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 <rfc xmlns:xi="http://www.w3.org/2001/XInclude" ipr="trust200902" docName="draft-ietf-mboned-driad-amt-discovery-13" number="8777" submissionType="IETF" category="std" updates="7450"> consensus="true" updates="7450" obsoletes="" xml:lang="en" tocInclude="true" sortRefs="true" symRefs="true" version="3">

   <!-- xml2rfc v2v3 conversion 2.39.0 -->
   <front>
     <title abbrev="DRIAD">DNS Reverse IP AMT (Automatic Automatic Multicast Tunneling)
     Tunneling (AMT) Discovery</title>
     <seriesInfo name="RFC" value="8777"/>
     <author initials="J." surname="Holland" fullname="Jake Holland">
       <organization>Akamai Technologies, Inc.</organization>
       <address>
	 <postal>
	   <street>150 Broadway</street>
          <city>Cambridge, MA 02144</city>
	   <city>Cambridge</city><region>MA</region><code>02144</code>
	   <country>United States of America</country>
	 </postal>
	 <email>jakeholland.net@gmail.com</email>
       </address>
     </author>
     <date year="2019" month="December" day="20"/> year="2020" month="April"/>
     <area>Ops</area>
     <workgroup>Mboned</workgroup>
    <keyword>Internet-Draft</keyword>

<keyword>example</keyword>

     <abstract>
       <t>This document updates RFC 7450 (Automatic 7450, "Automatic Multicast Tunneling, or AMT) Tunneling" (or AMT), by
 modifying the relay discovery process.  A new DNS resource record named
 AMTRELAY is defined for publishing AMT relays for source-specific multicast
 channels.  The reverse IP DNS zone for a multicast sender’s sender's IP address is
 configured to use AMTRELAY resource records to advertise a set of AMT relays
 that can receive and forward multicast traffic from that sender over an AMT
 tunnel.  Other extensions and clarifications to the relay discovery
 process are also defined.</t>
     </abstract>
   </front>
   <middle>
     <section anchor="intro" title="Introduction"> numbered="true" toc="default">
       <name>Introduction</name>
       <t>This document defines DNS Reverse IP AMT Discovery (DRIAD), a mechanism for
 AMT gateways to discover AMT relays that are capable of forwarding multicast
 traffic from a known source IP address.</t>
       <t>AMT (Automatic Multicast Tunneling) is defined in <xref target="RFC7450"/>, target="RFC7450" format="default"/> and provides
 a method to transport multicast traffic over a unicast tunnel, tunnel in order to
 traverse non-multicast-capable network segments.</t>

<t>Section 4.1.5 of <xref target="RFC7450"/> segments that are not multicast capable.</t>
       <t><xref target="RFC7450" sectionFormat="of" section="4.1.5"/> explains that the relay selection process
 for AMT is intended to be more flexible than the particular discovery method
 described in that document, and document. That section further explains that the selection process
 might need to depend on the source of the multicast traffic in some
 deployments, since a relay must be able to receive multicast traffic from the
 desired source in order to forward it.</t>

<t>That section
       <t><xref target="RFC7450" sectionFormat="of" section="4.1.5"/> goes on
       to suggest DNS-based queries as a possible solution. solution: DRIAD is a DNS-based solution, as suggested there. DNS based.  This solution also
 addresses the relay discovery issues in the “Disadvantages” "Disadvantages of this configuration" lists in Section
3.3 of Sections
 <xref target="RFC8313"/> target="RFC8313" sectionFormat="bare" section="3.3"/> and Section 3.4
 <xref target="RFC8313" sectionFormat="bare" section="3.4"/> of <xref
 target="RFC8313"/>.</t>
       <t>The goal for DRIAD is to enable multicast connectivity between separate
 multicast-enabled networks without preconfiguring any
 peering arrangements between the networks when neither the sending nor the receiving network
 is connected to a multicast-enabled backbone, without pre-configuring any
peering arrangement between the networks.</t> backbone.</t>
       <t>This document extends the relay discovery procedure described in Section
5.2.3.4 of <xref target="RFC7450"/>.</t> target="RFC7450" sectionFormat="of" section="5.2.3.4"/>.</t>
       <section anchor="background" title="Background"> numbered="true" toc="default">
	 <name>Background</name>
	 <t>The reader is assumed to be familiar with the basic DNS concepts
 described in <xref target="RFC1034"/>, target="RFC1034" format="default"/>, <xref target="RFC1035"/>, target="RFC1035"
 format="default"/>, and the subsequent documents that update them, particularly <xref target="RFC2181"/>.</t> target="RFC2181" format="default"/>.</t>
	 <t>The reader is also assumed to be familiar with the concepts and terminology
 regarding source-specific multicast as described in <xref target="RFC4607"/> target="RFC4607" format="default"/> and the
 use of IGMPv3 Internet Group Management Protocol Version 3 (IGMPv3) <xref target="RFC3376"/>
 target="RFC3376" format="default"/> and MLDv2 Multicast Listener Discovery Version 2
 (MLDv2) <xref target="RFC3810"/> target="RFC3810" format="default"/> for group management of
 source-specific multicast channels, as described in <xref target="RFC4604"/>.</t> target="RFC4604" format="default"/>.</t>
	 <t>The reader should also be familiar with AMT, particularly the terminology
 listed in Section 3.2 of Sections <xref target="RFC7450"/> target="RFC7450" sectionFormat="bare" section="3.2"/>
	 and Section 3.3 <xref target="RFC7450" sectionFormat="bare" section="3.3"/> of <xref target="RFC7450"/>.</t>
       </section>
       <section anchor="terminology" title="Terminology"> numbered="true" toc="default">
	 <name>Terminology</name>
	 <section anchor="relays-and-gateways" title="Relays numbered="true" toc="default">
	   <name>Relays and Gateways"> Gateways</name>
	   <t>When reading this document, it’s it's especially helpful to recall that once
 an AMT tunnel is established, the relay receives native multicast traffic
 and sends unicast tunnel-encapsulated traffic to the gateway, and the gateway. The gateway
 receives the tunnel-encapsulated packets, decapsulates them, and forwards
 them as native multicast packets, as illustrated in <xref target="figtunnel"/>.</t> target="figtunnel" format="default"/>.</t>
	   <figure title="AMT anchor="figtunnel">
	     <name>AMT Tunnel Illustration" anchor="figtunnel"><artwork><![CDATA[ Illustration</name>
	     <artwork name="" type="" align="left" alt=""><![CDATA[

  Multicast  +-----------+  Unicast  +-------------+  Multicast
 >---------> | AMT relay | >=======> | AMT gateway | >--------->
	     +-----------+           +-------------+
]]></artwork></figure>
 ]]></artwork>
	   </figure>
	 </section>
	 <section anchor="definitions" title="Definitions">

<texttable>
      <ttcol align='right'>Term</ttcol>
      <ttcol align='left'>Definition</ttcol>
      <c>(S,G)</c>
      <c>A numbered="true" toc="default">
	   <name>Definitions</name>
	   <table align="center">
	     <name>Definitions</name>
	     <thead>
	       <tr>
		 <th align="right">Term</th>
		 <th align="left">Definition</th>
	       </tr>
	     </thead>
	     <tbody>
	       <tr>
		 <td align="right">(S,G)</td>
		 <td align="left">A source-specific multicast channel, as described in <xref target="RFC4607"/>. target="RFC4607" format="default"/>. A pair of IP addresses with a source host IP and destination group IP.</c>
      <c>discovery broker</c>
      <c>A IP.</td>
	       </tr>
	       <tr>
		 <td align="right">CMTS</td>
		 <td align="left">Cable Modem Termination System</td>
	       </tr>
	       <tr>
		 <td align="right">discovery broker</td>
		 <td align="left">A broker or load balancer for AMT relay
		 discovery, as mentioned in section 4.2.1.1 of <xref target="RFC7450"/>.</c>
      <c>downstream</c>
      <c>Further
		 target="RFC7450" sectionFormat="of" section="4.2.1.1"/>.</td>
	       </tr>
	       <tr>
		 <td align="right">downstream</td>
		 <td align="left">Further from the source of traffic, as described in <xref target="RFC7450"/>.</c>
      <c>FQDN</c>
      <c>Fully target="RFC7450" format="default"/>.</td>
	       </tr>
	       <tr>
		 <td align="right">FQDN</td>
		 <td align="left">Fully Qualified Domain Name, as described in <xref target="RFC8499"/></c>
      <c>gateway</c>
      <c>An target="RFC8499" format="default"/>.</td>
	       </tr>
	       <tr>
		 <td align="right">gateway</td>
		 <td align="left">An AMT gateway, as described in <xref target="RFC7450"/></c>
      <c>L flag</c>
      <c>The “Limit” target="RFC7450" format="default"/>.</td>
	       </tr>
	       <tr>
		 <td align="right">L flag</td>
		 <td align="left">The "Limit" flag described in Section 5.1.4.4 of <xref target="RFC7450"/></c>
      <c>relay</c>
      <c>An target="RFC7450" sectionFormat="of" section="5.1.4.4"/>.</td>
	       </tr>
	       <tr>
		 <td align="right">OLT</td>
		 <td align="left">Optical Line Terminal</td>
	       </tr>
	       <tr>
		 <td align="right">relay</td>
		 <td align="left">An AMT relay, as described in <xref target="RFC7450"/></c>
      <c>RPF</c>
      <c>Reverse target="RFC7450" format="default"/>.</td>
	       </tr>
	       <tr>
		 <td align="right">RPF</td>
		 <td align="left">Reverse Path Forwarding, as described in <xref target="RFC5110"/></c>
      <c>RR</c>
      <c>A target="RFC5110" format="default"/>.</td>
	       </tr>
	       <tr>
		 <td align="right">RR</td>
		 <td align="left">A DNS Resource Record, as described in <xref target="RFC1034"/></c>
      <c>RRType</c>
      <c>A target="RFC1034" format="default"/>.</td>
	       </tr>
	       <tr>
		 <td align="right">RRType</td>
		 <td align="left">A DNS Resource Record Type, as described in <xref target="RFC1034"/></c>
      <c>SSM</c>
      <c>Source-specific target="RFC1034" format="default"/>.</td>
	       </tr>
	       <tr>
		 <td align="right">SSM</td>
		 <td align="left">Source-specific multicast, as described in <xref target="RFC4607"/></c>
      <c>upstream</c>
      <c>Closer target="RFC4607" format="default"/>.</td>
	       </tr>
	       <tr>
		 <td align="right">upstream</td>
		 <td align="left">Closer to the source of traffic, as described in <xref target="RFC7450"/>.</c>
      <c>CMTS</c>
      <c>Cable Modem Termination System</c>
      <c>OLT</c>
      <c>Optical Line Terminal</c>
</texttable>

<t>The target="RFC7450" format="default"/>.</td>
	       </tr>
	     </tbody>
	   </table>
	 </section>
<section anchor="reqs" numbered="true" toc="default">
<name>Requirements Language</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>
     <section anchor="relay-discovery-overview" title="Relay numbered="true" toc="default">
       <name>Relay Discovery Overview"> Overview</name>
       <section anchor="basic-mechanics" title="Basic Mechanics"> numbered="true" toc="default">
	 <name>Basic Mechanics</name>
	 <t>The AMTRELAY resource record (RR) defined in this document is used to
 publish the IP address or domain name of a set of AMT relays or discovery
 brokers that can receive, encapsulate, and forward multicast traffic from
 a particular sender.</t>
	 <t>The sender is the owner of the RR, RR and configures the zone so that it
 contains a set of RRs that provide the addresses or domain names of AMT
 relays (or discovery brokers that advertise relays) that can receive
 multicast IP traffic from that sender.</t>
	 <t>This enables AMT gateways in remote networks to discover an AMT relay that
 is capable of forwarding traffic from the sender.  This  This, in turn turn, enables those
 AMT gateways to receive the multicast traffic tunneled over a unicast AMT
 tunnel from those relays, relays and then to pass the multicast packets into
 networks or applications that are using the gateway to subscribe to traffic
 from that sender.</t>
	 <t>This mechanism only works for source-specific multicast (SSM) channels.  The
 source address of the (S,G) is reversed and used as an index into one of the
 reverse mapping trees (in-addr.arpa for IPv4, as described in Section 3.5 of <xref target="RFC1035"/>,
 target="RFC1035" sectionFormat="of" section="3.5"/>, or ip6.arpa for IPv6, as
	 described in Section 2.5 of <xref target="RFC3596"/>).</t> target="RFC3596" sectionFormat="of" section="2.5"/>).</t>
	 <t>This mechanism should be treated as an extension of the AMT relay discovery
 procedure described in Section 5.2.3.4 of <xref target="RFC7450"/>. target="RFC7450" sectionFormat="of" section="5.2.3.4"/>.  A gateway that
 supports this method of AMT relay discovery SHOULD <bcp14>SHOULD</bcp14> use this method
 whenever it’s it's performing the relay discovery procedure, and the source IP
 addresses for desired (S,G)s are known to the gateway, and conditions match
 the requirements outlined in <xref target="priority"/>.</t> target="priority" format="default"/>.</t>
	 <t>Some detailed example use cases are provided in <xref target="exampledeployments"/>, target="exampledeployments" format="default"/>, and
 other applicable example topologies appear in Section 3.3 of Sections <xref target="RFC8313"/>,
Section 3.4 of
 target="RFC8313" sectionFormat="bare" section="3.3"/>,
 <xref target="RFC8313"/>, target="RFC8313" sectionFormat="bare" section="3.4"/>, and Section 3.5 <xref
 target="RFC8313" sectionFormat="bare" section="3.5"/> of <xref target="RFC8313"/>.</t>
       </section>
       <section anchor="signaling-and-discovery" title="Signaling numbered="true" toc="default">
	 <name>Signaling and Discovery"> Discovery</name>
	 <t>This section describes a typical example of the end-to-end process for
 signaling a receiver’s receiver's join of an SSM channel that relies on an AMTRELAY
 RR.</t>
	 <t>The example in <xref target="figmessaging"/> target="figmessaging" format="default"/> contains 2 two multicast-enabled
 networks that are both connected to the internet with non-multicast-capable
links,
 links and which have no direct association with each other.</t>
	 <t>A content provider operates a sender, which is a source of multicast traffic
 inside a multicast-capable network.</t>
	 <t>An end user who is a customer of the content provider has a multicast-capable
internet service provider,
 Internet Service Provider (ISP), which operates a receiving network that uses an
 AMT gateway.  The AMT gateway is DRIAD-capable.</t> DRIAD capable.</t>
	 <t>The content provider provides the user with a receiving application that
 tries to subscribe to at least one (S,G).  This receiving application could could,
 for example example, be a file transfer system using FLUTE File Delivery over Unidirectional
 Transport (FLUTE) <xref target="RFC6726"/> or target="RFC6726" format="default"/>, a live
 video stream using RTP <xref target="RFC3550"/>, target="RFC3550" format="default"/>, or any other application that might
 subscribe to an SSM channel.</t>
	 <figure title="DRIAD Messaging" anchor="figmessaging"><artwork><![CDATA[ anchor="figmessaging">
	   <name>DRIAD Messaging</name>
	   <artwork name="" type="" align="left" alt=""><![CDATA[
		  +---------------+
		  |    Sender     |
   |    |         |  2001:db8::a  |
   |    |         +---------------+
   |Data|                 |
   |Flow|      Multicast  |
  \|    |/      Network   |
   \    /                 |        5: Propagate RPF for Join(S,G)
    \  /          +---------------+
     \/           |   AMT Relay relay   |
		  | 2001:db8:c::f |
		  +---------------+
			  |        4: Gateway connects to Relay,
				      sends Join(S,G) over tunnel
			  |
		 Unicast
		  Tunnel  |        3: --> DNS Query: type=AMTRELAY,
				  /        a.0.0.0.0.0.0.0.0.0.0.0.
      ^                   |      /         0.0.0.0.0.0.0.0.0.0.0.0.
      |                         /          8.b.d.0.1.0.0.2.ip6.arpa
      |                   |    /      <-- Response:
  Join/Leave       +-------------+         AMTRELAY=2001:db8:c::f
   Signals         | AMT gateway |
      |            +-------------+
      |                   |        2: Propagate RPF for Join(S,G)
      |        Multicast  |
		Network   |
        	          |     1: Join(S=2001:db8::a,G=ff3e::8000:d)
		   +-------------+
		   |   Receiver  |
		   |  (end user) |
		   +-------------+
]]></artwork></figure>
 ]]></artwork>
	 </figure>
	 <t>In this simple example, the sender IP is 2001:db8::a, it which is sending
 traffic to the group address ff3e::8000:d, and the relay IP is
 2001:db8::c:f.</t>
	 <t>The content provider has previously configured the DNS zone that
 contains the reverse IP domain name for the sender’s sender's IP address
 so that it provides an AMTRELAY RR with the relay’s relay's IP address.
(See address
 (see <xref target="rpformat"/> target="rpformat" format="default"/> for details about the AMTRELAY RR format and
semantics.)
 semantics).  As described in Section 2.5 of <xref target="RFC3596"/>, target="RFC3596"
 sectionFormat="of" section="2.5"/>, the
 reverse IP FQDN of the sender’s sender's address “2001:db8::a” "2001:db8::a" is:</t>

