Automatic Multicast Tunneling+1 541 343 6790gbumgard@gmail.comThis 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.The advantages and benefits provided by multicast technologies are
well known. There are a number of application areas that are ideal
candidates for the use of multicast, including media broadcasting, video
conferencing, collaboration, real-time data feeds, data replication, and
software updates. Unfortunately, many of these applications lack
multicast connectivity to networks that carry traffic generated by
multicast sources. The reasons for the lack of connectivity vary, but
are primarily the result of service provider policies and network
limitations.Automatic Multicast Tunneling (AMT) is a protocol that uses UDP-based
encapsulation to overcome the aforementioned lack of multicast
connectivity. AMT enables sites, hosts or applications that do not have
native multicast access to a network with multicast connectivity to a
source, to request and receive SSM and
ASM traffic from a network that does
provide multicast connectivity to that source.This document describes a protocol that may be used to deliver
multicast traffic from a multicast enabled network to sites that lack
multicast connectivity to the source network. This document does not
describe any methods for sourcing multicast traffic from isolated sites
as this topic is out of scope.AMT is not intended to be used as a substitute for native multicast,
especially in conditions or environments requiring high traffic flow.
AMT uses unicast replication to reach multiple receivers and the
bandwidth cost for this replication will be higher than that required if
the receivers were reachable via native multicast.AMT is designed to be deployed at the border of networks possessing
native multicast capabilities where access and provisioning can be
managed by the AMT service provider.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 .This document adopts the following definitions for use in
describing the protocol:A downstream
interface or connection that faces away from the multicast
distribution root or towards multicast receivers.An upstream
interface or connection that faces a multicast distribution root
or source.A non-broadcast multiple-access (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.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.A group membership database describes the current
multicast subscription state for an interface or system. See Section 3 in for a detailed
definition.The
multicast subscription state of a pseudo, virtual or physical
network interface. Often synonymous with group membership
database.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.The term "group membership protocol" is used as a
generic reference to the Internet Group Management (IGMP) (, , ) or Multicast Listener Discovery (, )
protocols.The
term "multicast protocol" is used as a generic reference to
multicast routing protocols used to join or leave multicast
distribution trees such as PIM-SM .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 ). 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.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 .AMT - Automatic Multicast Tunneling Protocol.ASM - Any-Source Multicast.DoS - Denial-of-Service (attack) and DDoS for
distributed-DoS.IGMP - Internet Group Management Protocol (v1, v2 and v3).IP - Internet Protocol (v4 and v6).MAC - Message Authentication Code (or Cookie).MLD - Multicast Listener Discovery protocol (v1 and v2).NAT - Network Address Translation (or translation node).NBMA - Non-Broadcast Multi-Access (network, interface or
mode)SSM - Source-Specific Multicast.PIM - Protocol Independent Multicast.This section provides an informative description of the protocol. A
normative description of the protocol and implementation requirements
may be found in section .The AMT protocol employs a client-server model in which a "gateway"
sends requests to receive specific multicast traffic to a "relay"
which 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.AMT relies on the Internet Group
Management (IGMP) and Multicast
Listener Discovery (MLD) 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, 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 and .A gateway runs the host portion of the IGMP and MLD protocols to
generate group membership updates that are sent via AMT messages to
a relay. A relay runs the router portion of the IGMP and MLD
protocols to process the group membership updates to produce the
required changes in multicast forwarding state. A relay uses AMT
messages to send incoming multicast IP datagrams to gateways
according to their current group membership state.The primary function of AMT is to provide the handshaking,
encapsulation and decapsulation required to transport the IGMP and
MLD messages and multicast IP datagrams between the gateways and
relays. The IGMP and MLD messages that are exchanged between
gateways and relays are encapsulated as complete IP datagrams within
