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<!DOCTYPE html>
<html lang="en" class="RFC BCP">
<head>
<meta charset="utf-8">
<meta content="Common,Latin" name="scripts">
<meta content="initial-scale=1.0" name="viewport">
<title>RFC 8704: Enhanced Feasible-Path Unicast Reverse Path Forwarding</title>
<meta content="Kotikalapudi Sriram" name="author">
<meta content="Doug Montgomery" name="author">
<meta content="Jeffrey Haas" name="author">
<meta content="
       
This document identifies a need for and proposes improvement of the unicast
Reverse Path Forwarding (uRPF) techniques (see RFC 3704) for detection and
mitigation of source address spoofing (see BCP 38). Strict uRPF is
inflexible about directionality, the loose uRPF is oblivious to
directionality, and the current feasible-path uRPF attempts to strike a
balance between the two (see RFC 3704). However, as shown in this document,
the existing feasible-path uRPF still has shortcomings. This document
describes enhanced feasible-path uRPF (EFP-uRPF) techniques that are more flexible (in a meaningful way) about directionality than the feasible-path uRPF (RFC 3704). The proposed EFP-uRPF methods aim to significantly reduce false positives regarding invalid detection in source address validation (SAV). Hence, they can potentially alleviate ISPs' concerns about the possibility of disrupting service for their customers and encourage greater deployment of uRPF techniques. This document updates RFC 3704.      
       
    " name="description">
<meta content="xml2rfc 2.39.0" name="generator">
<meta content="BGP, source address spoofing, source address validation (SAV), Reverse Path Forwarding (RPF), unicast RPF (uRPF), DDoS mitigation, BCP 38, BCP 84" name="keyword">
<meta content="8704" name="rfc.number">
<link href="rfc8704.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;
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  scroll-behavior: smooth;
}
.ears {
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}

/* headings */
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}
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svg {
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ul.compact, .ulCompact,
ol.compact, .olCompact {
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/* definition lists */
dl {
}
dl > dt {
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/* 
dl.nohang > dt {
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*/
dl > dd {
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/* links */
a {
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a[href] {
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}
a[href]:hover {
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figcaption a[href],
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/* XXX probably not this:
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#identifiers .authors .org {
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/* The prepared/rendered info at the very bottom of the page */
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}
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.docInfo .prepared {
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}

/* table of contents */
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nav.toc ul {
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}
nav.toc li {
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}
/* references */
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}
.references dd {
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.refInstance {
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}

.references .ascii {
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}

/* index */
.index ul {
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}
.index ul ul {
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}
.index li {
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  text-indent: -2em;
  padding-left: 2em;
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.indexIndex {
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}
.index a {
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}
/* make the index two-column on all but the smallest screens */
@media (min-width: 600px) {
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  .index ul ul {
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}

/* authors */
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address.vcard .nameRole {
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}
address.vcard .label {
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address.vcard .type {
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.alternative-contact {
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}
hr.addr {
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}

/* temporary notes */
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  right: 0.2em;
  padding: 0.2em;
  content: "The RFC Editor will remove this note";
  color: #9e2a00; /* Arlen: WCAG 2019 */
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}
.rfcEditorRemove {
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  background-color: #ffd; /* Arlen: WCAG 2019 */
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.cref {
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}
.crefSource {
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}
/* alternative layout for smaller screens */
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  }
  #title {
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  }
  h1 {
    font-size: 24px;
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  h2 {
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    margin-top: -18px;  /* provide offset for in-page anchors */
    padding-top: 38px;
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  #identifiers dd {
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  }
  #toc {
    position: fixed;
    z-index: 2;
    top: 0;
    right: 0;
    padding: 0;
    margin: 0;
    background-color: inherit;
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  #toc h2 {
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    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;
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  #toc h2::before { /* css hamburger */
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    height: 1px;
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    margin: 6px 0 0 0;
    background: white none repeat scroll 0 0;
    box-shadow: 0 4px 0 0 white, 0 8px 0 0 white;
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    border-left: 1px solid #ddd;
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}

/* alternative layout for wide screens */
@media screen and (min-width: 1024px) {
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    max-width: 724px;
    margin: 42px auto;
    padding-left: 1.5em;
    padding-right: 29em;
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  #toc {
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    right: 42px;
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    font-size: 1em;
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    overflow: auto;
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  img { /* future proofing */
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}

/* pagination */
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    font-size: 10pt;
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}

/* This is commented out here, as the string-set: doesn't
   pass W3C validation currently */
/*
.ears thead .left {
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.ears thead .center {
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*/

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}

@page {
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}

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


/* Top align author divs, to avoid names without organization dropping level with org names */
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/* Leave room in document info to show Internet-Draft on one line */
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/* Don't waste quite as much whitespace between label and value in doc info */
#identifiers dd {
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/* Give floating toc a background color (needed when it's a div inside section */
#toc {
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}

/* 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,
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    text-decoration: none;
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/* Give the bottom of the ToC some whitespace */
@media screen and (min-width: 1024px) {
  #toc {
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}

/* Style section numbers with more space between number and title */
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/* prevent monospace from becoming overly large */
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/* Fix the height/width aspect for ascii art*/
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/* Add styling for a link in the ToC that points to the top of the document */
a.toplink {
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}

/* Fix the dl styling to match the RFC 7992 attributes */
dl > dt,
dl.dlParallel > dt {
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}
dl.dlNewline > dt {
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}

/* 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 {
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}
table td.text-right,
table th.text-right {
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}

/* Make the alternative author contact informatio look less like just another
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.alternative-contact {
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}
address .non-ascii {
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}

/* With it being possible to set tables with alignment
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table {
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}

