-text {
  background-color: #f9f9f9;
  border: 1px solid #eee;
  border-radius: 3px;
  padding: 1em 1em 0;
  margin-bottom: 1.5em;
}
.alignRight.art-text pre {
  padding: 0;
}
.alignRight {
  margin: 1em 0;
}
.alignRight > *:first-child {
  border: none;
  margin: 0;
  float: right;
  clear: both;
}
.alignRight > *:nth-child(2) {
  clear: both;
  display: block;
  border: none;
}
svg {
  display: block;
}
.alignCenter.art-text {
  background-color: #f9f9f9;
  border: 1px solid #eee;
  border-radius: 3px;
  padding: 1em 1em 0;
  margin-bottom: 1.5em;
}
.alignCenter.art-text pre {
  padding: 0;
}
.alignCenter {
  margin: 1em 0;
}
.alignCenter > *:first-child {
  border: none;
  /* this isn't optimal, but it's an existence proof.  PrinceXML doesn't
     support flexbox yet.
  */
  display: table;
  margin: 0 auto;
}

/* lists */
ol, ul {
  padding: 0;
  margin: 0 0 1em 2em;
}
ol ol, ul ul, ol ul, ul ol {
  margin-left: 1em;
}
li {
  margin: 0 0 0.25em 0;
}
.ulCompact li {
  margin: 0;
}
ul.empty, .ulEmpty {
  list-style-type: none;
}
ul.empty li, .ulEmpty li {
  margin-top: 0.5em;
}
ul.compact, .ulCompact,
ol.compact, .olCompact {
  line-height: 100%;
  margin: 0 0 0 2em;
}

/* definition lists */
dl {
}
dl > dt {
  float: left;
  margin-right: 1em;
}
/* 
dl.nohang > dt {
  float: none;
}
*/
dl > dd {
  margin-bottom: .8em;
  min-height: 1.3em;
}
dl.compact > dd, .dlCompact > dd {
  margin-bottom: 0em;
}
dl > dd > dl {
  margin-top: 0.5em;
  margin-bottom: 0em;
}

/* links */
a {
  text-decoration: none;
}
a[href] {
  color: #22e; /* Arlen: WCAG 2019 */
}
a[href]:hover {
  background-color: #f2f2f2;
}
figcaption a[href],
a[href].selfRef {
  color: #222;
}
/* XXX probably not this:
a.selfRef:hover {
  background-color: transparent;
  cursor: default;
} */

/* Figures */
tt, code, pre, code {
  background-color: #f9f9f9;
  font-family: 'Roboto Mono', monospace;
}
pre {
  border: 1px solid #eee;
  margin: 0;
  padding: 1em;
}
img {
  max-width: 100%;
}
figure {
  margin: 0;
}
figure blockquote {
  margin: 0.8em 0.4em 0.4em;
}
figcaption {
  font-style: italic;
  margin: 0 0 1em 0;
}
@media screen {
  pre {
    overflow-x: auto;
    max-width: 100%;
    max-width: calc(100% - 22px);
  }
}

/* aside, blockquote */
aside, blockquote {
  margin-left: 0;
  padding: 1.2em 2em;
}
blockquote {
  background-color: #f9f9f9;
  color: #111; /* Arlen: WCAG 2019 */
  border: 1px solid #ddd;
  border-radius: 3px;
  margin: 1em 0;
}
cite {
  display: block;
  text-align: right;
  font-style: italic;
}

/* tables */
table {
  width: 100%;
  margin: 0 0 1em;
  border-collapse: collapse;
  border: 1px solid #eee;
}
th, td {
  text-align: left;
  vertical-align: top;
  padding: 0.5em 0.75em;
}
th {
  text-align: left;
  background-color: #e9e9e9;
}
tr:nth-child(2n+1) > td {
  background-color: #f5f5f5;
}
table caption {
  font-style: italic;
  margin: 0;
  padding: 0;
  text-align: left;
}
table p {
  /* XXX to avoid bottom margin on table row signifiers. If paragraphs should
     be allowed within tables more generally, it would be far better to select on a class. */
  margin: 0;
}

/* pilcrow */
a.pilcrow {
  color: #666; /* Arlen: AHDJ 2019 */
  text-decoration: none;
  visibility: hidden;
  user-select: none;
  -ms-user-select: none;
  -o-user-select:none;
  -moz-user-select: none;
  -khtml-user-select: none;
  -webkit-user-select: none;
  -webkit-touch-callout: none;
}
@media screen {
  aside:hover > a.pilcrow,
  p:hover > a.pilcrow,
  blockquote:hover > a.pilcrow,
  div:hover > a.pilcrow,
  li:hover > a.pilcrow,
  pre:hover > a.pilcrow {
    visibility: visible;
  }
  a.pilcrow:hover {
    background-color: transparent;
  }
}

/* misc */
hr {
  border: 0;
  border-top: 1px solid #eee;
}
.bcp14 {
  font-variant: small-caps;
}

.role {
  font-variant: all-small-caps;
}

/* info block */
#identifiers {
  margin: 0;
  font-size: 0.9em;
}
#identifiers dt {
  width: 3em;
  clear: left;
}
#identifiers dd {
  float: left;
  margin-bottom: 0;
}
#identifiers .authors .author {
  display: inline-block;
  margin-right: 1.5em;
}
#identifiers .authors .org {
  font-style: italic;
}

/* The prepared/rendered info at the very bottom of the page */
.docInfo {
  color: #666; /* Arlen: WCAG 2019 */
  font-size: 0.9em;
  font-style: italic;
  margin-top: 2em;
}
.docInfo .prepared {
  float: left;
}
.docInfo .prepared {
  float: right;
}

/* table of contents */
#toc  {
  padding: 0.75em 0 2em 0;
  margin-bottom: 1em;
}
nav.toc ul {
  margin: 0 0.5em 0 0;
  padding: 0;
  list-style: none;
}
nav.toc li {
  line-height: 1.3em;
  margin: 0.75em 0;
  padding-left: 1.2em;
  text-indent: -1.2em;
}
/* references */
.references dt {
  text-align: right;
  font-weight: bold;
  min-width: 7em;
}
.references dd {
  margin-left: 8em;
  overflow: auto;
}