AMT control messages. Multicast IP datagrams are replicated and
encapsulated in AMT data messages. All AMT messages are sent via
unicast UDP/IP.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.The pseudo-interface is conceptually a network interface on
which the gateway executes the host portion of the IPv4/IGMP (v2
or v3) and IPv6/MLD (v1 or v2) protocols. The multicast reception
state of the pseudo-interface is manipulated using the IGMP or MLD
service interface. The IGMP and MLD host protocols produce IP
datagrams containing group membership messages that the gateway
will send to the relay. The IGMP and MLD protocols also supply the
retransmission and timing behavior required for protocol
robustness.All AMT encapsulation, decapsulation and relay interaction is
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 and one or more
AMT gateway pseudo-interfaces.In this example, the host IP stack uses a virtual network
interface to interact with a gateway pseudo-interface
implementation.Use-cases for gateway functionality include: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.A virtual network interface or pseudo network
device driver that runs AMT on a physical network interface to
provide socket layer access to multicast traffic via the
IGMP/MLD service interface provided by the host IP stack.An
application or application component that implements and
executes IGMP/MLD and AMT internally to gain access to
multicast traffic.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.The pseudo-interface is conceptually a network interface on
which the relay runs the router portion of the IPv4/IGMPv3 and
IPv6/MLDv2 protocols. Relays do not send unsolicited IGMPv3/MLDv2
query messages to gateways so relays must consume or discard any
local queries normally generated by IGMPv3 or MLDv2. Note that the
protocol mandates the use of IGMPv3 and MLDv2 for query messages.
The AMT protocol is primarily intended for use in SSM applications
and relies on several values provided by IGMPv3/MLDv2 to control
gateway behavior.A relay maintains group membership state for each gateway
connected through the pseudo-interface as well as for the entire
pseudo-interface (if multiple gateways are managed via a single
interface). Multicast packets received on upstream interfaces on
the relay are routed to the pseudo-interface where they are
replicated, encapsulated and sent to interested gateways. Changes
in the pseudo-interface group membership state may trigger the
transmission of multicast protocol requests upstream towards a
given source or rendezvous point and cause changes in internal
routing/forwarding state.The relay pseudo-interface is a architectural abstraction used
to describe AMT protocol operation. For the purposes of this
document, the pseudo-interface is most easily viewed as an
interface to a single gateway - encapsulation, decapsulation, and
other AMT-specific processing occurs "within" the pseudo-interface
while forwarding and replication occur outside of it.An alternative view is to treat the pseudo-interface as a
non-broadcast multi-access (NBMA) network interface whose link
layer is the unicast-only network over which AMT messages are
exchanged with gateways. Individual gateways are conceptually
treated as logical NBMA links on the interface. In this
architectural model, group membership tracking, replication and
forwarding functions occur in the pseudo-interface.This document does not specify any particular architectural
solution - a relay developer may choose to implement and
distribute protocol functionality as required to take advantage of
existing relay platform services and architecture.Within this document, the term "relay" may be used as a generic
reference to an entity executing the relay protocol, a relay
pseudo-interface, or a relay device that has one or more network
interfaces with multicast connectivity to a native multicast
infrastructure, zero or more interfaces connected to a unicast
inter-network, and one or more relay pseudo-interfaces.Use-cases for relay functionality include:A
multicast router that runs AMT on a downstream interface to
provide gateway access to multicast traffic. A "relay router"
uses a multicast routing protocol (e.g. PIM-SM RFC4601) to construct a forwarding
path for multicast traffic by sending join and prune messages
to neighboring routers to join or leave multicast distribution
trees for a given SSM source or ASM rendezvous point.An IGMP/MLD proxy that runs AMT on a
downstream interface and host-mode IGMPv3/MLDv2 on a upstream
interface. This "relay proxy" sends group membership reports
to a local, multicast-enabled router to join and leave
specific SSM or ASM groups.The AMT protocol calls for a relay deployment model that uses
anycast addressing to pair gateways with relays.Under this approach, one or more relays advertise a route for the
same IP address prefix. To find a relay with which to communicate, a
gateway sends a message to an anycast IP address within that prefix.