/* 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 {
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/* 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 {
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  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 {
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  /* Avoid page breaks inside dl and author address entries */
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}
/* Avoid wrapping of URLs in references */
@media screen {
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}
/* Tweak the bcp14 keyword presentation */
.bcp14 {
  font-variant: small-caps;
  font-weight: bold;
  font-size: 0.9em;
}
/* Tweak the invisible space above H* in order not to overlay links in text above */
 h2 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  padding-top: 31px;
 }
 h3 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  padding-top: 24px;
 }
 h4 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  padding-top: 24px;
 }
/* Float artwork pilcrow to the right */
@media screen {
  .artwork a.pilcrow {
    display: block;
    line-height: 0.7;
    margin-top: 0.15em;
  }
}
/* Make pilcrows on dd visible */
@media screen {
  dd:hover > a.pilcrow {
    visibility: visible;
  }
}
/* Make the placement of figcaption match that of a table's caption
   by removing the figure's added bottom margin */
.alignLeft.art-text,
.alignCenter.art-text,
.alignRight.art-text {
   margin-bottom: 0;
}
.alignLeft,
.alignCenter,
.alignRight {
  margin: 1em 0 0 0;
}
/* In print, the pilcrow won't show on hover, so prevent it from taking up space,
   possibly even requiring a new line */
@media print {
  a.pilcrow {
    display: none;
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}
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<link href="https://dx.doi.org/10.17487/rfc8704" rel="alternate">
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<table class="ears">
<thead><tr>
<td class="left">RFC 8704</td>
<td class="center">Enhanced FP-uRPF</td>
<td class="right">February 2020</td>
</tr></thead>
<tfoot><tr>
<td class="left">Sriram, et al.</td>
<td class="center">Best Current Practice</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/rfc8704" class="eref">8704</a></dd>
<dt class="label-bcp">BCP:</dt>
<dd class="bcp">84</dd>
<dt class="label-updates">Updates:</dt>
<dd class="updates">
<a href="https://www.rfc-editor.org/rfc/rfc3704" class="eref">3704</a> </dd>
<dt class="label-category">Category:</dt>
<dd class="category">Best Current Practice</dd>
<dt class="label-published">Published:</dt>
<dd class="published">
<time datetime="2020-02" class="published">February 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">K. Sriram</div>
<div class="org">USA NIST</div>
</div>
<div class="author">
      <div class="author-name">D. Montgomery</div>
<div class="org">USA NIST</div>
</div>
<div class="author">
      <div class="author-name">J. Haas</div>
<div class="org">Juniper Networks, Inc.</div>
</div>
</dd>
</dl>
</div>
<h1 id="rfcnum">RFC 8704</h1>
<h1 id="title">Enhanced Feasible-Path Unicast Reverse Path Forwarding</h1>
<section id="section-abstract">
      <h2 id="abstract"><a href="#abstract" class="selfRef">Abstract</a></h2>
<p id="section-abstract-1">
This document identifies a need for and proposes improvement of the unicast
Reverse Path Forwarding (uRPF) techniques (see RFC 3704) for detection and
mitigation of source address spoofing (see BCP 38). Strict uRPF is
inflexible about directionality, the loose uRPF is oblivious to
directionality, and the current feasible-path uRPF attempts to strike a
balance between the two (see RFC 3704). However, as shown in this document,
the existing feasible-path uRPF still has shortcomings. This document
describes enhanced feasible-path uRPF (EFP-uRPF) techniques that are more flexible (in a meaningful way) about directionality than the feasible-path uRPF (RFC 3704). The proposed EFP-uRPF methods aim to significantly reduce false positives regarding invalid detection in source address validation (SAV). Hence, they can potentially alleviate ISPs' concerns about the possibility of disrupting service for their customers and encourage greater deployment of uRPF techniques. This document updates RFC 3704.<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 memo documents an Internet Best Current Practice.<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 BCPs 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/rfc8704">https://www.rfc-editor.org/info/rfc8704</a></span>.<a href="#section-boilerplate.1-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="copyright">
<section id="section-boilerplate.2">
        <h2 id="name-copyright-notice">
<a href="#name-copyright-notice" class="section-name selfRef">Copyright Notice</a>
        </h2>
<p id="section-boilerplate.2-1">
            Copyright (c) 2020 IETF Trust and the persons identified as the
            document authors. All rights reserved.<a href="#section-boilerplate.2-1" class="pilcrow">¶</a></p>
<p id="section-boilerplate.2-2">
            This document is subject to BCP 78 and the IETF Trust's Legal
            Provisions Relating to IETF Documents
            (<span><a href="https://trustee.ietf.org/license-info">https://trustee.ietf.org/license-info</a></span>) in effect on the date of
            publication of this document. Please review these documents
            carefully, as they describe your rights and restrictions with
            respect to this document. Code Components extracted from this
            document must include Simplified BSD License text as described in
            Section 4.e of the Trust Legal Provisions and are provided without
            warranty as described in the Simplified BSD License.<a href="#section-boilerplate.2-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="toc">
<section id="section-toc.1">
        <a href="#" onclick="scroll(0,0)" class="toplink">▲</a><h2 id="name-table-of-contents">
<a href="#name-table-of-contents" class="section-name selfRef">Table of Contents</a>
        </h2>
<nav class="toc"><ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.1">
            <p id="section-toc.1-1.1.1"><a href="#section-1" class="xref">1</a>.  <a href="#name-introduction" class="xref">Introduction</a><a href="#section-toc.1-1.1.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.1.2.1">
                <p id="section-toc.1-1.1.2.1.1"><a href="#section-1.1" class="xref">1.1</a>.  <a href="#name-terminology" class="xref">Terminology</a><a href="#section-toc.1-1.1.2.1.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.1.2.2">
                <p id="section-toc.1-1.1.2.2.1"><a href="#section-1.2" class="xref">1.2</a>.  <a href="#name-requirements-language" class="xref">Requirements Language</a><a href="#section-toc.1-1.1.2.2.1" class="pilcrow">¶</a></p>
</li>
</ul>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.2">
            <p id="section-toc.1-1.2.1"><a href="#section-2" class="xref">2</a>.  <a href="#name-review-of-existing-source-a" class="xref">Review of Existing Source Address Validation Techniques</a><a href="#section-toc.1-1.2.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.2.2.1">
                <p id="section-toc.1-1.2.2.1.1"><a href="#section-2.1" class="xref">2.1</a>.  <a href="#name-sav-using-access-control-li" class="xref">SAV Using Access Control List</a><a href="#section-toc.1-1.2.2.1.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.2.2.2">
                <p id="section-toc.1-1.2.2.2.1"><a href="#section-2.2" class="xref">2.2</a>.  <a href="#name-sav-using-strict-unicast-re" class="xref">SAV Using Strict Unicast Reverse Path Forwarding</a><a href="#section-toc.1-1.2.2.2.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.2.2.3">
                <p id="section-toc.1-1.2.2.3.1"><a href="#section-2.3" class="xref">2.3</a>.  <a href="#name-sav-using-feasible-path-uni" class="xref">SAV Using Feasible-Path Unicast Reverse Path Forwarding</a><a href="#section-toc.1-1.2.2.3.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.2.2.4">
                <p id="section-toc.1-1.2.2.4.1"><a href="#section-2.4" class="xref">2.4</a>.  <a href="#name-sav-using-loose-unicast-rev" class="xref">SAV Using Loose Unicast Reverse Path Forwarding</a><a href="#section-toc.1-1.2.2.4.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.2.2.5">
                <p id="section-toc.1-1.2.2.5.1"><a href="#section-2.5" class="xref">2.5</a>.  <a href="#name-sav-using-vrf-table" class="xref">SAV Using VRF Table</a><a href="#section-toc.1-1.2.2.5.1" class="pilcrow">¶</a></p>
</li>
</ul>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.3">
            <p id="section-toc.1-1.3.1"><a href="#section-3" class="xref">3</a>.  <a href="#name-sav-using-enhanced-feasible" class="xref">SAV Using Enhanced Feasible-Path uRPF</a><a href="#section-toc.1-1.3.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.3.2.1">
                <p id="section-toc.1-1.3.2.1.1"><a href="#section-3.1" class="xref">3.1</a>.  <a href="#name-description-of-the-method" class="xref">Description of the Method</a><a href="#section-toc.1-1.3.2.1.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.3.2.1.2.1">
                    <p id="section-toc.1-1.3.2.1.2.1.1"><a href="#section-3.1.1" class="xref">3.1.1</a>.  <a href="#name-algorithm-a-enhanced-feasib" class="xref">Algorithm A: Enhanced Feasible-Path uRPF</a><a href="#section-toc.1-1.3.2.1.2.1.1" class="pilcrow">¶</a></p>
</li>
</ul>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.3.2.2">
                <p id="section-toc.1-1.3.2.2.1"><a href="#section-3.2" class="xref">3.2</a>.  <a href="#name-operational-recommendations" class="xref">Operational Recommendations</a><a href="#section-toc.1-1.3.2.2.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.3.2.3">
                <p id="section-toc.1-1.3.2.3.1"><a href="#section-3.3" class="xref">3.3</a>.  <a href="#name-a-challenging-scenario" class="xref">A Challenging Scenario</a><a href="#section-toc.1-1.3.2.3.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.3.2.4">
                <p id="section-toc.1-1.3.2.4.1"><a href="#section-3.4" class="xref">3.4</a>.  <a href="#name-algorithm-b-enhanced-feasib" class="xref">Algorithm B: Enhanced Feasible-Path uRPF with Additional Flexibility across Customer Cone</a><a href="#section-toc.1-1.3.2.4.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.3.2.5">
                <p id="section-toc.1-1.3.2.5.1"><a href="#section-3.5" class="xref">3.5</a>.  <a href="#name-augmenting-rpf-lists-with-r" class="xref">Augmenting RPF Lists with ROA and IRR Data</a><a href="#section-toc.1-1.3.2.5.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.3.2.6">
                <p id="section-toc.1-1.3.2.6.1"><a href="#section-3.6" class="xref">3.6</a>.  <a href="#name-implementation-and-operatio" class="xref">Implementation and Operations Considerations</a><a href="#section-toc.1-1.3.2.6.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.3.2.6.2.1">
                    <p id="section-toc.1-1.3.2.6.2.1.1"><a href="#section-3.6.1" class="xref">3.6.1</a>.  <a href="#name-impact-on-fib-memory-size-r" class="xref">Impact on FIB Memory Size Requirement</a><a href="#section-toc.1-1.3.2.6.2.1.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.3.2.6.2.2">
                    <p id="section-toc.1-1.3.2.6.2.2.1"><a href="#section-3.6.2" class="xref">3.6.2</a>.  <a href="#name-coping-with-bgps-transient-" class="xref">Coping with BGP's Transient Behavior</a><a href="#section-toc.1-1.3.2.6.2.2.1" class="pilcrow">¶</a></p>
</li>
</ul>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.3.2.7">
                <p id="section-toc.1-1.3.2.7.1"><a href="#section-3.7" class="xref">3.7</a>.  <a href="#name-summary-of-recommendations" class="xref">Summary of Recommendations</a><a href="#section-toc.1-1.3.2.7.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.3.2.7.2.1">
                    <p id="section-toc.1-1.3.2.7.2.1.1"><a href="#section-3.7.1" class="xref">3.7.1</a>.  <a href="#name-applicability-of-the-efp-ur" class="xref">Applicability of the EFP-uRPF Method with Algorithm A</a><a href="#section-toc.1-1.3.2.7.2.1.1" class="pilcrow">¶</a></p>
</li>
</ul>
</li>
</ul>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.4">
            <p id="section-toc.1-1.4.1"><a href="#section-4" class="xref">4</a>.  <a href="#name-security-considerations" class="xref">Security Considerations</a><a href="#section-toc.1-1.4.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.5">
            <p id="section-toc.1-1.5.1"><a href="#section-5" class="xref">5</a>.  <a href="#name-iana-considerations" class="xref">IANA Considerations</a><a href="#section-toc.1-1.5.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.6">
            <p id="section-toc.1-1.6.1"><a href="#section-6" class="xref">6</a>.  <a href="#name-references" class="xref">References</a><a href="#section-toc.1-1.6.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.6.2.1">
                <p id="section-toc.1-1.6.2.1.1"><a href="#section-6.1" class="xref">6.1</a>.  <a href="#name-normative-references" class="xref">Normative References</a><a href="#section-toc.1-1.6.2.1.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.6.2.2">
                <p id="section-toc.1-1.6.2.2.1"><a href="#section-6.2" class="xref">6.2</a>.  <a href="#name-informative-references" class="xref">Informative References</a><a href="#section-toc.1-1.6.2.2.1" class="pilcrow">¶</a></p>
</li>
</ul>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.7">
            <p id="section-toc.1-1.7.1"><a href="#section-appendix.a" class="xref"></a><a href="#name-acknowledgements" class="xref">Acknowledgements</a><a href="#section-toc.1-1.7.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.8">
            <p id="section-toc.1-1.8.1"><a href="#section-appendix.b" class="xref"></a><a href="#name-authors-addresses" class="xref">Authors' Addresses</a><a href="#section-toc.1-1.8.1" class="pilcrow">¶</a></p>
</li>
</ul>
</nav>
</section>
</div>
<div id="intro">
<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">
Source address validation (SAV) refers to the detection and mitigation of source address (SA) spoofing <span>[<a href="#RFC2827" class="xref">RFC2827</a>]</span>. This document identifies a need for and proposes improvement of the unicast Reverse Path Forwarding (uRPF) techniques <span>[<a href="#RFC3704" class="xref">RFC3704</a>]</span> for SAV. Strict uRPF is inflexible about directionality (see <span>[<a href="#RFC3704" class="xref">RFC3704</a>]</span> for definitions), the loose uRPF is oblivious to directionality, and the current feasible-path uRPF attempts to strike a balance between the two <span>[<a href="#RFC3704" class="xref">RFC3704</a>]</span>. However, as shown in this document, the existing feasible-path uRPF still has shortcomings. Even with the feasible-path uRPF, ISPs are often apprehensive that they may be dropping customers' data packets with legitimate source addresses.<a href="#section-1-1" class="pilcrow">¶</a></p>
<p id="section-1-2">

