<?xml version="1.0" encoding="iso-8859-1"?>
<?rfc toc="yes"?>
<?rfc symrefs="yes" ?>
<?rfc sortrefs="yes" ?>
<?rfc compact="yes" ?>
<?rfc subcompact="no" ?> version='1.0' encoding='UTF-8'?>

<!DOCTYPE rfc [
  <!ENTITY nbsp    "&#160;">
  <!ENTITY zwsp   "&#8203;">
  <!ENTITY nbhy   "&#8209;">
  <!ENTITY wj     "&#8288;">
]>

<rfc xmlns:xi="http://www.w3.org/2001/XInclude" category="std" ipr="trust200902" docName="draft-ietf-bfd-unaffiliated-echo-14" number="9747" obsoletes="" updates="5880" consensus="true" submissionType="IETF"> submissionType="IETF" tocInclude="true" symRefs="true" sortRefs="true" version="3" xml:lang="en">

  <front>
    <title abbrev="Unaffiliated BFD Echo"> Unaffiliated Bidirectional Forwarding Detection (BFD) Echo </title>
    <seriesInfo name="RFC" value="9747"/>
    <author fullname="Weiqiang Cheng" initials="W" surname="Cheng">
      <organization>China Mobile</organization>
      <address>
        <postal>
        <street></street>
        <!-- Reorder these if your country does things differently -->
          <city>Beijing</city>
        <region></region>
        <code></code>
          <country>China</country>
        </postal>
        <phone></phone>
        <email>chengweiqiang@chinamobile.com</email>
        <!-- uri and facsimile elements may also be added -->
      </address>
    </author>
    <author fullname="Ruixue Wang" initials="R" surname="Wang">
      <organization>China Mobile</organization>
      <address>
        <postal>
        <street></street>
        <!-- Reorder these if your country does things differently -->
          <city>Beijing</city>
        <region></region>
        <code></code>
          <country>China</country>
        </postal>
        <phone></phone>
        <email>wangruixue@chinamobile.com</email>
        <!-- uri and facsimile elements may also be added -->
      </address>
    </author>
    <author fullname="Xiao Min" initials="X" surname="Min" role="editor">
      <organization>ZTE Corp.</organization>
      <address>
        <postal>
        <street></street>
        <!-- Reorder these if your country does things differently -->
          <city>Nanjing</city>
        <region></region>
        <code></code>
          <country>China</country>
        </postal>
       <phone></phone>
        <email>xiao.min2@zte.com.cn</email>
        <!-- uri and facsimile elements may also be added -->
      </address>
    </author>
    <author fullname="Reshad Rahman" initials="R" surname="Rahman">
      <organization>Equinix</organization>
      <address>
        <postal>
        <street></street>
        <!-- Reorder these if your country does things differently -->
          <city>Ottawa</city>
        <region></region>
        <code></code>
          <country>Canada</country>
        </postal>
        <phone></phone>
        <email>reshad@yahoo.com</email>
        <!-- uri and facsimile elements may also be added -->
      </address>
    </author>
    <author fullname="Raj Chetan Boddireddy" initials="R" surname="Boddireddy">
      <organization>Juniper Networks</organization>
      <address>
        <postal>
        <street></street>
        <!-- Reorder these if your country does things differently -->
        <city></city>
        <region></region>
        <code></code>
        <country></country>
        </postal>
        <phone></phone>
        <email>rchetan@juniper.net</email>
        <!-- uri and facsimile elements may also be added -->
      </address>
    </author>
    <date year="2024"/>

    <area>Routing</area>
    <workgroup>BFD Working Group</workgroup>

    <keyword>Request for Comments</keyword>
    <keyword>RFC</keyword>
    <keyword>Internet Draft</keyword>
    <keyword>I-D</keyword> year="2025" month="March"/>
    <area>RTG</area>
    <workgroup>bfd</workgroup>

<!-- [rfced] Please insert any keywords (beyond those that appear in
the title) for use on https://www.rfc-editor.org/search. -->

<keyword>example</keyword>

<!-- [rfced] Because this document updates RFC 5880, please
review the errata reported for RFC 5880
(https://www.rfc-editor.org/errata/rfc5880).
Please let us know if you agree that none of them are
relevant to the content of this document.
-->

    <abstract>
      <t>
	This document specifies an extension to the Bidirectional Forwarding Detection (BFD)
	protocol that enables the use of the BFD Echo function without the need for an associated
	BFD control session. This "Unaffiliated BFD Echo" mechanism allows rapid detection of
	forwarding path failures in networks where establishing BFD control sessions is impractical
	or undesirable. By decoupling the Echo function from the control plane, network devices can
	utilize BFD's fast failure detection capabilities in a simplified manner, enhancing network
	resiliency and operational efficiency.
      </t>
      <t>
	This document updates RFC 5880 by defining a new Unaffiliated BFD Echo mechanism.
      </t>
    </abstract>
  </front>
  <middle>

