File: rfc9187.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 9187: Sequence Number Extension for Windowed Protocols</title>
<meta content="Joe Touch" name="author">
<meta content="
       
   Sliding window protocols use finite sequence numbers to determine
   segment placement and order. These sequence number spaces wrap
   around and are reused during the operation of such protocols. This
   document describes a way to extend the size of these sequence
   numbers at the endpoints to avoid the impact of that wrap and reuse
   without transmitting additional information in the packet header.
   The resulting extended sequence numbers can be used at the endpoints
   in encryption and authentication algorithms to ensure input bit
   patterns do not repeat over the lifetime of a connection. 
    " name="description">
<meta content="xml2rfc 3.12.0" name="generator">
<meta content="TCP-AO" name="keyword">
<meta content="TCP" name="keyword">
<meta content="9187" name="rfc.number">
<!-- Generator version information:
  xml2rfc 3.12.0
    Python 3.6.15
    appdirs 1.4.4
    ConfigArgParse 1.4.1
    google-i18n-address 2.4.0
    html5lib 1.0.1
    intervaltree 3.0.2
    Jinja2 2.11.3
    kitchen 1.2.6
    lxml 4.4.2
    pycairo 1.15.1
    pycountry 19.8.18
    pyflakes 2.1.1
    PyYAML 5.4.1
    requests 2.24.0
    setuptools 40.5.0
    six 1.14.0
    WeasyPrint 52.5
-->
<link href="rfc9187.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.ulBare, li.ulBare {
  margin-left: 0em !important;
}
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;
}
/* Fix PDF info block run off issue */
@media print {
  #identifiers dd {
    float: none;
  }
}
#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 {
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<table class="ears">
<thead><tr>
<td class="left">RFC 9187</td>
<td class="center">Sequence Number Extension</td>
<td class="right">January 2022</td>
</tr></thead>
<tfoot><tr>
<td class="left">Touch</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">Independent Submission</dd>
<dt class="label-rfc">RFC:</dt>
<dd class="rfc"><a href="https://www.rfc-editor.org/rfc/rfc9187" class="eref">9187</a></dd>
<dt class="label-category">Category:</dt>
<dd class="category">Informational</dd>
<dt class="label-published">Published:</dt>
<dd class="published">
<time datetime="2022-01" class="published">January 2022</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">J. Touch</div>
<div class="org">Independent Consultant</div>
</div>
</dd>
</dl>
</div>
<h1 id="rfcnum">RFC 9187</h1>
<h1 id="title">Sequence Number Extension for Windowed Protocols</h1>
<section id="section-abstract">
      <h2 id="abstract"><a href="#abstract" class="selfRef">Abstract</a></h2>
<p id="section-abstract-1">
   Sliding window protocols use finite sequence numbers to determine
   segment placement and order. These sequence number spaces wrap
   around and are reused during the operation of such protocols. This
   document describes a way to extend the size of these sequence
   numbers at the endpoints to avoid the impact of that wrap and reuse
   without transmitting additional information in the packet header.
   The resulting extended sequence numbers can be used at the endpoints
   in encryption and authentication algorithms to ensure input bit
   patterns do not repeat over the lifetime of a connection.<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 is a contribution to the RFC Series, independently of any
            other RFC stream.  The RFC Editor has chosen to publish this
            document at its discretion and makes no statement about its value
            for implementation or deployment.  Documents approved for
            publication by the RFC Editor are not 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/rfc9187">https://www.rfc-editor.org/info/rfc9187</a></span>.<a href="#section-boilerplate.1-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="copyright">
<section id="section-boilerplate.2">
        <h2 id="name-copyright-notice">
<a href="#name-copyright-notice" class="section-name selfRef">Copyright Notice</a>
        </h2>
<p id="section-boilerplate.2-1">
            Copyright (c) 2022 IETF Trust and the persons identified as the
            document authors. All rights reserved.<a href="#section-boilerplate.2-1" class="pilcrow">¶</a></p>
<p id="section-boilerplate.2-2">
            This document is subject to BCP 78 and the IETF Trust's Legal
            Provisions Relating to IETF Documents
            (<span><a href="https://trustee.ietf.org/license-info">https://trustee.ietf.org/license-info</a></span>) in effect on the date of
