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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 8663: MPLS Segment Routing over IP</title>
<meta content="Xiaohu Xu" name="author">
<meta content="Stewart Bryant" name="author">
<meta content="Adrian Farrel" name="author">
<meta content="Syed Hassan" name="author">
<meta content="Wim Henderickx" name="author">
<meta content="Zhenbin Li" name="author">
<meta content="
       MPLS Segment Routing (SR-MPLS) is a method of source routing a packet
      through an MPLS data plane by imposing a stack of MPLS labels on the
      packet to specify the path together with any packet-specific
      instructions to be executed on it.
  
       SR-MPLS can be leveraged to realize a source-routing mechanism across
       MPLS, IPv4, and IPv6 data planes by using an MPLS label stack as a
       source-routing instruction set while making no changes to SR-MPLS
       specifications and interworking with SR-MPLS implementations. 
       This document describes how SR-MPLS-capable routers and IP-only
      routers can seamlessly coexist and interoperate through the use of
      SR-MPLS label stacks and IP encapsulation/tunneling such as MPLS-over-UDP
      as defined in RFC 7510. 
    " name="description">
<meta content="xml2rfc 2.35.0" name="generator">
<meta content="MPLS-SR-over-IP, SR-MPLS-over-IP, MPLS-SR-over-UDP, SR-MPLS-over-UDP" name="keyword">
<meta content="8663" name="rfc.number">
<link href="rfc8663.xml" type="application/rfc+xml" rel="alternate">
<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 {
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  padding: 1em 0 0.5em;
}
.author {
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}
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h2 {
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h3 {
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h4 {
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.alignRight {
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.alignRight > *:nth-child(2) {
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svg {
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.alignCenter.art-text pre {
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.alignCenter {
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.alignCenter > *:first-child {
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  */
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}

/* lists */
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ol ol, ul ul, ol ul, ul ol {
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}
li {
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.ulCompact li {
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ul.empty li, .ulEmpty li {
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ul.compact, .ulCompact,
ol.compact, .olCompact {
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}

/* definition lists */
dl {
}
dl > dt {
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}
/* 
dl.nohang > dt {
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}
*/
dl > dd {
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  min-height: 1.3em;
}
dl.compact > dd, .dlCompact > dd {
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}
dl > dd > dl {
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}

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

/* Figures */
tt, code, pre, code {
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pre {
  border: 1px solid #eee;
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}
img {
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}
figure {
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figure blockquote {
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}
figcaption {
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}
@media screen {
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}

/* aside, blockquote */
aside, blockquote {
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}
blockquote {
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cite {
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}

/* tables */
table {
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}
th, td {
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  vertical-align: top;
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}
th {
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tr:nth-child(2n+1) > td {
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table caption {
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}
table p {
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}

/* pilcrow */
a.pilcrow {
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}
@media screen {
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  p:hover > a.pilcrow,
  blockquote:hover > a.pilcrow,
  div:hover > a.pilcrow,
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  pre:hover > a.pilcrow {
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  a.pilcrow:hover {
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}

/* misc */
hr {
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}
.bcp14 {
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}

.role {
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}

/* info block */
#identifiers {
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}
#identifiers dt {
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}
#identifiers dd {
  float: left;
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}
#identifiers .authors .author {
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  margin-right: 1.5em;
}
#identifiers .authors .org {
  font-style: italic;
}

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

/* table of contents */
#toc  {
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  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%;
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  p {
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    widows: 3;
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  #toc, #n-introduction {
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  }
  #toc {
    border-top: none;
    padding-top: 0;
  }
  figure, pre {
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  figure {
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  h1, h2, h3, h4, h5, h6 {
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  }
  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;
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  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 {
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}

.ears tfoot .left {
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}

.ears tfoot .center {
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}

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

@page :first {
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  @top-left {
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    border: none;
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  @top-center {
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    border: none;
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  @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;
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  @top-left {
    content: string(ears-top-left);
    vertical-align: bottom;
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  @top-center {
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  @bottom-left {
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  @bottom-center {
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      vertical-align: top;
      border-top: solid 1px #ccc;
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  */

}

/* 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;
  }
}
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<link href="https://dx.doi.org/10.17487/rfc8663" rel="alternate">
  <link href="urn:issn:2070-1721" rel="alternate">
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<table class="ears">
<thead><tr>
<td class="left">RFC 8663</td>
<td class="center">SR-MPLS-over-IP</td>
<td class="right">December 2019</td>
</tr></thead>
<tfoot><tr>
<td class="left">Xu, et al.</td>
<td class="center">Standards Track</td>
<td class="right">[Page]</td>
</tr></tfoot>
</table>
<div id="external-metadata" class="document-information"></div>
<div id="internal-metadata" class="document-information">
<dl id="identifiers">
<dt class="label-stream">Stream:</dt>
<dd class="stream">Internet Engineering Task Force (IETF)</dd>
<dt class="label-rfc">RFC:</dt>
<dd class="rfc"><a href="https://www.rfc-editor.org/rfc/rfc8663" class="eref">8663</a></dd>
<dt class="label-category">Category:</dt>
<dd class="category">Standards Track</dd>
<dt class="label-published">Published:</dt>
<dd class="published">
<time datetime="2019-12" class="published">December 2019</time>
    </dd>
<dt class="label-issn">ISSN:</dt>
<dd class="issn">2070-1721</dd>
<dt class="label-authors">Authors:</dt>
<dd class="authors">
<div class="author">
      <div class="author-name">X. Xu</div>
<div class="org">Alibaba, Inc</div>
</div>
<div class="author">
      <div class="author-name">S. Bryant</div>
<div class="org">Futurewei Technologies</div>
</div>
<div class="author">
      <div class="author-name">A. Farrel</div>
<div class="org">Old Dog Consulting</div>
</div>
<div class="author">
      <div class="author-name">S. Hassan</div>
<div class="org">Cisco</div>
</div>
<div class="author">
      <div class="author-name">W. Henderickx</div>
<div class="org">Nokia</div>
</div>
<div class="author">
      <div class="author-name">Z. Li</div>
<div class="org">Huawei</div>
</div>
</dd>
</dl>
</div>
<h1 id="rfcnum">RFC 8663</h1>
<h1 id="title">MPLS Segment Routing over IP</h1>
<section id="section-abstract">
      <h2 id="abstract"><a href="#abstract" class="selfRef">Abstract</a></h2>
<p id="section-abstract-1">MPLS Segment Routing (SR-MPLS) is a method of source routing a packet
      through an MPLS data plane by imposing a stack of MPLS labels on the
      packet to specify the path together with any packet-specific
      instructions to be executed on it.
  
       SR-MPLS can be leveraged to realize a source-routing mechanism across
       MPLS, IPv4, and IPv6 data planes by using an MPLS label stack as a
       source-routing instruction set while making no changes to SR-MPLS
       specifications and interworking with SR-MPLS implementations.<a href="#section-abstract-1" class="pilcrow">¶</a></p>
<p id="section-abstract-2">This document describes how SR-MPLS-capable routers and IP-only
      routers can seamlessly coexist and interoperate through the use of
      SR-MPLS label stacks and IP encapsulation/tunneling such as MPLS-over-UDP
      as defined in RFC 7510.<a href="#section-abstract-2" class="pilcrow">¶</a></p>
</section>
<div id="status-of-memo">
<section id="section-boilerplate.1">
        <h2 id="name-status-of-this-memo">
<a href="#name-status-of-this-memo" class="section-name selfRef">Status of This Memo</a>
        </h2>
<p id="section-boilerplate.1-1">
            This is an Internet Standards Track document.<a href="#section-boilerplate.1-1" class="pilcrow">¶</a></p>
<p id="section-boilerplate.1-2">
            This document is a product of the Internet Engineering Task Force
            (IETF).  It represents the consensus of the IETF community.  It has
            received public review and has been approved for publication by
            the Internet Engineering Steering Group (IESG).  Further
            information on Internet Standards is available in Section 2 of 
            RFC 7841.<a href="#section-boilerplate.1-2" class="pilcrow">¶</a></p>
<p id="section-boilerplate.1-3">
            Information about the current status of this document, any
            errata, and how to provide feedback on it may be obtained at
            <span><a href="https://www.rfc-editor.org/info/rfc8663">https://www.rfc-editor.org/info/rfc8663</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) 2019 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-terminology" class="xref">Terminology</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-use-cases" class="xref">Use Cases</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-procedures-of-sr-mpls-over-" class="xref">Procedures of SR-MPLS-over-IP</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-forwarding-entry-constructi" class="xref">Forwarding Entry Construction</a><a href="#section-toc.1-1.3.2.1.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.3.2.1.2.1">
                    <p id="section-toc.1-1.3.2.1.2.1.1"><a href="#section-3.1.1" class="xref">3.1.1</a>.  <a href="#name-fib-construction-example" class="xref">FIB Construction Example</a><a href="#section-toc.1-1.3.2.1.2.1.1" class="pilcrow">¶</a></p>
</li>
                </ul>
</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-packet-forwarding-procedure" class="xref">Packet-Forwarding Procedures</a><a href="#section-toc.1-1.3.2.2.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.3.2.2.2.1">
                    <p id="section-toc.1-1.3.2.2.2.1.1"><a href="#section-3.2.1" class="xref">3.2.1</a>.  <a href="#name-packet-forwarding-with-penu" class="xref">Packet Forwarding with Penultimate Hop Popping</a><a href="#section-toc.1-1.3.2.2.2.1.1" class="pilcrow">¶</a></p>
</li>
                  <li class="toc ulEmpty" id="section-toc.1-1.3.2.2.2.2">
                    <p id="section-toc.1-1.3.2.2.2.2.1"><a href="#section-3.2.2" class="xref">3.2.2</a>.  <a href="#name-packet-forwarding-without-p" class="xref">Packet Forwarding without Penultimate Hop Popping</a><a href="#section-toc.1-1.3.2.2.2.2.1" class="pilcrow">¶</a></p>
</li>
                  <li class="toc ulEmpty" id="section-toc.1-1.3.2.2.2.3">
                    <p id="section-toc.1-1.3.2.2.2.3.1"><a href="#section-3.2.3" class="xref">3.2.3</a>.  <a href="#name-additional-forwarding-proce" class="xref">Additional Forwarding Procedures</a><a href="#section-toc.1-1.3.2.2.2.3.1" class="pilcrow">¶</a></p>
</li>
                </ul>
</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-iana-considerations" class="xref">IANA Considerations</a><a href="#section-toc.1-1.4.1" class="pilcrow">¶</a></p>
</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-security-considerations" class="xref">Security Considerations</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-references" class="xref">References</a><a href="#section-toc.1-1.6.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.6.2.1">
                <p id="section-toc.1-1.6.2.1.1"><a href="#section-6.1" class="xref">6.1</a>.  <a href="#name-normative-references" class="xref">Normative References</a><a href="#section-toc.1-1.6.2.1.1" class="pilcrow">¶</a></p>
</li>
              <li class="toc ulEmpty" id="section-toc.1-1.6.2.2">
                <p id="section-toc.1-1.6.2.2.1"><a href="#section-6.2" class="xref">6.2</a>.  <a href="#name-informative-references" class="xref">Informative References</a><a href="#section-toc.1-1.6.2.2.1" class="pilcrow">¶</a></p>
</li>
            </ul>
</li>
          <li class="toc ulEmpty" id="section-toc.1-1.7">
            <p id="section-toc.1-1.7.1"><a href="#section-appendix.a" class="xref"></a><a href="#name-acknowledgements" class="xref">Acknowledgements</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-appendix.b" class="xref"></a><a href="#name-contributors" class="xref">Contributors</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.c" class="xref"></a><a href="#name-authors-addresses" class="xref">Authors' Addresses</a><a href="#section-toc.1-1.9.1" 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">MPLS Segment Routing (SR-MPLS) <span>[<a href="#RFC8660" class="xref">RFC8660</a>]</span> is a method of source routing a packet through an
      MPLS data plane. This is achieved by the sender imposing a stack of MPLS
      labels that partially or completely specify the path that the packet is
      to take and any instructions to be executed on the packet as it passes
      through the network.

