wdiff rfc7450.alt-original rfc7450.txt

Network Working Group
Internet Engineering Task Force (IETF) G. Bumgardner
Internet-Draft
Intended status:
Request for Comments: 7450
Category: Standards Track December 1, 2014
Expires: June 4, February 2015
ISSN: 2070-1721

Automatic Multicast Tunneling
draft-ietf-mboned-auto-multicast-18

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

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This Internet-Draft will expire on June 4, 2015.
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 ....................................................3
2. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 3 ...................................................3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 .....................................................4
3.1. Requirements Notation . . . . . . . . . . . . . . . . . . 4 ......................................4
3.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4 ................................................4
3.3. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 5 ..............................................5
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6 ...............................................6
4.1. General Architecture . . . . . . . . . . . . . . . . . . 6 .......................................6
4.1.1. Relationship to IGMP and MLD Protocols . . . . . . . 7 ..............6
4.1.2. Gateways . . . . . . . . . . . . . . . . . . . . . . 8 ............................................7
4.1.3. Relays . . . . . . . . . . . . . . . . . . . . . . . 11 .............................................10
4.1.4. Deployment . . . . . . . . . . . . . . . . . . . . . 13 .........................................13
4.1.5. Discovery . . . . . . . . . . . . . . . . . . . . . . 15 ..........................................14
4.2. General Operation . . . . . . . . . . . . . . . . . . . . 16 .........................................15
4.2.1. Message Sequences . . . . . . . . . . . . . . . . . . 16 ..................................15
4.2.2. Tunneling . . . . . . . . . . . . . . . . . . . . . . 26 ..........................................26
5. Protocol Description . . . . . . . . . . . . . . . . . . . . 31 ...........................................31
5.1. Protocol Messages . . . . . . . . . . . . . . . . . . . . 31 .........................................31
5.1.1. Relay Discovery . . . . . . . . . . . . . . . . . . . 31 ....................................31
5.1.2. Relay Advertisement . . . . . . . . . . . . . . . . . 32 ................................32
5.1.3. Request . . . . . . . . . . . . . . . . . . . . . . . 34 ............................................34
5.1.4. Membership Query . . . . . . . . . . . . . . . . . . 36 ...................................35
5.1.5. Membership Update . . . . . . . . . . . . . . . . . . 39 ..................................39
5.1.6. Multicast Data . . . . . . . . . . . . . . . . . . . 42 .....................................41
5.1.7. Teardown . . . . . . . . . . . . . . . . . . . . . . 43 ...........................................43
5.2. Gateway Operation . . . . . . . . . . . . . . . . . . . . 46 .........................................45
5.2.1. IP/IGMP/MLD Protocol Requirements . . . . . . . . . . 46 ..................45
5.2.2. Pseudo-Interface Configuration . . . . . . . . . . . 47 .....................47
5.2.3. Gateway Service . . . . . . . . . . . . . . . . . . . 49 ....................................48
5.3. Relay Operation . . . . . . . . . . . . . . . . . . . . . 61 ...........................................61
5.3.1. IP/IGMP/MLD Protocol Requirements . . . . . . . . . . 61 ..................61
5.3.2. Startup . . . . . . . . . . . . . . . . . . . . . . . 62 ............................................61
5.3.3. Running . . . . . . . . . . . . . . . . . . . . . . . 62 ............................................62
5.3.4. Shutdown . . . . . . . . . . . . . . . . . . . . . . 73 ...........................................73
5.3.5. Response MAC Generation . . . . . . . . . . . . . . . 73 ............................73
5.3.6. Private Secret Generation . . . . . . . . . . . . . . 74 ..........................74
6. Security Considerations . . . . . . . . . . . . . . . . . . . 74 ........................................74
6.1. Relays . . . . . . . . . . . . . . . . . . . . . . . . . 75 ....................................................74
6.2. Gateways . . . . . . . . . . . . . . . . . . . . . . . . 76 ..................................................76
6.3. Encapsulated IP Packets . . . . . . . . . . . . . . . . . 76 ...................................76
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 77 ............................................77
7.1. IPv4 and IPv6 Anycast Prefix Allocation . . . . . . . . . 77 ...................77
7.1.1. IPv4 . . . . . . . . . . . . . . . . . . . . . . . . 77 ...............................................77
7.1.2. IPv6 . . . . . . . . . . . . . . . . . . . . . . . . 77 ...............................................78
7.2. UDP Port Number . . . . . . . . . . . . . . . . . . . . . 78 ...........................................78
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 80 .....................................................78
8.1. Normative References . . . . . . . . . . . . . . . . . . . 80 ......................................78
8.2. Informative References . . . . . . . . . . . . . . . . . . 81 ....................................79
Acknowledgments ...................................................81
Contributors ......................................................82
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .82 ..................................................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, 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 hosts, or applications that do not
have native multicast access to a network with multicast connectivity
to a source, to request and receive SSM Source-Specific Multicast (SSM)
[RFC4607] and ASM 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 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
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 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.