<figure><artwork><![CDATA[
	 <sourcecode name="" type=""><![CDATA[
 a.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.
                                                                arpa.
]]></artwork></figure>
 ]]></sourcecode>
	 <t>The sequence of events depicted in <xref target="figmessaging"/> target="figmessaging" format="default"/> is as follows:</t>

<t><list style="numbers">
  <t>The
	 <ol spacing="normal" type="1">
	   <li>The end user starts the app, which issues a join to the (S,G):
 (2001:db8::a, ff3e::8000:d).</t>
  <t>The ff3e::8000:d).</li>
	   <li>The join propagates with RPF through the receiver’s receiver's multicast-enabled
 network with PIM <xref target="RFC7761"/> target="RFC7761" format="default"/> or another
	   multicast routing mechanism, mechanism until the AMT gateway receives a signal to join the (S,G).</t> (S,G).</li>
	   <li>
	     <t>The AMT gateway performs a reverse DNS lookup for the AMTRELAY RRType,
	     RRType by sending an AMTRELAY RRType query for the reverse IP domain name
 for the sender’s sender's source IP address (the S from the (S,G)).  <vspace blankLines='1'/>  </t>
	     <t>
 The DNS resolver for the AMT gateway uses ordinary DNS recursive
 resolution until it has the authoritative result that the content
 provider configured, which informs the AMT gateway that the relay address
 is 2001:db8::c:f.</t>
  <t>The
	   </li>
	   <li>The AMT gateway performs AMT handshakes with the AMT relay as described
 in Section 4 of <xref target="RFC7450"/>, target="RFC7450" section="4" sectionFormat="of"/>, then forwards a Membership membership report to the
relay
 relay, indicating subscription to the (S,G).</t>
  <t>The (S,G).</li>
	   <li>The relay propagates the join through its network toward the sender,
	   sender and then forwards the appropriate AMT-encapsulated traffic to the
 gateway, which decapsulates and forwards it as a native multicast through
 its downstream network to the end user.</t>
</list></t> user.</li>
	 </ol>
	 <t>In the case of an IPv4 (S,G), the only difference in the AMT relay
 discovery process is the use of the in-addr.arpa reverse IP domain name,
 as described in Section 3.5 of <xref target="RFC1035"/>, target="RFC1035" sectionFormat="of" section="3.5"/>, instead of the in6.arpa
 domain name.  For example, if the (S,G) is (198.51.100.12, 232.252.0.2),
 the reverse IP FQDN for the AMTRELAY query would be
“12.100.51.198.in-addr.arpa.”.</t>
 "12.100.51.198.in-addr.arpa.".</t>
	 <t>Note that the address family of the AMT tunnel is independent of the
 address family for the multicast traffic.</t>
       </section>
       <section anchor="exampledeployments" title="Example Deployments"> numbered="true" toc="default">
	 <name>Example Deployments</name>
	 <section anchor="exrx" title="Example numbered="true" toc="default">
	   <name>Example Receiving Networks"> Networks</name>
	   <section anchor="exrxisp" title="Internet numbered="true" toc="default">
	     <name>Internet Service Provider"> Provider</name>
	     <t>One example of a receiving network is an Internet Service Provider (ISP)
 that offers multicast ingest services to its subscribers, illustrated in
 <xref target="figrxisp"/>.</t> target="figrxisp" format="default"/>.</t>
	     <t>In the example network below, subscribers can join (S,G)s with MLDv2 or
 IGMPv3 as described in <xref target="RFC4604"/>, target="RFC4604" format="default"/>, and the AMT gateway in this
 ISP can receive and forward multicast traffic from one of the example sending
 networks in <xref target="extx"/> target="extx" format="default"/> by discovering the appropriate AMT relays with a DNS
 lookup for the AMTRELAY RR with the reverse IP of the source in the (S,G).</t>
	     <figure title="Receiving anchor="figrxisp">
	       <name>Receiving ISP Example" anchor="figrxisp"><artwork><![CDATA[ Example</name>
	       <artwork name="" type="" align="left" alt=""><![CDATA[
		     Internet
		  ^            ^      Multicast-enabled
		  |            |      Receiving Network
	   +------|------------|-------------------------+
	   |      |            |                         |
	   |  +--------+   +--------+    +=========+     |
	   |  | Border |---| Border |    |   AMT   |     |
	   |  | Router |   | Router |    | gateway |     |
	   |  +--------+   +--------+    +=========+     |
	   |      |            |              |          |
	   |      +-----+------+-----------+--+          |
	   |            |                  |             |
	   |      +-------------+    +-------------+     |
	   |      | Agg Routers | .. | Agg Routers |     |
	   |      +-------------+    +-------------+     |
	   |            /     \ \     /         \        |
	   | +---------------+         +---------------+ |
	   | |Access Systems | ....... |Access Systems | |
	   | |(CMTS/OLT/etc.)|         |(CMTS/OLT/etc.)| |
	   | +---------------+         +---------------+ |
	   |        |                        |           |
	   +--------|------------------------|-----------+
		    |                        |
	      +---+-+-+---+---+        +---+-+-+---+---+
	      |   |   |   |   |        |   |   |   |   |
	     /-\ /-\ /-\ /-\ /-\      /-\ /-\ /-\ /-\ /-\
	     |_| |_| |_| |_| |_|      |_| |_| |_| |_| |_|

			    Subscribers
]]></artwork></figure>
 ]]></artwork>
	     </figure>
	   </section>
	   <section anchor="exoffice" title="Small Office"> numbered="true" toc="default">
	     <name>Small Office</name>
	     <t>Another example receiving network is a small branch office that regularly
 accesses some multicast content, illustrated in <xref target="figrxofficenm"/>.</t> target="figrxofficenm" format="default"/>.</t>
	     <t>This office has desktop devices that need to receive some multicast traffic,
 so an AMT gateway runs on a LAN with these devices, devices to pull traffic in
 through a non-multicast next-hop.</t> next hop.</t>
	     <t>The office also hosts some mobile devices that have AMT gateway instances
 embedded inside apps, apps in order to receive multicast traffic over their
 non-multicast wireless LAN.  (Note that the “Legacy Router” "Legacy Router" is a
 simplification that’s that's meant to describe a variety of possible conditions;
 for example example, it could be a device providing a split-tunnel VPN as described
 in <xref target="RFC7359"/>, target="RFC7359" format="default"/>, deliberately excluding multicast traffic for a VPN
 tunnel, rather than a device which that is incapable of multicast forwarding.)</t>
	     <figure title="Small anchor="figrxofficenm">
	       <name>Small Office (no multicast up)" anchor="figrxofficenm"><artwork><![CDATA[ (No Multicast Up)</name>
	       <artwork name="" type="" align="left" alt=""><![CDATA[
		  Internet
	       (non-multicast)
		      ^
		      |                  Office Network
	   +----------|----------------------------------+
	   |          |                                  |
	   |    +---------------+ (Wifi)   Mobile apps   |
	   |    | Modem+ | Wifi | - - - -  w/ embedded   |
	   |    | Router |  AP  |          AMT gateways  |
	   |    +---------------+                        |
	   |          |                                  |
	   |          |                                  |
	   |     +----------------+                      |
	   |     | Legacy Router  |                      |
	   |     |   (unicast)    |                      |
	   |     +----------------+                      |
	   |      /        |      \                      |
	   |     /         |       \                     |
	   | +--------+ +--------+ +--------+=========+  |
	   | | Phones | | ConfRm | | Desks  |   AMT   |  |
	   | | subnet | | subnet | | subnet | gateway |  |
	   | +--------+ +--------+ +--------+=========+  |
	   |                                             |
	   +---------------------------------------------+
]]></artwork></figure>
 ]]></artwork>
	     </figure>
	     <t>By adding an AMT relay to this office network as in <xref target="figrxoffice"/>, it’s target="figrxoffice" format="default"/>, it's
 possible to make use of multicast services from the example multicast-capable
 ISP in <xref target="exrxisp"/>.</t> target="exrxisp" format="default"/>.</t>
	     <figure title="Small anchor="figrxoffice">
	       <name>Small Office Example" anchor="figrxoffice"><artwork><![CDATA[ Example</name>
	       <artwork name="" type="" align="left" alt=""><![CDATA[
	    Multicast-capable ISP
		      ^
		      |                  Office Network
	   +----------|----------------------------------+
	   |          |                                  |
	   |    +---------------+ (Wifi)   Mobile apps   |
	   |    | Modem+ | Wifi | - - - -  w/ embedded   |
	   |    | Router |  AP  |          AMT gateways  |
	   |    +---------------+                        |
	   |          |               +=======+          |
	   |          +---Wired LAN---|  AMT  |          |
	   |          |               | relay |          |
	   |     +----------------+   +=======+          |
	   |     | Legacy Router  |                      |
	   |     |   (unicast)    |                      |
	   |     +----------------+                      |
	   |      /        |      \                      |
	   |     /         |       \                     |
	   | +--------+ +--------+ +--------+=========+  |
	   | | Phones | | ConfRm | | Desks  |   AMT   |  |
	   | | subnet | | subnet | | subnet | gateway |  |
	   | +--------+ +--------+ +--------+=========+  |
	   |                                             |
	   +---------------------------------------------+
]]></artwork></figure>
 ]]></artwork>
	     </figure>
	     <t>When multicast-capable networks are chained like this, with a network like
 the one in <xref target="figrxoffice"/> target="figrxoffice" format="default"/> receiving internet Internet services from a
 multicast-capable network like the one in <xref target="figrxisp"/>, it’s target="figrxisp" format="default"/>, it's important for
 AMT gateways to reach the more local AMT relay, relay in order to avoid
 accidentally tunneling multicast traffic from a more distant AMT relay with
unicast,
 unicast and failing to utilize the multicast transport capabilities of the
 network in <xref target="figrxisp"/>.</t> target="figrxisp" format="default"/>.</t>
	   </section>
	 </section>
	 <section anchor="extx" title="Example numbered="true" toc="default">
	   <name>Example Sending Networks"> Networks</name>
	   <section anchor="extxsnd" title="Sender-controlled Relays"> numbered="true" toc="default">
	     <name>Sender-Controlled Relays</name>
	     <t>When a sender network is also operating AMT relays to distribute multicast
 traffic, as in <xref target="figtxrelay"/>, target="figtxrelay" format="default"/>, each address could appear as an AMTRELAY RR
 for the reverse IP of the sender, or sender. Alternately, one or more domain names could appear
 in AMTRELAY RRs, and the AMT relay addresses can be discovered by finding
 A or AAAA records from those domain names.</t>
	     <figure title="Small anchor="figtxrelay">
	       <name>Small Office Example" anchor="figtxrelay"><artwork><![CDATA[ Example</name>
	       <artwork name="" type="" align="left" alt=""><![CDATA[
				       Sender Network
		 +-----------------------------------+
		 |                                   |
		 | +--------+   +=======+  +=======+ |
		 | | Sender |   |  AMT  |  |  AMT  | |
		 | +--------+   | relay |  | relay | |
		 |     |        +=======+  +=======+ |
		 |     |            |          |     |
		 |     +-----+------+----------+     |
		 |           |                       |
		 +-----------|-----------------------+
			     v
			  Internet
		       (non-multicast)
]]></artwork></figure>
 ]]></artwork>
	     </figure>
	   </section>
	   <section anchor="extxprv" title="Provider-controlled Relays"> numbered="true" toc="default">
	     <name>Provider-Controlled Relays</name>
	     <t>When an ISP offers a service to transmit outbound multicast traffic through
 a forwarding network, it might also offer AMT relays in order to reach
 receivers without multicast connectivity to the forwarding network, as in
 <xref target="figtxisp"/>. target="figtxisp" format="default"/>. In this case it’s case, it's recommended that the ISP also provide at
 least one domain name for the AMT relays for use with the AMTRELAY RR.</t>
	     <t>When the sender wishes to use the relays provided by the ISP for
 forwarding multicast traffic, an AMTRELAY RR should be configured to use
 the domain name provided by the ISP, ISP to allow for address reassignment of the
 relays without forcing the sender to reconfigure the corresponding AMTRELAY
 RRs.</t>
	     <figure title="Sending anchor="figtxisp">
	       <name>Sending ISP Example" anchor="figtxisp"><artwork><![CDATA[ Example</name>
	       <artwork name="" type="" align="left" alt=""><![CDATA[
		   +--------+
		   | Sender |
		   +---+----+        Multicast-enabled
		       |              Sending Network
	   +-----------|-------------------------------+
	   |           v                               |
	   |    +------------+     +=======+ +=======+ |
	   |    | Agg Router |     |  AMT  | |  AMT  | |
	   |    +------------+     | relay | | relay | |
	   |           |           +=======+ +=======+ |
	   |           |               |         |     |
	   |     +-----+------+--------+---------+     |
	   |     |            |                        |
	   | +--------+   +--------+                   |
	   | | Border |---| Border |                   |
	   | | Router |   | Router |                   |
	   | +--------+   +--------+                   |
	   +-----|------------|------------------------+
		 |            |
		 v            v
		    Internet
		 (non-multicast)
]]></artwork></figure>
 ]]></artwork>
	     </figure>
	   </section>
	 </section>
       </section>
     </section>
     <section anchor="relay-discovery-operation" title="Relay numbered="true" toc="default">
       <name>Relay Discovery Operation"> Operation</name>
       <section anchor="priority" title="Optimal numbered="true" toc="default">
	 <name>Optimal Relay Selection"> Selection</name>
	 <section anchor="overview" title="Overview"> numbered="true" toc="default">
	   <name>Overview</name>
	   <t>The reverse source IP DNS query of an AMTRELAY RR is a good way for a gateway
 to discover a relay that is known to the sender.</t>
	   <t>However, it is NOT *not* necessarily a good way to discover the best relay for that
 gateway to use, because the RR will only provide information about relays
 known to the source.</t>
	   <t>If there is an upstream relay in a network that is topologically closer to
 the gateway and is able to receive and forward multicast traffic from the sender,
 that relay is better for the gateway to use, use since more of the network path
 uses native multicast, allowing more chances for packet replication.  But since
 that relay is not known to the sender, it won’t won't be advertised in the sender’s sender's
 reverse IP DNS record.  An example network that illustrates this scenario is
 outlined in <xref target="figrxoffice"/> target="figrxoffice" format="default"/> from <xref target="exoffice"/>.</t>

