File: rfc8701.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 8701: Applying Generate Random Extensions And Sustain Extensibility (GREASE) to TLS Extensibility</title>
<meta content="David Benjamin" name="author">
<meta content="
       This document describes GREASE (Generate Random Extensions And Sustain
     Extensibility), a mechanism to prevent extensibility failures in the TLS
     ecosystem. It reserves a set of TLS protocol values that may be advertised
     to ensure peers correctly handle unknown values. 
    " name="description">
<meta content="xml2rfc 2.38.1" name="generator">
<meta content="TLS" name="keyword">
<meta content="GREASE" name="keyword">
<meta content="8701" name="rfc.number">
<link href="rfc8701.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;
  }

}
/* Avoid wrapping of URLs in references */
@media screen {
  .references a {
    white-space: nowrap;
  }
}
/* 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 */
}
/* Styling for title RFC Number */
h1#rfcnum {
  clear: both;
  margin: 0 0 -1em;
  padding: 1em 0 0 0;
}
/* Make .olPercent look the same as <ol><li> */
dl.olPercent > dd {
  margin: 0 0 0.25em 0;
  min-height: initial;
}
/* Give aside some styling to set it apart */
aside {
  border-left: 1px solid #ddd;
  margin: 1em 0 1em 2em;
  padding: 0.2em 2em;
}
aside > dl,
aside > ol,
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<table class="ears">
<thead><tr>
<td class="left">RFC 8701</td>
<td class="center">Applying Generate Random Extensions And Sustain Extensibility (GREASE) to TLS Extensibility</td>
<td class="right">January 2020</td>
</tr></thead>
<tfoot><tr>
<td class="left">Benjamin</td>
<td class="center">Informational</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/rfc8701" class="eref">8701</a></dd>
<dt class="label-category">Category:</dt>
<dd class="category">Informational</dd>
<dt class="label-published">Published:</dt>
<dd class="published">
<time datetime="2020-01" class="published">January 2020</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">D. Benjamin</div>
<div class="org">Google LLC</div>
</div>
</dd>
</dl>
</div>
<h1 id="rfcnum">RFC 8701</h1>
<h1 id="title">Applying Generate Random Extensions And Sustain Extensibility (GREASE) to TLS Extensibility</h1>
<section id="section-abstract">
      <h2 id="abstract"><a href="#abstract" class="selfRef">Abstract</a></h2>
<p id="section-abstract-1">This document describes GREASE (Generate Random Extensions And Sustain
     Extensibility), a mechanism to prevent extensibility failures in the TLS
     ecosystem. It reserves a set of TLS protocol values that may be advertised
     to ensure peers correctly handle unknown values.<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 document is not an Internet Standards Track specification; it is
            published for informational purposes.<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).  Not all documents
            approved by the IESG are candidates for any level of Internet
            Standard; see 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/rfc8701">https://www.rfc-editor.org/info/rfc8701</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) 2020 IETF Trust and the persons identified as the
            document authors. All rights reserved.<a href="#section-boilerplate.2-1" class="pilcrow">¶</a></p>
<p id="section-boilerplate.2-2">
            This document is subject to BCP 78 and the IETF Trust's Legal
            Provisions Relating to IETF Documents
            (<span><a href="https://trustee.ietf.org/license-info">https://trustee.ietf.org/license-info</a></span>) in effect on the date of
            publication of this document. Please review these documents
            carefully, as they describe your rights and restrictions with
            respect to this document. Code Components extracted from this
            document must include Simplified BSD License text as described in
            Section 4.e of the Trust Legal Provisions and are provided without
            warranty as described in the Simplified BSD License.<a href="#section-boilerplate.2-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="toc">
<section id="section-toc.1">
        <a href="#" onclick="scroll(0,0)" class="toplink">▲</a><h2 id="name-table-of-contents">
<a href="#name-table-of-contents" class="section-name selfRef">Table of Contents</a>
        </h2>
<nav class="toc"><ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.1">
            <p id="section-toc.1-1.1.1"><a href="#section-1" class="xref">1</a>.  <a href="#name-introduction" class="xref">Introduction</a><a href="#section-toc.1-1.1.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.1.2.1">
                <p id="section-toc.1-1.1.2.1.1"><a href="#section-1.1" class="xref">1.1</a>.  <a href="#name-requirements-language" class="xref">Requirements Language</a><a href="#section-toc.1-1.1.2.1.1" class="pilcrow">¶</a></p>
</li>
</ul>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.2">
