File: rfc8775.html

package info (click to toggle)
doc-rfc 20230121-1
links: PTS, VCS
area: non-free
in suites: bookworm, forky, sid, trixie
size: 1,609,944 kB
file content (2422 lines) | stat: -rw-r--r-- 102,984 bytes
parent folder | download | duplicates (2)
<!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 8775: PIM Designated Router Load Balancing</title>
<meta content="Yiqun Cai" name="author">
<meta content="Heidi Ou" name="author">
<meta content="Sri Vallepalli" name="author">
<meta content="Mankamana Mishra" name="author">
<meta content="Stig Venaas" name="author">
<meta content="Andy Green" name="author">
<meta content="
       On a multi-access network, one of the PIM-SM (PIM Sparse Mode)
      routers is elected as a
      Designated Router. One of the responsibilities of the Designated Router
      is to track local multicast listeners and forward data to these
      listeners if the group is operating in PIM-SM. This
      document specifies a modification to the PIM-SM protocol that
      allows more than one of the PIM-SM routers to take on this responsibility
      so that the forwarding load can be distributed among multiple routers.
       
    " name="description">
<meta content="xml2rfc 2.43.0" name="generator">
<meta content="Multicast" name="keyword">
<meta content="8775" name="rfc.number">
<link href="rfc8775.xml" rel="alternate" type="application/rfc+xml">
<link href="#copyright" rel="license">
<style type="text/css">/*

  NOTE: Changes at the bottom of this file overrides some earlier settings.

  Once the style has stabilized and has been adopted as an official RFC style,
  this can be consolidated so that style settings occur only in one place, but
  for now the contents of this file consists first of the initial CSS work as
  provided to the RFC Formatter (xml2rfc) work, followed by itemized and
  commented changes found necssary during the development of the v3
  formatters.

*/

/* fonts */
@import url('https://fonts.googleapis.com/css?family=Noto+Sans'); /* Sans-serif */
@import url('https://fonts.googleapis.com/css?family=Noto+Serif'); /* Serif (print) */
@import url('https://fonts.googleapis.com/css?family=Roboto+Mono'); /* Monospace */

@viewport {
  zoom: 1.0;
  width: extend-to-zoom;
}
@-ms-viewport {
  width: extend-to-zoom;
  zoom: 1.0;
}
/* general and mobile first */
html {
}
body {
  max-width: 90%;
  margin: 1.5em auto;
  color: #222;
  background-color: #fff;
  font-size: 14px;
  font-family: 'Noto Sans', Arial, Helvetica, sans-serif;
  line-height: 1.6;
  scroll-behavior: smooth;
}
.ears {
  display: none;
}

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

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

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

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

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

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

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

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

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

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

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

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

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

.refInstance {
  margin-bottom: 1.25em;
}

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

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

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

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

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

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

/* pagination */
@media print {
  body {

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

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

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

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

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

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

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

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

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

}

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


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

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

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

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

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

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

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

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

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


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

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

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

/* Make the alternative author contact informatio look less like just another
   author, and group it closer with the primary author contact information */
.alternative-contact {
  margin: 0.5em 0 0.25em 0;
}
address .non-ascii {
  margin: 0 0 0 2em;
}

/* With it being possible to set tables with alignment
  left, center, and right, { width: 100%; } does not make sense */
table {
  width: auto;
}

/* Avoid reference text that sits in a block with very wide left margin,
   because of a long floating dt label.*/
.references dd {
  overflow: visible;
}

/* Control caption placement */
caption {
  caption-side: bottom;
}

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

address.vcard {
  max-width: 30em;
  margin-right: auto;
}

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

/* Provide table alignment support.  We can't use the alignX classes above
   since they do unwanted things with caption and other styling. */
table.right {
 margin-left: auto;
 margin-right: 0;
}
table.center {
 margin-left: auto;
 margin-right: auto;
}
table.left {
 margin-left: 0;
 margin-right: auto;
}

/* Give the table caption label the same styling as the figcaption */
caption a[href] {
  color: #222;
}