            publication of this document. Please review these documents
            carefully, as they describe your rights and restrictions with
            respect to this document.<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 ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.1">
            <p id="section-toc.1-1.1.1" class="keepWithNext"><a href="#section-1" class="xref">1</a>.  <a href="#name-introduction" class="xref">Introduction</a></p>
</li>
          <li class="compact toc ulBare 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-background" class="xref">Background</a></p>
</li>
          <li class="compact toc ulBare 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-related-discussion" class="xref">Related Discussion</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.4">
            <p id="section-toc.1-1.4.1"><a href="#section-4" class="xref">4</a>.  <a href="#name-using-sne-in-protocol-desig" class="xref">Using SNE in Protocol Design</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.5">
            <p id="section-toc.1-1.5.1"><a href="#section-5" class="xref">5</a>.  <a href="#name-example-code" class="xref">Example Code</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.6">
            <p id="section-toc.1-1.6.1"><a href="#section-6" class="xref">6</a>.  <a href="#name-validation-suite" class="xref">Validation Suite</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7">
            <p id="section-toc.1-1.7.1"><a href="#section-7" class="xref">7</a>.  <a href="#name-security-considerations" class="xref">Security Considerations</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.8">
            <p id="section-toc.1-1.8.1"><a href="#section-8" class="xref">8</a>.  <a href="#name-iana-considerations" class="xref">IANA Considerations</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.9">
            <p id="section-toc.1-1.9.1"><a href="#section-9" class="xref">9</a>.  <a href="#name-informative-references" class="xref">Informative References</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.10">
            <p id="section-toc.1-1.10.1"><a href="#appendix-A" class="xref"></a><a href="#name-acknowledgments" class="xref">Acknowledgments</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.11">
            <p id="section-toc.1-1.11.1"><a href="#appendix-B" class="xref"></a><a href="#name-authors-address" class="xref">Author's Address</a></p>
</li>
        </ul>
</nav>
</section>
</div>
<div id="sect-1">
<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">
   Protocols use sequence numbers to maintain ordering and, in sliding
   window systems, to control the amount of outstanding unacknowledged
   information. These sequence numbers are finite and thus commonly
   wrap around during long connections, reusing past values.<a href="#section-1-1" class="pilcrow">¶</a></p>
<p id="section-1-2">
   It can be useful for protocols to keep track of this wrap around in
   a separate counter, such that the sequence number and counter
   together form an equivalent number space that need not wrap. This
   technique was introduced as "Sequence Number Extension" in the TCP Authentication Option (TCP-AO)
   <span>[<a href="#RFC5925" class="xref">RFC5925</a>]</span>. The example provided there was intended to introduce the
   concept, but the pseudocode provided is not complete.<a href="#section-1-2" class="pilcrow">¶</a></p>
<p id="section-1-3">
   This document presents the formal requirements for Sequence Number
   Extension (SNE), a code example, and a check sequence that can be
   used to validate this and alternate implementations. Sequence
   numbers are used in a variety of protocols to support loss
   detection, reordering, flow control, and congestion control.
   Limitations in the size of a sequence number protocol field can
   limit the ways in which these capabilities can be supported.<a href="#section-1-3" class="pilcrow">¶</a></p>
<p id="section-1-4">
   Under certain conditions, it is possible for both endpoints of a
   protocol to keep track of sequence number rollover and effectively
   extend the sequence number space without requiring modification of
   the sequence number field used within protocol messages. These
   conditions assume that the received sequence numbers never vary by
   more than half the size of the space of the field used in messages,
   i.e., they never hop forward or backward by more than half that
   space. This constraint is typical in sliding window protocols, such
   as TCP. However, although both ends can track rollover