This message is routed to the topologically-nearest relay that has
advertised the prefix. The relay that receives the message responds
by sending its unicast address back to the gateway. The gateway uses
this address as the destination address for any messages it
subsequently sends to the relay.The use of anycast addressing provides the following benefits:
Relays may be deployed at multiple locations within a single
multicast-enabled network. Relays might be installed "near"
gateways to reduce bandwidth requirements, latency and limit the
number of gateways that might be serviced by a single relay.Relays may be added or removed at any time thereby allowing
staged deployment, scaling and hot-swapping - the relay
discovery process will always return the nearest operational
relay.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.Ideally, the AMT protocol would provide a universal solution
for connecting receivers to multicast sources - that any gateway
could be used to access any globally advertised multicast source
via publicly-accessible, widely-deployed relays. Unfortunately,
today's Internet does not yet allow this, because many relays will
lack native multicast access to sources even though they may be
globally accessible via unicast.In these cases, a provider may deploy relays within their own
source network to allow for multicast distribution within that
network. Gateways that use these relays must use a
provider-specific relay discovery mechanism or a private anycast
address to obtain access to these relays.AMT relies on UDP to provide best-effort delivery of multicast
data to gateways. Neither AMT or the UDP protocol provide 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 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.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:The anycast address prefix used to route
discovery messages to a relay.The anycast destination address used when
sending discovery messages.The
unicast IP address obtained as a result of the discovery
process.The selection of an anycast Relay Discovery Address may be
source-dependent, as a relay located via relay discovery must have
multicast connectivity to a desired source.Similarly, the selection of a unicast Relay Address may be
source-dependent, as a relay contacted by a gateway to supply
multicast traffic must have native multicast connectivity to the
traffic sourceMethods that might be used to perform source-specific or
group-specific relay selection are highly implementation-dependent
and are not further addressed by this document. Possible
approaches include the use of static lookup tables, DNS-based
queries, or a provision of a service interface that accepts join
requests on (S,G,relay-discovery-address) or (S,G,relay-address)
tuples.IANA has assigned an address prefix for use in advertising and
discovering publicly accessible relays.A relay discovery address is constructed from the address
prefix by setting the low-order octet of the prefix address to 1
(for both IPv4 and IPv6).Public relays must advertise a route to the address prefix
(e.g. via BGP ) and configure an
interface to respond to the relay discovery address.The IANA address assignments are discussed in .The AMT protocol defines the following messages for control and
encapsulation. These messages are exchanged as UDP/IP datagrams, one
message per datagram.Sent by
gateways to solicit a Relay Advertisement from any relay. Used
to find a relay with which to communicate.Sent
by relays as a response to a Relay Discovery message. Used to
deliver a relay address to a gateway.Sent by gateways
to solicit a Membership Query message from a relay.Sent by
relays as a response to a Request message. Used to deliver an
encapsulated IGMPv3 or MLDv2 query message to the gateway.Sent
by gateways to deliver an encapsulated IGMP or MLD
report/leave/done message to a relay.Sent by
relays to deliver an encapsulated IP multicast datagram or
datagram fragment to a gateway.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.The following sequence describes how the Relay Discovery and
Relay Advertisement messages are used to find a relay with which
to communicate: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 network node that advertises that
address.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.When the gateway receives the Relay Advertisement message
it verifies that the nonce matches the one sent in the Relay
Discovery message, and if it does, uses the relay address
carried by the Relay Advertisement as the destination address
for subsequent AMT messages.Note that the responder need not be a relay - the responder may
obtain a relay address by some other means and return the result
in the Relay Advertisement (i.e., the responder is a load-balancer
or broker).There exists a significant difference between normal IGMP and
MLD behavior and that required by AMT. An IGMP/MLD router acting
as a querier normally transmits query messages on a network
interface to construct and refresh group membership state for the
connected network. These query messages are multicast to all
IGMP/MLD enabled hosts on the network. Each host responds by
multicasting report messages that describe their current multicast
reception state.However, AMT does not allow relays to send unsolicited query
messages to gateways, as the set of active gateways may be unknown
to the relay and potentially quite large. Instead, AMT requires
each gateway to periodically send a message to a relay to solicit
a general-query response. A gateway accomplishes this by sending a
Request message to a relay. The relay responds by sending
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 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 or