.refInstance {
  margin-bottom: 1.25em;
}

.references .ascii {
  margin-bottom: 0.25em;
}

/* index */
.index ul {
  margin: 0 0 0 1em;
  padding: 0;
  list-style: none;
}
.index ul ul {
  margin: 0;
}
.index li {
  margin: 0;
  text-indent: -2em;
  padding-left: 2em;
  padding-bottom: 5px;
}
.indexIndex {
  margin: 0.5em 0 1em;
}
.index a {
  font-weight: 700;
}
/* make the index two-column on all but the smallest screens */
@media (min-width: 600px) {
  .index ul {
    -moz-column-count: 2;
    -moz-column-gap: 20px;
  }
  .index ul ul {
    -moz-column-count: 1;
    -moz-column-gap: 0;
  }
}

/* authors */
address.vcard {
  font-style: normal;
  margin: 1em 0;
}

address.vcard .nameRole {
  font-weight: 700;
  margin-left: 0;
}
address.vcard .label {
  font-family: "Noto Sans",Arial,Helvetica,sans-serif;
  margin: 0.5em 0;
}
address.vcard .type {
  display: none;
}
.alternative-contact {
  margin: 1.5em 0 1em;
}
hr.addr {
  border-top: 1px dashed;
  margin: 0;
  color: #ddd;
  max-width: calc(100% - 16px);
}

/* temporary notes */
.rfcEditorRemove::before {
  position: absolute;
  top: 0.2em;
  right: 0.2em;
  padding: 0.2em;
  content: "The RFC Editor will remove this note";
  color: #9e2a00; /* Arlen: WCAG 2019 */
  background-color: #ffd; /* Arlen: WCAG 2019 */
}
.rfcEditorRemove {
  position: relative;
  padding-top: 1.8em;
  background-color: #ffd; /* Arlen: WCAG 2019 */
  border-radius: 3px;
}
.cref {
  background-color: #ffd; /* Arlen: WCAG 2019 */
  padding: 2px 4px;
}
.crefSource {
  font-style: italic;
}
/* alternative layout for smaller screens */
@media screen and (max-width: 1023px) {
  body {
    padding-top: 2em;
  }
  #title {
    padding: 1em 0;
  }
  h1 {
    font-size: 24px;
  }
  h2 {
    font-size: 20px;
    margin-top: -18px;  /* provide offset for in-page anchors */
    padding-top: 38px;
  }
  #identifiers dd {
    max-width: 60%;
  }
  #toc {
    position: fixed;
    z-index: 2;
    top: 0;
    right: 0;
    padding: 0;
    margin: 0;
    background-color: inherit;
    border-bottom: 1px solid #ccc;
  }
  #toc h2 {
    margin: -1px 0 0 0;
    padding: 4px 0 4px 6px;
    padding-right: 1em;
    min-width: 190px;
    font-size: 1.1em;
    text-align: right;
    background-color: #444;
    color: white;
    cursor: pointer;
  }
  #toc h2::before { /* css hamburger */
    float: right;
    position: relative;
    width: 1em;
    height: 1px;
    left: -164px;
    margin: 6px 0 0 0;
    background: white none repeat scroll 0 0;
    box-shadow: 0 4px 0 0 white, 0 8px 0 0 white;
    content: "";
  }
  #toc nav {
    display: none;
    padding: 0.5em 1em 1em;
    overflow: auto;
    height: calc(100vh - 48px);
    border-left: 1px solid #ddd;
  }
}

/* alternative layout for wide screens */
@media screen and (min-width: 1024px) {
  body {
    max-width: 724px;
    margin: 42px auto;
    padding-left: 1.5em;
    padding-right: 29em;
  }
  #toc {
    position: fixed;
    top: 42px;
    right: 42px;
    width: 25%;
    margin: 0;
    padding: 0 1em;
    z-index: 1;
  }
  #toc h2 {
    border-top: none;
    border-bottom: 1px solid #ddd;
    font-size: 1em;
    font-weight: normal;
    margin: 0;
    padding: 0.25em 1em 1em 0;
  }
  #toc nav {
    display: block;
    height: calc(90vh - 84px);
    bottom: 0;
    padding: 0.5em 0 0;
    overflow: auto;
  }
  img { /* future proofing */
    max-width: 100%;
    height: auto;
  }
}

/* pagination */
@media print {
  body {

    width: 100%;
  }
  p {
    orphans: 3;
    widows: 3;
  }
  #n-copyright-notice {
    border-bottom: none;
  }
  #toc, #n-introduction {
    page-break-before: always;
  }
  #toc {
    border-top: none;
    padding-top: 0;
  }
  figure, pre {
    page-break-inside: avoid;
  }
  figure {
    overflow: scroll;
  }
  h1, h2, h3, h4, h5, h6 {
    page-break-after: avoid;
  }
  h2+*, h3+*, h4+*, h5+*, h6+* {
    page-break-before: avoid;
  }
  pre {
    white-space: pre-wrap;
    word-wrap: break-word;
    font-size: 10pt;
  }
  table {
    border: 1px solid #ddd;
  }
  td {
    border-top: 1px solid #ddd;
  }
}

/* This is commented out here, as the string-set: doesn't
   pass W3C validation currently */
/*
.ears thead .left {
  string-set: ears-top-left content();
}

.ears thead .center {
  string-set: ears-top-center content();
}

.ears thead .right {
  string-set: ears-top-right content();
}

.ears tfoot .left {
  string-set: ears-bottom-left content();
}

.ears tfoot .center {
  string-set: ears-bottom-center content();
}

.ears tfoot .right {
  string-set: ears-bottom-right content();
}
*/

@page :first {
  padding-top: 0;
  @top-left {
    content: normal;
    border: none;
  }
  @top-center {
    content: normal;
    border: none;
  }
  @top-right {
    content: normal;
    border: none;
  }
}