<t>It’s target="exoffice" format="default"/>.</t>
	   <t>It's only appropriate for an AMT gateway to discover an AMT relay by querying
 an AMTRELAY RR owned by a sender when all of these conditions are met:</t>

<t><list style="numbers">
  <t>The
	   <ol spacing="normal" type="1">
	     <li>The gateway needs to propagate a join of an (S,G) over AMT, AMT because in
 the gateway’s gateway's network, no RPF next hop toward the source can
 propagate a native multicast join of the (S,G); and</t>
  <t>The (S,G);</li>
	     <li>The gateway is not already connected to a relay that forwards multicast
 traffic from the source of the (S,G); and</t>
  <t>The (S,G);</li>
	     <li>The gateway is not configured to use a particular IP address for AMT
 discovery, or a relay discovered with that IP is not able to forward
 traffic from the source of the (S,G); and</t>
  <t>The (S,G);</li>
	     <li>The gateway is not able to find an upstream AMT relay with DNS-SD
	     DNS-based Service Discovery (DNS-SD)
 <xref target="RFC6763"/>, target="RFC6763" format="default"/> using “_amt._udp” "_amt._udp" as the Service section of the queries, or a
 relay discovered this way is not able to forward traffic from the source of
 the (S,G) (as described in <xref target="trafficabsent"/> or target="trafficabsent" format="default"/> and <xref target="loaded"/>); and</t>
  <t>The target="loaded" format="counter"/>); and</li>
	     <li>The gateway is not able to find an upstream AMT relay with the well-known
 anycast addresses from Section 7 of <xref target="RFC7450"/>.</t>
</list></t> target="RFC7450" sectionFormat="of" section="7"/>.</li>
	   </ol>
	   <t>When all of the above conditions are met, the gateway has no path within its local
 network that can receive multicast traffic from the source IP of the (S,G).</t>
	   <t>In this situation, the best way to find a relay that can forward the
 required traffic is to use information that comes from the operator of the
 sender.  When the sender has configured an AMTRELAY RR, gateways can use the
 DRIAD mechanism defined in this document to discover the relay information
 provided by the sender.</t>
	   <t>Note that the DNS-SD service is not source-specific, so even though
when available, several methods of finding a more local source of
multicast traffic connectivity above conditions are preferred designed to using prefer the use of
	   a local AMT relay
provided by an AMTRELAY RR, a gateway further if one can be discovered.  However, note also
	   that the network upstream of the locally discovered relay would
	   still be
using need to receive traffic from the AMTRELAY RR sender of the (S,G) in order
	   to forward it.  Therefore, unless the upstream network has a peering contains the
	   sender or has a multicast-capable peering with a network that provides an alternative end-to-end multicast transport path for can
	   forward traffic from the (S,G)’s traffic.</t> sender, the upstream network might still
	   use AMT to ingest the traffic from a network that can receive
	   traffic from the sender.  If this is the case, the upstream AMT
	   gateway could still rely on the AMTRELAY RR provided by the sender,
	   even though the AMTRELAY RR is not directly used by gateways
	   topologically closer to the receivers.  For a concrete example of
	   such a situation, consider the network in <xref target="figrxoffice"/> connected as one
	   of the customers to the network in <xref target="figrxisp"/>.</t>
	 </section>
	 <section anchor="ordering" title="Preference Ordering"> numbered="true" toc="default">
	   <name>Preference Ordering</name>
	   <t>This section defines a preference ordering for relay addresses during
 the relay discovery process.  Gateways are encouraged to implement a
 Happy Eyeballs <xref target="RFC8305"/> algorithm to try candidate relays concurrently,
 concurrently (see <xref target="happy"/>), but even
 gateways that do not implement a Happy Eyeballs algorithm SHOULD <bcp14>SHOULD</bcp14> use
 this ordering, except as noted.</t>
	   <t>When establishing an AMT tunnel to forward multicast data, it’s it's
 very important for the discovery process to prioritize the network
 topology considerations ahead of address selection considerations, considerations in
 order to gain the packet replication benefits from using multicast
 instead of unicast tunneling in the multicast-capable portions of the
 network path.</t>
	   <t>The intent of the advice and requirements in this section is to describe
 how a gateway should make use of the concurrency provided by a Happy
 Eyeballs algorithm to reduce the join latency, latency while still prioritizing
 network efficiency considerations over Address Selection address selection considerations.</t>

<t>Section 4 of <xref target="RFC8305"/>
	   <t><xref target="RFC8305" sectionFormat="of" section="4"/> requires a Happy Eyeballs algorithm to sort
 the addresses with the Destination Address Selection defined in Section
6 of <xref target="RFC6724"/>, target="RFC6724" sectionFormat="of" section="6"/>, but for the above reasons, that requirement is superseded
 in the AMT discovery use case by the following considerations:</t>

<t><list style="symbols">
	   <ul spacing="normal">
	     <li>
	       <t>Prefer Local Relays  <vspace blankLines='1'/>
<xref target="figrxoffice"/>  </t>
	       <t><xref target="figrxoffice" format="default"/> and <xref target="exoffice"/> target="exoffice" format="default"/> provide a motivating example to prefer
  DNS-SD <xref target="RFC6763"/> target="RFC6763" format="default"/> for discovery strictly ahead of using the AMTRELAY RR
  controlled by the sender for AMT discovery.  <vspace blankLines='1'/> discovery.</t>
	       <t>
 For this reason, it’s RECOMMENDED it's <bcp14>RECOMMENDED</bcp14> that AMT gateways by default perform
  service discovery using DNS Service Discovery (DNS-SD) <xref target="RFC6763"/> target="RFC6763" format="default"/> for
  _amt._udp.&lt;domain&gt; (with &lt;domain&gt; chosen as described in Section 11 of <xref target="RFC6763"/>) target="RFC6763" sectionFormat="of" section="11"/>) and use the AMT relays discovered that way in preference to
  AMT relays discoverable via the mechanism defined in this document
  (DRIAD).</t>
	     </li>
	     <li>
	       <t>Prefer Relays Managed by the Containing Network  <vspace blankLines='1'/>  </t>
	       <t>
 When no local relay is discoverable with DNS-SD, it still may be the
  case that a relay local to the receiver is operated by the network
  providing transit services to the receiver.  <vspace blankLines='1'/>  </t>
	       <t>
 In this case, when the network cannot make the relay discoverable via
  DNS-SD, the network SHOULD <bcp14>SHOULD</bcp14> use the well-known anycast addresses from
 Section 7 of <xref target="RFC7450"/> target="RFC7450" format="default" section="7"/> to route discovery traffic to the relay most
  appropriate to the receiver’s gateway.  <vspace blankLines='1'/> receiver's gateway.</t>
	       <t>
 Accordingly, the gateway SHOULD <bcp14>SHOULD</bcp14> by default discover a relay with the
  well-known AMT anycast addresses as the second preference next-best option after DNS-SD
  when searching for a local relay.</t>
	     </li>
	     <li>
	       <t>Let Sender Manage Relay Provisioning  <vspace blankLines='1'/>  </t>
	       <t>
 A related motivating example is provided by considering a sender whose
  traffic can be forwarded by relays in a sender-controlled network
  like <xref target="figtxrelay"/> target="figtxrelay" format="default"/> in <xref target="extxsnd"/>, target="extxsnd" format="default"/> and also by relays in a
  provider-controlled network like <xref target="figtxisp"/> target="figtxisp" format="default"/> in <xref target="extxprv"/>, target="extxprv" format="default"/>, with
  different cost and scalability profiles for the different options.  <vspace blankLines='1'/>  </t>
	       <t>
 In this example about the sending-side network, the precedence field
  described in <xref target="rrdef-precedence"/> target="rrdef-precedence" format="default"/> is a critical method of control so
  that senders can provide the appropriate guidance to gateways
  during the discovery process, process in order to manage load and failover
  scenarios in a manner that operates well with the sender’s sender's
  provisioning strategy for horizontal scaling of AMT relays.  <vspace blankLines='1'/>  </t>
	       <t>
 Therefore, after DNS-SD, the precedence from the RR MUST <bcp14>MUST</bcp14> be used for
  sorting preference ahead of the Destination Address Selection ordering
  from Section 6 of <xref target="RFC6724"/>, target="RFC6724" section="6" sectionFormat="of"/> so that only relay IPs with the same
  precedence are directly compared according to the Destination Address
  Selection ordering.</t>
</list></t>
	     </li>
	   </ul>
	   <t>Accordingly, AMT gateways SHOULD <bcp14>SHOULD</bcp14> by default prefer relays in this
	   order:</t>

<figure><artwork><![CDATA[
   1. DNS-SD
   2. Anycast
 <ol>
 <li>DNS-SD</li>
 <li>Anycast addresses from Section 7 of [RFC7450]
   3. DRIAD
]]></artwork></figure> <xref target="RFC7450" sectionFormat="of" section="7"/></li>
 <li>DRIAD</li>
 </ol>
	   <t>This default behavior MAY <bcp14>MAY</bcp14> be overridden by administrative configuration where
 other behavior is more appropriate for the gateway within its network.</t>
	   <t>Among relay addresses that have an equivalent preference as described above, a
 Happy Eyeballs algorithm for AMT SHOULD <bcp14>SHOULD</bcp14> use the Destination Address Selection
 defined in Section 6 of <xref target="RFC6724"/>.</t> target="RFC6724" sectionFormat="of" section="6"/>.</t>
	   <t>Among relay addresses that still have an equivalent preference after the
 above orderings, a gateway SHOULD <bcp14>SHOULD</bcp14> make a non-deterministic choice (such as
 a pseudorandom selection) for relay preference ordering, ordering in order to
 support load balancing by DNS configurations that provide many relay
 options.</t>
	   <t>The gateway MAY <bcp14>MAY</bcp14> introduce a bias in the non-deterministic choice according
 to information obtained out of band or from a historical record about
network topology, timing information, or the response to a probing
mechanism, that indicates some expected benefits from selecting some relays in
 preference to others. Details about the structure and collection of this
 information are out of scope for this document, document but could, for example, be
 obtained by out-of-band methods or from a historical record about
 network topology, timing information, or the response to a probing
 mechanism. A gateway in possession of such information MAY <bcp14>MAY</bcp14> use it to prefer topologically closer relays.</t>
	   <t>Within the above constraints, gateways MAY <bcp14>MAY</bcp14> make use of other considerations
 from Section 4 of <xref target="RFC8305"/>, target="RFC8305" sectionFormat="of" section="4"/>, such as the address family interleaving
 strategies, to produce a final ordering of candidate relay addresses.</t>
	   <t>Note also that certain relay addresses might be excluded from consideration
 by the hold-down timers described in <xref target="trafficabsent"/> target="trafficabsent" format="default"/> or <xref target="loaded"/>. target="loaded" format="counter"/>.  These
 relays constitute “unusable destinations” "unusable destinations" under Rule 1 of the Destination
 Address Selection, Selection and are also not part of the superseding considerations
 described above.</t>
	   <t>The discovery and connection process for the relay addresses in the above
 described ordering MAY <bcp14>MAY</bcp14> operate in parallel, subject to delays prescribed
 by the Happy Eyeballs requirements described in Section 5 of <xref target="RFC8305"/>
 target="RFC8305" sectionFormat="of" section="5"/>
 for successively launched concurrent connection attempts.</t>
	 </section>
	 <section anchor="connecting-to-multiple-relays" title="Connecting numbered="true" toc="default">
	   <name>Connecting to Multiple Relays"> Relays</name>
	   <t>In some deployments, it may be useful for a gateway to connect to
 multiple upstream relays and subscribe to the same traffic, traffic in order to
 support an active/active failover model.  A gateway SHOULD NOT <bcp14>SHOULD NOT</bcp14> be
 configured to do so without guaranteeing that adequate bandwidth is
 available.</t>
	   <t>A gateway configured to do this SHOULD <bcp14>SHOULD</bcp14> still use the same preference
ordering preference-ordering logic from <xref target="ordering"/> target="ordering" format="default"/> for each connection.  (Note that
 this ordering allows for overriding by explicit administrative
 configuration where required.)</t>
	 </section>
       </section>
       <section anchor="happy" title="Happy Eyeballs"> numbered="true" toc="default">
	 <name>Happy Eyeballs</name>
	 <section anchor="overview-1" title="Overview"> numbered="true" toc="default">
	   <name>Overview</name>
	   <t>Often, multiple choices of relay will exist for a gateway using DRIAD
 for relay discovery.  Happy Eyeballs <xref target="RFC8305"/> target="RFC8305" format="default"/> provides a widely
 deployed and generalizable strategy for probing multiple possible
 connections in parallel, therefore parallel. Therefore, it is RECOMMENDED <bcp14>RECOMMENDED</bcp14> that DRIAD-capable
 gateways implement a Happy Eyeballs algorithm to support fast discovery
 of the most preferred available relay, relay by probing multiple relays
 concurrently.</t>
	   <t>The parallel discovery logic of a Happy Eyeballs algorithm serves to
 reduce join latency for the initial join of an SSM channel.  This section
 and the preference ordering of relays defined in <xref target="ordering"/> taken target="ordering"
 format="default"/> together provide guidance on use of a Happy Eyeballs algorithm for the
 case of establishing AMT connections.</t>
	   <t>Note that according to the definition in <xref target="connection-def"/> target="connection-def" format="default"/> of this
 document, establishing the connection occurs before sending a membership
 report.  As described in Section 5 of <xref target="RFC8305"/>, target="RFC8305"
 sectionFormat="of" section="5"/>, only one of the
 successful connections will be used, and the others are all canceled
 or ignored.  In the context of an AMT connection, this means the gateway
 will send the membership reports that subscribe to traffic only for the
 chosen connection, connection after the Happy Eyeballs algorithm resolves.</t>
	 </section>
	 <section anchor="algorithm-guidelines" title="Algorithm Guidelines"> numbered="true" toc="default">
	   <name>Algorithm Guidelines</name>
	   <t>During the “Initiation "Initiation of asynchronous DNS queries” queries" phase
	   described in
Section 3 of <xref target="RFC8305"/>), target="RFC8305" sectionFormat="of" section="3"/>, a gateway attempts to resolve the domain names
 listed in <xref target="priority"/>. target="priority" format="default"/>.  This consists of resolving the SRV queries for
 DNS-SD domains for the AMT service, as well as the AMTRELAY query for the
	   reverse IP domain defined in this document.</t>