   unambiguously, doing so can be surprisingly complex. This document
   provides examples and test cases to simplify that process.<a href="#section-1-4" class="pilcrow">¶</a></p>
<p id="section-1-5">
   This document is intended for protocol designers who seek to use
   larger sequence numbers at the endpoints without needing to extend
   the sequence number field used in messages, such as for
   authentication protocols, e.g., TCP-AO <span>[<a href="#RFC5925" class="xref">RFC5925</a>]</span>. Use of extended
   sequence numbers should be part of a protocol specification so that
   both endpoints can ensure they comply with the requirements needed
   to enable their use in both locations.<a href="#section-1-5" class="pilcrow">¶</a></p>
<p id="section-1-6">
 The remainder of this document describes how SNE can be supported and provides the
 pseudocode to
   demonstrate how received messages can unambiguously determine the
   appropriate extension value, as long as the reordering is
   constrained. <a href="#sect-2" class="xref">Section 2</a>  provides background on the concept. <a href="#sect-3" class="xref">Section 3</a> discusses currently known uses of SNE. <a href="#sect-4" class="xref">Section 4</a> discusses how SNE
   is used in protocol design and how it differs from in-band use of
   sequence numbers. <a href="#sect-5" class="xref">Section 5</a> provides a framework for testing SNE
   implementations, including example code for the SNE function, and 
   <a href="#sect-6" class="xref">Section 6</a> provides a sequence that can be used by that code for
   validation. <a href="#sect-7" class="xref">Section 7</a> concludes with a discussion of security
   issues.<a href="#section-1-6" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sect-2">
<section id="section-2">
      <h2 id="name-background">
<a href="#section-2" class="section-number selfRef">2. </a><a href="#name-background" class="section-name selfRef">Background</a>
      </h2>
<p id="section-2-1">
   Protocols use sequence numbers to maintain message order. The
   transmitter typically increments them either once per message or by
   the length of the message. The receiver uses them to reorder
   messages and detect gaps due to inferred loss.<a href="#section-2-1" class="pilcrow">¶</a></p>
<p id="section-2-2">
   Sequence numbers are represented within those messages (e.g., in the
   headers) as values of a finite, unsigned number space. This space is
   typically represented in a fixed-length bit string, whose values
   range from 0..(2<sup>N</sup>)-1, inclusive.<a href="#section-2-2" class="pilcrow">¶</a></p>
<p id="section-2-3">
   The use of finite representations has repercussions on the use of
   these values at both the transmitter and receiver. Without
   additional constraints, when the number space "wraps around", it
   would be impossible for the receiver to distinguish between the uses
   of the same value.<a href="#section-2-3" class="pilcrow">¶</a></p>
<p id="section-2-4">
   As a consequence, additional constraints are required. Transmitters
   are typically required to limit reuse until they can assume that
   receivers would successfully differentiate the two uses of the same
   value. The receiver always interprets values it sees based on the
   assumption that successive values never differ by just under half
   the number space. A receiver cannot detect an error in that
   sequence, but it will incorrectly interpret numbers if reordering
   violates this constraint.<a href="#section-2-4" class="pilcrow">¶</a></p>
<p id="section-2-5">
   The constraint requires that "forward" values advance the values by
   less than half the sequence number space, ensuring that receivers
   never experience a series of values that violate that rule.<a href="#section-2-5" class="pilcrow">¶</a></p>
<p id="section-2-6">
   We define a sequence space as follows:<a href="#section-2-6" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-2-7.1">An unsigned integer within the range of 0..(2<sup>N</sup>)-1, i.e., for N bits.<a href="#section-2-7.1" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-2-7.2">An operation that increments values in that space by K, where K &lt; 2<sup>(N-1)</sup>, i.e., less than half the range. This operation is used exclusively by the transmitter.<a href="#section-2-7.2" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-2-7.3">An operation that compares two values in that space to determine
     their order, e.g., where X &lt; Y implies that X comes before Y.<a href="#section-2-7.3" class="pilcrow">¶</a>
</li>
      </ul>
<p id="section-2-8">
