path: root/doc/rfc/rfc7450.txt

Internet Engineering Task Force (IETF)                     G. Bumgardner
Request for Comments: 7450                                 February 2015
Category: Standards Track
ISSN: 2070-1721


                     Automatic Multicast Tunneling

Abstract

   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.

   The AMT protocol is specifically designed to support rapid deployment
   by requiring minimal changes to existing network infrastructure.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7450.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1. Introduction ....................................................3
   2. Applicability ...................................................3
   3. Terminology .....................................................4
      3.1. Requirements Notation ......................................4
      3.2. Definitions ................................................4
      3.3. Abbreviations ..............................................5
   4. Protocol Overview ...............................................6
      4.1. General Architecture .......................................6
           4.1.1. Relationship to IGMP and MLD Protocols ..............6
           4.1.2. Gateways ............................................7
           4.1.3. Relays .............................................10
           4.1.4. Deployment .........................................13
           4.1.5. Discovery ..........................................14
      4.2. General Operation .........................................15
           4.2.1. Message Sequences ..................................15
           4.2.2. Tunneling ..........................................26
   5. Protocol Description ...........................................31
      5.1. Protocol Messages .........................................31
           5.1.1. Relay Discovery ....................................31
           5.1.2. Relay Advertisement ................................32
           5.1.3. Request ............................................34
           5.1.4. Membership Query ...................................35
           5.1.5. Membership Update ..................................39
           5.1.6. Multicast Data .....................................41
           5.1.7. Teardown ...........................................43
      5.2. Gateway Operation .........................................45
           5.2.1. IP/IGMP/MLD Protocol Requirements ..................45
           5.2.2. Pseudo-Interface Configuration .....................47
           5.2.3. Gateway Service ....................................48
      5.3. Relay Operation ...........................................61
           5.3.1. IP/IGMP/MLD Protocol Requirements ..................61
           5.3.2. Startup ............................................61
           5.3.3. Running ............................................62
           5.3.4. Shutdown ...........................................73
           5.3.5. Response MAC Generation ............................73
           5.3.6. Private Secret Generation ..........................74
   6. Security Considerations ........................................74
      6.1. Relays ....................................................74
      6.2. Gateways ..................................................76
      6.3. Encapsulated IP Packets ...................................76
   7. IANA Considerations ............................................77
      7.1. IPv4 and IPv6 Anycast Prefix Allocation ...................77
           7.1.1. IPv4 ...............................................77
           7.1.2. IPv6 ...............................................78
      7.2. UDP Port Number ...........................................78




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   8. References .....................................................78
      8.1. Normative References ......................................78
      8.2. Informative References ....................................79
   Acknowledgments ...................................................81
   Contributors ......................................................82
   Author's Address ..................................................82

1.  Introduction

   The advantages and benefits provided by multicast technologies are
   well known.  There are a number of application areas that are ideal
   candidates for the use of multicast, including media broadcasting,
   video conferencing, collaboration, real-time data feeds, data
   replication, and software updates.  Unfortunately, many of these
   applications lack multicast connectivity to networks that carry
   traffic generated by multicast sources.  The reasons for the lack of
   connectivity vary but are primarily the result of service provider
   policies and network limitations.

   Automatic Multicast Tunneling (AMT) is a protocol that uses UDP-based
   encapsulation to overcome the aforementioned lack of multicast
   connectivity.  AMT enables sites, hosts, or applications that do not
   have native multicast access to a network with multicast connectivity
   to a source, to request and receive Source-Specific Multicast (SSM)
   [RFC4607] and Any-Source Multicast (ASM) [RFC1112] traffic from a
   network that does provide multicast connectivity to that source.

2.  Applicability

   This document describes a protocol that may be used to deliver
   multicast traffic from a multicast-enabled network to sites that lack
   multicast connectivity to the source network.  This document does not
   describe any methods for sourcing multicast traffic from isolated
   sites, as this topic is out of scope.

   AMT is not intended to be used as a substitute for native multicast,
   especially in conditions or environments requiring high traffic flow.
   AMT uses unicast replication to reach multiple receivers, and the
   bandwidth cost for this replication will be higher than that required
   if the receivers were reachable via native multicast.

   AMT is designed to be deployed at the border of networks possessing
   native multicast capabilities where access and provisioning can be
   managed by the AMT service provider.







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3.  Terminology

3.1.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.2.  Definitions

   This document adopts the following definitions for use in describing
   the protocol:

   Downstream:
      A downstream interface or connection that faces away from the
      multicast distribution root or towards multicast receivers.

   Upstream:
      An upstream interface or connection that faces a multicast
      distribution root or source.

   Non-Broadcast Multi-Access (NBMA):
      An NBMA network or interface is one to which multiple network
      nodes (hosts or routers) are attached, but where packets are
      transmitted directly from one node to another node over a virtual
      circuit or physical link.  NBMA networks do not support multicast
      or broadcast traffic -- a node that sources multicast traffic must
      replicate the multicast packets for separate transmission to each
      node that has requested the multicast traffic.

   Multicast Receiver:
      An entity that requests and receives multicast traffic.  A
      receiver may be a router, host, application, or application
      component.  The method by which a receiver transmits group
      membership requests and receives multicast traffic varies
      according to receiver type.

   Group Membership Database:
      A group membership database describes the current multicast
      subscription state (also referred to as "reception state") for an
      interface or system.  See Section 3 of [RFC3376] for a detailed
      definition.

   Reception State:
      The multicast subscription state of a pseudo-interface, virtual
      interface, or physical network interface.  Often synonymous with
      group membership database.




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   Subscription:
      A group or state entry in a group membership database or reception
      state table.  The presence of a subscription entry indicates
      membership in an IP multicast group.

   Group Membership Protocol:
      The term "group membership protocol" is used as a generic
      reference to the Internet Group Management Protocol (IGMP)
      [RFC1112] [RFC2236] [RFC3376] or the Multicast Listener Discovery
      protocol [RFC2710] [RFC3810].

   Multicast Protocol:
      The term "multicast protocol" is used as a generic reference to
      multicast routing protocols used to join or leave multicast
      distribution trees, such as Protocol Independent Multicast -
      Sparse Mode (PIM-SM) [RFC4601].

   Network Address Translation (NAT):
      Network Address Translation is the process of modifying the source
      IP address and port numbers carried by an IP packet while
      transiting a network node (see [RFC2663]).  Intervening NAT
      devices may change the source address and port carried by messages
      sent from an AMT gateway to an AMT relay, possibly producing
      changes in protocol state and behavior.

   Anycast:
      A network addressing and routing method in which packets from a
      single sender are routed to the topologically nearest node in a
      group of potential receivers all identified by the same
      destination address.  See [RFC4786].

3.3.  Abbreviations

      AMT - Automatic Multicast Tunneling protocol.

      ASM - Any-Source Multicast.

      DoS - Denial-of-Service (attack) and DDoS for distributed DoS.

      IGMP - Internet Group Management Protocol (v1, v2, and v3).

      IP - Internet Protocol (v4 and v6).

      MAC - Message Authentication Code (or Cookie).

      MLD - Multicast Listener Discovery protocol (v1 and v2).

      NAT - Network Address Translation (or translation node).



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      NBMA - Non-Broadcast Multi-Access (network, interface, or mode).

      PIM - Protocol Independent Multicast.

      SSM - Source-Specific Multicast.

4.  Protocol Overview

   This section provides an informative description of the protocol.  A
   normative description of the protocol and implementation requirements
   may be found in Section 5.

4.1.  General Architecture

   Isolated Site |    Unicast Network   |  Native Multicast
                 |      (Internet)      |
                 |                      |
                 |                      |
                 |   Group Membership   |
      +-------+ =========================> +-------+ Multicast +------+
      |Gateway|  |                      |  | Relay |<----//----|Source|
      +-------+ <========================= +-------+           +------+
                 |   Multicast Data     |
                 |                      |
                 |                      |

                     Figure 1: Basic AMT Architecture

   The AMT protocol employs a client-server model in which a "gateway"
   sends requests to receive specific multicast traffic to a "relay"
   that responds by delivering the requested multicast traffic back to
   the gateway.

   Gateways are generally deployed within networks that lack multicast
   support or lack connectivity to a multicast-enabled network
   containing multicast sources of interest.

   Relays are deployed within multicast-enabled networks that contain,
   or have connectivity to, multicast sources.

4.1.1.  Relationship to IGMP and MLD Protocols

   AMT relies on the Internet Group Management Protocol (IGMP) [RFC3376]
   and the Multicast Listener Discovery (MLD) protocol [RFC3810] to
   provide the functionality required to manage, communicate, and act on
   changes in multicast group membership.  A gateway or relay
   implementation does not necessarily require a fully functional,
   conforming implementation of IGMP or MLD to adhere to this



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   specification, but the protocol description that appears in this
   document assumes that this is the case.  The minimum functional and
   behavioral requirements for the IGMP and MLD protocols are described
   in Sections 5.2.1 and 5.3.1.

               Gateway                          Relay

                 General _____         _____
     ___________  Query |     |       |     | Query  ___________
    |           |<------|     |       |     |<------|           |
    | Host-Mode |       | AMT |       | AMT |       |Router-Mode|
    | IGMP/MLD  |       |     |  UDP  |     |       | IGMP/MLD  |
    |___________|------>|     |<----->|     |------>|___________|
                 Report |     |       |     | Report
             Leave/Done |     |       |     | Leave/Done
                        |     |       |     |
    IP Multicast <------|     |       |     |<------ IP Multicast
                        |_____|       |_____|

          Figure 2: Multicast Reception State Managed by IGMP/MLD

   A gateway runs the host portion of the IGMP and MLD protocols to
   generate group membership updates that are sent via AMT messages to a
   relay.  A relay runs the router portion of the IGMP and MLD protocols
   to process the group membership updates to produce the required
   changes in multicast forwarding state.  A relay uses AMT messages to
   send incoming multicast IP datagrams to gateways according to their
   current group membership state.

   The primary function of AMT is to provide the handshaking,
   encapsulation, and decapsulation required to transport the IGMP and
   MLD messages and multicast IP datagrams between the gateways and
   relays.  The IGMP and MLD messages that are exchanged between
   gateways and relays are encapsulated as complete IP datagrams within
   AMT control messages.  Multicast IP datagrams are replicated and
   encapsulated in AMT data messages.  All AMT messages are sent via
   unicast UDP/IP.

4.1.2.  Gateways

   The downstream side of a gateway services one or more receivers --
   the gateway accepts group membership requests from receivers and
   forwards requested multicast traffic back to those receivers.  The
   gateway functionality may be directly implemented in the host
   requesting the multicast service or within an application running on
   a host.





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   The upstream side of a gateway connects to relays.  A gateway sends
   encapsulated IGMP and MLD messages to a relay to indicate an interest
   in receiving specific multicast traffic.

4.1.2.1.  Architecture

   Each gateway possesses a logical pseudo-interface:

     join/leave ---+                   +----------+
                   |                   |          |
                   V      IGMPv3/MLDv2 |          |
              +---------+ General Query|          |   AMT
              |IGMP/MLD |<-------------|   AMT    | Messages +------+
              |Host-Mode|              | Gateway  |<-------->|UDP/IP|
              |Protocol |------------->|Pseudo-I/F|          +------+
              +---------+   IGMP/MLD   |          |             ^
                             Report    |          |             |
                           Leave/Done  |          |             V
    IP Multicast <---------------------|          |           +---+
                                       +----------+           |I/F|
                                                              +---+

                  Figure 3: AMT Gateway Pseudo-Interface

   The pseudo-interface is conceptually a network interface on which the
   gateway executes the host portion of the IPv4/IGMP (v2 or v3) and
   IPv6/MLD (v1 or v2) protocols.  The multicast reception state of the
   pseudo-interface is manipulated using the IGMP or MLD service
   interface.  The IGMP and MLD host protocols produce IP datagrams
   containing group membership messages that the gateway will send to
   the relay.  The IGMP and MLD protocols also supply the retransmission
   and timing behavior required for protocol robustness.

   All AMT encapsulation, decapsulation, and relay interaction are
   assumed to occur within the pseudo-interface.

   A gateway host or application may create separate interfaces for
   IPv4/IGMP and IPv6/MLD.  A gateway host or application may also
   require additional pseudo-interfaces for each source or domain-
   specific relay address.

   Within this document, the term "gateway" may be used as a generic
   reference to an entity executing the gateway protocol, a gateway
   pseudo-interface, or a gateway device that has one or more interfaces
   connected to a unicast internetwork and one or more AMT gateway
   pseudo-interfaces.





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   The following diagram illustrates how an existing host IP stack
   implementation might be used to provide AMT gateway functionality to
   a multicast application:

           +-----------------------------------------------------+
           |Host                                                 |
           |    ______________________________________           |
           |   |                                      |          |
           |   |    ___________________________       |          |
           |   |   |                           |      |          |
           |   |   |                           v      |          |
           |   |   |        +-----------+  +--------------+      |
           |   |   |        |Application|  |  AMT Daemon  |      |
           |   |   |        +-----------+  +--------------+      |
           |   |   | join/leave |   ^ data        ^ AMT          |
           |   |   |            |   |             |              |
           |   |   |       +----|---|-------------|-+            |
           |   |   |       |  __|   |_________    | |            |
           |   |   |       | |                |   | |            |
           |   |   |       | |       Sockets  |   | |            |
           |   |   |       +-|------+-------+-|---|-+            |
           |   |   |       | | IGMP |  TCP  | |UDP| |            |
           |   |   |       +-|------+-------+-|---|-+            |
           |   |   |       | | ^       IP     |   | |            |
           |   |   |       | | |  ____________|   | |            |
           |   |   |       | | | |                | |            |
           |   |   |       +-|-|-|----------------|-+            |
           |   |   |         | | |                |              |
           |   |   | IP(IGMP)| | |IP(UDP(data))   |IP(UDP(AMT))  |
           |   |   |         v | |                v              |
           |   |   |     +-----------+          +---+            |
           |   |   |     |Virtual I/F|          |I/F|            |
           |   |   |     +-----------+          +---+            |
           |   |   |         |   ^                ^              |
           |   |   | IP(IGMP)|   |IP(UDP(data))   |              |
           |   |   |_________|   |IP(IGMP)        |              |
           |   |                 |                |              |
           |   |_________________|                |              |
           |                                      |              |
           +--------------------------------------|--------------+
                                                  v
                                              AMT Relay

            Figure 4: Virtual Interface Implementation Example

   In this example, the host IP stack uses a virtual network interface
   to interact with a gateway pseudo-interface implementation.




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4.1.2.2.  Use Cases

   Use cases for gateway functionality include the following:

   IGMP/MLD Proxy
      An IGMP/MLD proxy that runs AMT on an upstream interface and
      router-mode IGMP/MLD on downstream interfaces to provide host
      access to multicast traffic via the IGMP and MLD protocols.

   Virtual Network Interface
      A virtual network interface or pseudo-network device driver that
      runs AMT on a physical network interface to provide socket-layer
      access to multicast traffic via the IGMP/MLD service interface
      provided by the host IP stack.

   Application
      An application or application component that implements and
      executes IGMP/MLD and AMT internally to gain access to multicast
      traffic.

4.1.3.  Relays

   The downstream side of a relay services gateways -- the relay accepts
   encapsulated IGMP and MLD group membership messages from gateways and
   encapsulates and forwards the requested multicast traffic back to
   those gateways.

   The upstream side of a relay communicates with a native multicast
   infrastructure -- the relay sends join and prune/leave requests
   towards multicast sources and accepts requested multicast traffic
   from those sources.




















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4.1.3.1.  Architecture

   Each relay possesses a logical pseudo-interface:

                                       +------------------------------+
                     +--------+        | Multicast Control Plane      |
                     |        |IGMP/MLD|                              |
                     |        | Query* | +------------+  +----------+ |
                     |        |<---//----|IGMPv3/MLDv2|  |Multicast | |
              AMT    |        |        | |Router-Mode |->|Routing   |<->
   +------+ Messages | AMT    |----//--->|Protocol    |  |Protocol  | |
   |UDP/IP|<-------->| Relay  |IGMP/MLD| +------------+  +----------+ |
   +------+          | Pseudo-| Report |      |               |       |
      ^              | I/F    | Leave/ +------|---------------|-------+
      |              |        |  Done         |               |
      |              |        |               v               |
      V              |        | IP        +-----------+       |
    +---+            |        | Multicast |Multicast  |<------+
    |I/F|            |        |<---//-----|Forwarding |
    +---+            +--------+           |Plane      |<--- IP Multicast
                                          +-----------+

    * Queries, if generated, are consumed by the pseudo-interface.

            Figure 5: AMT Relay Pseudo-Interface (Router-Based)

   The pseudo-interface is conceptually a network interface on which the
   relay runs the router portion of the IPv4/IGMPv3 and IPv6/MLDv2
   protocols.  Relays do not send unsolicited IGMPv3/MLDv2 query
   messages to gateways so relays must consume or discard any local
   queries normally generated by IGMPv3 or MLDv2.  Note that the
   protocol mandates the use of IGMPv3 and MLDv2 for query messages.
   The AMT protocol is primarily intended for use in SSM applications
   and relies on several values provided by IGMPv3/MLDv2 to control
   gateway behavior.