    <section title="Introduction">
    <section>
      <name>Introduction</name>
      <t>
	To minimize the impact of device and link faults on services and to improve network availability
	in single-hop scenarios, a network device needs the capability to quickly detect communication
	faults with adjacent devices. Prompt detection allows for timely remedial actions to ensure
	service continuity.
      </t>
      <t>
	BFD <xref target="RFC5880"/> provides a low-overhead, short-interval method for detecting faults
	on the communication path between adjacent forwarding engines, which may include interfaces, data
	links, and the forwarding engines themselves. BFD offers a unified mechanism to monitor any media
	and protocol layers in real time.
      </t>
      <t>
	BFD defines two primary modes-Asynchronous modes -- Asynchronous mode and Demand mode-to mode -- to accommodate various deployment
	scenarios. Additionally, it supports an Echo function that reduces the level of BFD support required
	in device implementations, as described in Section 3.2 of <xref section="3.2" sectionFormat="of" target="RFC5880"/>. When the Echo
	function is activated, the local system sends BFD Echo packets, and the remote system loops back the
	received Echo packets through the forwarding path, as described in Section 5 of <xref section="5" sectionFormat="of" target="RFC5880"/>
	and Section 4 of <xref section="4" sectionFormat="of" target="RFC5881"/>. If several consecutive BFD Echo packets are not received
	by the local system, the BFD session is declared Down.
      </t>
      <t>
	There are two typical scenarios when using the BFD Echo function:
	<list style="symbols">
      </t>
      <ul spacing="normal">
        <li>
          <t>
	Full BFD protocol capability with adjunct Echo function (Affiliated BFD Echo): This scenario requires
	both the local device and the adjacent device to support the full BFD protocol. This operation remains
	unchanged from <xref target="RFC5880"/>.
          </t>
        </li>
        <li>
          <t>
	BFD Echo-Only method without full BFD protocol capability (Unaffiliated BFD Echo): This scenario requires
	only the local device to support sending and demultiplexing BFD Control packets. In this case, BFD
	Control packets are sent over the BFD Echo port, and the processing procedures for Asynchronous mode
	are used with the modifications specified in this document. Note that this method requires the local device
	to send packets with one of its own IP addresses as the destination address, upon receipt of which the adjacent
	device loops them back to the local device. Also note that this method monitors the connectivity to a device
	over a specific interface and does not verify the availability of a specific IP address at that device.
          </t>
    </list>
        </li>
      </ul>
      <t>
	This document specifies the Unaffiliated BFD Echo scenario.
      </t>
      <t>
	Section 5 of
	<xref section="5" sectionFormat="of" target="RFC5880"/> indicates that the payload of an Affiliated BFD Echo packet is a local
	matter and,
	matter; therefore, its contents are outside the scope of that specification. This document, however,
	specifies the contents of the Unaffiliated BFD Echo packet and the procedures for handling them. While this
	may appear to contravene Section 5 of <xref section="5" sectionFormat="of" target="RFC5880"/>, the core behavior in that RFC states that the
	contents of BFD Echo packets are a local matter; this document is defining that "local matter". Regarding the
	selection of IP addresses, the rules stated in Section 4 of <xref section="4" sectionFormat="of" target="RFC5881"/> are applicable to the
	encapsulation of an Unaffiliated BFD Echo packet.
      </t>
      <t>
	Section 6.2.2 of <xref target="BBF-TR-146"/> describes a use case for the Unaffiliated BFD Echo.
      </t>
      <t>
	This document updates <xref target="RFC5880"/> by defining a new method of BFD Echo-only operation which only
	impacts the sender of BFD Echo packets sender without requiring an implementation to support the BFD protocol at the
	loop-back
	loopback device, such that any IP forwarder can loop-back loop back the BFD Echo packets. It specifies the use of the
	Unaffiliated BFD Echo over IPv4 and IPv6 for a single IP hop. The reason why it cannot be used for multihop
	paths is that the Unaffiliated BFD Echo packets would be looped back by the first hop. A full description of
	the updates to <xref target="RFC5880"/> is provided in Section 3. <xref target="updates-to-rfc-5880"/>.
      </t>