This document describes enhanced feasible-path uRPF (EFP-uRPF)
techniques that aim to be more flexible (in a meaningful way) about
directionality than the feasible-path uRPF. It is based on the
principle that if BGP updates for multiple prefixes with the same
origin AS were received on different interfaces (at border routers), then incoming data packets with source addresses in any of those prefixes should be accepted on any of those interfaces (presented in <a href="#newtech" class="xref">Section 3</a>). For some challenging ISP-customer scenarios (see <a href="#challenge" class="xref">Section 3.3</a>), this document also describes a more relaxed version of the enhanced feasible-path uRPF technique (presented in <a href="#algB" class="xref">Section 3.4</a>). Implementation and operations considerations are discussed in <a href="#impl" class="xref">Section 3.6</a>.<a href="#section-1-2" class="pilcrow">¶</a></p>
<p id="section-1-3">
Throughout this document, the routes under consideration are assumed to have been vetted based on prefix filtering <span>[<a href="#RFC7454" class="xref">RFC7454</a>]</span> and possibly origin validation <span>[<a href="#RFC6811" class="xref">RFC6811</a>]</span>.<a href="#section-1-3" class="pilcrow">¶</a></p>
<p id="section-1-4">
The EFP-uRPF methods aim to significantly reduce false positives regarding invalid detection in SAV. They are expected to add greater operational robustness and efficacy to uRPF while minimizing ISPs' concerns about accidental service disruption for their customers. It is expected that this will encourage more deployment of uRPF to help realize its Denial of Service (DoS) and Distributed DoS (DDoS) prevention benefits network wide.<a href="#section-1-4" class="pilcrow">¶</a></p>
<section id="section-1.1">
        <h3 id="name-terminology">
<a href="#section-1.1" class="section-number selfRef">1.1. </a><a href="#name-terminology" class="section-name selfRef">Terminology</a>
        </h3>
<p id="section-1.1-1">
The Reverse Path Forwarding (RPF) list is the list of permissible source-address prefixes for incoming data packets on a given interface.<a href="#section-1.1-1" class="pilcrow">¶</a></p>
<p id="section-1.1-2">
Peering relationships considered in this document are provider-to-customer
(P2C), customer-to-provider (C2P), and peer-to-peer (P2P). Here,
"provider" refers to a transit provider. The first two are transit relationships. A peer connected via a P2P link is known as a lateral peer (non-transit).<a href="#section-1.1-2" class="pilcrow">¶</a></p>
<p id="section-1.1-3">
AS A's customer cone is A plus all the ASes that can be reached from A following only P2C links <span>[<a href="#Luckie" class="xref">Luckie</a>]</span>.<a href="#section-1.1-3" class="pilcrow">¶</a></p>
<p id="section-1.1-4">
A stub AS is an AS that does not have any customers or lateral peers. In this document, a single-homed stub AS is one that has a single transit provider and a multihomed stub AS is one that has multiple (two or more) transit providers.<a href="#section-1.1-4" class="pilcrow">¶</a></p>
</section>
<section id="section-1.2">
        <h3 id="name-requirements-language">
<a href="#section-1.2" class="section-number selfRef">1.2. </a><a href="#name-requirements-language" class="section-name selfRef">Requirements Language</a>
        </h3>
<p id="section-1.2-1">
    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-1.2-1" class="pilcrow">¶</a></p>
</section>
</section>
</div>
<div id="review">
<section id="section-2">
      <h2 id="name-review-of-existing-source-a">
<a href="#section-2" class="section-number selfRef">2. </a><a href="#name-review-of-existing-source-a" class="section-name selfRef">Review of Existing Source Address Validation Techniques</a>
      </h2>
<p id="section-2-1">
There are various existing techniques for the mitigation of DoS/DDoS
attacks with spoofed addresses <span>[<a href="#RFC2827" class="xref">RFC2827</a>]</span> <span>[<a href="#RFC3704" class="xref">RFC3704</a>]</span>.  SAV is performed in network edge devices, such as
border routers, Cable Modem Termination Systems (CMTS) <span>[<a href="#RFC4036" class="xref">RFC4036</a>]</span>, and Packet Data Network Gateways (PDN-GWs) in mobile
networks <span>[<a href="#Firmin" class="xref">Firmin</a>]</span>.  Ingress Access Control List (ACL) and uRPF are techniques employed for implementing SAV <span>[<a href="#RFC2827" class="xref">RFC2827</a>]</span> <span>[<a href="#RFC3704" class="xref">RFC3704</a>]</span> <span>[<a href="#ISOC" class="xref">ISOC</a>]</span>.<a href="#section-2-1" class="pilcrow">¶</a></p>
<div id="acl">
<section id="section-2.1">
        <h3 id="name-sav-using-access-control-li">
<a href="#section-2.1" class="section-number selfRef">2.1. </a><a href="#name-sav-using-access-control-li" class="section-name selfRef">SAV Using Access Control List</a>
        </h3>
<p id="section-2.1-1">
Ingress/egress ACLs are maintained to list acceptable
(or alternatively, unacceptable) prefixes for the source addresses in the
incoming/outgoing Internet Protocol (IP) packets. Any packet with a source
address that fails the filtering criteria is dropped. The ACLs for the
ingress/egress filters need to be maintained to keep them up to
date. Updating the ACLs is an operator-driven manual process; hence,
it is operationally difficult or infeasible.<a href="#section-2.1-1" class="pilcrow">¶</a></p>
<p id="section-2.1-2">
Typically, the egress ACLs in access aggregation devices (e.g., CMTS, PDN-GW)
permit source addresses only from the address spaces (prefixes) that are
associated with the interface on which the customer network is connected. Ingress ACLs are typically deployed on border routers and drop ingress packets when the source address is spoofed (e.g., belongs to obviously disallowed prefix blocks, IANA special-purpose prefixes <span>[<a href="#SPAR-v4" class="xref">SPAR-v4</a>]</span><span>[<a href="#SPAR-v6" class="xref">SPAR-v6</a>]</span>, provider's own prefixes, etc.).<a href="#section-2.1-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="surpf">
<section id="section-2.2">
        <h3 id="name-sav-using-strict-unicast-re">
<a href="#section-2.2" class="section-number selfRef">2.2. </a><a href="#name-sav-using-strict-unicast-re" class="section-name selfRef">SAV Using Strict Unicast Reverse Path Forwarding</a>
        </h3>
<p id="section-2.2-1">
Note: In the figures (scenarios) in this section and the subsequent sections,
the following terminology is used:<a href="#section-2.2-1" class="pilcrow">¶</a></p>
<ul>
<li id="section-2.2-2.1">
"fails" means drops packets with legitimate
source addresses.<a href="#section-2.2-2.1" class="pilcrow">¶</a>
</li>
<li id="section-2.2-2.2">
 "works (but not desirable)" means passes all packets with
legitimate source addresses but is oblivious to directionality.<a href="#section-2.2-2.2" class="pilcrow">¶</a>
</li>
<li id="section-2.2-2.3">
"works best" means passes all packets with legitimate source addresses with no
(or minimal) compromise of directionality.<a href="#section-2.2-2.3" class="pilcrow">¶</a>
</li>
<li id="section-2.2-2.4">
The notation Pi[ASn ASm ...] denotes a BGP update with prefix Pi and an
AS_PATH as shown in the square brackets.<a href="#section-2.2-2.4" class="pilcrow">¶</a>
</li>
</ul>
<p id="section-2.2-3">
In the strict uRPF method, an ingress packet
at a border router is accepted only if the Forwarding Information Base (FIB)
contains a prefix that encompasses the source address and forwarding
information for that prefix points back to the interface over which the packet
was received. In other words, the reverse path for routing to the source
address (if it were used as a destination address) should use the same
interface over which the packet was received. It is well known that this
method has limitations when networks are multihomed, routes are not
symmetrically announced to all transit providers, and there is asymmetric
routing of data packets. Asymmetric routing occurs (see <a href="#fig1" class="xref">Figure 1</a>) when a customer AS announces one prefix (P1) to one
transit provider (ISP-a) and a different prefix (P2) to another transit
provider (ISP-b) but routes data packets with source addresses in the second
prefix (P2) to the first transit provider (ISP-a) or vice versa. Then, data
packets with a source address in prefix P2 that are received at AS2
directly from AS1
will get dropped.  Further, data packets with a source address in prefix P1 that originate from AS1 and traverse via AS3 to AS2 will also get dropped at AS2.<a href="#section-2.2-3" class="pilcrow">¶</a></p>
<span id="name-scenario-1-for-illustration"></span><div id="fig1">
<figure id="figure-1">
          <div class="artwork art-text alignLeft" id="section-2.2-4.1">
<pre>
           +------------+ ---- P1[AS2 AS1] ---&gt; +------------+
           | AS2(ISP-a) | &lt;----P2[AS3 AS1] ---- | AS3(ISP-b) |
           +------------+                       +------------+ 
                    /\                             /\
                     \                             /
                      \                           /
                       \                         /
                 P1[AS1]\                       /P2[AS1]
                         \                     /
                        +-----------------------+
                        |      AS1(customer)    |
                        +-----------------------+
                          P1, P2 (prefixes originated)