done message is passed to the gateway pseudo-interface. These
messages may be produced as a result of the aforementioned
general-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.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: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.When the relay receives a Request message, it generates a
message authentication code (MAC) by computing a hash value
from message source IP address, source UDP port, request nonce
and a private secret. The relay then sends a Membership Query
message to the gateway that contains the request nonce, the
MAC, and an IGMPv3 or MLDv2 general query.When the gateway receives a Membership Query message, it
verifies that the request nonce matches the one sent in the
last Request, and if it does, the gateway saves the request
nonce and MAC for use in sending subsequent Membership Update
messages. The gateway starts a timer whose expiration will
trigger the transmission of a new Request message and extracts
the encapsulated general query message for processing by the
IGMP or MLD protocol. The query timer duration is specified by
the relay in the Querier's Query Interval Code (QQIC) field in
the IGMPv3 or MLDv2 general query. The QQIC field is defined
in Section 4.1.7 of and Section
5.1.9 of ).The gateway's IGMP or MLD protocol implementation processes
the general query to produce a current-state report.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).When the relay receives a Membership Update message, it
computes a MAC from the message source IP address, source UDP
port, request nonce and a private secret. The relay accepts
the Membership Update message if the received MAC matches the
computed MAC, otherwise the message is ignored. If the message
is accepted, the relay may proceed to allocate, refresh, or
modify tunnel state. This includes making any group
membership, routing and forwarding state changes and issuing
any upstream protocol requests required to satisfy the state
change. The diagram illustrates two scenarios: The gateway has not previously reported any group
subscriptions and the report does not contain any group
subscriptions, so the relay takes no action.The gateway has previously reported a group
subscription so the current-state report lists all current
subscriptions. The relay responds by refreshing tunnel or
group state and resetting any related timers.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 IGMP/MLD service
interface (as described in Section 2 of or Section 3 of ). The IGMP/MLD protocol responds by
generating an IGMP or MLD state-change message.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
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
and Section 5.1.8 in.When the relay receives a Membership Update message, it
again computes a MAC from the message source IP address,
source UDP port, request nonce and a private secret. The relay
accepts the Membership Update message if the received MAC
matches the computed MAC, otherwise the message is ignored. If
the message is accepted, the relay processes the encapsulated
IGMP/MLD and allocates, modifies or deletes tunnel state
accordingly. This includes making any group membership,
routing and forwarding state changes and issuing any upstream
protocol requests required to satisfy the state change. The
diagram illustrates two scenarios:The gateway wishes to add a group subscription.The gateway wishes to delete a previously reported
group subscription.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.When the gateway receives a Multicast Data message, it
extracts the multicast packet from the message and passes it
on to the appropriate receivers.When the query timer expires the gateway sends a new
Request message to the relay to start a new membership update
cycle.The MAC-based source-authentication mechanism described above
provides a simple defense against malicious attempts to exhaust
relay resources via source-address spoofing. Flooding a relay with
spoofed Request or Membership Update messages may consume
computational resources and network bandwidth, but will not result
in the allocation of state because the Request message is
stateless and spoofed Membership Update messages will fail
source-authentication and be rejected by the relay.A relay will only allocate new tunnel state if the IGMP/MLD
report carried by the Membership Update message creates one or
more group subscriptions.A relay deallocates tunnel state after one of the following
events; the gateway sends a Membership Update message containing a
report that results in the deletion of all remaining group
subscriptions, the IGMP/MLD state expires (due to lack of refresh
by the gateway), or the relay receives a valid Teardown message
from the gateway (See ).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).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 ) to stop the transmission of
undeliverable or duplicate multicast data messages. Gateway
support for the Teardown message is optional - gateways are not
required to send them and may instead rely on group membership to
expire on the relay.The following sequence describes how the Membership Query and
Teardown message are used to detect an address change and stop the
delivery of Multicast Data messages to an address:A gateway sends a Request message containing a random nonce
to the relay.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.When the gateway receives a Membership Query message that
carries the gateway address fields, it compares the gateway IP
address and 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.The gateway sends a new Request message to the relay.