    <section title="Conventions
      <section>
        <name>Conventions Used in This Document"> Document</name>
        <t>
    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT",
	"RECOMMENDED", "NOT RECOMMENDED", "MAY", "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
    "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL NOT</bcp14>",
    "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>",
    "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
    "<bcp14>MAY</bcp14>", and   "OPTIONAL" "<bcp14>OPTIONAL</bcp14>" in this document are to be
    interpreted as described in BCP 14 BCP&nbsp;14 <xref target="RFC2119"/> <xref
    target="RFC8174"/> when, and only when, they appear in all capitals, as
    shown here.</t> here.
        </t>
      </section>
    </section>
    <section title="Unaffiliated anchor="unaffiliated-bfd-echo-procedures">
      <name>Unaffiliated BFD Echo Procedures"> Procedures</name>
      <t>
	This section specifies the Unaffiliated BFD Echo procedures.
      </t>
      <figure anchor="Figure_1" title="Unaffiliated anchor="Figure_1">
        <name>Unaffiliated BFD Echo diagram"> Echo</name>
        <artset>
          <artwork type="svg">
      <svg xmlns="http://www.w3.org/2000/svg" version="1.1" height="272" width="512" viewBox="0 0 512 272" class="diagram" text-anchor="middle" font-family="monospace" font-size="13px" stroke-linecap="round">
              <path d="M 8,32 L 8,240" fill="none" stroke="black"/>
              <path d="M 40,64 L 40,192" fill="none" stroke="black"/>
              <path d="M 144,32 L 144,88" fill="none" stroke="black"/>
              <path d="M 144,104 L 144,136" fill="none" stroke="black"/>
              <path d="M 144,184 L 144,240" fill="none" stroke="black"/>
              <path d="M 368,32 L 368,136" fill="none" stroke="black"/>
              <path d="M 368,184 L 368,240" fill="none" stroke="black"/>
              <path d="M 384,144 L 384,176" fill="none" stroke="black"/>
              <path d="M 504,32 L 504,240" fill="none" stroke="black"/>
              <path d="M 8,32 L 144,32" fill="none" stroke="black"/>
              <path d="M 368,32 L 504,32" fill="none" stroke="black"/>
              <path d="M 48,64 L 136,64" fill="none" stroke="black"/>
              <path d="M 136,96 L 160,96" fill="none" stroke="black"/>
              <path d="M 136,144 L 376,144" fill="none" stroke="black"/>
              <path d="M 128,176 L 376,176" fill="none" stroke="black"/>
              <path d="M 48,192 L 136,192" fill="none" stroke="black"/>
              <path d="M 8,240 L 144,240" fill="none" stroke="black"/>
              <path d="M 368,240 L 504,240" fill="none" stroke="black"/>
              <polygon class="arrowhead" points="168,96 156,90.4 156,101.6" fill="black" transform="rotate(0,160,96)"/>
              <polygon class="arrowhead" points="136,176 124,170.4 124,181.6" fill="black" transform="rotate(180,128,176)"/>
              <g class="text">
                <text x="76" y="20">Device A</text>
                <text x="436" y="20">Device B</text>
                <text x="92" y="84">Unaffiliated</text>
                <text x="84" y="100">BFD Echo</text>
                <text x="80" y="116">Session</text>
                <text x="256" y="132">Unaffiliated BFD Echo</text>
                <text x="408" y="148">BFD</text>
                <text x="144" y="164">|</text>
                <text x="424" y="164">packets</text>
                <text x="420" y="180">looped</text>
                <text x="72" y="260">BFD supported</text>
                <text x="440" y="260">BFD not supported</text>
              </g>
            </svg>
          </artwork>
          <artwork type="ascii-art" align="left"><![CDATA[
        Device A                                     Device B
   +----------------+                           +----------------+
   |                |                           |                |
   |   |------------|                           |                |
   |   |Unaffiliated|                           |                |
   |   | BFD Echo  --->                         |                |
   |   | Session    |                           |                |
   |   |            |   Unaffiliated BFD Echo   |                |
   |   |           -------------------------------| BFD          |
   |   |            |                             | packets      |
   |   |          <-------------------------------| looped       |
   |   |------------|                           |                |
   |                |                           |                |
   |                |                           |                |
   +----------------+                           +----------------+
     BFD supported                               BFD not supported
     ]]></artwork>
        </artset>
      </figure>
      <t>
    As shown in Figure 1, <xref target="Figure_1"/>, device A supports BFD, whereas device B is a regular IP forwarder that does not support
	BFD. Device A would send Unaffiliated BFD Echo packets, and after receiving the Unaffiliated BFD Echo packets
	sent from device A, the one-hop-away BFD peer device B immediately loops them back by normal IP forwarding, this forwarding. This
	allows device A to rapidly detect a connectivity loss to device B. Note that device B would not intercept any
	received Unaffiliated BFD Echo packet or parse any BFD protocol field within the Unaffiliated BFD Echo packet.
      </t>
      <t>
    An Unaffiliated BFD Echo session is not actually a BFD session because there is no coordination of BFD protocol
	state between the two link ends: the remote end does not support BFD and so cannot engage in a BFD session.
<!--[rfced] Please clarify this sentence, especially "from its own
standpoint". Is this about the local end's standpoint?

Original:
   The local end as an initiator may regard
   the Unaffiliated BFD Echo session as a BFD session from its own
   standpoint.

Perhaps:
   From the standpoint of the local end (as an initiator),
   the Unaffiliated BFD Echo session may be regarded as a BFD session.

Or:
   The local end (with the viewpoint of the initiator) may regard
   the Unaffiliated BFD Echo session as a BFD session.
-->
The
	local end as an initiator may regard the Unaffiliated BFD Echo session as a BFD session from its own standpoint.
      </t>
      <t>
	For the Unaffiliated Echo procedure, an Unaffiliated BFD Echo session is established on device A. The session
	MUST
	<bcp14>MUST</bcp14> adhere to the BFD state machine specified in Section 6.2 of <xref section="6.2" sectionFormat="of" target="RFC5880"/>, with the exception
	that the received state is not derived from BFD Control packets originating from the remote system, but rather
	from packets that are generated by the local system and looped back from the remote system. Consequently, the
	AdminDown state is not utilized in Unaffiliated BFD Echo.
      </t>
      <t>
	BFD Control packets are transmitted and received as Unaffiliated BFD Echo packets, using UDP destination port
	3785, as defined in <xref target="RFC5881"/>. The standard procedures for BFD Asynchronous sessions are applied
	to the looped BFD Control packets, including packet validation and authentication, in accordance with <xref target="RFC5880"/>.
      </t>
      <t>
	Once an Unaffiliated BFD Echo session is created on device A, it starts sending Unaffiliated BFD Echo packets.
<!--[rfced] May we update this sentence to two sentences as follows
for clarity?  Specifically, the updates are assuming:
- the "once" phrase applies to the latter part.
- "is conformed to" means "conforms to".