          Consider data packets received at AS2
          (1) from AS1 with a source address (SA) in P2, or
          (2) from AS3 that originated from AS1 with a SA in P1:
                    * Strict uRPF fails
                    * Feasible-path uRPF fails
                    * Loose uRPF works (but not desirable)
                    * Enhanced feasible-path uRPF works best 
</pre>
</div>
<figcaption><a href="#figure-1" class="selfRef">Figure 1</a>:
<a href="#name-scenario-1-for-illustration" class="selfRef">Scenario 1 for Illustration of Efficacy of uRPF Schemes</a>
          </figcaption></figure>
</div>
</section>
</div>
<div id="fp-urpf">
<section id="section-2.3">
        <h3 id="name-sav-using-feasible-path-uni">
<a href="#section-2.3" class="section-number selfRef">2.3. </a><a href="#name-sav-using-feasible-path-uni" class="section-name selfRef">SAV Using Feasible-Path Unicast Reverse Path Forwarding</a>
        </h3>
<p id="section-2.3-1">
The feasible-path uRPF technique helps partially overcome the problem
identified with the strict uRPF in the multihoming case. The feasible-path
uRPF is similar to the strict uRPF, but in addition to inserting the best-path
prefix, additional prefixes from alternative announced routes are also
included in the RPF list. This method relies on either (a) announcements for
the same prefixes (albeit some may be prepended to effect lower
preference) propagating to all transit providers performing feasible-path uRPF checks or
(b) announcement of an aggregate less-specific prefix to all transit providers
while announcing more-specific prefixes (covered by the less-specific prefix)
to different transit providers as needed for traffic engineering.<a href="#section-2.3-1" class="pilcrow">¶</a></p>
<p id="section-2.3-2">As an
example, in the multihoming scenario (see Scenario 2 in <a href="#fig2" class="xref">Figure 2</a>), if the customer AS announces routes for both prefixes
(P1, P2) to both transit providers (with suitable prepends if needed for
traffic engineering), then the feasible-path uRPF method works. It should be
mentioned that the feasible-path uRPF works in this scenario only if customer
routes are preferred at AS2 and AS3 over a shorter non-customer
route. However, the feasible-path uRPF method has limitations as well. One
form of limitation naturally occurs when the recommendation (a) or (b)
mentioned above regarding propagation of prefixes is not followed.<a href="#section-2.3-2" class="pilcrow">¶</a></p>
<p id="section-2.3-3">Another
form of limitation can be described as follows. In Scenario 2 (described here,
illustrated in <a href="#fig2" class="xref">Figure 2</a>), it is possible that
the second transit provider (ISP-b or AS3) does not propagate the prepended
route for prefix P1 to the first transit provider (ISP-a or AS2). This is
because AS3's decision policy permits giving priority to a shorter route to
prefix P1 via a lateral peer (AS2) over a longer route learned directly from
the customer (AS1). In such a scenario, AS3 would not send any route
announcement for prefix P1 to AS2 (over the P2P link). Then, a data packet
with a source address in prefix P1 that originates from AS1 and traverses via AS3 to AS2 will get dropped at AS2.<a href="#section-2.3-3" class="pilcrow">¶</a></p>
<span id="name-scenario-2-for-illustration"></span><div id="fig2">
<figure id="figure-2">
          <div class="artwork art-text alignLeft" id="section-2.3-4.1">
<pre>                   
          +------------+  routes for P1, P2   +------------+
          | AS2(ISP-a) |&lt;--------------------&gt;| AS3(ISP-b) |
          +------------+        (P2P)         +------------+
                    /\                            /\
                     \                            /
               P1[AS1]\                          /P2[AS1]
                       \                        /
         P2[AS1 AS1 AS1]\                      /P1[AS1 AS1 AS1]
                         \                    /
                        +-----------------------+
                        |      AS1(customer)    |
                        +-----------------------+
                          P1, P2 (prefixes originated)

        Consider data packets received at AS2 via AS3
        that originated from AS1 and have a source address in P1:
        * Feasible-path uRPF works (if the customer route to P1
          is preferred at AS3 over the shorter path)
        * Feasible-path uRPF fails (if the shorter path to P1
          is preferred at AS3 over the customer route)
        * Loose uRPF works (but not desirable)
        * Enhanced feasible-path uRPF works best
</pre>
</div>
<figcaption><a href="#figure-2" class="selfRef">Figure 2</a>:
<a href="#name-scenario-2-for-illustration" class="selfRef">Scenario 2 for Illustration of Efficacy of uRPF Schemes</a>
          </figcaption></figure>
</div>
</section>
</div>
<div id="lurpf">
<section id="section-2.4">
        <h3 id="name-sav-using-loose-unicast-rev">
<a href="#section-2.4" class="section-number selfRef">2.4. </a><a href="#name-sav-using-loose-unicast-rev" class="section-name selfRef">SAV Using Loose Unicast Reverse Path Forwarding</a>
        </h3>
<p id="section-2.4-1">
In the loose uRPF method, an ingress packet
at the border router is accepted only if the FIB has one or more prefixes that
encompass the source address. That is, a packet is dropped if no route exists
in the FIB for the source address. Loose uRPF sacrifices directionality.  It only drops packets if the source address is unreachable in the current FIB (e.g., IANA special-purpose prefixes <span>[<a href="#SPAR-v4" class="xref">SPAR-v4</a>]</span><span>[<a href="#SPAR-v6" class="xref">SPAR-v6</a>]</span>, unallocated, allocated but currently not routed).<a href="#section-2.4-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="vrf">
<section id="section-2.5">
        <h3 id="name-sav-using-vrf-table">
<a href="#section-2.5" class="section-number selfRef">2.5. </a><a href="#name-sav-using-vrf-table" class="section-name selfRef">SAV Using VRF Table</a>
        </h3>
<p id="section-2.5-1">
The Virtual Routing and Forwarding (VRF) technology <span>[<a href="#RFC4364" class="xref">RFC4364</a>]</span> <span>[<a href="#Juniper" class="xref">Juniper</a>]</span> allows a router
to maintain multiple routing table instances separate from the global Routing
Information Base (RIB). External BGP (eBGP) peering sessions send specific
routes to be stored in a dedicated VRF table. The uRPF process queries the VRF
table (instead of the FIB) for source address validation. A VRF table can be dedicated per eBGP peer and used for uRPF for only that peer, resulting in strict mode operation.  For implementing loose uRPF on an interface, the corresponding VRF table would be global, i.e., contains the same routes as in the FIB.<a href="#section-2.5-1" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="newtech">
<section id="section-3">
      <h2 id="name-sav-using-enhanced-feasible">
<a href="#section-3" class="section-number selfRef">3. </a><a href="#name-sav-using-enhanced-feasible" class="section-name selfRef">SAV Using Enhanced Feasible-Path uRPF</a>
      </h2>
<div id="descrip">
<section id="section-3.1">
        <h3 id="name-description-of-the-method">
<a href="#section-3.1" class="section-number selfRef">3.1. </a><a href="#name-description-of-the-method" class="section-name selfRef">Description of the Method</a>
        </h3>
<p id="section-3.1-1"> 
The enhanced feasible-path uRPF (EFP-uRPF) method adds greater operational robustness and efficacy to existing uRPF methods discussed in <a href="#review" class="xref">Section 2</a>. That is because it avoids dropping legitimate data packets and compromising directionality. The method is based on the principle that if BGP updates for multiple prefixes with the same origin AS were received on different interfaces (at border routers), then incoming data packets with source addresses in any of those prefixes should be accepted on any of those interfaces. The EFP-uRPF method can be best explained with an example, as follows:<a href="#section-3.1-1" class="pilcrow">¶</a></p>
<p id="section-3.1-2">
Let us say, in its Adj-RIBs-In <span>[<a href="#RFC4271" class="xref">RFC4271</a>]</span>, a border router of ISP-A has the set of prefixes {Q1, Q2, Q3}, each of which has AS-x as its origin and AS-x is in ISP-A's customer cone. In this set, the border router received the route for prefix Q1 over a customer-facing interface while it learned the routes for prefixes Q2 and Q3 from a lateral peer and an upstream transit provider, respectively. In this example scenario, the enhanced feasible-path uRPF method requires Q1, Q2, and Q3 be included in the RPF list for the customer interface under consideration.<a href="#section-3.1-2" class="pilcrow">¶</a></p>
<p id="section-3.1-3">
Thus, the EFP-uRPF method gathers feasible paths
for customer interfaces in a more precise way (as compared to the feasible-path uRPF) so that all legitimate packets are accepted while the directionality property is not compromised.<a href="#section-3.1-3" class="pilcrow">¶</a></p>
<p id="section-3.1-4">
The above-described EFP-uRPF method is recommended to be applied on customer
interfaces. It can also 
be extended to create the RPF lists for lateral peer
interfaces. That is, the EFP-uRPF method can be applied (and loose uRPF
avoided) on lateral peer interfaces. That will help to avoid compromising directionality for lateral peer interfaces (which is inevitable with loose uRPF; see <a href="#lurpf" class="xref">Section 2.4</a>).<a href="#section-3.1-4" class="pilcrow">¶</a></p>
<p id="section-3.1-5"> 
Looking back at Scenarios 1 and 2 (Figures <a href="#fig1" class="xref">1</a> and <a href="#fig2" class="xref">2</a>), the EFP-uRPF method works better than the other uRPF
methods. Scenario 3 (<a href="#fig3" class="xref">Figure 3</a>) further
illustrates the enhanced feasible-path uRPF method with a more concrete
example. In this scenario, the focus is on operation of the EFP-uRPF
at ISP4 (AS4). ISP4 learns a route for prefix P1 via a
C2P interface from customer ISP2 (AS2). This route for P1 has origin
AS1. ISP4 also learns a route for P2 via another C2P interface from customer
ISP3 (AS3). Additionally, AS4 learns a route for P3 via a lateral P2P interface from ISP5 (AS5). Routes for all three prefixes have the same
origin AS (i.e., AS1). Using the enhanced feasible-path uRPF scheme and given the
commonality of the origin AS across the routes for P1, P2, and P3, AS4 includes
all of these prefixes in the RPF list for the customer interfaces (from AS2
and AS3).<a href="#section-3.1-5" class="pilcrow">¶</a></p>
<span id="name-scenario-3-for-illustration"></span><div id="fig3">
<figure id="figure-3">
          <div class="artwork art-text alignLeft" id="section-3.1-6.1">
<pre>
                 +----------+   P3[AS5 AS1]  +------------+
                 | AS4(ISP4)|&lt;---------------|  AS5(ISP5) |
                 +----------+      (P2P)     +------------+
                     /\   /\                        /\
                     /     \                        /
         P1[AS2 AS1]/       \P2[AS3 AS1]           /
              (C2P)/         \(C2P)               /
                  /           \                  /
           +----------+    +----------+         /
           | AS2(ISP2)|    | AS3(ISP3)|        /
           +----------+    +----------+       /
                    /\           /\          /
                     \           /          /
               P1[AS1]\         /P2[AS1]   /P3[AS1]
                  (C2P)\       /(C2P)     /(C2P)
                        \     /          /
                     +----------------+ /
                     |  AS1(customer) |/
                     +----------------+
                          P1, P2, P3 (prefixes originated)