   We assume that both sides begin with the same initial value, which can be
   anywhere in the space. That value is either assumed (e.g., 0) before the
   protocol begins or coordinated before other messages are exchanged (as
   with TCP Initial Sequence Numbers (ISNs) <span>[<a href="#RFC0793" class="xref">RFC0793</a>]</span>). It is assumed that the receiver always receives values that
   are always within (2<sup>N</sup>)-1, but the successive received values never jump
   forward or backward by more than 2<sup>(N-1)</sup>-1, i.e., just under half the
   range.<a href="#section-2-8" class="pilcrow">¶</a></p>
<p id="section-2-9">
   No other operations are supported. The transmitter is not permitted
   to "backup", such that values are always used in "increment" order.
   The receiver cannot experience loss or gaps larger than 2<sup>(N-1)</sup>-1,
   which is typically enforced either by assumption or by explicit
   endpoint coordination.<a href="#section-2-9" class="pilcrow">¶</a></p>
<p id="section-2-10">
   An SNE is a separate number space that
   can be combined with the sequence number to create a larger number
   space that need not wrap around during a connection.<a href="#section-2-10" class="pilcrow">¶</a></p>
<p id="section-2-11">
   On the transmit side, SNE is trivially accomplished by incrementing a local
   counter once each time the sequence number increment "wraps" around or by
   keeping a larger local sequence number whose least-significant part is the
   message sequence number and most-significant part can be considered the
   SNE. The transmitter typically does not need to maintain an SNE except when
   used in local computations, such as for Message Authentication Codes (MACs) in TCP-AO <span>[<a href="#RFC5925" class="xref">RFC5925</a>]</span>.<a href="#section-2-11" class="pilcrow">¶</a></p>
<p id="section-2-12">
   The goal of this document is to demonstrate that SNE can be
   accomplished on the receiver side without transmitting additional
   information in messages. It defines the stateful function
   compute_sne() as follows:<a href="#section-2-12" class="pilcrow">¶</a></p>
<p style="margin-left: 3.0em" id="section-2-13">SNE = compute_sne(seqno)<a href="#section-2-13" class="pilcrow">¶</a></p>
<p id="section-2-14">The compute_sne() function accepts the sequence number seen in a
   received message
   and computes the corresponding SNE. The function includes persistent
   local state that tracks the largest currently received SNE and seqno
   combination. The concatenation of SNE and seqno emulates the
   equivalent larger sequence number space that can avoid wrap around.<a href="#section-2-14" class="pilcrow">¶</a></p>
<p id="section-2-15">
   Note that the function defined here is capable of receiving any
   series of seqno values and computing their correct corresponding
   SNE, as long as the series never "jumps" more than half the number
   space "backward" from the largest value seen "forward".<a href="#section-2-15" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sect-3">
<section id="section-3">
      <h2 id="name-related-discussion">
<a href="#section-3" class="section-number selfRef">3. </a><a href="#name-related-discussion" class="section-name selfRef">Related Discussion</a>
      </h2>
<p id="section-3-1">
   The DNS uses sequence numbers to determine when a Start of Authority
   (SOA) serial number is more recent than a previous one, even
   considering sequence space wrap <span>[<a href="#RFC1034" class="xref">RFC1034</a>]</span><span>[<a href="#RFC1035" class="xref">RFC1035</a>]</span>. The use of
   wrapped sequence numbers for sliding windows in network protocols
   was first described as a sequence number space <span>[<a href="#IEN74" class="xref">IEN74</a>]</span>.<a href="#section-3-1" class="pilcrow">¶</a></p>
<p id="section-3-2">
   A more recent discussion describes this as "serial number arithmetic" and defines a comparison operator it claimed was missing
   in IEN-74 <span>[<a href="#RFC1982" class="xref">RFC1982</a>]</span>. That document defines two operations: addition
   (presumably shifting the window forward) and comparison (defining
   the order of two values). Addition is defined in that document as
   limited to values within the range of 0..windowsize/2-1. Comparison is
   defined in that
   document by a set of equations therein, but that document does not
   provide a way for a receiver to compute the correct equivalent SNE,
   especially including the potential for sequence number reordering,
   as is demonstrated in this document.<a href="#section-3-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sect-4">