      SR-MPLS uses an MPLS label stack to encode a sequence of source-routing
      instructions. This can be used to realize a source-routing mechanism
      that can operate across MPLS, IPv4, and IPv6 data planes. This approach
      makes no changes to SR-MPLS specifications and allows interworking with
      SR-MPLS implementations. More specifically, the source-routing
      instructions in a source-routed packet could be
      uniformly encoded as an MPLS label stack regardless of whether the
      underlay is IPv4, IPv6 (including Segment Routing for IPv6 (SRv6) <span>[<a href="#RFC8354" class="xref">RFC8354</a>]</span>), or MPLS.<a href="#section-1-1" class="pilcrow">¶</a></p>
<p id="section-1-2">This document describes how SR-MPLS-capable routers and IP-only
      routers can seamlessly coexist and interoperate through the use of
      SR-MPLS label stacks and IP encapsulation/tunneling such as MPLS-over-UDP
      <span>[<a href="#RFC7510" class="xref">RFC7510</a>]</span>.<a href="#section-1-2" class="pilcrow">¶</a></p>
<p id="section-1-3"><a href="#usecases" class="xref">Section 2</a> describes various use
      cases for tunneling SR-MPLS over IP. <a href="#procs" class="xref">Section 3</a> describes a typical application scenario and how the
      packet forwarding happens.<a href="#section-1-3" class="pilcrow">¶</a></p>
<div id="Abbreviations_Terminology">
<section id="section-1.1">
        <h3 id="name-terminology">
<a href="#section-1.1" class="section-number selfRef">1.1. </a><a href="#name-terminology" class="section-name selfRef">Terminology</a>
        </h3>
<p id="section-1.1-1">This memo makes use of the terms defined in <span>[<a href="#RFC3031" class="xref">RFC3031</a>]</span> and <span>[<a href="#RFC8660" class="xref">RFC8660</a>]</span>.<a href="#section-1.1-1" class="pilcrow">¶</a></p>
<p id="section-1.1-2">
    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-1.1-2" class="pilcrow">¶</a></p>
</section>
</div>
</section>
<div id="usecases">
<section id="section-2">
      <h2 id="name-use-cases">
<a href="#section-2" class="section-number selfRef">2. </a><a href="#name-use-cases" class="section-name selfRef">Use Cases</a>
      </h2>
<p id="section-2-1">Tunneling SR-MPLS using IPv4 and/or IPv6 (including SRv6) tunnels is
      useful at least in the use cases listed below. In all cases, this can be
      enabled using an IP tunneling mechanism such as MPLS-over-UDP as described
      in <span>[<a href="#RFC7510" class="xref">RFC7510</a>]</span>. The tunnel selected <span class="bcp14">MUST</span> have its remote
      endpoint (destination) address equal to the address of the next
      node capable of SR-MPLS identified as being on the SR path (i.e., the
      egress of the active segment). The local endpoint (source) address is
      set to an address of the encapsulating node. <span>[<a href="#RFC7510" class="xref">RFC7510</a>]</span>
      gives further advice on how to set the source address if the UDP
      zero-checksum mode is used with MPLS-over-UDP. Using UDP as the
      encapsulation may be particularly beneficial because it is agnostic of
      the underlying transport.<a href="#section-2-1" class="pilcrow">¶</a></p>
<ul>
<li id="section-2-2.1">
          <p id="section-2-2.1.1">Incremental deployment of the SR-MPLS technology may be
          facilitated by tunneling SR-MPLS packets across parts of a network
          that are not SR-MPLS as shown in <a href="#islandsFig" class="xref">Figure 1</a>. This
          demonstrates how islands of SR-MPLS may be connected across a legacy
          network. It may be particularly useful for joining sites (such as
          data centers).<a href="#section-2-2.1.1" class="pilcrow">¶</a></p>
<span id="name-sr-mpls-over-udp-to-tunnel-"></span><div id="islandsFig">
<figure id="figure-1">
            <div class="artwork art-text alignLeft" id="section-2-2.1.2.1">
<pre>
                   ________________________
    _______       (                        )       _______
   (       )     (        IP Network        )     (       )
  ( SR-MPLS )   (                            )   ( SR-MPLS )
 (  Network  ) (                              ) (  Network  )
(         --------                          --------         )
(        | Border |    SR-in-UDP Tunnel    | Border |        )
(        | Router |========================| Router |        )
(        |   R1   |                        |   R2   |        )
(         --------                          --------         )
 (           ) (                              ) (           )
  (         )   (                            )   (         )
   (_______)     (                          )     (_______)
                  (________________________)
</pre>
</div>
<figcaption><a href="#figure-1" class="selfRef">Figure 1</a>:
<a href="#name-sr-mpls-over-udp-to-tunnel-" class="selfRef">SR-MPLS-over-UDP to Tunnel between SR-MPLS Sites</a>
            </figcaption></figure>
</div>
</li>
        <li id="section-2-2.2">If the encoding of entropy <span>[<a href="#RFC6790" class="xref">RFC6790</a>]</span> is desired, IP-tunneling mechanisms that allow the
        encoding of entropy, such as MPLS-over-UDP encapsulation <span>[<a href="#RFC7510" class="xref">RFC7510</a>]</span> where the source port of the UDP
        header is used as an entropy field, may be used to maximize the
        utilization of Equal-Cost Multipath (ECMP) and/or Link Aggregation
        Groups (LAGs), especially when it is difficult to make use of the
        entropy-label mechanism. This is to be contrasted with <span>[<a href="#RFC4023" class="xref">RFC4023</a>]</span> where MPLS-over-IP does not provide
        for an entropy mechanism. Refer to <span>[<a href="#RFC8662" class="xref">RFC8662</a>]</span>) for more discussion about using entropy labels in
        SR-MPLS.<a href="#section-2-2.2" class="pilcrow">¶</a>
</li>
        <li id="section-2-2.3">
          <p id="section-2-2.3.1">Tunneling MPLS over IP provides a technology that enables Segment
          Routing (SR) in an IPv4 and/or IPv6 network where the routers do not
          support SRv6 capabilities <span>[<a href="#I-D.ietf-6man-segment-routing-header" class="xref">IPv6-SRH</a>]</span> and
          where MPLS forwarding is not an option. This is shown in <a href="#transitionFig" class="xref">Figure 2</a>.<a href="#section-2-2.3.1" class="pilcrow">¶</a></p>
<span id="name-sr-mpls-enabled-within-an-i"></span><div id="transitionFig">
<figure id="figure-2">
            <div class="artwork art-text alignLeft" id="section-2-2.3.2.1">
<pre>
                __________________________________
             __(           IP Network             )__
          __(                                        )__
         (               --        --        --         )
    --------   --   --  |SR|  --  |SR|  --  |SR|  --   --------
   | Ingress| |IR| |IR| |  | |IR| |  | |IR| |  | |IR| | Egress|
--&gt;| Router |===========|  |======|  |======|  |======| Router|--&gt;
   |   SR   | |  | |  | |  | |  | |  | |  | |  | |  | |   SR  |
    --------   --   --  |  |  --  |  |  --  |  |  --   --------
         (__             --        --        --       __)
            (__                                    __)
               (__________________________________)