   A relay maintains group membership state for each gateway connected
   through the pseudo-interface as well as for the entire
   pseudo-interface (if multiple gateways are managed via a single
   interface).  Multicast packets received on upstream interfaces on the
   relay are routed to the pseudo-interface where they are replicated,
   encapsulated, and sent to interested gateways.  Changes in the
   pseudo-interface group membership state may trigger the transmission
   of multicast protocol requests upstream towards a given source or
   rendezvous point and cause changes in internal routing/forwarding
   state.





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   The relay pseudo-interface is an architectural abstraction used to
   describe AMT protocol operation.  For the purposes of this document,
   the pseudo-interface is most easily viewed as an interface to a
   single gateway -- encapsulation, decapsulation, and other
   AMT-specific processing occurs "within" the pseudo-interface while
   forwarding and replication occur outside of it.

   An alternative view is to treat the pseudo-interface as a
   non-broadcast multi-access (NBMA) network interface whose link layer
   is the unicast-only network over which AMT messages are exchanged
   with gateways.  Individual gateways are conceptually treated as
   logical NBMA links on the interface.  In this architectural model,
   group membership tracking, replication, and forwarding functions
   occur in the pseudo-interface.

   This document does not specify any particular architectural solution
   -- a relay developer may choose to implement and distribute protocol
   functionality as required to take advantage of existing relay
   platform services and architecture.

   Within this document, the term "relay" may be used as a generic
   reference to an entity executing the relay protocol, a relay
   pseudo-interface, or a relay device that has one or more network
   interfaces with multicast connectivity to a native multicast
   infrastructure, zero or more interfaces connected to a unicast
   internetwork, and one or more relay pseudo-interfaces.

4.1.3.2.  Use Cases

   Use cases for relay functionality include the following:

   Multicast Router
      A multicast router that runs AMT on a downstream interface to
      provide gateway access to multicast traffic.  A "relay router"
      uses a multicast routing protocol (e.g., PIM-SM [RFC4601]) to
      construct a forwarding path for multicast traffic by sending join
      and prune messages to neighboring routers to join or leave
      multicast distribution trees for a given SSM source or ASM
      rendezvous point.

   IGMP/MLD Proxy Router
      An IGMP/MLD proxy that runs AMT on a downstream interface and
      host-mode IGMPv3/MLDv2 on an upstream interface.  This "relay
      proxy" sends group membership reports to a local, multicast-
      enabled router to join and leave specific SSM or ASM groups.






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4.1.4.  Deployment

   The AMT protocol calls for a relay deployment model that uses anycast
   addressing [RFC1546] [RFC4291] to pair gateways with relays.

   Under this approach, one or more relays advertise a route for the
   same IP address prefix.  To find a relay with which to communicate, a
   gateway sends a message to an anycast IP address within that prefix.
   This message is routed to the topologically nearest relay that has
   advertised the prefix.  The relay that receives the message responds
   by sending its unicast address back to the gateway.  The gateway uses
   this address as the destination address for any messages it
   subsequently sends to the relay.

   The use of anycast addressing provides the following benefits:

   o  Relays may be deployed at multiple locations within a single
      multicast-enabled network.  Relays might be installed "near"
      gateways to reduce bandwidth requirements and latency and to limit
      the number of gateways that might be serviced by a single relay.

   o  Relays may be added or removed at any time, thereby allowing
      staged deployment, scaling, and hot-swapping -- the relay
      discovery process will always return the nearest operational
      relay.

   o  Relays may take themselves offline when they exhaust resources
      required to service additional gateways.  Existing gateway
      connections may be preserved, but new gateway requests would be
      routed to the next-nearest relay.

4.1.4.1.  Public versus Private

   Ideally, the AMT protocol would provide a universal solution for
   connecting receivers to multicast sources, so that any gateway could
   be used to access any globally advertised multicast source via
   publicly accessible, widely deployed relays.  Unfortunately, today's
   Internet does not yet allow this, because many relays will lack
   native multicast access to sources even though they may be globally
   accessible via unicast.

   In these cases, a provider may deploy relays within their own source
   network to allow for multicast distribution within that network.
   Gateways that use these relays must use a provider-specific relay
   discovery mechanism or a private anycast address to obtain access to
   these relays.





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4.1.4.2.  Congestion Considerations

   AMT relies on UDP to provide best-effort delivery of multicast data
   to gateways.  Neither AMT nor UDP provides the congestion control
   mechanisms required to regulate the flow of data messages passing
   through a network.  While congestion remediation might be provided by
   multicast receiver applications via multicast group selection or
   upstream reporting mechanisms, there are no means by which to ensure
   that such mechanisms are employed.  To limit the possible congestion
   across a network or wider Internet, AMT service providers are
   expected to deploy AMT relays near the provider's network border and
   its interface with edge routers.  The provider must limit relay
   address advertisements to those edges to prevent distant gateways
   from being able to access a relay and potentially generate flows that
   consume or exceed the capacity of intervening links.

4.1.5.  Discovery

   To execute the gateway portion of the protocol, a gateway requires a
   unicast IP address of an operational relay.  This address may be
   obtained using a number of methods -- it may be statically assigned
   or dynamically chosen via some form of relay discovery process.

   As described in the previous section, the AMT protocol provides a
   relay discovery method that relies on anycast addressing.  Gateways
   are not required to use AMT relay discovery, but all relay
   implementations must support it.

   The AMT protocol uses the following terminology when describing the
   discovery process:

   Relay Discovery Address Prefix:
      The anycast address prefix used to route discovery messages to a
      relay.

   Relay Discovery Address:
      The anycast destination address used when sending discovery
      messages.

   Relay Address:
      The unicast IP address obtained as a result of the discovery
      process.

4.1.5.1.  Relay Discovery Address Selection

   The selection of an anycast Relay Discovery Address may be source
   dependent, as a relay located via relay discovery must have multicast
   connectivity to a desired source.



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   Similarly, the selection of a unicast Relay Address may be source
   dependent, as a relay contacted by a gateway to supply multicast
   traffic must have native multicast connectivity to the traffic
   source.

   Methods that might be used to perform source-specific or
   group-specific relay selection are highly implementation dependent
   and are not further addressed by this document.  Possible approaches
   include the use of static lookup tables, DNS-based queries, or a
   provision of a service interface that accepts join requests on
   (S,G,relay-discovery-address) or (S,G,relay-address) tuples.

4.1.5.2.  Relay Discovery Address Prefix

   IANA has assigned IPv4 and IPv6 address prefixes for use in
   advertising and discovering publicly accessible relays.

   A Relay Discovery Address is constructed from an address prefix by
   setting the low-order octet of the prefix address to 1 (for both IPv4
   and IPv6).  All remaining addresses within each prefix are reserved
   for future use.

   Public relays must advertise a route to the address prefix (e.g., via
   BGP [RFC4271]) and configure an interface to respond to the Relay
   Discovery Address.

   The discovery address prefixes are described in Section 7.

4.2.  General Operation

4.2.1.  Message Sequences

   The AMT protocol defines the following messages for control and
   encapsulation.  These messages are exchanged as UDP/IP datagrams, one
   message per datagram.

   Relay Discovery:
      Sent by gateways to solicit a Relay Advertisement from any relay.
      Used to find a relay with which to communicate.

   Relay Advertisement:
      Sent by relays as a response to a Relay Discovery message.  Used
      to deliver a Relay Address to a gateway.

   Request:
      Sent by gateways to solicit a Membership Query message from a
      relay.




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   Membership Query:
      Sent by relays as a response to a Request message.  Used to
      deliver an encapsulated IGMPv3 or MLDv2 query message to the
      gateway.

   Membership Update:
      Sent by gateways to deliver an encapsulated IGMP or MLD
      report/leave/done message to a relay.

   Multicast Data:
      Sent by relays to deliver an encapsulated IP multicast datagram or
      datagram fragment to a gateway.

   Teardown:
      Sent by gateways to stop the delivery of Multicast Data messages
      requested in an earlier Membership Update message.

   The following sections describe how these messages are exchanged to
   execute the protocol.

4.2.1.1.  Relay Discovery Sequence

                       Gateway               Relay
                       -------               -----
                          :                    :
                          |                    |
                      [1] |Relay Discovery     |
                          |------------------->|
                          |                    |
                          | Relay Advertisement| [2]
                          |<-------------------|
                      [3] |                    |
                          :                    :

                  Figure 6: AMT Relay Discovery Sequence
















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   The following sequence describes how the Relay Discovery and Relay
   Advertisement messages are used to find a relay with which to
   communicate:

   1.  The gateway sends a Relay Discovery message containing a random
       nonce to the Relay Discovery Address.  If the Relay Discovery
       Address is an anycast address, the message is routed to the
       topologically nearest network node that advertises that address.

   2.  The node receiving the Relay Discovery message sends a Relay
       Advertisement message back to the source of the Relay Discovery
       message.  The message carries a copy of the nonce contained in
       the Relay Discovery message and the unicast IP address of a
       relay.

   3.  When the gateway receives the Relay Advertisement message, it
       verifies that the nonce matches the one sent in the Relay
       Discovery message and, if it does, uses the Relay Address carried
       by the Relay Advertisement as the destination address for
       subsequent AMT messages.

   Note that the responder need not be a relay -- the responder may
   obtain a Relay Address by some other means and return the result in
   the Relay Advertisement (i.e., the responder is a load-balancer or
   broker).

4.2.1.2.  Membership Update Sequence

   There exists a significant difference between normal IGMP and MLD
   behavior and that required by AMT.  An IGMP/MLD router acting as a
   querier normally transmits query messages on a network interface to
   construct and refresh group membership state for the connected
   network.  These query messages are multicast to all IGMP/MLD-enabled
   hosts on the network.  Each host responds by multicasting report
   messages that describe their current multicast reception state.

   However, AMT does not allow relays to send unsolicited query messages
   to gateways, as the set of active gateways may be unknown to the
   relay and potentially quite large.  Instead, AMT requires each
   gateway to periodically send a message to a relay to solicit a query
   response.  A gateway accomplishes this by sending a Request message
   to a relay.  The relay responds by sending a Membership Query message
   back to the gateway.  The Membership Query message carries an
   encapsulated query that is processed by the IGMP or MLD protocol
   implementation on the gateway to produce a membership/listener
   report.  Each time the gateway receives a Membership Query message,
   it starts a timer whose expiration will trigger the start of a new
   Request->Membership Query message exchange.  This timer-driven



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   sequence is used to mimic the transmission of a periodic query by an
   IGMP/MLD router.  This query cycle may continue indefinitely once
   started by sending the initial Request message.

   A membership update occurs when an IGMP or MLD report, leave, or done
   message is passed to the gateway pseudo-interface.  These messages
   may be produced as a result of the aforementioned query processing or
   as a result of receiver interaction with the IGMP/MLD service
   interface.  Each report is encapsulated and sent to the relay after
   the gateway has successfully established communication with the relay
   via a Request and Membership Query message exchange.  If a report is
   passed to the pseudo-interface before the gateway has received a
   Membership Query message from the relay, the gateway may discard the
   report or queue the report for delivery after a Membership Query is
   received.  Subsequent IGMP/MLD report/leave/done messages that are
   passed to the pseudo-interface are immediately encapsulated and
   transmitted to the relay.


































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           IGMP/MLD             Pseudo-I/F              Relay
           --------             ----------              -----
              :                     :                     :
              |                     |       Request       |
              |                    1|-------------------->|
              |                     |  Membership Query   |2
    Query     |                     |       Q(0,{})       |
    Timer     |         Start      3|<--------------------|
     (QT)<--------------------------|                     |
              |        Q(0,{})      |                     |
              |<--------------------|                     |
             4|         R({})       |  Membership Update  |
              |-------------------->|5       R({})        |
              |                     |====================>|6a
    Join(S,G) :                     :                     :
   ()-------->|7 R({G:ALLOW({S})})  |  Membership Update  |
              |-------------------->|8  R({G:ALLOW({S})}) |
              |                     |====================>|9a  Join(S,G)
              |                     |                     |---------->()
              :                     :                     :
              |         ------------|---------------------|------------
              |        |            |                     |            |
              |        |            |    Multicast Data   |  IP(S,G)   |
              |        |            |       IP(S,G)     10|<--------() |
              |        |  IP(S,G) 11|<====================|            |
              |        | ()<--------|                     |            |
              |        |            |                     |            |
              :         ------------:---------------------:------------
              |       Expired       |                     |
     (QT)-------------------------->|12      Request      |
              |                    1|-------------------->|
              |                     |  Membership Query   |2
              |                     |       Q(0,{})       |
              |        Start       3|<--------------------|
     (QT)<--------------------------|                     |
              |       Q(0,{})       |                     |
              |<--------------------|                     |
             4| R({G:INCLUDE({S})}) |  Membership Update  |
              |-------------------->|5 R({G:INCLUDE({S})})|
              |                     |====================>|6b
   Leave(S,G) :                     :                     :
   ()-------->|7 R({G:BLOCK({S})})  |  Membership Update  |
              |-------------------->|8  R({G:BLOCK({S})}) |
              |                     |====================>|9b Prune(S,G)
              |                     |                     |---------->()
              :                     :                     :

        Figure 7: Membership Update Sequence (IGMPv3/MLDv2 Example)



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   The following sequence describes how the Request, Membership Query,
   and Membership Update messages are used to report current group
   membership state or changes in group membership state:

   1.   A gateway sends a Request message to the relay that contains a
        random nonce and a flag indicating whether the relay should
        return an IGMPv3 or MLDv2 General Query.

   2.   When the relay receives a Request message, it generates a
        message authentication code (MAC), typically, by computing a
        hash digest from the message source IP address, source UDP port,
        request nonce, and a private secret.  The relay then sends a
        Membership Query message to the gateway that contains the
        request nonce, the MAC, and an IGMPv3 or MLDv2 General Query.

   3.   When the gateway receives a Membership Query message, it
        verifies that the request nonce matches the one sent in the last
        Request, and if it does, the gateway saves the request nonce and
        MAC for use in sending subsequent Membership Update messages.
        The gateway starts a timer whose expiration will trigger the
        transmission of a new Request message and extracts the
        encapsulated General Query message for processing by the IGMP or
        MLD protocol.  The query timer duration is specified by the
        relay in the Querier's Query Interval Code (QQIC) field in the
        IGMPv3 or MLDv2 General Query.  The QQIC field is defined in
        Section 4.1.7 of [RFC3376] and Section 5.1.9 of [RFC3810]).

   4.   The gateway's IGMP or MLD protocol implementation processes the
        General Query to produce a current-state report.

   5.   When an IGMP or MLD report is passed to the pseudo-interface,
        the gateway encapsulates the report in a Membership Update
        message and sends it to the relay.  The request nonce and MAC
        fields in the Membership Update are assigned the values from the
        last Membership Query message received for the corresponding
        group membership protocol (IGMPv3 or MLDv2).

   6.   When the relay receives a Membership Update message, it computes
        a MAC from the message source IP address, source UDP port,
        request nonce, and a private secret.  The relay accepts the
        Membership Update message if the received MAC matches the
        computed MAC; otherwise, the message is ignored.  If the message
        is accepted, the relay may proceed to allocate, refresh, or
        modify tunnel state.  This includes making any group membership,







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        routing, and forwarding state changes, and also issuing any
        upstream protocol requests required to satisfy the state change.
        The diagram illustrates two scenarios:

        A.  The gateway has not previously reported any group
            subscriptions and the report does not contain any group
            subscriptions, so the relay takes no action.

        B.  The gateway has previously reported a group subscription, so
            the current-state report lists all current subscriptions.
            The relay responds by refreshing tunnel or group state and
            resetting any related timers.

   7.   A receiver indicates to the gateway that it wishes to join
        (allow) or leave (block) specific multicast traffic.  This
        request is typically made using some form of IGMP/MLD service
        interface (as described in Section 2 of [RFC3376] and Section 3
        of [RFC3810]).  The IGMP/MLD protocol responds by generating an
        IGMP or MLD state-change message.

   8.   When an IGMP or MLD report/leave/done message is passed to the
        pseudo-interface, the gateway encapsulates the message in a
        Membership Update message and sends it to the relay.  The
        request nonce and MAC fields in the Membership Update are
        assigned the values from the last Membership Query message
        received for the corresponding group membership protocol (IGMP
        or MLD).

        The IGMP and MLD protocols may generate multiple messages to
        provide robustness against packet loss -- each of these must be
        encapsulated in a new Membership Update message and sent to the
        relay.  The Querier's Robustness Variable (QRV) field in the
        last IGMP/MLD query delivered to the IGMP/MLD protocol is
        typically used to specify the number of repetitions (i.e., the
        host adopts the QRV value as its own Robustness Variable value).
        The QRV field is defined in Section 4.1.6 of [RFC3376] and
        Section 5.1.8 of [RFC3810].