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 (NMBA):
A non-broadcast multiple-access (NBMA) (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 in of [RFC3376] for a detailed
definition.

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

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])
[RFC1112] [RFC2236] [RFC3376] or the Multicast Listener Discovery ([RFC2710],
[RFC3810]) protocols.
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 trees, such as PIM-SM 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 (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. protocol.

ASM - Any-Source Multicast.

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

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

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

SSM - Source-Specific Multicast. 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 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"
which
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] protocols 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, fully functional,
conforming implementation of IGMP or MLD to adhere to this
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 Section Sections 5.2.1 and
Section 5.3.1.

Gateway Relay

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

Figure 2: Multicast Reception State Managed By 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
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.

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|
|Host-Mode| | Gateway |<-------->|UDP/IP|
|Protocol |------------->|Pseudo I/F| |------------->|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 decapsulation, and relay interaction is 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 inter-network internetwork and one or more AMT gateway
pseudo-interfaces.

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.

4.1.2.2. Use-Cases

Use-cases Use Cases

Use cases for gateway functionality include: 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 pseudo-network device driver that
runs AMT on a physical network interface to provide socket layer 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.

4.1.3.1. Architecture

Each relay possesses a logical pseudo-interface:

+------------------------------+
+--------+ | Multicast Control Plane |
| |IGMP/MLD| |
| | Query* | +------------+ +----------+ |
| |<---//----|IGMPv3/MLDv2| |Multicast | |
AMT | | | |Router Mode |Router-Mode |->|Routing |<->
+------+ Messages | AMT |----//--->|Protocol | |Protocol | |
|UDP/IP|<-------->| Relay |IGMP/MLD| +------------+ +----------+ |
+------+ | Pseudo | 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
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
encapsulated, and sent to interested gateways. Changes in the pseudo-
interface
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.

The relay pseudo-interface is a 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
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 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,
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 inter-network,
internetwork, and one or more relay pseudo-interfaces.

4.1.3.2. Use-Cases

Use-cases Use Cases

Use cases for relay functionality include: 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. (e.g., PIM-SM RFC4601 [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 a 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.

4.1.4. Deployment

The AMT protocol calls for a relay deployment model that uses anycast
addressing [RFC1546][RFC4291] [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 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, 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 time, thereby allowing
staged deployment, scaling 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 versus Private

Ideally, the AMT protocol would provide a universal solution for
connecting receivers to multicast sources - sources, so that any gateway could
be used to access any globally advertised multicast source via publicly-
publicly accessible, widely-deployed 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.

4.1.4.2. Congestion Considerations

AMT relies on UDP to provide best-effort delivery of multicast data
to gateways. Neither AMT or the nor UDP protocol provide 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- source
dependent, as a relay located via relay discovery must have multicast
connectivity to a desired source.

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

Methods that might be used to perform source-specific or group-
specific
group-specific relay selection are highly implementation-dependent 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)
(S,G,relay-discovery-address) or (S,G,relay-address) tuples.

4.1.5.2. IANA-Assigned Relay Discovery Address Prefix

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

A relay discovery address Relay Discovery Address is constructed from the 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. (e.g., via
BGP [RFC4271]) and configure an interface to respond to the relay
discovery address. Relay
Discovery Address.

The IANA discovery address assignments prefixes are discussed 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 Relay Address to a gateway.

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

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
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
topologically-nearest 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 message, it
verifies that the nonce matches the one sent in the Relay
Discovery message, and message and, if it does, uses the relay address 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 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 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
general-query 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 general 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 message,
it starts a timer whose expiration will trigger the start of a new
Request->Membership Query message exchange. This timer-driven
sequence is used to mimic the transmission of a periodic general 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 leave, or done
message is passed to the gateway pseudo-interface. These messages
may be produced as a result of the aforementioned general-query 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.

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)
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. 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 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. 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 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. 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
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 nonce, and a private secret. The relay accepts the
Membership Update message if the received MAC matches the
computed MAC, otherwise 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,
routing
routing, and forwarding state changes 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 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] or 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 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 in of [RFC3376] and
Section 5.1.8
in 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 nonce, and a private secret. The relay accepts
the Membership Update message if the received MAC matches the
computed MAC, otherwise MAC; otherwise, the message is ignored. If the message
is accepted, the relay processes the encapsulated IGMP/MLD and
allocates, modifies modifies, or deletes tunnel state accordingly. This
includes making any group membership, routing routing, and forwarding
state changes 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 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, bandwidth but will not result in the allocation
of state state, because the Request message is stateless and spoofed
Membership Update messages will fail source-authentication 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; 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 (see
Section 4.2.1.3).