  Key:
    IR : IP-only Router
    SR : SR-MPLS-capable Router
    == : SR-MPLS-over-UDP Tunnel
</pre>
</div>
<figcaption><a href="#figure-2" class="selfRef">Figure 2</a>:
<a href="#name-sr-mpls-enabled-within-an-i" class="selfRef">SR-MPLS Enabled within an IP Network</a>
            </figcaption></figure>
</div>
</li>
      </ul>
</section>
</div>
<div id="procs">
<section id="section-3">
      <h2 id="name-procedures-of-sr-mpls-over-">
<a href="#section-3" class="section-number selfRef">3. </a><a href="#name-procedures-of-sr-mpls-over-" class="section-name selfRef">Procedures of SR-MPLS-over-IP</a>
      </h2>
<p id="section-3-1">This section describes the construction of forwarding information
      base (FIB) entries and the forwarding behavior that allow the deployment
      of SR-MPLS when some routers in the network are IP only (i.e., do not
      support SR-MPLS). Note that the examples in Sections <a href="#fibeg" class="xref">3.1.1</a> and <a href="#fwd" class="xref">3.2</a> assume that
      OSPF or IS-IS is enabled; in fact, other mechanisms of discovery and
      advertisement could be used including other routing protocols (such as
      BGP) or a central controller.<a href="#section-3-1" class="pilcrow">¶</a></p>
<div id="fib">
<section id="section-3.1">
        <h3 id="name-forwarding-entry-constructi">
<a href="#section-3.1" class="section-number selfRef">3.1. </a><a href="#name-forwarding-entry-constructi" class="section-name selfRef">Forwarding Entry Construction</a>
        </h3>
<p id="section-3.1-1">This subsection describes how to construct the forwarding
        information base (FIB) entry on an SR-MPLS-capable router when some or
        all of the next hops along the shortest path towards a prefix Segment
        Identifier (Prefix-SID) are IP-only routers. <a href="#fibeg" class="xref">Section 3.1.1</a>
        provides a concrete example of how the process applies when using OSPF
        or IS-IS.<a href="#section-3.1-1" class="pilcrow">¶</a></p>
<p id="section-3.1-2">Consider router A that receives a labeled packet with top label
        L(E) that corresponds to the Prefix-SID SID(E) of prefix P(E)
        advertised by router E. Suppose the i-th next-hop router (termed NHi)
        along the shortest path from router A toward SID(E) is not SR-MPLS
        capable while both routers A and E are SR-MPLS capable. The following
        processing steps apply:<a href="#section-3.1-2" class="pilcrow">¶</a></p>
<ul>
<li id="section-3.1-3.1">Router E is SR-MPLS capable, so it advertises a Segment Routing
            Global Block (SRGB). The SRGB is defined in <span>[<a href="#RFC8402" class="xref">RFC8402</a>]</span>.
            There are a number of ways that the advertisement can be achieved
            including IGPs, BGP, and configuration/management protocols. For
            example, see <span>[<a href="#I-D.ietf-bess-datacenter-gateway" class="xref">DC-GATEWAY</a>]</span>.<a href="#section-3.1-3.1" class="pilcrow">¶</a>
</li>
          <li id="section-3.1-3.2">When Router E advertises the Prefix-SID SID(E) of prefix P(E), it <span class="bcp14">MUST</span>
also advertise the egress endpoint address and the encapsulation type of any
tunnel used to reach E.  This information is flooded domain wide.<a href="#section-3.1-3.2" class="pilcrow">¶</a>
</li>
          <li id="section-3.1-3.3">If A and E are in different routing domains, then the information <span class="bcp14">MUST</span>
            be flooded into both domains. How this is achieved depends on the
            advertisement mechanism being used. The objective is that router A
            knows the characteristics of router E that originated the
            advertisement of SID(E).<a href="#section-3.1-3.3" class="pilcrow">¶</a>
</li>
          <li id="section-3.1-3.4">
            <p id="section-3.1-3.4.1">Router A programs the FIB entry for prefix P(E) corresponding
            to the SID(E) according to whether a pop or swap action is advertised
            for the prefix. The resulting action may be:<a href="#section-3.1-3.4.1" class="pilcrow">¶</a></p>
<ul>
<li id="section-3.1-3.4.2.1">pop the top label<a href="#section-3.1-3.4.2.1" class="pilcrow">¶</a>
</li>
              <li id="section-3.1-3.4.2.2">swap the top label to a value equal to SID(E) plus the
                  lower bound of the SRGB of E<a href="#section-3.1-3.4.2.2" class="pilcrow">¶</a>
</li>
            </ul>
</li>
        </ul>
<p id="section-3.1-4">Once constructed, the FIB can be used by a router to tell it how to
        process packets. It encapsulates the packets according to the
        appropriate encapsulation advertised for the segment and then sends
        the packets towards the next hop NHi.<a href="#section-3.1-4" class="pilcrow">¶</a></p>
<div id="fibeg">
<section id="section-3.1.1">
          <h4 id="name-fib-construction-example">
<a href="#section-3.1.1" class="section-number selfRef">3.1.1. </a><a href="#name-fib-construction-example" class="section-name selfRef">FIB Construction Example</a>
          </h4>
<p id="section-3.1.1-1">This section is non-normative and provides a worked example of how
          a FIB might be constructed using OSPF and IS-IS extensions. It is based
          on the process described in <a href="#fib" class="xref">Section 3.1</a>.<a href="#section-3.1.1-1" class="pilcrow">¶</a></p>
<ul>
<li id="section-3.1.1-2.1">Router E is SR-MPLS capable, so it advertises a Segment Routing
              Global Block (SRGB) using
              <span>[<a href="#RFC8665" class="xref">RFC8665</a>]</span> or
              <span>[<a href="#RFC8667" class="xref">RFC8667</a>]</span>.<a href="#section-3.1.1-2.1" class="pilcrow">¶</a>
</li>
            <li id="section-3.1.1-2.2">When Router E advertises the Prefix-SID SID(E) of prefix P(E),
              it also advertises the encapsulation endpoint address and the tunnel
              type of any tunnel used to reach E using
              <span>[<a href="#I-D.ietf-isis-encapsulation-cap" class="xref">ISIS-ENCAP</a>]</span> or
              <span>[<a href="#I-D.ietf-ospf-encapsulation-cap" class="xref">OSPF-ENCAP</a>]</span>.<a href="#section-3.1.1-2.2" class="pilcrow">¶</a>
</li>
            <li id="section-3.1.1-2.3">
              <p id="section-3.1.1-2.3.1">If A and E are in different domains, then the information is
              flooded into both domains and any intervening domains.<a href="#section-3.1.1-2.3.1" class="pilcrow">¶</a></p>
<ul>
<li id="section-3.1.1-2.3.2.1">The OSPF Tunnel Encapsulations TLV
                  <span>[<a href="#I-D.ietf-ospf-encapsulation-cap" class="xref">OSPF-ENCAP</a>]</span> or the IS-IS
                  Tunnel Encapsulation Type sub-TLV
                  <span>[<a href="#I-D.ietf-isis-encapsulation-cap" class="xref">ISIS-ENCAP</a>]</span> is flooded
                  domain wide.<a href="#section-3.1.1-2.3.2.1" class="pilcrow">¶</a>
</li>
                <li id="section-3.1.1-2.3.2.2">The OSPF SID/Label Range TLV
                  <span>[<a href="#RFC8665" class="xref">RFC8665</a>]</span> or
                  the IS-IS SR-Capabilities sub-TLV
                  <span>[<a href="#RFC8667" class="xref">RFC8667</a>]</span> is
                  advertised domain wide so that router A knows the
                  characteristics of router E.<a href="#section-3.1.1-2.3.2.2" class="pilcrow">¶</a>
</li>
                <li id="section-3.1.1-2.3.2.3">
                  <p id="section-3.1.1-2.3.2.3.1">When router E advertises the prefix P(E):<a href="#section-3.1.1-2.3.2.3.1" class="pilcrow">¶</a></p>
<ul>
<li id="section-3.1.1-2.3.2.3.2.1">If router E is running IS-IS, it uses the extended
                      reachability TLV (TLVs 135, 235, 236, 237) and associates
                      the IPv4/IPv6 or IPv4/IPv6 Source Router ID sub-TLV(s)
                      <span>[<a href="#RFC7794" class="xref">RFC7794</a>]</span>.<a href="#section-3.1.1-2.3.2.3.2.1" class="pilcrow">¶</a>
</li>
                    <li id="section-3.1.1-2.3.2.3.2.2">If router E is running OSPF, it uses the OSPFv2 Extended