            <p id="section-toc.1-1.2.1"><a href="#section-2" class="xref">2</a>.  <a href="#name-grease-values" class="xref">GREASE Values</a><a href="#section-toc.1-1.2.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.3">
            <p id="section-toc.1-1.3.1"><a href="#section-3" class="xref">3</a>.  <a href="#name-client-initiated-extension-" class="xref">Client-Initiated Extension Points</a><a href="#section-toc.1-1.3.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.3.2.1">
                <p id="section-toc.1-1.3.2.1.1"><a href="#section-3.1" class="xref">3.1</a>.  <a href="#name-client-behavior" class="xref">Client Behavior</a><a href="#section-toc.1-1.3.2.1.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.3.2.2">
                <p id="section-toc.1-1.3.2.2.1"><a href="#section-3.2" class="xref">3.2</a>.  <a href="#name-server-behavior" class="xref">Server Behavior</a><a href="#section-toc.1-1.3.2.2.1" class="pilcrow">¶</a></p>
</li>
</ul>
</li>
<li class="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-server-initiated-extension-" class="xref">Server-Initiated Extension Points</a><a href="#section-toc.1-1.4.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" 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-server-behavior-2" class="xref">Server Behavior</a><a href="#section-toc.1-1.4.2.1.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" 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-client-behavior-2" class="xref">Client Behavior</a><a href="#section-toc.1-1.4.2.2.1" class="pilcrow">¶</a></p>
</li>
</ul>
</li>
<li class="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-sending-grease-values" class="xref">Sending GREASE Values</a><a href="#section-toc.1-1.5.1" class="pilcrow">¶</a></p>
</li>
<li class="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-iana-considerations" class="xref">IANA Considerations</a><a href="#section-toc.1-1.6.1" class="pilcrow">¶</a></p>
</li>
<li class="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-considerations" class="xref">Security Considerations</a><a href="#section-toc.1-1.7.1" class="pilcrow">¶</a></p>
</li>
<li class="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-normative-references" class="xref">Normative References</a><a href="#section-toc.1-1.8.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.9">
            <p id="section-toc.1-1.9.1"><a href="#section-appendix.a" class="xref"></a><a href="#name-acknowledgments" class="xref">Acknowledgments</a><a href="#section-toc.1-1.9.1" class="pilcrow">¶</a></p>
</li>
<li class="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><a href="#section-toc.1-1.10.1" class="pilcrow">¶</a></p>
</li>
</ul>
</nav>
</section>
</div>
<section id="section-1">
      <h2 id="name-introduction">
<a href="#section-1" class="section-number selfRef">1. </a><a href="#name-introduction" class="section-name selfRef">Introduction</a>
      </h2>
<p id="section-1-1">The TLS protocol <span>[<a href="#RFC8446" class="xref">RFC8446</a>]</span> includes several points of
extensibility, including the list of cipher suites and several lists of extensions.
The values transmitted in these lists identify implementation capabilities. TLS follows
a model where one side, usually the client, advertises capabilities, and the
peer, usually the server, selects them. The responding side must ignore unknown
values so that new capabilities may be introduced to the ecosystem while
maintaining interoperability.<a href="#section-1-1" class="pilcrow">¶</a></p>
<p id="section-1-2">However, bugs may cause an implementation to reject unknown
      values. It will interoperate with existing peers, so the mistake may
      spread through the ecosystem unnoticed. Later, when new values are
      defined, updated peers will discover that the metaphorical joint in the
      protocol has rusted shut and the new values cannot be deployed without
      interoperability failures.<a href="#section-1-2" class="pilcrow">¶</a></p>
<p id="section-1-3">To avoid this problem, this document reserves some currently unused values for
TLS implementations to advertise at random. Correctly implemented peers will ignore
these values and interoperate. Peers that do not tolerate unknown values will
fail to interoperate, revealing the mistake before it is
widespread.<a href="#section-1-3" class="pilcrow">¶</a></p>
<p id="section-1-4">In keeping with the rusted joint metaphor, this technique is called "GREASE"
(Generate Random Extensions And Sustain Extensibility).<a href="#section-1-4" class="pilcrow">¶</a></p>
<section id="section-1.1">
        <h3 id="name-requirements-language">
<a href="#section-1.1" class="section-number selfRef">1.1. </a><a href="#name-requirements-language" class="section-name selfRef">Requirements Language</a>
        </h3>
<p id="section-1.1-1">
    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-1.1-1" class="pilcrow">¶</a></p>
</section>
</section>
<section id="section-2">
      <h2 id="name-grease-values">