@page {
  size: A4;
  margin-bottom: 45mm;
  padding-top: 20px;
  /* The follwing is commented out here, but set appropriately by in code, as
     the content depends on the document */
  /*
  @top-left {
    content: 'Internet-Draft';
    vertical-align: bottom;
    border-bottom: solid 1px #ccc;
  }
  @top-left {
    content: string(ears-top-left);
    vertical-align: bottom;
    border-bottom: solid 1px #ccc;
  }
  @top-center {
    content: string(ears-top-center);
    vertical-align: bottom;
    border-bottom: solid 1px #ccc;
  }
  @top-right {
    content: string(ears-top-right);
    vertical-align: bottom;
    border-bottom: solid 1px #ccc;
  }
  @bottom-left {
    content: string(ears-bottom-left);
    vertical-align: top;
    border-top: solid 1px #ccc;
  }
  @bottom-center {
    content: string(ears-bottom-center);
    vertical-align: top;
    border-top: solid 1px #ccc;
  }
  @bottom-right {
      content: '[Page ' counter(page) ']';
      vertical-align: top;
      border-top: solid 1px #ccc;
  }
  */

}

/* Changes introduced to fix issues found during implementation */
/* Make sure links are clickable even if overlapped by following H* */
a {
  z-index: 2;
}
/* Separate body from document info even without intervening H1 */
section {
  clear: both;
}


/* Top align author divs, to avoid names without organization dropping level with org names */
.author {
  vertical-align: top;
}

/* Leave room in document info to show Internet-Draft on one line */
#identifiers dt {
  width: 8em;
}

/* Don't waste quite as much whitespace between label and value in doc info */
#identifiers dd {
  margin-left: 1em;
}

/* Give floating toc a background color (needed when it's a div inside section */
#toc {
  background-color: white;
}

/* Make the collapsed ToC header render white on gray also when it's a link */
@media screen and (max-width: 1023px) {
  #toc h2 a,
  #toc h2 a:link,
  #toc h2 a:focus,
  #toc h2 a:hover,
  #toc a.toplink,
  #toc a.toplink:hover {
    color: white;
    background-color: #444;
    text-decoration: none;
  }
}

/* Give the bottom of the ToC some whitespace */
@media screen and (min-width: 1024px) {
  #toc {
    padding: 0 0 1em 1em;
  }
}

/* Style section numbers with more space between number and title */
.section-number {
  padding-right: 0.5em;
}

/* prevent monospace from becoming overly large */
tt, code, pre, code {
  font-size: 95%;
}

/* Fix the height/width aspect for ascii art*/
pre.sourcecode,
.art-text pre {
  line-height: 1.12;
}


/* Add styling for a link in the ToC that points to the top of the document */
a.toplink {
  float: right;
  margin-right: 0.5em;
}

/* Fix the dl styling to match the RFC 7992 attributes */
dl > dt,
dl.dlParallel > dt {
  float: left;
  margin-right: 1em;
}
dl.dlNewline > dt {
  float: none;
}

/* Provide styling for table cell text alignment */
table td.text-left,
table th.text-left {
  text-align: left;
}
table td.text-center,
table th.text-center {
  text-align: center;
}
table td.text-right,
table th.text-right {
  text-align: right;
}

/* 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
  left, center, and right, { width: 100%; } does not make sense */
table {
  width: auto;
}

/* 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 {
  caption-side: bottom;
}

/* Limit the width of the author address vcard, so names in right-to-left
   script don't end up on the other side of the page. */

address.vcard {
  max-width: 30em;
  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] {
  color: #222;
}

@media print {
  .toplink {
    display: none;
  }

  /* avoid overwriting the top border line with the ToC header */
  #toc {
    padding-top: 1px;
  }

  /* Avoid page breaks inside dl and author address entries */
  .vcard {
    page-break-inside: avoid;
  }