                      Prefix Opaque Link-State Advertisement (LSA) <span>[<a href="#RFC7684" class="xref">RFC7684</a>]</span> and sets the
                      flooding scope to Autonomous System (AS) wide.<a href="#section-3.1.1-2.3.2.3.2.2" class="pilcrow">¶</a>
</li>
                  </ul>
</li>
                <li id="section-3.1.1-2.3.2.4">If router E is running IS-IS and advertises the IS-IS
                  Router CAPABILITY TLV (TLV 242) <span>[<a href="#RFC7981" class="xref">RFC7981</a>]</span>, it sets the
                  "Router ID" field to a valid value or includes an IPv6
                  TE Router ID sub-TLV (TLV 12), or it does both. The "S" bit
                  (flooding scope) of the IS-IS Router CAPABILITY TLV (TLV 242) is set
                  to "1".<a href="#section-3.1.1-2.3.2.4" class="pilcrow">¶</a>
</li>
              </ul>
</li>
            <li id="section-3.1.1-2.4">
              <p id="section-3.1.1-2.4.1">Router A programs the FIB entry for prefix P(E) corresponding
              to the SID(E) according to whether a pop or swap action is advertised
              for the prefix as follows:<a href="#section-3.1.1-2.4.1" class="pilcrow">¶</a></p>
<ul>
<li id="section-3.1.1-2.4.2.1">
                  <p id="section-3.1.1-2.4.2.1.1">If the No-PHP (NP) Flag in OSPF or the Persistent (P) Flag in IS-IS is clear:<a href="#section-3.1.1-2.4.2.1.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty">
<li class="ulEmpty" id="section-3.1.1-2.4.2.1.2.1">pop the top label<a href="#section-3.1.1-2.4.2.1.2.1" class="pilcrow">¶</a>
</li>
                  </ul>
</li>
                <li id="section-3.1.1-2.4.2.2">
                  <p id="section-3.1.1-2.4.2.2.1">If the No-PHP (NP) Flag in OSPF or the Persistent (P) Flag in IS-IS is set:<a href="#section-3.1.1-2.4.2.2.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty">
<li class="ulEmpty" id="section-3.1.1-2.4.2.2.2.1">swap the top label to a value equal to SID(E) plus the
                      lower bound of the SRGB of E<a href="#section-3.1.1-2.4.2.2.2.1" class="pilcrow">¶</a>
</li>
                  </ul>
</li>
              </ul>
</li>
          </ul>
<p id="section-3.1.1-3">When forwarding the packet according to the constructed FIB entry, the
          router encapsulates the packet according to the encapsulation as advertised
          using the mechanisms described in <span>[<a href="#I-D.ietf-isis-encapsulation-cap" class="xref">ISIS-ENCAP</a>]</span>
          or <span>[<a href="#I-D.ietf-ospf-encapsulation-cap" class="xref">OSPF-ENCAP</a>]</span>. It then sends the
          packets towards the next hop NHi.<a href="#section-3.1.1-3" class="pilcrow">¶</a></p>
<p id="section-3.1.1-4">Note that <span>[<a href="#RFC7510" class="xref">RFC7510</a>]</span> specifies the use of port number 6635
          to indicate that the payload of a UDP packet is MPLS, and port number 6636 for
          MPLS-over-UDP utilizing DTLS. However, <span>[<a href="#I-D.ietf-isis-encapsulation-cap" class="xref">ISIS-ENCAP</a>]</span>
          and <span>[<a href="#I-D.ietf-ospf-encapsulation-cap" class="xref">OSPF-ENCAP</a>]</span> provide dynamic protocol
          mechanisms to configure the use of any Dynamic Port for a tunnel that uses UDP
          encapsulation. Nothing in this document prevents the use of an IGP or any other
          mechanism to negotiate the use of a Dynamic Port when UDP encapsulation is used
          for SR-MPLS, but if no such mechanism is used, then the port numbers specified in
          <span>[<a href="#RFC7510" class="xref">RFC7510</a>]</span> are used.<a href="#section-3.1.1-4" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="fwd">
<section id="section-3.2">
        <h3 id="name-packet-forwarding-procedure">
<a href="#section-3.2" class="section-number selfRef">3.2. </a><a href="#name-packet-forwarding-procedure" class="section-name selfRef">Packet-Forwarding Procedures</a>
        </h3>
<p id="section-3.2-1"><span>[<a href="#RFC7510" class="xref">RFC7510</a>]</span> specifies an IP-based encapsulation for
        MPLS, i.e., MPLS-over-UDP. This approach is applicable where IP-based
        encapsulation for MPLS is required and further fine-grained load
        balancing of MPLS packets over IP networks over 
        ECMP and/or LAGs is also required. This
        section provides details about the forwarding procedure when
        UDP encapsulation is adopted for SR-MPLS-over-IP. Other encapsulation
        and tunneling mechanisms can be applied using similar techniques,
        but for clarity, this section uses UDP encapsulation as the exemplar.<a href="#section-3.2-1" class="pilcrow">¶</a></p>
<p id="section-3.2-2">Nodes that are SR-MPLS capable can process SR-MPLS packets. Not all
        of the nodes in an SR-MPLS domain are SR-MPLS capable. Some nodes may
        be "legacy routers" that cannot handle SR-MPLS packets but can forward
        IP packets. A node capable of SR-MPLS <span class="bcp14">MAY</span> advertise its capabilities
        using the IGP as described in <a href="#procs" class="xref">Section 3</a>. There are six
        types of nodes in an SR-MPLS domain:<a href="#section-3.2-2" class="pilcrow">¶</a></p>
<ul>
<li id="section-3.2-3.1">Domain ingress nodes that receive packets and encapsulate them
            for transmission across the domain. Those packets may be any
            payload protocol including native IP packets or packets that are
            already MPLS encapsulated.<a href="#section-3.2-3.1" class="pilcrow">¶</a>
</li>
          <li id="section-3.2-3.2">Legacy transit nodes that are IP routers but that are not
          SR-MPLS capable (i.e., are not able to perform Segment
          Routing).<a href="#section-3.2-3.2" class="pilcrow">¶</a>
</li>
          <li id="section-3.2-3.3">Transit nodes that are SR-MPLS capable but that are not
            identified by a SID in the SID stack.<a href="#section-3.2-3.3" class="pilcrow">¶</a>
</li>
          <li id="section-3.2-3.4">Transit nodes that are SR-MPLS capable and need to perform
            SR-MPLS routing because they are identified by a SID in the SID
            stack.<a href="#section-3.2-3.4" class="pilcrow">¶</a>
</li>
          <li id="section-3.2-3.5">The penultimate node capable of SR-MPLS on the path that processes
            the last SID on the stack on behalf of the domain egress node.<a href="#section-3.2-3.5" class="pilcrow">¶</a>
</li>
          <li id="section-3.2-3.6">The domain egress node that forwards the payload packet for
            ultimate delivery.<a href="#section-3.2-3.6" class="pilcrow">¶</a>
</li>
        </ul>
<div id="phpfwd">
<section id="section-3.2.1">
          <h4 id="name-packet-forwarding-with-penu">
<a href="#section-3.2.1" class="section-number selfRef">3.2.1. </a><a href="#name-packet-forwarding-with-penu" class="section-name selfRef">Packet Forwarding with Penultimate Hop Popping</a>
          </h4>
<p id="section-3.2.1-1">The description in this section assumes that the label associated
          with each Prefix-SID is advertised by the owner of the Prefix-SID as
          a Penultimate Hop-Popping (PHP) label. That is, if one of the IGP
          flooding mechanisms is used, the NP-Flag in OSPF or the P-Flag in
          IS-IS associated with the Prefix-SID is not set.<a href="#section-3.2.1-1" class="pilcrow">¶</a></p>
<span id="name-packet-forwarding-example-w"></span><div id="phpfwdeg">
<figure id="figure-3">
            <div class="artwork art-text alignCenter" id="section-3.2.1-2.1">
<pre>
 +-----+       +-----+       +-----+       +-----+       +-----+
 |  A  +-------+  B  +-------+  C  +-------+  D  +-------+  H  |
 +-----+       +--+--+       +--+--+       +--+--+       +-----+
                  |             |             |
                  |             |             |
               +--+--+       +--+--+       +--+--+
               |  E  +-------+  F  +-------+  G  |
               +-----+       +-----+       +-----+