<section id="section-4">
      <h2 id="name-using-sne-in-protocol-desig">
<a href="#section-4" class="section-number selfRef">4. </a><a href="#name-using-sne-in-protocol-desig" class="section-name selfRef">Using SNE in Protocol Design</a>
      </h2>
<p id="section-4-1">
   As noted in the introduction, message sequence numbers enable
   reordering, loss detection, flow control, and congestion control.
   They are also used to differentiate otherwise potentially identical
   messages that might repeat as part of a sequence or stream.<a href="#section-4-1" class="pilcrow">¶</a></p>
<p id="section-4-2">
   The size of the sequence number field used within transferred messages
   defines the ability of a protocol to tolerate reordering and gaps,
   notably limited to half the space of that field. For example, a field of 8
   bits can reorder and detect losses of smaller than 2<sup>7</sup>, i.e., 127
   messages. When used for these purposes -- reordering, loss detection,
   flow control, and congestion control -- the size of the field defines
   the limits of those capabilities.<a href="#section-4-2" class="pilcrow">¶</a></p>
<p id="section-4-3">
   Sequence numbers are also used to differentiate messages; when used
   this way, they can be problematic if they repeat for otherwise
   identical messages. Protocols using sequence numbers tolerate that
   repetition because they are aware of the rollover of these sequence
   number spaces at both endpoints. In some cases, it can be useful to
   track this rollover and use the rollover count as an extension to
   the sequence number, e.g., to differentiate authentication MACs.
   This SNE is never transmitted in
   messages; the existing rules of sequence numbers ensure both ends can
   keep track unambiguously -- both for new messages and reordered
   messages.<a href="#section-4-3" class="pilcrow">¶</a></p>
<p id="section-4-4">
   The constraints required to use SNE have already been presented as
   background in <a href="#sect-2" class="xref">Section 2</a>. The transmitter must never send messages
   out of sequence beyond half the range of the sequence number field
   used in messages. A receiver uses this assumption to interpret
   whether received numbers are part of pre-wrap sequences or post-wrap
   sequences. Note that a receiver cannot enforce or detect if the
   transmitter has violated these assumptions on its own; it relies on
   explicit coordination to ensure this property is maintained, such as
   the exchange of acknowledgements.<a href="#section-4-4" class="pilcrow">¶</a></p>
<p id="section-4-5">
   SNEs are intended for use when it is helpful for both ends to
   unambiguously determine whether the sequence number in a message has
   wrapped and whether a received message is pre-wrap or post-wrap for
   each such wrap. This can be used by both endpoints to ensure all
   messages of arbitrarily long sequences can be differentiated, e.g.,
   ensuring unique MACs.<a href="#section-4-5" class="pilcrow">¶</a></p>
<p id="section-4-6">
   SNE does not extend the actual sequence space of a protocol or
   (thus) its tolerance to reordering or gaps. It also cannot improve
   its dynamic range for flow control or congestion control, although
   there are other somewhat related methods that can, such as window
   scaling <span>[<a href="#RFC7323" class="xref">RFC7323</a>]</span> (which increases range at the expense of
   granularity).<a href="#section-4-6" class="pilcrow">¶</a></p>
<p id="section-4-7">
   SNE is not needed if messages are already unique over the entirety
   of a transfer sequence, e.g., either because the sequence number
   field used in its messages never wrap around or because other fields
   provide that disambiguation, such as timestamps.<a href="#section-4-7" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sect-5">
<section id="section-5">
      <h2 id="name-example-code">
<a href="#section-5" class="section-number selfRef">5. </a><a href="#name-example-code" class="section-name selfRef">Example Code</a>
      </h2>
<p id="section-5-1">
   The following C code is provided as a verified example of SNE
   from 16 to 32 bits. The code includes both the
   framework used for validation and the compute_sne() function, the
   latter of which can be used operationally.<a href="#section-5-1" class="pilcrow">¶</a></p>
<p id="section-5-2">
   A correct test will indicate "OK" for each test. An incorrect test
      will indicate "ERROR" where applicable.<a href="#section-5-2" class="pilcrow">¶</a></p>
<div id="section-5-3">
<pre class="lang-c sourcecode">&lt;CODE BEGINS&gt; file "compute_sne.c"