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

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

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

}
/* Avoid wrapping of URLs in references */
@media screen {
  .references a {
    white-space: nowrap;
  }
}
/* Tweak the bcp14 keyword presentation */
.bcp14 {
  font-variant: small-caps;
  font-weight: bold;
  font-size: 0.9em;
}
/* Tweak the invisible space above H* in order not to overlay links in text above */
 h2 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  padding-top: 31px;
 }
 h3 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  padding-top: 24px;
 }
 h4 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  padding-top: 24px;
 }
/* Float artwork pilcrow to the right */
@media screen {
  .artwork a.pilcrow {
    display: block;
    line-height: 0.7;
    margin-top: 0.15em;
  }
}
/* Make pilcrows on dd visible */
@media screen {
  dd:hover > a.pilcrow {
    visibility: visible;
  }
}
/* Make the placement of figcaption match that of a table's caption
   by removing the figure's added bottom margin */
.alignLeft.art-text,
.alignCenter.art-text,
.alignRight.art-text {
   margin-bottom: 0;
}
.alignLeft,
.alignCenter,
.alignRight {
  margin: 1em 0 0 0;
}
/* In print, the pilcrow won't show on hover, so prevent it from taking up space,
   possibly even requiring a new line */
@media print {
  a.pilcrow {
    display: none;
  }
}
/* Styling for the external metadata */
div#external-metadata {
  background-color: #eee;
  padding: 0.5em;
  margin-bottom: 0.5em;
  display: none;
}
div#internal-metadata {
  padding: 0.5em;                       /* to match the external-metadata padding */
}
/* Styling for title RFC Number */
h1#rfcnum {
  clear: both;
  margin: 0 0 -1em;
  padding: 1em 0 0 0;
}
/* Make .olPercent look the same as <ol><li> */
dl.olPercent > dd {
  margin-bottom: 0.25em;
  min-height: initial;
}
/* Give aside some styling to set it apart */
aside {
  border-left: 1px solid #ddd;
  margin: 1em 0 1em 2em;
  padding: 0.2em 2em;
}
aside > dl,
aside > ol,
aside > ul,
aside > table,
aside > p {
  margin-bottom: 0.5em;
}
/* Additional page break settings */
@media print {
  figcaption, table caption {
    page-break-before: avoid;
  }
}
/* Font size adjustments for print */
@media print {
  body  { font-size: 10pt;      line-height: normal; max-width: 96%; }
  h1    { font-size: 1.72em;    padding-top: 1.5em; } /* 1*1.2*1.2*1.2 */
  h2    { font-size: 1.44em;    padding-top: 1.5em; } /* 1*1.2*1.2 */
  h3    { font-size: 1.2em;     padding-top: 1.5em; } /* 1*1.2 */
  h4    { font-size: 1em;       padding-top: 1.5em; }
  h5, h6 { font-size: 1em;      margin: initial; padding: 0.5em 0 0.3em; }
}
/* Sourcecode margin in print, when there's no pilcrow */
@media print {
  .artwork,
  .sourcecode {
    margin-bottom: 1em;
  }
}
/* Avoid narrow tables forcing too narrow table captions, which may render badly */
table {
  min-width: 20em;
}
/* ol type a */
ol.type-a { list-style-type: lower-alpha; }
ol.type-A { list-style-type: upper-alpha; }
ol.type-i { list-style-type: lower-roman; }
ol.type-I { list-style-type: lower-roman; }
/* Apply the print table and row borders in general, on request from the RPC,
and increase the contrast between border and odd row background sligthtly */
table {
  border: 1px solid #ddd;
}
td {
  border-top: 1px solid #ddd;
}
tr:nth-child(2n+1) > td {
  background-color: #f8f8f8;
}
/* Use style rules to govern display of the TOC. */
@media screen and (max-width: 1023px) {
  #toc nav { display: none; }
  #toc.active nav { display: block; }
}
/* Add support for keepWithNext */
.keepWithNext {
  break-after: avoid-page;
  break-after: avoid-page;
}
/* Add support for keepWithPrevious */
.keepWithPrevious {
  break-before: avoid-page;
}
/* Change the approach to avoiding breaks inside artwork etc. */
figure, pre, table, .artwork, .sourcecode  {
  break-before: avoid-page;
  break-after: auto;
}
/* Avoid breaks between <dt> and <dd> */
dl {
  break-inside: auto;
}
dt {
  break-before: auto;
  break-inside: avoid-page;
  break-after: avoid-page;
}
dd {
  break-before: avoid-page;
  break-inside: avoid-page;
  break-after: auto;
}
dd.break {
  margin-bottom: 0;
  min-height: 0;
  break-before: auto;
  break-inside: auto;
  break-after: auto;
}
/* Undo break-before ToC */
@media print {
  #toc {
    break-before: auto;
  }
}</style>
<link href="rfc-local.css" rel="stylesheet" type="text/css">
<link href="https://dx.doi.org/10.17487/rfc8775" rel="alternate">
  <link href="urn:issn:2070-1721" rel="alternate">
  <link href="https://datatracker.ietf.org/doc/draft-ietf-pim-drlb-15" rel="prev">
  </head>
<body>
<script src="https://www.rfc-editor.org/js/metadata.min.js"></script>
<table class="ears">
<thead><tr>
<td class="left">RFC 8775</td>
<td class="center">PIM Designated Router Load Balancing</td>
<td class="right">April 2020</td>
</tr></thead>
<tfoot><tr>
<td class="left">Cai, 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/rfc8775" class="eref">8775</a></dd>
<dt class="label-category">Category:</dt>
<dd class="category">Standards Track</dd>
<dt class="label-published">Published:</dt>
<dd class="published">
<time datetime="2020-04" class="published">April 2020</time>
    </dd>
<dt class="label-issn">ISSN:</dt>
<dd class="issn">2070-1721</dd>
<dt class="label-authors">Authors:</dt>
<dd class="authors">
<div class="author">
      <div class="author-name">Y. Cai</div>
<div class="org">Alibaba Group</div>
</div>
<div class="author">
      <div class="author-name">H. Ou</div>
<div class="org">Alibaba Group</div>
</div>
<div class="author">
      <div class="author-name">S. Vallepalli</div>
</div>
<div class="author">
      <div class="author-name">M. Mishra</div>
<div class="org">Cisco Systems, Inc.</div>
</div>
<div class="author">
      <div class="author-name">S. Venaas</div>
<div class="org">Cisco Systems, Inc.</div>
</div>
<div class="author">
      <div class="author-name">A. Green</div>
<div class="org">British Telecom</div>
</div>
</dd>
</dl>
</div>
<h1 id="rfcnum">RFC 8775</h1>
<h1 id="title">PIM Designated Router Load Balancing</h1>
<section id="section-abstract">
      <h2 id="abstract"><a href="#abstract" class="selfRef">Abstract</a></h2>
<p id="section-abstract-1">On a multi-access network, one of the PIM-SM (PIM Sparse Mode)
      routers is elected as a
      Designated Router. One of the responsibilities of the Designated Router
      is to track local multicast listeners and forward data to these
      listeners if the group is operating in PIM-SM. This
      document specifies a modification to the PIM-SM protocol that
      allows more than one of the PIM-SM routers to take on this responsibility
      so that the forwarding load can be distributed among multiple routers.<a href="#section-abstract-1" class="pilcrow">¶</a></p>
</section>
<div id="status-of-memo">
<section id="section-boilerplate.1">
        <h2 id="name-status-of-this-memo">
<a href="#name-status-of-this-memo" class="section-name selfRef">Status of This Memo</a>
        </h2>
<p id="section-boilerplate.1-1">
            This is an Internet Standards Track document.<a href="#section-boilerplate.1-1" class="pilcrow">¶</a></p>
<p id="section-boilerplate.1-2">
            This document is a product of the Internet Engineering Task Force
            (IETF).  It represents the consensus of the IETF community.  It has
            received public review and has been approved for publication by
            the Internet Engineering Steering Group (IESG).  Further
            information on Internet Standards is available in Section 2 of 
            RFC 7841.<a href="#section-boilerplate.1-2" class="pilcrow">¶</a></p>
<p id="section-boilerplate.1-3">
            Information about the current status of this document, any
            errata, and how to provide feedback on it may be obtained at
            <span><a href="https://www.rfc-editor.org/info/rfc8775">https://www.rfc-editor.org/info/rfc8775</a></span>.<a href="#section-boilerplate.1-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="copyright">
<section id="section-boilerplate.2">
        <h2 id="name-copyright-notice">
<a href="#name-copyright-notice" class="section-name selfRef">Copyright Notice</a>
        </h2>
<p id="section-boilerplate.2-1">
            Copyright (c) 2020 IETF Trust and the persons identified as the
            document authors. All rights reserved.<a href="#section-boilerplate.2-1" class="pilcrow">¶</a></p>
<p id="section-boilerplate.2-2">
            This document is subject to BCP 78 and the IETF Trust's Legal
            Provisions Relating to IETF Documents
            (<span><a href="https://trustee.ietf.org/license-info">https://trustee.ietf.org/license-info</a></span>) in effect on the date of
            publication of this document. Please review these documents
            carefully, as they describe your rights and restrictions with
            respect to this document. Code Components extracted from this
            document must include Simplified BSD License text as described in
            Section 4.e of the Trust Legal Provisions and are provided without
            warranty as described in the Simplified BSD License.<a href="#section-boilerplate.2-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="toc">
<section id="section-toc.1">
        <a href="#" onclick="scroll(0,0)" class="toplink">▲</a><h2 id="name-table-of-contents">
<a href="#name-table-of-contents" class="section-name selfRef">Table of Contents</a>
        </h2>
<nav class="toc"><ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.1">
            <p id="section-toc.1-1.1.1" class="keepWithNext"><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>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.2">
            <p id="section-toc.1-1.2.1" class="keepWithNext"><a href="#section-2" class="xref">2</a>.  <a href="#name-terminology" class="xref">Terminology</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" class="keepWithNext"><a href="#section-3" class="xref">3</a>.  <a href="#name-applicability" class="xref">Applicability</a><a href="#section-toc.1-1.3.1" class="pilcrow">¶</a></p>
</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-functional-overview" class="xref">Functional Overview</a><a href="#section-toc.1-1.4.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.4.2.1">
                <p id="section-toc.1-1.4.2.1.1"><a href="#section-4.1" class="xref">4.1</a>.  <a href="#name-gdr-candidates" class="xref">GDR Candidates</a><a href="#section-toc.1-1.4.2.1.1" class="pilcrow">¶</a></p>
</li>
</ul>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.5">
            <p id="section-toc.1-1.5.1"><a href="#section-5" class="xref">5</a>.  <a href="#name-protocol-specification" class="xref">Protocol Specification</a><a href="#section-toc.1-1.5.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.5.2.1">
                <p id="section-toc.1-1.5.2.1.1"><a href="#section-5.1" class="xref">5.1</a>.  <a href="#name-hash-mask-and-hash-algorith" class="xref">Hash Mask and Hash Algorithm</a><a href="#section-toc.1-1.5.2.1.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.5.2.2">
                <p id="section-toc.1-1.5.2.2.1"><a href="#section-5.2" class="xref">5.2</a>.  <a href="#name-modulo-hash-algorithm" class="xref">Modulo Hash Algorithm</a><a href="#section-toc.1-1.5.2.2.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.5.2.2.2.1">
                    <p id="section-toc.1-1.5.2.2.2.1.1"><a href="#section-5.2.1" class="xref">5.2.1</a>.  <a href="#name-modulo-hash-algorithm-examp" class="xref">Modulo Hash Algorithm Examples</a><a href="#section-toc.1-1.5.2.2.2.1.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.5.2.2.2.2">