      +--------+
      |IP(A-&gt;E)|
      +--------+                 +--------+        +--------+
      |  UDP   |                 |IP(E-&gt;G)|        |IP(G-&gt;H)|
      +--------+                 +--------+        +--------+
      |  L(G)  |                 |  UDP   |        |  UDP   |
      +--------+                 +--------+        +--------+
      |  L(H)  |                 |  L(H)  |        |Exp Null|
      +--------+                 +--------+        +--------+
      | Packet |     ---&gt;        | Packet |  ---&gt;  | Packet |
      +--------+                 +--------+        +--------+
</pre>
</div>
<figcaption><a href="#figure-3" class="selfRef">Figure 3</a>:
<a href="#name-packet-forwarding-example-w" class="selfRef">Packet-Forwarding Example with PHP</a>
            </figcaption></figure>
</div>
<p id="section-3.2.1-3">In the example shown in <a href="#phpfwdeg" class="xref">Figure 3</a>, assume that
          routers A, E, G, and H are capable of SR-MPLS while the remaining
          routers (B, C, D, and F) are only capable of forwarding IP packets.
          Routers A, E, G, and H advertise their Segment Routing related
          information, such as via IS-IS or OSPF.<a href="#section-3.2.1-3" class="pilcrow">¶</a></p>
<p id="section-3.2.1-4">Now assume that router A (the Domain ingress) wants to send a
          packet to router H (the Domain egress) via the explicit path
          {E-&gt;G-&gt;H}. Router A will impose an MPLS label stack on the
          packet that corresponds to that explicit path. Since the next hop
          toward router E is only IP capable (B is a legacy transit node),
          router A replaces the top label (that indicated router E) with a
          UDP-based tunnel for MPLS (i.e., MPLS-over-UDP <span>[<a href="#RFC7510" class="xref">RFC7510</a>]</span>) to router E and then sends the packet. In other
          words, router A pops the top label and then encapsulates the MPLS
          packet in a UDP tunnel to router E.<a href="#section-3.2.1-4" class="pilcrow">¶</a></p>
<p id="section-3.2.1-5">When the IP-encapsulated MPLS packet arrives at router E (which
          is a transit node capable of SR-MPLS), router E strips the IP-based
          tunnel header and then processes the decapsulated MPLS packet. The top
          label indicates that the packet must be forwarded toward router G.
          Since the next hop toward router G is only IP capable, router E
          replaces the current top label with an MPLS-over-UDP tunnel toward
          router G and sends it out. That is, router E pops the top label and
          then encapsulates the MPLS packet in a UDP tunnel to router G.<a href="#section-3.2.1-5" class="pilcrow">¶</a></p>
<p id="section-3.2.1-6">When the packet arrives at router G, router G will strip the
          IP-based tunnel header and then process the decapsulated MPLS
          packet. The top label indicates that the packet must be forwarded
          toward router H. Since the next hop toward router H is only
          IP capable (D is a legacy transit router), router G would replace
          the current top label with an MPLS-over-UDP tunnel toward router H
          and send it out. However, since router G reaches the bottom of the
          label stack (G is the penultimate node capable of SR-MPLS on the path),
          this would leave the original packet that router A wanted to send to
          router H encapsulated in UDP as if it was MPLS (i.e., with a UDP
          header and destination port indicating MPLS) even though the
          original packet could have been any protocol. That is, the final
          SR-MPLS has been popped exposing the payload packet.<a href="#section-3.2.1-6" class="pilcrow">¶</a></p>
<p id="section-3.2.1-7">To handle this, when a router (here it is router G) pops the
          final SR-MPLS label, it inserts an explicit NULL label <span>[<a href="#RFC3032" class="xref">RFC3032</a>]</span> before encapsulating the packet in an
          MPLS-over-UDP tunnel toward router H and sending it out. That is,
          router G pops the top label, discovers it has reached the bottom of
          stack, pushes an explicit NULL label, and then encapsulates the MPLS
          packet in a UDP tunnel to router H.<a href="#section-3.2.1-7" class="pilcrow">¶</a></p>
</section>
</div>
<div id="nophpfwd">
<section id="section-3.2.2">
          <h4 id="name-packet-forwarding-without-p">
<a href="#section-3.2.2" class="section-number selfRef">3.2.2. </a><a href="#name-packet-forwarding-without-p" class="section-name selfRef">Packet Forwarding without Penultimate Hop Popping</a>
          </h4>
<p id="section-3.2.2-1"><a href="#nophpfwdeg" class="xref">Figure 4</a> demonstrates the packet walk in the
          case where the label associated with each Prefix-SID advertised by
          the owner of the Prefix-SID is not a Penultimate Hop-Popping (PHP)
          label (e.g., the NP-Flag in OSPF or the P-Flag in IS-IS
          associated with the Prefix-SID is set). Apart from the PHP function,
          the roles of the routers are unchanged from <a href="#phpfwd" class="xref">Section 3.2.1</a>.<a href="#section-3.2.2-1" class="pilcrow">¶</a></p>
<span id="name-packet-forwarding-example-wi"></span><div id="nophpfwdeg">
<figure id="figure-4">
            <div class="artwork art-text alignCenter" id="section-3.2.2-2.1">
<pre>
 +-----+       +-----+       +-----+        +-----+        +-----+
 |  A  +-------+  B  +-------+  C  +--------+  D  +--------+  H  |
 +-----+       +--+--+       +--+--+        +--+--+        +-----+
                  |             |              |
                  |             |              |
               +--+--+       +--+--+        +--+--+
               |  E  +-------+  F  +--------+  G  |
               +-----+       +-----+        +-----+