   9.   When the relay receives a Membership Update message, it again
        computes a MAC from the message source IP address, source UDP
        port, request nonce, and a private secret.  The relay accepts
        the Membership Update message if the received MAC matches the
        computed MAC; otherwise, the message is ignored.  If the message
        is accepted, the relay processes the encapsulated IGMP/MLD and
        allocates, modifies, or deletes tunnel state accordingly.  This
        includes making any group membership, routing, and forwarding





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        state changes, and also issuing any upstream protocol requests
        required to satisfy the state change.  The diagram illustrates
        two scenarios:

        A.  The gateway wishes to add a group subscription.

        B.  The gateway wishes to delete a previously reported group
            subscription.

   10.  Multicast datagrams transmitted from a source travel through the
        native multicast infrastructure to the relay.  When the relay
        receives a multicast IP datagram that carries a source and
        destination address for which a gateway has expressed an
        interest in receiving (via the Membership Update message), it
        encapsulates the datagram into a Multicast Data message and
        sends it to the gateway using the source IP address and UDP port
        carried by the Membership Update message as the destination
        address.

   11.  When the gateway receives a Multicast Data message, it extracts
        the multicast packet from the message and passes it on to the
        appropriate receivers.

   12.  When the query timer expires, the gateway sends a new Request
        message to the relay to start a new membership update cycle.

   The MAC-based source-authentication mechanism described above
   provides a simple defense against malicious attempts to exhaust relay
   resources via source-address spoofing.  Flooding a relay with spoofed
   Request or Membership Update messages may consume computational
   resources and network bandwidth but will not result in the allocation
   of state, because the Request message is stateless and spoofed
   Membership Update messages will fail source authentication and be
   rejected by the relay.

   A relay will only allocate new tunnel state if the IGMP/MLD report
   carried by the Membership Update message creates one or more group
   subscriptions.

   A relay deallocates tunnel state after one of the following events:
   the gateway sends a Membership Update message containing a report
   that results in the deletion of all remaining group subscriptions,
   the IGMP/MLD state expires (due to lack of refresh by the gateway),
   or the relay receives a valid Teardown message from the gateway (see
   Section 4.2.1.3).






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   A gateway that accepts or reports group subscriptions for both IPv4
   and IPv6 addresses will send separate Request and Membership Update
   messages for each protocol (IPv4/IGMP and IPv6/MLD).

4.2.1.3.  Teardown Sequence

   A gateway sends a Teardown message to a relay to request that it stop
   delivering Multicast Data messages to a tunnel endpoint created by an
   earlier Membership Update message.  This message is intended to be
   used following a gateway address change (see Section 4.2.2.1) to stop
   the transmission of undeliverable or duplicate Multicast Data
   messages.  Gateway support for the Teardown message is RECOMMENDED.
   Gateways are not required to send them and may instead rely on group
   membership to expire on the relay.





































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                      Gateway                  Relay
                      -------                  -----
                         :        Request        :
                     [1] |           N           |
                         |---------------------->|
                         |    Membership Query   | [2]
                         |    N,MAC,gADDR,gPORT  |
                         |<======================|
                     [3] |   Membership Update   |
                         |   ({G:INCLUDE({S})})  |
                         |======================>|
                         |                       |
    ---------------------:-----------------------:---------------------
   |                     |                       |                     |
   |                     |    *Multicast Data    |  *IP Packet(S,G)    |
   |                     |      gADDR,gPORT      |<-----------------() |
   |    *IP Packet(S,G)  |<======================|                     |
   | ()<-----------------|                       |                     |
   |                     |                       |                     |
    ---------------------:-----------------------:---------------------
                         ~                       ~
                         ~        Request        ~
                     [4] |           N'          |
                         |---------------------->|
                         |   Membership Query    | [5]
                         | N',MAC',gADDR',gPORT' |
                         |<======================|
                     [6] |                       |
                         |       Teardown        |
                         |   N,MAC,gADDR,gPORT   |
                         |---------------------->|
                         |                       | [7]
                         |   Membership Update   |
                         |  ({G:INCLUDE({S})})   |
                         |======================>|
                         |                       |
    ---------------------:-----------------------:---------------------
   |                     |                       |                     |
   |                     |    *Multicast Data    |  *IP Packet(S,G)    |
   |                     |     gADDR',gPORT'     |<-----------------() |
   |    *IP Packet (S,G) |<======================|                     |
   | ()<-----------------|                       |                     |
   |                     |                       |                     |
    ---------------------:-----------------------:---------------------
                         |                       |
                         :                       :

        Figure 8: Teardown Message Sequence (IGMPv3/MLDv2 Example)



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   The following sequence describes how the Membership Query and
   Teardown messages are used to detect an address change and stop the
   delivery of Multicast Data messages to an address:

   1.  A gateway sends a Request message containing a random nonce to
       the relay.

   2.  The relay sends a Membership Query message to the gateway that
       contains the source IP address (gADDR) and source UDP port
       (gPORT) values from the Request message.  These values will be
       used to identify the tunnel should one be created by a subsequent
       Membership Update message.

   3.  When the gateway receives a Membership Query message that carries
       the gateway address fields, it compares the gateway IP address
       and UDP port number values with those received in the previous
       Membership Query (if any).  If these values do not match, this
       indicates that the Request message arrived at the relay carrying
       a different source address than the one sent previously.  At this
       point in the sequence, no change in source address or port has
       occurred.

   4.  The gateway sends a new Request message to the relay.  However,
       this Request message arrives at the relay carrying a different
       source address than that of the previous Request due to some
       change in network interface, address assignment, network
       topology, or NAT mapping.

   5.  The relay again responds by sending a Membership Query message to
       the gateway that contains the new source IP address (gADDR') and
       source UDP port (gPORT') values from the Request message.

   6.  When the gateway receives the Membership Query message, it
       compares the gateway address and port number values against those
       returned in the previous Membership Query message.

   7.  If the reported address or port has changed, the gateway sends a
       Teardown message to the relay that contains the request nonce,
       MAC, gateway IP address, and gateway port number returned in the
       earlier Membership Query message.  The gateway may send the
       Teardown message multiple times where the number of repetitions
       is governed by the Querier's Robustness Variable (QRV) value
       contained in the IGMPv3/MLDv2 General Query carried by the
       original Membership Query (see Section 4.1.6 of [RFC3376] and
       Section 5.1.8 of [RFC3810]).  The gateway continues to process
       the new Membership Query message as usual.





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   8.  When the relay receives a Teardown message, it computes a MAC
       from the message source IP address, source UDP port, request
       nonce, and a private secret.  The relay accepts the Teardown
       message if the received MAC matches the computed MAC; otherwise,
       the message is ignored.  If the message is accepted, the relay
       makes any group membership, routing, and forwarding state changes
       required to stop the transmission of Multicast Data messages to
       that address.

4.2.1.4.  Timeout and Retransmission

   The AMT protocol does not establish any requirements regarding what
   actions a gateway should take if it fails to receive a response from
   a relay.  A gateway implementation may wait for an indefinite period
   of time to receive a response, may set a time limit on how long to
   wait for a response, may retransmit messages should the time limit be
   reached, may limit the number of retransmissions, or may simply
   report an error.

   For example, a gateway may retransmit a Request message if it fails
   to receive a Membership Query or expected Multicast Data messages
   within some time period.  If the gateway fails to receive any
   response to a Request after several retransmissions or within some
   maximum period of time, it may reenter the relay discovery phase in
   an attempt to find a new relay.  This topic is addressed in more
   detail in Section 5.2.

4.2.2.  Tunneling

   From the standpoint of a relay, an AMT "tunnel" is identified by the
   IP address and UDP port pair used as the destination address for
   sending encapsulated multicast IP datagrams to a gateway.  In this
   document, we refer to this address as the tunnel endpoint address.

   A gateway sends a Membership Update message to a relay to add or
   remove group subscriptions to a tunnel endpoint.  The tunnel endpoint
   is identified by the source IP address and source UDP port carried by
   the Membership Update message when it arrives at a relay (this
   address may differ from that carried by the message when it exited
   the gateway as a result of network address translation).

   The Membership Update messages sent by a single gateway host may
   originate from several source addresses or ports -- each unique
   combination represents a unique tunnel endpoint.  A single gateway
   host may legitimately create and accept traffic on multiple tunnel
   endpoints, e.g., the gateway may use separate ports for the IPv4/IGMP
   and IPv6/MLD protocols.




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   A tunnel is "created" when a gateway sends a Membership Update
   message containing an IGMP or MLD membership report that creates one
   or more group subscriptions when none currently existed for that
   tunnel endpoint address.

   A tunnel ceases to exist when all group subscriptions for a tunnel
   endpoint are deleted.  This may occur as a result of the following
   events:

   o  The gateway sends an IGMP or MLD report, leave, or done message to
      the relay that deletes the last group subscription linked to the
      tunnel endpoint.

   o  The gateway sends a Teardown message to the relay that causes it
      to delete any and all subscriptions bound to the tunnel endpoint.

   o  The relay stops receiving updates from the gateway until such time
      that per-group or per-tunnel timers expire, causing the relay to
      delete the subscriptions.

   The tunneling approach described above conceptually transforms a
   unicast-only internetwork into an NBMA link layer, over which
   multicast traffic may be delivered.  Each relay, plus the set of all
   gateways using the relay, together may be thought of as being on a
   separate logical NBMA link, where the "link layer" address is a UDP/
   IP address-port pair provided by the Membership Update message.

4.2.2.1.  Address Roaming

   As described above, each time a relay receives a Membership Update
   message from a new source address-port pair, the group subscriptions
   described by that message apply to the tunnel endpoint identified by
   that address.

   This can cause problems for a gateway if the address carried by the
   messages it sends to a relay changes unexpectedly.  These changes may
   cause the relay to transmit duplicate, undeliverable, or unrequested
   traffic back towards the gateway or an intermediate device.  This may
   create congestion and have negative consequences for the gateway, its
   network, or multicast receivers and in some cases may also produce a
   significant amount of ICMP traffic directed back towards the relay by
   a NAT, router, or gateway host.









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   There are several scenarios in which the address carried by messages
   sent by a gateway may change without that gateway's knowledge -- for
   example, when:

   o  The message originates from a different interface on a gateway
      that possesses multiple interfaces.

   o  The DHCP assignment for a gateway interface changes.

   o  The gateway roams to a different wireless network.

   o  The address mapping applied by an intervening network-translation
      device (NAT) changes as a result of mapping expiration or routing
      changes in a multihomed network.

   In the case where the address change occurs between the transmission
   of a Request message and subsequent Membership Update messages, the
   relay will simply ignore any Membership Update messages from the new
   address because MAC authentication will fail (see Section 4.2.1.2).
   The relay may continue to transmit previously requested traffic, but
   no duplication will occur, i.e., the possibility for the delivery of
   duplicate traffic does not arise until a Request message is received
   from the new address.

   The protocol provides a method for a gateway to detect an address
   change and explicitly request that the relay stop sending traffic to
   a previous address.  This process involves the Membership Query and
   Teardown messages and is described in Section 4.2.1.3.

4.2.2.2.  Network Address Translation

   The messages sent by a gateway to a relay may be subject to network
   address translation (NAT) -- the source IP address and UDP port
   carried by an IP packet sent by the gateway may be modified multiple
   times before arriving at the relay.  In the most restrictive form of
   NAT, the NAT device will create a new mapping for each combination of
   source and destination IP address and UDP port.  In this case,
   bidirectional communication can only be conducted by sending outgoing
   packets to the source address and port carried by the last incoming
   packet.











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            Membership Update                 Membership Update
            src: iADDR:iPORT                  src: eADDR:ePORT
            dst: rADDR:rPORT                  dst: rADDR:rPORT
                               +---------+
                               |   NAT   |
        +---------+           +-----------+          +---------+
        |         |---------->|           |--------->|         |
        | Gateway |           |  Mapping  |          |  Relay  |
        |         |<----------|           |<---------|         |
        +---------+           +-----------+          +---------+
                               |         |
                               +---------+
            Multicast Data                    Multicast Data
            src: rADDR:rPORT                  src: rADDR:rPORT
            dst: iADDR:iPORT                  dst: eADDR:ePORT

               Figure 9: Network Address Translation in AMT

   AMT provides automatic NAT traversal by using the source IP address
   and UDP port carried by the Membership Update message as received at
   the relay as the destination address for any Multicast Data messages
   the relay sends back as a result.

   The NAT mapping created by a Membership Update message will
   eventually expire unless it is refreshed by a passing message.  This
   refresh will occur each time the gateway performs the periodic update
   required to refresh group state within the relay (see
   Section 4.2.1.2).

4.2.2.3.  UDP Encapsulation

                Gateway                              Relay

           IP:IGMP                                       IP:IGMP
              |    AMT:IP:IGMP               AMT:IP:IGMP    |
              |         |                         |         |
              |         |   IP:UDP:AMT:IP:IGMP    |         |
    _______   |   ___   |   ______   |   ______   |   ___   |   _______
   |IGMP|IP|  v  |AMT|  v  |UDP|IP|  v  |IP|UDP|  v  |AMT|  v  |IP|IGMP|
   |    |  |     |   |     |   |  |     |  |   |     |   |     |  |    |
   |    |<------------------------------------------------------->|    |
   |____|  |     |   |     |   |  |     |  |   |     |   |     |  |____|
   |       |<--------------------------------------------------|       |
   |_______|  ^  |___|  ^  |___|__|  ^  |__|___|  ^  |___|  ^  |_______|
              |         |            |            |         |
             IP      AMT:IP    IP:UDP:AMT:IP    AMT:IP      IP

                       Figure 10: AMT Encapsulation



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   The IGMP and MLD messages used in AMT are exchanged as complete IP
   datagrams.  These IP datagrams are encapsulated in AMT messages that
   are transmitted using UDP.  The same holds true for multicast traffic
   -- each multicast IP datagram or datagram fragment that arrives at
   the relay is encapsulated in an AMT message and transmitted to one or
   more gateways via UDP.

   The IP protocol of the encapsulated packets need not match the IP
   protocol used to send the AMT messages.  AMT messages sent via IPv4
   may carry IPv6/MLD packets, and AMT messages sent via IPv6 may carry
   IPv4/IGMP packets.

   The Checksum field contained in the UDP header of the messages
   requires special consideration.  Of primary concern is the cost of
   computing a checksum on each replicated multicast packet after it is
   encapsulated for delivery to a gateway.  Many routing/forwarding
   platforms do not possess the capability to compute checksums on
   UDP-encapsulated packets, as they may not have access to the entire
   datagram.

   To avoid placing an undue burden on the relay platform, the protocol
   specifically allows zero-valued UDP checksums on the Multicast Data
   messages.  This is not an issue in UDP over IPv4, as the UDP Checksum
   field may be set to zero.  However, this is a problem for UDP over
   IPv6, as that protocol requires a valid, non-zero checksum in UDP
   datagrams [RFC2460].  Messages sent over IPv6 with a UDP checksum of
   zero may fail to reach the gateway.  This is a well-known issue for
   UDP-based tunneling protocols and is described in [RFC6936].  A
   recommended solution is described in [RFC6935].

4.2.2.4.  UDP Fragmentation

   Naive encapsulation of multicast IP datagrams within AMT data
   messages may produce UDP datagrams that might require fragmentation
   if their size exceeds the MTU of the network path between the relay
   and a gateway.  Many multicast applications, especially those related
   to media streaming, are designed to deliver independent data samples
   in separate packets, without fragmentation, to ensure that some
   number of complete samples can be delivered even in the presence of
   packet loss.  To prevent or reduce undesirable fragmentation, the AMT
   protocol describes specific procedures for handling multicast
   datagrams whose encapsulation might exceed the Path MTU.  These
   procedures are described in Section 5.3.3.6.








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5.  Protocol Description

   This section provides a normative description of the AMT protocol.

5.1.  Protocol Messages

   The AMT protocol defines seven message types for control and
   encapsulation.  These messages are assigned the following names and
   numeric identifiers:

                  +--------------+---------------------+
                  | Message Type | Message Name        |
                  +--------------+---------------------+
                  |      1       | Relay Discovery     |
                  |      2       | Relay Advertisement |
                  |      3       | Request             |
                  |      4       | Membership Query    |
                  |      5       | Membership Update   |
                  |      6       | Multicast Data      |
                  |      7       | Teardown            |
                  +--------------+---------------------+

   These messages are exchanged as IPv4 or IPv6 UDP datagrams.

5.1.1.  Relay Discovery

   A Relay Discovery message is used to solicit a response from a relay
   in the form of a Relay Advertisement message.

   The UDP/IP datagram containing this message MUST carry a valid,
   non-zero UDP checksum and carry the following IP address and UDP port
   values:

   Source IP Address - The IP address of the gateway interface on which
      the gateway will listen for a relay response.  Note: The value of
      this field may be changed as a result of network address
      translation before arriving at the relay.