                    <p id="section-toc.1-1.5.2.2.2.2.1"><a href="#section-5.2.2" class="xref">5.2.2</a>.  <a href="#name-limitations" class="xref">Limitations</a><a href="#section-toc.1-1.5.2.2.2.2.1" class="pilcrow">¶</a></p>
</li>
</ul>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.5.2.3">
                <p id="section-toc.1-1.5.2.3.1"><a href="#section-5.3" class="xref">5.3</a>.  <a href="#name-pim-hello-options" class="xref">PIM Hello Options</a><a href="#section-toc.1-1.5.2.3.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.5.2.3.2.1">
                    <p id="section-toc.1-1.5.2.3.2.1.1"><a href="#section-5.3.1" class="xref">5.3.1</a>.  <a href="#name-pim-dr-load-balancing-capab" class="xref">PIM DR Load-Balancing Capability (DRLB-Cap) Hello Option</a><a href="#section-toc.1-1.5.2.3.2.1.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.5.2.3.2.2">
                    <p id="section-toc.1-1.5.2.3.2.2.1"><a href="#section-5.3.2" class="xref">5.3.2</a>.  <a href="#name-pim-dr-load-balancing-list-" class="xref">PIM DR Load-Balancing List (DRLB-List) Hello Option</a><a href="#section-toc.1-1.5.2.3.2.2.1" class="pilcrow">¶</a></p>
</li>
</ul>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.5.2.4">
                <p id="section-toc.1-1.5.2.4.1"><a href="#section-5.4" class="xref">5.4</a>.  <a href="#name-pim-dr-operation" class="xref">PIM DR Operation</a><a href="#section-toc.1-1.5.2.4.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.5.2.5">
                <p id="section-toc.1-1.5.2.5.1"><a href="#section-5.5" class="xref">5.5</a>.  <a href="#name-pim-gdr-candidate-operation" class="xref">PIM GDR Candidate Operation</a><a href="#section-toc.1-1.5.2.5.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.5.2.6">
                <p id="section-toc.1-1.5.2.6.1"><a href="#section-5.6" class="xref">5.6</a>.  <a href="#name-drlb-list-hello-option-proc" class="xref">DRLB-List Hello Option Processing</a><a href="#section-toc.1-1.5.2.6.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.5.2.7">
                <p id="section-toc.1-1.5.2.7.1"><a href="#section-5.7" class="xref">5.7</a>.  <a href="#name-pim-assert-modification" class="xref">PIM Assert Modification</a><a href="#section-toc.1-1.5.2.7.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.5.2.8">
                <p id="section-toc.1-1.5.2.8.1"><a href="#section-5.8" class="xref">5.8</a>.  <a href="#name-backward-compatibility" class="xref">Backward Compatibility</a><a href="#section-toc.1-1.5.2.8.1" class="pilcrow">¶</a></p>
</li>
</ul>
</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-operational-considerations" class="xref">Operational Considerations</a><a href="#section-toc.1-1.6.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.7">
            <p id="section-toc.1-1.7.1"><a href="#section-7" class="xref">7</a>.  <a href="#name-iana-considerations" class="xref">IANA Considerations</a><a href="#section-toc.1-1.7.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.7.2.1">
                <p id="section-toc.1-1.7.2.1.1"><a href="#section-7.1" class="xref">7.1</a>.  <a href="#name-initial-registry" class="xref">Initial Registry</a><a href="#section-toc.1-1.7.2.1.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.7.2.2">
                <p id="section-toc.1-1.7.2.2.1"><a href="#section-7.2" class="xref">7.2</a>.  <a href="#name-assignment-of-new-hash-algo" class="xref">Assignment of New Hash Algorithms</a><a href="#section-toc.1-1.7.2.2.1" class="pilcrow">¶</a></p>
</li>
</ul>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.8">
            <p id="section-toc.1-1.8.1"><a href="#section-8" class="xref">8</a>.  <a href="#name-security-considerations" class="xref">Security Considerations</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-9" class="xref">9</a>.  <a href="#name-references" class="xref">References</a><a href="#section-toc.1-1.9.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.9.2.1">
                <p id="section-toc.1-1.9.2.1.1"><a href="#section-9.1" class="xref">9.1</a>.  <a href="#name-normative-references" class="xref">Normative References</a><a href="#section-toc.1-1.9.2.1.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.9.2.2">
                <p id="section-toc.1-1.9.2.2.1"><a href="#section-9.2" class="xref">9.2</a>.  <a href="#name-informative-references" class="xref">Informative References</a><a href="#section-toc.1-1.9.2.2.1" class="pilcrow">¶</a></p>
</li>
</ul>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.10">
            <p id="section-toc.1-1.10.1"><a href="#section-appendix.a" class="xref"></a><a href="#name-acknowledgements" class="xref">Acknowledgements</a><a href="#section-toc.1-1.10.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.11">
            <p id="section-toc.1-1.11.1"><a href="#section-appendix.b" class="xref"></a><a href="#name-authors-addresses" class="xref">Authors' Addresses</a><a href="#section-toc.1-1.11.1" class="pilcrow">¶</a></p>
</li>
</ul>
</nav>
</section>
</div>
<section id="section-1">
      <h2 id="name-introduction">
<a href="#section-1" class="section-number selfRef">1. </a><a href="#name-introduction" class="section-name selfRef">Introduction</a>
      </h2>
<p id="section-1-1">On a multi-access LAN (such as an Ethernet) with one or more PIM-SM
      (PIM Sparse Mode) <span>[<a href="#RFC7761" class="xref">RFC7761</a>]</span> routers, one
      of the PIM-SM 
      routers is elected as a Designated Router (DR). The PIM DR has two
      responsibilities in the PIM-SM protocol. For any active sources on a LAN,
      the PIM DR is responsible for registering with the Rendezvous Point (RP)
      if the group is operating in PIM-SM. Also, the PIM DR is responsible for
      tracking local multicast listeners and forwarding data to these
      listeners if the group is operating in PIM-SM.<a href="#section-1-1" class="pilcrow">¶</a></p>
<p id="section-1-2">Consider the following LAN in <a href="#LAN-REC" class="xref">Figure 1</a>:<a href="#section-1-2" class="pilcrow">¶</a></p>
<span id="name-lan-with-receivers"></span><div id="LAN-REC">
<figure id="figure-1">
        <div class="artwork art-text alignLeft" id="section-1-3.1">
<pre>
                          (core networks)
                           |     |     |
                           |     |     |
                          R1    R2     R3
                           |     |     |
                           ----(LAN)----
                                 |
                                 |
                         (many receivers)
</pre>
</div>
<figcaption><a href="#figure-1" class="selfRef">Figure 1</a>:
<a href="#name-lan-with-receivers" class="selfRef">LAN with Receivers</a>
        </figcaption></figure>
</div>
<p id="section-1-4">Assume R1 is elected as the DR.  According to the
      PIM-SM protocol, R1 will be responsible for forwarding traffic
      to that LAN on behalf of all local members. In addition to keeping
      track of membership reports, R1 is also responsible for
      initiating the creation of source and/or shared trees towards the
      senders or the RPs. The membership reports would be IGMP or Multicast
      Listener Discovery (MLD)
      messages. This applies to any versions of the IGMP and MLD protocols.
      The most recent versions are IGMPv3 <span>[<a href="#RFC3376" class="xref">RFC3376</a>]</span> and
      MLDv2 <span>[<a href="#RFC3810" class="xref">RFC3810</a>]</span>.<a href="#section-1-4" class="pilcrow">¶</a></p>
<p id="section-1-5">Having a single router acting as DR and being responsible for
      data-plane forwarding leads to several issues.  One of the issues is
      that the
      aggregated bandwidth will be limited to what R1 can handle with
      regards to capacity of incoming links, the interface on the LAN,
      and total forwarding capacity. It is very common that a LAN consists of
      switches that run IGMP/MLD or PIM snooping <span>[<a href="#RFC4541" class="xref">RFC4541</a>]</span>. 
      This allows the forwarding of multicast packets to be
      restricted only to segments leading to receivers that have indicated
      their interest in multicast groups using either IGMP or MLD.  The
      emergence of the switched Ethernet allows the aggregated bandwidth to
      exceed, sometimes by a large number, that of a single link.  For
      example, let us modify <a href="#LAN-REC" class="xref">Figure 1</a> and
      introduce an Ethernet switch in <a href="#LAN-SWITCH" class="xref">Figure 2</a>.<a href="#section-1-5" class="pilcrow">¶</a></p>
<span id="name-lan-with-ethernet-switch"></span><div id="LAN-SWITCH">
<figure id="figure-2">
        <div class="artwork art-text alignLeft" id="section-1-6.1">
<pre>
                         (core networks)
                          |     |     |
                          |     |     |
                         R1    R2     R3
                          |     |     |
                       +=gi1===gi2===gi3=+
                       +                 +
                       +      switch     +
                       +                 +
                       +=gi4===gi5===gi6=+
                          |     |     |
                         H1    H2     H3
</pre>
</div>
<figcaption><a href="#figure-2" class="selfRef">Figure 2</a>:
<a href="#name-lan-with-ethernet-switch" class="selfRef">LAN with Ethernet Switch</a>
        </figcaption></figure>
</div>
<p id="section-1-7">Let us assume that each individual link is a Gigabit Ethernet.  Each
      router (R1, R2, and R3) and the switch have enough forwarding capacity
      to handle hundreds of gigabits of data.<a href="#section-1-7" class="pilcrow">¶</a></p>
<p id="section-1-8">Let us further assume that each of the hosts requests 500 Mbps of
      unique multicast data. This totals to 1.5 Gbps of data, which is less
      than what each switch or the combined uplink bandwidth across the
      routers can handle, even under failure of a single router.<a href="#section-1-8" class="pilcrow">¶</a></p>
<p id="section-1-9"> On the other hand, the link between R1 and switch, via port gi1, can
      only handle a throughput of 1 Gbps.  And if R1 is the only DR (the
      PIM DR elected using the procedure defined by <span>[<a href="#RFC7761" class="xref">RFC7761</a>]</span>),
      at least 500 Mbps worth of data will be lost because the only link that
      can be used to draw the traffic from the routers to the switch is via
      gi1. In other words, the entire network's throughput is limited by the
      single connection between the PIM DR and the switch (or LAN, as in
      <a href="#LAN-REC" class="xref">Figure 1</a>).<a href="#section-1-9" class="pilcrow">¶</a></p>
<p id="section-1-10">Another important issue is related to failover.  If R1 is the only
      forwarder on a shared LAN, when R1
      goes out of service, multicast forwarding for the entire LAN has
      to be rebuilt by the newly elected PIM DR.  However, if there were a
      way that allowed multiple routers to forward to the LAN for
      different groups, failure of one of the routers would only lead to
      disruption to a subset of the flows, therefore improving the overall
      resilience of the network.<a href="#section-1-10" class="pilcrow">¶</a></p>
<p id="section-1-11">This document specifies a modification to the PIM-SM protocol
      that allows more than one of these routers, called Group Designated
      Routers (GDRs), to be selected so that the forwarding load can be
      distributed among a number of routers.<a href="#section-1-11" class="pilcrow">¶</a></p>