}
/* Tweak the bcp14 keyword presentation */
.bcp14 {
  font-variant: small-caps;
  font-weight: bold;
  font-size: 0.9em;
}
/* Tweak the invisible space above H* in order not to overlay links in text above */
 h2 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  padding-top: 31px;
 }
 h3 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  padding-top: 24px;
 }
 h4 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  padding-top: 24px;
 }
/* Float artwork pilcrow to the right */
@media screen {
  .artwork a.pilcrow {
    display: block;
    line-height: 0.7;
    margin-top: 0.15em;
  }
}
/* Make pilcrows on dd visible */
@media screen {
  dd:hover > a.pilcrow {
    visibility: visible;
  }
}
/* 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,
   possibly even requiring a new line */
@media print {
  a.pilcrow {
    display: none;
  }
}
/* Styling for the external metadata */
div#external-metadata {
  background-color: #eee;
  padding: 0.5em;
  margin-bottom: 0.5em;
  display: none;
}
div#internal-metadata {
  padding: 0.5em;                       /* to match the external-metadata padding */
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<table class="ears">
<thead><tr>
<td class="left">RFC 8999</td>
<td class="center">QUIC Invariants</td>
<td class="right">May 2021</td>
</tr></thead>
<tfoot><tr>
<td class="left">Thomson</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/rfc8999" class="eref">8999</a></dd>
<dt class="label-category">Category:</dt>
<dd class="category">Standards Track</dd>
<dt class="label-published">Published:</dt>
<dd class="published">
<time datetime="2021-05" class="published">May 2021</time>
    </dd>
<dt class="label-issn">ISSN:</dt>
<dd class="issn">2070-1721</dd>
<dt class="label-authors">Author:</dt>
<dd class="authors">
<div class="author">
      <div class="author-name">M. Thomson</div>
<div class="org">Mozilla</div>
</div>
</dd>
</dl>
</div>
<h1 id="rfcnum">RFC 8999</h1>
<h1 id="title">Version-Independent Properties of QUIC</h1>
<section id="section-abstract">
      <h2 id="abstract"><a href="#abstract" class="selfRef">Abstract</a></h2>
<p id="section-abstract-1">This document defines the properties of the QUIC transport protocol that are
common to all versions of the protocol.<a href="#section-abstract-1" 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/rfc8999">https://www.rfc-editor.org/info/rfc8999</a></span>.<a href="#section-boilerplate.1-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="copyright">
<section id="section-boilerplate.2">
        <h2 id="name-copyright-notice">
<a href="#name-copyright-notice" class="section-name selfRef">Copyright Notice</a>
        </h2>
<p id="section-boilerplate.2-1">
            Copyright (c) 2021 IETF Trust and the persons identified as the
            document authors. All rights reserved.<a href="#section-boilerplate.2-1" class="pilcrow">¶</a></p>
<p id="section-boilerplate.2-2">
            This document is subject to BCP 78 and the IETF Trust's Legal
            Provisions Relating to IETF Documents
            (<span><a href="https://trustee.ietf.org/license-info">https://trustee.ietf.org/license-info</a></span>) in effect on the date of
            publication of this document. Please review these documents
            carefully, as they describe your rights and restrictions with
            respect to this document. Code Components extracted from this
            document must include Simplified BSD License text as described in
            Section 4.e of the Trust Legal Provisions and are provided without
            warranty as described in the Simplified BSD License.<a href="#section-boilerplate.2-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="toc">
<section id="section-toc.1">
        <a href="#" onclick="scroll(0,0)" class="toplink">▲</a><h2 id="name-table-of-contents">
<a href="#name-table-of-contents" class="section-name selfRef">Table of Contents</a>
        </h2>
<nav class="toc"><ul class="compact toc ulEmpty">
<li class="compact toc ulEmpty" id="section-toc.1-1.1">
            <p id="section-toc.1-1.1.1" class="keepWithNext"><a href="#section-1" class="xref">1</a>.  <a href="#name-an-extremely-abstract-descr" class="xref">An Extremely Abstract Description of QUIC</a></p>
</li>
          <li class="compact toc ulEmpty" id="section-toc.1-1.2">
            <p id="section-toc.1-1.2.1" class="keepWithNext"><a href="#section-2" class="xref">2</a>.  <a href="#name-fixed-properties-of-all-qui" class="xref">Fixed Properties of All QUIC Versions</a></p>
</li>
          <li class="compact toc ulEmpty" id="section-toc.1-1.3">
            <p id="section-toc.1-1.3.1" class="keepWithNext"><a href="#section-3" class="xref">3</a>.  <a href="#name-conventions-and-definitions" class="xref">Conventions and Definitions</a></p>
</li>
          <li class="compact toc ulEmpty" id="section-toc.1-1.4">
            <p id="section-toc.1-1.4.1"><a href="#section-4" class="xref">4</a>.  <a href="#name-notational-conventions" class="xref">Notational Conventions</a></p>
</li>
          <li class="compact toc ulEmpty" id="section-toc.1-1.5">
            <p id="section-toc.1-1.5.1"><a href="#section-5" class="xref">5</a>.  <a href="#name-quic-packets" class="xref">QUIC Packets</a></p>
<ul class="compact toc ulEmpty">
<li class="compact toc ulEmpty" 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-long-header" class="xref">Long Header</a></p>
</li>
              <li class="compact toc ulEmpty" 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-short-header" class="xref">Short Header</a></p>
</li>
              <li class="compact toc ulEmpty" 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-connection-id" class="xref">Connection ID</a></p>
</li>
              <li class="compact toc ulEmpty" 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-version" class="xref">Version</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulEmpty" id="section-toc.1-1.6">
            <p id="section-toc.1-1.6.1"><a href="#section-6" class="xref">6</a>.  <a href="#name-version-negotiation" class="xref">Version Negotiation</a></p>
</li>
          <li class="compact toc ulEmpty" id="section-toc.1-1.7">
            <p id="section-toc.1-1.7.1"><a href="#section-7" class="xref">7</a>.  <a href="#name-security-and-privacy-consid" class="xref">Security and Privacy Considerations</a></p>
</li>
          <li class="compact toc ulEmpty" id="section-toc.1-1.8">
            <p id="section-toc.1-1.8.1"><a href="#section-8" class="xref">8</a>.  <a href="#name-references" class="xref">References</a></p>
<ul class="compact toc ulEmpty">
<li class="compact toc ulEmpty" id="section-toc.1-1.8.2.1">
                <p id="section-toc.1-1.8.2.1.1"><a href="#section-8.1" class="xref">8.1</a>.  <a href="#name-normative-references" class="xref">Normative References</a></p>
</li>
              <li class="compact toc ulEmpty" id="section-toc.1-1.8.2.2">
                <p id="section-toc.1-1.8.2.2.1"><a href="#section-8.2" class="xref">8.2</a>.  <a href="#name-informative-references" class="xref">Informative References</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulEmpty" id="section-toc.1-1.9">