#include &lt;stdio.h&gt;
#include &lt;sys/param.h&gt;

#define distance(x,y)   (((x)&lt;(y))?((y)-(x)):((x)-(y)))

#define SNEDEBUG 1

// This is the core code, stand-alone, to compute SNE from seqno
// &gt;&gt; replace this function with your own code to test alternates
unsigned long compute_sne(unsigned long seqno) {
    // INPUT: 32-bit unsigned sequence number (low bits)
    // OUTPUT: 32-bit unsigned SNE (high bits)

    // variables used in this code example to compute SNE:

    static unsigned long
      RCV_SNE = 0;        // high-watermark SNE
    static int
      RCV_SNE_FLAG = 1;   // set during first half rollover
                          // (prevents re-rollover)
    static unsigned long
      RCV_PREV_SEQ = 0;   // high-watermark SEQ
    unsigned long
      holdSNE;            // temp copy of output

    holdSNE = RCV_SNE;                // use current SNE to start
    if (distance(seqno,RCV_PREV_SEQ) &lt; 0x80000000) {
        // both in same SNE range?
        if ((seqno &gt;= 0x80000000) &amp;&amp; (RCV_PREV_SEQ &lt; 0x80000000)) {
            // jumps fwd over N/2?
            RCV_SNE_FLAG = 0;         // reset wrap increment flag
        }
        RCV_PREV_SEQ = MAX(seqno,RCV_PREV_SEQ);
                                      // move prev forward if needed
    } else {
            // both in diff SNE ranges
            if (seqno &lt; 0x80000000) {
                // jumps forward over zero?
                RCV_PREV_SEQ = seqno; // update prev
                if (RCV_SNE_FLAG == 0) {
                    // first jump over zero? (wrap)
                    RCV_SNE_FLAG = 1;
                              // set flag so we increment once
                    RCV_SNE = RCV_SNE + 1;
                              // increment window
                    holdSNE = RCV_SNE;
                              // use updated SNE value
                }
            } else {
                // jump backward over zero
                holdSNE = RCV_SNE - 1;
                              // use pre-rollover SNE value
            }
    }
    #ifdef SNEDEBUG
    fprintf(stderr,"state RCV_SNE_FLAG =        %1d\n",
      RCV_SNE_FLAG);
    fprintf(stderr,"state      RCV_SNE = %08lx\n", RCV_SNE);
    fprintf(stderr,"state RCV_PREV_SEQ = %08lx\n", RCV_PREV_SEQ);
    #endif
    return holdSNE;
}

int main() {
    // variables used as input and output:
    unsigned long SEG_SEQ;        // input - received SEQ
    unsigned long SNE;            // output - SNE corresponding
                                  // to received SEQ

    // variables used to validate the computed SNE:
    unsigned long SEG_HIGH;       // input - xmitter side SNE
                                  // -&gt; SNE should match this value
    unsigned long long BIG_PREV;  // prev 64-bit total seqno
    unsigned long long BIG_THIS = 0;  // current 64-bit total seqno
                                  // -&gt; THIS, PREV should never jump
                                  //    more than half the SEQ space

   char *prompt = "Input hex numbers only (0x is optional)\n\n")
                  "\tHex input\n"
                  "\t(2 hex numbers separated by whitespace,"
                  "each with 8 or fewer digits)";

    fprintf(stderr,"%s\n",prompt);

    while (scanf("%lx %lx",&amp;SEG_HIGH,&amp;SEG_SEQ) == 2) {
        BIG_PREV = BIG_THIS;
        BIG_THIS = (((unsigned long long)SEG_HIGH) &lt;&lt; 32)
                  | ((unsigned long long)SEG_SEQ);