	   <t>Each of the SRV and AMTRELAY responses might contain one contain:</t>

	   <ul
	   spacing="normal"> <li>one
	   or more IP
addresses, addresses (as with type 1 or type 2 AMTRELAY responses,
	   responses or when the SRV Additional Data section of the
	   SRV response contains the address records for the target,
	   as urged by <xref target="RFC2782"/>), or they might
contain target="RFC2782" format="default"/>),
	   or</li>
	   <li>
	   only domain names (as with type 3
	   responses from <xref target="rtype"/>, target="rtype" format="default"/> or
	   an SRV response without an additional data section).</t> section).</li></ul>
	   <t>When present, IP addresses in the initial response provide resolved
 destination address candidates for the “Sorting "Sorting of resolved
 destination addresses” addresses" phase described in Section 4 of <xref target="RFC8305"/>), target="RFC8305"
 sectionFormat="of" section="4"/>),
 whereas domain names without IP addresses in the initial response result
 in another set of queries for AAAA and A records, whose responses provide
 the candidate resolved destination addresses.</t>
	   <t>Since the SRV or AMTRELAY responses don’t don't have a bound on the count of
 queries that might be generated aside from the bounds imposed by the
 DNS resolver, it’s it's important for the gateway to provide a rate limit on
 the DNS queries.  The DNS query functionality is expected to follow
 ordinary standards and best practices for DNS clients.  A gateway MAY <bcp14>MAY</bcp14>
 use an existing DNS client implementation that does so, so and MAY <bcp14>MAY</bcp14> rely on
 that client’s rate limiting client's rate-limiting logic to avoid issuing excessive queries.
 Otherwise, a gateway MUST <bcp14>MUST</bcp14> provide a rate limit for the DNS queries, and
 its default settings SHOULD NOT <bcp14>SHOULD NOT</bcp14> permit more than 10 queries for any
 100-millisecond period (though this MAY <bcp14>MAY</bcp14> be overridable by the administrative
 configuration).</t>
	   <t>As the resolved IP addresses arrive, the Happy Eyeballs algorithm
 sorts them according to the requirements and recommendations given in
 <xref target="ordering"/>, target="ordering" format="default"/> and attempts connections with the corresponding relays
 under the algorithm restrictions and guidelines given in <xref target="RFC8305"/> target="RFC8305" format="default"/> for
 the “Establishment "Establishment of one connection, which cancels all other attempts” attempts"
 phase.  As described in Section 3 of <xref target="RFC8305"/>, target="RFC8305" sectionFormat="of" section="3"/>, DNS resolution is
 treated as asynchronous, and connection initiation does not wait
 for lagging DNS responses.</t>
	 </section>
	 <section anchor="connection-def" title="Connection Definition">

<t>Section 5 of <xref target="RFC8305"/> numbered="true" toc="default">
	   <name>Connection Definition</name>
	   <t><xref target="RFC8305" sectionFormat="of" section="5"/>
	   non-normatively describes success at a successful connection attempt as “generally "generally when the TCP handshake completes”.</t> completes".</t>
	   <t>There is no normative definition of a connection in the AMT specification
 <xref target="RFC7450"/>, target="RFC7450" format="default"/>, and there is no TCP connection involved in an AMT tunnel.</t>
	   <t>However, the concept of an AMT connection in the context of a Happy
 Eyeballs algorithm is a useful one, and so this section provides the
 following normative definition:</t>

<t><list style="symbols">
  <t>An
	   <ul spacing="normal">

	     <li>An AMT connection is established successfully when the gateway receives
 from a newly discovered relay a valid Membership Query message
(Section 5.1.4 of <xref target="RFC7450"/>)
 (<xref target="RFC7450" sectionFormat="of" section="5.1.4"/>) that does not have the L flag set.</t>
</list></t> set.</li>
	   </ul>
	   <t>See <xref target="loaded"/> target="loaded" format="default"/> of this document for further information about the
 relevance of the L flag to the establishment of a Happy Eyeballs
 connection.  See <xref target="flowhealth"/> target="flowhealth" format="default"/> for an overview of how to respond
 if the connection does not provide multicast connectivity to the
 source.</t>
	   <t>To “cancel” "cancel" this kind of AMT connection for the Happy Eyeballs algorithm,
 a gateway that has not sent a membership report with a subscription
 would simply stop sending AMT packets for that connection.  A
 gateway only sends a membership report to a connection it has chosen as
 the most preferred available connection.</t>
	 </section>
       </section>
       <section anchor="guidelines-for-restarting-discovery" title="Guidelines numbered="true" toc="default">
	 <name>Guidelines for Restarting Discovery"> Discovery</name>
	 <section anchor="overview-2" title="Overview">

<t>It’s numbered="true" toc="default">
	   <name>Overview</name>
	   <t>It's expected that gateways deployed in different environments will use a
 variety of heuristics to decide when it’s it's appropriate to restart the relay
 discovery process, process in order to meet different performance goals (for example,
 to fulfill different kinds of service level agreements).</t>
	   <t>In general, restarting the discovery process is always safe for
 the gateway and relay during any of the events listed in this section, section
 but may cause a disruption in the forwarded traffic if the discovery
 process results in choosing a different relay, relay because this changes
 the RPF forwarding tree for the multicast traffic upstream of the gateway.
 This is likely to result in some dropped or duplicated packets from channels
	   actively being tunneled from the old relay to the gateway.</t>
	   <t>The degree of impact on the traffic from choosing a different relay may
 depend on network conditions between the gateway and the new relay, as well
 as the network conditions and topology between the sender and the new relay,
 as this may cause the relay to propagate a new RPF join toward the sender.</t>
	   <t>Balancing the expected impact on the tunneled traffic against likely
 or observed problems with an existing connection to the relay is the goal
 of the heuristics that gateways use to determine when to restart the
 discovery process.</t>
	   <t>The non-normative advice in this section should be treated as guidelines to
 operators and implementors working with AMT systems that can use DRIAD as
 part of the relay discovery process.</t>
	 </section>
	 <section anchor="updates-to-restarting-events" title="Updates numbered="true" toc="default">
	   <name>Updates to Restarting Events">

<t>Section 5.2.3.4.1 of <xref target="RFC7450"/> Events</name>
	   <t><xref target="RFC7450" sectionFormat="of" section="5.2.3.4.1"/> lists several events that may cause a
 gateway to start or restart the discovery procedure.</t>
	   <t>This document provides some updates and recommendations regarding the
 handling of these and similar events.  The first 5 five events are copied
 here and numbered for easier reference, and the remaining 4 four events are
 newly added for consideration in this document:</t>

<t><list style="numbers">
  <t>When
	   <ol spacing="normal" type="1">
	     <li>When a gateway pseudo-interface is started (enabled).</t>
  <t>When (enabled).</li>
	     <li>When the gateway wishes to report a group subscription when none
 currently exist.</t>
  <t>Before exists.</li>
	     <li>Before sending the next Request message in a membership update
cycle.</t>
  <t>After
 cycle.</li>
	     <li>After the gateway fails to receive a response to a Request
message.</t>
  <t>After
 message.</li>
	     <li>After the gateway receives a Membership Query message with the
 L flag set to 1.</t>
  <t>When 1.</li>
	     <li>When the gateway wishes to report an (S,G) subscription with a source
 address that does not currently have other group subscriptions.</t>
  <t>When subscriptions.</li>
            <li>When there is a network change detected, detected; for example example, when a gateway is
operating inside an end user device or application, application and the device
joins a different network, network or when the domain portion of a DNS-SD
domain name changes in response to a DHCP message or administrative
configuration.</t>
  <t>When
configuration.</li>
            <li>When substantial loss, persistent congestion, or network overload is
detected in the stream of AMT packets from a relay.</t>
  <t>When relay.</li>
            <li>When the gateway has reported one or more (S,G) subscriptions, subscriptions but
no traffic is received from the source for some timeout.  (See timeout (see
<xref target="trafficabsent"/>).</t>
</list></t> target="trafficabsent" format="default"/>).</li>
          </ol>
          <t>This list is not exhaustive, nor are any of the listed events strictly
required to always force a restart of the discovery process.</t>
          <t>Note that during event #1, a gateway may use DNS-SD, DNS-SD but does not
have sufficient information to use DRIAD, since no source is known.</t>
        </section>
        <section anchor="stability" title="Tunnel Stability"> numbered="true" toc="default">
          <name>Tunnel Stability</name>
          <t>In general, subscribers to active traffic flows that are being forwarded
by an AMT gateway are less likely to experience a degradation in service
(for example, from missing or duplicated packets) when the gateway continues
using the same relay, relay as long as the relay is not overloaded and the network
conditions remain stable.</t>
          <t>Therefore, gateways SHOULD <bcp14>SHOULD</bcp14> avoid performing a full restart of the discovery
process during routine cases of event #3 (sending a new Request message),
since it occurs frequently in normal operation.</t>
          <t>However, see Sections <xref target="flowhealth"/>, target="flowhealth" format="counter"/>, <xref target="discoverymessage"/>, target="discoverymessage" format="counter"/>, and <xref target="ancient"/> target="ancient" format="counter"/> for more
information about exceptional cases when it may be appropriate to use
event #3.</t>
        </section>
        <section anchor="flowhealth" title="Traffic Health"> numbered="true" toc="default">
          <name>Traffic Health</name>
          <section anchor="trafficabsent" title="Absence numbered="true" toc="default">
            <name>Absence of Traffic"> Traffic</name>
            <t>If a gateway indicates one or more (S,G) subscriptions in a Membership
Update message, message but no traffic for any of the (S,G)s is received in a
reasonable time, it’s it's appropriate for the gateway to restart the
discovery process.</t>
            <t>If the gateway restarts the discovery process multiple times consecutively
for this reason, the timeout period SHOULD <bcp14>SHOULD</bcp14> be adjusted to provide a random
exponential back-off.</t>
            <t>The RECOMMENDED <bcp14>RECOMMENDED</bcp14> timeout is a random value in the range
[initial_timeout, MIN(initial_timeout * 2^retry_count, maximum_timeout)],
with a RECOMMENDED <bcp14>RECOMMENDED</bcp14> initial_timeout of 4 seconds and a RECOMMENDED <bcp14>RECOMMENDED</bcp14>
maximum_timeout of 120 seconds (which is the recommended minimum NAT
mapping timeout described in <xref target="RFC4787"/>).</t> target="RFC4787" format="default"/>).</t>
            <t>Note that the recommended initial_timeout is larger than the initial
timout
timeout recommended in the similar algorithm from Section 5.2.3.4.3 of <xref target="RFC7450"/>. target="RFC7450" sectionFormat="of" section="5.2.3.4.3"/>.  This is to provide time for RPF Join propagation in the
sending network.  Although the timeout values may be administratively
adjusted to support performance requirements, operators are advised to
consider the possibility of join propagation delays between the sender
and the relay when choosing an appropriate timeout value.</t>
            <t>Gateways restarting the discovery process because of an absence of
traffic MUST <bcp14>MUST</bcp14> use a hold-down timer that removes this relay from
consideration during subsequent rounds of discovery while active.
The hold-down SHOULD <bcp14>SHOULD</bcp14> last for no less than 3 minutes and no more than
10 minutes.</t>
          </section>
          <section anchor="loss-and-congestion" title="Loss numbered="true" toc="default">
            <name>Loss and Congestion"> Congestion</name>
            <t>In some gateway deployments, it is also feasible to monitor the health of
traffic flows through the gateway, gateway -- for example example, by detecting the rate of
packet loss by communicating out of band with receivers, receivers or monitoring the
packets of known protocols with sequence numbers.  Where feasible,
it’s
it's encouraged for gateways to use such traffic health information to
trigger a restart of the discovery process during event #3 (before
sending a new Request message).</t>
            <t>However, to avoid synchronized rediscovery by many gateways simultaneously
after if a transient network event upstream of a that affects the tunneled
	    multicast stream -- as opposed to an event that affects the tunnel
	    connection between the relay results in
many receivers detecting and gateway -- occurs, poor flow health at the same time, it’s recommended
to add
	    detection could be triggered for many gateways simultaneously. In
	    this situation, adding a random delay before restarting the discovery process in this case.</t> to avoid synchronized
	    rediscovery by many gateways is recommended.</t>
            <t>The span of the random portion of the delay should be no less than 10
seconds by default, default but may be administratively configured
to support different performance requirements.</t>
          </section>
          <section anchor="ancient" title="Ancient numbered="true" toc="default">
            <name>Ancient Discovery Information"> Information</name>
            <t>In most cases, a gateway actively receiving healthy traffic from a relay
that has not indicated load with the L flag should prefer to remain
connected to the same relay, as described in <xref target="stability"/>.</t> target="stability" format="default"/>.</t>
            <t>However, a relay that appears healthy but has been forwarding traffic for
days or weeks may have an increased chance of becoming unstable.  Gateways
may benefit from restarting the discovery process during event #3 (before
sending a Request message) after the expiration of a long-term timeout, timeout on
the order of multiple hours, hours or even days in some deployments.</t>
            <t>It may be beneficial for such timers to consider the amount of traffic
currently being forwarded, forwarded and to give a higher probability of restarting
discovery during periods with an unusually low data rate, rate to reduce the
impact on active traffic while still avoiding relying on the results of a
very old discovery.</t>
            <t>Other issues may also be worth considering as part of this heuristic; for
example, if the DNS expiry time of the record that was used to discover
the current relay has not passed, the long term long-term timer might be restarted
without restarting the discovery process.</t>
          </section>
        </section>
        <section anchor="loaded" title="Relay numbered="true" toc="default">
          <name>Relay Loaded or Shutting Down"> Down</name>
          <t>The L flag (see Section 5.1.4.4 of <xref target="RFC7450"/>) target="RFC7450" sectionFormat="of" section="5.1.4.4"/>) is the preferred mechanism for
a relay to signal overloading or a graceful shutdown to gateways.</t>
          <t>A gateway that supports handling of the L flag should generally restart the
discovery process when it processes a Membership Query packet with the
L flag set.  If an L flag is received while a concurrent Happy Eyeballs
discovery process is under way underway for multiple candidate relays (<xref target="happy"/>), target="happy" format="default"/>),
the relay sending the L flag SHOULD NOT <bcp14>SHOULD NOT</bcp14> be considered for the relay selection.</t>
          <t>It is also RECOMMENDED <bcp14>RECOMMENDED</bcp14> that gateways avoid choosing a relay
that has recently sent an L flag, with approximately a 10-minute hold-down.
Gateways SHOULD <bcp14>SHOULD</bcp14> treat this hold-down timer in the same way as the hold-down
in <xref target="trafficabsent"/>, target="trafficabsent" format="default"/> so that the relay is removed from consideration
for short-term subsequent short-term rounds of discovery.</t>
        </section>
        <section anchor="discoverymessage" title="Relay numbered="true" toc="default">
          <name>Relay Discovery Messages vs. Restarting Discovery"> Discovery</name>
          <t>All AMT relays are required by <xref target="RFC7450"/> target="RFC7450"
	  format="default"/> to support handling of
Relay Discovery messages (e.g. (e.g., in Section 5.3.3.2 of <xref target="RFC7450"/>).</t> target="RFC7450" sectionFormat="of" section="5.3.3.2"/>).</t>
          <t>So a gateway with an existing connection to a relay can send a Relay
Discovery message to the unicast address of that AMT relay.  Under stable
conditions with an unloaded relay, it’s it's expected that the relay will
return its own unicast address in the Relay Advertisement, Advertisement in response
to such a Relay Discovery message.  Since this will not result in the
gateway changing to another relay unless the relay directs the gateway
away, this is a reasonable exception to the advice against handling event #3
described in <xref target="stability"/>.</t> target="stability" format="default"/>.</t>
          <t>This behavior is discouraged for gateways that do support the L flag, flag to
avoid sending unnecessary packets over the network.</t>
          <t>However, gateways that do not support the L flag may be able to avoid a
disruption in the forwarded traffic by sending such Relay Discovery
messages regularly.  When a relay is under load or has started a graceful
shutdown, it may respond with a different relay address, which the gateway
can use to connect to a different relay.  This kind of coordinated handoff
will likely result in a smaller disruption to the traffic than if the relay
simply stops responding to Request messages, messages and stops forwarding traffic.</t>
          <t>This style of Relay Discovery message (one sent to the unicast address
of a relay that’s that's already forwarding traffic to this gateway) SHOULD NOT <bcp14>SHOULD NOT</bcp14> be
considered a full restart of the relay discovery process.  It is RECOMMENDED
for <bcp14>RECOMMENDED</bcp14>
that gateways to support the L flag, but for gateways that do not support the
L flag, sending this message during event #3 may help mitigate service
degradation when relays become unstable.</t>
        </section>
        <section anchor="independent-discovery-per-traffic-source" title="Independent numbered="true" toc="default">
          <name>Independent Discovery Per per Traffic Source"> Source</name>
          <t>Relays discovered via the AMTRELAY RR are source-specific relay addresses, addresses and
may use different pseudo-interfaces from each other and from relays
discovered via DNS-SD or via a non-source-specific address, as described in
Section 4.1.2.1 of
<xref target="RFC7450"/>.</t> target="RFC7450" sectionFormat="of" section="4.1.2.1"/>.</t>
          <t>Restarting the discovery process for one pseudo-interface does not require
restarting the discovery process for other pseudo-interfaces.  Gateway
heuristics about restarting the discovery process should operate
independently for different tunnels to relays, relays when responding to events
that are specific to the different tunnels.</t>
        </section>
      </section>
      <section anchor="dns-configuration" title="DNS Configuration">