Original:
   Unaffiliated BFD Echo
   packets with zeroed "Your Discriminator" field are demultiplexed to
   the proper session based on the source IP address or UDP source port,
   once the remote system loops back the local discriminator, all
   further received packets are demultiplexed based on the "Your
   Discriminator" field only, which is conformed to the procedure
   specified in Section 6.3 of [RFC5880].

Perhaps:
   Unaffiliated BFD Echo
   packets with zeroed "Your Discriminator" field are demultiplexed to
   the proper session based on the source IP address or UDP source port.
   After the remote system loops back the local discriminator, all
   further received packets are demultiplexed based on the "Your
   Discriminator" field only, which conforms to the procedure
   specified in Section 6.3 of [RFC5880].
-->
	Unaffiliated BFD Echo packets with zeroed "Your Discriminator" field are demultiplexed to the proper session based on
	the source IP address or UDP source port, once the remote system loops back the local discriminator, all further
	received packets are demultiplexed based on the "Your Discriminator" field only, which is conformed to the procedure
	specified in <xref section="6.3" sectionFormat="of" target="RFC5880"/>. An Unaffiliated BFD Echo packet follows the same encapsulation
	rules as for a BFD Echo packet as specified in Section 4 of <xref section="4" sectionFormat="of" target="RFC5881"/>. All Unaffiliated BFD Echo packets
	for the session MUST <bcp14>MUST</bcp14> be sent with a TTL or Hop Limit value of 255. Received packets MUST <bcp14>MUST</bcp14> have a TTL or Hop Limit
	value of 254 (similar to Appendix A of <xref section="A" sectionFormat="of" target="RFC5082"/> to verify against a configured number of hops); otherwise,
	the received packets MUST <bcp14>MUST</bcp14> be dropped.
      </t>
      <t>
<!--[rfced] We note that quotation marks are not used around the
field names in RFC 5880. Do you want to keep the quotation marks
within this document?

Current:
   "Your Discriminator" field
   "Desired Min TX Interval" [field]
   "Required Min RX Interval" field
   "Required Min Echo RX Interval" field
-->
	In the context of an Unaffiliated BFD Echo packet, the "Desired Min TX Interval" and "Required Min RX Interval" fields,
	as defined in <xref target="RFC5880"/>, MUST <bcp14>MUST</bcp14> be populated with a specific value to prevent the potential exposure of
	uninitialized memory. It is RECOMMENDED <bcp14>RECOMMENDED</bcp14> that these fields be set to a value of 1 second (1,000,000 microseconds). However,
	upon receipt, these values MUST <bcp14>MUST</bcp14> be ignored and MUST NOT <bcp14>MUST NOT</bcp14> be used in the calculation of the Detection Time.
      </t>
      <t>
	The "Required Min Echo RX Interval" field, as defined in <xref target="RFC5880"/>, MUST <bcp14>MUST</bcp14> be populated with a specific value
	to prevent the potential exposure of uninitialized memory. It is RECOMMENDED <bcp14>RECOMMENDED</bcp14> that this field be set to 0. However, this value
	MUST
	<bcp14>MUST</bcp14> be ignored upon receipt. The transmission interval for Unaffiliated BFD Echo packets when in the Up state MUST <bcp14>MUST</bcp14> be
	provisioned on device A.
      </t>
      <t>
	The functionality of the Unaffiliated BFD Echo feature is dependent on device B performing IP forwarding. While this capability
	is typically expected to be supported on routers, it may not be enabled by default on hosts. The method for provisioning device
	B to loop back Unaffiliated BFD Echo packets is outside the scope of this document.
      </t>
      <t>
	Similar to what's specified in <xref target="RFC5880"/>, the Unaffiliated BFD Echo session begins with the
	periodic, slow transmission of Unaffiliated BFD Echo packets. The slow transmission rate should be no greater than
	one packet per second, until the session on device A is Up. After the session is Up, the provisioned transmission interval is
	used. When the Unaffiliated BFD Echo session on device A goes Down, the slow transmission rate is resumed. The "Detect Mult"
	defined in <xref target="RFC5880"/> MUST <bcp14>MUST</bcp14> be set to a value provisioned on device A. When the bfd.SessionState is
	Up and a "Detect Mult" number of Unaffiliated BFD Echo packets have not arrived at device A as they should, the device
	A "MUST "<bcp14>MUST</bcp14> set bfd.SessionState to Down and bfd.LocalDiag to 2 (Echo Function Failed)", as specified in Section 6.8.5
	of
	<xref section="6.8.5" sectionFormat="of" target="RFC5880"/>.
      </t>
      <t>
	In summary, the Unaffiliated BFD Echo packet reuses the format of the BFD Control packet defined in <xref target="RFC5880"/>,
	and the fields within the Unaffiliated BFD Echo packet are populated as follows:
	    <list style='symbols'>
      </t>
      <ul spacing="normal">
        <li>
          <t>My Discriminator: MUST <bcp14>MUST</bcp14> be set to the provisioned local discriminator.</t>
        </li>
        <li>
          <t>Your Discriminator: MUST <bcp14>MUST</bcp14> initially be set to 0, and then MUST <bcp14>MUST</bcp14> be set to the value of "My Discriminator" looped back
		 from the remote system.</t>
        </li>
        <li>
          <t>Desired Min TX Interval: MUST <bcp14>MUST</bcp14> be set to a specific value, with a suggested value of 1 second (1,000,000 microseconds).</t>
        </li>
        <li>
          <t>Required Min RX Interval: MUST <bcp14>MUST</bcp14> be set to a specific value, with a suggested value of 1 second (1,000,000 microseconds).</t>
        </li>
        <li>
          <t>Required Min Echo RX Interval: MUST <bcp14>MUST</bcp14> be set to a specific value, with a suggested value of 0.</t>
        </li>
        <li>
          <t>Detect Mult: MUST <bcp14>MUST</bcp14> be set to the provisioned maximum allowable number of consecutively lost Unaffiliated BFD Echo packets.</t>
	    </list>
	</t>
        </li>
      </ul>
    </section>
    <section title="Updates anchor="updates-to-rfc-5880">
      <name>Updates to RFC 5880"> 5880</name>
      <t>
	The Unaffiliated BFD Echo described in this document reuses the BFD
	Echo function as described in <xref target="RFC5880"/> and <xref
	target="RFC5881"/>, but does not require BFD Asynchronous or Demand
	mode. In the Unaffiliated BFD Echo operation, only the local system
	has the BFD protocol enabled, while the remote system simply loops
	back the received BFD Echo packets as ordinary data packets, without
	engaging in the BFD protocol.
      </t>
      <t>
	This document updates <xref target="RFC5880"/> with respect to its
	descriptions on the BFD Echo function as follows.
      </t>
      <t>
	The 4th paragraph of Section 3.2 of <xref section="3.2" sectionFormat="of" target="RFC5880"/> is
	updated as below:
      </t>