            <p id="section-toc.1-1.9.1"><a href="#section-appendix.a" class="xref">Appendix A</a>.  <a href="#name-incorrect-assumptions" class="xref">Incorrect Assumptions</a></p>
</li>
          <li class="compact toc ulEmpty" id="section-toc.1-1.10">
            <p id="section-toc.1-1.10.1"><a href="#section-appendix.b" class="xref"></a><a href="#name-authors-address" class="xref">Author's Address</a></p>
</li>
        </ul>
</nav>
</section>
</div>
<div id="an-extremely-abstract-description-of-quic">
<section id="section-1">
      <h2 id="name-an-extremely-abstract-descr">
<a href="#section-1" class="section-number selfRef">1. </a><a href="#name-an-extremely-abstract-descr" class="section-name selfRef">An Extremely Abstract Description of QUIC</a>
      </h2>
<p id="section-1-1">QUIC is a connection-oriented protocol between two endpoints.  Those endpoints
exchange UDP datagrams.  These UDP datagrams contain QUIC packets.  QUIC
endpoints use QUIC packets to establish a QUIC connection, which is shared
protocol state between those endpoints.<a href="#section-1-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="fixed-properties-of-all-quic-versions">
<section id="section-2">
      <h2 id="name-fixed-properties-of-all-qui">
<a href="#section-2" class="section-number selfRef">2. </a><a href="#name-fixed-properties-of-all-qui" class="section-name selfRef">Fixed Properties of All QUIC Versions</a>
      </h2>
<p id="section-2-1">In addition to providing secure, multiplexed transport, QUIC <span>[<a href="#QUIC-TRANSPORT" class="xref">QUIC-TRANSPORT</a>]</span>
allows for the option to negotiate a version.  This allows the protocol to
change over time in response to new requirements.  Many characteristics of the
protocol could change between versions.<a href="#section-2-1" class="pilcrow">¶</a></p>
<p id="section-2-2">This document describes the subset of QUIC that is intended to remain stable as
new versions are developed and deployed.  All of these invariants are
independent of the IP version.<a href="#section-2-2" class="pilcrow">¶</a></p>
<p id="section-2-3">The primary goal of this document is to ensure that it is possible to deploy new
versions of QUIC.  By documenting the properties that cannot change, this
document aims to preserve the ability for QUIC endpoints to negotiate changes to
any other aspect of the protocol.  As a consequence, this also guarantees a
minimal amount of information that is made available to entities other than
endpoints.  Unless specifically prohibited in this document, any aspect of the
protocol can change between different versions.<a href="#section-2-3" class="pilcrow">¶</a></p>
<p id="section-2-4"><a href="#bad-assumptions" class="xref">Appendix A</a> contains a non-exhaustive list of some incorrect assumptions
that might be made based on knowledge of QUIC version 1; these do not apply to
every version of QUIC.<a href="#section-2-4" class="pilcrow">¶</a></p>
</section>
</div>
<div id="conventions-and-definitions">
<section id="section-3">
      <h2 id="name-conventions-and-definitions">
<a href="#section-3" class="section-number selfRef">3. </a><a href="#name-conventions-and-definitions" class="section-name selfRef">Conventions and Definitions</a>
      </h2>
<p id="section-3-1">The key words "<span class="bcp14">MUST</span>", "<span class="bcp14">MUST NOT</span>", "<span class="bcp14">REQUIRED</span>", "<span class="bcp14">SHALL</span>", "<span class="bcp14">SHALL NOT</span>", "<span class="bcp14">SHOULD</span>",
"<span class="bcp14">SHOULD NOT</span>", "<span class="bcp14">RECOMMENDED</span>", "<span class="bcp14">NOT RECOMMENDED</span>", "<span class="bcp14">MAY</span>", and "<span class="bcp14">OPTIONAL</span>" in this
document are to be interpreted as described in BCP 14 <span>[<a href="#RFC2119" class="xref">RFC2119</a>]</span> <span>[<a href="#RFC8174" class="xref">RFC8174</a>]</span>
when, and only when, they appear in all capitals, as shown here.<a href="#section-3-1" class="pilcrow">¶</a></p>
<p id="section-3-2">This document defines requirements on future QUIC versions, even where normative
language is not used.<a href="#section-3-2" class="pilcrow">¶</a></p>
<p id="section-3-3">This document uses terms and notational conventions from <span>[<a href="#QUIC-TRANSPORT" class="xref">QUIC-TRANSPORT</a>]</span>.<a href="#section-3-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="notational-conventions">
<section id="section-4">
      <h2 id="name-notational-conventions">
<a href="#section-4" class="section-number selfRef">4. </a><a href="#name-notational-conventions" class="section-name selfRef">Notational Conventions</a>
      </h2>
<p id="section-4-1">The format of packets is described using the notation defined in this section.
This notation is the same as that used in <span>[<a href="#QUIC-TRANSPORT" class="xref">QUIC-TRANSPORT</a>]</span>.<a href="#section-4-1" class="pilcrow">¶</a></p>
<p id="section-4-2">Complex fields are named and then followed by a list of fields surrounded by a
pair of matching braces. Each field in this list is separated by commas.<a href="#section-4-2" class="pilcrow">¶</a></p>
<p id="section-4-3">Individual fields include length information, plus indications about fixed
value, optionality, or repetitions. Individual fields use the following
notational conventions, with all lengths in bits:<a href="#section-4-3" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlParallel" id="section-4-4">
        <dt id="section-4-4.1">x (A):</dt>
        <dd style="margin-left: 1.5em" id="section-4-4.2">
          <p id="section-4-4.2.1">Indicates that x is A bits long<a href="#section-4-4.2.1" class="pilcrow">¶</a></p>
</dd>
        <dd class="break"></dd>
<dt id="section-4-4.3">x (A..B):</dt>
        <dd style="margin-left: 1.5em" id="section-4-4.4">
          <p id="section-4-4.4.1">Indicates that x can be any length from A to B; A can be omitted to indicate
a minimum of zero bits, and B can be omitted to indicate no set upper limit;
values in this format always end on a byte boundary<a href="#section-4-4.4.1" class="pilcrow">¶</a></p>
</dd>
        <dd class="break"></dd>
<dt id="section-4-4.5">x (L) = C:</dt>
        <dd style="margin-left: 1.5em" id="section-4-4.6">
          <p id="section-4-4.6.1">Indicates that x has a fixed value of C; the length of x is described by
L, which can use any of the length forms above<a href="#section-4-4.6.1" class="pilcrow">¶</a></p>
</dd>
        <dd class="break"></dd>
<dt id="section-4-4.7">x (L) ...:</dt>
        <dd style="margin-left: 1.5em" id="section-4-4.8">
          <p id="section-4-4.8.1">Indicates that x is repeated zero or more times and that each instance has a
length of L<a href="#section-4-4.8.1" class="pilcrow">¶</a></p>
</dd>
      <dd class="break"></dd>
</dl>
<p id="section-4-5">This document uses network byte order (that is, big endian) values.  Fields
are placed starting from the high-order bits of each byte.<a href="#section-4-5" class="pilcrow">¶</a></p>