         Consider that data packets (sourced from AS1)
         may be received at AS4 with a source address
         in P1, P2, or P3 via any of the neighbors (AS2, AS3, AS5):
         * Feasible-path uRPF fails
         * Loose uRPF works (but not desirable)
         * Enhanced feasible-path uRPF works best
</pre>
</div>
<figcaption><a href="#figure-3" class="selfRef">Figure 3</a>:
<a href="#name-scenario-3-for-illustration" class="selfRef">Scenario 3 for Illustration of Efficacy of uRPF Schemes</a>
          </figcaption></figure>
</div>
<div id="algA">
<section id="section-3.1.1">
          <h4 id="name-algorithm-a-enhanced-feasib">
<a href="#section-3.1.1" class="section-number selfRef">3.1.1. </a><a href="#name-algorithm-a-enhanced-feasib" class="section-name selfRef">Algorithm A: Enhanced Feasible-Path uRPF</a>
          </h4>
<p id="section-3.1.1-1">
The underlying algorithm in the solution method described above (<a href="#descrip" class="xref">Section 3.1</a>) can be specified as follows (to be implemented in a transit AS):<a href="#section-3.1.1-1" class="pilcrow">¶</a></p>
<ol start="1" type="1" class="normal" id="section-3.1.1-2">
            <li id="section-3.1.1-2.1">
Create the set of unique origin ASes considering only the routes in the Adj-RIBs-In of customer interfaces. Call it Set A = {AS1, AS2, ...,  ASn}.<a href="#section-3.1.1-2.1" class="pilcrow">¶</a>
</li>
<li id="section-3.1.1-2.2">
Considering all routes in Adj-RIBs-In for all interfaces (customer, lateral peer, and transit provider), form the set of unique prefixes that have a common origin AS1. Call it Set X1.<a href="#section-3.1.1-2.2" class="pilcrow">¶</a>
</li>
<li id="section-3.1.1-2.3">
Include Set X1 in the RPF list on all customer interfaces on which one or more of the prefixes in Set X1 were received.<a href="#section-3.1.1-2.3" class="pilcrow">¶</a>
</li>
<li id="section-3.1.1-2.4">
Repeat Steps 2 and 3 for each of the remaining ASes in Set A (i.e., for ASi, where i = 2, ..., n).<a href="#section-3.1.1-2.4" class="pilcrow">¶</a>
</li>
</ol>
<p id="section-3.1.1-3">
The above algorithm can also be extended to apply the EFP-uRPF method to
lateral peer interfaces. However, it is left up to the operator to decide
whether they should apply the EFP-uRPF or loose uRPF method on lateral peer interfaces. The loose uRPF method is recommended to be applied on transit provider interfaces.<a href="#section-3.1.1-3" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="recomm">
<section id="section-3.2">
        <h3 id="name-operational-recommendations">
<a href="#section-3.2" class="section-number selfRef">3.2. </a><a href="#name-operational-recommendations" class="section-name selfRef">Operational Recommendations</a>
        </h3>
<p id="section-3.2-1">
The following operational recommendations will make the operation of the enhanced feasible-path uRPF robust:<a href="#section-3.2-1" class="pilcrow">¶</a></p>
<p id="section-3.2-2"> 
For multihomed stub AS:<a href="#section-3.2-2" class="pilcrow">¶</a></p>
<ul>
<li id="section-3.2-3.1"> 
A multihomed stub AS should announce at least one of the prefixes it originates to each of its transit provider ASes. 
(It is understood that a single-homed stub AS would announce all prefixes it originates to its sole transit provider AS.)<a href="#section-3.2-3.1" class="pilcrow">¶</a>
</li>
</ul>
<p id="section-3.2-4"> 
For non-stub AS:<a href="#section-3.2-4" class="pilcrow">¶</a></p>
<ul>
<li id="section-3.2-5.1"> 
A non-stub AS should also announce at least one of the prefixes it originates to each of its transit provider ASes.<a href="#section-3.2-5.1" class="pilcrow">¶</a>
</li>
<li id="section-3.2-5.2"> 
Additionally, from the routes it has learned from customers, a non-stub AS <span class="bcp14">SHOULD</span> announce at least one route per origin AS to each of its transit provider ASes.<a href="#section-3.2-5.2" class="pilcrow">¶</a>
</li>
</ul>
</section>
</div>
<div id="challenge">
<section id="section-3.3">
        <h3 id="name-a-challenging-scenario">
<a href="#section-3.3" class="section-number selfRef">3.3. </a><a href="#name-a-challenging-scenario" class="section-name selfRef">A Challenging Scenario</a>
        </h3>
<p id="section-3.3-1"> 
It should be observed that in the absence of ASes adhering to above
recommendations, the following example scenario, which poses
a challenge for the enhanced feasible-path uRPF (as well as for traditional
feasible-path uRPF), may be constructed. In the scenario illustrated in <a href="#fig4" class="xref">Figure 4</a>, since routes for neither P1 nor P2 are propagated on the AS2-AS4 interface (due to the presence of NO_EXPORT Community), the enhanced feasible-path uRPF at AS4 will reject data packets received on that interface with source addresses in P1 or P2. (For a little more complex example scenario, see slide #10 in <span>[<a href="#Sriram-URPF" class="xref">Sriram-URPF</a>]</span>.)<a href="#section-3.3-1" class="pilcrow">¶</a></p>
<span id="name-illustration-of-a-challengi"></span><div id="fig4">
<figure id="figure-4">
          <div class="artwork art-text alignLeft" id="section-3.3-2.1">
<pre>
                 +----------+  
                 | AS4(ISP4)|                       
                 +----------+       
                     /\   /\                        
                     /     \  P1[AS3 AS1]                                  
      P1 and P2 not /       \ P2[AS3 AS1]           
        propagated /         \ (C2P)                
          (C2P)   /           \                  
           +----------+    +----------+         
           | AS2(ISP2)|    | AS3(ISP3)|                        
           +----------+    +----------+       
                    /\           /\          
                     \           / P1[AS1]          
    P1[AS1] NO_EXPORT \         / P2[AS1]              
    P2[AS1] NO_EXPORT  \       / (C2P)     
                 (C2P)  \     /          
                     +----------------+ 
                     |  AS1(customer) |
                     +----------------+
                          P1, P2 (prefixes originated)
         