   Source UDP Port - The UDP port number on which the gateway will
      listen for a relay response.  Note: The value of this field may be
      changed as a result of network address translation before arriving
      at the relay.

   Destination IP Address - An anycast or unicast IP address, i.e., the
      Relay Discovery Address advertised by a relay.

   Destination UDP Port - The AMT port number (see Section 7.2).




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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  V=0  |Type=1 |     Reserved                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Discovery Nonce                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 11: Relay Discovery Message Format

5.1.1.1.  Version (V)

   The protocol version number for this message is 0.

5.1.1.2.  Type

   The type number for this message is 1.

5.1.1.3.  Reserved

   Reserved bits that MUST be set to zero by the gateway and ignored by
   the relay.

5.1.1.4.  Discovery Nonce

   A 32-bit random value generated by the gateway and echoed by the
   relay in a Relay Advertisement message.  This value is used by the
   gateway to correlate Relay Advertisement messages with Relay
   Discovery messages.  Discovery nonce generation is described in
   Section 5.2.3.4.5.

5.1.2.  Relay Advertisement

   The Relay Advertisement message is used to supply a gateway with a
   unicast IP address of a relay.  A relay sends this message to a
   gateway when it receives a Relay Discovery message from that gateway.

   The UDP/IP datagram containing this message MUST carry a valid,
   non-zero UDP checksum and carry the following IP address and UDP port
   values:

   Source IP Address - The destination IP address carried by the Relay
      Discovery message (i.e., the Relay Discovery Address advertised by
      the relay).

   Source UDP Port - The destination UDP port carried by the Relay
      Discovery message (i.e., the AMT port number).




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   Destination IP Address - The source IP address carried by the Relay
      Discovery message.  Note: The value of this field may be changed
      as a result of network address translation before arriving at the
      gateway.

   Destination UDP Port - The source UDP port carried by the Relay
      Discovery message.  Note: The value of this field may be changed
      as a result of network address translation before arriving at the
      gateway.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  V=0  |Type=2 |                   Reserved                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Discovery Nonce                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                  Relay Address (IPv4 or IPv6)                 ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 12: Relay Advertisement Message Format

5.1.2.1.  Version (V)

   The protocol version number for this message is 0.

5.1.2.2.  Type

   The type number for this message is 2.

5.1.2.3.  Reserved

   Reserved bits that MUST be set to zero by the relay and ignored by
   the gateway.

5.1.2.4.  Discovery Nonce

   A 32-bit value copied from the Discovery Nonce field
   (Section 5.1.1.4) contained in the Relay Discovery message.  The
   gateway uses this value to match a Relay Advertisement to a Relay
   Discovery message.








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5.1.2.5.  Relay Address

   The unicast IPv4 or IPv6 address of the relay.  A gateway uses the
   length of the UDP datagram containing the Relay Advertisement message
   to determine the address family, i.e., length - 8 = 4 (IPv4) or 16
   (IPv6).  The relay returns an IP address for the protocol used to
   send the Relay Discovery message, i.e., an IPv4 address for an IPv4
   Relay Discovery Address or an IPv6 address for an IPv6 Relay
   Discovery Address.

5.1.3.  Request

   A gateway sends a Request message to a relay to solicit a Membership
   Query response.

   The successful delivery of this message marks the start of the first
   stage in the three-way handshake used to create or update state
   within a relay.

   The UDP/IP datagram containing this message MUST carry a valid,
   non-zero UDP checksum and carry the following IP address and UDP port
   values:

   Source IP Address - The IP address of the gateway interface on which
      the gateway will listen for a response from the relay.  Note: The
      value of this field may be changed as a result of network address
      translation before arriving at the relay.

   Source UDP Port - The UDP port number on which the gateway will
      listen for a response from the relay.  Note: The value of this
      field may be changed as a result of network address translation
      before arriving at the relay.

   Destination IP Address - The unicast IP address of the relay.

   Destination UDP Port - The AMT port number.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  V=0  |Type=3 |   Reserved  |P|            Reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Request Nonce                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 13: Request Message Format





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5.1.3.1.  Version (V)

   The protocol version number for this message is 0.

5.1.3.2.  Type

   The type number for this message is 3.

5.1.3.3.  Reserved

   Reserved bits that MUST be set to zero by the gateway and ignored by
   the relay.

5.1.3.4.  P Flag

   The P flag is set to indicate which group membership protocol the
   gateway wishes the relay to use in the Membership Query response:

   Value   Meaning

     0     The relay MUST respond with a Membership Query message that
           contains an IPv4 packet carrying an IGMPv3 General Query
           message.
     1     The relay MUST respond with a Membership Query message that
           contains an IPv6 packet carrying an MLDv2 General Query
           message.

5.1.3.5.  Request Nonce

   A 32-bit random value generated by the gateway and echoed by the
   relay in a Membership Query message.  This value is used by the relay
   to compute the Response MAC value and is used by the gateway to
   correlate Membership Query messages with Request messages.  Request
   Nonce generation is described in Section 5.2.3.5.6.

5.1.4.  Membership Query

   A relay sends a Membership Query message to a gateway to solicit a
   Membership Update response, but only after receiving a Request
   message from the gateway.

   The successful delivery of this message to a gateway marks the start
   of the second stage in the three-way handshake used to create or
   update tunnel state within a relay.







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   The UDP/IP datagram containing this message MUST carry a valid,
   non-zero UDP checksum and carry the following IP address and UDP port
   values:

   Source IP Address - The destination IP address carried by the Request
      message (i.e., the unicast IP address of the relay).

   Source UDP Port - The destination UDP port carried by the Request
      message (i.e., the AMT port number).

   Destination IP Address - The source IP address carried by the Request
      message.  Note: The value of this field may be changed as a result
      of network address translation before arriving at the gateway.

   Destination UDP Port - The source UDP port carried by the Request
      message.  Note: The value of this field may be changed as a result
      of network address translation before arriving at the gateway.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  V=0  |Type=4 | Reserved  |L|G|         Response MAC          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Request Nonce                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |               Encapsulated General Query Message              |
   ~                 IPv4:IGMPv3(Membership Query)                 ~
   |                  IPv6:MLDv2(Listener Query)                   |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Gateway Port Number       |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   +                                                               +
   |                Gateway IP Address (IPv4 or IPv6)              |
   +                                                               +
   |                                                               |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 14: Membership Query Message Format






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5.1.4.1.  Version (V)

   The protocol version number for this message is 0.

5.1.4.2.  Type

   The type number for this message is 4.

5.1.4.3.  Reserved

   Reserved bits that MUST be set to zero by the relay and ignored by
   the gateway.

5.1.4.4.  Limit (L) Flag

   A 1-bit flag set to 1 to indicate that the relay is NOT accepting
   Membership Update messages from new gateway tunnel endpoints and that
   it will ignore any that are.  A value of 0 has no special
   significance -- the relay may or may not be accepting Membership
   Update messages from new gateway tunnel endpoints.  A gateway checks
   this flag before attempting to create new group subscription state on
   the relay to determine whether it should restart relay discovery.  A
   gateway that has already created group subscriptions on the relay may
   ignore this flag.  Support for this flag is RECOMMENDED.

5.1.4.5.  Gateway Address (G) Flag

   A 1-bit flag set to 0 to indicate that the message does NOT carry the
   Gateway Port Number and Gateway IP Address fields, and 1 to indicate
   that it does.  A relay implementation that supports the optional
   teardown procedure (see Section 5.3.3.5) SHOULD set this flag as well
   as the Gateway Port Number and Gateway IP Address field values.  If a
   relay sets this flag, it MUST also include the Gateway Port Number
   and Gateway IP Address fields in the message.  A gateway
   implementation that does not support the optional teardown procedure
   (see Section 5.2.3.7) MAY ignore this flag and the Gateway Address
   fields if they are present.

5.1.4.6.  Response MAC

   A 48-bit source authentication value generated by the relay as
   described in Section 5.3.5.  The gateway echoes this value in
   subsequent Membership Update messages to allow the relay to verify
   that the sender of a Membership Update message was the intended
   receiver of a Membership Query sent by the relay.






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5.1.4.7.  Request Nonce

   A 32-bit value copied from the Request Nonce field (Section 5.1.3.5)
   carried by a Request message.  The relay will have included this
   value in the Response MAC computation.  The gateway echoes this value
   in subsequent Membership Update messages.  The gateway also uses this
   value to match a Membership Query to a Request message.

5.1.4.8.  Encapsulated General Query Message

   An IP-encapsulated IGMP or MLD message generated by the relay.  This
   field will contain one of the following IP datagrams:

      IPv4:IGMPv3 Membership Query

      IPv6:MLDv2 Listener Query

   The source address carried by the query message should be set as
   described in Section 5.3.3.3.

   The Querier's Query Interval Code (QQIC) field in the General Query
   is used by a relay to specify the time offset a gateway should use to
   schedule a new three-way handshake to refresh the group membership
   state within the relay (current time + Query Interval).  The QQIC
   field is defined in Section 4.1.7 of [RFC3376] and Section 5.1.9 of
   [RFC3810].

   The Querier's Robustness Variable (QRV) field in the General Query is
   used by a relay to specify the number of times a gateway should
   retransmit unsolicited membership reports, encapsulated within
   Membership Update messages, and, optionally, the number of times to
   send a Teardown message.  The QRV field is defined in Section 4.1.6
   of [RFC3376] and Section 5.1.8 of [RFC3810].

5.1.4.9.  Gateway Address Fields

   The Gateway Port Number and Gateway Address fields are present in the
   Membership Query message if, and only if, the G flag is set.

   A gateway need not parse the encapsulated IP datagram to determine
   the position of these fields within the UDP datagram containing the
   Membership Query message -- if the G flag is set, the gateway may
   simply subtract the total length of the fields (18 bytes) from the
   total length of the UDP datagram to obtain the offset.







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5.1.4.9.1.  Gateway Port Number

   A 16-bit UDP port number containing a UDP port value.

   The relay sets this field to the value of the UDP source port of the
   Request message that triggered the Query message.

5.1.4.9.2.  Gateway IP Address

   A 16-byte IP address that, when combined with the value contained in
   the Gateway Port Number field, forms the gateway endpoint address
   that the relay will use to identify the tunnel instance, if any,
   created by a subsequent Membership Update message.  This field may
   contain an IPv6 address or an IPv4 address stored as an
   IPv4-compatible IPv6 address, where the IPv4 address is prefixed with
   96 bits set to zero (see [RFC4291]).  This address must match that
   used by the relay to compute the value stored in the Response MAC
   field.

5.1.5.  Membership Update

   A gateway sends a Membership Update message to a relay to report a
   change in group membership state, or to report the current group
   membership state in response to receiving a Membership Query message.
   The gateway encapsulates the IGMP or MLD message as an IP datagram
   within a Membership Update message and sends it to the relay, where
   it may (see below) be decapsulated and processed by the relay to
   update group membership and forwarding state.

   A gateway cannot send a Membership Update message until it receives a
   Membership Query from a relay, because the gateway must copy the
   Request Nonce and Response MAC values carried by a Membership Query
   into any subsequent Membership Update messages it sends back to that
   relay.  These values are used by the relay to verify that the sender
   of the Membership Update message was the recipient of the Membership
   Query message from which these values were copied.

   The successful delivery of this message to the relay marks the start
   of the final stage in the three-way handshake.  This stage concludes
   when the relay successfully verifies that the sender of the
   Membership Update message was the recipient of a Membership Query
   message sent earlier.  At this point, the relay may proceed to
   process the encapsulated IGMP or MLD message to create or update
   group membership and forwarding state on behalf of the gateway.







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   The UDP/IP datagram containing this message MUST carry a valid,
   non-zero UDP checksum and carry the following IP address and UDP port
   values:

   Source IP Address - The IP address of the gateway interface on which
      the gateway will listen for Multicast Data messages from the
      relay.  The address must be the same address used to send the
      initial Request message, or the message will be ignored.  Note:
      The value of this field may be changed as a result of network
      address translation before arriving at the relay.

   Source UDP Port - The UDP port number on which the gateway will
      listen for Multicast Data messages from the relay.  This port must
      be the same port used to send the initial Request message, or the
      message will be ignored.  Note: The value of this field may be
      changed as a result of network address translation before arriving
      at the relay.

   Destination IP Address - The unicast IP address of the relay.

   Destination UDP Port - The AMT port number.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  V=0  |Type=5 |  Reserved     |        Response MAC           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Request Nonce                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |         Encapsulated Group Membership Update Message          |
   ~           IPv4:IGMP(Membership Report|Leave Group)            ~
   |            IPv6:MLD(Listener Report|Listener Done)            |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 15: Membership Update Message Format

5.1.5.1.  Version (V)

   The protocol version number for this message is 0.

5.1.5.2.  Type

   The type number for this message is 5.




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5.1.5.3.  Reserved

   Reserved bits that MUST be set to zero by the gateway and ignored by
   the relay.

5.1.5.4.  Response MAC

   A 48-bit value copied from the Response MAC field (Section 5.1.4.6)
   in a Membership Query message.  Used by the relay to perform source
   authentication.

5.1.5.5.  Request Nonce

   A 32-bit value copied from the Request Nonce field in a Request or
   Membership Query message.  Used by the relay to perform source
   authentication.

5.1.5.6.  Encapsulated Group Membership Update Message

   An IP-encapsulated IGMP or MLD message produced by the host-mode IGMP
   or MLD protocol running on a gateway pseudo-interface.  This field
   will contain one of the following IP datagrams:

      IPv4:IGMPv2 Membership Report

      IPv4:IGMPv2 Leave Group

      IPv4:IGMPv3 Membership Report

      IPv6:MLDv1 Multicast Listener Report

      IPv6:MLDv1 Multicast Listener Done

      IPv6:MLDv2 Multicast Listener Report

   The source address carried by the message should be set as described
   in Section 5.2.1.

5.1.6.  Multicast Data

   A relay sends a Multicast Data message to deliver a multicast IP
   datagram or datagram fragment to a gateway.

   The Checksum field in the UDP header of this message MAY contain a
   value of zero when sent over IPv4 but SHOULD, if possible, contain a
   valid, non-zero value when sent over IPv6 (see Section 4.2.2.3).





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   The UDP/IP datagram containing this message MUST carry the following
   IP address and UDP port values:

   Source IP Address - The unicast IP address of the relay.

   Source UDP Port - The AMT port number.

   Destination IP Address - A tunnel endpoint IP address, i.e., the
      source IP address carried by the Membership Update message sent by
      a gateway to indicate an interest in receiving the multicast
      packet.  Note: The value of this field may be changed as a result
      of network address translation before arriving at the gateway.

   Destination UDP Port - A tunnel endpoint UDP port, i.e., the source
      UDP port carried by the Membership Update message sent by a
      gateway to indicate an interest in receiving the multicast packet.
      Note: The value of this field may be changed as a result of
      network address translation before arriving at the gateway.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  V=0  |Type=6 |    Reserved   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   ~                     IP Multicast Packet                       ~
   |                                                               |
   +                - - - - - - - - - - - - - - - - - - - - - - - -+
   |               :               :               :               :
   +-+-+-+-+-+-+-+-+- - - - - - - - - - - - - - - - - - - - - - - -

                 Figure 16: Multicast Data Message Format

5.1.6.1.  Version (V)

   The protocol version number for this message is 0.

5.1.6.2.  Type

   The type number for this message is 6.

5.1.6.3.  Reserved

   Reserved bits that MUST be set to zero by the relay and ignored by
   the gateway.






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5.1.6.4.  IP Multicast Data

   A complete IPv4 or IPv6 multicast datagram or datagram fragment.

5.1.7.  Teardown

   A gateway sends a Teardown message to a relay to request that it stop
   sending Multicast Data messages to a tunnel endpoint created by an
   earlier Membership Update message.  A gateway sends this message when
   it detects that a Request message sent to the relay carries an
   address that differs from that carried by a previous Request message.
   The gateway uses the Gateway IP Address and Gateway Port Number
   fields in the Membership Query message to detect these address
   changes.

   To provide backwards compatibility with early implementations of the
   AMT protocol, support for this message and associated procedures is
   considered OPTIONAL -- gateways are not required to send this
   message, and relays are not required to act upon it.

   The UDP/IP datagram containing this message MUST carry a valid,
   non-zero UDP checksum and carry the following IP address and UDP port
   values:

   Source IP Address - The IP address of the gateway interface used to
      send the message.  This address may differ from that used to send
      earlier messages.  Note: The value of this field may be changed as
      a result of network address translation before arriving at the
      relay.

   Source UDP Port - The UDP port number.  This port number may differ
      from that used to send earlier messages.  Note: The value of this
      field may be changed as a result of network address translation
      before arriving at the relay.

   Destination IP Address - The unicast IP address of the relay.

   Destination UDP Port - The AMT port number.