      +--------+
      |IP(A-&gt;E)|
      +--------+                 +--------+
      |  UDP   |                 |IP(E-&gt;G)|
      +--------+                 +--------+        +--------+
      |  L(E)  |                 |  UDP   |        |IP(G-&gt;H)|
      +--------+                 +--------+        +--------+
      |  L(G)  |                 |  L(G)  |        |  UDP   |
      +--------+                 +--------+        +--------+
      |  L(H)  |                 |  L(H)  |        |  L(H)  |
      +--------+                 +--------+        +--------+
      | Packet |     ---&gt;        | Packet |  ---&gt;  | Packet |
      +--------+                 +--------+        +--------+
</pre>
</div>
<figcaption><a href="#figure-4" class="selfRef">Figure 4</a>:
<a href="#name-packet-forwarding-example-wi" class="selfRef">Packet-Forwarding Example without PHP</a>
            </figcaption></figure>
</div>
<p id="section-3.2.2-3">As can be seen from the figure, the SR-MPLS label for each
          segment is left in place until the end of the segment where it is
          popped and the next instruction is processed.<a href="#section-3.2.2-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="addnlfwd">
<section id="section-3.2.3">
          <h4 id="name-additional-forwarding-proce">
<a href="#section-3.2.3" class="section-number selfRef">3.2.3. </a><a href="#name-additional-forwarding-proce" class="section-name selfRef">Additional Forwarding Procedures</a>
          </h4>
<dl class="dlParallel" id="section-3.2.3-1">
            <dt id="section-3.2.3-1.1">Non-MPLS Interfaces:</dt>
            <dd id="section-3.2.3-1.2">Although the description in
              the previous two sections is based on the use of Prefix-SIDs,
              tunneling SR-MPLS packets is useful when the top label of a
              received SR-MPLS packet indicates an Adjacency SID and the
              corresponding adjacent node to that Adjacency SID is not capable
              of MPLS forwarding but can still process SR-MPLS packets. In
              this scenario, the top label would be replaced by an IP tunnel
              toward that adjacent node and then forwarded over the
              corresponding link indicated by the Adjacency SID.<a href="#section-3.2.3-1.2" class="pilcrow">¶</a>
</dd>
<dt id="section-3.2.3-1.3">When to Use IP-Based Tunnels:</dt>
            <dd id="section-3.2.3-1.4">The description in
              the previous two sections is based on the assumption that
              an MPLS-over-UDP tunnel is used when the next hop towards the next
              segment is not MPLS enabled. However, even in the case where the
              next hop towards the next segment is MPLS capable, an
              MPLS-over-UDP tunnel towards the next segment could still be
              used instead due to local policies. For instance, in the example
              as described in <a href="#nophpfwdeg" class="xref">Figure 4</a>, assume F is now a
       transit node capable of SR-MPLS while all the other assumptions
              remain unchanged; since F is not identified by a SID in the stack
              and an MPLS-over-UDP tunnel is preferred to an MPLS LSP
              according to local policies, router E replaces the current
              top label with an MPLS-over-UDP tunnel toward router G and sends
              it out. (Note that if an MPLS LSP was preferred, the packet
              would be forwarded as native SR-MPLS.)<a href="#section-3.2.3-1.4" class="pilcrow">¶</a>
</dd>
<dt id="section-3.2.3-1.5">IP Header Fields:</dt>
            <dd id="section-3.2.3-1.6">When encapsulating an MPLS
              packet in UDP, the resulting packet is further encapsulated in
              IP for transmission. IPv4 or IPv6 may be used according to the
              capabilities of the network. The address fields are set as
              described in <a href="#usecases" class="xref">Section 2</a>. The other IP header
              fields (such as the ECN field <span>[<a href="#RFC6040" class="xref">RFC6040</a>]</span>, the
              Differentiated Services Code Point (DSCP) <span>[<a href="#RFC2983" class="xref">RFC2983</a>]</span>, or IPv6 Flow Label) on each UDP-encapsulated
              segment <span class="bcp14">SHOULD</span> be configurable according to the operator's
              policy; they may be copied from the header of the incoming
              packet; they may be promoted from the header of the payload
              packet; they may be set according to instructions programmed to
              be associated with the SID; or they may be configured dependent
              on the outgoing interface and payload. The TTL field setting in
              the encapsulating packet header is handled as described in
              <span>[<a href="#RFC7510" class="xref">RFC7510</a>]</span>, which refers to <span>[<a href="#RFC4023" class="xref">RFC4023</a>]</span>.<a href="#section-3.2.3-1.6" class="pilcrow">¶</a>
</dd>
<dt id="section-3.2.3-1.7">Entropy and ECMP:</dt>
            <dd id="section-3.2.3-1.8">When encapsulating an MPLS
              packet with an IP tunnel header that is capable of encoding
              entropy (such as <span>[<a href="#RFC7510" class="xref">RFC7510</a>]</span>), the corresponding
              entropy field (the source port in the case of a UDP tunnel) <span class="bcp14">MAY</span>
              be filled with an entropy value that is generated by the
              encapsulator to uniquely identify a flow. However, what
              constitutes a flow is locally determined by the encapsulator. For
              instance, if the MPLS label stack contains at least one entropy
              label and the encapsulator is capable of reading that entropy
              label, the entropy label value could be directly copied to the
              source port of the UDP header. Otherwise, the encapsulator may
              have to perform a hash on the whole label stack or the five-tuple
              of the SR-MPLS payload if the payload is determined as an IP packet.
              To avoid recalculating the hash or hunting for the entropy label
              each time the packet is encapsulated in a UDP tunnel, it <span class="bcp14">MAY</span> be
              desirable that the entropy value contained in the incoming
              packet (i.e., the UDP source port value) is retained when
              stripping the UDP header and is reused as the entropy value of
              the outgoing packet.<a href="#section-3.2.3-1.8" class="pilcrow">¶</a>
</dd>
<dt id="section-3.2.3-1.9">Congestion Considerations:</dt>
            <dd id="section-3.2.3-1.10">
              <span><a href="https://www.rfc-editor.org/rfc/rfc7510#section-5" class="relref">Section 5</a> of [<a href="#RFC7510" class="xref">RFC7510</a>]</span> provides a detailed analysis of the
              implications of congestion in MPLS-over-UDP systems and builds
              on <span><a href="https://www.rfc-editor.org/rfc/rfc8085#section-3.1.3" class="relref">Section 3.1.3</a> of [<a href="#RFC8085" class="xref">RFC8085</a>]</span>, which describes
              the congestion implications of UDP tunnels. All of those
              considerations apply to SR-MPLS-over-UDP tunnels as described
              in this document. In particular, it should be noted that the
              traffic carried in SR-MPLS flows is likely to be IP traffic.<a href="#section-3.2.3-1.10" class="pilcrow">¶</a>
</dd>
</dl>
</section>
</div>
</section>
</div>
</section>
</div>
<div id="IANA">
<section id="section-4">
      <h2 id="name-iana-considerations">
<a href="#section-4" class="section-number selfRef">4. </a><a href="#name-iana-considerations" class="section-name selfRef">IANA Considerations</a>
      </h2>
<p id="section-4-1">This document has no IANA actions.<a href="#section-4-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="Security">
<section id="section-5">
      <h2 id="name-security-considerations">
<a href="#section-5" class="section-number selfRef">5. </a><a href="#name-security-considerations" class="section-name selfRef">Security Considerations</a>
      </h2>
<p id="section-5-1">The security consideration of <span>[<a href="#RFC8354" class="xref">RFC8354</a>]</span> (which redirects
      the reader to <span>[<a href="#RFC5095" class="xref">RFC5095</a>]</span>) and <span>[<a href="#RFC7510" class="xref">RFC7510</a>]</span>
      apply. DTLS <span>[<a href="#RFC6347" class="xref">RFC6347</a>]</span> <span class="bcp14">SHOULD</span> be used where security is
      needed on an SR-MPLS-over-UDP segment including when the IP segment crosses
      the public Internet or some other untrusted environment. <span>[<a href="#RFC8402" class="xref">RFC8402</a>]</span>
      provides security considerations for Segment Routing, and <span><a href="https://www.rfc-editor.org/rfc/rfc8402#section-8.1" class="relref">Section 8.1</a> of [<a href="#RFC8402" class="xref">RFC8402</a>]</span> is particularly applicable to SR-MPLS.<a href="#section-5-1" class="pilcrow">¶</a></p>
<p id="section-5-2">It is difficult for an attacker to pass a raw MPLS-encoded packet
      into a network, and operators have considerable experience in excluding
      such packets at the network boundaries, for example, by excluding all
      packets that are revealed to be carrying an MPLS packet as the payload
      of IP tunnels. Further discussion of MPLS security is found in
      <span>[<a href="#RFC5920" class="xref">RFC5920</a>]</span>.<a href="#section-5-2" class="pilcrow">¶</a></p>
<p id="section-5-3">It is easy for a network ingress node to detect any attempt to smuggle an IP
      packet into the network since it would see that the UDP destination port
      was set to MPLS, and such filtering <span class="bcp14">SHOULD</span> be applied. If, however, the
      mechanisms described in <span>[<a href="#RFC8665" class="xref">RFC8665</a>]</span>
      or <span>[<a href="#RFC8667" class="xref">RFC8667</a>]</span> are applied,
      a wider variety of UDP port numbers might be in use making port filtering
      harder.<a href="#section-5-3" class="pilcrow">¶</a></p>
<p id="section-5-4">SR packets not having a destination address terminating in the network
      would be transparently carried and would pose no different security risk to
      the network under consideration than any other traffic.<a href="#section-5-4" class="pilcrow">¶</a></p>
<p id="section-5-5">Where control-plane techniques are used (as described in <a href="#procs" class="xref">Section 3</a>), it is important that these protocols are adequately