       Consider that data packets (sourced from AS1) 
       may be received at AS4 with a source address 
       in P1 or P2 via AS2:       
       * Feasible-path uRPF fails
       * Loose uRPF works (but not desirable)
       * Enhanced feasible-path uRPF with Algorithm A fails
       * Enhanced feasible-path uRPF with Algorithm B works best
</pre>
</div>
<figcaption><a href="#figure-4" class="selfRef">Figure 4</a>:
<a href="#name-illustration-of-a-challengi" class="selfRef">Illustration of a Challenging Scenario</a>
          </figcaption></figure>
</div>
</section>
</div>
<div id="algB">
<section id="section-3.4">
        <h3 id="name-algorithm-b-enhanced-feasib">
<a href="#section-3.4" class="section-number selfRef">3.4. </a><a href="#name-algorithm-b-enhanced-feasib" class="section-name selfRef">Algorithm B: Enhanced Feasible-Path uRPF with Additional Flexibility across Customer Cone</a>
        </h3>
<p id="section-3.4-1">
Adding further flexibility to the enhanced feasible-path uRPF method can help address the potential limitation identified above using the scenario in <a href="#fig4" class="xref">Figure 4</a> (<a href="#challenge" class="xref">Section 3.3</a>). In the following, "route" refers to a route currently existing in the Adj-RIBs-In. Including the additional degree of flexibility, the modified algorithm called Algorithm B (implemented in a transit AS) can be described as follows:<a href="#section-3.4-1" class="pilcrow">¶</a></p>
<ol start="1" type="1" class="normal" id="section-3.4-2">
          <li id="section-3.4-2.1"> 
Create the set of all directly connected customer interfaces. Call it Set I = {I1, I2, ..., Ik}.<a href="#section-3.4-2.1" class="pilcrow">¶</a>
</li>
<li id="section-3.4-2.2"> 
Create the set of all unique prefixes for which routes exist in Adj-RIBs-In for the interfaces in Set I. Call it Set P = {P1, P2, ..., Pm}.<a href="#section-3.4-2.2" class="pilcrow">¶</a>
</li>
<li id="section-3.4-2.3"> 
Create the set of all unique origin ASes seen in the routes that exist in Adj-RIBs-In for the interfaces in Set I. Call it Set A = {AS1, AS2, ..., ASn}.<a href="#section-3.4-2.3" class="pilcrow">¶</a>
</li>
<li id="section-3.4-2.4"> 
Create the set of all unique prefixes for which routes exist in Adj-RIBs-In of all lateral peer and transit provider interfaces such that each of the routes has its origin AS belonging in Set A. Call it Set Q = {Q1, Q2, ..., Qj}.<a href="#section-3.4-2.4" class="pilcrow">¶</a>
</li>
<li id="section-3.4-2.5"> 
Then, Set Z = Union(P,Q) is the RPF list that is applied for every customer interface in Set I.<a href="#section-3.4-2.5" class="pilcrow">¶</a>
</li>
</ol>
<p id="section-3.4-3"> 
When Algorithm B (which is more flexible than Algorithm A) is employed on
customer interfaces, the type of limitation identified in <a href="#fig4" class="xref">Figure 4</a> (<a href="#challenge" class="xref">Section 3.3</a>) is overcome
and the method works. The directionality property is minimally compromised,
but the proposed EFP-uRPF method with Algorithm B is still a much better choice (for the scenario under consideration) than applying the loose uRPF method, which is oblivious to directionality.<a href="#section-3.4-3" class="pilcrow">¶</a></p>
<p id="section-3.4-4"> 
So, applying the EFP-uRPF method with Algorithm B is recommended on customer interfaces for the challenging scenarios, such as those described in <a href="#challenge" class="xref">Section 3.3</a>.<a href="#section-3.4-4" class="pilcrow">¶</a></p>
</section>
</div>
<div id="augment">
<section id="section-3.5">
        <h3 id="name-augmenting-rpf-lists-with-r">
<a href="#section-3.5" class="section-number selfRef">3.5. </a><a href="#name-augmenting-rpf-lists-with-r" class="section-name selfRef">Augmenting RPF Lists with ROA and IRR Data</a>
        </h3>
<p id="section-3.5-1">
It is worth emphasizing that an indirect part of the proposal in this document
is that RPF filters may be augmented from secondary sources. Hence, the
construction of RPF lists using a method proposed in this document (Algorithm
A or B) can be augmented with data from Route Origin Authorization (ROA) <span>[<a href="#RFC6482" class="xref">RFC6482</a>]</span>, as well as Internet Routing Registry
(IRR) data. Special care should be exercised when using IRR data because it is
not always accurate or trusted.  In the EFP-uRPF method with Algorithm A (see <a href="#algA" class="xref">Section 3.1.1</a>), if a ROA includes prefix Pi and ASj, then augment
the RPF list of each customer interface on which at least one route with
origin ASj was received with prefix Pi. In the EFP-uRPF method with Algorithm B, if ASj
belongs in Set A (see Step #3 <a href="#algB" class="xref">Section 3.4</a>) and if a ROA includes prefix Pi and ASj,
then augment the RPF list Z in Step 5 of Algorithm B with prefix Pi. Similar procedures can be followed with reliable IRR data as well. This will help make the RPF lists more robust about source addresses that may be legitimately used by customers of the ISP.<a href="#section-3.5-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="impl">
<section id="section-3.6">
        <h3 id="name-implementation-and-operatio">
<a href="#section-3.6" class="section-number selfRef">3.6. </a><a href="#name-implementation-and-operatio" class="section-name selfRef">Implementation and Operations Considerations</a>
        </h3>
<div id="rpfsize">
<section id="section-3.6.1">
          <h4 id="name-impact-on-fib-memory-size-r">
<a href="#section-3.6.1" class="section-number selfRef">3.6.1. </a><a href="#name-impact-on-fib-memory-size-r" class="section-name selfRef">Impact on FIB Memory Size Requirement</a>
          </h4>
<p id="section-3.6.1-1">
The existing RPF checks in edge routers take advantage of existing
line card implementations to perform the RPF functions. For
implementation of the enhanced feasible-path uRPF, the general
necessary feature would be to extend the line cards to take arbitrary
RPF lists that are not necessarily the same as the existing FIB
contents. In the algorithms (Sections <a href="#algA" class="xref">3.1.1</a> and <a href="#algB" class="xref">3.4</a>) described here, the RPF lists are constructed by applying a set of rules to all received BGP routes (not just those selected as best path and installed in the FIB). The concept of uRPF querying an RPF list (instead of the FIB) is similar to uRPF querying a VRF table (see <a href="#vrf" class="xref">Section 2.5</a>).<a href="#section-3.6.1-1" class="pilcrow">¶</a></p>
<p id="section-3.6.1-2">
The techniques described in this document require that there should be additional memory (i.e., ternary content-addressable memory (TCAM)) available to store the RPF lists in line cards. For an ISP's AS, the RPF list size for each line card will roughly equal the total number of originated prefixes from ASes in its customer cone (assuming Algorithm B in <a href="#algB" class="xref">Section 3.4</a> is used). (Note: EFP-uRPF with Algorithm A (see <a href="#algA" class="xref">Section 3.1.1</a>) requires much less memory than EFP-uRPF with Algorithm B.)<a href="#section-3.6.1-2" class="pilcrow">¶</a></p>
<p id="section-3.6.1-3">
The following table shows the measured customer cone sizes in number of prefixes originated (from all ASes in the customer cone) for various types of ISPs <span>[<a href="#Sriram-RIPE63" class="xref">Sriram-RIPE63</a>]</span>:<a href="#section-3.6.1-3" class="pilcrow">¶</a></p>
<span id="name-customer-cone-sizes-prefixe"></span><div id="ccsize">
<table class="center" id="table-1">
            <caption>
<a href="#table-1" class="selfRef">Table 1</a>:
<a href="#name-customer-cone-sizes-prefixe" class="selfRef">Customer Cone Sizes (# Prefixes) for Various Types of ISPs</a>
            </caption>
<thead>
              <tr>
                <th class="text-left" rowspan="1" colspan="1">Type of ISP</th>
                <th class="text-left" rowspan="1" colspan="1">Measured Customer Cone Size in # Prefixes (in turn this is an
      estimate for RPF list size on the line card)</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td class="text-left" rowspan="1" colspan="1">Very Large Global ISP #1</td>
                <td class="text-left" rowspan="1" colspan="1">32393</td>
              </tr>
              <tr>
                <td class="text-left" rowspan="1" colspan="1">Very Large Global ISP #2</td>
                <td class="text-left" rowspan="1" colspan="1">29528</td>
              </tr>
              <tr>
                <td class="text-left" rowspan="1" colspan="1">Large Global ISP</td>
                <td class="text-left" rowspan="1" colspan="1">20038</td>
              </tr>
              <tr>
                <td class="text-left" rowspan="1" colspan="1">Mid-size Global ISP</td>
                <td class="text-left" rowspan="1" colspan="1">8661</td>
              </tr>
              <tr>
                <td class="text-left" rowspan="1" colspan="1">Regional ISP (in Asia)</td>
                <td class="text-left" rowspan="1" colspan="1">1101</td>
              </tr>
            </tbody>
          </table>
</div>
<p id="section-3.6.1-5">
For some super large global ISPs that are at the core of the Internet, the customer cone size (# prefixes) can be as high as a few hundred thousand <span>[<a href="#CAIDA" class="xref">CAIDA</a>]</span>, but uRPF is most effective when deployed at ASes at the edges of the Internet where the customer cone sizes are smaller, as shown in <a href="#ccsize" class="xref">Table 1</a>.<a href="#section-3.6.1-5" class="pilcrow">¶</a></p>
<p id="section-3.6.1-6">
A very large global ISP's router line card is likely to have a FIB size large enough to accommodate 2 million routes <span>[<a href="#Cisco1" class="xref">Cisco1</a>]</span>. Similarly, the line cards in routers corresponding to a large global ISP, a midsize global ISP, and a regional ISP are likely to have FIB sizes large enough to accommodate about 1 million, 0.5 million, and 100k routes, respectively <span>[<a href="#Cisco2" class="xref">Cisco2</a>]</span>. Comparing these FIB size numbers with the corresponding RPF list size numbers in <a href="#ccsize" class="xref">Table 1</a>, it can be surmised that the conservatively estimated RPF list size is only a small fraction of the anticipated FIB memory size under relevant ISP scenarios. What is meant here by relevant ISP scenarios is that only smaller ISPs (and possibly some midsize and regional ISPs) are expected to implement the proposed EFP-uRPF method since it is most effective closer to the edges of the Internet.<a href="#section-3.6.1-6" class="pilcrow">¶</a></p>