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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  V=0  |Type=7 |  Reserved     |         Response MAC          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Request Nonce                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Gateway Port Number       |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   +                                                               +
   |              Gateway IP Address (IPv4 or IPv6)                |
   +                                                               +
   |                                                               |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 17: Membership Teardown Message Format

5.1.7.1.  Version (V)

   The protocol version number for this message is 0.

5.1.7.2.  Type

   The type number for this message is 7.

5.1.7.3.  Reserved

   Reserved bits that MUST be set to zero by the gateway and ignored by
   the relay.

5.1.7.4.  Response MAC

   A 48-bit value copied from the Response MAC field (Section 5.1.4.6)
   in the last Membership Query message the relay sent to the gateway
   endpoint address of the tunnel to be torn down.  The gateway endpoint
   address is provided by the Gateway IP Address and Gateway Port Number
   fields carried by the Membership Query message.  The relay validates
   the Teardown message by comparing this value with one computed from
   the Gateway IP Address field, Gateway Port Number field, Request
   Nonce field, and a private secret (just as it does in the Membership
   Update message).





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5.1.7.5.  Request Nonce

   A 32-bit value copied from the Request Nonce field (Section 5.1.4.7)
   in the last Membership Query message the relay sent to the gateway
   endpoint address of the tunnel to be torn down.  The gateway endpoint
   address is provided by the Gateway IP Address and Gateway Port Number
   fields carried by the Membership Query message.  This value must
   match that used by the relay to compute the value stored in the
   Response MAC field.

5.1.7.6.  Gateway Port Number

   A 16-bit UDP port number that, when combined with the value contained
   in the Gateway IP Address field, forms the tunnel endpoint address
   that the relay will use to identify the tunnel instance to tear down.
   The relay provides this value to the gateway using the Gateway Port
   Number field (Section 5.1.4.9.1) in a Membership Query message.  This
   port number must match that used by the relay to compute the value
   stored in the Response MAC field.

5.1.7.7.  Gateway IP Address

   A 16-byte IP address that, when combined with the value contained in
   the Gateway Port Number field, forms the tunnel endpoint address that
   the relay will use to identify the tunnel instance to tear down.  The
   relay provides this value to the gateway using the Gateway IP Address
   field (Section 5.1.4.9.2) in a Membership Query message.  This field
   may contain an IPv6 address or an IPv4 address stored as an
   IPv4-compatible IPv6 address, where the IPv4 address is prefixed with
   96 bits set to zero (see [RFC4291]).  This address must match that
   used by the relay to compute the value stored in the Response MAC
   field.

5.2.  Gateway Operation

   The following sections describe gateway implementation requirements.
   A non-normative discussion of gateway operation may be found in
   Section 4.2.

5.2.1.  IP/IGMP/MLD Protocol Requirements

   Gateway operation requires a subset of host-mode IPv4/IGMP and IPv6/
   MLD functionality to provide group membership tracking, query
   processing, and report generation.  A gateway MAY use IGMPv2 (ASM),
   IGMPv3 (ASM and SSM), MLDv1 (ASM), or MLDv2 (ASM and SSM).






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   An application with embedded gateway functionality must provide its
   own implementation of this subset of the IPv4/IGMP and IPv6/MLD
   protocols.  The service interface used to manipulate group membership
   state need not match that described in the IGMP and MLD
   specifications, but the actions taken as a result SHOULD be similar
   to those described in Section 5.1 of [RFC3376] and Section 6.1 of
   [RFC3810].  The gateway application will likely need to implement
   many of the same functions as a host IP stack, including checksum
   verification, dispatching, datagram filtering and forwarding, and IP
   encapsulation/decapsulation.

   The encapsulated IGMP datagrams generated by a gateway MUST conform
   to the descriptions found in Section 4 of [RFC3376].  These datagrams
   MUST possess the IP headers, header options, and header values called
   for in [RFC3376], with the following exception: a gateway MAY use any
   source address value in an IGMP report datagram, including the
   "unspecified" address (all octets are zero).  This exception is made
   because a gateway pseudo-interface might not possess a valid IPv4
   address, and even if an address has been assigned to the interface,
   that address might not be a valid link-local source address on any
   relay interface.  It is for this reason that a relay must accept
   encapsulated IGMP reports regardless of the source address they
   carry.  See Section 5.3.1.

   The encapsulated MLD messages generated by a gateway MUST conform to
   the description found in Section 5 of [RFC3810].  These datagrams
   MUST possess the IP headers, header options, and header values called
   for in [RFC3810], with the following exception: a gateway MAY use any
   source address value in an MLD report datagram, including the
   "unspecified" address (all octets are zero).  This exception is made
   because a gateway pseudo-interface might not possess a valid IPv6
   address, and even if an address has been assigned to the interface,
   that address might not be a valid link-local source address on any
   relay interface.  As with IGMP, it is for this reason that a relay
   must accept encapsulated MLD reports regardless of the source address
   they carry.  See Section 5.3.1.

   The gateway IGMP/MLD implementation SHOULD retransmit unsolicited
   membership state-change reports and merge new state-change reports
   with pending reports as described in Section 5.1 of [RFC3376] and
   Section 6.1 of [RFC3810].  The number of retransmissions is specified
   by the relay in the Querier's Robustness Variable (QRV) field in the
   last General Query forwarded by the pseudo-interface.  See
   Section 4.1.6 of [RFC3376] and Section 5.1.8 of [RFC3810].







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   The gateway IGMP/MLD implementation SHOULD handle General Query
   messages as described in Section 5.2 of [RFC3376] and Section 6.2 of
   [RFC3810] but MAY ignore the Max Resp Code (Maximum Response Code)
   field value and generate a current-state report without any delay.

   An IPv4 gateway implementation MUST accept IPv4 datagrams that carry
   the General Query variant of the IGMPv3 Membership Query message, as
   described in Section 4 of [RFC3376].  The gateway MUST accept the
   IGMP datagram regardless of the IP source address carried by that
   datagram.

   An IPv6 gateway implementation MUST accept IPv6 datagrams that carry
   the General Query variant of the MLDv2 Multicast Listener Query
   message, as described in Section 5 of [RFC3810].  The gateway MUST
   accept the MLD datagram regardless of the IP source address carried
   by that datagram.

5.2.2.  Pseudo-Interface Configuration

   A gateway host may possess or create multiple gateway
   pseudo-interfaces, each with a unique configuration that describes a
   binding to a specific IP protocol, Relay Address, Relay Discovery
   Address, or upstream network interface.

5.2.2.1.  Relay Discovery Address

   If a gateway implementation uses AMT relay discovery to obtain a
   Relay Address, it must first be supplied with a Relay Discovery
   Address.  The Relay Discovery Address may be an anycast or unicast
   address.  A gateway implementation may rely on a static address
   assignment or some form of dynamic address discovery.  This
   specification does not require that a gateway implementation use any
   particular method to obtain a Relay Discovery Address -- an
   implementation may employ any method that returns a suitable Relay
   Discovery Address.

5.2.2.2.  Relay Address

   Before a gateway implementation can execute the AMT protocol to
   request and receive multicast traffic, it must be supplied with a
   unicast Relay Address.  A gateway implementation may rely on static
   address assignment or support some form of dynamic address discovery.
   This specification does not require the use of any particular method
   to obtain a Relay Address -- an implementation may employ any method
   that returns a suitable Relay Address.






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5.2.2.3.  Upstream Interface Selection

   A gateway host that possesses multiple network interfaces or
   addresses may allow for an explicit selection of the interface to use
   when communicating with a relay.  The selection might be made to
   satisfy connectivity, tunneling, or IP protocol requirements.

5.2.2.4.  Optional Retransmission Parameters

   A gateway implementation that supports retransmission MAY require the
   following information:

   Discovery Timeout
      Initial time to wait for a response to a Relay Discovery message.

   Maximum Relay Discovery Retransmission Count
      Maximum number of Relay Discovery retransmissions to allow before
      terminating relay discovery and reporting an error.

   Request Timeout
      Initial time to wait for a response to a Request message.

   Maximum Request Retransmission Count
      Maximum number of Request retransmissions to allow before
      abandoning a relay and restarting relay discovery or reporting an
      error.

   Maximum Retries Count for "Destination Unreachable"
      The maximum number of times a gateway should attempt to send the
      same Request or Membership Update message after receiving an ICMP
      Destination Unreachable message.

5.2.3.  Gateway Service

   In the following descriptions, a gateway pseudo-interface is treated
   as a passive entity managed by a gateway service.  The gateway
   pseudo-interface provides the state, and the gateway service provides
   the processing.  The term "gateway" is used when describing service
   behavior with respect to a single pseudo-interface.

5.2.3.1.  Startup

   When a gateway pseudo-interface is started, the gateway service
   begins listening for AMT messages sent to the UDP endpoint(s)
   associated with the pseudo-interface and for any locally generated
   IGMP/MLD messages passed to the pseudo-interface.  The handling of
   these messages is described below.




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   When the pseudo-interface is enabled, the gateway service MAY:

   o  Optionally execute the relay discovery procedure described in
      Section 5.2.3.4.

   o  Optionally execute the membership query procedure described in
      Section 5.2.3.5 to start the periodic membership update cycle.

5.2.3.2.  Handling AMT Messages

   A gateway MUST ignore any datagram it receives that cannot be
   interpreted as a Relay Advertisement, Membership Query, or Multicast
   Data message.  The handling of Relay Advertisement, Membership Query,
   and Multicast Data messages is addressed in the sections that follow.

   A gateway that conforms to this specification MUST ignore any message
   with a Version field value other than zero.

   While listening for AMT messages, a gateway may be notified that an
   ICMP Destination Unreachable message was received as a result of an
   AMT message transmission.  Handling of ICMP Destination Unreachable
   messages is described in Section 5.2.3.9.

5.2.3.3.  Handling Multicast Data Messages

   A gateway may receive Multicast Data messages after it sends a
   Membership Update message to a relay that adds a group subscription.
   The gateway may continue to receive Multicast Data messages long
   after the gateway sends a Membership Update message that deletes
   existing group subscriptions.  The gateway MUST be prepared to
   receive these messages at any time but MAY ignore them or discard
   their contents if the gateway no longer has any interest in receiving
   the multicast datagrams contained within them.

   A gateway MUST ignore a Multicast Data message if it fails to satisfy
   any of the following requirements:

   o  The source IP address and UDP port carried by the Multicast Data
      message MUST be equal to the destination IP address and UDP port
      carried by the matching Membership Update message (i.e., the
      current Relay Address).

   o  The destination address carried by the encapsulated IP datagram
      MUST fall within the multicast address allocation assigned to the
      relevant IP protocol, i.e., 224.0.0.0/4 for IPv4 and ff00::/8
      for IPv6.





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   The gateway extracts the encapsulated IP datagram and forwards it to
   the local IP protocol implementation for checksum verification,
   fragmented datagram reassembly, source and group filtering, and
   transport-layer protocol processing.

   Because AMT uses UDP encapsulation to deliver multicast datagrams to
   gateways, it qualifies as a tunneling protocol subject to the
   limitations described in [RFC6936].  If supported, a gateway SHOULD
   employ the solution described in [RFC6936] to ensure that the local
   IP stack does not discard IPv6 datagrams with zero checksums.  If
   Multicast Data message datagrams are processed directly within the
   gateway (instead of the host IP stack), the gateway MUST NOT discard
   any of these datagrams because they carry a UDP checksum of zero.

5.2.3.4.  Relay Discovery Procedure

   This section describes gateway requirements related to the relay
   discovery message sequence described in Section 4.2.1.1.

5.2.3.4.1.  Starting Relay Discovery

   A gateway may start or restart the relay discovery procedure in
   response to the following events:

   o  When a gateway pseudo-interface is started (enabled).

   o  When the gateway wishes to report a group subscription when none
      currently exist.

   o  Before sending the next Request message in a membership update
      cycle, i.e., each time the query timer expires (see below).

   o  After the gateway fails to receive a response to a Request
      message.

   o  After the gateway receives a Membership Query message with the
      L flag set to 1.

5.2.3.4.2.  Sending a Relay Discovery Message

   A gateway sends a Relay Discovery message to a relay to start the
   relay discovery process.

   The gateway MUST send the Relay Discovery message using the current
   Relay Discovery Address and AMT port number as the destination.  The
   Discovery Nonce value in the Relay Discovery message MUST be computed
   as described in Section 5.2.3.4.5.




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   The gateway MUST save a copy of the Relay Discovery message or save
   the Discovery Nonce value for possible retransmission and
   verification of a Relay Advertisement response.

   When a gateway sends a Relay Discovery message, it may be notified
   that an ICMP Destination Unreachable message was received as a result
   of an earlier AMT message transmission.  Handling of ICMP Destination
   Unreachable messages is described in Section 5.2.3.9.

5.2.3.4.3.  Waiting for a Relay Advertisement Message

   A gateway MAY retransmit a Relay Discovery message if it does not
   receive a matching Relay Advertisement message within some timeout
   period.  If the gateway retransmits the message multiple times, the
   timeout period SHOULD be adjusted to provide a random exponential
   back-off.  The RECOMMENDED timeout is a random value in the range
   [initial_timeout, MIN(initial_timeout * 2^retry_count,
   maximum_timeout)], with a RECOMMENDED initial_timeout of 1 second and
   a RECOMMENDED maximum_timeout of 120 seconds (which is the
   recommended minimum NAT mapping timeout described in [RFC4787]).

5.2.3.4.4.  Handling a Relay Advertisement Message

   When a gateway receives a Relay Advertisement message, it must first
   determine whether it should accept or ignore the message.  A gateway
   MUST ignore a Relay Advertisement message if it fails to satisfy any
   of the following requirements:

   o  The gateway MUST be waiting for a Relay Advertisement message.

   o  The Discovery Nonce value contained in the Relay Advertisement
      message MUST be equal to the Discovery Nonce value contained in
      the Relay Discovery message.

   o  The source IP address and UDP port of the Relay Advertisement
      message MUST be equal to the destination IP address and UDP port
      of the matching Relay Discovery message.

   Once a gateway receives a Relay Advertisement response to a Relay
   Discovery message, it SHOULD ignore any other Relay Advertisements
   that arrive on the AMT interface until it sends a new Relay Discovery
   message.

   If a gateway executes the relay discovery procedure at the start of
   each membership update cycle and the Relay Address returned in the
   latest Relay Advertisement message differs from the address returned
   in a previous Relay Advertisement message, then the gateway SHOULD
   send a Teardown message (if supported) to the old Relay Address,



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   using information from the last Membership Query message received
   from that relay, as described in Section 5.2.3.7.  This behavior is
   illustrated in the following diagram.

                     Gateway              Relay-1
                     -------              -------
                        :                    :
     Query      Expired |                    |
     Timer (QT)-------->|                    |
                        |  Relay Discovery   |
                        |------------------->|
                        |                    |
                        | Relay Advertisement|
                        |<-------------------|
                        |                    |
                        |      Request       |
                        |------------------->|
                        |                    |
                        |  Membership Query  |
                        |<===================|
                  Start |                    |
           (QT)<--------| Membership Update  |
                        |===================>|
                        |                    |
                        ~                    ~             Relay-2
                Expired |                    |             -------
           (QT)-------->|                    |                :
                        |  Relay Discovery   |                |
                        |------------------------------------>|
                        |                    |                |
                        | Relay Advertisement|                |
                        |<------------------------------------|
                        |                    |                |
                        |     Teardown       |                |
                        |------------------->|                |
                        |                    |                |
                        |      Request       |                |
                        |------------------------------------>|
                        |                    |                |
                        |  Membership Query  |                |
                        |<====================================|
                  Start |                    |                |
           (QT)<--------| Membership Update  |                |
                        |====================================>|
                        |                    |                |
                        :                    :                :

              Figure 18: Teardown after Relay Address Change



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5.2.3.4.5.  Discovery Nonce Generation

   The discovery nonce MUST be a random, non-zero 32-bit value and, if
   possible, SHOULD be computed using a cryptographically secure
   pseudorandom number generator.  A new nonce SHOULD be generated each
   time the gateway restarts the relay discovery process.  The same
   nonce SHOULD be used when retransmitting a Relay Discovery message.

5.2.3.5.  Membership Query Procedure

   This section describes gateway requirements related to the membership
   update message sequence described in Section 4.2.1.2.

5.2.3.5.1.  Starting the Membership Update Cycle

   A gateway may send a Request message to start a membership update
   cycle (following the optional relay discovery procedure) in response
   to the following events:

   o  When the gateway pseudo-interface is activated.

   o  When the gateway wishes to report a group subscription when none
      currently exist.

   Starting the membership update cycle when a gateway pseudo-interface
   is started provides several benefits:

   o  Better performance by allowing state-change reports to be sent as
      they are generated, thus minimizing the time to join.

   o  More robustness by relying on unsolicited state-change reports to
      update group membership state rather than the current-state
      reports generated by the membership update cycle.  Unsolicited
      state-change reports are typically retransmitted multiple times
      while current-state reports are not.

   o  Simplified implementation by eliminating any need to queue IGMP/
      MLD messages for delivery after a Membership Query is received,
      since the IGMP/MLD state-change messages may be sent as they are
      generated.