<t>Often numbered="true" toc="default">
        <name>DNS Configuration</name>
        <t>Often, an AMT gateway will only have access to the source and group IP addresses
of the desired traffic, traffic and will not know any other name for the source of the
traffic.  Because of this, typically typically, the best way of looking up AMTRELAY RRs
will be by using the source IP address as an index into one of the reverse
mapping trees (in-addr.arpa for IPv4, as described in Section 3.5 of <xref target="RFC1035"/>, target="RFC1035" sectionFormat="of" section="3.5"/>, or ip6.arpa for IPv6, as described in Section 2.5 of <xref target="RFC3596"/>).</t> target="RFC3596" sectionFormat="of" section="2.5"/>).</t>
        <t>Therefore, it is RECOMMENDED <bcp14>RECOMMENDED</bcp14> that AMTRELAY RRs be added to reverse IP
zones as appropriate.  AMTRELAY records MAY <bcp14>MAY</bcp14> also appear in other zones,
since this may be necessary to perform delegation from the reverse zones
(see
(see, for example Section 5.2 of example, <xref target="RFC2317"/>), target="RFC2317" sectionFormat="of" section="5.2"/>), but the use case enabled
by this document requires a reverse IP mapping for the source from an
(S,G) in order to be useful to most AMT gateways.  This document does
not define semantics for the use of AMTRELAY records obtained in a way
other than by a reverse IP lookup.</t>
        <t>When performing the AMTRELAY RR lookup, any CNAMEs or DNAMEs found MUST <bcp14>MUST</bcp14> be
followed.  This is necessary to support zone delegation.  Some examples
outlining this need are described in <xref target="RFC2317"/>.</t> target="RFC2317" format="default"/>.</t>
        <t>See Sections <xref target="rrdef"/> target="rrdef" format="counter"/> and <xref target="rpformat"/> target="rpformat" format="counter"/> for a detailed explanation of the contents
for
of a DNS Zone zone file.</t>
      </section>
      <section anchor="waiting-for-dns-resolution" title="Waiting numbered="true" toc="default">
        <name>Waiting for DNS resolution">

<t>The DNS Resolution</name>
        <t>DNS query functionality is expected to follow ordinary standards and best
practices for DNS clients.  A gateway MAY <bcp14>MAY</bcp14> use an existing DNS client
implementation that does so, so and MAY <bcp14>MAY</bcp14> rely on that client’s client's retry logic
to determine the timeouts between retries.</t>
        <t>Otherwise, a gateway MAY re-send <bcp14>MAY</bcp14> resend a DNS query if it does not receive an
appropriate DNS response within some timeout period.  If the gateway retries
multiple times, the timeout period SHOULD <bcp14>SHOULD</bcp14> be adjusted to provide a random
exponential back-off.</t>
        <t>As with the waiting process for the Relay Advertisement message from
Section 5.2.3.4.3 of
<xref target="RFC7450"/>, target="RFC7450" sectionFormat="of" section="5.2.3.4.3"/>, the RECOMMENDED <bcp14>RECOMMENDED</bcp14> timeout is a random value
in the range [initial_timeout, MIN(initial_timeout * 2^retry_count,
maximum_timeout)], with a RECOMMENDED <bcp14>RECOMMENDED</bcp14> initial_timeout of 1 second and
a RECOMMENDED <bcp14>RECOMMENDED</bcp14> maximum_timeout of 120 seconds.</t>
      </section>
    </section>
    <section anchor="rrdef" title="AMTRELAY numbered="true" toc="default">
      <name>AMTRELAY Resource Record Definition"> Definition</name>
      <section anchor="amtrelay-rrtype" title="AMTRELAY RRType"> numbered="true" toc="default">
        <name>AMTRELAY RRType</name>
        <t>The AMTRELAY RRType has the mnemonic AMTRELAY and type code 260 (decimal).</t>
        <t>The AMTRELAY RR is class independent.</t>
      </section>
      <section anchor="amtrelay-rdata-format" title="AMTRELAY numbered="true" toc="default">
        <name>AMTRELAY RData Format"> Format</name>
        <t>The AMTRELAY RData consists of a an 8-bit precedence field, a 1-bit
“Discovery Optional”
"Discovery Optional" field, a 7-bit type field, and a variable
length relay field.</t>

<figure><artwork><![CDATA[
        <artwork name="" type="" align="left" alt=""><![CDATA[
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   precedence  |D|    type     |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
~                            relay                              ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork></figure>
]]></artwork>
        <section anchor="rrdef-precedence" title="RData numbered="true" toc="default">
          <name>RData Format - Precedence"> Precedence</name>
          <t>This is an 8-bit precedence for this record.  It is interpreted in
the same way as the PREFERENCE field described in Section 3.3.9 of <xref target="RFC1035"/>.</t> target="RFC1035" sectionFormat="of" section="3.3.9"/>.</t>
          <t>Relays listed in AMTRELAY records with a lower value for precedence are to be
attempted first.</t>
        </section>
        <section anchor="rrdef-dbit" title="RData numbered="true" toc="default">
          <name>RData Format - Discovery Optional (D-bit)"> (D-bit)</name>
          <t>The D bit D-bit is a “Discovery Optional” "Discovery Optional" flag.</t>
          <t>If the D bit D-bit is set to 0, a gateway using this RR MUST <bcp14>MUST</bcp14> perform AMT relay
discovery as described in Section 4.2.1.1 of <xref target="RFC7450"/>, target="RFC7450" sectionFormat="of" section="4.2.1.1"/> rather than directly
sending an AMT Request message to the relay.</t>
          <t>That is, the gateway MUST <bcp14>MUST</bcp14> receive an AMT Relay Advertisement message (Section
5.1.2 of <xref target="RFC7450"/>) (<xref target="RFC7450" sectionFormat="of" section="5.1.2"/>) for an address before sending an AMT Request message
(Section 5.1.3 of <xref target="RFC7450"/>)
(<xref target="RFC7450" sectionFormat="of" section="5.1.3"/>) to that address. Before receiving the Relay
Advertisement message, this record has only indicated that the address can be
used for AMT relay discovery, not for a Request message.  This is necessary for
devices that are not fully functional AMT relays, relays but rather load balancers or
brokers, as mentioned in Section 4.2.1.1 of <xref target="RFC7450"/>.</t> target="RFC7450" sectionFormat="of" section="4.2.1.1"/>.</t>
          <t>If the D bit D-bit is set to 1, the gateway MAY <bcp14>MAY</bcp14> send an AMT Request message directly
to the discovered relay address without first sending an AMT Discovery message.</t>
          <t>This bit should be set according to advice from the AMT relay operator. The
D bit MUST
D-bit <bcp14>MUST</bcp14> be set to zero when no information is available from the AMT relay
operator about its suitability.</t>
        </section>
        <section anchor="rtype" title="RData numbered="true" toc="default">
          <name>RData Format - Type"> Type</name>
          <t>The type field indicates the format of the information that
is stored in the relay field.</t>
          <t>The following values are defined:</t>

<t><list style="symbols">
  <t>type
          <ul spacing="normal">
            <li>type = 0:
The relay field is empty (0 bytes).</t>
  <t>type bytes).</li>
            <li>type = 1:
The relay field contains a 4-octet IPv4 address.</t>
  <t>type address.</li>
            <li>type = 2:
The relay field contains a 16-octet IPv6 address.</t>
  <t>type address.</li>
            <li>type = 3:
The relay field contains a wire-encoded domain name. The wire-encoded
format is self-describing, so the length is implicit. The domain name
MUST NOT
<bcp14>MUST NOT</bcp14> be compressed.  (See Section 3.3 of compressed (see <xref target="RFC1035"/> target="RFC1035" sectionFormat="of"
section="3.3"/> and Section 4 of <xref target="RFC3597"/>.)</t>
</list></t> target="RFC3597" sectionFormat="of" section="4"/>).</li>
          </ul>
          <t>RRs with an undefined value in the Type field SHOULD NOT <bcp14>SHOULD NOT</bcp14> be considered
by receiving gateways for AMT relay discovery.</t>
        </section>
        <section anchor="rdformat" title="RData numbered="true" toc="default">
          <name>RData Format - Relay"> Relay</name>
          <t>The relay field is the address or domain name of the AMT relay. It is
formatted according to the type field.</t>
          <t>When the type field is 0, the length of the relay field is 0, and it
indicates that no AMT relay should be used for multicast traffic from this
source.</t>
          <t>When the type field is 1, the length of the relay field is 4 octets, and a
32-bit IPv4 address is present. This is an IPv4 address as described in
Section 3.4.1 of
<xref target="RFC1035"/>. target="RFC1035" sectionFormat="of" section="3.4.1"/>. This is a 32-bit number in network byte
order.</t>
          <t>When the type field is 2, the length of the relay field is 16 octets, and
a 128-bit IPv6 address is present. This is an IPv6 address as described in
Section 2.2 of
<xref target="RFC3596"/>. target="RFC3596" sectionFormat="of" section="2.2"/>. This is a 128-bit number in network byte order.</t>
          <t>When the type field is 3, the relay field is a normal wire-encoded domain
name, as described in Section 3.3 of <xref target="RFC1035"/>. Compression MUST NOT be
used, for target="RFC1035" sectionFormat="of"
section="3.3"/>. For the reasons given in Section 4 of <xref target="RFC3597"/>.</t> target="RFC3597"
sectionFormat="of" section="4"/>, compression <bcp14>MUST NOT</bcp14> be
used.</t>
          <t>For a type 3 record, the D-bit and preference fields carry over to all
A or AAAA records for the domain name.  There is no difference in the
result of the discovery process when it’s it's obtained by type 1 or type 2
AMTRELAY records with identical D-bit and preference fields, fields vs. when
the result is obtained by a type 3 AMTRELAY record that resolves
to the same set of IPv4 and IPv6 addresses via A and AAAA lookups.</t>
        </section>
      </section>
      <section anchor="rpformat" title="AMTRELAY numbered="true" toc="default">
        <name>AMTRELAY Record Presentation Format"> Format</name>
        <section anchor="representation-of-amtrelay-rrs" title="Representation numbered="true" toc="default">
          <name>Representation of AMTRELAY RRs"> RRs</name>
          <t>AMTRELAY RRs may appear in a zone data master file. The precedence, D-bit,
relay type, and relay fields are REQUIRED.</t> <bcp14>REQUIRED</bcp14>.</t>
          <t>If the relay type field is 0, the relay field MUST <bcp14>MUST</bcp14> be “.”.</t> ".".</t>
          <t>The presentation for the record is as follows:</t>

<figure><artwork><![CDATA[
          <sourcecode name="" type=""><![CDATA[
  IN AMTRELAY precedence D-bit type relay
]]></artwork></figure>
]]></sourcecode>
        </section>
        <section anchor="examples" title="Examples"> numbered="true" toc="default">
          <name>Examples</name>
          <t>In a DNS authoritative nameserver that understands the AMTRELAY type,
the zone might contain a set of entries like this:</t>