	<t>
	<list>
	<t>
	OLD TEXT<br/>An

      <t>OLD TEXT</t>
      <blockquote>An adjunct to both modes is the Echo function.
	</t>
	<t>
	NEW TEXT<br/>An function.</blockquote>

      <t>NEW TEXT</t>
      <blockquote>An adjunct to both modes is the Echo function, which can also be running independently.
	</t>
	<t>
	OLD TEXT<br/>Since independently.</blockquote>

      <t>OLD TEXT</t>
      <blockquote>Since the Echo function is handling the task of detection,
      the rate of periodic transmission of Control packets may be reduced (in
      the case of Asynchronous mode) or eliminated completely (in the case of
      Demand mode).
	</t>
	<t>
	NEW TEXT<br/>Since mode).</blockquote>

      <t>NEW TEXT</t>
      <blockquote>Since the Echo function is handling the task of detection,
      the rate of periodic transmission of Control packets may be reduced (in
      the case of Asynchronous mode) or eliminated completely (in the case of
      Demand mode).  The Echo function may also be used independently, with
      neither Asynchronous nor Demand mode.
	</t>
	</list>
	</t> mode.</blockquote>

      <t>
	The 3rd and 9th paragraphs of Section 6.1 of <xref section="6.1" sectionFormat="of" target="RFC5880"/> are updated as below:
      </t>

    <t>
	<list>
	<t>
	OLD TEXT<br/>Once

      <t>OLD TEXT</t>
      <blockquote>Once the BFD session is Up, a system can choose to start the
      Echo function if it desires and the other system signals that it will
      allow it. The rate of transmission of Control packets is typically kept
      low when the Echo function is active.
	</t>
	<t>
	NEW TEXT<br/>When active.</blockquote>

      <t>NEW TEXT</t>
      <blockquote>When a system is running with Asynchronous or Demand mode,
      once the BFD session is Up, it can choose to start the Echo function if
      it desires and the other system signals that it will allow it. The rate
      of transmission of Control packets is typically kept low for
      Asynchronous mode or eliminated completely for Demand mode when the Echo
      function is active.
	</t>
	<t>
	OLD TEXT<br/>If active.</blockquote>

      <t>OLD TEXT</t>
      <blockquote>If the session goes Down, the transmission of Echo packets
      (if any) ceases, and the transmission of Control packets goes back to
      the slow
    rate.
	</t>
	<t>
	NEW TEXT<br/>In rate.</blockquote>

      <t>NEW TEXT</t>
      <blockquote>In Asynchronous mode or Demand mode, if the session goes
      Down, the transmission of Echo packets (if any) ceases, and the
      transmission of Control packets goes back to the slow rate.
	</t>
	</list>
	</t> rate.</blockquote>

      <t>
	The 2nd paragraph of Section 6.4 of <xref section="6.4" sectionFormat="of" target="RFC5880"/> is updated as below:
      </t>

    <t>
	<list>
	<t>
	OLD TEXT<br/>When

      <t>OLD TEXT</t>
      <blockquote>When a system is using the Echo function, it is advantageous
      to choose a sedate reception rate for Control packets, since liveness
      detection is being handled by the Echo packets. This can be controlled
      by manipulating the Required Min RX Interval field (see section 6.8.3).
	</t>
	<t>
	NEW TEXT<br/>When 6.8.3).</blockquote>

      <t>NEW TEXT</t>
      <blockquote>When a system is using the Echo function with Asynchronous
      mode, it is advantageous to choose a sedate reception rate for Control
      packets, since liveness detection is being handled by the Echo
      packets. This can be controlled by manipulating the Required Min RX
      Interval field (see section 6.8.3).  Note that a system operating in
      Demand mode would direct the remote system to cease the periodic
      transmission of BFD Control packets, by setting the Demand (D) bit in
      its BFD Control packets.
	</t>
	</list>
	</t> packets.</blockquote>