   However, this approach places an additional load on relays, as a
   gateway will send periodic requests even when it has no multicast
   subscriptions.  To reduce load on a relay, a gateway SHOULD only send
   a Membership Update message while it has active group subscriptions.
   A relay will still need to compute a Response MAC for each Request





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   but will not be required to recompute it a second time to
   authenticate a Membership Update message that contains no
   subscriptions.

5.2.3.5.2.  Sending a Request Message

   A gateway sends a Request message to a relay to solicit a Membership
   Query response and start the membership update cycle.

   A gateway constructs a Request message containing a Request Nonce
   value computed as described in Section 5.2.3.5.6.  The gateway MUST
   set the P flag in the Request message to identify the protocol the
   gateway wishes the relay to use for the General Query response.

   A gateway MUST send a Request message using the current Relay Address
   and AMT port number as the destination.

   A gateway MUST save a copy of the Request message or save the Request
   Nonce and P flag values for possible retransmission and verification
   of a Membership Query response.

   When a gateway sends a Request message, it may be notified that an
   ICMP Destination Unreachable message was received as a result of an
   earlier AMT message transmission.  Handling of ICMP Destination
   Unreachable messages is described in Section 5.2.3.9.

5.2.3.5.3.  Waiting for a Membership Query Message

   A gateway MAY retransmit a Request message if it does not receive a
   matching Membership Query message within some timeout period.  If the
   gateway retransmits the message multiple times, the timeout period
   SHOULD be adjusted to provide a random exponential back-off.  The
   RECOMMENDED timeout is a random value in the range [initial_timeout,
   MIN(initial_timeout * 2^retry_count, maximum_timeout)], with a
   RECOMMENDED initial_timeout of 1 second and a RECOMMENDED
   maximum_timeout of 120 seconds (which is the recommended minimum NAT
   mapping timeout described in [RFC4787]).

   If a gateway that uses relay discovery does not receive a Membership
   Query within a specified time period or after a specified number of
   retries, the gateway SHOULD stop waiting for a Membership Query
   message and restart relay discovery to locate another relay.









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5.2.3.5.4.  Handling a Membership Query Message

   When a gateway receives a Membership Query message, it must first
   determine whether it should accept or ignore the message.  A gateway
   MUST ignore a Membership Query message, or the encapsulated IP
   datagram within it, if the message fails to satisfy any of the
   following requirements:

   o  The gateway MUST be waiting for a Membership Query message.

   o  The Request Nonce value contained in the Membership Query MUST
      equal the Request Nonce value contained in the Request message.

   o  The source IP address and UDP port of the Membership Query MUST
      equal the destination IP address and UDP port of the matching
      Request message (i.e., the current Relay Address).

   o  The encapsulated IP datagram MUST carry an IGMPv3 or MLDv2
      message.  The protocol MUST match the protocol identified by the
      P flag in the Request message.

   o  The IGMPv3 or MLDv2 message MUST be a General Query message.

   o  The total length of the encapsulated IP datagram as computed from
      the lengths contained in the datagram header(s) MUST NOT exceed
      the available field length within the Membership Query message.

   Once a gateway receives a Membership Query response to a Request
   message, it SHOULD ignore any other Membership Query messages that
   arrive on the AMT interface until it sends a new Request message.

   The gateway MUST save the Membership Query message, or the Request
   Nonce, Response MAC, Gateway IP Address, and Gateway Port Number
   fields for use in sending subsequent Membership Update and Teardown
   messages.

   The gateway extracts the encapsulated IP datagram and forwards it to
   the local IP protocol implementation for checksum verification and
   dispatching to the IGMP or MLD implementation running on the
   pseudo-interface.  The gateway MUST NOT forward any octets that might
   exist between the encapsulated IP datagram and the end of the message
   or Gateway Address fields.

   The MLD protocol specification indicates that senders should use a
   link-local source IP address in message datagrams.  This requirement
   must be relaxed for AMT because gateways and relays do not normally
   share a common subnet.  For this reason, a gateway implementation
   MUST accept MLD (and IGMP) query message datagrams regardless of the



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   source IP address they carry.  This may require additional processing
   on the part of the gateway that might be avoided if the relay and
   gateway use the IPv4 and IPv6 addresses allocated for use in
   AMT-encapsulated control packets as described in Section 5.2.1.

   The gateway MUST start a timer that will trigger the next iteration
   of the membership update cycle by executing the membership query
   procedure.  The gateway SHOULD compute the timer duration from the
   Querier's Query Interval Code carried by the General Query.  A
   gateway MAY use a smaller timer duration if required to refresh a NAT
   mapping that would otherwise time out.  A gateway MAY use a larger
   timer duration if it has no group subscriptions to report.

   If the gateway supports the Teardown message and the G flag is set in
   the Membership Query message, the gateway MUST compare the Gateway IP
   Address and Gateway Port Number on the new Membership Query message
   with the values carried by the previous Membership Query message.  If
   either value has changed, the gateway MUST send a Teardown message to
   the relay as described in Section 5.2.3.7.

   If the L flag is set in the Membership Query message, the relay is
   reporting that it is NOT accepting Membership Update messages that
   create new tunnel endpoints and will simply ignore any that do.  If
   the L flag is set and the gateway is not currently reporting any
   group subscriptions to the relay, the gateway SHOULD stop sending
   periodic Request messages and restart the relay discovery procedure
   (if discovery is enabled) to find a new relay with which to
   communicate.  Even if the L flag is set, the gateway MAY continue to
   send updates if it has previously reported group subscriptions to the
   relay, one or more subscriptions still exist, and the gateway
   endpoint address has not changed since the last Membership Query was
   received (see previous paragraph).

5.2.3.5.5.  Handling Query Timer Expiration

   When the query timer (started in the previous step) expires, the
   gateway should execute the membership query procedure again to
   continue the membership update cycle.

5.2.3.5.6.  Request Nonce Generation

   The Request Nonce MUST be a random value and, if possible, SHOULD be
   computed using a cryptographically secure pseudorandom number
   generator.  A new nonce MUST be generated each time the gateway
   starts the membership query process.  The same nonce SHOULD be used
   when retransmitting a Request message.





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5.2.3.6.  Membership Update Procedure

   This section describes gateway requirements related to the membership
   update message sequence described in Section 4.2.1.2.

   The membership update process is primarily driven by the host-mode
   IGMP or MLD protocol implementation running on the gateway
   pseudo-interface.  The IGMP and MLD protocols produce current-state
   reports in response to General Query messages generated by the
   pseudo-interface via AMT and produce state-change reports in response
   to receiver requests made using the IGMP or MLD service interface.

5.2.3.6.1.  Handling an IGMP/MLD IP Datagram

   The gateway pseudo-interface MUST accept the following IP datagrams
   from the IPv4/IGMP and IPv6/MLD protocols running on the
   pseudo-interface:

   o  IPv4 datagrams that carry an IGMPv2 or IGMPv3 Membership Report or
      an IGMPv2 Leave Group message as described in Section 4 of
      [RFC3376].

   o  IPv6 datagrams that carry an MLDv1 or MLDv2 Multicast Listener
      Report or an MLDv1 Multicast Listener Done message as described in
      Section 5 of [RFC3810].

   The gateway must be prepared to receive these messages any time the
   pseudo-interface is running.  The gateway MUST ignore any datagrams
   not listed above.

   A gateway that waits to start a membership update cycle until after
   it receives a datagram containing an IGMP/MLD state-change message
   MAY:

   o  Discard IGMP or MLD datagrams until it receives a Membership Query
      message, at which time it processes the Membership Query message
      as normal to eventually produce a current-state report on the
      pseudo-interface, which describes the end state (RECOMMENDED).

   o  Insert IGMP or MLD datagrams into a queue for transmission after
      it receives a Membership Query message.

   If and when a gateway receives a Membership Query message (for IGMP
   or MLD), it sends any queued or incoming IGMP or MLD datagrams to the
   relay as described in the next section.






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5.2.3.6.2.  Sending a Membership Update Message

   A gateway cannot send a Membership Update message to a relay until it
   has received a Membership Query message from a relay.  If the gateway
   has not yet located a relay with which to communicate, it MUST first
   execute the relay discovery procedure described in Section 5.2.3.4 to
   obtain a Relay Address.  If the gateway has a Relay Address but has
   not yet received a Membership Query message, it MUST first execute
   the membership query procedure described in Section 5.2.3.5 to obtain
   a Request Nonce and Response MAC that can be used to send a
   Membership Update message.

   Once a gateway possesses a valid Relay Address, Request Nonce, and
   Response MAC, it may encapsulate the IP datagram containing the IGMP/
   MLD message into a Membership Update message.  The gateway MUST copy
   the Request Nonce and Response MAC values from the last Membership
   Query received from the relay into the corresponding fields in the
   Membership Update.  The gateway MUST send the Membership Update
   message using the Relay Address and AMT port number as the
   destination.

   When a gateway sends a Membership Update message, it may be notified
   that an ICMP Destination Unreachable message was received as a result
   of an earlier AMT message transmission.  Handling of ICMP Destination
   Unreachable messages is described in Section 5.2.3.9.

5.2.3.7.  Teardown Procedure

   This section describes gateway requirements related to the teardown
   message sequence described in Section 4.2.1.3.

   Gateway support for the Teardown message is RECOMMENDED.

   A gateway that supports Teardown SHOULD make use of Teardown
   functionality if it receives a Membership Query message from a relay
   that has the G flag set to indicate that it contains valid Gateway
   Address fields.

5.2.3.7.1.  Handling a Membership Query Message

   As described in Section 5.2.3.5.4, if a gateway supports the Teardown
   message, has reported active group subscriptions, and receives a
   Membership Query message with the G flag set, the gateway MUST
   compare the Gateway IP Address and Gateway Port Number on the new
   Membership Query message with the values carried by the previous
   Membership Query message.  If either value has changed, the gateway
   MUST send a Teardown message as described in the next section.




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5.2.3.7.2.  Sending a Teardown Message

   A gateway sends a Teardown message to a relay to request that it stop
   delivering Multicast Data messages to the gateway and delete any
   group memberships created by the gateway.

   When a gateway constructs a Teardown message, it MUST copy the
   Request Nonce, Response MAC, Gateway IP Address, and Gateway Port
   Number fields from the Membership Query message that provided the
   Response MAC for the last Membership Update message sent, into the
   corresponding fields of the Teardown message.

   A gateway MUST send the Teardown message using the Relay Address and
   AMT port number as the destination.  A gateway MAY send the Teardown
   message multiple times for robustness.  The gateway SHOULD use the
   Querier's Robustness Variable (QRV) field contained in the query
   encapsulated within the last Membership Query to set the limit on the
   number of retransmissions (see Section 4.1.6 of [RFC3376] and
   Section 5.1.8 of [RFC3810]).  If the gateway sends the Teardown
   message multiple times, it SHOULD insert a delay between each
   transmission using the timing algorithm employed in IGMP/MLD for
   transmitting unsolicited state-change reports.  The RECOMMENDED
   default delay value is 1 second.

   When a gateway sends a Teardown message, it may be notified that an
   ICMP Destination Unreachable message was received as a result of an
   earlier AMT message transmission.  Handling of ICMP Destination
   Unreachable messages is described in Section 5.2.3.9.

5.2.3.8.  Shutdown

   When a gateway pseudo-interface is stopped and the gateway has
   existing group subscriptions, the gateway SHOULD either:

   o  Send a Teardown message to the relay as described in
      Section 5.2.3.7, but only if the gateway supports the Teardown
      message and the current relay is returning Gateway Address fields
      in Membership Query messages, or

   o  Send a Membership Update message to the relay that will delete
      existing group subscriptions.

5.2.3.9.  Handling ICMP Destination Unreachable Responses

   A gateway may receive an ICMP Destination Unreachable message
   [RFC0792] after sending an AMT message.  Whether the gateway is
   notified that an ICMP message was received is highly dependent on
   firewall and gateway IP stack behavior and gateway implementation.



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   If the reception of an ICMP Destination Unreachable message is
   reported to the gateway while waiting to receive an AMT message, the
   gateway may respond as follows, depending on platform capabilities
   and which outgoing message triggered the ICMP response:

   1.  The gateway MAY simply abandon the current relay and restart
       relay discovery (if used).  This is the least desirable approach,
       as it does not allow for transient network changes.

   2.  If the last message sent was a Relay Discovery or Request
       message, the gateway MAY simply ignore the ICMP response and
       continue waiting for incoming AMT messages.  If the gateway is
       configured to retransmit Relay Discovery or Request messages, the
       normal retransmission behavior for those messages is preserved to
       prevent the gateway from prematurely abandoning a relay.

   3.  If the last message sent was a Membership Update message, the
       gateway MAY start a new membership update and associated Request
       retransmission cycle.

   If the reception of an ICMP Destination Unreachable message is
   reported to the gateway when attempting to transmit a new AMT
   message, the gateway may respond as follows, depending on platform
   capabilities and which outgoing message triggered the ICMP response:

   1.  The gateway MAY simply abandon the current relay and restart
       relay discovery (if used).  This is the least desirable approach,
       as it does not allow for transient network changes.

   2.  If the last message sent was a Relay Discovery, Request, or
       Teardown message, the gateway MAY attempt to transmit the new
       message.  If the gateway is configured to retransmit Relay
       Discovery, Request, or Teardown messages, the normal
       retransmission behavior for those messages is preserved to
       prevent the gateway from prematurely abandoning a relay.

   3.  If the last message sent was a Membership Update message, the
       gateway SHOULD start a new membership update and associated
       Request retransmission cycle.












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5.3.  Relay Operation

   The following sections describe relay implementation requirements.  A
   non-normative discussion of relay operation may be found in
   Section 4.2.

5.3.1.  IP/IGMP/MLD Protocol Requirements

   A relay requires a subset of router-mode IGMP and MLD functionality
   to provide group membership tracking and report processing.

   A relay accessible via IPv4 MUST support IPv4/IGMPv3 and MAY support
   IPv6/MLDv2.  A relay accessible via IPv6 MUST support IPv6/MLDv2 and
   MAY support IPv4/IGMPv3.

   A relay MUST apply the forwarding rules described in Section 6.3 of
   [RFC3376] and Section 7.3 of [RFC3810].

   A relay MUST handle incoming reports as described in Section 6.4 of
   [RFC3376] and Section 7.4 of [RFC3810], with the exception that
   actions that lead to queries MAY be modified to eliminate query
   generation.  A relay MUST accept IGMP and MLD report datagrams
   regardless of the IP source address carried by those datagrams.

   All other aspects of IGMP/MLD router behavior, such as the handling
   of queries, querier election, etc., are not used or required for
   relay operation.

5.3.2.  Startup

   If a relay is deployed for anycast discovery, the relay MUST
   advertise an anycast Relay Discovery Address Prefix into the unicast
   routing system of the anycast domain.  An address within that prefix,
   i.e., a Relay Discovery Address, MUST be assigned to a relay
   interface.

   A unicast IPv4 and/or IPv6 address MUST be assigned to the relay
   interface that will be used to send and receive AMT control and data
   messages.  This address or addresses are returned in Relay
   Advertisement messages.

   The remaining details of relay "startup" are highly implementation
   dependent and are not addressed in this document.








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5.3.3.  Running

   When a relay is started, it begins listening for AMT messages on the
   interface to which the unicast Relay Address(es) has been assigned,
   i.e., the address returned in Relay Advertisement messages.

5.3.3.1.  Handling AMT Messages

   A relay MUST ignore any message other than a Relay Discovery,
   Request, Membership Update, or Teardown message.  The handling of
   Relay Discovery, Request, Membership Update, and Teardown messages is
   addressed in the sections that follow.

   Support for the Teardown message is OPTIONAL.  If a relay does not
   support the Teardown message, it MUST also ignore this message.

   A relay that conforms to this specification MUST ignore any message
   with a Version field value other than zero.

5.3.3.2.  Handling a Relay Discovery Message

   This section describes relay requirements related to the relay
   discovery message sequence described in Section 4.2.1.1.

   A relay MUST accept and respond to Relay Discovery messages sent to
   an anycast Relay Discovery Address or the unicast Relay Address.  If
   a relay receives a Relay Discovery message sent to its unicast
   address, it MUST respond just as it would if the message had been
   sent to its anycast Relay Discovery Address.

   When a relay receives a Relay Discovery message, it responds by
   sending a Relay Advertisement message back to the source of the Relay
   Discovery message.

   The relay MUST use the source IP address and UDP port number of the
   Relay Discovery message as the destination IP address and UDP port
   number for the Relay Advertisement message.  The source IP address
   and UDP port number carried by the Relay Advertisement message MUST
   match the destination IP address and UDP port number of the Relay
   Discovery message to ensure successful NAT traversal.

   The relay MUST copy the value contained in the Discovery Nonce field
   of the Relay Discovery message into the Discovery Nonce field in the
   Relay Advertisement message.