<p id="section-4-6"><a href="#fig-ex-format" class="xref">Figure 1</a> shows an example structure:<a href="#section-4-6" class="pilcrow">¶</a></p>
<span id="name-example-format"></span><div id="fig-ex-format">
<figure id="figure-1">
        <div class="artwork art-text alignLeft" id="section-4-7.1">
<pre>
Example Structure {
  One-bit Field (1),
  7-bit Field with Fixed Value (7) = 61,
  Arbitrary-Length Field (..),
  Variable-Length Field (8..24),
  Repeated Field (8) ...,
}
</pre>
</div>
<figcaption><a href="#figure-1" class="selfRef">Figure 1</a>:
<a href="#name-example-format" class="selfRef">Example Format</a>
        </figcaption></figure>
</div>
</section>
</div>
<div id="quic-packets">
<section id="section-5">
      <h2 id="name-quic-packets">
<a href="#section-5" class="section-number selfRef">5. </a><a href="#name-quic-packets" class="section-name selfRef">QUIC Packets</a>
      </h2>
<p id="section-5-1">QUIC endpoints exchange UDP datagrams that contain one or more QUIC packets.
This section describes the invariant characteristics of a QUIC packet.  A
version of QUIC could permit multiple QUIC packets in a single UDP datagram, but
the invariant properties only describe the first packet in a datagram.<a href="#section-5-1" class="pilcrow">¶</a></p>
<p id="section-5-2">QUIC defines two types of packet headers: long and short.  Packets with a long
header are identified by the most significant bit of the first byte being set;
packets with a short header have that bit cleared.<a href="#section-5-2" class="pilcrow">¶</a></p>
<p id="section-5-3">QUIC packets might be integrity protected, including the header.  However, QUIC
Version Negotiation packets are not integrity protected; see <a href="#vn" class="xref">Section 6</a>.<a href="#section-5-3" class="pilcrow">¶</a></p>
<p id="section-5-4">Aside from the values described here, the payload of QUIC packets is
version specific and of arbitrary length.<a href="#section-5-4" class="pilcrow">¶</a></p>
<div id="long-header">
<section id="section-5.1">
        <h3 id="name-long-header">
<a href="#section-5.1" class="section-number selfRef">5.1. </a><a href="#name-long-header" class="section-name selfRef">Long Header</a>
        </h3>
<p id="section-5.1-1">Long headers take the form described in <a href="#fig-long" class="xref">Figure 2</a>.<a href="#section-5.1-1" class="pilcrow">¶</a></p>
<span id="name-quic-long-header"></span><div id="fig-long">
<figure id="figure-2">
          <div class="artwork art-text alignLeft" id="section-5.1-2.1">
<pre>
Long Header Packet {
  Header Form (1) = 1,
  Version-Specific Bits (7),
  Version (32),
  Destination Connection ID Length (8),
  Destination Connection ID (0..2040),
  Source Connection ID Length (8),
  Source Connection ID (0..2040),
  Version-Specific Data (..),
}
</pre>
</div>
<figcaption><a href="#figure-2" class="selfRef">Figure 2</a>:
<a href="#name-quic-long-header" class="selfRef">QUIC Long Header</a>
          </figcaption></figure>
</div>
<p id="section-5.1-3">A QUIC packet with a long header has the high bit of the first byte set to 1.
All other bits in that byte are version specific.<a href="#section-5.1-3" class="pilcrow">¶</a></p>
<p id="section-5.1-4">The next four bytes include a 32-bit Version field.  Versions are described in
<a href="#version" class="xref">Section 5.4</a>.<a href="#section-5.1-4" class="pilcrow">¶</a></p>
<p id="section-5.1-5">The next byte contains the length in bytes of the Destination Connection ID
field that follows it.  This length is encoded as an 8-bit unsigned integer.
The Destination Connection ID field follows the Destination Connection ID Length
field and is between 0 and 255 bytes in length.  Connection IDs are described in
<a href="#connection-id" class="xref">Section 5.3</a>.<a href="#section-5.1-5" class="pilcrow">¶</a></p>
<p id="section-5.1-6">The next byte contains the length in bytes of the Source Connection ID field
that follows it.  This length is encoded as an 8-bit unsigned integer.  The
Source Connection ID field follows the Source Connection ID Length field and is
between 0 and 255 bytes in length.<a href="#section-5.1-6" class="pilcrow">¶</a></p>
<p id="section-5.1-7">The remainder of the packet contains version-specific content.<a href="#section-5.1-7" class="pilcrow">¶</a></p>
</section>
</div>
<div id="short-header">
<section id="section-5.2">
        <h3 id="name-short-header">
<a href="#section-5.2" class="section-number selfRef">5.2. </a><a href="#name-short-header" class="section-name selfRef">Short Header</a>
        </h3>
<p id="section-5.2-1">Short headers take the form described in <a href="#fig-short" class="xref">Figure 3</a>.<a href="#section-5.2-1" class="pilcrow">¶</a></p>
<span id="name-quic-short-header"></span><div id="fig-short">
<figure id="figure-3">
          <div class="artwork art-text alignLeft" id="section-5.2-2.1">
<pre>
Short Header Packet {
  Header Form (1) = 0,
  Version-Specific Bits (7),
  Destination Connection ID (..),
  Version-Specific Data (..),
}
</pre>
</div>
<figcaption><a href="#figure-3" class="selfRef">Figure 3</a>:
<a href="#name-quic-short-header" class="selfRef">QUIC Short Header</a>
          </figcaption></figure>
</div>
<p id="section-5.2-3">A QUIC packet with a short header has the high bit of the first byte set to 0.<a href="#section-5.2-3" class="pilcrow">¶</a></p>
<p id="section-5.2-4">A QUIC packet with a short header includes a Destination Connection ID
immediately following the first byte.  The short header does not include the
Destination Connection ID Length, Source Connection ID Length, Source Connection
ID, or Version fields.  The length of the Destination Connection ID is not
encoded in packets with a short header and is not constrained by this
specification.<a href="#section-5.2-4" class="pilcrow">¶</a></p>
<p id="section-5.2-5">The remainder of the packet has version-specific semantics.<a href="#section-5.2-5" class="pilcrow">¶</a></p>
</section>
</div>
<div id="connection-id">
<section id="section-5.3">
        <h3 id="name-connection-id">
<a href="#section-5.3" class="section-number selfRef">5.3. </a><a href="#name-connection-id" class="section-name selfRef">Connection ID</a>
        </h3>
<p id="section-5.3-1">A connection ID is an opaque field of arbitrary length.<a href="#section-5.3-1" class="pilcrow">¶</a></p>
<p id="section-5.3-2">The primary function of a connection ID is to ensure that changes in addressing
at lower protocol layers (UDP, IP, and below) do not cause packets for a QUIC
connection to be delivered to the wrong QUIC endpoint.  The connection ID
is used by endpoints and the intermediaries that support them to ensure that
each QUIC packet can be delivered to the correct instance of an endpoint.  At
the endpoint, the connection ID is used to identify the QUIC connection for
which the packet is intended.<a href="#section-5.3-2" class="pilcrow">¶</a></p>
<p id="section-5.3-3">The connection ID is chosen by each endpoint using version-specific methods.
Packets for the same QUIC connection might use different connection ID values.<a href="#section-5.3-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="version">