</section>
<section id="section-2">
      <h2 id="name-terminology">
<a href="#section-2" class="section-number selfRef">2. </a><a href="#name-terminology" class="section-name selfRef">Terminology</a>
      </h2>
<p id="section-2-1">
    The key words "<span class="bcp14">MUST</span>", "<span class="bcp14">MUST NOT</span>",
    "<span class="bcp14">REQUIRED</span>", "<span class="bcp14">SHALL</span>", "<span class="bcp14">SHALL NOT</span>", "<span class="bcp14">SHOULD</span>", "<span class="bcp14">SHOULD NOT</span>",
    "<span class="bcp14">RECOMMENDED</span>", "<span class="bcp14">NOT RECOMMENDED</span>", 
    "<span class="bcp14">MAY</span>", and "<span class="bcp14">OPTIONAL</span>" in this document are
    to be interpreted as 
    described in BCP 14 <span>[<a href="#RFC2119" class="xref">RFC2119</a>]</span> <span>[<a href="#RFC8174" class="xref">RFC8174</a>]</span>
    when, and only when, they appear in all capitals, as shown here.<a href="#section-2-1" class="pilcrow">¶</a></p>
<p id="section-2-2">With respect to PIM-SM, this document follows the terminology that
      has been defined in <span>[<a href="#RFC7761" class="xref">RFC7761</a>]</span>.<a href="#section-2-2" class="pilcrow">¶</a></p>
<p id="section-2-3"> This document also introduces the following new acronyms:<a href="#section-2-3" class="pilcrow">¶</a></p>
<dl class="dlParallel" id="section-2-4">
        <dt id="section-2-4.1"> GDR: Group Designated Router.</dt>
<dd id="section-2-4.2">For each multicast
   flow, either a (*,G) for Any-Source Multicast (ASM) or an (S,G)
   for Source-Specific Multicast (SSM) <span>[<a href="#RFC4607" class="xref">RFC4607</a>]</span>, 
   a hash algorithm (described below) is used to select one of the
   routers as a GDR.  The GDR is responsible for initiating the
   forwarding tree building process for the corresponding multicast
   flow.<a href="#section-2-4.2" class="pilcrow">¶</a>
</dd>
<dd class="break"></dd>
<dt id="section-2-4.3">GDR Candidate:</dt>
<dd id="section-2-4.4">a router that has the potential to
          become a GDR. There might be multiple GDR Candidates on a LAN,
          but only one can become the GDR for a specific multicast flow.<a href="#section-2-4.4" class="pilcrow">¶</a>
</dd>
<dd class="break"></dd>
</dl>
</section>
<section id="section-3">
      <h2 id="name-applicability">
<a href="#section-3" class="section-number selfRef">3. </a><a href="#name-applicability" class="section-name selfRef">Applicability</a>
      </h2>
<p id="section-3-1">The extension specified in this document applies to
      PIM-SM routers acting as last-hop routers (there are directly connected
      receivers). It does not alter the behavior of a PIM DR or any other
      routers on the first-hop network (directly connected sources).
      This is because the source tree is built using the IP address of the
      sender, not the IP address of the PIM DR that sends PIM registers
      towards the RP.  The load balancing between first-hop routers can be
      achieved naturally if an IGP provides equal cost multiple paths
      (which it usually does in practice).  Also, distributing the load to do
      source registration does not justify the additional complexity required
      to support it.<a href="#section-3-1" class="pilcrow">¶</a></p>
</section>
<section id="section-4">
      <h2 id="name-functional-overview">
<a href="#section-4" class="section-number selfRef">4. </a><a href="#name-functional-overview" class="section-name selfRef">Functional Overview</a>
      </h2>
<p id="section-4-1">In the PIM DR election as defined in <span>[<a href="#RFC7761" class="xref">RFC7761</a>]</span>, when 
      multiple routers are connected to a multi-access LAN (for
      example, an Ethernet), one of them is elected to act as PIM DR.  The
      PIM DR is responsible for sending local Join/Prune messages towards the
      RP or source. In order to elect the PIM DR, each PIM router on the LAN
      examines the received PIM Hello messages and compares its own DR
      priority and IP address with those of its neighbors.  The router with
      the highest DR priority is the PIM DR.  If there are multiple such
      routers, their IP addresses are used as the tiebreaker, as described
      in <span>[<a href="#RFC7761" class="xref">RFC7761</a>]</span>.<a href="#section-4-1" class="pilcrow">¶</a></p>
<p id="section-4-2">
        In order to share forwarding load among last-hop routers, besides the
        normal PIM DR election, one or more GDRs are elected on the
 multi-access LAN.  There is only one PIM DR on the multi-access
        LAN, but there might be multiple GDR Candidates.<a href="#section-4-2" class="pilcrow">¶</a></p>
<p id="section-4-3">For each multicast flow, that is, (*,G) for ASM and (S,G) for SSM,
      a hash algorithm (<a href="#maskalgo" class="xref">Section 5.1</a>) is used to
      select one of the routers to be the GDR.
      The new DR Load-Balancing Capability (DRLB-Cap) PIM Hello Option is
      used to announce the Capability, as well as the hash algorithm type.
      Routers with the new DRLB-Cap Option advertised in their PIM Hello,
      using the same GDR election hash algorithm and the same DR priority as
      the PIM DR, are considered as GDR Candidates.<a href="#section-4-3" class="pilcrow">¶</a></p>
<p id="section-4-4">Hash masks are defined for Source, Group, and RP, separately, in
      order to handle PIM ASM/SSM.  The masks, as well as a sorted list of GDR
      Candidate addresses, are announced by the DR in a new DR Load-Balancing
      List (DRLB-List) PIM Hello Option.<a href="#section-4-4" class="pilcrow">¶</a></p>
<p id="section-4-5">A hash algorithm based on the announced Source, Group, or RP masks
      allows one GDR to be assigned to a corresponding multicast state.
      That GDR is responsible for initiating the creation of the
      multicast forwarding tree for multicast traffic.<a href="#section-4-5" class="pilcrow">¶</a></p>
<section id="section-4.1">
        <h3 id="name-gdr-candidates">
<a href="#section-4.1" class="section-number selfRef">4.1. </a><a href="#name-gdr-candidates" class="section-name selfRef">GDR Candidates</a>
        </h3>
<p id="section-4.1-1">GDR is the new concept introduced by this specification.  GDR
        Candidates are routers eligible for GDR election on the LAN.  To
        become a GDR Candidate, a router must have the same DR priority and
 run the same GDR election hash algorithm as the DR on the LAN.<a href="#section-4.1-1" class="pilcrow">¶</a></p>
<p id="section-4.1-2">For example, assume there are 4 routers on the LAN: R1, R2, R3, and
        R4, each announcing a DRLB-Cap Option. R1, R2, and R3 have the same
 DR priority, while R4's DR priority is less preferred.
        In this example, R4 will not be eligible for GDR election, because R4
        will not become a PIM DR unless all of R1, R2, and R3 go out of
        service.<a href="#section-4.1-2" class="pilcrow">¶</a></p>
<p id="section-4.1-3">Furthermore, assume router R1 wins the PIM DR election, R1 and R2
        advertise the same hash algorithm for GDR election, while R3 advertises
 a different one. In this case, only R1 and R2 will be eligible for GDR
        election, while R3 will not.<a href="#section-4.1-3" class="pilcrow">¶</a></p>
<p id="section-4.1-4">As a DR, R1 will include its own Load-Balancing Hash Masks and 
        the identity of R1 and R2 (the GDR Candidates) in its DRLB-List Hello
        Option.<a href="#section-4.1-4" class="pilcrow">¶</a></p>
</section>
</section>
<section id="section-5">
      <h2 id="name-protocol-specification">
<a href="#section-5" class="section-number selfRef">5. </a><a href="#name-protocol-specification" class="section-name selfRef">Protocol Specification</a>
      </h2>
<div id="maskalgo">
<section id="section-5.1">
        <h3 id="name-hash-mask-and-hash-algorith">
<a href="#section-5.1" class="section-number selfRef">5.1. </a><a href="#name-hash-mask-and-hash-algorith" class="section-name selfRef">Hash Mask and Hash Algorithm</a>
        </h3>
<p id="section-5.1-1">A hash mask is used to extract a number of bits from the
        corresponding IP address field (32 for IPv4, 128 for IPv6) and
 calculate a hash value.  A hash value is used to select a GDR from GDR
        Candidates advertised by the PIM DR. Hash masks allow for certain flows
 to always be forwarded by the same GDR, by ignoring certain bits in the
 hash value calculation, so that the hash values are the same. For
 example, 0.0.255.0 defines a
        hash mask for an IPv4 address that masks the first, second, and
        fourth octets, which means that only the third octet will
 influence the hash value computed. Note that the masks need not
 be a contiguous set of bits. For example, for IPv4, 15.15.15.15 would be a
 valid mask.<a href="#section-5.1-1" class="pilcrow">¶</a></p>
<p id="section-5.1-2">
   In the text below, a hash mask is, in some places, said to be zero.
   A hash mask is zero if no bits are set, that is,
   0.0.0.0 for IPv4 and :: for IPv6. Also, a hash mask is said to be
   an all-bits-set mask if it is 255.255.255.255 for IPv4 or
   ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff for IPv6.<a href="#section-5.1-2" class="pilcrow">¶</a></p>
<p id="section-5.1-3">There are three hash masks defined:<a href="#section-5.1-3" class="pilcrow">¶</a></p>
<ul>
<li id="section-5.1-4.1">RP Hash Mask<a href="#section-5.1-4.1" class="pilcrow">¶</a>
</li>
<li id="section-5.1-4.2">Source Hash Mask<a href="#section-5.1-4.2" class="pilcrow">¶</a>
</li>
<li id="section-5.1-4.3">Group Hash Mask<a href="#section-5.1-4.3" class="pilcrow">¶</a>
</li>
</ul>
<p id="section-5.1-5">The hash masks need to be configured on the PIM routers that can
        potentially become a PIM DR, unless the implementation provides
        default hash mask values.
        An implementation <span class="bcp14">SHOULD</span> have default hash mask values as follows.
 The default RP Hash Mask <span class="bcp14">SHOULD</span> be zero (no bits set). The default
 Source and Group Hash Masks <span class="bcp14">SHOULD</span> both be all-bits-set masks.
 These default values are likely acceptable for most deployments and
 simplify configuration. There is only a need to use other masks if
 one needs to ensure that certain flows are forwarded by the same GDR.<a href="#section-5.1-5" class="pilcrow">¶</a></p>
<p id="section-5.1-6">
   The DRLB-List Hello Option contains a list of GDR Candidates.
   The first one listed has ordinal number 0, the second listed
   ordinal number 1, and the last one has ordinal number N - 1 if
   there are N candidates listed. The hash value computed will be
   the ordinal number of the GDR Candidate that is acting as GDR for
   the flow in question.<a href="#section-5.1-6" class="pilcrow">¶</a></p>
<p id="section-5.1-7">The input to be hashed is determined as follows:<a href="#section-5.1-7" class="pilcrow">¶</a></p>
<ul>
<li id="section-5.1-8.1">If the group is in ASM mode and the RP Hash Mask announced by
            the PIM DR is not zero (at least one bit is set), calculate the
     value of hashvalue_RP (<a href="#algorithm" class="xref">Section 5.2</a>) to determine
     the GDR.<a href="#section-5.1-8.1" class="pilcrow">¶</a>
</li>
<li id="section-5.1-8.2">If the group is in ASM mode and the RP Hash Mask announced by
            the PIM DR is zero (no bits are set), obtain the value of
            hashvalue_Group (<a href="#algorithm" class="xref">Section 5.2</a>) to determine the
     GDR.<a href="#section-5.1-8.2" class="pilcrow">¶</a>
</li>
<li id="section-5.1-8.3">If the group is in SSM mode, use