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 (see Section 4.2.2.1) to stop
the transmission of undeliverable or duplicate multicast data Multicast Data
messages. Gateway support for the Teardown message is optional -
gateways RECOMMENDED.
Gateways are not required to send them and may instead rely on group
membership to expire on the relay.

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)
The following sequence describes how the Membership Query and
Teardown message 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
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 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 Querier's Robustness Variable (QRV) value
contained in the IGMPv3/MLDv2 general query General Query carried by the
original Membership Query (See (see Section 4.1.6 in of [RFC3376] and
Section 5.1.8 in of [RFC3810]). The gateway continues to process
the new Membership Query message as usual.

8. When the relay receives a Teardown message, it computes a MAC
from the message source IP address, source UDP port, request
nonce
nonce, and a private secret. The relay accepts the Teardown
message if the received MAC matches the computed MAC, otherwise MAC; otherwise,
the message is ignored. If the message is accepted, the relay
makes any group membership, routing 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. This In this
document, we refer to this address is referred here 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.

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 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 inter-network 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 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, receivers and in some cases, cases may also produce a
significant amount of ICMP traffic directed back towards the relay by
a NAT, router router, or gateway host.

There are several scenarios in which the address carried by messages
sent by a gateway may change without that gateway's knowledge, as 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- network-translation
device (NAT) changes as a result of mapping expiration or routing
changes in a multi-homed 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, bi-
directional
bidirectional communication can only be conducted by sending outgoing
packets to the source address and port carried by the last incoming
packet.

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 (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
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 packets, and AMT messages sent via IPv6 may carry
IPv4/IGMP packets.

The checksum 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
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 Multicast Data
messages. This is not an issue in UDP over IPv4 IPv4, as the UDP checksum Checksum
field may be set to zero. However, this is a problem for UDP over
IPv6
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 well-known issue for
UDP-based tunneling protocols that and is described in [RFC6936]. A
recommended solution is described in [RFC6935].

4.2.2.4. UDP Fragmentation

Naive encapsulation of a multicast IP datagrams within an 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 Path MTU. These
procedures are described in Section 5.3.3.6.

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
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 IANA-assigned AMT port number (See (see Section 7.2).

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
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 IANA-assigned AMT port number).

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.

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; 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 relay address for an IPv4 discovery address
Relay Discovery Address or an IPv6 relay address for an IPv6 discovery
address. 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
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 IANA-assigned 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

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" 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 General Query
message.
1 The relay MUST respond with a Membership Query message that
contains an IPv6 packet carrying an MLDv2 general query 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
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 second stage in the three-way handshake used to create or
update tunnel state within a relay.

The UDP/IP datagram containing this message MUST carry a valid, non-
zero
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 IANA-assigned 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

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 (see Section 5.3.3.5) SHOULD set this flag and 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
(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.

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 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 in of [RFC3376] and Section 5.1.9 in of
[RFC3810].

The Querier's Robustness Variable (QRV) field in the general query 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 and, optionally, the number of times to
send a Teardown message. The QRV field is defined in Section 4.1.6
in
of [RFC3376] and Section 5.1.8 in 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" 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 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.

5.1.4.9.1. Gateway Port Number

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

The Relay 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 (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 a it receives a
Membership Query from a relay 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.

The UDP/IP datagram containing this message MUST carry a valid, non-
zero
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 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 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 IANA-assigned AMT port number.

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.

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 of 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 an a multicast IP
datagram or datagram fragment to a gateway.

The checksum 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 (see Section 4.2.2.3).

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 IANA-assigned 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

Bits

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

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
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
message, and relays are not required to act upon it.

The UDP/IP datagram containing this message MUST carry a valid, non-
zero
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 IANA-assigned 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=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, Address field, Gateway Port Number, Number field, Request
Nonce fields field, and a private secret (just as it does in the Membership
Update message).

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

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 options, and header values called
for in [RFC3376], with the following exception; exception: a gateway MAY use any
source address value in an IGMP report datagram datagram, including the
"unspecified" address (all octets are zero ). 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 options, and header values called
for in [RFC3810], with the following exception; exception: a gateway MAY use any
source address value in an MLD report datagram datagram, including the
"unspecified" address (all octets are zero ). 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 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 General Query forwarded by the pseudo-interface. See
Section 4.1.6 in of [RFC3376] and Section 5.1.8 in of [RFC3810].