<section id="section-5.4">
        <h3 id="name-version">
<a href="#section-5.4" class="section-number selfRef">5.4. </a><a href="#name-version" class="section-name selfRef">Version</a>
        </h3>
<p id="section-5.4-1">The Version field contains a 4-byte identifier.  This value can be used by
endpoints to identify a QUIC version.  A Version field with a value of
0x00000000 is reserved for version negotiation; see <a href="#vn" class="xref">Section 6</a>.  All other values
are potentially valid.<a href="#section-5.4-1" class="pilcrow">¶</a></p>
<p id="section-5.4-2">The properties described in this document apply to all versions of QUIC. A
protocol that does not conform to the properties described in this document is
not QUIC.  Future documents might describe additional properties that apply to
a specific QUIC version or to a range of QUIC versions.<a href="#section-5.4-2" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="vn">
<section id="section-6">
      <h2 id="name-version-negotiation">
<a href="#section-6" class="section-number selfRef">6. </a><a href="#name-version-negotiation" class="section-name selfRef">Version Negotiation</a>
      </h2>
<p id="section-6-1">A QUIC endpoint that receives a packet with a long header and a version it
either does not understand or does not support might send a Version Negotiation
packet in response.  Packets with a short header do not trigger version
negotiation.<a href="#section-6-1" class="pilcrow">¶</a></p>
<p id="section-6-2">A Version Negotiation packet sets the high bit of the first byte, and thus it
conforms with the format of a packet with a long header as defined in
<a href="#long-header" class="xref">Section 5.1</a>.  A Version Negotiation packet is identifiable as such by the
Version field, which is set to 0x00000000.<a href="#section-6-2" class="pilcrow">¶</a></p>
<span id="name-version-negotiation-packet"></span><div id="version-negotiation-format">
<figure id="figure-4">
        <div class="artwork art-text alignLeft" id="section-6-3.1">
<pre>
Version Negotiation Packet {
  Header Form (1) = 1,
  Unused (7),
  Version (32) = 0,
  Destination Connection ID Length (8),
  Destination Connection ID (0..2040),
  Source Connection ID Length (8),
  Source Connection ID (0..2040),
  Supported Version (32) ...,
}
</pre>
</div>
<figcaption><a href="#figure-4" class="selfRef">Figure 4</a>:
<a href="#name-version-negotiation-packet" class="selfRef">Version Negotiation Packet</a>
        </figcaption></figure>
</div>
<p id="section-6-4">Only the most significant bit of the first byte of a Version Negotiation packet
has any defined value.  The remaining 7 bits, labeled "Unused", can be set to
any value when sending and <span class="bcp14">MUST</span> be ignored on receipt.<a href="#section-6-4" class="pilcrow">¶</a></p>
<p id="section-6-5">After the Source Connection ID field, the Version Negotiation packet contains a
list of Supported Version fields, each identifying a version that the endpoint
sending the packet supports.  A Version Negotiation packet contains no other
fields.  An endpoint <span class="bcp14">MUST</span> ignore a packet that contains no Supported Version
fields or contains a truncated Supported Version value.<a href="#section-6-5" class="pilcrow">¶</a></p>
<p id="section-6-6">Version Negotiation packets do not use integrity or confidentiality protection.
Specific QUIC versions might include protocol elements that allow endpoints to
detect modification or corruption in the set of supported versions.<a href="#section-6-6" class="pilcrow">¶</a></p>
<p id="section-6-7">An endpoint <span class="bcp14">MUST</span> include the value from the Source Connection ID field of the
packet it receives in the Destination Connection ID field.  The value for the
Source Connection ID field <span class="bcp14">MUST</span> be copied from the Destination Connection ID
field of the received packet, which is initially randomly selected by a client.
Echoing both connection IDs gives clients some assurance that the server
received the packet and that the Version Negotiation packet was not generated by
an attacker that is unable to observe packets.<a href="#section-6-7" class="pilcrow">¶</a></p>
<p id="section-6-8">An endpoint that receives a Version Negotiation packet might change the version
that it decides to use for subsequent packets.  The conditions under which an
endpoint changes its QUIC version will depend on the version of QUIC that it
chooses.<a href="#section-6-8" class="pilcrow">¶</a></p>
<p id="section-6-9">See <span>[<a href="#QUIC-TRANSPORT" class="xref">QUIC-TRANSPORT</a>]</span> for a more thorough description of how an endpoint that
supports QUIC version 1 generates and consumes a Version Negotiation packet.<a href="#section-6-9" class="pilcrow">¶</a></p>
</section>
</div>
<div id="security-and-privacy-considerations">
<section id="section-7">
      <h2 id="name-security-and-privacy-consid">
<a href="#section-7" class="section-number selfRef">7. </a><a href="#name-security-and-privacy-consid" class="section-name selfRef">Security and Privacy Considerations</a>
      </h2>
<p id="section-7-1">It is possible that middleboxes could observe traits of a specific version of
QUIC and assume that when other versions of QUIC exhibit similar traits the same
underlying semantic is being expressed.  There are potentially many such traits;
see <a href="#bad-assumptions" class="xref">Appendix A</a>.  Some effort has been made to either eliminate or
obscure some observable traits in QUIC version 1, but many of these remain.
Other QUIC versions might make different design decisions and so exhibit
different traits.<a href="#section-7-1" class="pilcrow">¶</a></p>
<p id="section-7-2">The QUIC version number does not appear in all QUIC packets, which means that
reliably extracting information from a flow based on version-specific traits
requires that middleboxes retain state for every connection ID they see.<a href="#section-7-2" class="pilcrow">¶</a></p>
<p id="section-7-3">The Version Negotiation packet described in this document is not
integrity protected; it only has modest protection against insertion by
attackers.  An endpoint <span class="bcp14">MUST</span> authenticate the semantic content of a Version
Negotiation packet if it attempts a different QUIC version as a result.<a href="#section-7-3" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-8">
      <h2 id="name-references">
<a href="#section-8" class="section-number selfRef">8. </a><a href="#name-references" class="section-name selfRef">References</a>
      </h2>
<section id="section-8.1">
        <h3 id="name-normative-references">
<a href="#section-8.1" class="section-number selfRef">8.1. </a><a href="#name-normative-references" class="section-name selfRef">Normative References</a>
        </h3>
<dl class="references">
<dt id="RFC2119">[RFC2119]</dt>
        <dd>