            hashvalue_SG (<a href="#algorithm" class="xref">Section 5.2</a>) to determine the GDR.<a href="#section-5.1-8.3" class="pilcrow">¶</a>
</li>
</ul>
<p id="section-5.1-9">
   A simple modulo hash algorithm is defined in this document.
          However, to allow another hash algorithm to be used, a 1-octet
          "Hash Algorithm" field is included in the DRLB-Cap Hello Option to
          specify the hash algorithm used by the router.<a href="#section-5.1-9" class="pilcrow">¶</a></p>
<p id="section-5.1-10">If different hash algorithms are advertised among the routers
   on a LAN, only the routers advertising the same hash algorithm
   as the DR (as well as having the same DR priority as the DR) are
   eligible for GDR election.<a href="#section-5.1-10" class="pilcrow">¶</a></p>
</section>
</div>
<div id="algorithm">
<section id="section-5.2">
        <h3 id="name-modulo-hash-algorithm">
<a href="#section-5.2" class="section-number selfRef">5.2. </a><a href="#name-modulo-hash-algorithm" class="section-name selfRef">Modulo Hash Algorithm</a>
        </h3>
<p id="section-5.2-1">
   As part of computing the hash, the notation LSZC(hash_mask) is used
   to denote the number of zeroes
   counted from the least significant bit of a hash mask
   hash_mask. As an example, LSZC(255.255.128) is 7 and
   LSZC(ffff:8000::) is 111. If all bits are set, LSZC will
   be 0. If the mask is zero, then
   LSZC will be 32 for IPv4 and 128 for IPv6.<a href="#section-5.2-1" class="pilcrow">¶</a></p>
<p id="section-5.2-2">
   The number of GDR Candidates is denoted as GDRC.<a href="#section-5.2-2" class="pilcrow">¶</a></p>
<p id="section-5.2-3">
   The idea behind the modulo hash algorithm is, in simple terms,
   that the corresponding mask is applied to a value, then the result
   is shifted right LSZC(mask) bits so that the least significant bits
   that were masked out are not considered. Then, this result is masked
   by 0xffffffff, keeping only the last 32 bits of the result 
   (this only makes a difference for IPv6). Finally, the hash value is
   this result modulo the number of GDR Candidates (GDRC).<a href="#section-5.2-3" class="pilcrow">¶</a></p>
<p id="section-5.2-4">
   The modulo hash algorithm, for computing the values hashvalue_RP,
          hashvalue_Group, and hashvalue_SG, is defined as follows.<a href="#section-5.2-4" class="pilcrow">¶</a></p>
<p id="section-5.2-5">
   hashvalue_RP is calculated as:<a href="#section-5.2-5" class="pilcrow">¶</a></p>
<div class="artwork art-text alignLeft" id="section-5.2-6">
<pre>
   (((RP_address &amp; RP_mask) &gt;&gt; LSZC(RP_mask)) &amp; 0xffffffff) % GDRC
</pre><a href="#section-5.2-6" class="pilcrow">¶</a>
</div>
<ul class="ulEmpty">
<li class="ulEmpty" id="section-5.2-7.1">RP_address is the address of the RP defined for the group,
 and RP_mask is the RP Hash Mask.<a href="#section-5.2-7.1" class="pilcrow">¶</a>
</li>
</ul>
<p id="section-5.2-8">
   hashvalue_Group is calculated as:<a href="#section-5.2-8" class="pilcrow">¶</a></p>
<div class="artwork art-text alignLeft" id="section-5.2-9">
<pre>
   (((Group_address &amp; Group_mask) &gt;&gt; LSZC(Group_mask)) &amp; 0xffffffff)
   % GDRC
</pre><a href="#section-5.2-9" class="pilcrow">¶</a>
</div>
<ul class="ulEmpty">
<li class="ulEmpty" id="section-5.2-10.1">
       Group_address is the group address, and Group_mask is the
       Group Hash Mask.<a href="#section-5.2-10.1" class="pilcrow">¶</a>
</li>
</ul>
<p id="section-5.2-11">
   hashvalue_SG is calculated as:<a href="#section-5.2-11" class="pilcrow">¶</a></p>
<div class="artwork art-text alignLeft" id="section-5.2-12">
<pre>
   ((((Source_address &amp; Source_mask) &gt;&gt; LSZC(Source_mask)) &amp;
   0xffffffff) ^ (((Group_address &amp; Group_mask) &gt;&gt; LSZC(Group_mask))
   &amp; 0xffffffff)) % GDRC
</pre><a href="#section-5.2-12" class="pilcrow">¶</a>
</div>
<ul class="ulEmpty">
<li class="ulEmpty" id="section-5.2-13.1">
       Group_address is the group address, and Group_mask is the
       Group Hash Mask.<a href="#section-5.2-13.1" class="pilcrow">¶</a>
</li>
</ul>
<section id="section-5.2.1">
          <h4 id="name-modulo-hash-algorithm-examp">
<a href="#section-5.2.1" class="section-number selfRef">5.2.1. </a><a href="#name-modulo-hash-algorithm-examp" class="section-name selfRef">Modulo Hash Algorithm Examples</a>
          </h4>
<p id="section-5.2.1-1">To help illustrate the algorithm, consider this example.
   Router X with IPv4 address 203.0.113.1 receives a DRLB-List
   Hello Option from the DR that announces RP Hash
   Mask 0.0.255.0 and a list of GDR Candidates, sorted by IP
   addresses from high to low: 203.0.113.3, 203.0.113.2, and
   203.0.113.1.  The ordinal number assigned to those addresses
   would be:<a href="#section-5.2.1-1" class="pilcrow">¶</a></p>
<p id="section-5.2.1-2">
          0 for 203.0.113.3; 1 for 203.0.113.2; 2 for 203.0.113.1
   (Router X).<a href="#section-5.2.1-2" class="pilcrow">¶</a></p>
<p id="section-5.2.1-3">Assume there are 2 RPs: RP1 192.0.2.1 for Group1 and RP2
   198.51.100.2 for Group2.  Following the modulo hash algorithm:<a href="#section-5.2.1-3" class="pilcrow">¶</a></p>
<ul>
<li id="section-5.2.1-4.1">LSZC(0.0.255.0) is 8, and GDRC is 3.
   The hashvalue_RP for Group1 with RP RP1 is:<a href="#section-5.2.1-4.1" class="pilcrow">¶</a>
</li>
</ul>
<ul class="ulEmpty">
<li class="ulEmpty" id="section-5.2.1-5.1">
              <div class="artwork art-text alignLeft" id="section-5.2.1-5.1.1">
<pre>
(((192.0.2.1 &amp; 0.0.255.0) &gt;&gt; 8) &amp; 0xffffffff % 3)
= 2 % 3
= 2
</pre><a href="#section-5.2.1-5.1.1" class="pilcrow">¶</a>
</div>
</li>
<li class="ulEmpty" id="section-5.2.1-5.2">This  matches the ordinal number assigned to Router X.
   Router X will be the GDR for Group1.<a href="#section-5.2.1-5.2" class="pilcrow">¶</a>
</li>
</ul>
<ul>
<li id="section-5.2.1-6.1">The hashvalue_RP for Group2 with RP RP2 is:<a href="#section-5.2.1-6.1" class="pilcrow">¶</a>
</li>
</ul>
<ul class="ulEmpty">
<li class="ulEmpty" id="section-5.2.1-7.1">
              <div class="artwork art-text alignLeft" id="section-5.2.1-7.1.1">
<pre>
(((198.51.100.2 &amp; 0.0.255.0) &gt;&gt; 8) &amp; 0xffffffff % 3)
= 100 % 3
= 1
</pre><a href="#section-5.2.1-7.1.1" class="pilcrow">¶</a>
</div>
</li>
<li class="ulEmpty" id="section-5.2.1-7.2">This is different from the ordinal number of Router X (2).
   Hence, Router X will not be GDR for Group2.<a href="#section-5.2.1-7.2" class="pilcrow">¶</a>
</li>
</ul>
<p id="section-5.2.1-8">For IPv6, consider this example, similar to the above.
   Router X with IPv6 address fe80::1 receives a DRLB-List
   Hello Option from the DR that announces RP Hash
   Mask ::ffff:ffff:ffff:0 and a list of GDR Candidates, sorted by IP
   addresses from high to low: fe80::3, fe80::2, and fe80::1.
   The ordinal number assigned to those addresses would be:<a href="#section-5.2.1-8" class="pilcrow">¶</a></p>
<ul class="ulEmpty">
<li class="ulEmpty" id="section-5.2.1-9.1">0 for fe80::3; 1 for fe80::2; 2 for fe80::1 (Router X).<a href="#section-5.2.1-9.1" class="pilcrow">¶</a>
</li>
</ul>
<p id="section-5.2.1-10">Assume there are 2 RPs: RP1 2001:db8::1:0:5678:1 for Group1 and
   RP2 2001:db8::1:0:1234:2 for Group2.
   Following the modulo hash algorithm:<a href="#section-5.2.1-10" class="pilcrow">¶</a></p>
<ul>
<li id="section-5.2.1-11.1">LSZC(::ffff:ffff:ffff:0) is 16, and GDRC is 3.
   The hashvalue_RP for Group1 with RP RP1 is:<a href="#section-5.2.1-11.1" class="pilcrow">¶</a>
</li>
</ul>
<ul class="ulEmpty">
<li class="ulEmpty" id="section-5.2.1-12.1">
              <div class="artwork art-text alignLeft" id="section-5.2.1-12.1.1">
<pre>
(((2001:db8::1:0:5678:1 &amp; ::ffff:ffff:ffff:0) &gt;&gt; 16) &amp;
 0xffffffff % 3)
= ((::1:0:5678:0 &gt;&gt; 16) &amp; 0xffffffff % 3)
= (::1:0:5678 &amp; 0xffffffff % 3)
= ::5678 % 3
= 2
</pre><a href="#section-5.2.1-12.1.1" class="pilcrow">¶</a>
</div>
</li>
<li class="ulEmpty" id="section-5.2.1-12.2">This matches the ordinal number assigned to Router X.
   Router X will be the GDR for Group1.<a href="#section-5.2.1-12.2" class="pilcrow">¶</a>
</li>
</ul>
<ul>
<li id="section-5.2.1-13.1">The hashvalue_RP for Group2 with RP RP2 is:<a href="#section-5.2.1-13.1" class="pilcrow">¶</a>
</li>
</ul>
<ul class="ulEmpty">
<li class="ulEmpty" id="section-5.2.1-14.1">
              <div class="artwork art-text alignLeft" id="section-5.2.1-14.1.1">
<pre>
(((2001:db8::1:0:1234:1 &amp; ::ffff:ffff:ffff:0) &gt;&gt; 16) &amp;
 0xffffffff % 3)
= ((::1:0:1234:0 &gt;&gt; 16) &amp; 0xffffffff % 3)
= (::1:0:1234 &amp; 0xffffffff % 3)
= ::1234 % 3
= 1
</pre><a href="#section-5.2.1-14.1.1" class="pilcrow">¶</a>
</div>
</li>
<li class="ulEmpty" id="section-5.2.1-14.2">This is different from the ordinal number of Router X (2).
   Hence, Router X will not be GDR for Group2.<a href="#section-5.2.1-14.2" class="pilcrow">¶</a>
</li>
</ul>
</section>
<section id="section-5.2.2">
          <h4 id="name-limitations">
<a href="#section-5.2.2" class="section-number selfRef">5.2.2. </a><a href="#name-limitations" class="section-name selfRef">Limitations</a>
          </h4>
<p id="section-5.2.2-1">
            The modulo hash algorithm has poor failover characteristics when
            a shared LAN has more than two GDRs. In the
            case of more than two GDRs on a LAN, when one GDR fails, all
            of the groups may be reassigned to a different GDR, even if
     they were not assigned to the failed GDR. However, many
     deployments use only two routers on a shared LAN for redundancy
     purposes. Future work may define new hash algorithms where only
     groups assigned to the failed GDR get reassigned.<a href="#section-5.2.2-1" class="pilcrow">¶</a></p>
<p id="section-5.2.2-2">The modulo hash algorithm will use, at most, 32 consecutive bits of
   the input addresses for its computation. Exactly which bits are
   used of the source, group, or RP addresses depend on the respective
   masks. This limitation may be an issue for IPv6 deployments,
   since not all bits of the IPv6 addresses are considered. If this
   causes operational issues, a new hash algorithm would need to be
   defined.<a href="#section-5.2.2-2" class="pilcrow">¶</a></p>
</section>
</section>
</div>
<section id="section-5.3">
        <h3 id="name-pim-hello-options">
<a href="#section-5.3" class="section-number selfRef">5.3. </a><a href="#name-pim-hello-options" class="section-name selfRef">PIM Hello Options</a>
        </h3>
<p id="section-5.3-1">PIM routers include a new option, called
        "Load-Balancing Capability (DRLB-Cap)", in their PIM Hello messages.<a href="#section-5.3-1" class="pilcrow">¶</a></p>
<p id="section-5.3-2">Besides this DRLB-Cap Hello Option, the elected PIM DR also
 includes a new "DR Load-Balancing List (DRLB-List) Hello Option".
 The DRLB-List Hello Option consists of three hash masks, as defined
 above, and also a list of GDR Candidate addresses on the LAN. It is
 recommended that the GDR Candidate addresses are sorted in descending
 order. This ensures that when using algorithms, such as the modulo hash
 algorithm in this document, that it is predictable which GDR is
 responsible for which groups, regardless of the order the DR learned
 about the candidates.<a href="#section-5.3-2" class="pilcrow">¶</a></p>
<section id="section-5.3.1">
          <h4 id="name-pim-dr-load-balancing-capab">
<a href="#section-5.3.1" class="section-number selfRef">5.3.1. </a><a href="#name-pim-dr-load-balancing-capab" class="section-name selfRef">PIM DR Load-Balancing Capability (DRLB-Cap) Hello Option</a>
          </h4>
<span id="name-pim-dr-load-balancing-capabi"></span><div id="PIM-CAP">
<figure id="figure-3">