<a href="#section-2" class="section-number selfRef">2. </a><a href="#name-grease-values" class="section-name selfRef">GREASE Values</a>
      </h2>
<p id="section-2-1">This document reserves a number of TLS protocol values, referred to as
     GREASE values. These values were allocated sparsely to discourage server
     implementations from conditioning on them. For convenience, they were also
     chosen so all types share a number scheme with a consistent pattern while
     avoiding collisions with any existing applicable registries in TLS.<a href="#section-2-1" class="pilcrow">¶</a></p>
<p id="section-2-2">The following values are reserved as GREASE values for cipher suites
     and Application-Layer Protocol Negotiation (ALPN) <span>[<a href="#RFC7301" class="xref">RFC7301</a>]</span> identifiers:<a href="#section-2-2" class="pilcrow">¶</a></p>
<ul class="ulEmpty">
<li class="ulEmpty" id="section-2-3.1">{0x0A,0x0A}<a href="#section-2-3.1" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-3.2">{0x1A,0x1A}<a href="#section-2-3.2" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-3.3">{0x2A,0x2A}<a href="#section-2-3.3" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-3.4">{0x3A,0x3A}<a href="#section-2-3.4" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-3.5">{0x4A,0x4A}<a href="#section-2-3.5" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-3.6">{0x5A,0x5A}<a href="#section-2-3.6" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-3.7">{0x6A,0x6A}<a href="#section-2-3.7" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-3.8">{0x7A,0x7A}<a href="#section-2-3.8" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-3.9">{0x8A,0x8A}<a href="#section-2-3.9" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-3.10">{0x9A,0x9A}<a href="#section-2-3.10" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-3.11">{0xAA,0xAA}<a href="#section-2-3.11" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-3.12">{0xBA,0xBA}<a href="#section-2-3.12" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-3.13">{0xCA,0xCA}<a href="#section-2-3.13" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-3.14">{0xDA,0xDA}<a href="#section-2-3.14" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-3.15">{0xEA,0xEA}<a href="#section-2-3.15" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-3.16">{0xFA,0xFA}<a href="#section-2-3.16" class="pilcrow">¶</a>
</li>
</ul>
<p id="section-2-4">The following values are reserved as GREASE values for extensions,
        named groups, signature algorithms, and versions:<a href="#section-2-4" class="pilcrow">¶</a></p>
<ul class="ulEmpty">
<li class="ulEmpty" id="section-2-5.1">0x0A0A<a href="#section-2-5.1" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-5.2">0x1A1A<a href="#section-2-5.2" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-5.3">0x2A2A<a href="#section-2-5.3" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-5.4">0x3A3A<a href="#section-2-5.4" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-5.5">0x4A4A<a href="#section-2-5.5" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-5.6">0x5A5A<a href="#section-2-5.6" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-5.7">0x6A6A<a href="#section-2-5.7" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-5.8">0x7A7A<a href="#section-2-5.8" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-5.9">0x8A8A<a href="#section-2-5.9" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-5.10">0x9A9A<a href="#section-2-5.10" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-5.11">0xAAAA<a href="#section-2-5.11" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-5.12">0xBABA<a href="#section-2-5.12" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-5.13">0xCACA<a href="#section-2-5.13" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-5.14">0xDADA<a href="#section-2-5.14" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-5.15">0xEAEA<a href="#section-2-5.15" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-5.16">0xFAFA<a href="#section-2-5.16" class="pilcrow">¶</a>
</li>
</ul>
<p id="section-2-6">The values allocated above are thus no longer available for use as TLS or
        DTLS <span>[<a href="#RFC6347" class="xref">RFC6347</a>]</span> version numbers.<a href="#section-2-6" class="pilcrow">¶</a></p>
<p id="section-2-7">The following values are reserved as GREASE values for PskKeyExchangeModes:<a href="#section-2-7" class="pilcrow">¶</a></p>
<ul class="ulEmpty">
<li class="ulEmpty" id="section-2-8.1">0x0B<a href="#section-2-8.1" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-8.2">0x2A<a href="#section-2-8.2" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-8.3">0x49<a href="#section-2-8.3" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-8.4">0x68<a href="#section-2-8.4" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-8.5">0x87<a href="#section-2-8.5" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-8.6">0xA6<a href="#section-2-8.6" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-8.7">0xC5<a href="#section-2-8.7" class="pilcrow">¶</a>