        // given SEG_SEQ, compute SNE
        SNE = compute_sne(SEG_SEQ);

        fprintf(stderr,"           SEG_SEQ = %08lx\n", SEG_SEQ);
        fprintf(stderr,"               SNE = %08lx\n", SNE);
        fprintf(stderr,"          SEG_HIGH = %08lx %s\n",SEG_HIGH,
                (SEG_HIGH == SNE)? " - OK" : " - ERROR !!!!!!!");
        fprintf(stderr,"\t\tthe jump was %16llx %s %s\n",
                distance(BIG_PREV,BIG_THIS),
                ((BIG_PREV &lt; BIG_THIS)?"+":"-"),
                (((distance(BIG_PREV,BIG_THIS)) &gt; 0x7FFFFFFF)
                 ? "ILLEGAL JUMP" : "."));
        fprintf(stderr,"\n");
        fprintf(stderr,"\n");

        fprintf(stderr,"%s\n",prompt);

    }
}

&lt;CODE ENDS&gt;</pre><a href="#section-5-3" class="pilcrow">¶</a>
</div>
</section>
</div>
<div id="sect-6">
<section id="section-6">
      <h2 id="name-validation-suite">
<a href="#section-6" class="section-number selfRef">6. </a><a href="#name-validation-suite" class="section-name selfRef">Validation Suite</a>
      </h2>
<p id="section-6-1">
   The following numbers are used to validate SNE
   variants and are shown in the order they legitimately could be
   received. Each line represents a single 64-bit number, represented
   as two hexadecimal 32-bit numbers with a space between. The numbers
   are formatted for use in the example code provided in <a href="#sect-5" class="xref">Section 5</a>.<a href="#section-6-1" class="pilcrow">¶</a></p>
<p id="section-6-2">
   A correctly operating extended sequence number system can receive
   the least-significant half (the right side) and compute the correct
   most-significant half (the left side) correctly. It specifically
   tests both forward and backward jumps in received values that
   represent legitimate reordering.<a href="#section-6-2" class="pilcrow">¶</a></p>
<div id="section-6-3">
<pre class="lang-test-vector sourcecode">
00000000 00000000
00000000 30000000
00000000 90000000
00000000 70000000
00000000 a0000000
00000001 00000001
00000000 e0000000
00000001 00000000
00000001 7fffffff
00000001 00000000
00000001 50000000
00000001 80000000
00000001 00000001
00000001 40000000
00000001 90000000
00000001 b0000000
00000002 0fffffff
00000002 20000000
00000002 90000000
00000002 70000000
00000002 A0000000
00000003 00004000
00000002 D0000000
00000003 20000000
00000003 90000000
00000003 70000000
00000003 A0000000
00000004 00004000
00000003 D0000000
</pre><a href="#section-6-3" class="pilcrow">¶</a>
</div>
</section>
</div>
<div id="sect-7">
<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">
   Sequence numbers and their extensions can be useful in a variety of
   security contexts. Because the extension part (most-significant
   half) is determined by the previously exchanged sequence values
   (least-significant half), the extension should not be considered as
   adding entropy for the purposes of message authentication or
   encryption.<a href="#section-7-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sect-8">
<section id="section-8">
      <h2 id="name-iana-considerations">
<a href="#section-8" class="section-number selfRef">8. </a><a href="#name-iana-considerations" class="section-name selfRef">IANA Considerations</a>
      </h2>
<p id="section-8-1">This document has no IANA actions.<a href="#section-8-1" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-9">
      <h2 id="name-informative-references">
<a href="#section-9" class="section-number selfRef">9. </a><a href="#name-informative-references" class="section-name selfRef">Informative References</a>
      </h2>
<dl class="references">
<dt id="IEN74">[IEN74]</dt>
      <dd>
<span class="refAuthor">Plummmer, W.</span>, <span class="refTitle">"Sequence Number Arithmetic"</span>, <span class="refContent">IEN-74</span>, <time datetime="1978-09" class="refDate">September 1978</time>. </dd>
<dd class="break"></dd>
<dt id="RFC0793">[RFC0793]</dt>
      <dd>