<span class="refAuthor">Bradner, S.</span>, <span class="refTitle">"Key words for use in RFCs to Indicate Requirement Levels"</span>, <span class="seriesInfo">BCP 14</span>, <span class="seriesInfo">RFC 2119</span>, <span class="seriesInfo">DOI 10.17487/RFC2119</span>, <time datetime="1997-03" class="refDate">March 1997</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc2119">https://www.rfc-editor.org/info/rfc2119</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="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>
</dl>
</section>
<section id="section-8.2">
        <h3 id="name-informative-references">
<a href="#section-8.2" class="section-number selfRef">8.2. </a><a href="#name-informative-references" class="section-name selfRef">Informative References</a>
        </h3>
<dl class="references">
<dt id="QUIC-TLS">[QUIC-TLS]</dt>
        <dd>
<span class="refAuthor">Thomson, M., Ed.</span> and <span class="refAuthor">S. Turner, Ed.</span>, <span class="refTitle">"Using TLS to Secure QUIC"</span>, <span class="seriesInfo">RFC 9001</span>, <span class="seriesInfo">DOI 10.17487/RFC9001</span>, <time datetime="2021-05" class="refDate">May 2021</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc9001">https://www.rfc-editor.org/info/rfc9001</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="QUIC-TRANSPORT">[QUIC-TRANSPORT]</dt>
        <dd>
<span class="refAuthor">Iyengar, J., Ed.</span> and <span class="refAuthor">M. Thomson, Ed.</span>, <span class="refTitle">"QUIC: A UDP-Based Multiplexed and Secure Transport"</span>, <span class="seriesInfo">RFC 9000</span>, <span class="seriesInfo">DOI 10.17487/RFC9000</span>, <time datetime="2021-05" class="refDate">May 2021</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc9000">https://www.rfc-editor.org/info/rfc9000</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC5116">[RFC5116]</dt>
      <dd>
<span class="refAuthor">McGrew, D.</span>, <span class="refTitle">"An Interface and Algorithms for Authenticated Encryption"</span>, <span class="seriesInfo">RFC 5116</span>, <span class="seriesInfo">DOI 10.17487/RFC5116</span>, <time datetime="2008-01" class="refDate">January 2008</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc5116">https://www.rfc-editor.org/info/rfc5116</a>&gt;</span>. </dd>
<dd class="break"></dd>
</dl>
</section>
</section>
<div id="bad-assumptions">
<section id="section-appendix.a">
      <h2 id="name-incorrect-assumptions">
<a href="#section-appendix.a" class="section-number selfRef">Appendix A. </a><a href="#name-incorrect-assumptions" class="section-name selfRef">Incorrect Assumptions</a>
      </h2>
<p id="section-appendix.a-1">There are several traits of QUIC version 1 <span>[<a href="#QUIC-TRANSPORT" class="xref">QUIC-TRANSPORT</a>]</span> that are not
protected from observation but are nonetheless considered to be changeable when
a new version is deployed.<a href="#section-appendix.a-1" class="pilcrow">¶</a></p>
<p id="section-appendix.a-2">This section lists a sampling of incorrect assumptions that might be made about
QUIC based on knowledge of QUIC version 1.  Some of these statements are not
even true for QUIC version 1.  This is not an exhaustive list; it is intended to
be illustrative only.<a href="#section-appendix.a-2" class="pilcrow">¶</a></p>
<p id="section-appendix.a-3">
        <strong>Any and all of the following statements can be false for a given QUIC
version:</strong><a href="#section-appendix.a-3" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-appendix.a-4.1">QUIC uses TLS <span>[<a href="#QUIC-TLS" class="xref">QUIC-TLS</a>]</span>, and some TLS messages are visible on the wire.<a href="#section-appendix.a-4.1" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-appendix.a-4.2">QUIC long headers are only exchanged during connection establishment.<a href="#section-appendix.a-4.2" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-appendix.a-4.3">Every flow on a given 5-tuple will include a connection establishment phase.<a href="#section-appendix.a-4.3" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-appendix.a-4.4">The first packets exchanged on a flow use the long header.<a href="#section-appendix.a-4.4" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-appendix.a-4.5">The last packet before a long period of quiescence might be assumed
to contain only an acknowledgment.<a href="#section-appendix.a-4.5" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-appendix.a-4.6">QUIC uses an Authenticated Encryption with Associated Data (AEAD) function
(AEAD_AES_128_GCM; see <span>[<a href="#RFC5116" class="xref">RFC5116</a>]</span>) to protect the packets it exchanges
during connection establishment.<a href="#section-appendix.a-4.6" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-appendix.a-4.7">QUIC packet numbers are encrypted and appear as the first encrypted bytes.<a href="#section-appendix.a-4.7" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-appendix.a-4.8">QUIC packet numbers increase by one for every packet sent.<a href="#section-appendix.a-4.8" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-appendix.a-4.9">QUIC has a minimum size for the first handshake packet sent by a client.<a href="#section-appendix.a-4.9" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-appendix.a-4.10">QUIC stipulates that a client speak first.<a href="#section-appendix.a-4.10" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-appendix.a-4.11">QUIC packets always have the second bit of the first byte (0x40) set.<a href="#section-appendix.a-4.11" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-appendix.a-4.12">A QUIC Version Negotiation packet is only sent by a server.<a href="#section-appendix.a-4.12" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-appendix.a-4.13">A QUIC connection ID changes infrequently.<a href="#section-appendix.a-4.13" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-appendix.a-4.14">QUIC endpoints change the version they speak if they are sent a Version
Negotiation packet.<a href="#section-appendix.a-4.14" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-appendix.a-4.15">The Version field in a QUIC long header is the same in both directions.<a href="#section-appendix.a-4.15" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-appendix.a-4.16">A QUIC packet with a particular value in the Version field means that the
corresponding version of QUIC is in use.<a href="#section-appendix.a-4.16" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-appendix.a-4.17">Only one connection at a time is established between any pair of QUIC
endpoints.<a href="#section-appendix.a-4.17" class="pilcrow">¶</a>
</li>
      </ul>
</section>
</div>
<div id="authors-addresses">
<section id="section-appendix.b">
      <h2 id="name-authors-address">
<a href="#name-authors-address" class="section-name selfRef">Author's Address</a>
      </h2>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Martin Thomson</span></div>
<div dir="auto" class="left"><span class="org">Mozilla</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:mt@lowentropy.net" class="email">mt@lowentropy.net</a>
</div>
</address>
</section>
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