            <div class="artwork art-text alignCenter" id="section-5.3.1-1.1">
<pre>
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Type = 34           |         Length = 4            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Reserved                  |Hash Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</pre>
</div>
<figcaption><a href="#figure-3" class="selfRef">Figure 3</a>:
<a href="#name-pim-dr-load-balancing-capabi" class="selfRef">PIM DR Load-Balancing Capability Hello Option</a>
            </figcaption></figure>
</div>
<dl class="dlParallel" id="section-5.3.1-2">
            <dt id="section-5.3.1-2.1">Type:</dt>
<dd id="section-5.3.1-2.2">34<a href="#section-5.3.1-2.2" class="pilcrow">¶</a>
</dd>
<dd class="break"></dd>
<dt id="section-5.3.1-2.3">Length:</dt>
<dd id="section-5.3.1-2.4">4<a href="#section-5.3.1-2.4" class="pilcrow">¶</a>
</dd>
<dd class="break"></dd>
<dt id="section-5.3.1-2.5">Reserved:</dt>
<dd id="section-5.3.1-2.6">Transmitted as zero, ignored on receipt.<a href="#section-5.3.1-2.6" class="pilcrow">¶</a>
</dd>
<dd class="break"></dd>
<dt id="section-5.3.1-2.7">Hash Algorithm:</dt>
<dd id="section-5.3.1-2.8">Hash algorithm type. A value listed in the
       IANA "PIM Designated Router Load-Balancing Hash Algorithms"
       registry. 0 is used for the hash algorithm defined in this
       document.<a href="#section-5.3.1-2.8" class="pilcrow">¶</a>
</dd>
<dd class="break"></dd>
</dl>
<p id="section-5.3.1-3">This DRLB-Cap Hello Option <span class="bcp14">MUST</span> be advertised by routers on
          all interfaces where DR Load Balancing is enabled. Note that the
   option is included, at most, once.<a href="#section-5.3.1-3" class="pilcrow">¶</a></p>
</section>
<section id="section-5.3.2">
          <h4 id="name-pim-dr-load-balancing-list-">
<a href="#section-5.3.2" class="section-number selfRef">5.3.2. </a><a href="#name-pim-dr-load-balancing-list-" class="section-name selfRef">PIM DR Load-Balancing List (DRLB-List) Hello Option</a>
          </h4>
<span id="name-pim-dr-load-balancing-list-h"></span><div id="PIM-LIST">
<figure id="figure-4">
            <div class="artwork art-text alignCenter" id="section-5.3.2-1.1">
<pre>
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Type = 35           |         Length                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Group Mask                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Source Mask                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            RP Mask                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    GDR Candidate Address(es)                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</pre>
</div>
<figcaption><a href="#figure-4" class="selfRef">Figure 4</a>:
<a href="#name-pim-dr-load-balancing-list-h" class="selfRef">PIM DR Load-Balancing List Hello Option</a>
            </figcaption></figure>
</div>
<dl class="dlParallel" id="section-5.3.2-2">
            <dt id="section-5.3.2-2.1">Type:</dt>
<dd id="section-5.3.2-2.2">35<a href="#section-5.3.2-2.2" class="pilcrow">¶</a>
</dd>
<dd class="break"></dd>
<dt id="section-5.3.2-2.3">Length:</dt>
<dd id="section-5.3.2-2.4">(3 + n) x (4 or 16) bytes, where n is the number
              of GDR Candidates.<a href="#section-5.3.2-2.4" class="pilcrow">¶</a>
</dd>
<dd class="break"></dd>
<dt id="section-5.3.2-2.5">Group Mask (32/128 bits):</dt>
<dd id="section-5.3.2-2.6">Mask applied to group addresses
       as part of hash computation.<a href="#section-5.3.2-2.6" class="pilcrow">¶</a>
</dd>
<dd class="break"></dd>
<dt id="section-5.3.2-2.7"> Source Mask (32/128 bits):</dt>
<dd id="section-5.3.2-2.8">Mask applied to source addresses
       as part of hash computation.<a href="#section-5.3.2-2.8" class="pilcrow">¶</a>
</dd>
<dd class="break"></dd>
<dt id="section-5.3.2-2.9">RP Mask (32/128 bits):</dt>
<dd id="section-5.3.2-2.10">Mask applied to RP addresses
       as part of hash computation.<a href="#section-5.3.2-2.10" class="pilcrow">¶</a>
</dd>
<dd class="break"></dd>
</dl>
<p id="section-5.3.2-3">All masks <span class="bcp14">MUST</span> have the same number of bits as the IP
      source address in the PIM Hello IP header.<a href="#section-5.3.2-3" class="pilcrow">¶</a></p>
<dl class="dlParallel" id="section-5.3.2-4">
            <dt id="section-5.3.2-4.1">GDR Candidate Address(es) (32/128 bits):</dt>
<dd id="section-5.3.2-4.2">
              <p id="section-5.3.2-4.2.1">List of GDR Candidate(s)<a href="#section-5.3.2-4.2.1" class="pilcrow">¶</a></p>
<p id="section-5.3.2-4.2.2">All addresses <span class="bcp14">MUST</span> be in the same address family as the
 PIM Hello IP header. It is recommended that the addresses are
 sorted in descending order.<a href="#section-5.3.2-4.2.2" class="pilcrow">¶</a></p>
<p id="section-5.3.2-4.2.3">If the "Interface ID" option, as specified in
 <span>[<a href="#RFC6395" class="xref">RFC6395</a>]</span>, is present in a GDR Candidate's
 PIM Hello message and the "Router Identifier" portion is
 non-zero:<a href="#section-5.3.2-4.2.3" class="pilcrow">¶</a></p>
<ul>
<li id="section-5.3.2-4.2.4.1">For IPv4, the "GDR Candidate Address" will be set directly
                  to the "Router Identifier".<a href="#section-5.3.2-4.2.4.1" class="pilcrow">¶</a>
</li>
<li id="section-5.3.2-4.2.4.2">For IPv6, the "GDR Candidate Address" will be 96 bits of
 zeroes, followed by the 32 bit Router Identifier.<a href="#section-5.3.2-4.2.4.2" class="pilcrow">¶</a>
</li>
</ul>
<p id="section-5.3.2-4.2.5">If the "Interface ID" option is not present in a GDR
 Candidate's PIM Hello message or if the "Interface ID"
 option is present but the "Router Identifier" field is zero,
 the "GDR Candidate Address" will be the IPv4 or IPv6 source
 address of the PIM Hello message.<a href="#section-5.3.2-4.2.5" class="pilcrow">¶</a></p>
<p id="section-5.3.2-4.2.6">This DRLB-List Hello Option <span class="bcp14">MUST</span> only be advertised by the
 elected PIM DR. It <span class="bcp14">MUST</span> be ignored if received from a non-DR.
 The option <span class="bcp14">MUST</span> also be ignored if the hash masks are not
 the correct number of bits or GDR Candidate addresses are in
 the wrong address family.<a href="#section-5.3.2-4.2.6" class="pilcrow">¶</a></p>
</dd>
<dd class="break"></dd>
</dl>
</section>
</section>
<section id="section-5.4">
        <h3 id="name-pim-dr-operation">
<a href="#section-5.4" class="section-number selfRef">5.4. </a><a href="#name-pim-dr-operation" class="section-name selfRef">PIM DR Operation</a>
        </h3>
<p id="section-5.4-1">The DR election process is still the same as defined in
 <span>[<a href="#RFC7761" class="xref">RFC7761</a>]</span>. The DR advertises the new DRLB-List Hello
 Option, which contains mask values from user configuration (or default
 values), followed by a list of GDR Candidate addresses. Note that
 if a router included the "Interface ID" option in the hello message
 and the Router ID is non-zero, the Router ID will be used to form the
 GDR Candidate address of the router, as discussed in the previous
 section. It is recommended that the list be sorted from the highest
 value to the lowest value.  The reason for sorting the list is to
 make the behavior deterministic, regardless of the order in which the
 DR learns of new candidates.  Note that, as for non-DR routers, the DR
 also advertises the DRLB-Cap Hello Option to indicate its ability to
 support the new functionality and the type of GDR election hash
 algorithm it uses.<a href="#section-5.4-1" class="pilcrow">¶</a></p>
<p id="section-5.4-2">If a PIM DR receives a neighbor DRLB-Cap Hello Option that
 contains the same hash algorithm as the DR and the neighbor has the
 same DR priority as the DR, PIM DR <span class="bcp14">SHOULD</span> consider the neighbor as a
 GDR Candidate and insert the GDR Candidate's Address into the
 list of the DRLB-List Option. However, the DR may have policies
 limiting which or the number of GDR Candidates to
 include. Likewise, the DR <span class="bcp14">SHOULD</span> include itself in the list of GDR
 Candidates, but it is permissible not to do so, for instance, if there
 is some policy restricting the candidate set.<a href="#section-5.4-2" class="pilcrow">¶</a></p>
<p id="section-5.4-3">If a PIM neighbor included in the list expires, stops announcing
 the DRLB-Cap Hello Option, changes DR priority, changes hash algorithm,
 or otherwise becomes ineligible as a candidate, the DR <span class="bcp14">SHOULD</span>
 immediately send a triggered hello with a new list in the DRLB-List
 option, excluding the neighbor.<a href="#section-5.4-3" class="pilcrow">¶</a></p>
<p id="section-5.4-4">If a new router becomes eligible as a candidate, there is no
 urgency in sending out an updated list. An updated list <span class="bcp14">SHOULD</span> be
 included in the next hello.<a href="#section-5.4-4" class="pilcrow">¶</a></p>
</section>
<section id="section-5.5">
        <h3 id="name-pim-gdr-candidate-operation">
<a href="#section-5.5" class="section-number selfRef">5.5. </a><a href="#name-pim-gdr-candidate-operation" class="section-name selfRef">PIM GDR Candidate Operation</a>
        </h3>
<p id="section-5.5-1">When an IGMP/MLD report is received, a hash algorithm is used by
 the GDR Candidates to determine which router is going to be responsible
 for building forwarding trees on behalf of the host.<a href="#section-5.5-1" class="pilcrow">¶</a></p>
<p id="section-5.5-2">The router <span class="bcp14">MUST</span> include the DRLB-Cap Hello Option in all PIM Hello
 messages sent on the interface.  Note that the presence of the
 DRLB-Cap Option in the PIM Hello does not guarantee that the router
 will be considered as a GDR Candidate.  Once the DR election is done,
 the DRLB-List Hello Option is received from the current PIM DR
 containing a list of the selected GDR Candidates.<a href="#section-5.5-2" class="pilcrow">¶</a></p>
<p id="section-5.5-3">A router only acts as a GDR Candidate if it is included in the GDR
        Candidate list of the DRLB-List Hello Option. See next section for
 details.<a href="#section-5.5-3" class="pilcrow">¶</a></p>
</section>
<section id="section-5.6">
        <h3 id="name-drlb-list-hello-option-proc">
<a href="#section-5.6" class="section-number selfRef">5.6. </a><a href="#name-drlb-list-hello-option-proc" class="section-name selfRef">DRLB-List Hello Option Processing</a>
        </h3>
<p id="section-5.6-1">
   This section discusses processing of the DRLB-List Hello Option,
   including the case where it was received in the previous hello
   but not in the current hello.
   All routers <span class="bcp14">MUST</span> ignore the DRLB-List Hello Option if it is
   received from a PIM router that is not the DR. The option <span class="bcp14">MUST</span>
   only be processed by routers that are announcing the DRLB-Cap Option
   and only if the hash algorithm announced by the DR is the same as
   the local announcement.
   All GDR Candidates <span class="bcp14">MUST</span> use the hash masks advertised
   in the Option, 
   even if they differ from those the candidate was configured with.
   The DR <span class="bcp14">MUST</span> also process its own DRLB-List Hello Option.<a href="#section-5.6-1" class="pilcrow">¶</a></p>
<p id="section-5.6-2">A router stores the latest option contents that were announced,
 if any, and deletes the previous contents. The router <span class="bcp14">MUST</span> also
 compare the new contents with any previous contents and, if there
 are any changes, continue processing as below. Note that if the
 option does not pass the above checks, the below processing <span class="bcp14">MUST</span> be