</li>
<li class="ulEmpty" id="section-2-8.8">0xE4<a href="#section-2-8.8" class="pilcrow">¶</a>
</li>
</ul>
</section>
<section id="section-3">
      <h2 id="name-client-initiated-extension-">
<a href="#section-3" class="section-number selfRef">3. </a><a href="#name-client-initiated-extension-" class="section-name selfRef">Client-Initiated Extension Points</a>
      </h2>
<p id="section-3-1">Most extension points in TLS are offered by the client and selected by
the server. This section details client and server behavior around
GREASE values for these.<a href="#section-3-1" class="pilcrow">¶</a></p>
<section id="section-3.1">
        <h3 id="name-client-behavior">
<a href="#section-3.1" class="section-number selfRef">3.1. </a><a href="#name-client-behavior" class="section-name selfRef">Client Behavior</a>
        </h3>
<p id="section-3.1-1">When sending a ClientHello, a client <span class="bcp14">MAY</span> behave as follows:<a href="#section-3.1-1" class="pilcrow">¶</a></p>
<ul>
<li id="section-3.1-2.1">A client <span class="bcp14">MAY</span> select one or more GREASE cipher suite values and advertise them in the "cipher_suites" field.<a href="#section-3.1-2.1" class="pilcrow">¶</a>
</li>
<li id="section-3.1-2.2">A client <span class="bcp14">MAY</span> select one or more GREASE extension values and advertise them as extensions with varying length and contents.<a href="#section-3.1-2.2" class="pilcrow">¶</a>
</li>
<li id="section-3.1-2.3">A client <span class="bcp14">MAY</span> select one or more GREASE named group values and advertise them in the "supported_groups" extension, if sent. It <span class="bcp14">MAY</span> also send KeyShareEntry values for a subset of those selected in the "key_share" extension. For each of these, the "key_exchange" field <span class="bcp14">MAY</span> be any value.<a href="#section-3.1-2.3" class="pilcrow">¶</a>
</li>
<li id="section-3.1-2.4">A client <span class="bcp14">MAY</span> select one or more GREASE signature algorithm values and advertise them in the "signature_algorithms" or "signature_algorithms_cert" extensions, if sent.<a href="#section-3.1-2.4" class="pilcrow">¶</a>
</li>
<li id="section-3.1-2.5">A client <span class="bcp14">MAY</span> select one or more GREASE version values and advertise them in the "supported_versions" extension, if sent.<a href="#section-3.1-2.5" class="pilcrow">¶</a>
</li>
<li id="section-3.1-2.6">A client <span class="bcp14">MAY</span> select one or more GREASE PskKeyExchangeMode values and advertise them in the "psk_key_exchange_modes" extension, if sent.<a href="#section-3.1-2.6" class="pilcrow">¶</a>
</li>
<li id="section-3.1-2.7">A client <span class="bcp14">MAY</span> select one or more GREASE ALPN identifiers and advertise them in the "application_layer_protocol_negotiation" extension, if sent.<a href="#section-3.1-2.7" class="pilcrow">¶</a>
</li>
</ul>
<p id="section-3.1-3">Clients <span class="bcp14">MUST</span> reject GREASE values when negotiated by
        the server.  In particular, the client <span class="bcp14">MUST</span> fail the
        connection if a GREASE value appears in any of the following:<a href="#section-3.1-3" class="pilcrow">¶</a></p>
<ul>
<li id="section-3.1-4.1">The "version" value in a ServerHello or HelloRetryRequest<a href="#section-3.1-4.1" class="pilcrow">¶</a>
</li>
<li id="section-3.1-4.2">The "cipher_suite" value in a ServerHello<a href="#section-3.1-4.2" class="pilcrow">¶</a>
</li>
<li id="section-3.1-4.3">Any ServerHello extension<a href="#section-3.1-4.3" class="pilcrow">¶</a>
</li>
<li id="section-3.1-4.4">Any HelloRetryRequest, EncryptedExtensions, or Certificate extension in TLS 1.3<a href="#section-3.1-4.4" class="pilcrow">¶</a>
</li>
<li id="section-3.1-4.5">The "namedcurve" value in a ServerKeyExchange for an Ephemeral
   Elliptic Curve Diffie-Hellman (ECDHE) cipher in TLS 1.2 <span>[<a href="#RFC5246" class="xref">RFC5246</a>]</span> or earlier<a href="#section-3.1-4.5" class="pilcrow">¶</a>
</li>
<li id="section-3.1-4.6">The signature algorithm in a ServerKeyExchange signature in TLS 1.2 or earlier<a href="#section-3.1-4.6" class="pilcrow">¶</a>
</li>
<li id="section-3.1-4.7">The signature algorithm in a server CertificateVerify signature in TLS 1.3<a href="#section-3.1-4.7" class="pilcrow">¶</a>
</li>
</ul>
<p id="section-3.1-5">Note that this can be implemented without special processing on the client. The client
     is already required to reject unknown server-selected values, so it
     may leave GREASE values as unknown and reuse the existing logic.<a href="#section-3.1-5" class="pilcrow">¶</a></p>
</section>
<section id="section-3.2">
        <h3 id="name-server-behavior">
<a href="#section-3.2" class="section-number selfRef">3.2. </a><a href="#name-server-behavior" class="section-name selfRef">Server Behavior</a>
        </h3>
<p id="section-3.2-1">
When processing a ClientHello, servers <span class="bcp14">MUST NOT</span> treat GREASE values differently from any unknown value.