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   If the Relay Discovery message was received as an IPv4 datagram, the
   relay MUST return an IPv4 address in the Relay Address field of the
   Relay Advertisement message.  If the Relay Discovery message was
   received as an IPv6 datagram, the relay MUST return an IPv6 address
   in the Relay Address field.

5.3.3.3.  Handling a Request Message

   This section describes relay requirements related to the membership
   query portion of the message sequence described in Section 4.2.1.2.

   When a relay receives a Request message, it responds by sending a
   Membership Query message back to the source of the Request message.

   The relay MUST use the source IP address and UDP port of the Request
   message as the destination IP address and UDP port for the Membership
   Query message.  The source IP address and UDP port carried by the
   Membership Query MUST match the destination IP address and UDP port
   of the Request to ensure successful NAT traversal.

   The relay MUST return the value contained in the Request Nonce field
   of the Request message in the Request Nonce field of the Membership
   Query message.  The relay MUST compute a MAC value, as described in
   Section 5.3.5, and return that value in the Response MAC field of the
   Membership Query message.

   If a relay supports the Teardown message, it MUST set the G flag in
   the Membership Query message and return the source IP address and UDP
   port carried by the Request message in the corresponding Gateway IP
   Address and Gateway Port Number fields.  If the relay does not
   support the Teardown message, it SHOULD NOT set these fields, as this
   may cause the gateway to generate unnecessary Teardown messages.

   If the P flag in the Request message is 0, the relay MUST return an
   IPv4-encapsulated IGMPv3 General Query in the Membership Query
   message.  If the P flag is 1, the relay MUST return an
   IPv6-encapsulated MLDv2 General Query in the Membership Query
   message.

   If the relay is not accepting Membership Update messages that create
   new tunnel endpoints due to resource limitations, it SHOULD set the
   L flag in the Membership Query message to notify the gateway of this
   state.  Support for the L flag is OPTIONAL.  See Section 5.3.3.8.








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   The encapsulated IGMPv3 General Query datagrams generated by a relay
   MUST conform to the descriptions found in Section 4.1 of [RFC3376].
   These datagrams MUST possess the IP headers, header options, and
   header values called for in [RFC3376], with the following exception:
   a relay MAY use any source IP address for an IGMP General Query
   datagram, including the "unspecified" address (all octets are zero).
   This exception is made because any source address that a relay might
   normally send may not be a valid link-local address on any gateway
   interface.  It is for this reason that a gateway must accept
   encapsulated IGMP queries regardless of the source address they
   carry.  See Section 5.2.1.

   The encapsulated MLDv2 General Query datagrams generated by a relay
   MUST conform to the descriptions found in Section 5.1 of [RFC3810].
   These datagrams MUST possess the IP headers, header options, and
   header values called for in [RFC3810], with the following exception:
   a relay MAY use any source IP address for an MLD General Query
   datagram, including the "unspecified" address (all octets are zero).
   This exception is made because any source address that a relay might
   normally send may not be a valid link-local address on any gateway
   interface.  As with IGMP, it is for this reason that a gateway must
   accept encapsulated MLD queries regardless of the source address they
   carry.  See Section 5.2.1.

   A relay MUST set the Querier's Query Interval Code (QQIC) field in
   the General Query to supply the gateway with a suggested time
   duration to use for the membership query timer.  The QQIC field is
   defined in Section 4.1.7 of [RFC3376] and Section 5.1.9 of [RFC3810].
   A relay MAY adjust this value to affect the rate at which the Request
   messages are sent from a gateway.  However, a gateway is allowed to
   use a shorter duration than the duration specified in the QQIC field,
   so a relay may be limited in its ability to spread out Requests
   coming from a gateway.

   A relay MUST set the Querier's Robustness Variable (QRV) field in the
   General Query to a non-zero value.  This value SHOULD be greater than
   one.  If a gateway retransmits membership state-change messages, it
   will retransmit them (Robustness Variable - 1) times.  The QRV field
   is defined in Section 4.1.6 of [RFC3376] and Section 5.1.8 of
   [RFC3810].

   A relay SHOULD set the Maximum Response Code field in the General
   Query to a value of 1 to trigger an immediate response from the
   gateway (some host IGMP/MLD implementations may not accept a value of
   zero).  A relay SHOULD NOT use the IGMPv3/MLDv2 Query Response
   Interval variable, if available, to generate the Maximum Response
   Code field value, as the Query Response Interval variable is used in
   setting the duration of group state timers and must not be set to



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   such a small value.  The Maximum Response Code field is defined in
   Section 4.1.1 of [RFC3376] and Section 5.1.3 of [RFC3810].  See
   Section 5.3.3.7.

5.3.3.4.  Handling a Membership Update Message

   This section describes relay requirements related to the membership
   update portion of the message sequence described in Section 4.2.1.2.

   When a relay receives a Membership Update message, it must first
   determine whether it should accept or ignore the message.  A relay
   MUST NOT make any changes to group membership and forwarding state if
   the message fails to satisfy any of the following requirements:

   o  The IP datagram encapsulated within the message MUST be one of the
      following:

      *  IPv4 datagram carrying an IGMPv2 or IGMPv3 Membership Report
         message.

      *  IPv4 datagram carrying an IGMPv2 Leave Group message.

      *  IPv6 datagram carrying an MLDv1 or MLDv2 Multicast Listener
         Report message.

      *  IPv6 datagram carrying MLDv1 Multicast Listener Done message.

   o  The encapsulated IP datagram MUST satisfy the IP header
      requirements for the IGMP or MLD message type as described in
      Section 4 of [RFC3376], Section 2 of [RFC2236], Section 5 of
      [RFC3810], and Section 3 of [RFC2710], with the following
      exception: a relay MUST accept an IGMP or MLD message regardless
      of the IP source address carried by the datagram.

   o  The total length of the encapsulated IP datagram as computed from
      the lengths contained in the datagram header(s) MUST NOT exceed
      the available field length within the Membership Update message.

   o  The computed checksums for the encapsulated IP datagram and its
      payload MUST match the values contained therein.  Checksum
      computation and verification vary by protocol; see [RFC0791] for
      IPv4, [RFC3376] for IGMPv3, and [RFC4443] for MLD (ICMPv6).

   o  If processing of the encapsulated IGMP or MLD message would result
      in an allocation of new state or a modification of existing state,
      the relay MUST authenticate the source of the message by verifying
      that the value contained in the Response MAC field equals the MAC
      value computed from the fields in the Membership Update message



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      datagram.  If a time-varying private secret is used in the
      computation of a Response MAC, the relay MUST retain the previous
      version of the private secret for use in authenticating Membership
      Updates sent during the subsequent query interval.  If the first
      attempt at Response MAC authentication fails, the relay MUST
      attempt to authenticate the Response MAC using the previous
      private secret value unless 2 * query_interval time has elapsed
      since the private secret change.  See Section 5.3.5.

   A relay MAY skip source authentication to reduce the computational
   cost of handling Membership Update messages if the relay can make a
   trivial determination that the IGMP/MLD message carried by the
   Membership Update message will produce no changes in group membership
   or forwarding state.  The relay does not need to compute and compare
   MAC values if it finds there are no group subscriptions for the
   source of the Membership Update message and either of the following
   is true:

   o  The encapsulated IP datagram is an IGMPv3 Membership Report or
      MLDv2 Multicast Listener Report message that contains no group
      records.  This may often be the case for gateways that
      continuously repeat the membership update cycle even though they
      have no group subscriptions to report.

   o  The encapsulated IP datagram is an IGMPv2 Leave Group or MLDv1
      Multicast Listener Done message.

   The IGMP and MLD protocol specifications indicate that senders SHOULD
   use a link-local source IP address in message datagrams.  This
   requirement must be relaxed for AMT because gateways and relays do
   not share a common subnet.  For this reason, a relay implementation
   MUST accept IGMP and MLD datagrams regardless of the source IP
   address they carry.

   Once a relay has determined that the Membership Update message is
   valid, it processes the encapsulated IGMP or MLD message to update
   group membership state and communicates with the multicast protocol
   to update forwarding state and possibly send multicast protocol
   messages towards upstream routers.  The relay MUST ignore any octets
   that might exist between the encapsulated IP datagram and the end of
   the Membership Update message.

   As described in Section 4.2.2, a relay uses the source IP address and
   source UDP port carried by a Membership Update message to identify a
   tunnel endpoint.  A relay uses the tunnel endpoint as the destination
   address for any Multicast Data messages it sends as a result of the





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   group membership and forwarding state created by processing the IGMP/
   MLD messages contained in Membership Update messages received from
   the endpoint.

   If a Membership Update message originates from a new endpoint, the
   relay MUST determine whether it can accept updates from a new
   endpoint.  If a relay has been configured with a limit on the total
   number of endpoints, or a limit on the total number of endpoints for
   a given source address, then the relay MAY ignore the Membership
   Update message and possibly withdraw any Relay Discovery Address
   Prefix announcement that it might have made.  See Section 5.3.3.8.

   A relay MUST maintain some form of group membership database for each
   endpoint.  The per-endpoint databases are used to update a forwarding
   table containing entries that map a (*,G) or (S,G) subscription to a
   list of tunnel endpoints.

   A relay MUST maintain some form of group membership database
   representing a merger of the group membership databases of all
   endpoints.  The merged group membership database is used to update
   upstream multicast forwarding state.

   A relay MUST maintain a forwarding table that maps each unique (*,G)
   and (S,G) subscription to a list of tunnel endpoints.  A relay uses
   this forwarding table to provide the destination address when
   performing UDP/IP encapsulation of the incoming multicast IP
   datagrams to form Multicast Data messages.

   If a group filter mode for a group entry on a tunnel endpoint is
   EXCLUDE, the relay SHOULD NOT forward datagrams that originate from
   sources in the filter source list unless the relay architecture does
   not readily support source filtering.  A relay MAY ignore the source
   list if necessary because gateways are expected to do their own
   source filtering.

5.3.3.5.  Handling a Teardown Message

   This section describes relay requirements related to the teardown
   message sequence described in Section 4.2.1.3.

   When a relay (that supports the Teardown message) receives a Teardown
   message, it MUST first authenticate the source of the Teardown
   message by verifying that the Response MAC carried by the Teardown
   message is equal to a MAC value computed from the fields carried by
   the Teardown message.  The method used to compute the MAC differs
   from that used to generate and validate the Membership Query and
   Membership Update messages in that the source IP address and source
   UDP port number used to compute the MAC are taken from the Gateway IP



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   Address and Gateway Port Number fields in the Teardown message rather
   than from the IP and UDP headers in the datagram that carries the
   Teardown message.  The MAC computation is described in Section 5.3.5.
   A relay MUST ignore a Teardown message if the computed MAC does not
   equal the value of the Response MAC field.

   If a relay determines that a Teardown message is authentic, it MUST
   immediately stop transmitting Multicast Data messages to the endpoint
   identified by the Gateway IP Address and Gateway Port Number fields
   in the message.  The relay MUST eventually delete any group
   membership and forwarding state associated with the endpoint but MAY
   delay doing so to allow a gateway to recreate group membership state
   on a new endpoint and thereby avoid making unnecessary (temporary)
   changes in upstream routing/forwarding state.

   The state changes made by a relay when processing a Teardown message
   MUST be identical to those that would be made if the relay had
   received an IGMP/MLD report that would cause the IGMP or MLD protocol
   to delete all existing group records in the group membership database
   associated with the endpoint.  The processing of the Teardown message
   should trigger or mimic the normal interaction between IGMP or MLD
   and a multicast protocol to produce required changes in forwarding
   state and possibly send prune/leave messages towards upstream
   routers.

5.3.3.6.  Handling Multicast IP Datagrams

   When a multicast IP datagram is forwarded to the relay
   pseudo-interface, the relay MUST, for each gateway that has expressed
   an interest in receiving the datagram, encapsulate the IP datagram
   into a Multicast Data message or messages and send that message or
   messages to the gateway.  This process is highly implementation
   dependent but conceptually requires the following steps:

   o  Use the IP datagram source and destination address to look up the
      appropriate (*,G) or (S,G) entry in the endpoint forwarding table
      created for the pseudo-interface as a result of IGMP/MLD
      processing.

   o  Possibly replicate the datagram for each gateway endpoint listed
      for that (*,G) or (S,G) entry.










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   o  If the multicast IP datagram size exceeds the Tunnel MTU as
      determined according to the procedure described in
      Section 5.3.3.6.1, the relay must execute the procedure described
      in Section 5.3.3.6.2.

   o  Encapsulate and transmit the IP datagram according to the
      procedure described in Section 5.3.3.6.3.

   The relay pseudo-interface MUST ignore any other IP datagrams
   forwarded to the pseudo-interface.

5.3.3.6.1.  Path and Tunnel MTU

   A relay MUST compute a Tunnel MTU (TMTU) value for each AMT tunnel
   that originates on the relay.  A relay will use the TMTU value to
   determine whether an incoming multicast IP datagram can be delivered
   downstream in a Membership Data message without fragmentation.  A
   relay MUST compute the TMTU by subtracting the size of the Membership
   Data message headers (IP, UDP, and AMT) from the current Path MTU
   (PMTU) associated with each AMT tunnel.  The relay MUST maintain a
   PMTU value on a per-tunnel or per-relay basis.  A relay MUST support
   one or both of the following methods for determining the PMTU value:

   o  The relay MAY provide a configuration option that establishes a
      fixed PMTU that will be applied to all AMT tunnels originating at
      the relay.

   o  The relay MAY dynamically adjust PMTU value(s) in response to
      receipt of ICMP/ICMPv6 Datagram Too Big messages as described in
      [RFC1191] and [RFC1981].

   If a relay supports dynamic adjustment of per-tunnel or per-relay
   PMTU values in response to ICMP messages, the relay MUST provide a
   configuration option that disables this feature and also provide a
   configuration option that establishes a minimum PMTU for all tunnels.
   These configuration options may be used to mitigate certain types of
   denial-of-service attacks (see Section 6).  When dynamic PMTU
   adjustments are disabled, the PMTU for all tunnels MUST default to
   the Link MTU (first hop) on the downstream interface.












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5.3.3.6.2.  MTU Filtering Procedure

   This section defines procedures that a relay must execute when it
   receives a multicast datagram whose size is greater than the Tunnel
   MTU of the tunnel or tunnels through which it must be delivered.

5.3.3.6.2.1.  IPv4 Multicast IP Datagrams

   If the DF bit in the multicast datagram header is set to 1 (Don't
   Fragment), the relay MUST discard the packet and, if the datagram
   originated from an SSM source, send an ICMPv4 [RFC0792] Destination
   Unreachable message to the source, with code 4 (fragmentation needed
   and DF set).  The ICMP Destination Unreachable message MUST contain a
   Next-Hop MTU (as specified by [RFC1191]), and the relay MUST set the
   Next-Hop MTU to the TMTU associated with the tunnel or tunnels.  If
   the DF bit in the multicast datagram header is set to 0 (May
   Fragment), the relay MUST fragment the datagram and encapsulate each
   fragment within Multicast Data messages for transmission through the
   tunnel or tunnels.  This ensures that gateways will receive complete,
   non-fragmented Multicast Data messages, containing fragmented
   multicast datagram payloads.  The relay SHOULD avoid generating a
   separate ICMP message for each tunnel but instead send a single ICMP
   message with a Next-Hop MTU equal to the smallest TMTU of all tunnels
   to which the datagram was to be forwarded.

5.3.3.6.2.2.  IPv6 Multicast IP Datagrams

   The relay MUST discard the packet and, if the datagram originated
   from an SSM source, send an ICMPv6 [RFC4443] Packet Too Big message
   to the payload source.  The MTU specified in the Packet Too Big
   message MUST be equal to the TMTU associated with the tunnel or
   tunnels.  The relay SHOULD avoid generating a separate ICMPv6 message
   for each tunnel but instead send a single ICMPv6 message with a
   Next-Hop MTU equal to the smallest TMTU of all tunnels to which the
   datagram was to be forwarded.

5.3.3.6.3.  Encapsulation Procedure

   A relay encapsulates a multicast IP datagram in a UDP/IP Membership
   Data message, using the tunnel endpoint UDP/IP address as the
   destination address and the unicast Relay Address and port number as
   the source UDP/IP address.  To ensure successful NAT traversal, the
   source address and port MUST match the destination address and port
   carried by the Membership Update message sent by the gateway to
   create the forwarding table entry.






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   If possible, the relay SHOULD compute a valid, non-zero checksum for
   the UDP datagram carrying the Multicast Data message.  See
   Section 4.2.2.3.

   The following sections describe additional requirements related to
   the IP protocol of the tunnel and that of the multicast IP datagram.