<figure><artwork><![CDATA[
          <sourcecode name="" type=""><![CDATA[
    $ORIGIN 100.51.198.in-addr.arpa.
    12     IN AMTRELAY  10 0 1 203.0.113.15
    12     IN AMTRELAY  10 0 2 2001:db8::15
    12     IN AMTRELAY 128 1 3 amtrelays.example.com.
]]></artwork></figure>
]]></sourcecode>
          <t>This configuration advertises an IPv4 discovery address, an IPv6
discovery address, and a domain name for AMT relays which that can receive
traffic from the source 198.51.100.12.  The IPv4 and IPv6 addresses
are configured with a D-bit of 0 (meaning discovery is mandatory, as
described in <xref target="rrdef-dbit"/>), target="rrdef-dbit" format="default"/>) and a precedence 10 (meaning they’re they're
preferred ahead of the last entry, which has precedence 128).</t>
          <t>For zone files in name servers that don’t don't support the AMTRELAY RRType
natively, it’s it's possible to use the format for unknown RR types, as
described in <xref target="RFC3597"/>. target="RFC3597" format="default"/>.  This approach would replace the AMTRELAY
entries in the example above with the entries below:</t>

<figure><artwork><![CDATA[
          <sourcecode name="" type=""><![CDATA[
    10   IN TYPE260  \# (
           6  ; length
           0a ; precedence=10
           01 ; D=0, relay type=1, an IPv4 address
           cb00710f ) ; 203.0.113.15
    10   IN TYPE260  \# (
           18 ; length
           0a ; precedence=10
           02 ; D=0, relay type=2, an IPv6 address
           20010db800000000000000000000000f ) ; 2001:db8::15
    10   IN TYPE260  \# (
           24 ; length
           80 ; precedence=128
           83 ; D=1, relay type=3, a wire-encoded domain name
         09616d7472656c617973076578616d706c6503636f6d ) ; domain name
]]></artwork></figure>
]]></sourcecode>
          <t>See <xref target="extranslate"/> target="extranslate" format="default"/> for more details.</t>
        </section>
      </section>
    </section>
    <section anchor="iana" title="IANA Considerations"> numbered="true" toc="default">
      <name>IANA Considerations</name>
      <t>This document updates the IANA Registry for DNS Resource "Resource Record Types (RR) TYPEs" registry
by assigning type 260 to the AMTRELAY record.</t>
      <t>This document creates a new registry named “AMTRELAY "AMTRELAY Resource Record
Parameters”,
Parameters" with a sub-registry subregistry for the “Relay "Relay Type Field”. Field".  The initial
values in the sub-registry subregistry are:</t>

<figure><artwork><![CDATA[
         +-------+---------------------------------------+
         | Value | Description                           |
         +-------+---------------------------------------+
         |   0   | No relay is present.                  |
         |   1   | A
<table align="left">
         <name>Initial Contents of the &quot;Relay Type Field&quot; Registry</name>
  <thead>
          <tr>
            <th align="left">Value</th>
            <th align="left">Description</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td align="left">0</td>
            <td align="left">No relay is present</td>
          </tr>
          <tr>
            <td align="left">1</td>
            <td align="left">A 4-byte IPv4 address is present      |
         |   2   | A present</td>
          </tr>
<tr>
            <td align="left">2</td>
            <td align="left">A 16-byte IPv6 address is present     |
         |   3   | A present</td>
</tr>
<tr>
            <td align="left">3</td>
            <td align="left">A wire-encoded domain name is present |
         | 4-255 | Unassigned                            |
         +-------+---------------------------------------+
]]></artwork></figure> present</td>
</tr>
<tr>
            <td align="left">4-255</td>
            <td align="left">Unassigned</td>
</tr>
        </tbody>
</table>
      <t>Values 0, 1, 2, and 3 are further explained in Sections <xref target="rtype"/> target="rtype" format="counter"/> and <xref target="rdformat"/>. target="rdformat" format="counter"/>.
Relay type numbers 4 through 255 can be assigned with a policy of
Specification Required (as described in <xref target="RFC8126"/>).</t> target="RFC8126" format="default"/>).</t>
    </section>
    <section anchor="security-considerations" title="Security Considerations"> numbered="true" toc="default">
      <name>Security Considerations</name>
      <section anchor="use-of-amt" title="Use numbered="true" toc="default">
        <name>Use of AMT"> AMT</name>
        <t>This document defines a mechanism that enables a more widespread and
automated use of AMT, even without access to a multicast backbone.
Operators of networks and applications that include a DRIAD-capable
AMT gateway are advised to carefully consider the security considerations
in Section 6 of <xref target="RFC7450"/>.</t> target="RFC7450" sectionFormat="of" section="6"/>.</t>
        <t>AMT gateway operators also are encouraged to take appropriate steps to
ensure the integrity of the data received via AMT, for example example, by the
opportunistic use of IPSec IPsec <xref target="RFC4301"/> target="RFC4301" format="default"/> to secure traffic received from AMT
relays,
relays when IPSECKEY records <xref target="RFC4025"/> target="RFC4025" format="default"/> are available or when a trust
relationship with the AMT relays can be otherwise established and secured.</t>
        <t>Note that AMT does not itself provide any integrity protection on for
Multicast Data packets (Section 5.1.6 of <xref target="RFC7450"/>), (<xref target="RFC7450" sectionFormat="of" section="5.1.6"/>), so absent
protections like those mentioned above, even an off-path attacker who
discovers the gateway IP, the relay IP, and the relay source port for
an active AMT connection can inject multicast data packets for a
joined (S,G) into the data stream if he can get data packets delivered
to the gateway IP that spoof the relay as the source.</t>
      </section>
      <section anchor="record-spoofing" title="Record-spoofing"> numbered="true" toc="default">
        <name>Record-Spoofing</name>
        <t>The AMTRELAY resource record contains information that SHOULD <bcp14>SHOULD</bcp14> be
communicated to the DNS client without being modified.  The
method used to ensure the result was unmodified is up to the client.</t>
        <t>There must be a trust relationship between the end consumer of this
resource record and the DNS server.  This relationship may be end-to-end
DNSSEC validation, validation or a secure connection to a trusted DNS server that
provides end-to-end safety, safety to prevent record-spoofing of the response
from the trusted server.  The connection to the trusted server can use
any secure channel, such as with a TSIG <xref target="RFC2845"/> target="RFC2845" format="default"/> or SIG(0) <xref target="RFC2931"/> target="RFC2931" format="default"/>
channel, a secure local channel on the host, DNS over TLS <xref target="RFC7858"/>, target="RFC7858" format="default"/>,
DNS over HTTPS <xref target="RFC8484"/>, target="RFC8484" format="default"/>, or some other mechanism that provides
authentication of the RR.</t>
        <t>If an AMT gateway accepts a maliciously crafted AMTRELAY record,
the result could be a Denial of Service, Service or receivers processing
multicast traffic from a source under the attacker’s attacker's control.</t>
      </section>
      <section anchor="congestion" title="Congestion"> numbered="true" toc="default">
        <name>Congestion</name>
        <t>Multicast traffic, particularly interdomain multicast traffic, carries
some congestion risks, as described in Section 4 of <xref target="RFC8085"/>.</t> target="RFC8085" sectionFormat="of" section="4"/>.</t>
        <t>Application implementors and network operators that use AMT gateways
are advised to take precautions precautions, including monitoring of application
traffic behavior, traffic authentication at ingest, rate-limiting of
multicast traffic, and the use of circuit-breaker techniques such as
those described in Section 3.1.10 of <xref target="RFC8085"/> target="RFC8085" sectionFormat="of" section="3.1.10"/> and similar
protections at the network level, level in order to ensure network health
in the event of misconfiguration, poorly written applications that
don’t
don't follow UDP congestion control principles, or a deliberate attack.</t>

<t>Section 4.1.4.2 of <xref target="RFC7450"/>
        <t><xref target="RFC7450" sectionFormat="of" section="4.1.4.2"/> and Section 6.1 of <xref target="RFC7450"/> target="RFC7450" sectionFormat="of" section="6.1"/>
provide some further considerations and advice about mitigating
congestion risk.</t>
      </section>
    </section>
  </middle>
  <back>
    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.1034.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.1035.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2181.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2782.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3376.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3596.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3597.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3810.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4604.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4607.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6724.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6763.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7450.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8085.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8305.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8499.xml"/>
      </references>
      <references>
        <name>Informative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2317.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2845.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2931.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3550.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4025.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4301.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4787.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5110.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6726.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7359.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7761.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7858.xml"/>
 <xi:include
     href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8126.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8313.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8484.xml"/>
      </references>
    </references>
    <section anchor="acknowledgements" title="Acknowledgements">

<t>This specification was inspired by anchor="extranslate" numbered="true" toc="default">
      <name>Unknown RRType Construction</name>
      <t>In a DNS resolver that understands the previous work of Doug Nortz,
Robert Sayko, David Segelstein, and Percy Tarapore, presented in
the MBONED working group at IETF 93.</t>

<t>Thanks to Jeff Goldsmith, Toerless Eckert, Mikael Abrahamsson, Lenny
Giuliano, Mark Andrews, Sandy Zheng, Kyle Rose, Ben Kaduk, Bill
Atwood, Tim Chown, Christian Worm Mortensen, Warren Kumari, Dan
Romanescu, Bernard Aboba, Carlos Pignataro, Niclas Comstedt,
Mirja Kühlewind, Henning Rogge, Éric Vyncke, Barry Lieba,
Roman Danyliw, Alissa Cooper, Suresh Krishnan, Adam Roach,
and Daniel Franke for their very helpful reviews and comments.</t>

</section>

  </middle>

  <back>

    <references title='Normative References'>

<reference  anchor="RFC1034" target='https://www.rfc-editor.org/info/rfc1034'>
<front>
<title>Domain names - concepts and facilities</title>
<author initials='P.V.' surname='Mockapetris' fullname='P.V. Mockapetris'><organization /></author>
<date year='1987' month='November' />
<abstract><t>This RFC is the revised basic definition of The Domain Name System.  It obsoletes RFC-882.  This memo describes the domain style names and their used for host address look up and electronic mail forwarding.  It discusses the clients and servers in the domain name system and the protocol used between them.</t></abstract>
</front>
<seriesInfo name='STD' value='13'/>
<seriesInfo name='RFC' value='1034'/>
<seriesInfo name='DOI' value='10.17487/RFC1034'/>
</reference>

<reference  anchor="RFC1035" target='https://www.rfc-editor.org/info/rfc1035'>
<front>
<title>Domain names - implementation and specification</title>
<author initials='P.V.' surname='Mockapetris' fullname='P.V. Mockapetris'><organization /></author>
<date year='1987' month='November' />
<abstract><t>This RFC is the revised specification of the protocol and format used in the implementation of the Domain Name System.  It obsoletes RFC-883. This memo documents the details of the domain name client - server communication.</t></abstract>
</front>
<seriesInfo name='STD' value='13'/>
<seriesInfo name='RFC' value='1035'/>
<seriesInfo name='DOI' value='10.17487/RFC1035'/>
</reference>

<reference  anchor="RFC2119" target='https://www.rfc-editor.org/info/rfc2119'>
<front>
<title>Key words for use in RFCs to Indicate Requirement Levels</title>
<author initials='S.' surname='Bradner' fullname='S. Bradner'><organization /></author>
<date year='1997' month='March' />
<abstract><t>In many standards track documents several words are used to signify the requirements in the specification.  These words are often capitalized. This document defines these words as they should be interpreted in IETF documents.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t></abstract>
</front>
<seriesInfo name='BCP' value='14'/>
<seriesInfo name='RFC' value='2119'/>
<seriesInfo name='DOI' value='10.17487/RFC2119'/>
</reference>

<reference  anchor="RFC2181" target='https://www.rfc-editor.org/info/rfc2181'>
<front>
<title>Clarifications to the DNS Specification</title>
<author initials='R.' surname='Elz' fullname='R. Elz'><organization /></author>
<author initials='R.' surname='Bush' fullname='R. Bush'><organization /></author>
<date year='1997' month='July' />
<abstract><t>This document considers some areas that have been identified as problems with the specification of the Domain Name System, and proposes remedies for the defects identified. [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='2181'/>
<seriesInfo name='DOI' value='10.17487/RFC2181'/>
</reference>

<reference  anchor="RFC2782" target='https://www.rfc-editor.org/info/rfc2782'>
<front>
<title>A DNS RR for specifying the location of services (DNS SRV)</title>
<author initials='A.' surname='Gulbrandsen' fullname='A. Gulbrandsen'><organization /></author>
<author initials='P.' surname='Vixie' fullname='P. Vixie'><organization /></author>
<author initials='L.' surname='Esibov' fullname='L. Esibov'><organization /></author>
<date year='2000' month='February' />
<abstract><t>This document describes a DNS RR which specifies the location of the server(s) for a specific protocol and domain.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='2782'/>
<seriesInfo name='DOI' value='10.17487/RFC2782'/>
</reference>

<reference  anchor="RFC3376" target='https://www.rfc-editor.org/info/rfc3376'>
<front>
<title>Internet Group Management Protocol, Version 3</title>
<author initials='B.' surname='Cain' fullname='B. Cain'><organization /></author>
<author initials='S.' surname='Deering' fullname='S. Deering'><organization /></author>
<author initials='I.' surname='Kouvelas' fullname='I. Kouvelas'><organization /></author>
<author initials='B.' surname='Fenner' fullname='B. Fenner'><organization /></author>
<author initials='A.' surname='Thyagarajan' fullname='A. Thyagarajan'><organization /></author>
<date year='2002' month='October' />
</front>
<seriesInfo name='RFC' value='3376'/>
<seriesInfo name='DOI' value='10.17487/RFC3376'/>
</reference>

<reference  anchor="RFC3596" target='https://www.rfc-editor.org/info/rfc3596'>
<front>
<title>DNS Extensions to Support IP Version 6</title>
<author initials='S.' surname='Thomson' fullname='S. Thomson'><organization /></author>
<author initials='C.' surname='Huitema' fullname='C. Huitema'><organization /></author>
<author initials='V.' surname='Ksinant' fullname='V. Ksinant'><organization /></author>
<author initials='M.' surname='Souissi' fullname='M. Souissi'><organization /></author>
<date year='2003' month='October' />
<abstract><t>This document defines the changes that need to be made to the Domain Name System (DNS) to support hosts running IP version 6 (IPv6).  The changes include a resource record type to store an IPv6 address, a domain to support lookups based on an IPv6 address, and updated definitions of existing query types that return Internet addresses as part of additional section processing.  The extensions are designed to be compatible with existing applications and, in particular, DNS implementations themselves.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='STD' value='88'/>
<seriesInfo name='RFC' value='3596'/>
<seriesInfo name='DOI' value='10.17487/RFC3596'/>
</reference>