Servers <span class="bcp14">MUST NOT</span> negotiate any GREASE value when offered in a ClientHello.
Servers <span class="bcp14">MUST</span> correctly ignore unknown values in a ClientHello and attempt to negotiate with one of the remaining parameters.
(There may not be any known parameters remaining, in which case parameter negotiation will fail.)<a href="#section-3.2-1" class="pilcrow">¶</a></p>
<p id="section-3.2-2">Note that these requirements are restatements or corollaries of
     existing server requirements in TLS.<a href="#section-3.2-2" class="pilcrow">¶</a></p>
</section>
</section>
<section id="section-4">
      <h2 id="name-server-initiated-extension-">
<a href="#section-4" class="section-number selfRef">4. </a><a href="#name-server-initiated-extension-" class="section-name selfRef">Server-Initiated Extension Points</a>
      </h2>
<p id="section-4-1">Some extension points are offered by the server and selected by the
client. This section details client and server behavior around GREASE
values for these.<a href="#section-4-1" class="pilcrow">¶</a></p>
<section id="section-4.1">
        <h3 id="name-server-behavior-2">
<a href="#section-4.1" class="section-number selfRef">4.1. </a><a href="#name-server-behavior-2" class="section-name selfRef">Server Behavior</a>
        </h3>
<p id="section-4.1-1">When sending a CertificateRequest in TLS 1.3, a server <span class="bcp14">MAY</span> behave as follows:<a href="#section-4.1-1" class="pilcrow">¶</a></p>
<ul>
<li id="section-4.1-2.1">A server <span class="bcp14">MAY</span> select one or more GREASE extension values and advertise them as extensions with varying length and contents.<a href="#section-4.1-2.1" class="pilcrow">¶</a>
</li>
<li id="section-4.1-2.2">A server <span class="bcp14">MAY</span> select one or more GREASE signature algorithm values and advertise them in the "signature_algorithms" or "signature_algorithms_cert" extensions, if present.<a href="#section-4.1-2.2" class="pilcrow">¶</a>
</li>
</ul>
<p id="section-4.1-3">When sending a NewSessionTicket message in TLS 1.3, a server <span class="bcp14">MAY</span> select one or more GREASE extension values and advertise them as extensions with varying length and contents.<a href="#section-4.1-3" class="pilcrow">¶</a></p>
<p id="section-4.1-4">Servers <span class="bcp14">MUST</span> reject GREASE values when negotiated by the client.
In particular, the server <span class="bcp14">MUST</span> fail the connection if a GREASE
value appears in any of the following:<a href="#section-4.1-4" class="pilcrow">¶</a></p>
<ul>
<li id="section-4.1-5.1">Any Certificate extension in TLS 1.3<a href="#section-4.1-5.1" class="pilcrow">¶</a>
</li>
<li id="section-4.1-5.2">The signature algorithm in a client CertificateVerify signature<a href="#section-4.1-5.2" class="pilcrow">¶</a>
</li>
</ul>
<p id="section-4.1-6">Note that this can be implemented without special processing on the server. The server
     is already required to reject unknown client-selected values, so it
     may leave GREASE values as unknown and reuse the existing logic.<a href="#section-4.1-6" class="pilcrow">¶</a></p>
</section>
<section id="section-4.2">
        <h3 id="name-client-behavior-2">
<a href="#section-4.2" class="section-number selfRef">4.2. </a><a href="#name-client-behavior-2" class="section-name selfRef">Client Behavior</a>
        </h3>
<p id="section-4.2-1">
When processing a CertificateRequest or NewSessionTicket, clients <span class="bcp14">MUST NOT</span> treat GREASE values differently from any unknown value.