      <t>
	The 2nd paragraph of Section 6.8 of <xref section="6.8" sectionFormat="of" target="RFC5880"/> is updated as below:
      </t>

    <t>
	<list>
	<t>
	OLD TEXT<br/>When

      <t>OLD TEXT</t>
      <blockquote>When a system is said to have "the Echo function active" it
      means that the system is sending BFD Echo packets, implying that the
      session is Up and the other system has signaled its willingness to loop
      back Echo packets.
	</t>
	<t>
	NEW TEXT<br/>When packets.</blockquote>

      <t>NEW TEXT</t>
      <blockquote>When a system in Asynchronous or Demand mode is said to have
      "the Echo function active" it means that the system is sending BFD Echo
      packets, implying that the session is Up and the other system has
      signaled its willingness to loop back Echo packets.
	</t>
	</list>
	</t> packets.</blockquote>

      <t>
	The 7th paragraph of Section 6.8.3 of <xref section="6.8.3" sectionFormat="of" target="RFC5880"/> is updated as below:
      </t>

    <t>
	<list>
	<t>
	OLD TEXT<br/>When

      <t>OLD TEXT</t>
      <blockquote>When the Echo function is active, a system SHOULD
      <bcp14>SHOULD</bcp14> set bfd.RequiredMinRxInterval to a value of not
      less than one second (1,000,000 microseconds). This is intended to keep
      received BFD Control traffic at a negligible level, since the actual
      detection function is being performed using BFD Echo packets.
	</t>
	<t>
	NEW TEXT<br/>When
      packets.</blockquote>

      <t>NEW TEXT</t>
      <blockquote>When the Echo function is active with Asynchronous mode, a
      system SHOULD <bcp14>SHOULD</bcp14> set bfd.RequiredMinRxInterval to a value of
      not less than one second (1,000,000 microseconds). This is intended to
      keep received BFD Control traffic at a negligible level, since the
      actual detection function is being performed using BFD Echo packets. A
      system operating in Demand mode would not receive BFD Control traffic.
	</t>
	</list>
	</t> traffic.</blockquote>

      <t>
	The 1st and 2nd paragraphs of Section 6.8.9 of <xref section="6.8.9" sectionFormat="of" target="RFC5880"/> are updated as below:
      </t>

    <t>
	<list>
	<t>
<!--[rfced] Regarding the updates to Section 6.8.9 of RFC 5880
a) Because these are two contiguous paragraphs in RFC 5880, we suggest
they be together rather than separate. Is this acceptable?
b) Because "except as follows" conveys meaning to the reader (i.e., they
need to read the subsequent text in RFC 5880), we suggest including it in
the OLD and NEW TEXT.

Suggested:
   The 1st and 2nd paragraphs of Section 6.8.9 of [RFC5880] are updated
   as below:

   OLD TEXT<br/>BFD TEXT

   |  BFD Echo packets MUST NOT be transmitted when bfd.SessionState is
   |  not Up.  BFD Echo packets MUST NOT be transmitted unless the last
   |  BFD Control packet received from the remote system contains a
   |  nonzero value in Required Min Echo RX Interval.
	</t>
	<t>
   |
   |  BFD Echo packets MAY be transmitted when bfd.SessionState is Up.
   |  The interval between transmitted BFD Echo packets MUST NOT be less
   |  than the value advertised by the remote system in Required Min
   |  Echo RX Interval, except as follows: [...]

   NEW TEXT<br/>When TEXT

   |  When a system is using the Echo function with either Asynchronous
   |  or Demand mode, BFD Echo packets MUST NOT be transmitted when
   |  bfd.SessionState is not Up, and BFD Echo packets MUST NOT be
   |  transmitted unless the last BFD Control packet received from the
   |  remote system contains a nonzero value in Required Min Echo RX
   |  Interval.
	</t>
	<t>
	OLD TEXT<br/>BFD
   |
   |  When a system is using the Echo function with either Asynchronous
   |  or Demand mode, BFD Echo packets MAY be transmitted when
   |  bfd.SessionState is Up.  The Up, and the interval between transmitted BFD
   |  Echo packets MUST NOT be less than the value advertised by the
   |  remote system in Required Min Echo RX Interval, except as follows:
   |  [...]
-->

      <t>OLD TEXT</t>
      <blockquote>BFD Echo packets <bcp14>MUST NOT</bcp14> be transmitted when
      bfd.SessionState is not Up.  BFD Echo packets <bcp14>MUST NOT</bcp14> be
      transmitted unless the last BFD Control packet received from the remote
      system contains a nonzero value in Required Min Echo RX
    Interval...
	</t>
	<t>
	NEW TEXT<br/>When Interval.</blockquote>

      <t>NEW TEXT</t>
      <blockquote>When a system is using the Echo function with either
      Asynchronous or Demand mode, BFD Echo packets MAY <bcp14>MUST NOT</bcp14> be
      transmitted when bfd.SessionState is not Up, and BFD Echo packets
      <bcp14>MUST NOT</bcp14> be transmitted unless the last BFD Control
      packet received from the remote system contains a nonzero value in
      Required Min Echo RX Interval.</blockquote>

      <t>OLD TEXT</t>
      <blockquote>BFD Echo packets <bcp14>MAY</bcp14> be transmitted when
      bfd.SessionState is Up.  The interval between transmitted BFD Echo
      packets MUST NOT <bcp14>MUST NOT</bcp14> be less than the value advertised by the
      remote system in Required Min Echo RX
    Interval...
	</t>
	</list>
	</t> Interval...</blockquote>