<reference  anchor="RFC3597" target='https://www.rfc-editor.org/info/rfc3597'>
<front>
<title>Handling of Unknown DNS Resource Record (RR) Types</title>
<author initials='A.' surname='Gustafsson' fullname='A. Gustafsson'><organization /></author>
<date year='2003' month='September' />
<abstract><t>Extending the Domain Name System (DNS) with new Resource Record (RR) types currently requires changes to name server software.  This document specifies the changes necessary to allow future DNS implementations to handle new RR types transparently.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='3597'/>
<seriesInfo name='DOI' value='10.17487/RFC3597'/>
</reference>

<reference  anchor="RFC3810" target='https://www.rfc-editor.org/info/rfc3810'>
<front>
<title>Multicast Listener Discovery Version 2 (MLDv2) for IPv6</title>
<author initials='R.' surname='Vida' fullname='R. Vida' role='editor'><organization /></author>
<author initials='L.' surname='Costa' fullname='L. Costa' role='editor'><organization /></author>
<date year='2004' month='June' />
<abstract><t>This document updates RFC 2710, and it specifies Version 2 of the ulticast Listener Discovery Protocol (MLDv2).  MLD is used by an IPv6 router to discover the presence of multicast listeners on directly attached links, and to discover which multicast addresses are of interest to those neighboring nodes.  MLDv2 is designed to be interoperable with MLDv1.  MLDv2 adds the ability for a node to report interest in listening to packets with a particular multicast address only from specific source addresses or from all sources except for specific source addresses.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='3810'/>
<seriesInfo name='DOI' value='10.17487/RFC3810'/>
</reference>

<reference  anchor="RFC4604" target='https://www.rfc-editor.org/info/rfc4604'>
<front>
<title>Using Internet Group Management Protocol Version 3 (IGMPv3) and Multicast Listener Discovery Protocol Version 2 (MLDv2) for Source-Specific Multicast</title>
<author initials='H.' surname='Holbrook' fullname='H. Holbrook'><organization /></author>
<author initials='B.' surname='Cain' fullname='B. Cain'><organization /></author>
<author initials='B.' surname='Haberman' fullname='B. Haberman'><organization /></author>
<date year='2006' month='August' />
<abstract><t>The Internet Group Management Protocol Version 3 (IGMPv3) and the Multicast Listener Discovery Protocol Version 2 (MLDv2) are protocols that allow a host to inform its neighboring routers of its desire to receive IPv4 and IPv6 multicast transmissions, respectively. Source-specific multicast (SSM) is a form of multicast in which a receiver is required to specify both the network-layer address of the source and the multicast destination address in order to receive the multicast transmission.  This document defines the notion of an &quot;SSM-aware&quot; router and host, and clarifies and (in some cases) modifies the behavior of IGMPv3 and MLDv2 on SSM-aware routers and hosts to accommodate source-specific multicast.  This document updates the IGMPv3 and MLDv2 specifications.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='4604'/>
<seriesInfo name='DOI' value='10.17487/RFC4604'/>
</reference>

<reference  anchor="RFC4607" target='https://www.rfc-editor.org/info/rfc4607'>
<front>
<title>Source-Specific Multicast for IP</title>
<author initials='H.' surname='Holbrook' fullname='H. Holbrook'><organization /></author>
<author initials='B.' surname='Cain' fullname='B. Cain'><organization /></author>
<date year='2006' month='August' />
<abstract><t>IP version 4 (IPv4) addresses in the 232/8 (232.0.0.0 to 232.255.255.255) range are designated as source-specific multicast (SSM) destination addresses and are reserved for use by source-specific applications and protocols.  For IP version 6 (IPv6), the address prefix FF3x::/32 is reserved for source-specific multicast use.  This document defines an extension to the Internet network service that applies to datagrams sent to SSM addresses and defines the host and router requirements to support this extension.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='4607'/>
<seriesInfo name='DOI' value='10.17487/RFC4607'/>
</reference>

<reference  anchor="RFC6724" target='https://www.rfc-editor.org/info/rfc6724'>
<front>
<title>Default Address Selection for Internet Protocol Version 6 (IPv6)</title>
<author initials='D.' surname='Thaler' fullname='D. Thaler' role='editor'><organization /></author>
<author initials='R.' surname='Draves' fullname='R. Draves'><organization /></author>
<author initials='A.' surname='Matsumoto' fullname='A. Matsumoto'><organization /></author>
<author initials='T.' surname='Chown' fullname='T. Chown'><organization /></author>
<date year='2012' month='September' />
<abstract><t>This document describes two algorithms, one for source address selection and one for destination address selection.  The algorithms specify default behavior for all Internet Protocol version 6 (IPv6) implementations.  They do not override choices made by applications or upper-layer protocols, nor do they preclude the development of more advanced mechanisms for address selection.  The two algorithms share a common context, including an optional mechanism for allowing administrators to provide policy that can override the default behavior.  In dual-stack implementations, the destination address selection algorithm can consider both IPv4 and IPv6 addresses -- depending on the available source addresses, the algorithm might prefer IPv6 addresses over IPv4 addresses, or vice versa.</t><t>Default address selection as defined in this specification applies to all IPv6 nodes, including both hosts and routers.  This document obsoletes RFC 3484.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='6724'/>
<seriesInfo name='DOI' value='10.17487/RFC6724'/>
</reference>

<reference  anchor="RFC6763" target='https://www.rfc-editor.org/info/rfc6763'>
<front>
<title>DNS-Based Service Discovery</title>
<author initials='S.' surname='Cheshire' fullname='S. Cheshire'><organization /></author>
<author initials='M.' surname='Krochmal' fullname='M. Krochmal'><organization /></author>
<date year='2013' month='February' />
<abstract><t>This document specifies how DNS resource records are named and structured to facilitate service discovery.  Given a type of service that a client is looking for, and a domain in which the client is looking for that service, this mechanism allows clients to discover a list of named instances of that desired service, using standard DNS queries. This mechanism is referred to as DNS-based Service Discovery, or DNS-SD.</t></abstract>
</front>
<seriesInfo name='RFC' value='6763'/>
<seriesInfo name='DOI' value='10.17487/RFC6763'/>
</reference>

<reference  anchor="RFC7450" target='https://www.rfc-editor.org/info/rfc7450'>
<front>
<title>Automatic Multicast Tunneling</title>
<author initials='G.' surname='Bumgardner' fullname='G. Bumgardner'><organization /></author>
<date year='2015' month='February' />
<abstract><t>This document describes Automatic Multicast Tunneling (AMT), a protocol for delivering multicast traffic from sources in a multicast-enabled network to receivers that lack multicast connectivity to the source network.  The protocol uses UDP encapsulation and unicast replication to provide this functionality.</t><t>The AMT protocol is specifically designed to support rapid deployment by requiring minimal changes to existing network infrastructure.</t></abstract>
</front>
<seriesInfo name='RFC' value='7450'/>
<seriesInfo name='DOI' value='10.17487/RFC7450'/>
</reference>

<reference  anchor="RFC8085" target='https://www.rfc-editor.org/info/rfc8085'>
<front>
<title>UDP Usage Guidelines</title>
<author initials='L.' surname='Eggert' fullname='L. Eggert'><organization /></author>
<author initials='G.' surname='Fairhurst' fullname='G. Fairhurst'><organization /></author>
<author initials='G.' surname='Shepherd' fullname='G. Shepherd'><organization /></author>
<date year='2017' month='March' />
<abstract><t>The User Datagram Protocol (UDP) provides a minimal message-passing transport that has no inherent congestion control mechanisms.  This document provides guidelines on the use of UDP for the designers of applications, tunnels, and other protocols that use UDP.  Congestion control guidelines are a primary focus, but the document also provides guidance on other topics, including message sizes, reliability, checksums, middlebox traversal, the use of Explicit Congestion Notification (ECN), Differentiated Services Code Points (DSCPs), and ports.</t><t>Because congestion control is critical to the stable operation of the Internet, applications and other protocols that choose to use UDP as an Internet transport must employ mechanisms to prevent congestion collapse and to establish some degree of fairness with concurrent traffic.  They may also need to implement additional mechanisms, depending on how they use UDP.</t><t>Some guidance is also applicable to the design of other protocols (e.g., protocols layered directly on IP or via IP-based tunnels), especially when these protocols do not themselves provide congestion control.</t><t>This document obsoletes RFC 5405 and adds guidelines for multicast UDP usage.</t></abstract>
</front>
<seriesInfo name='BCP' value='145'/>
<seriesInfo name='RFC' value='8085'/>
<seriesInfo name='DOI' value='10.17487/RFC8085'/>
</reference>

<reference  anchor="RFC8305" target='https://www.rfc-editor.org/info/rfc8305'>
<front>
<title>Happy Eyeballs Version 2: Better Connectivity Using Concurrency</title>
<author initials='D.' surname='Schinazi' fullname='D. Schinazi'><organization /></author>
<author initials='T.' surname='Pauly' fullname='T. Pauly'><organization /></author>
<date year='2017' month='December' />
<abstract><t>Many communication protocols operating over the modern Internet use hostnames.  These often resolve to multiple IP addresses, each of which may have different performance and connectivity characteristics.  Since specific addresses or address families (IPv4 or IPv6) may be blocked, broken, or sub-optimal on a network, clients that attempt multiple connections in parallel have a chance of establishing a connection more quickly.  This document specifies requirements for algorithms that reduce this user-visible delay and provides an example algorithm, referred to as &quot;Happy Eyeballs&quot;.  This document obsoletes the original algorithm description in RFC 6555.</t></abstract>
</front>
<seriesInfo name='RFC' value='8305'/>
<seriesInfo name='DOI' value='10.17487/RFC8305'/>
</reference>

<reference  anchor="RFC8174" target='https://www.rfc-editor.org/info/rfc8174'>
<front>
<title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</title>
<author initials='B.' surname='Leiba' fullname='B. Leiba'><organization /></author>
<date year='2017' month='May' />
<abstract><t>RFC 2119 specifies common key words that may be used in protocol  specifications.  This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the  defined special meanings.</t></abstract>
</front>
<seriesInfo name='BCP' value='14'/>
<seriesInfo name='RFC' value='8174'/>
<seriesInfo name='DOI' value='10.17487/RFC8174'/>
</reference>

<reference  anchor="RFC8499" target='https://www.rfc-editor.org/info/rfc8499'>
<front>
<title>DNS Terminology</title>
<author initials='P.' surname='Hoffman' fullname='P. Hoffman'><organization /></author>
<author initials='A.' surname='Sullivan' fullname='A. Sullivan'><organization /></author>
<author initials='K.' surname='Fujiwara' fullname='K. Fujiwara'><organization /></author>
<date year='2019' month='January' />
<abstract><t>The Domain Name System (DNS) is defined in literally dozens of different RFCs.  The terminology used by implementers and developers of DNS protocols, and by operators of DNS systems, has sometimes changed in the decades since the DNS was first defined.  This document gives current definitions for many of the terms used in the DNS in a single document.</t><t>This document obsoletes RFC 7719 and updates RFC 2308.</t></abstract>
</front>
<seriesInfo name='BCP' value='219'/>
<seriesInfo name='RFC' value='8499'/>
<seriesInfo name='DOI' value='10.17487/RFC8499'/>
</reference>

    </references>

    <references title='Informative References'>

<reference  anchor="RFC2317" target='https://www.rfc-editor.org/info/rfc2317'>
<front>
<title>Classless IN-ADDR.ARPA delegation</title>
<author initials='H.' surname='Eidnes' fullname='H. Eidnes'><organization /></author>
<author initials='G.' surname='de Groot' fullname='G. de Groot'><organization /></author>
<author initials='P.' surname='Vixie' fullname='P. Vixie'><organization /></author>
<date year='1998' month='March' />
<abstract><t>This document describes a way to do IN-ADDR.ARPA delegation on non-octet boundaries for address spaces covering fewer than 256 addresses.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t></abstract>
</front>
<seriesInfo name='BCP' value='20'/>
<seriesInfo name='RFC' value='2317'/>
<seriesInfo name='DOI' value='10.17487/RFC2317'/>
</reference>

<reference  anchor="RFC2845" target='https://www.rfc-editor.org/info/rfc2845'>
<front>
<title>Secret Key Transaction Authentication for DNS (TSIG)</title>
<author initials='P.' surname='Vixie' fullname='P. Vixie'><organization /></author>
<author initials='O.' surname='Gudmundsson' fullname='O. Gudmundsson'><organization /></author>
<author initials='D.' surname='Eastlake 3rd' fullname='D. Eastlake 3rd'><organization /></author>
<author initials='B.' surname='Wellington' fullname='B. Wellington'><organization /></author>
<date year='2000' month='May' />
<abstract><t>This protocol allows for transaction level authentication using shared secrets and one way hashing.  It can be used to authenticate dynamic updates as coming from an approved client, or to authenticate responses as coming from an approved recursive name server.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='2845'/>
<seriesInfo name='DOI' value='10.17487/RFC2845'/>
</reference>

<reference  anchor="RFC2931" target='https://www.rfc-editor.org/info/rfc2931'>
<front>
<title>DNS Request and Transaction Signatures ( SIG(0)s )</title>
<author initials='D.' surname='Eastlake 3rd' fullname='D. Eastlake 3rd'><organization /></author>
<date year='2000' month='September' />
<abstract><t>This document describes the minor but non-interoperable changes in Request and Transaction signature resource records ( SIG(0)s ) that implementation experience has deemed necessary.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='2931'/>
<seriesInfo name='DOI' value='10.17487/RFC2931'/>
</reference>

<reference  anchor="RFC3550" target='https://www.rfc-editor.org/info/rfc3550'>
<front>
<title>RTP: A Transport Protocol for Real-Time Applications</title>
<author initials='H.' surname='Schulzrinne' fullname='H. Schulzrinne'><organization /></author>
<author initials='S.' surname='Casner' fullname='S. Casner'><organization /></author>
<author initials='R.' surname='Frederick' fullname='R. Frederick'><organization /></author>
<author initials='V.' surname='Jacobson' fullname='V. Jacobson'><organization /></author>
<date year='2003' month='July' />
<abstract><t>This memorandum describes RTP, the real-time transport protocol.  RTP provides end-to-end network transport functions suitable for applications transmitting real-time data, such as audio, video or simulation data, over multicast or unicast network services.  RTP does not address resource reservation and does not guarantee quality-of- service for real-time services.  The data transport is augmented by a control protocol (RTCP) to allow monitoring of the data delivery in a manner scalable to large multicast networks, and to provide minimal control and identification functionality.  RTP and RTCP are designed to be independent of the underlying transport and network layers.  The protocol supports the use of RTP-level translators and mixers. Most of the text in this memorandum is identical to RFC 1889 which it obsoletes.  There are no changes in the packet formats on the wire, only changes to the rules and algorithms governing how the protocol is used. The biggest change is an enhancement to the scalable timer algorithm for calculating when to send RTCP packets in order to minimize transmission in excess of the intended rate when many participants join a session simultaneously.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='STD' value='64'/>
<seriesInfo name='RFC' value='3550'/>
<seriesInfo name='DOI' value='10.17487/RFC3550'/>
</reference>