Clients <span class="bcp14">MUST NOT</span> negotiate any GREASE value when offered by the server.
Clients <span class="bcp14">MUST</span> correctly ignore unknown values offered by the server and attempt to negotiate with one of the remaining parameters.
(There may not be any known parameters remaining, in which case parameter negotiation will fail.)<a href="#section-4.2-1" class="pilcrow">¶</a></p>
<p id="section-4.2-2">Note that these requirements are restatements or corollaries of
     existing client requirements in TLS.<a href="#section-4.2-2" class="pilcrow">¶</a></p>
</section>
</section>
<section id="section-5">
      <h2 id="name-sending-grease-values">
<a href="#section-5" class="section-number selfRef">5. </a><a href="#name-sending-grease-values" class="section-name selfRef">Sending GREASE Values</a>
      </h2>
<p id="section-5-1">Implementations advertising GREASE values <span class="bcp14">SHOULD</span> select them at random.
This is intended to encourage implementations to ignore all unknown values
rather than any individual value. Implementations <span class="bcp14">MUST</span> honor protocol
specifications when sending GREASE values. For instance, <span><a href="https://www.rfc-editor.org/rfc/rfc8446#section-4.2" class="relref">Section 4.2</a> of [<a href="#RFC8446" class="xref">RFC8446</a>]</span> forbids duplicate extension types within a single
extension block. Implementations sending multiple GREASE extensions in a single
block must therefore ensure the same value is not selected twice.<a href="#section-5-1" class="pilcrow">¶</a></p>
<p id="section-5-2">Implementations <span class="bcp14">SHOULD</span> balance diversity in GREASE advertisements with determinism.
  For example, a client that randomly varies GREASE value positions for each
  connection may only fail against a broken server with some probability. This
  risks the failure being masked by automatic retries. A client that positions
  GREASE values deterministically over a period of time (such as a single
  software release) stresses fewer cases but is more likely to detect bugs from
  those cases.<a href="#section-5-2" class="pilcrow">¶</a></p>
</section>
<div id="IANA">
<section id="section-6">
      <h2 id="name-iana-considerations">
<a href="#section-6" class="section-number selfRef">6. </a><a href="#name-iana-considerations" class="section-name selfRef">IANA Considerations</a>
      </h2>
<p id="section-6-1">This document updates the "TLS Cipher Suites" registry, available at
     <span>&lt;<a href="https://www.iana.org/assignments/tls-parameters">https://www.iana.org/assignments/tls-parameters</a>&gt;</span>:<a href="#section-6-1" class="pilcrow">¶</a></p>
<span id="name-additions-to-the-tls-cipher"></span><table class="center" id="table-1">
        <caption>
<a href="#table-1" class="selfRef">Table 1</a>:
<a href="#name-additions-to-the-tls-cipher" class="selfRef">Additions to the TLS Cipher Suites Registry</a>
        </caption>
<thead>
          <tr>
            <th class="text-center" rowspan="1" colspan="1">Value</th>
            <th class="text-center" rowspan="1" colspan="1">Description</th>
            <th class="text-center" rowspan="1" colspan="1">DTLS-OK</th>
            <th class="text-center" rowspan="1" colspan="1">Recommended</th>
            <th class="text-center" rowspan="1" colspan="1">Reference</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">{0x0A,0x0A}</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">{0x1A,0x1A}</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">{0x2A,0x2A}</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">{0x3A,0x3A}</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">{0x4A,0x4A}</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">{0x5A,0x5A}</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">{0x6A,0x6A}</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">{0x7A,0x7A}</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">{0x8A,0x8A}</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">{0x9A,0x9A}</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">{0xAA,0xAA}</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">{0xBA,0xBA}</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">{0xCA,0xCA}</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">{0xDA,0xDA}</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">{0xEA,0xEA}</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">{0xFA,0xFA}</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
        </tbody>
      </table>
<p id="section-6-3">This document updates the "TLS Supported Groups" registry, available at
<span>&lt;<a href="https://www.iana.org/assignments/tls-parameters">https://www.iana.org/assignments/tls-parameters</a>&gt;</span>:<a href="#section-6-3" class="pilcrow">¶</a></p>
<span id="name-additions-to-the-tls-suppor"></span><table class="center" id="table-2">
        <caption>
<a href="#table-2" class="selfRef">Table 2</a>:
<a href="#name-additions-to-the-tls-suppor" class="selfRef">Additions to the TLS Supported Groups Registry</a>
        </caption>
<thead>
          <tr>
            <th class="text-center" rowspan="1" colspan="1">Value</th>
            <th class="text-center" rowspan="1" colspan="1">Description</th>
            <th class="text-center" rowspan="1" colspan="1">DTLS-OK</th>
            <th class="text-center" rowspan="1" colspan="1">Recommended</th>