      <t>NEW TEXT</t>
      <blockquote>When a system is using the Echo function with either
      Asynchronous or Demand mode, BFD Echo packets <bcp14>MAY</bcp14> be
      transmitted when bfd.SessionState is Up, and the interval between
      transmitted BFD Echo packets <bcp14>MUST NOT</bcp14> be less than the
      value advertised by the remote system in Required Min Echo RX
      Interval...</blockquote>

    </section>

    <section title="Operational Considerations">
    <section>
      <name>Operational Considerations</name>
      <t>
    All Operational Considerations operational considerations from <xref target="RFC5880"/> apply. Since this mechanism leverages existing BFD machinery,
	particularly periodic pacing of traffic based on configuration, there's no real possibility to create congestion.
<!--[rfced] Please clarify this sentence, in particular,
"would be counter productive to check".

Original:
   Moreover, creating congestion would be counter
   productive to check the bidirectional connectivity.

Perhaps:
   Moreover, creating congestion would be
   counterproductive to checking the bidirectional connectivity.
-->
Moreover,
	creating congestion would be counterproductive to check the bidirectional connectivity.
      </t>
      <t>
    Some devices that would benefit from the use of BFD may be unable to support the full BFD protocol. Examples of such
	devices include servers running virtual machines, or Internet of Things (IoT) devices. By using Unaffiliated BFD
	Echo, these devices only need to support a basic loopback function.
      </t>
      <t>
    As specified in Section 2 <xref target="unaffiliated-bfd-echo-procedures"/> of this document, some configuration is needed to make the Unaffiliated BFD Echo work,
	although the configuration won't go beyond the scope of <xref target="RFC5880"/>.
<!--[rfced] FYI, we updated this sentence to three sentences as follows;
please review whether the text conveys the intended meaning. In
particular, please review whether "other BFD system" was intended as
singular (as below) or plural (perhaps you intended "coexist with
other types of BFD sessions").

Original:
   At a BFD-enabled BFD-
   enabled local system, the Unaffiliated BFD Echo session can coexist
   with other type of BFD session, in which scenario the remote system
   for the Unaffiliated BFD Echo session must be different from the
   remote system for other type of BFD session, and the local system's
   discriminators for different BFD sessions must be different, at the
   same time it's not necessary for the local system to differentiate
   the Unaffiliated BFD Echo session from other type of BFD session.

Current:
   At a BFD-
   enabled local system, the Unaffiliated BFD Echo session can coexist
   with another type of BFD session.  In that scenario, the remote
   system for the Unaffiliated BFD Echo session must be different from
   the remote system for the other type of BFD session, and the local
   system's discriminators for different BFD sessions must be different.
   At the same time, it's not necessary for the local system to
   differentiate the Unaffiliated BFD Echo session from the other type
   of BFD session.
-->
At a BFD-enabled local system, the
	Unaffiliated BFD Echo session can coexist with another type of BFD session. In that scenario, the remote system for the
	Unaffiliated BFD Echo session must be different from the remote system for the other type of BFD session, and the local
	system's discriminators for different BFD sessions must be different. At the same time, it's not necessary for the local
	system to differentiate the Unaffiliated BFD Echo session from the other type of BFD session.
      </t>
    </section>

    <section title="Security Considerations">
    <section>
      <name>Security Considerations</name>
      <t>
	All Security Considerations security considerations from <xref target="RFC5880"/> and <xref target="RFC5881"/> apply.
      </t>
      <t>
	Unaffiliated BFD Echo requires the remote device to loop Unaffiliated BFD Echo packets. In order to provide this
	service, the remote device cannot make use of Unicast Strict Reverse Path Forwarding (RPF) <xref target="RFC3704"/>,
	otherwise the Unaffiliated BFD Echo packets might not pass the RPF check at the remote device.
      </t>
      <t>
	As described in Section 5 of <xref section="5" sectionFormat="of" target="RFC5880"/>, BFD Echo packets may be spoofed. Specifically for Unaffiliated
	BFD Echo, a DoS attacker may send spoofed Unaffiliated BFD Echo packets to the loop-back loopback device, so some form of
	authentication SHOULD <bcp14>SHOULD</bcp14> be included. Considering the Unaffiliated BFD Echo packets in this document are also BFD
	Control packets, the "Authentication Section" as defined in <xref target="RFC5880"/> for a BFD Control packet is
	RECOMMENDED
	<bcp14>RECOMMENDED</bcp14> to be included within the Unaffiliated BFD Echo packet.
      </t>
      <t>
	As stated in Section 2, <xref target="unaffiliated-bfd-echo-procedures"/>, in order to avoid unset values being a potential vector for disclosure of uninitialized
	memory, all fields of the Unaffiliated BFD Echo packet MUST <bcp14>MUST</bcp14> be populated with a certain value, even if some of the
	fields are ignored on receipt.
      </t>
    </section>

    <section title="IANA Considerations">
    <section>
      <name>IANA Considerations</name>
      <t> This document has no IANA action requested.</t> actions.</t>
    </section>

  </middle>
  <back>

    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5880.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5881.xml"/>
      </references>
      <references>
        <name>Informative References</name>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.3704.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5082.xml"/>

<!-- [rfced] FYI, we have updated this reference to match
what was available at the URL, as shown below.