</section>
</div>
<div id="hyst">
<section id="section-3.6.2">
          <h4 id="name-coping-with-bgps-transient-">
<a href="#section-3.6.2" class="section-number selfRef">3.6.2. </a><a href="#name-coping-with-bgps-transient-" class="section-name selfRef">Coping with BGP's Transient Behavior</a>
          </h4>
<p id="section-3.6.2-1">
BGP routing announcements can exhibit transient behavior. Routes may be
withdrawn temporarily and then reannounced due to transient conditions, such
as BGP session reset or link failure recovery. To cope with this, hysteresis should be introduced in the maintenance of the RPF lists. Deleting entries from the RPF lists <span class="bcp14">SHOULD</span> be delayed by a predetermined amount (the value based on operational experience) when responding to route withdrawals. This should help suppress the effects due to the transients in BGP.<a href="#section-3.6.2-1" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="summ_recomm">
<section id="section-3.7">
        <h3 id="name-summary-of-recommendations">
<a href="#section-3.7" class="section-number selfRef">3.7. </a><a href="#name-summary-of-recommendations" class="section-name selfRef">Summary of Recommendations</a>
        </h3>
<p id="section-3.7-1">
Depending on the scenario, an ISP or enterprise AS operator should follow one of the following recommendations concerning uRPF/SAV:<a href="#section-3.7-1" class="pilcrow">¶</a></p>
<ol start="1" type="1" class="normal" id="section-3.7-2">
          <li id="section-3.7-2.1"> 
For directly connected networks, i.e., subnets directly connected to the AS, the AS under consideration <span class="bcp14">SHOULD</span> perform ACL-based SAV.<a href="#section-3.7-2.1" class="pilcrow">¶</a>
</li>
<li id="section-3.7-2.2"> 
For a directly connected single-homed stub AS (customer), the AS under consideration <span class="bcp14">SHOULD</span> perform SAV based on the strict uRPF method.<a href="#section-3.7-2.2" class="pilcrow">¶</a>
</li>
<li id="section-3.7-2.3">
            <p id="section-3.7-2.3.1"> 
For all other scenarios:<a href="#section-3.7-2.3.1" class="pilcrow">¶</a></p>
<ul>
<li id="section-3.7-2.3.2.1">
The EFP-uRPF method with Algorithm B (see <a href="#algB" class="xref">Section 3.4</a>) <span class="bcp14">SHOULD</span> be applied on customer interfaces.<a href="#section-3.7-2.3.2.1" class="pilcrow">¶</a>
</li>
<li id="section-3.7-2.3.2.2">
The loose uRPF method <span class="bcp14">SHOULD</span> be applied on lateral peer and transit provider interfaces.<a href="#section-3.7-2.3.2.2" class="pilcrow">¶</a>
</li>
</ul>
</li>
</ol>
<p id="section-3.7-3">
It is also recommended that prefixes from registered ROAs and IRR route objects that include ASes in an ISP's customer cone <span class="bcp14">SHOULD</span> be used to augment the pertaining RPF lists (see <a href="#augment" class="xref">Section 3.5</a> for details).<a href="#section-3.7-3" class="pilcrow">¶</a></p>
<div id="discuss">
<section id="section-3.7.1">
          <h4 id="name-applicability-of-the-efp-ur">
<a href="#section-3.7.1" class="section-number selfRef">3.7.1. </a><a href="#name-applicability-of-the-efp-ur" class="section-name selfRef">Applicability of the EFP-uRPF Method with Algorithm A</a>
          </h4>
<p id="section-3.7.1-1">The EFP-uRPF method with Algorithm A is not mentioned in the above set of recommendations. It is an alternative to EFP-uRPF with Algorithm B and can be used in limited circumstances. The EFP-uRPF method with Algorithm A is expected to work fine if an ISP deploying it has only multihomed stub customers. It is trivially equivalent to strict uRPF if an ISP deploys it for a single-homed stub customer. More generally, it is also expected to work fine when there is absence of limitations, such as those described in <a href="#challenge" class="xref">Section 3.3</a>. However, caution is required for use of EFP-uRPF with Algorithm A because even if the limitations are not expected at the time of deployment, the vulnerability to change in conditions exists. It may be difficult for an ISP to know or track the extent of use of NO_EXPORT (see <a href="#challenge" class="xref">Section 3.3</a>) on routes within its customer cone. If an ISP decides to use EFP-uRPF with Algorithm A, it should make its direct customers aware of the operational recommendations in <a href="#recomm" class="xref">Section 3.2</a>. This means that the ISP notifies direct customers that at least one prefix originated by each AS in the direct customer's customer cone must propagate to the ISP.<a href="#section-3.7.1-1" class="pilcrow">¶</a></p>
<p id="section-3.7.1-2">
On a lateral peer interface, an ISP may choose to apply the EFP-uRPF method with Algorithm A (with appropriate modification of the algorithm). This is because stricter forms of uRPF (than the loose uRPF) may be considered applicable by some ISPs on interfaces with lateral peers.<a href="#section-3.7.1-2" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
</section>
</div>
<div id="seccon">
<section id="section-4">
      <h2 id="name-security-considerations">
<a href="#section-4" class="section-number selfRef">4. </a><a href="#name-security-considerations" class="section-name selfRef">Security Considerations</a>
      </h2>
<p id="section-4-1">
The security considerations in BCP 38 <span>[<a href="#RFC2827" class="xref">RFC2827</a>]</span> and RFC 3704 <span>[<a href="#RFC3704" class="xref">RFC3704</a>]</span> apply
for this document as well. In addition, if considering using the EFP-uRPF method with Algorithm A, an ISP or AS operator should be aware of the applicability considerations and potential vulnerabilities discussed in <a href="#discuss" class="xref">Section 3.7.1</a>.<a href="#section-4-1" class="pilcrow">¶</a></p>
<p id="section-4-2"> 
In augmenting RPF lists with ROA (and possibly reliable IRR) information (see
<a href="#augment" class="xref">Section 3.5</a>), a trade-off is made in favor of
reducing false positives (regarding invalid detection in SAV) at the expense
of another slight risk. The other risk being that a malicious actor at another
AS in the neighborhood within the customer cone might take advantage (of the
augmented prefix) to some extent. This risk also exists even with normal
announced prefixes (i.e., without ROA augmentation) for any uRPF method other
than the strict uRPF. However, the risk is mitigated if the transit provider of the other AS in question is performing SAV.<a href="#section-4-2" class="pilcrow">¶</a></p>
<p id="section-4-3"> 
Though not within the scope of this document, security hardening of routers and other supporting systems (e.g., Resource PKI (RPKI) and ROA management systems) against compromise is extremely important. The compromise of those systems can affect the operation and performance of the SAV methods described in this document.<a href="#section-4-3" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-5">
      <h2 id="name-iana-considerations">
<a href="#section-5" class="section-number selfRef">5. </a><a href="#name-iana-considerations" class="section-name selfRef">IANA Considerations</a>
      </h2>
<p id="section-5-1">This document has no IANA actions.<a href="#section-5-1" class="pilcrow">¶</a></p>
</section>
<section id="section-6">
      <h2 id="name-references">
<a href="#section-6" class="section-number selfRef">6. </a><a href="#name-references" class="section-name selfRef">References</a>
      </h2>
<section id="section-6.1">
        <h3 id="name-normative-references">
<a href="#section-6.1" class="section-number selfRef">6.1. </a><a href="#name-normative-references" class="section-name selfRef">Normative References</a>
        </h3>
<dl class="references">
<dt id="RFC2119">[RFC2119]</dt>
<dd>
<span class="refAuthor">Bradner, S.</span>, <span class="refTitle">"Key words for use in RFCs to Indicate Requirement Levels"</span>, <span class="seriesInfo">BCP 14</span>, <span class="seriesInfo">RFC 2119</span>, <span class="seriesInfo">DOI 10.17487/RFC2119</span>, <time datetime="1997-03">March 1997</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc2119">https://www.rfc-editor.org/info/rfc2119</a>&gt;</span>. </dd>
<dt id="RFC2827">[RFC2827]</dt>
<dd>
<span class="refAuthor">Ferguson, P.</span><span class="refAuthor"> and D. Senie</span>, <span class="refTitle">"Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing"</span>, <span class="seriesInfo">BCP 38</span>, <span class="seriesInfo">RFC 2827</span>, <span class="seriesInfo">DOI 10.17487/RFC2827</span>, <time datetime="2000-05">May 2000</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc2827">https://www.rfc-editor.org/info/rfc2827</a>&gt;</span>. </dd>
<dt id="RFC3704">[RFC3704]</dt>
<dd>
<span class="refAuthor">Baker, F.</span><span class="refAuthor"> and P. Savola</span>, <span class="refTitle">"Ingress Filtering for Multihomed Networks"</span>, <span class="seriesInfo">BCP 84</span>, <span class="seriesInfo">RFC 3704</span>, <span class="seriesInfo">DOI 10.17487/RFC3704</span>, <time datetime="2004-03">March 2004</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc3704">https://www.rfc-editor.org/info/rfc3704</a>&gt;</span>. </dd>
<dt id="RFC4271">[RFC4271]</dt>
<dd>
<span class="refAuthor">Rekhter, Y., Ed.</span><span class="refAuthor">, Li, T., Ed.</span><span class="refAuthor">, and S. Hares, Ed.</span>, <span class="refTitle">"A Border Gateway Protocol 4 (BGP-4)"</span>, <span class="seriesInfo">RFC 4271</span>, <span class="seriesInfo">DOI 10.17487/RFC4271</span>, <time datetime="2006-01">January 2006</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4271">https://www.rfc-editor.org/info/rfc4271</a>&gt;</span>. </dd>
<dt id="RFC8174">[RFC8174]</dt>
<dd>
<span class="refAuthor">Leiba, B.</span>, <span class="refTitle">"Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"</span>, <span class="seriesInfo">BCP 14</span>, <span class="seriesInfo">RFC 8174</span>, <span class="seriesInfo">DOI 10.17487/RFC8174</span>, <time datetime="2017-05">May 2017</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8174">https://www.rfc-editor.org/info/rfc8174</a>&gt;</span>. </dd>
</dl>
</section>
<section id="section-6.2">
        <h3 id="name-informative-references">
<a href="#section-6.2" class="section-number selfRef">6.2. </a><a href="#name-informative-references" class="section-name selfRef">Informative References</a>