            <th class="text-center" rowspan="1" colspan="1">Reference</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">2570</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">6682</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">10794</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">14906</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">19018</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">23130</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">27242</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">31354</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">35466</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">39578</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">43690</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">47802</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">51914</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">56026</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">60138</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">64250</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">Y</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
        </tbody>
      </table>
<p id="section-6-5">This document updates the "TLS ExtensionType Values" registry, available at
<span>&lt;<a href="https://www.iana.org/assignments/tls-extensiontype-values">https://www.iana.org/assignments/tls-extensiontype-values</a>&gt;</span>:<a href="#section-6-5" class="pilcrow">¶</a></p>
<span id="name-additions-to-the-tls-extens"></span><table class="center" id="table-3">
        <caption>
<a href="#table-3" class="selfRef">Table 3</a>:
<a href="#name-additions-to-the-tls-extens" class="selfRef">Additions to the TLS ExtensionType Values Registry</a>
        </caption>
<thead>
          <tr>
            <th class="text-center" rowspan="1" colspan="1">Value</th>
            <th class="text-center" rowspan="1" colspan="1">Extension Name</th>
            <th class="text-center" rowspan="1" colspan="1">TLS 1.3</th>
            <th class="text-center" rowspan="1" colspan="1">Recommended</th>
            <th class="text-center" rowspan="1" colspan="1">Reference</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">2570</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">CH, CR, NST</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">6682</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">CH, CR, NST</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">10794</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">CH, CR, NST</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">14906</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">CH, CR, NST</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">19018</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">CH, CR, NST</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">23130</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">CH, CR, NST</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">27242</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">CH, CR, NST</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">31354</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">CH, CR, NST</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">35466</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">CH, CR, NST</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">39578</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">CH, CR, NST</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">43690</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">CH, CR, NST</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">47802</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">CH, CR, NST</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">51914</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">CH, CR, NST</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">56026</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">CH, CR, NST</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">60138</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">CH, CR, NST</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">64250</td>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">CH, CR, NST</td>
            <td class="text-center" rowspan="1" colspan="1">N</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
        </tbody>
      </table>
<p id="section-6-7">This document updates the "TLS Application-Layer Protocol Negotiation
       (ALPN) Protocol IDs" registry, available at
       <span>&lt;<a href="https://www.iana.org/assignments/tls-extensiontype-values">https://www.iana.org/assignments/tls-extensiontype-values</a>&gt;</span>:<a href="#section-6-7" class="pilcrow">¶</a></p>
<span id="name-additions-to-the-tls-applic"></span><table class="center" id="table-4">
        <caption>
<a href="#table-4" class="selfRef">Table 4</a>:
<a href="#name-additions-to-the-tls-applic" class="selfRef">Additions to the TLS Application-Layer Protocol Negotiation
 (ALPN) Protocol IDs Registry</a>
        </caption>
<thead>
          <tr>
            <th class="text-center" rowspan="1" colspan="1">Protocol</th>
            <th class="text-center" rowspan="1" colspan="1">Identification Sequence</th>
            <th class="text-center" rowspan="1" colspan="1">Reference</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">0x0A 0x0A</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">0x1A 0x1A</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">0x2A 0x2A</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">0x3A 0x3A</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">0x4A 0x4A</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">0x5A 0x5A</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">0x6A 0x6A</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">0x7A 0x7A</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">0x8A 0x8A</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">0x9A 0x9A</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">0xAA 0xAA</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">0xBA 0xBA</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">0xCA 0xCA</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">0xDA 0xDA</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">0xEA 0xEA</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">Reserved</td>