<span class="refAuthor">Postel, J.</span>, <span class="refTitle">"Transmission Control Protocol"</span>, <span class="seriesInfo">STD 7</span>, <span class="seriesInfo">RFC 793</span>, <span class="seriesInfo">DOI 10.17487/RFC0793</span>, <time datetime="1981-09" class="refDate">September 1981</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc793">https://www.rfc-editor.org/info/rfc793</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC1034">[RFC1034]</dt>
      <dd>
<span class="refAuthor">Mockapetris, P.</span>, <span class="refTitle">"Domain names - concepts and facilities"</span>, <span class="seriesInfo">STD 13</span>, <span class="seriesInfo">RFC 1034</span>, <span class="seriesInfo">DOI 10.17487/RFC1034</span>, <time datetime="1987-11" class="refDate">November 1987</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc1034">https://www.rfc-editor.org/info/rfc1034</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC1035">[RFC1035]</dt>
      <dd>
<span class="refAuthor">Mockapetris, P.</span>, <span class="refTitle">"Domain names - implementation and specification"</span>, <span class="seriesInfo">STD 13</span>, <span class="seriesInfo">RFC 1035</span>, <span class="seriesInfo">DOI 10.17487/RFC1035</span>, <time datetime="1987-11" class="refDate">November 1987</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc1035">https://www.rfc-editor.org/info/rfc1035</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC1982">[RFC1982]</dt>
      <dd>
<span class="refAuthor">Elz, R.</span> and <span class="refAuthor">R. Bush</span>, <span class="refTitle">"Serial Number Arithmetic"</span>, <span class="seriesInfo">RFC 1982</span>, <span class="seriesInfo">DOI 10.17487/RFC1982</span>, <time datetime="1996-08" class="refDate">August 1996</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc1982">https://www.rfc-editor.org/info/rfc1982</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC5925">[RFC5925]</dt>
      <dd>
<span class="refAuthor">Touch, J.</span>, <span class="refAuthor">Mankin, A.</span>, and <span class="refAuthor">R. Bonica</span>, <span class="refTitle">"The TCP Authentication Option"</span>, <span class="seriesInfo">RFC 5925</span>, <span class="seriesInfo">DOI 10.17487/RFC5925</span>, <time datetime="2010-06" class="refDate">June 2010</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc5925">https://www.rfc-editor.org/info/rfc5925</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7323">[RFC7323]</dt>
    <dd>
<span class="refAuthor">Borman, D.</span>, <span class="refAuthor">Braden, B.</span>, <span class="refAuthor">Jacobson, V.</span>, and <span class="refAuthor">R. Scheffenegger, Ed.</span>, <span class="refTitle">"TCP Extensions for High Performance"</span>, <span class="seriesInfo">RFC 7323</span>, <span class="seriesInfo">DOI 10.17487/RFC7323</span>, <time datetime="2014-09" class="refDate">September 2014</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7323">https://www.rfc-editor.org/info/rfc7323</a>&gt;</span>. </dd>
<dd class="break"></dd>
</dl>
</section>
<div id="sect-10">
<section id="appendix-A">
      <h2 id="name-acknowledgments">
<a href="#name-acknowledgments" class="section-name selfRef">Acknowledgments</a>
      </h2>
<p id="appendix-A-1">
   The need for this document was first noted by <span class="contact-name">Juhamatti Kuusisaari</span>
   in April 2020 during discussions of the pseudocode in RFC 5925.<a href="#appendix-A-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="authors-addresses">
<section id="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">Joe Touch</span></div>
<div dir="auto" class="left">
<span class="locality">Manhattan Beach</span>, <span class="region">CA</span> <span class="postal-code">90266</span>
</div>
<div dir="auto" class="left"><span class="country-name">United States of America</span></div>
<div class="tel">
<span>Phone:</span>
<a href="tel:+1%20(310)%20560-0334" class="tel">+1 (310) 560-0334</a>
</div>
<div class="email">
<span>Email:</span>
<a href="mailto:touch@strayalpha.com" class="email">touch@strayalpha.com</a>
</div>
</address>
</section>
</div>
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