For the URL, would you prefer to use the original
(A) https://www.broadband-forum.org/technical/download/TR-146.pdf
(which redirects to a wiki page titled "Broadband Forum Published
Resources") or
(B) https://www.broadband-forum.org/pdfs/tr-146-1-0-0.pdf
(which is the document itself)?

Original:
   [BBF-TR-146]
              Broadband Forum, "BBF Technical Report - Subscriber
              Sessions Issue 1", 2013, <https://www.broadband-
              forum.org/technical/download/TR-146.pdf>.

Current:
   [BBF-TR-146]
              Broadband Forum, "TR-146: Subscriber Sessions", Broadband
              Forum Technical Report, TR-146, Issue 1, May 2013,
              <https://www.broadband-forum.org/technical/download/TR-
              146.pdf>.
-->
        <reference anchor="BBF-TR-146" target="https://www.broadband-forum.org/technical/download/TR-146.pdf">
          <front>
            <title>TR-146: Subscriber Sessions</title>
            <author>
              <organization>Broadband Forum</organization>
            </author>
            <date month="May" year="2013"/>
          </front>
	  <refcontent>Broadband Forum Technical Report, TR-146, Issue 1</refcontent>
        </reference>

      </references>
    </references>

    <section title="Acknowledgements"> numbered="false">
      <name>Acknowledgements</name>

      <t> The authors would like to acknowledge Ketan Talaulikar, Greg Mirsky, Santosh Pallagatti, Aijun Wang, Eric Vyncke,
	Adrian Farrel, Tim Wicinski, Dhruv Dhody, Stephen Farrell, Gunter <contact fullname="Ketan
      Talaulikar"/>, <contact fullname="Greg Mirsky"/>, <contact
      fullname="Santosh Pallagatti"/>, <contact fullname="Aijun Wang"/>,
      <contact fullname="Éric Vyncke"/>, <contact fullname="Adrian Farrel"/>,
      <contact fullname="Tim Wicinski"/>, <contact fullname="Dhruv Dhody"/>,
      <contact fullname="Stephen Farrell"/>, <contact fullname="Gunter Van de Velde, Gyan Mishra, Brian Trammell, Gorry Fairhurst,
	Mahesh Jethanandani, John Scudder, Murray Kucherawy, and Zaheduzzaman Sarker Velde"/>, <contact fullname="Gyan Mishra"/>, <contact fullname="Brian Trammell"/>, <contact fullname="Gorry Fairhurst"/>, <contact
      fullname="Mahesh Jethanandani"/>, <contact fullname="John Scudder"/>,
      <contact fullname="Murray Kucherawy"/>, and <contact
      fullname="Zaheduzzaman Sarker"/> for their careful review reviews and very
      helpful comments.</t>
      <t> The authors would like to acknowledge Jeff Haas <contact fullname="Jeff Haas"/> for his guidance, insightful review, and very helpful
      comments.</t>
      <t> The authors would like to acknowledge Erik Auerswald <contact fullname="Erik Auerswald"/> for his insightful comments during the discussion of this
      document.</t>
      <t> The authors would like to acknowledge Detao Zhao <contact fullname="Detao Zhao"/> for the very helpful discussion.</t>
    </section>

    <section title="Contributors">
    <t>Liu Aihua<br/>ZTE<br/>Email: liu.aihua@zte.com.cn</t>
    <t>Qian Xin<br/>ZTE<br/>Email: qian.xin2@zte.com.cn</t>
    <t>Zhao Yanhua<br/>ZTE<br/>Email: zhao.yanhua3@zte.com.cn</t> numbered="false">
      <name>Contributors</name>
    <contact fullname="Liu Aihua">
      <organization>ZTE</organization>
      <address>
        <email>liu.aihua@zte.com.cn</email>
      </address>
    </contact>

    <contact fullname="Qian Xin">
      <organization>ZTE</organization>
      <address>
        <email>qian.xin2@zte.com.cn</email>
      </address>
    </contact>

    <contact fullname="Zhao Yanhua">
      <organization>ZTE</organization>
      <address>
        <email>zhao.yanhua3@zte.com.cn</email>
      </address>
    </contact>

    </section>

</middle>

<back>

    <references title="Normative References">
     <?rfc include="reference.RFC.2119"?>
     <?rfc include="reference.RFC.8174"?>
     <?rfc include="reference.RFC.5880"?>
     <?rfc include="reference.RFC.5881"?>
    </references>

	<references title="Informative References">
     <?rfc include="reference.RFC.3704"?>
     <?rfc include="reference.RFC.5082"?>
     <reference anchor="BBF-TR-146"
                 target="https://www.broadband-forum.org/technical/download/TR-146.pdf">
        <front>
          <title>BBF Technical Report - Subscriber Sessions Issue 1</title>

          <author>
            <organization>Broadband Forum</organization>
          </author>

          <date year="2013"/>
        </front>
     </reference>
    </references>

  </back>
<!-- [rfced] Please review the "Inclusive Language" portion of the online
Style Guide <https://www.rfc-editor.org/styleguide/part2/#inclusive_language>
and let us know if any changes are needed.  Updates of this nature typically
result in more precise language, which is helpful for readers.

Note that our script did not flag any words in particular, but this should
still be reviewed as a best practice.
-->

</rfc>