            <td class="text-center" rowspan="1" colspan="1">0xFA 0xFA</td>
            <td class="text-center" rowspan="1" colspan="1">[RFC8701]</td>
          </tr>
        </tbody>
      </table>
</section>
</div>
<div id="Security">
<section id="section-7">
      <h2 id="name-security-considerations">
<a href="#section-7" class="section-number selfRef">7. </a><a href="#name-security-considerations" class="section-name selfRef">Security Considerations</a>
      </h2>
<p id="section-7-1">GREASE values cannot be negotiated, so they do not directly impact the
     security of TLS connections.<a href="#section-7-1" class="pilcrow">¶</a></p>
<p id="section-7-2">Historically, when interoperability problems arise in deploying new TLS
     features, implementations have used a fallback retry on error with the
     feature disabled. This allows an active attacker to silently disable the
     new feature. By preventing a class of such interoperability problems,
     GREASE reduces the need for this kind of fallback. Implementations <span class="bcp14">SHOULD NOT</span> retry with GREASE disabled on connection failure. While allowing an
     attacker to disable GREASE is unlikely to have immediate security
     consequences, such a fallback would prevent GREASE from defending against
     extensibility failures.<a href="#section-7-2" class="pilcrow">¶</a></p>
<p id="section-7-3">If an implementation does not select GREASE values at random, it is 
     possible it will allow for fingerprinting of the implementation or 
     perhaps even of individual users. This can result in a negative impact to
     a user's privacy.<a href="#section-7-3" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-8">
      <h2 id="name-normative-references">
<a href="#section-8" class="section-number selfRef">8. </a><a href="#name-normative-references" class="section-name selfRef">Normative References</a>
      </h2>
<dl class="references">
<dt id="RFC2119">[RFC2119]</dt>
<dd>
<span class="refAuthor">Bradner, S.</span>, <span class="refTitle">"Key words for use in RFCs to Indicate Requirement Levels"</span>, <span class="seriesInfo">BCP 14</span>, <span class="seriesInfo">RFC 2119</span>, <span class="seriesInfo">DOI 10.17487/RFC2119</span>, <time datetime="1997-03">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>
<dt id="RFC5246">[RFC5246]</dt>
<dd>
<span class="refAuthor">Dierks, T.</span><span class="refAuthor"> and E. Rescorla</span>, <span class="refTitle">"The Transport Layer Security (TLS) Protocol Version 1.2"</span>, <span class="seriesInfo">RFC 5246</span>, <span class="seriesInfo">DOI 10.17487/RFC5246</span>, <time datetime="2008-08">August 2008</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc5246">https://www.rfc-editor.org/info/rfc5246</a>&gt;</span>. </dd>
<dt id="RFC6347">[RFC6347]</dt>
<dd>
<span class="refAuthor">Rescorla, E.</span><span class="refAuthor"> and N. Modadugu</span>, <span class="refTitle">"Datagram Transport Layer Security Version 1.2"</span>, <span class="seriesInfo">RFC 6347</span>, <span class="seriesInfo">DOI 10.17487/RFC6347</span>, <time datetime="2012-01">January 2012</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6347">https://www.rfc-editor.org/info/rfc6347</a>&gt;</span>. </dd>
<dt id="RFC7301">[RFC7301]</dt>
<dd>
<span class="refAuthor">Friedl, S.</span><span class="refAuthor">, Popov, A.</span><span class="refAuthor">, Langley, A.</span><span class="refAuthor">, and E. Stephan</span>, <span class="refTitle">"Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension"</span>, <span class="seriesInfo">RFC 7301</span>, <span class="seriesInfo">DOI 10.17487/RFC7301</span>, <time datetime="2014-07">July 2014</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7301">https://www.rfc-editor.org/info/rfc7301</a>&gt;</span>. </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">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>
<dt id="RFC8446">[RFC8446]</dt>
<dd>
<span class="refAuthor">Rescorla, E.</span>, <span class="refTitle">"The Transport Layer Security (TLS) Protocol Version 1.3"</span>, <span class="seriesInfo">RFC 8446</span>, <span class="seriesInfo">DOI 10.17487/RFC8446</span>, <time datetime="2018-08">August 2018</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8446">https://www.rfc-editor.org/info/rfc8446</a>&gt;</span>. </dd>
</dl>
</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">
The author would like to thank <span class="contact-name">Adam Langley</span>, <span class="contact-name">Nick Harper</span>, and <span class="contact-name">Steven Valdez</span> for their
feedback and suggestions. In addition, the rusted joint metaphor is originally
due to <span class="contact-name">Adam Langley</span>.<a href="#section-appendix.a-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="authors-addresses">
<section id="section-appendix.b">
      <h2 id="name-authors-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">David Benjamin</span></div>
<div dir="auto" class="left"><span class="org">Google LLC</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:davidben@google.com" class="email">davidben@google.com</a>
</div>
</address>
</section>
</div>
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