5.3.3.6.3.1.  Tunneling over IPv4

   When a relay delivers an IPv4 payload over an IPv4 tunnel and the
   DF bit in the payload header is set to 1 (Don't Fragment), the relay
   MUST set the DF bit in the Multicast Data IP header to 1.  When a
   relay delivers an IPv4 payload over an IPv4 tunnel and the DF bit in
   the payload header is set to 0 (May Fragment), by default, the relay
   MUST set the DF bit in the Multicast Data IP header to 1.  However, a
   relay MAY provide a configuration option that allows the DF bit to be
   copied from the payload header to the Multicast Data IP header to
   allow downstream fragmentation of the Multicast Data message.  When a
   relay delivers an IPv6 payload over an IPv4 tunnel, the relay MUST
   set the DF bit in the Multicast Data IP header to 1.  The relay MUST
   NOT transmit a Multicast Data message with an IP header in which the
   MF (More Fragments) bit is set to 1.

5.3.3.6.3.2.  Tunneling over IPv6

   When tunneling over IPv6, a relay MUST NOT emit a Multicast Data
   message datagram containing an IPv6 fragment header.

5.3.3.6.4.  Handling Destination Unreachable Messages

   If a relay receives a sequence of ICMP or ICMPv6 Destination
   Unreachable messages (excluding ICMP code 4; see below) in response
   to transmission of a sequence of AMT Multicast Data messages to a
   gateway, the relay SHOULD discontinue sending messages to that
   gateway and shut down the tunnel for that gateway.

   Handling of ICMP Destination Unreachable messages with code 4,
   "fragmentation needed and DF set" (i.e., "Datagram Too Big") is
   covered in Section 5.3.3.6.1.  If a relay provides this capability,
   it MUST provide a configuration option that indicates what number of
   sequential Destination Unreachable messages can be received and
   ignored before the relay will automatically shut down a tunnel.









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5.3.3.7.  State Timers

   A relay MUST maintain a timer or timers whose expiration will trigger
   the removal of any group subscriptions and forwarding state
   previously created for a gateway endpoint should the gateway fail to
   refresh the group membership state within a specified time interval.

   A relay MAY use a variant of the IGMPv3/MLDv2 state management
   protocol described in Section 6 of [RFC3376] or Section 7 of
   [RFC3810] or may maintain a per-endpoint timer to trigger the
   deletion of group membership state.

   If a per-endpoint timer is used, the relay MUST restart this timer
   each time it receives a new Membership Update message from the
   gateway endpoint.

   The endpoint timer duration MAY be computed from tunable IGMP/MLD
   variables as follows:

   ((Robustness_Variable) * (Query_Interval)) + Query_Response_Interval

   If IGMP/MLD default values are used for these variables, the gateway
   will time out after 125s * 2 + 10s = 260s.  The timer duration MUST
   be greater than the query interval suggested in the last Membership
   Query message sent to the gateway endpoint.

   Regardless of the timers used (IGMPv3/MLDv2 or endpoint), the
   Query_Response_Interval value SHOULD be greater than or equal to 10s
   to allow for packet loss and round-trip time in the Request/
   Membership Query message exchange.

5.3.3.8.  Relay Resource Management

   A relay may be configured with various service limits to ensure a
   minimum level of performance for gateways that connect to it.

   If a relay has determined that it has reached or exceeded maximum
   allowable capacity or has otherwise exhausted resources required to
   support additional gateways, it SHOULD withdraw any Relay Discovery
   Address Prefix it has advertised into the unicast internetwork and
   SHOULD set the L flag in any Membership Query messages it returns to
   gateways while in this state.

   If the relay receives an update from a gateway that adds group
   membership or forwarding state for an endpoint that has already
   reached maximum allowable state entries, the relay SHOULD continue to
   accept updates from the gateway but ignore any group membership/
   forwarding state additions requested by that gateway.



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   If the relay receives an update from a gateway that would create a
   new tunnel endpoint for a source IP address that has already reached
   the maximum allowable number of endpoints (maximum UDP ports), it
   should simply ignore the Membership Update.

5.3.4.  Shutdown

   The following steps should be treated as an abstract description of
   the shutdown procedure for a relay:

   o  Withdraw the Relay Discovery Address Prefix advertisement
      (if used).

   o  Stop listening for Relay Discovery messages.

   o  Stop listening for control messages from gateways.

   o  Stop sending data messages to gateways.

   o  Delete all AMT group membership and forwarding state created on
      the relay, coordinating with the multicast routing protocol to
      update the group membership state on upstream interfaces as
      required.

5.3.5.  Response MAC Generation

   A Response MAC value is computed by the relay.  A Response MAC
   computation is required in the following situations:

   o  To generate a Response MAC value from a Request message for
      inclusion in a Membership Query message.

   o  To generate a Response MAC value from a Membership Update message
      for use in authenticating the Response MAC carried within that
      message.

   o  To generate a Response MAC value from a Teardown message to
      authenticate the Response MAC carried within that message.

   Gateways treat the Response MAC field as an opaque value, so a relay
   implementation may generate the MAC using any method available to it.
   The RECOMMENDED method for computing the Response MAC is to compute a
   cryptographically secure hash or keyed-hash digest from the following
   values:

   o  The source IP address of the message (or Teardown Gateway IP
      Address field).




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   o  The source UDP port of the message (or Teardown Gateway Port
      Number field).

   o  The Request Nonce contained in the message.

   o  A private secret or key known only to the relay.

5.3.6.  Private Secret Generation

   If the relay implementation uses a private secret (or key) to compute
   the Response MAC value, the relay SHOULD periodically compute a new
   private secret.  The RECOMMENDED maximum interval is 2 hours.  A
   relay MUST retain the prior secret for use in verifying MAC values
   that were sent to gateways just prior to the use of the new secret.

6.  Security Considerations

   AMT is not intended to be a strongly secure protocol.  In general,
   the protocol provides the same level of security and robustness as is
   provided by the UDP, IGMP, and MLD protocols on which it relies.  The
   lack of strong security features can be largely attributed to the
   desire to make the protocol lightweight by minimizing the state and
   computation required to service a single gateway, thereby allowing a
   relay to service a larger number of gateways.

   Many of the threats and vectors described in [RFC3552] may be
   employed against the protocol to launch various types of denial-of-
   service attacks that can affect the functioning of gateways or their
   ability to locate and communicate with a relay.  These scenarios are
   described below.

   As is the case for UDP, IGMP, and MLD, the AMT protocol provides no
   mechanisms for ensuring message delivery or integrity.  The protocol
   does not provide confidentiality -- multicast groups, sources, and
   streams requested by a gateway are sent in the clear.

   The protocol does use a three-way handshake to provide trivial source
   authentication for state allocation and updates (see below).  The
   protocol also requires gateways and relays to ignore malformed
   messages and those messages that do not carry expected address
   values, protocol payload types, or content.

6.1.  Relays

   The three-way handshake provided by the membership update message
   sequence (see Section 4.2.1.2) provides a defense against source-
   spoofing-based resource-exhaustion attacks on a relay by requiring
   source authentication before state allocation.  However, in an effort



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   to consume computational resources, attackers may still attempt to
   flood a relay with Request and Membership Update messages to force
   the relay to make the MAC authentication computations.
   Implementations may choose to limit the frequency with which a relay
   responds to Request messages sent from a single IP address or IP
   address and UDP port pair, but support for this functionality is not
   required.  The three-way handshake provides no defense against an
   eavesdropping or man-in-the-middle attacker.

   Attackers that execute the gateway protocol may consume relay
   resources by instantiating a large number of tunnels or joining a
   large number of multicast streams.  A relay implementation should
   provide a mechanism for limiting the number of tunnels (Multicast
   Data message destinations) that can be created for a single gateway
   source address.  Relays should also provide a means for limiting the
   number of joins per tunnel instance as a defense against these
   attacks.

   Relays may withdraw their AMT anycast prefix advertisement when they
   reach configured maximum capacity or exhaust required resources.
   This behavior allows gateways to use the relay discovery process to
   find the next topologically nearest relay that has advertised the
   prefix.  This behavior also allows a successful resource-exhaustion
   attack to propagate from one relay to the next until all relays
   reachable using the anycast address have effectively been taken
   offline.  This behavior may also be used to acquire the unicast
   addresses for individual relays that can then be used to launch a
   DDoS attack on all of the relays without using the relay discovery
   process.  To prevent wider disruption of AMT-based distribution
   networks, relay anycast address advertisements can be limited to
   specific administrative routing domains.  This will isolate such
   attacks to a single domain.

   The Path and Tunnel MTU adjustment (discovery) procedure described in
   Section 5.3.3.6.1 is vulnerable to two denial-of-service attacks (see
   Section 8 of [RFC1191] for details).  Both attacks are based on a
   malicious party sending forged ICMPv4 Destination Unreachable or
   ICMPv6 Packet Too Big messages to a host.  In the first attack, the
   forged message indicates an inordinately small Path MTU.  In the
   second attack, the forged message indicates an inordinately large
   Path MTU.  In both cases, throughput is adversely affected.  In order
   to mitigate such attacks, relay implementations MUST include a
   configuration option to disable Path MTU adjustments on AMT tunnels.








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6.2.  Gateways

   A passive eavesdropper may launch a denial-of-service attack on a
   gateway by capturing a Membership Query or Membership Update message
   and using the Request Nonce and message authentication code carried
   by the captured message to send a spoofed Membership Update or
   Teardown message to the relay.  The spoofed messages may be used to
   modify or destroy group membership state associated with the gateway,
   thereby changing or interrupting the multicast traffic flows.

   A passive eavesdropper may also spoof Multicast Data messages in an
   attempt to overload the gateway or to disrupt or supplant existing
   traffic flows.  A properly implemented gateway will filter Multicast
   Data messages that do not originate from the expected Relay Address
   and should filter non-multicast packets and multicast IP packets
   whose group or source addresses are not included in the current
   reception state for the gateway pseudo-interface.

   An active eavesdropper may launch a man-in-the-middle attack in which
   messages normally exchanged between a gateway and relay are
   intercepted, modified, spoofed, or discarded by the attacker.  The
   attacker may deny access to, modify, or replace requested multicast
   traffic.  The AMT protocol provides no means for detecting or
   defending against a man-in-the-middle attack -- any such
   functionality must be provided by multicast receiver applications
   through independent detection and validation of incoming multicast
   datagrams.

   The anycast discovery technique for finding relays (see
   Section 4.1.4) introduces a risk that a rogue router or a rogue
   Autonomous System (AS) could introduce a bogus route to a specific
   Relay Discovery Address Prefix and thus divert or absorb Relay
   Discovery messages sent by gateways.  Network managers must guarantee
   the integrity of their routing to a particular Relay Discovery
   Address Prefix in much the same way that they guarantee the integrity
   of all other routes.

6.3.  Encapsulated IP Packets

   An attacker forging or modifying a Membership Query or Membership
   Update message may attempt to embed something other than an IGMP or
   MLD message within the encapsulated IP packet carried by these
   messages in an effort to introduce these into the recipient's IP
   stack.  A properly implemented gateway or relay will ignore any such
   messages and may further choose to ignore Membership Query messages
   that do not contain IGMP/MLD General Query or Membership Update
   messages that do not contain IGMP/MLD membership reports.




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   Properly implemented gateways and relays will also filter
   encapsulated IP packets that appear corrupted or truncated by
   verifying packet length and checksums.

7.  IANA Considerations

7.1.  IPv4 and IPv6 Anycast Prefix Allocation

   The following unicast prefixes have been assigned to provide anycast
   routing of Relay Discovery messages to public AMT relays as described
   in Section 4.1.4.  Address assignments within these prefixes are
   described in Section 4.1.5.2.

7.1.1.  IPv4

   IANA has assigned 192.52.193.0/24 from the "IANA IPv4 Special-Purpose
   Address Registry".  The block has been registered as follows:

                 +----------------------+----------------+
                 | Attribute            | Value          |
                 +----------------------+----------------+
                 | Address Block        |192.52.193.0/24 |
                 | Name                 | AMT            |
                 | RFC                  | [RFC7450]      |
                 | Allocation Date      | 2014-12        |
                 | Termination Date     | N/A            |
                 | Source               | True           |
                 | Destination          | True           |
                 | Forwardable          | True           |
                 | Global               | True           |
                 | Reserved-by-Protocol | False          |
                 +----------------------+----------------+



















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7.1.2.  IPv6

   IANA has registered the following special-purpose address block for
   IPv6 anycast AMT relay discovery.

                 +----------------------+----------------+
                 | Attribute            | Value          |
                 +----------------------+----------------+
                 | Address Block        | 2001:3::/32    |
                 | Name                 | AMT            |
                 | RFC                  | [RFC7450]      |
                 | Allocation Date      | 2014-12        |
                 | Termination Date     | N/A            |
                 | Source               | True           |
                 | Destination          | True           |
                 | Forwardable          | True           |
                 | Global               | True           |
                 | Reserved-by-Protocol | False          |
                 +----------------------+----------------+

7.2.  UDP Port Number

   The UDP port number 2268 has been reserved with IANA for use in the
   implementation and deployment of AMT.  The protocol described by this
   document continues to use this port number according to the intent of
   the original request.  IANA has updated the assignee, contact, and
   reference fields for this port number in accordance with this
   document.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
              Thyagarajan, "Internet Group Management Protocol,
              Version 3", RFC 3376, October 2002,
              <http://www.rfc-editor.org/info/rfc3376>.

   [RFC3810]  Vida, R., Ed., and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              June 2004, <http://www.rfc-editor.org/info/rfc3810>.






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   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006,
              <http://www.rfc-editor.org/info/rfc4291>.

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, August 2006,
              <http://www.rfc-editor.org/info/rfc4607>.

   [RFC4787]  Audet, F., Ed., and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, January 2007,
              <http://www.rfc-editor.org/info/rfc4787>.

8.2.  Informative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981, <http://www.rfc-editor.org/info/rfc0791>.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981,
              <http://www.rfc-editor.org/info/rfc0792>.

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, August 1989,
              <http://www.rfc-editor.org/info/rfc1112>.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990, <http://www.rfc-editor.org/info/rfc1191>.

   [RFC1546]  Partridge, C., Mendez, T., and W. Milliken, "Host
              Anycasting Service", RFC 1546, November 1993,
              <http://www.rfc-editor.org/info/rfc1546>.

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996,
              <http://www.rfc-editor.org/info/rfc1981>.

   [RFC2236]  Fenner, W., "Internet Group Management Protocol,
              Version 2", RFC 2236, November 1997,
              <http://www.rfc-editor.org/info/rfc2236>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998,
              <http://www.rfc-editor.org/info/rfc2460>.







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   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663, August 1999,
              <http://www.rfc-editor.org/info/rfc2663>.

   [RFC2710]  Deering, S., Fenner, W., and B. Haberman, "Multicast
              Listener Discovery (MLD) for IPv6", RFC 2710,
              October 1999, <http://www.rfc-editor.org/info/rfc2710>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              July 2003, <http://www.rfc-editor.org/info/rfc3552>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              January 2006, <http://www.rfc-editor.org/info/rfc4271>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", RFC 4443,
              March 2006, <http://www.rfc-editor.org/info/rfc4443>.

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601, August 2006,
              <http://www.rfc-editor.org/info/rfc4601>.

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, December 2006,
              <http://www.rfc-editor.org/info/rfc4786>.

   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935, April 2013,
              <http://www.rfc-editor.org/info/rfc6935>.

   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, April 2013,
              <http://www.rfc-editor.org/info/rfc6936>.












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Acknowledgments

   The author would like to thank the following individuals for their
   suggestions, comments, and corrections:

      Mark Altom
      Toerless Eckert
      Marshall Eubanks
      Gorry Fairhurst
      Dino Farinacci
      Lenny Giuliano
      Andy Huang
      Tom Imburgia
      Patricia McCrink
      Han Nguyen
      Doug Nortz
      Pekka Savola
      Robert Sayko
      Greg Shepherd
      Steve Simlo
      Mohit Talwar
      Lorenzo Vicisano
      Kurt Windisch
      John Zwiebel

   The anycast discovery mechanism described in this document is based
   on similar work done by the NGTrans WG for obtaining automatic IPv6
   connectivity without explicit tunnels ("6to4").  Tony Ballardie
   provided helpful discussion that inspired this document.

   Juniper Networks was instrumental in funding several versions of this
   document as well as an open source implementation.



















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Contributors

   The following people provided significant contributions to the design
   of the protocol and earlier versions of this specification:

      Amit Aggarwal
      Microsoft Corporation
      One Microsoft Way
      Redmond, WA  98052-6399
      United States
      EMail: amitag@microsoft.com

      Thomas Morin
      Orange
      2, avenue Pierre Marzin
      Lannion  22300
      France
      EMail: thomas.morin@orange.com

      Dirk Ooms
      OneSparrow
      Robert Molsstraat 11; 2018 Antwerp
      Belgium
      EMail: dirk@onesparrow.com

      Tom Pusateri
      !j
      Wake Forest, NC
      United States
      EMail: pusateri@bangj.com

      Dave Thaler
      Microsoft Corporation
      One Microsoft Way
      Redmond, WA  98052-6399
      United States
      EMail: dthaler@microsoft.com

Author's Address

   Gregory Bumgardner

   Phone: +1 541 343 6790
   EMail: gbumgard@gmail.com







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