        </h3>
<dl class="references">
<dt id="CAIDA">[CAIDA]</dt>
<dd>
<span class="refAuthor">CAIDA</span>, <span class="refTitle">"Information for AS 174 (COGENT-174)"</span>, <time datetime="2019-10">October 2019</time>, <span>&lt;<a href="https://spoofer.caida.org/as.php?asn=174">https://spoofer.caida.org/as.php?asn=174</a>&gt;</span>. </dd>
<dt id="Cisco1">[Cisco1]</dt>
<dd>
<span class="refAuthor">Cisco</span>, <span class="refTitle">"Internet Routing Table Growth Causes %ROUTING-FIB-4-RSRC_LOW Message on Trident-Based Line Cards"</span>, <time datetime="2014-01">January 2014</time>, <span>&lt;<a href="https://www.cisco.com/c/en/us/support/docs/routers/asr-9000-series-aggregation-services-routers/116999-problem-line-card-00.html">https://www.cisco.com/c/en/us/support/docs/routers/asr-9000-series-aggregation-services-routers/116999-problem-line-card-00.html</a>&gt;</span>. </dd>
<dt id="Cisco2">[Cisco2]</dt>
<dd>
<span class="refAuthor">Cisco</span>, <span class="refTitle">"Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 5.x (Chapter 15: 'Managing the Unicast RIB and FIB')"</span>, <time datetime="2018-03">March 2018</time>, <span>&lt;<a href="https://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/5_x/nx-os/unicast/configuration/guide/l3_cli_nxos/l3_NewChange.html">https://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/5_x/nx-os/unicast/configuration/guide/l3_cli_nxos/l3_NewChange.html</a>&gt;</span>. </dd>
<dt id="Firmin">[Firmin]</dt>
<dd>
<span class="refAuthor">Firmin, F.</span>, <span class="refTitle">"The Evolved Packet Core"</span>, <span>&lt;<a href="https://www.3gpp.org/technologies/keywords-acronyms/100-the-evolved-packet-core">https://www.3gpp.org/technologies/keywords-acronyms/100-the-evolved-packet-core</a>&gt;</span>. </dd>
<dt id="ISOC">[ISOC]</dt>
<dd>
<span class="refAuthor">Internet Society</span>, <span class="refTitle">"Addressing the challenge of IP spoofing"</span>, <time datetime="2015-09">September 2015</time>, <span>&lt;<a href="https://www.internetsociety.org/resources/doc/2015/addressing-the-challenge-of-ip-spoofing/">https://www.internetsociety.org/resources/doc/2015/addressing-the-challenge-of-ip-spoofing/</a>&gt;</span>. </dd>
<dt id="Juniper">[Juniper]</dt>
<dd>
<span class="refAuthor">Juniper Networks</span>, <span class="refTitle">"Creating Unique VPN Routes Using VRF Tables"</span>, <time datetime="2019-05">May 2019</time>, <span>&lt;<a href="https://www.juniper.net/documentation/en_US/junos/topics/topic-map/l3-vpns-routes-vrf-tables.html#id-understanding-virtual-routing-and-forwarding-tables">https://www.juniper.net/documentation/en_US/junos/topics/topic-map/l3-vpns-routes-vrf-tables.html#id-understanding-virtual-routing-and-forwarding-tables</a>&gt;</span>. </dd>
<dt id="Luckie">[Luckie]</dt>
<dd>
<span class="refAuthor">Luckie, M.</span><span class="refAuthor">, Huffaker, B.</span><span class="refAuthor">, Dhamdhere, A.</span><span class="refAuthor">, Giotsas, V.</span><span class="refAuthor">, and kc. claffy</span>, <span class="refTitle">"AS Relationships, customer cones, and validation"</span>, <span class="refContent">In Proceedings of the 2013 Internet Measurement Conference</span>, <span class="seriesInfo">DOI 10.1145/2504730.2504735</span>, <time datetime="2013-10">October 2013</time>, <span>&lt;<a href="https://dl.acm.org/doi/10.1145/2504730.2504735">https://dl.acm.org/doi/10.1145/2504730.2504735</a>&gt;</span>. </dd>
<dt id="RFC4036">[RFC4036]</dt>
<dd>
<span class="refAuthor">Sawyer, W.</span>, <span class="refTitle">"Management Information Base for Data Over Cable Service Interface Specification (DOCSIS) Cable Modem Termination Systems for Subscriber Management"</span>, <span class="seriesInfo">RFC 4036</span>, <span class="seriesInfo">DOI 10.17487/RFC4036</span>, <time datetime="2005-04">April 2005</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4036">https://www.rfc-editor.org/info/rfc4036</a>&gt;</span>. </dd>
<dt id="RFC4364">[RFC4364]</dt>
<dd>
<span class="refAuthor">Rosen, E.</span><span class="refAuthor"> and Y. Rekhter</span>, <span class="refTitle">"BGP/MPLS IP Virtual Private Networks (VPNs)"</span>, <span class="seriesInfo">RFC 4364</span>, <span class="seriesInfo">DOI 10.17487/RFC4364</span>, <time datetime="2006-02">February 2006</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4364">https://www.rfc-editor.org/info/rfc4364</a>&gt;</span>. </dd>
<dt id="RFC6482">[RFC6482]</dt>
<dd>
<span class="refAuthor">Lepinski, M.</span><span class="refAuthor">, Kent, S.</span><span class="refAuthor">, and D. Kong</span>, <span class="refTitle">"A Profile for Route Origin Authorizations (ROAs)"</span>, <span class="seriesInfo">RFC 6482</span>, <span class="seriesInfo">DOI 10.17487/RFC6482</span>, <time datetime="2012-02">February 2012</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6482">https://www.rfc-editor.org/info/rfc6482</a>&gt;</span>. </dd>
<dt id="RFC6811">[RFC6811]</dt>
<dd>
<span class="refAuthor">Mohapatra, P.</span><span class="refAuthor">, Scudder, J.</span><span class="refAuthor">, Ward, D.</span><span class="refAuthor">, Bush, R.</span><span class="refAuthor">, and R. Austein</span>, <span class="refTitle">"BGP Prefix Origin Validation"</span>, <span class="seriesInfo">RFC 6811</span>, <span class="seriesInfo">DOI 10.17487/RFC6811</span>, <time datetime="2013-01">January 2013</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6811">https://www.rfc-editor.org/info/rfc6811</a>&gt;</span>. </dd>
<dt id="RFC7454">[RFC7454]</dt>
<dd>
<span class="refAuthor">Durand, J.</span><span class="refAuthor">, Pepelnjak, I.</span><span class="refAuthor">, and G. Doering</span>, <span class="refTitle">"BGP Operations and Security"</span>, <span class="seriesInfo">BCP 194</span>, <span class="seriesInfo">RFC 7454</span>, <span class="seriesInfo">DOI 10.17487/RFC7454</span>, <time datetime="2015-02">February 2015</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7454">https://www.rfc-editor.org/info/rfc7454</a>&gt;</span>. </dd>
<dt id="SPAR-v4">[SPAR-v4]</dt>
<dd>
<span class="refAuthor">IANA</span>, <span class="refTitle">"IANA IPv4 Special-Purpose Address Registry"</span>, <span>&lt;<a href="https://www.iana.org/assignments/iana-ipv4-special-registry/">https://www.iana.org/assignments/iana-ipv4-special-registry/</a>&gt;</span>. </dd>
<dt id="SPAR-v6">[SPAR-v6]</dt>
<dd>
<span class="refAuthor">IANA</span>, <span class="refTitle">"IANA IPv6 Special-Purpose Address Registry"</span>, <span>&lt;<a href="https://www.iana.org/assignments/iana-ipv6-special-registry/">https://www.iana.org/assignments/iana-ipv6-special-registry/</a>&gt;</span>. </dd>
<dt id="Sriram-RIPE63">[Sriram-RIPE63]</dt>
<dd>
<span class="refAuthor">Sriram, K.</span><span class="refAuthor"> and R. Bush</span>, <span class="refTitle">"Estimating CPU Cost of BGPSEC on a Router"</span>, <span class="refContent">Presented at RIPE 63 and at the SIDR WG meeting at
     IETF 83</span>, <time datetime="2012-03">March 2012</time>, <span>&lt;<a href="http://www.ietf.org/proceedings/83/slides/slides-83-sidr-7.pdf">http://www.ietf.org/proceedings/83/slides/slides-83-sidr-7.pdf</a>&gt;</span>. </dd>
<dt id="Sriram-URPF">[Sriram-URPF]</dt>
<dd>
<span class="refAuthor">Sriram, K.</span><span class="refAuthor">, Montgomery, D.</span><span class="refAuthor">, and J. Haas</span>, <span class="refTitle">"Enhanced Feasible-Path Unicast Reverse Path Filtering"</span>, <span class="refContent">Presented at the OPSEC WG meeting at IETF 101</span>, <time datetime="2018-03">March 2018</time>, <span>&lt;<a href="https://datatracker.ietf.org/meeting/101/materials/slides-101-opsec-draft-sriram-opsec-urpf-improvements-00">https://datatracker.ietf.org/meeting/101/materials/slides-101-opsec-draft-sriram-opsec-urpf-improvements-00</a>&gt;</span>. </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">Sandy Murphy</span>,
<span class="contact-name">Alvaro Retana</span>,
<span class="contact-name">Job Snijders</span>,
<span class="contact-name">Marco Marzetti</span>,
<span class="contact-name">Marco d'Itri</span>,
<span class="contact-name">Nick Hilliard</span>,
<span class="contact-name">Gert Doering</span>,
<span class="contact-name">Fred Baker</span>,
<span class="contact-name">Igor Gashinsky</span>,
<span class="contact-name">Igor Lubashev</span>,
<span class="contact-name">Andrei Robachevsky</span>,
<span class="contact-name">Barry Greene</span>,
<span class="contact-name">Amir Herzberg</span>,
<span class="contact-name">Ruediger Volk</span>,
<span class="contact-name">Jared Mauch</span>,
<span class="contact-name">Oliver Borchert</span>,
<span class="contact-name">Mehmet Adalier</span>, and
<span class="contact-name">Joel Jaeggli</span> for comments and suggestions. The comments and suggestions received from the IESG reviewers are also much appreciated.<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">Kotikalapudi Sriram</span></div>
<div dir="auto" class="left"><span class="org">USA National Institute of Standards and Technology</span></div>
<div dir="auto" class="left"><span class="street-address">100 Bureau Drive</span></div>
<div dir="auto" class="left">
<span class="locality">Gaithersburg</span>, <span class="region">MD</span> <span class="postal-code">20899</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:ksriram@nist.gov" class="email">ksriram@nist.gov</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Doug Montgomery</span></div>
<div dir="auto" class="left"><span class="org">USA National Institute of Standards and Technology</span></div>
<div dir="auto" class="left"><span class="street-address">100 Bureau Drive</span></div>
<div dir="auto" class="left">
<span class="locality">Gaithersburg</span>, <span class="region">MD</span> <span class="postal-code">20899</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:dougm@nist.gov" class="email">dougm@nist.gov</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Jeffrey Haas</span></div>
<div dir="auto" class="left"><span class="org">Juniper Networks, Inc.</span></div>
<div dir="auto" class="left"><span class="street-address">1133 Innovation Way</span></div>
<div dir="auto" class="left">
<span class="locality">Sunnyvale</span>, <span class="region">CA</span> <span class="postal-code">94089</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:jhaas@juniper.net" class="email">jhaas@juniper.net</a>
</div>
</address>
</section>
</div>
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