 done as if the option was not announced.<a href="#section-5.6-2" class="pilcrow">¶</a></p>
<p id="section-5.6-3">
   If the contents of the DRLB-List Option, the masks, or the candidate
   list differ from the previously saved copy, it is received for the
   first time, or it is no longer being received or accepted, the
   option <span class="bcp14">MUST</span> be processed as below.<a href="#section-5.6-3" class="pilcrow">¶</a></p>
<ol start="1" type="1" class="normal type-1" id="section-5.6-4">
          <li id="section-5.6-4.1">
            <p id="section-5.6-4.1.1">If the local router is included in the "GDR Candidate
            Address(es)" field, it will look for its own address, or if it
     announces a non-zero Router ID, its own Router ID. For each of the
            groups or source and group pairs, if the group is in SSM mode
            with local receiver interest, the router <span class="bcp14">MUST</span> run
            the hash algorithm to determine which of them is for the GDR.<a href="#section-5.6-4.1.1" class="pilcrow">¶</a></p>
<ul>
<li id="section-5.6-4.1.2.1">If there is no change in the GDR status, then no further
       action is required.<a href="#section-5.6-4.1.2.1" class="pilcrow">¶</a>
</li>
<li id="section-5.6-4.1.2.2">If the router becomes the new GDR, then a multicast
       forwarding tree <span class="bcp14">MUST</span> be built <span>[<a href="#RFC7761" class="xref">RFC7761</a>]</span>.<a href="#section-5.6-4.1.2.2" class="pilcrow">¶</a>
</li>
<li id="section-5.6-4.1.2.3">
       If the router is no longer the GDR, then it uses an Assert as
              explained in <a href="#assert" class="xref">Section 5.7</a>.<a href="#section-5.6-4.1.2.3" class="pilcrow">¶</a>
</li>
</ul>
</li>
<li id="section-5.6-4.2">
            <p id="section-5.6-4.2.1">If one of the following occurs:<a href="#section-5.6-4.2.1" class="pilcrow">¶</a></p>
<ul>
<li id="section-5.6-4.2.2.1">the local router is not included in the "GDR Candidate
              Address(es)" field,<a href="#section-5.6-4.2.2.1" class="pilcrow">¶</a>
</li>
<li id="section-5.6-4.2.2.2">the DRLB-List Hello Option is no longer included in the DR's
              Hello, or<a href="#section-5.6-4.2.2.2" class="pilcrow">¶</a>
</li>
<li id="section-5.6-4.2.2.3">the DR's Neighbor Liveness Timer expires [RFC7761],<a href="#section-5.6-4.2.2.3" class="pilcrow">¶</a>
</li>
</ul>
<p id="section-5.6-4.2.3">
       then for each group (or each source and group pair if the group
       is in SSM mode) with local receiver interest, for which the
       router is the GDR, the router uses an Assert as explained in
       <a href="#assert" class="xref">Section 5.7</a>.<a href="#section-5.6-4.2.3" class="pilcrow">¶</a></p>
</li>
</ol>
</section>
<div id="assert">
<section id="section-5.7">
        <h3 id="name-pim-assert-modification">
<a href="#section-5.7" class="section-number selfRef">5.7. </a><a href="#name-pim-assert-modification" class="section-name selfRef">PIM Assert Modification</a>
        </h3>
<p id="section-5.7-1">GDR changes may occur due to configuration change,
 GDR Candidates going down, and also new routers coming up and
 becoming GDR Candidates. This may occur while flows are being
 forwarded. If the GDR for an active flow changes, there is likely
 to be some disruption, such as packet loss or duplicates.
 By using asserts, packet loss is minimized while allowing a small
 amount of duplicates.<a href="#section-5.7-1" class="pilcrow">¶</a></p>
<p id="section-5.7-2">When a router stops acting as the GDR for a group, or source and
 group pair if SSM, it <span class="bcp14">MUST</span> set the Assert metric preference to maximum
 (0x7fffffff) and the Assert metric to one less than maximum
 (0xfffffffe). That is, whenever it sends or receives an Assert for the
 group, it must use these values as the metric preference and metric
 rather than the values provided by the unicast routing protocol.<a href="#section-5.7-2" class="pilcrow">¶</a></p>
<p id="section-5.7-3">The rest of this section is just for illustration purposes and
 not part of the protocol definition.<a href="#section-5.7-3" class="pilcrow">¶</a></p>
<p id="section-5.7-4">To illustrate the behavior when there is a GDR change, consider
 the following scenario where there are two flows:
        G1 and G2.  R1 is the GDR for G1, and R2 is the GDR for G2.
        When R3 comes up, it is possible that R3 becomes GDR for both
        G1 and G2; hence, R3 starts to build the forwarding tree for G1 and
        G2.  If R1 and R2 stop forwarding before R3 completes the process,
        packet loss might occur.  On the other hand, if R1 and R2 continue
        forwarding while R3 is building the forwarding trees, duplicates
        might occur.<a href="#section-5.7-4" class="pilcrow">¶</a></p>
<p id="section-5.7-5">When the role of GDR changes as above, instead of immediately
        stopping forwarding, R1 and R2 continue forwarding to G1 and G2
        respectively, while, at the same time, R3 build forwarding trees for
        G1 and G2.  This will lead to PIM Asserts.<a href="#section-5.7-5" class="pilcrow">¶</a></p>
<p id="section-5.7-6">For G1, using the functionality described in this document, R1
 and R3 determine the new GDR, which is R3.  With the modified Assert
 behavior, R1 sets its Assert metric to the near maximum value, as discussed
 above.  That will make R3, which has normal metric in its Assert,
 the Assert winner.<a href="#section-5.7-6" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-5.8">
        <h3 id="name-backward-compatibility">
<a href="#section-5.8" class="section-number selfRef">5.8. </a><a href="#name-backward-compatibility" class="section-name selfRef">Backward Compatibility</a>
        </h3>
<p id="section-5.8-1">In the case of a hybrid Ethernet shared LAN (where some PIM routers
 support the functionality defined in this document and some do not):<a href="#section-5.8-1" class="pilcrow">¶</a></p>
<ul>
<li id="section-5.8-2.1">If the DR does not support the new functionality, then there
   will be no load balancing.<a href="#section-5.8-2.1" class="pilcrow">¶</a>
</li>
<li id="section-5.8-2.2">If non-DR routers do not support the new functionality, they
   will not be considered as GDR Candidate and will not take part
   in load balancing. Load balancing may still happen on the link.<a href="#section-5.8-2.2" class="pilcrow">¶</a>
</li>
</ul>
</section>
</section>
<section id="section-6">
      <h2 id="name-operational-considerations">
<a href="#section-6" class="section-number selfRef">6. </a><a href="#name-operational-considerations" class="section-name selfRef">Operational Considerations</a>
      </h2>
<p id="section-6-1">
 An administrator needs to consider what the total bandwidth
 requirements are and find a set of routers that together have
 enough available capacity while making sure that each of the routers
 can handle its part, assuming that the traffic is distributed
 roughly equally among the routers. Ideally, one should also have
 enough bandwidth to handle the case where at least one router fails.
 All routers should have reachability to the sources and
 RPs, if applicable, that are not via the LAN.<a href="#section-6-1" class="pilcrow">¶</a></p>
<p id="section-6-2">Care must be taken when choosing what hash masks to configure. One
      would typically configure the same masks on all the routers so that
      they are the same, regardless of which router is elected as DR. The
      default masks are likely suitable for most deployment. The RP Hash
      Mask must be configured (the default is no bits set) if one wishes to
      hash based on the RP address rather than the group address for ASM.
      The default masks will use the entire group addresses, and source
      addresses if SSM, as part of the hash. An administrator may set other
      masks that mask out part of the addresses to ensure that certain
      flows always get hashed to the same router. How this is achieved depends
      on how the group addresses are allocated.<a href="#section-6-2" class="pilcrow">¶</a></p>
<p id="section-6-3">
 Only the routers announcing the same hash algorithm as the DR
        would be considered as GDR Candidates. Network administrators
        need to make sure that the desired set of routers announce the
        same algorithm. Migration between different algorithms is
        not considered in this document.<a href="#section-6-3" class="pilcrow">¶</a></p>
</section>
<section id="section-7">
      <h2 id="name-iana-considerations">
<a href="#section-7" class="section-number selfRef">7. </a><a href="#name-iana-considerations" class="section-name selfRef">IANA Considerations</a>
      </h2>
<p id="section-7-1">IANA has made these assignments in the "PIM-Hello Options" registry:
      value 34 for the PIM DR Load-Balancing Capability (DRLB-Cap) Hello
      Option (with Length of 4), and value 35 for the PIM DR Load-Balancing
      List (DRLB-List) Hello Option (with variable Length).<a href="#section-7-1" class="pilcrow">¶</a></p>
<p id="section-7-2"> 
      Per this document, IANA has created a registry called
      "PIM Designated Router Load-Balancing Hash Algorithms" in the
      "Protocol Independent Multicast (PIM)" branch of the registry tree.
      The registry lists hash algorithms for use by PIM Designated Router
      Load Balancing.<a href="#section-7-2" class="pilcrow">¶</a></p>
<section id="section-7.1">
        <h3 id="name-initial-registry">
<a href="#section-7.1" class="section-number selfRef">7.1. </a><a href="#name-initial-registry" class="section-name selfRef">Initial Registry</a>
        </h3>
<p id="section-7.1-1">
          The initial content of the registry is as follows.<a href="#section-7.1-1" class="pilcrow">¶</a></p>
<div id="initial-reg">
<table class="center" id="table-1">
          <caption><a href="#table-1" class="selfRef">Table 1</a></caption>
<thead>
            <tr>
              <th class="text-left" rowspan="1" colspan="1">Type</th>
              <th class="text-left" rowspan="1" colspan="1">Name</th>
              <th class="text-left" rowspan="1" colspan="1">Reference</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td class="text-left" rowspan="1" colspan="1">0</td>
              <td class="text-left" rowspan="1" colspan="1">Modulo</td>
              <td class="text-left" rowspan="1" colspan="1">RFC 8775</td>
            </tr>
            <tr>
              <td class="text-left" rowspan="1" colspan="1">1-255</td>
              <td class="text-left" rowspan="1" colspan="1">Unassigned</td>
              <td class="text-left" rowspan="1" colspan="1"></td>
            </tr>
          </tbody>
        </table>
</div>
</section>
<section id="section-7.2">
        <h3 id="name-assignment-of-new-hash-algo">
<a href="#section-7.2" class="section-number selfRef">7.2. </a><a href="#name-assignment-of-new-hash-algo" class="section-name selfRef">Assignment of New Hash Algorithms</a>
        </h3>
<p id="section-7.2-1">Assignment of new hash algorithms is done according to the "IETF
        Review" procedure; see <span>[<a href="#RFC8126" class="xref">RFC8126</a>]</span>.<a href="#section-7.2-1" class="pilcrow">¶</a></p>
</section>
</section>
<section id="section-8">
      <h2 id="name-security-considerations">
<a href="#section-8" class="section-number selfRef">8. </a><a href="#name-security-considerations" class="section-name selfRef">Security Considerations</a>
      </h2>
<p id="section-8-1">Security of the new DR Load-Balancing PIM Hello Options is only
      guaranteed by the security of PIM Hello messages, so the security
      considerations for PIM Hello messages, as described in PIM-SM