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

An IPv4 gateway implementation MUST accept IPv4 datagrams that carry
the general query 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 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,
pseudo-interfaces, each with a unique configuration that describes a
binding to a specific IP protocol, relay address, relay discovery address 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,
Relay Address, it must first be supplied with a relay discovery
address. Relay Discovery
Address. The relay discovery address 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 - Relay Discovery Address -- an
implementation may employ any method that returns a suitable relay
discovery address. 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. 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 - Relay Address -- an implementation may employ any method
that returns a suitable relay address. Relay Address.

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 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 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".
Destination Unreachable message.

5.2.3. Gateway Service

In the following descriptions, a gateway pseudo interface pseudo-interface is treated
as a passive entity managed by a gateway service. The gateway
pseudo-interface provides the state 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 locally generated
IGMP/MLD messages passed to the pseudo-interface. The handling of
these messages is described below.

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, 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). 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 ff00::/8
for IPv6.

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
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 IANA-assigned 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.

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 an 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 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 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, Relay Address,
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 after Relay Address Change

5.2.3.4.5. Discovery Nonce Generation

The discovery nonce MUST be a random, non-zero, non-zero 32-bit value, and value and, if
possible, SHOULD be computed using a cryptographically secure pseudo
random
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 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, Request
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" P flag in the Request message to identify the protocol the
gateway wishes the relay to use for the general query General Query response.

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

A gateway MUST save a copy of the Request message or save the Request
Nonce and P-flag 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 an 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.

5.2.3.5.4. Handling a Membership Query Message

When a gateway receives a Membership Query message 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). Relay Address).

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

o The IGMPv3 or MLDv2 message MUST be a general query 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 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.
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
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
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. General Query. A
gateway MAY use a smaller timer duration if required to refresh a NAT
mapping that would otherwise timeout. 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 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 changed, the gateway MUST send a Teardown message to
the relay as described in Section 5.2.3.7.

If the L-flag 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 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. The Even if the L flag is set, the gateway MAY continue to
send updates even if the
L-flag is set, if it has previously reported group subscriptions to the
relay, one or more subscriptions still exist 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 Request Nonce MUST be a random value, and value and, if possible, SHOULD be
computed using a cryptographically secure pseudo random 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.

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.
pseudo-interface. The IGMP and MLD protocols produce current-state
reports in response to general queries 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:
pseudo-interface:

o IPv4 datagrams that carry an IGMPv2, 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
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) MLD), it sends any queued or incoming IGMP or MLD datagrams to the
relay as described in the next section.

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. Relay Address. If the gateway has a relay address, 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 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 IANA-assigned 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" G flag set to indicate that it contains valid gateway
address 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" 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 changed, the gateway
MUST send a Teardown message as described in the next section.

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 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
IANA-assigned
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 (see Section 4.1.6 in of [RFC3376] and
Section 5.1.7 in 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,
message and the current relay is returning gateway address 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" 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.

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 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 approach,
as it does not allow for transient network changes.

2. If the last message sent was a Relay Discovery, Request Request, or
Teardown message, the gateway MAY attempt to transmit the new
message. If the gateway is configured to retransmit Relay
Discovery, Request 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.

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] [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- implementation
dependent and are not addressed in this document.

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 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 anycast Relay Discovery Address or the unicast relay address. 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 discovery address. Relay Discovery Address.

When a relay receives a Relay Discovery message 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. The relay MUST use port
number for the destination Relay Advertisement message. The source IP address
and UDP port
of number carried by the Relay Discovery as Advertisement message MUST
match the source 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.

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 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 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 message, it SHOULD NOT set these fields fields, as this
may cause the gateway to generate unnecessary Teardown messages.

If the P-flag P flag in the Request message is 0, the relay MUST return an
IPv4-encapsulated IGMPv3 general query General Query in the Membership Query
message. If the P-flag P flag is 1, the relay MUST return an
IPv6-encapsulated MLDv2 general query 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
L flag in the Membership Query message to notify the gateway of this
state. Support for the L-flag L flag is OPTIONAL. See Section 5.3.3.8.

The encapsulated IGMPv3 general query 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 options, and
header values called for in [RFC3376], with the following exception; exception:
a relay MAY use any source IP address for an IGMP general query
datagram 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 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 options, and
header values called for in [RFC3810], with the following exception; exception:
a relay MAY use any source IP address for an MLD general query
datagram 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 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 in of [RFC3376] and Section 5.1.9 in 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
General Query to a non-zero value. This value SHOULD be greater than
one. If a gateway retransmits membership state change state-change messages, it
will retransmit them (robustness variable (Robustness Variable - 1) times. The QRV field
is defined in Section 4.1.6 in of [RFC3376] and Section 5.1.8 in of
[RFC3810].