      secured for the environment in which they are run as discussed in
      <span>[<a href="#RFC6862" class="xref">RFC6862</a>]</span> and <span>[<a href="#RFC5920" class="xref">RFC5920</a>]</span>.<a href="#section-5-5" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-6">
      <h2 id="name-references">
<a href="#section-6" class="section-number selfRef">6. </a><a href="#name-references" class="section-name selfRef">References</a>
      </h2>
<section id="section-6.1">
        <h3 id="name-normative-references">
<a href="#section-6.1" class="section-number selfRef">6.1. </a><a href="#name-normative-references" class="section-name selfRef">Normative References</a>
        </h3>
<dl class="references">
<dt id="RFC2119">[RFC2119]</dt>
        <dd>
<span class="refAuthor">Bradner, S.</span>, <span class="refTitle">"Key words for use in RFCs to Indicate Requirement Levels"</span>, <span class="seriesInfo">BCP 14</span>, <span class="seriesInfo">RFC 2119</span>, <span class="seriesInfo">DOI 10.17487/RFC2119</span>, <time datetime="1997-03">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="RFC3031">[RFC3031]</dt>
        <dd>
<span class="refAuthor">Rosen, E.</span><span class="refAuthor">, Viswanathan, A.</span><span class="refAuthor">, and R. Callon</span>, <span class="refTitle">"Multiprotocol Label Switching Architecture"</span>, <span class="seriesInfo">RFC 3031</span>, <span class="seriesInfo">DOI 10.17487/RFC3031</span>, <time datetime="2001-01">January 2001</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc3031">https://www.rfc-editor.org/info/rfc3031</a>&gt;</span>. </dd>
<dt id="RFC3032">[RFC3032]</dt>
        <dd>
<span class="refAuthor">Rosen, E.</span><span class="refAuthor">, Tappan, D.</span><span class="refAuthor">, Fedorkow, G.</span><span class="refAuthor">, Rekhter, Y.</span><span class="refAuthor">, Farinacci, D.</span><span class="refAuthor">, Li, T.</span><span class="refAuthor">, and A. Conta</span>, <span class="refTitle">"MPLS Label Stack Encoding"</span>, <span class="seriesInfo">RFC 3032</span>, <span class="seriesInfo">DOI 10.17487/RFC3032</span>, <time datetime="2001-01">January 2001</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc3032">https://www.rfc-editor.org/info/rfc3032</a>&gt;</span>. </dd>
<dt id="RFC4023">[RFC4023]</dt>
        <dd>
<span class="refAuthor">Worster, T.</span><span class="refAuthor">, Rekhter, Y.</span><span class="refAuthor">, and E. Rosen, Ed.</span>, <span class="refTitle">"Encapsulating MPLS in IP or Generic Routing Encapsulation (GRE)"</span>, <span class="seriesInfo">RFC 4023</span>, <span class="seriesInfo">DOI 10.17487/RFC4023</span>, <time datetime="2005-03">March 2005</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4023">https://www.rfc-editor.org/info/rfc4023</a>&gt;</span>. </dd>
<dt id="RFC5095">[RFC5095]</dt>
        <dd>
<span class="refAuthor">Abley, J.</span><span class="refAuthor">, Savola, P.</span><span class="refAuthor">, and G. Neville-Neil</span>, <span class="refTitle">"Deprecation of Type 0 Routing Headers in IPv6"</span>, <span class="seriesInfo">RFC 5095</span>, <span class="seriesInfo">DOI 10.17487/RFC5095</span>, <time datetime="2007-12">December 2007</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc5095">https://www.rfc-editor.org/info/rfc5095</a>&gt;</span>. </dd>
<dt id="RFC6040">[RFC6040]</dt>
        <dd>
<span class="refAuthor">Briscoe, B.</span>, <span class="refTitle">"Tunnelling of Explicit Congestion Notification"</span>, <span class="seriesInfo">RFC 6040</span>, <span class="seriesInfo">DOI 10.17487/RFC6040</span>, <time datetime="2010-11">November 2010</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6040">https://www.rfc-editor.org/info/rfc6040</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="RFC7510">[RFC7510]</dt>
        <dd>
<span class="refAuthor">Xu, X.</span><span class="refAuthor">, Sheth, N.</span><span class="refAuthor">, Yong, L.</span><span class="refAuthor">, Callon, R.</span><span class="refAuthor">, and D. Black</span>, <span class="refTitle">"Encapsulating MPLS in UDP"</span>, <span class="seriesInfo">RFC 7510</span>, <span class="seriesInfo">DOI 10.17487/RFC7510</span>, <time datetime="2015-04">April 2015</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7510">https://www.rfc-editor.org/info/rfc7510</a>&gt;</span>. </dd>
<dt id="RFC7684">[RFC7684]</dt>
        <dd>
<span class="refAuthor">Psenak, P.</span><span class="refAuthor">, Gredler, H.</span><span class="refAuthor">, Shakir, R.</span><span class="refAuthor">, Henderickx, W.</span><span class="refAuthor">, Tantsura, J.</span><span class="refAuthor">, and A. Lindem</span>, <span class="refTitle">"OSPFv2 Prefix/Link Attribute Advertisement"</span>, <span class="seriesInfo">RFC 7684</span>, <span class="seriesInfo">DOI 10.17487/RFC7684</span>, <time datetime="2015-11">November 2015</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7684">https://www.rfc-editor.org/info/rfc7684</a>&gt;</span>. </dd>
<dt id="RFC7794">[RFC7794]</dt>
        <dd>
<span class="refAuthor">Ginsberg, L., Ed.</span><span class="refAuthor">, Decraene, B.</span><span class="refAuthor">, Previdi, S.</span><span class="refAuthor">, Xu, X.</span><span class="refAuthor">, and U. Chunduri</span>, <span class="refTitle">"IS-IS Prefix Attributes for Extended IPv4 and IPv6 Reachability"</span>, <span class="seriesInfo">RFC 7794</span>, <span class="seriesInfo">DOI 10.17487/RFC7794</span>, <time datetime="2016-03">March 2016</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7794">https://www.rfc-editor.org/info/rfc7794</a>&gt;</span>. </dd>
<dt id="RFC7981">[RFC7981]</dt>
        <dd>
<span class="refAuthor">Ginsberg, L.</span><span class="refAuthor">, Previdi, S.</span><span class="refAuthor">, and M. Chen</span>, <span class="refTitle">"IS-IS Extensions for Advertising Router Information"</span>, <span class="seriesInfo">RFC 7981</span>, <span class="seriesInfo">DOI 10.17487/RFC7981</span>, <time datetime="2016-10">October 2016</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7981">https://www.rfc-editor.org/info/rfc7981</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="RFC8402">[RFC8402]</dt>
        <dd>
<span class="refAuthor">Filsfils, C., Ed.</span><span class="refAuthor">, Previdi, S., Ed.</span><span class="refAuthor">, Ginsberg, L.</span><span class="refAuthor">, Decraene, B.</span><span class="refAuthor">, Litkowski, S.</span><span class="refAuthor">, and R. Shakir</span>, <span class="refTitle">"Segment Routing Architecture"</span>, <span class="seriesInfo">RFC 8402</span>, <span class="seriesInfo">DOI 10.17487/RFC8402</span>, <time datetime="2018-07">July 2018</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8402">https://www.rfc-editor.org/info/rfc8402</a>&gt;</span>. </dd>
<dt id="RFC8660">[RFC8660]</dt>
      <dd>
<span class="refAuthor">Bashandy, A.</span><span class="refAuthor">, Filsfils, C.</span><span class="refAuthor">, Previdi, S.</span><span class="refAuthor">, Decraene, B.</span><span class="refAuthor">, Litkowski, S.</span><span class="refAuthor">, and R. Shakir</span>, <span class="refTitle">"Segment Routing with the MPLS Data Plane"</span>, <span class="seriesInfo">RFC 8660</span>, <span class="seriesInfo">DOI 10.17487/RFC8660</span>, <time datetime="2019-12">December 2019</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8660">https://www.rfc-editor.org/info/rfc8660</a>&gt;</span>. </dd>
</dl>
</section>
<section id="section-6.2">
        <h3 id="name-informative-references">
<a href="#section-6.2" class="section-number selfRef">6.2. </a><a href="#name-informative-references" class="section-name selfRef">Informative References</a>
        </h3>
<dl class="references">
<dt id="I-D.ietf-bess-datacenter-gateway">[DC-GATEWAY]</dt>
        <dd>
<span class="refAuthor">Farrel, A.</span><span class="refAuthor">, Drake, J.</span><span class="refAuthor">, Rosen, E.</span><span class="refAuthor">, Patel, K.</span><span class="refAuthor">, and L. Jalil</span>, <span class="refTitle">"Gateway Auto-Discovery and Route Advertisement for Segment Routing Enabled Domain Interconnection"</span>, <span class="refContent">Work in Progress</span>, <span class="seriesInfo">Internet-Draft, draft-ietf-bess-datacenter-gateway-04</span>, <time datetime="2019-08-21">21 August 2019</time>, <span>&lt;<a href="https://tools.ietf.org/html/draft-ietf-bess-datacenter-gateway-04">https://tools.ietf.org/html/draft-ietf-bess-datacenter-gateway-04</a>&gt;</span>. </dd>
<dt id="I-D.ietf-6man-segment-routing-header">[IPv6-SRH]</dt>
        <dd>
<span class="refAuthor">Filsfils, C.</span><span class="refAuthor">, Dukes, D.</span><span class="refAuthor">, Previdi, S.</span><span class="refAuthor">, Leddy, J.</span><span class="refAuthor">, Matsushima, S.</span><span class="refAuthor">, and D. Voyer</span>, <span class="refTitle">"IPv6 Segment Routing Header (SRH)"</span>, <span class="refContent">Work in Progress</span>, <span class="seriesInfo">Internet-Draft, draft-ietf-6man-segment-routing-header-26</span>, <time datetime="2019-10-22">22 October 2019</time>, <span>&lt;<a href="https://tools.ietf.org/html/draft-ietf-6man-segment-routing-header-26">https://tools.ietf.org/html/draft-ietf-6man-segment-routing-header-26</a>&gt;</span>. </dd>
<dt id="I-D.ietf-isis-encapsulation-cap">[ISIS-ENCAP]</dt>
        <dd>
<span class="refAuthor">Xu, X.</span><span class="refAuthor">, Decraene, B.</span><span class="refAuthor">, Raszuk, R.</span><span class="refAuthor">, Chunduri, U.</span><span class="refAuthor">, Contreras, L.</span><span class="refAuthor">, and L. Jalil</span>, <span class="refTitle">"Advertising Tunnelling Capability in IS-IS"</span>, <span class="refContent">Work in Progress</span>, <span class="seriesInfo">Internet-Draft, draft-ietf-isis-encapsulation-cap-01</span>, <time datetime="2017-04-24">24 April 2017</time>, <span>&lt;<a href="https://tools.ietf.org/html/draft-ietf-isis-encapsulation-cap-01">https://tools.ietf.org/html/draft-ietf-isis-encapsulation-cap-01</a>&gt;</span>. </dd>
<dt id="I-D.ietf-ospf-encapsulation-cap">[OSPF-ENCAP]</dt>
        <dd>
<span class="refAuthor">Xu, X.</span><span class="refAuthor">, Decraene, B.</span><span class="refAuthor">, Raszuk, R.</span><span class="refAuthor">, Contreras, L.</span><span class="refAuthor">, and L. Jalil</span>, <span class="refTitle">"The Tunnel Encapsulations OSPF Router Information"</span>, <span class="refContent">Work in Progress</span>, <span class="seriesInfo">Internet-Draft, draft-ietf-ospf-encapsulation-cap-09</span>, <time datetime="2017-10-10">10 October 2017</time>, <span>&lt;<a href="https://tools.ietf.org/html/draft-ietf-ospf-encapsulation-cap-09">https://tools.ietf.org/html/draft-ietf-ospf-encapsulation-cap-09</a>&gt;</span>. </dd>