      <span>[<a href="#RFC7761" class="xref">RFC7761</a>]</span>, apply here.<a href="#section-8-1" class="pilcrow">¶</a></p>
<p id="section-8-2">If the DR is subverted, it could omit or add certain GDRs or
      announce an unsupported algorithm. If another router is subverted, it
      could be made DR and cause similar issues. While these issues are
      specific to this specification, they are not that different from existing
      attacks, such as subverting a DR and lowering the DR priority, causing a
      different router to become the DR.<a href="#section-8-2" class="pilcrow">¶</a></p>
<p id="section-8-3">If, for any reason, the DR includes a GDR in the announced list that
      announces a different algorithm from what the DR announces, the GDR
      is required to ignore the announcement, and there will be no router
      acting as the DR for the flows that hash to that GDR.<a href="#section-8-3" class="pilcrow">¶</a></p>
<p id="section-8-4">If a GDR is subverted, it could potentially be made to stop forwarding
      all the traffic it is expected to forward. This is also similar today to
      if a DR is subverted.<a href="#section-8-4" class="pilcrow">¶</a></p>
<p id="section-8-5">An administrator may be able to achieve the desired load balancing
      of known flows, but an attacker may send a single high rate flow that
      is served by a single GDR or send multiple flows that are expected to
      be hashed to the same GDR.<a href="#section-8-5" class="pilcrow">¶</a></p>
</section>
<section id="section-9">
      <h2 id="name-references">
<a href="#section-9" class="section-number selfRef">9. </a><a href="#name-references" class="section-name selfRef">References</a>
      </h2>
<section id="section-9.1">
        <h3 id="name-normative-references">
<a href="#section-9.1" class="section-number selfRef">9.1. </a><a href="#name-normative-references" class="section-name selfRef">Normative References</a>
        </h3>
<dl class="references">
<dt id="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>
<dd class="break"></dd>
<dt id="RFC6395">[RFC6395]</dt>
<dd>
<span class="refAuthor">Gulrajani, S.</span><span class="refAuthor"> and S. Venaas</span>, <span class="refTitle">"An Interface Identifier (ID) Hello Option for PIM"</span>, <span class="seriesInfo">RFC 6395</span>, <span class="seriesInfo">DOI 10.17487/RFC6395</span>, <time datetime="2011-10">October 2011</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6395">https://www.rfc-editor.org/info/rfc6395</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7761">[RFC7761]</dt>
<dd>
<span class="refAuthor">Fenner, B.</span><span class="refAuthor">, Handley, M.</span><span class="refAuthor">, Holbrook, H.</span><span class="refAuthor">, Kouvelas, I.</span><span class="refAuthor">, Parekh, R.</span><span class="refAuthor">, Zhang, Z.</span><span class="refAuthor">, and L. Zheng</span>, <span class="refTitle">"Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)"</span>, <span class="seriesInfo">STD 83</span>, <span class="seriesInfo">RFC 7761</span>, <span class="seriesInfo">DOI 10.17487/RFC7761</span>, <time datetime="2016-03">March 2016</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7761">https://www.rfc-editor.org/info/rfc7761</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8126">[RFC8126]</dt>
<dd>
<span class="refAuthor">Cotton, M.</span><span class="refAuthor">, Leiba, B.</span><span class="refAuthor">, and T. Narten</span>, <span class="refTitle">"Guidelines for Writing an IANA Considerations Section in RFCs"</span>, <span class="seriesInfo">BCP 26</span>, <span class="seriesInfo">RFC 8126</span>, <span class="seriesInfo">DOI 10.17487/RFC8126</span>, <time datetime="2017-06">June 2017</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8126">https://www.rfc-editor.org/info/rfc8126</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8174">[RFC8174]</dt>
<dd>
<span class="refAuthor">Leiba, B.</span>, <span class="refTitle">"Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"</span>, <span class="seriesInfo">BCP 14</span>, <span class="seriesInfo">RFC 8174</span>, <span class="seriesInfo">DOI 10.17487/RFC8174</span>, <time datetime="2017-05">May 2017</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8174">https://www.rfc-editor.org/info/rfc8174</a>&gt;</span>. </dd>
<dd class="break"></dd>
</dl>
</section>
<section id="section-9.2">
        <h3 id="name-informative-references">
<a href="#section-9.2" class="section-number selfRef">9.2. </a><a href="#name-informative-references" class="section-name selfRef">Informative References</a>
        </h3>
<dl class="references">
<dt id="RFC3376">[RFC3376]</dt>
<dd>
<span class="refAuthor">Cain, B.</span><span class="refAuthor">, Deering, S.</span><span class="refAuthor">, Kouvelas, I.</span><span class="refAuthor">, Fenner, B.</span><span class="refAuthor">, and A. Thyagarajan</span>, <span class="refTitle">"Internet Group Management Protocol, Version 3"</span>, <span class="seriesInfo">RFC 3376</span>, <span class="seriesInfo">DOI 10.17487/RFC3376</span>, <time datetime="2002-10">October 2002</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc3376">https://www.rfc-editor.org/info/rfc3376</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC3810">[RFC3810]</dt>
<dd>
<span class="refAuthor">Vida, R., Ed.</span><span class="refAuthor"> and L. Costa, Ed.</span>, <span class="refTitle">"Multicast Listener Discovery Version 2 (MLDv2) for IPv6"</span>, <span class="seriesInfo">RFC 3810</span>, <span class="seriesInfo">DOI 10.17487/RFC3810</span>, <time datetime="2004-06">June 2004</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc3810">https://www.rfc-editor.org/info/rfc3810</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC4541">[RFC4541]</dt>
<dd>
<span class="refAuthor">Christensen, M.</span><span class="refAuthor">, Kimball, K.</span><span class="refAuthor">, and F. Solensky</span>, <span class="refTitle">"Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches"</span>, <span class="seriesInfo">RFC 4541</span>, <span class="seriesInfo">DOI 10.17487/RFC4541</span>, <time datetime="2006-05">May 2006</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4541">https://www.rfc-editor.org/info/rfc4541</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC4607">[RFC4607]</dt>
<dd>
<span class="refAuthor">Holbrook, H.</span><span class="refAuthor"> and B. Cain</span>, <span class="refTitle">"Source-Specific Multicast for IP"</span>, <span class="seriesInfo">RFC 4607</span>, <span class="seriesInfo">DOI 10.17487/RFC4607</span>, <time datetime="2006-08">August 2006</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4607">https://www.rfc-editor.org/info/rfc4607</a>&gt;</span>. </dd>
<dd class="break"></dd>
</dl>
</section>
</section>
<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">
        The authors would like to thank <span class="contact-name">Steve Simlo</span> and
 <span class="contact-name">Taki Millonis</span> for
        helping with the original idea; <span class="contact-name">Alia Atlas</span>,
 <span class="contact-name">Bill Atwood</span>, <span class="contact-name">Joe Clarke</span>,
 <span class="contact-name">Alissa Cooper</span>, <span class="contact-name">Jake  Holland</span>, <span class="contact-name">Bharat Joshi</span>, <span class="contact-name">Anish Kachinthaya</span>, 
 <span class="contact-name">Anvitha Kachinthaya</span>, <span class="contact-name">Benjamin  Kaduk</span>, <span class="contact-name">Mirja Kühlewind</span>, <span class="contact-name">Barry Leiba</span>, 
 <span class="contact-name">Ben Niven-Jenkins</span>, <span class="contact-name">Alvaro  Retana</span>, <span class="contact-name">Adam Roach</span>,
 <span class="contact-name">Michael Scharf</span>, <span class="contact-name">Éric  Vyncke</span>, and <span class="contact-name">Carl Wallace</span>
 for reviews and comments; and <span class="contact-name">Toerless Eckert</span>
 and <span class="contact-name">Rishabh Parekh</span> for helpful conversation on
 the document.<a href="#section-appendix.a-1" class="pilcrow">¶</a></p>
</section>
<div id="authors-addresses">
<section id="section-appendix.b">
      <h2 id="name-authors-addresses">
<a href="#name-authors-addresses" class="section-name selfRef">Authors' Addresses</a>
      </h2>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Yiqun Cai</span></div>
<div dir="auto" class="left"><span class="org">Alibaba Group</span></div>
<div dir="auto" class="left"><span class="street-address">520 Almanor Avenue</span></div>
<div dir="auto" class="left">
<span class="locality">Sunnyvale</span>, <span class="region">CA</span> <span class="postal-code">94085</span>
</div>
<div dir="auto" class="left"><span class="country-name">United States of America</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:yiqun.cai@alibaba-inc.com" class="email">yiqun.cai@alibaba-inc.com</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Heidi Ou</span></div>
<div dir="auto" class="left"><span class="org">Alibaba Group</span></div>
<div dir="auto" class="left"><span class="street-address">520 Almanor Avenue</span></div>
<div dir="auto" class="left">
<span class="locality">Sunnyvale</span>, <span class="region">CA</span> <span class="postal-code">94085</span>
</div>
<div dir="auto" class="left"><span class="country-name">United States of America</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:heidi.ou@alibaba-inc.com" class="email">heidi.ou@alibaba-inc.com</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Sri Vallepalli</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:vallepal@yahoo.com" class="email">vallepal@yahoo.com</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Mankamana Mishra</span></div>
<div dir="auto" class="left"><span class="org">Cisco Systems, Inc.</span></div>
<div dir="auto" class="left"><span class="street-address">821 Alder Drive,</span></div>
<div dir="auto" class="left">
<span class="locality">Milpitas</span>, <span class="region">CA</span> <span class="postal-code">95035</span>
</div>
<div dir="auto" class="left"><span class="country-name">United States of America</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:mankamis@cisco.com" class="email">mankamis@cisco.com</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Stig Venaas</span></div>
<div dir="auto" class="left"><span class="org">Cisco Systems, Inc.</span></div>
<div dir="auto" class="left"><span class="street-address">Tasman Drive</span></div>
<div dir="auto" class="left">
<span class="locality">San Jose</span>, <span class="region">CA</span> <span class="postal-code">95134</span>
</div>
<div dir="auto" class="left"><span class="country-name">United States of America</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:stig@cisco.com" class="email">stig@cisco.com</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Andy Green</span></div>
<div dir="auto" class="left"><span class="org">British Telecom</span></div>
<div dir="auto" class="left"><span class="street-address">Adastral Park</span></div>
<div dir="auto" class="left"><span class="locality">Ipswich</span></div>
<div dir="auto" class="left"><span class="postal-code">IP5 2RE</span></div>
<div dir="auto" class="left"><span class="country-name">United Kingdom</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:andy.da.green@bt.com" class="email">andy.da.green@bt.com</a>
</div>
</address>
</section>
</div>
<script>const toc = document.getElementById("toc");
toc.querySelector("h2").addEventListener("click", e => {
  toc.classList.toggle("active");
});
toc.querySelector("nav").addEventListener("click", e => {
  toc.classList.remove("active");
});
</script>
</body>
</html>