A relay SHOULD set the Maximum Response Code field in the general
query 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 value, as the Query Response Interval variable is used in
setting the duration of group state timers and must not be set to
such a small value. The Maximum Response Code field is defined in
Section 4.1.1 in of [RFC3376] and Section 5.1.3 in 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 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 -
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 varies vary by protocol; See 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 Membership 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
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 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 membership 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 messages 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
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 an 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
Address and Gateway Port Number field 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 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, 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 as 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,
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,
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.

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 Datagram Too Big" 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
denial-of-service attacks (See (Section 6)). (see Section 6). When dynamic PMTU
adjustments are disabled, the PMTU for all tunnels MUST default to
the Link MTU (first-hop) (first hop) on the downstream interface.

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 type equal to code 4 (fragmentation needed
and DF set). The ICMP Destination Unreachable message MUST contain an next-hop a
Next-Hop MTU (as specified by [RFC1191]) [RFC1191]), and the relay MUST set the next-hop
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, tunnel but instead send a single ICMP
message with a Next-hop 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, tunnel but instead send a single ICMPv6 message with a
Next-hop
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 Relay Address and IANA-assigned
AMT 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.

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, tunnel and the
DF
Bit 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, tunnel and the DF Bit 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 a 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 of type
"Destination Unreachable" (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 shutdown shut down the tunnel for that gateway (Handling gateway.

Handling of ICMP "Destination Unreachable" Destination Unreachable messages with code 4,
"fragmentation required" needed and DF set" (i.e., "Datagram Too Big") is
covered in Section 5.3.3.6.1). 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" Destination Unreachable messages can be received and
ignored before the relay will automatically shutdown shut down a tunnel.

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],
[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 timeout 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 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.

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
cryptographically secure hash or keyed-hash digest from the following
values:

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

o The Source source UDP port of the message (or Teardown Gateway Port
Number field) 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 secured secure protocol. In general,
the protocol provides the same level of security and robustness as is
provided by the UDP, IGMP IGMP, and MLD protocols on which it relies. The
lack of strong security features can largely be largely attributed to the
desire to make the protocol light-weight 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 IGMP, and MLD, the AMT protocol provides no
mechanisms for ensuring message delivery or integrity. The protocol
does not provide confidentiality - -- multicast groups, sources 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
or
values, protocol payload types types, or content.

6.1. Relays

The three-way handshake provided by the membership update message
sequence (See (Section 4.2.1.2)) (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
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 in an effort to consume computational resources. 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 topologically nearest relay that has advertised the
prefix. This behavior also allows a successful resource exhaustion 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 which 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
network,
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 denial-of-service attacks (see
Section 8 of [RFC1191] for details). Both attacks are based upon 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.

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 Request Nonce and message authentication code carried
by the captured message to send a spoofed a 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 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 spoofed, or discarded by the attacker. The
attacker may deny access to, modify 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 AS
Autonomous System (AS) could introduce a bogus route to a specific
Relay Discovery Address
prefix, 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 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 a IGMP/MLD general queries General Query or Membership Update
messages that do not contain IGMP/MLD membership reports.

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 Relay Discovery messages to public AMT Relays 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

We suggest that

IANA assign an x.x.x.x/24 has assigned 192.52.193.0/24 from the "IANA IPv4 Recovered Special-Purpose
Address Space Registry, but any /24 which has been unassigned and
unadvertised for at least twelve months is acceptable. Registry". The block
should be has been registered as follows:

7.1.2. IPv6

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

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 should assign has updated the assignee, contact, and
reference fields for this port number to AMT
upon acceptance of in accordance with this I-D.
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. 2002,
<http://www.rfc-editor.org/info/rfc3376>.

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

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

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

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

8.2. Informative References

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

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

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

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

[RFC1546] Partridge, C., Mendez, T., and W. Milliken, "Host
Anycasting Service", RFC 1546, November 1993. 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.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. 1996,
<http://www.rfc-editor.org/info/rfc1981>.

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

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

[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999. 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. 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. 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. 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. 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. 2006,
<http://www.rfc-editor.org/info/rfc4601>.

[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, December 2006. 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. 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.

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
USA
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
USA
Email: pusateri@bangj.com

Dave Thaler
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
USA
Email: dthaler@microsoft.com 2013,
<http://www.rfc-editor.org/info/rfc6936>.

Acknowledgments

The authors 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
draft
document as well as an open source implementation.

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:
EMail: gbumgard@gmail.com