<dt id="RFC2983">[RFC2983]</dt>
        <dd>
<span class="refAuthor">Black, D.</span>, <span class="refTitle">"Differentiated Services and Tunnels"</span>, <span class="seriesInfo">RFC 2983</span>, <span class="seriesInfo">DOI 10.17487/RFC2983</span>, <time datetime="2000-10">October 2000</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc2983">https://www.rfc-editor.org/info/rfc2983</a>&gt;</span>. </dd>
<dt id="RFC5920">[RFC5920]</dt>
        <dd>
<span class="refAuthor">Fang, L., Ed.</span>, <span class="refTitle">"Security Framework for MPLS and GMPLS Networks"</span>, <span class="seriesInfo">RFC 5920</span>, <span class="seriesInfo">DOI 10.17487/RFC5920</span>, <time datetime="2010-07">July 2010</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc5920">https://www.rfc-editor.org/info/rfc5920</a>&gt;</span>. </dd>
<dt id="RFC6790">[RFC6790]</dt>
        <dd>
<span class="refAuthor">Kompella, K.</span><span class="refAuthor">, Drake, J.</span><span class="refAuthor">, Amante, S.</span><span class="refAuthor">, Henderickx, W.</span><span class="refAuthor">, and L. Yong</span>, <span class="refTitle">"The Use of Entropy Labels in MPLS Forwarding"</span>, <span class="seriesInfo">RFC 6790</span>, <span class="seriesInfo">DOI 10.17487/RFC6790</span>, <time datetime="2012-11">November 2012</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6790">https://www.rfc-editor.org/info/rfc6790</a>&gt;</span>. </dd>
<dt id="RFC6862">[RFC6862]</dt>
        <dd>
<span class="refAuthor">Lebovitz, G.</span><span class="refAuthor">, Bhatia, M.</span><span class="refAuthor">, and B. Weis</span>, <span class="refTitle">"Keying and Authentication for Routing Protocols (KARP) Overview, Threats, and Requirements"</span>, <span class="seriesInfo">RFC 6862</span>, <span class="seriesInfo">DOI 10.17487/RFC6862</span>, <time datetime="2013-03">March 2013</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6862">https://www.rfc-editor.org/info/rfc6862</a>&gt;</span>. </dd>
<dt id="RFC8085">[RFC8085]</dt>
        <dd>
<span class="refAuthor">Eggert, L.</span><span class="refAuthor">, Fairhurst, G.</span><span class="refAuthor">, and G. Shepherd</span>, <span class="refTitle">"UDP Usage Guidelines"</span>, <span class="seriesInfo">BCP 145</span>, <span class="seriesInfo">RFC 8085</span>, <span class="seriesInfo">DOI 10.17487/RFC8085</span>, <time datetime="2017-03">March 2017</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8085">https://www.rfc-editor.org/info/rfc8085</a>&gt;</span>. </dd>
<dt id="RFC8354">[RFC8354]</dt>
        <dd>
<span class="refAuthor">Brzozowski, J.</span><span class="refAuthor">, Leddy, J.</span><span class="refAuthor">, Filsfils, C.</span><span class="refAuthor">, Maglione, R., Ed.</span><span class="refAuthor">, and M. Townsley</span>, <span class="refTitle">"Use Cases for IPv6 Source Packet Routing in Networking (SPRING)"</span>, <span class="seriesInfo">RFC 8354</span>, <span class="seriesInfo">DOI 10.17487/RFC8354</span>, <time datetime="2018-03">March 2018</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8354">https://www.rfc-editor.org/info/rfc8354</a>&gt;</span>. </dd>
<dt id="RFC8662">[RFC8662]</dt>
        <dd>
<span class="refAuthor">Kini, S.</span><span class="refAuthor">, Kompella, K.</span><span class="refAuthor">, Sivabalan, S.</span><span class="refAuthor">, Litkowski, S.</span><span class="refAuthor">, Shakir, R.</span><span class="refAuthor">, and J. Tantsura</span>, <span class="refTitle">"Entropy Label for Source Packet Routing in Networking (SPRING) Tunnels"</span>, <span class="seriesInfo">RFC 8662</span>, <span class="seriesInfo">DOI 10.17487/RFC8662</span>, <time datetime="2019-12">December 2019</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8662">https://www.rfc-editor.org/info/rfc8662</a>&gt;</span>. </dd>
<dt id="RFC8665">[RFC8665]</dt>
        <dd>
<span class="refAuthor">Psenak, P., Ed.</span><span class="refAuthor">, Previdi, S., Ed.</span><span class="refAuthor">, Filsfils, C.</span><span class="refAuthor">, Gredler, H.</span><span class="refAuthor">, Shakir, R.</span><span class="refAuthor">, Henderickx, W.</span><span class="refAuthor">, and J. Tantsura</span>, <span class="refTitle">"OSPF Extensions for Segment Routing"</span>, <span class="seriesInfo">RFC 8665</span>, <span class="seriesInfo">DOI 10.17487/RFC8665</span>, <time datetime="2019-12">December 2019</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8665">https://www.rfc-editor.org/info/rfc8665</a>&gt;</span>. </dd>
<dt id="RFC8667">[RFC8667]</dt>
      <dd>
<span class="refAuthor">Previdi, S., Ed.</span><span class="refAuthor">, Ginsberg, L., Ed.</span><span class="refAuthor">, Filsfils, C.</span><span class="refAuthor">, Bashandy, A.</span><span class="refAuthor">, Gredler, H.</span><span class="refAuthor">, and B. Decraene</span>, <span class="refTitle">"IS-IS Extensions for Segment Routing"</span>, <span class="seriesInfo">RFC 8667</span>, <span class="seriesInfo">DOI 10.17487/RFC8667</span>, <time datetime="2019-12">December 2019</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8667">https://www.rfc-editor.org/info/rfc8667</a>&gt;</span>. </dd>
</dl>
</section>
</section>
<div id="Acknowledgements">
<section id="section-appendix.a">
      <h2 id="name-acknowledgements">
<a href="#name-acknowledgements" class="section-name selfRef">Acknowledgements</a>
      </h2>
<p id="section-appendix.a-1">Thanks to Joel Halpern, Bruno Decraene, Loa Andersson,
      Ron Bonica, Eric Rosen, Jim Guichard, Gunter Van De Velde,
      Andy Malis, Robert Sparks, and Al Morton for their insightful
      comments on this document.<a href="#section-appendix.a-1" class="pilcrow">¶</a></p>
<p id="section-appendix.a-2">Additional thanks to Mirja Kuehlewind, Alvaro Retana, Spencer Dawkins,
      Benjamin Kaduk, Martin Vigoureux, Suresh Krishnan, and Eric Vyncke
      for careful reviews and resulting comments.<a href="#section-appendix.a-2" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-appendix.b">
      <h2 id="name-contributors">
<a href="#name-contributors" class="section-name selfRef">Contributors</a>
      </h2>
<div class="artwork art-text alignLeft" id="section-appendix.b-1">
<pre>
Ahmed Bashandy
Individual
Email: abashandy.ietf@gmail.com
</pre><a href="#section-appendix.b-1" class="pilcrow">¶</a>
</div>
<div class="artwork art-text alignLeft" id="section-appendix.b-2">
<pre>
Clarence Filsfils
Cisco
Email: cfilsfil@cisco.com
</pre><a href="#section-appendix.b-2" class="pilcrow">¶</a>
</div>
<div class="artwork art-text alignLeft" id="section-appendix.b-3">
<pre>
John Drake
Juniper
Email: jdrake@juniper.net
</pre><a href="#section-appendix.b-3" class="pilcrow">¶</a>
</div>
<div class="artwork art-text alignLeft" id="section-appendix.b-4">
<pre>
Shaowen Ma
Mellanox Technologies
Email: mashaowen@gmail.com
</pre><a href="#section-appendix.b-4" class="pilcrow">¶</a>
</div>
<div class="artwork art-text alignLeft" id="section-appendix.b-5">
<pre>
Mach Chen
Huawei
Email: mach.chen@huawei.com
</pre><a href="#section-appendix.b-5" class="pilcrow">¶</a>
</div>
<div class="artwork art-text alignLeft" id="section-appendix.b-6">
<pre>
Hamid Assarpour
Broadcom
Email:hamid.assarpour@broadcom.com
</pre><a href="#section-appendix.b-6" class="pilcrow">¶</a>
</div>
<div class="artwork art-text alignLeft" id="section-appendix.b-7">
<pre>
Robert Raszuk
Bloomberg LP
Email: robert@raszuk.net
</pre><a href="#section-appendix.b-7" class="pilcrow">¶</a>
</div>
<div class="artwork art-text alignLeft" id="section-appendix.b-8">
<pre>
Uma Chunduri
Huawei
Email: uma.chunduri@gmail.com
</pre><a href="#section-appendix.b-8" class="pilcrow">¶</a>
</div>
<div class="artwork art-text alignLeft" id="section-appendix.b-9">
<pre>
Luis M. Contreras
Telefonica I+D
Email: luismiguel.contrerasmurillo@telefonica.com
</pre><a href="#section-appendix.b-9" class="pilcrow">¶</a>
</div>
<div class="artwork art-text alignLeft" id="section-appendix.b-10">
<pre>
Luay Jalil
Verizon
Email: luay.jalil@verizon.com
</pre><a href="#section-appendix.b-10" class="pilcrow">¶</a>
</div>
<div class="artwork art-text alignLeft" id="section-appendix.b-11">
<pre>
Gunter Van De Velde
Nokia
Email: gunter.van_de_velde@nokia.com
</pre><a href="#section-appendix.b-11" class="pilcrow">¶</a>
</div>
<div class="artwork art-text alignLeft" id="section-appendix.b-12">
<pre>
Tal Mizrahi
Marvell
Email: talmi@marvell.com
</pre><a href="#section-appendix.b-12" class="pilcrow">¶</a>
</div>
<div class="artwork art-text alignLeft" id="section-appendix.b-13">
<pre>
Jeff Tantsura
Apstra, Inc.
Email: jefftant.ietf@gmail.com
</pre><a href="#section-appendix.b-13" class="pilcrow">¶</a>
</div>
</section>
<div id="authors-addresses">
<section id="section-appendix.c">
      <h2 id="name-authors-addresses">
<a href="#name-authors-addresses" class="section-name selfRef">Authors' Addresses</a>
      </h2>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Xiaohu Xu</span></div>
<div dir="auto" class="left"><span class="org">Alibaba, Inc</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:xiaohu.xxh@alibaba-inc.com" class="email">xiaohu.xxh@alibaba-inc.com</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Stewart Bryant</span></div>
<div dir="auto" class="left"><span class="org">Futurewei Technologies</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:stewart.bryant@gmail.com" class="email">stewart.bryant@gmail.com</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Adrian Farrel</span></div>
<div dir="auto" class="left"><span class="org">Old Dog Consulting</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:adrian@olddog.co.uk" class="email">adrian@olddog.co.uk</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Syed Hassan</span></div>
<div dir="auto" class="left"><span class="org">Cisco</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:shassan@cisco.com" class="email">shassan@cisco.com</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Wim Henderickx</span></div>
<div dir="auto" class="left"><span class="org">Nokia</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:wim.henderickx@nokia.com" class="email">wim.henderickx@nokia.com</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Zhenbin Li</span></div>
<div dir="auto" class="left"><span class="org">Huawei</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:lizhenbin@huawei.com" class="email">lizhenbin@huawei.com</a>
</div>
</address>
</section>
</div>
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