<reference  anchor="RFC4025" target='https://www.rfc-editor.org/info/rfc4025'>
<front>
<title>A Method for Storing IPsec Keying Material in DNS</title>
<author initials='M.' surname='Richardson' fullname='M. Richardson'><organization /></author>
<date year='2005' month='March' />
<abstract><t>This document describes a new resource record for the Domain Name System (DNS).  This record may be used to store public keys for use in IP security (IPsec) systems.  The record also includes provisions for indicating what system should be contacted when an IPsec tunnel is established with the entity in question. </t><t> This record replaces the functionality of the sub-type #4 of the KEY Resource Record, which has been obsoleted by RFC 3445.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='4025'/>
<seriesInfo name='DOI' value='10.17487/RFC4025'/>
</reference>

<reference  anchor="RFC4301" target='https://www.rfc-editor.org/info/rfc4301'>
<front>
<title>Security Architecture for the Internet Protocol</title>
<author initials='S.' surname='Kent' fullname='S. Kent'><organization /></author>
<author initials='K.' surname='Seo' fullname='K. Seo'><organization /></author>
<date year='2005' month='December' />
<abstract><t>This document describes an updated version of the &quot;Security Architecture for IP&quot;, which is designed to provide security services for traffic at the IP layer.  This document obsoletes RFC 2401 (November 1998).  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='4301'/>
<seriesInfo name='DOI' value='10.17487/RFC4301'/>
</reference>

<reference  anchor="RFC4787" target='https://www.rfc-editor.org/info/rfc4787'>
<front>
<title>Network Address Translation (NAT) Behavioral Requirements for Unicast UDP</title>
<author initials='F.' surname='Audet' fullname='F. Audet' role='editor'><organization /></author>
<author initials='C.' surname='Jennings' fullname='C. Jennings'><organization /></author>
<date year='2007' month='January' />
<abstract><t>This document defines basic terminology for describing different types of Network Address Translation (NAT) behavior when handling Unicast UDP and also defines a set of requirements that would allow many applications, such as multimedia communications or online gaming, to work consistently.  Developing NATs that meet this set of requirements will greatly increase the likelihood that these applications will function properly.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t></abstract>
</front>
<seriesInfo name='BCP' value='127'/>
<seriesInfo name='RFC' value='4787'/>
<seriesInfo name='DOI' value='10.17487/RFC4787'/>
</reference>

<reference  anchor="RFC5110" target='https://www.rfc-editor.org/info/rfc5110'>
<front>
<title>Overview of the Internet Multicast Routing Architecture</title>
<author initials='P.' surname='Savola' fullname='P. Savola'><organization /></author>
<date year='2008' month='January' />
<abstract><t>This document describes multicast routing architectures that are currently deployed on the Internet.  This document briefly describes those protocols and references their specifications.</t><t>This memo also reclassifies several older RFCs to Historic.  These RFCs describe multicast routing protocols that were never widely deployed or have fallen into disuse.  This memo provides information for the Internet community.</t></abstract>
</front>
<seriesInfo name='RFC' value='5110'/>
<seriesInfo name='DOI' value='10.17487/RFC5110'/>
</reference>

<reference  anchor="RFC6726" target='https://www.rfc-editor.org/info/rfc6726'>
<front>
<title>FLUTE - File Delivery over Unidirectional Transport</title>
<author initials='T.' surname='Paila' fullname='T. Paila'><organization /></author>
<author initials='R.' surname='Walsh' fullname='R. Walsh'><organization /></author>
<author initials='M.' surname='Luby' fullname='M. Luby'><organization /></author>
<author initials='V.' surname='Roca' fullname='V. Roca'><organization /></author>
<author initials='R.' surname='Lehtonen' fullname='R. Lehtonen'><organization /></author>
<date year='2012' month='November' />
<abstract><t>This document defines File Delivery over Unidirectional Transport (FLUTE), a protocol for the unidirectional delivery of files over the Internet, which is particularly suited to multicast networks.  The specification builds on Asynchronous Layered Coding, the base protocol designed for massively scalable multicast distribution. This document obsoletes RFC 3926.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='6726'/>
<seriesInfo name='DOI' value='10.17487/RFC6726'/>
</reference>

<reference  anchor="RFC7359" target='https://www.rfc-editor.org/info/rfc7359'>
<front>
<title>Layer 3 Virtual Private Network (VPN) Tunnel Traffic Leakages in Dual-Stack Hosts/Networks</title>
<author initials='F.' surname='Gont' fullname='F. Gont'><organization /></author>
<date year='2014' month='August' />
<abstract><t>The subtle way in which the IPv6 and IPv4 protocols coexist in typical networks, together with the lack of proper IPv6 support in popular Virtual Private Network (VPN) tunnel products, may inadvertently result in VPN tunnel traffic leakages.  That is, traffic meant to be transferred over an encrypted and integrity- protected VPN tunnel may leak out of such a tunnel and be sent in the clear on the local network towards the final destination.  This document discusses some scenarios in which such VPN tunnel traffic leakages may occur as a result of employing IPv6-unaware VPN software.  Additionally, this document offers possible mitigations for this issue.</t></abstract>
</front>
<seriesInfo name='RFC' value='7359'/>
<seriesInfo name='DOI' value='10.17487/RFC7359'/>
</reference>

<reference  anchor="RFC7761" target='https://www.rfc-editor.org/info/rfc7761'>
<front>
<title>Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)</title>
<author initials='B.' surname='Fenner' fullname='B. Fenner'><organization /></author>
<author initials='M.' surname='Handley' fullname='M. Handley'><organization /></author>
<author initials='H.' surname='Holbrook' fullname='H. Holbrook'><organization /></author>
<author initials='I.' surname='Kouvelas' fullname='I. Kouvelas'><organization /></author>
<author initials='R.' surname='Parekh' fullname='R. Parekh'><organization /></author>
<author initials='Z.' surname='Zhang' fullname='Z. Zhang'><organization /></author>
<author initials='L.' surname='Zheng' fullname='L. Zheng'><organization /></author>
<date year='2016' month='March' />
<abstract><t>This document specifies Protocol Independent Multicast - Sparse Mode (PIM-SM).  PIM-SM is a multicast routing protocol that can use the underlying unicast routing information base or a separate multicast-capable routing information base.  It builds unidirectional shared trees rooted at a Rendezvous Point (RP) per group, and it optionally creates shortest-path trees per source.</t><t>This document obsoletes RFC 4601 by replacing it, addresses the errata filed against it, removes the optional (*,*,RP), PIM Multicast Border Router features and authentication using IPsec that lack sufficient deployment experience (see Appendix A), and moves the PIM specification to Internet Standard.</t></abstract>
</front>
<seriesInfo name='STD' value='83'/>
<seriesInfo name='RFC' value='7761'/>
<seriesInfo name='DOI' value='10.17487/RFC7761'/>
</reference>

<reference  anchor="RFC7858" target='https://www.rfc-editor.org/info/rfc7858'>
<front>
<title>Specification for DNS over Transport Layer Security (TLS)</title>
<author initials='Z.' surname='Hu' fullname='Z. Hu'><organization /></author>
<author initials='L.' surname='Zhu' fullname='L. Zhu'><organization /></author>
<author initials='J.' surname='Heidemann' fullname='J. Heidemann'><organization /></author>
<author initials='A.' surname='Mankin' fullname='A. Mankin'><organization /></author>
<author initials='D.' surname='Wessels' fullname='D. Wessels'><organization /></author>
<author initials='P.' surname='Hoffman' fullname='P. Hoffman'><organization /></author>
<date year='2016' month='May' />
<abstract><t>This document describes the use of Transport Layer Security (TLS) to provide privacy for DNS.  Encryption provided by TLS eliminates opportunities for eavesdropping and on-path tampering with DNS queries in the network, such as discussed in RFC 7626.  In addition, this document specifies two usage profiles for DNS over TLS and provides advice on performance considerations to minimize overhead from using TCP and TLS with DNS.</t><t>This document focuses on securing stub-to-recursive traffic, as per the charter of the DPRIVE Working Group.  It does not prevent future applications of the protocol to recursive-to-authoritative traffic.</t></abstract>
</front>
<seriesInfo name='RFC' value='7858'/>
<seriesInfo name='DOI' value='10.17487/RFC7858'/>
</reference>

<reference  anchor="RFC8126" target='https://www.rfc-editor.org/info/rfc8126'>
<front>
<title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
<author initials='M.' surname='Cotton' fullname='M. Cotton'><organization /></author>
<author initials='B.' surname='Leiba' fullname='B. Leiba'><organization /></author>
<author initials='T.' surname='Narten' fullname='T. Narten'><organization /></author>
<date year='2017' month='June' />
<abstract><t>Many protocols make use of points of extensibility that use constants to identify various protocol parameters.  To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper.  For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA).</t><t>To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed.  This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry.</t><t>This is the third edition of this document; it obsoletes RFC 5226.</t></abstract>
</front>
<seriesInfo name='BCP' value='26'/>
<seriesInfo name='RFC' value='8126'/>
<seriesInfo name='DOI' value='10.17487/RFC8126'/>
</reference>

<reference  anchor="RFC8313" target='https://www.rfc-editor.org/info/rfc8313'>
<front>
<title>Use of Multicast across Inter-domain Peering Points</title>
<author initials='P.' surname='Tarapore' fullname='P. Tarapore' role='editor'><organization /></author>
<author initials='R.' surname='Sayko' fullname='R. Sayko'><organization /></author>
<author initials='G.' surname='Shepherd' fullname='G. Shepherd'><organization /></author>
<author initials='T.' surname='Eckert' fullname='T. Eckert' role='editor'><organization /></author>
<author initials='R.' surname='Krishnan' fullname='R. Krishnan'><organization /></author>
<date year='2018' month='January' />
<abstract><t>This document examines the use of Source-Specific Multicast (SSM) across inter-domain peering points for a specified set of deployment scenarios.  The objectives are to (1) describe the setup process for multicast-based delivery across administrative domains for these scenarios and (2) document supporting functionality to enable this process.</t></abstract>
</front>
<seriesInfo name='BCP' value='213'/>
<seriesInfo name='RFC' value='8313'/>
<seriesInfo name='DOI' value='10.17487/RFC8313'/>
</reference>

<reference  anchor="RFC8484" target='https://www.rfc-editor.org/info/rfc8484'>
<front>
<title>DNS Queries over HTTPS (DoH)</title>
<author initials='P.' surname='Hoffman' fullname='P. Hoffman'><organization /></author>
<author initials='P.' surname='McManus' fullname='P. McManus'><organization /></author>
<date year='2018' month='October' />
<abstract><t>This document defines a protocol for sending DNS queries and getting DNS responses over HTTPS.  Each DNS query-response pair is mapped into an HTTP exchange.</t></abstract>
</front>
<seriesInfo name='RFC' value='8484'/>
<seriesInfo name='DOI' value='10.17487/RFC8484'/>
</reference>

    </references>

<section anchor="extranslate" title="Unknown RRType construction">

<t>In a DNS resolver that understands the AMTRELAY type, AMTRELAY type, the zone file might
contain this line:</t>

<figure><artwork><![CDATA[
      <sourcecode name="" type=""><![CDATA[
  IN AMTRELAY 128 0 3 amtrelays.example.com.
]]></artwork></figure>
]]></sourcecode>
      <t>In order to translate this example to appear as an unknown RRType
as defined in <xref target="RFC3597"/>, target="RFC3597" format="default"/>, one could run the following program:</t>

<figure><artwork><![CDATA[
<CODE BEGINS>
      <sourcecode name="" type="python" markers="true"><![CDATA[
  $ cat translate.py
  #!/usr/bin/env python3
  import sys
  name=sys.argv[1]
  wire=''
  for dn in name.split('.'):
    if len(dn) > 0:
      wire += ('%02x' % len(dn))
      wire += (''.join('%02x'%ord(x) for x in dn))
  print(len(wire)//2) + 2
  print(wire)

  $ ./translate.py amtrelays.example.com
  24
  09616d7472656c617973076578616d706c6503636f6d
<CODE ENDS>
]]></artwork></figure>
]]></sourcecode>
      <t>The length of the RData and the hex string for the domain name
“amtrelays.example.com”
"amtrelays.example.com" are the outputs of this program.</t>

<t>22 is the
      <t>The length of the wire-encoded domain name, name is 22, so 2 was added to
that value (1 for the precedence field and 1 for the combined D-bit and
relay type fields) to get the full length 24 of the RData.  For the 2
octets ahead of the domain name, we encode the precedence, D-bit, and
relay type fields, as described in <xref target="rrdef"/>.</t> target="rrdef" format="default"/>.</t>
      <t>This results in a zone file entry like this:</t>

<figure><artwork><![CDATA[

      <artwork name="" type="" align="left" alt=""><![CDATA[
  IN TYPE260  \# ( 24 ; length
          80 ; precedence = 128
          03 ; D-bit=0, relay type=3 (wire-encoded domain name)
        09616d7472656c617973076578616d706c6503636f6d ) ; domain name
]]></artwork></figure>
]]></artwork>
    </section>

    <section anchor="acknowledgements" numbered="false" toc="default">
      <name>Acknowledgements</name>
      <t>This specification was inspired by the previous work of <contact
      fullname="Doug Nortz"/>, <contact fullname="Robert Sayko"/>, <contact fullname="David Segelstein"/>, and <contact fullname="Percy Tarapore"/>, presented in
the MBONED Working Group at IETF 93.</t>
      <t>Thanks to <contact fullname="Jeff Goldsmith"/>, <contact fullname="Toerless Eckert"/>, <contact fullname="Mikael Abrahamsson"/>, <contact fullname="Lenny
Giuliano"/>, <contact fullname="Mark Andrews"/>, <contact fullname="Sandy
Zheng"/>, <contact fullname="Kyle Rose"/>, <contact fullname="Ben Kaduk"/>,
      <contact fullname="Bill Atwood"/>, <contact fullname="Tim Chown"/>, <contact fullname="Christian Worm Mortensen"/>, <contact fullname="Warren Kumari"/>, <contact fullname="Dan
Romanescu"/>, <contact fullname="Bernard Aboba"/>, <contact fullname="Carlos
Pignataro"/>, <contact fullname="Niclas Comstedt"/>, <contact fullname="Mirja Kühlewind"/>, <contact fullname="Henning Rogge"/>, <contact fullname="Eric Vyncke"/>, <contact fullname="Barry Lieba"/>,
<contact fullname="Roman Danyliw"/>, <contact fullname="Alissa Cooper"/>,
<contact fullname="Suresh Krishnan"/>, <contact fullname="Adam Roach"/>,
and <contact fullname="Daniel Franke"/> for their very helpful reviews and comments.</t>
    </section>
  </back>

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