However, this Request message arrives at the relay carrying a
different source address than that of the previous Request due
to some change in network interface, address assignment,
network topology or NAT mapping.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.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.If the reported address or port has changed, the gateway
sends a Teardown message to the relay that contains the
request nonce, MAC, gateway IP address and gateway port number
returned in the earlier Membership Query message. The gateway
may send the Teardown message multiple times where the number
of repetitions is governed by the Querier Robustness Variable
(QRV) value contained in the IGMPv3/MLDv2 general query
carried by the original Membership Query (See Section 4.1.6 in and Section 5.1.8 in). The gateway
continues to process the new Membership Query message as
usual.When the relay receives a Teardown message, it computes a
MAC from the message source IP address, source UDP port,
request nonce and a private secret. The relay accepts the
Teardown message if the received MAC matches the computed MAC,
otherwise the message is ignored. If the message is accepted,
the relay makes any group membership, routing and forwarding
state changes required to stop the transmission of Multicast
Data messages to that address.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 .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
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:The gateway sends an IGMP or MLD report, leave or done
message to the relay that deletes the last group subscription
linked to the tunnel endpoint.The gateway sends a Teardown message to the relay that causes
it to delete any and all subscriptions bound to the tunnel
endpoint.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 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.As described above, each time a relay receives a Membership
Update message from a new source address-port pair, the group
subscriptions described by that message apply to the tunnel
endpoint identified by that address.This can cause problems for a gateway if the address carried by
the messages it sends to a relay changes unexpectedly. These
changes may cause the relay to transmit duplicate, undeliverable
or unrequested traffic back towards the gateway or an intermediate
device. This may create congestion and have negative consequences
for the gateway, its network, or multicast receivers, and in some
cases, may also produce a significant amount of ICMP traffic
directed back towards the relay by a NAT, router or gateway
host.There are several scenarios in which the address carried by
messages sent by a gateway may change without that gateway's
knowledge, as for example, when:The message originates from a different interface on a
gateway that possesses multiple interfaces.The DHCP assignment for a gateway interface changes.The gateway roams to a different wireless network.The address mapping applied by an intervening
network-translation-device (NAT) changes as a result of
mapping expiration or routing changes in a multi-homed
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 ).
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 .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 communication can only be
conducted by sending outgoing packets to the source address and
port carried by the last incoming packet.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 ).The IGMP and MLD messages used in AMT are exchanged as complete
IP datagrams. These IP datagrams are encapsulated in AMT messages
that are transmitted using UDP. The same holds true for multicast
traffic - each multicast IP datagram or datagram fragment that
arrives at the relay is encapsulated in an AMT message and
transmitted to one or more gateways via UDP.The IP protocol of the encapsulated packets need not match the
IP protocol used to send the AMT messages. AMT messages sent via
IPv4 may carry IPv6/MLD packets and AMT messages sent via IPv6 may
carry IPv4/IGMP packets.The checksum field contained in the UDP header of the messages
requires special consideration. Of primary concern is the cost of
computing a checksum on each replicated multicast packet after it
is encapsulated for delivery to a gateway. Many routing/forwarding
platforms do not possess the capability to compute checksums on
UDP encapsulated packets as they may not have access to the entire
datagram.To avoid placing an undue burden on the relay platform, the
protocol specifically allows zero-valued UDP checksums on the
multicast data messages. This is not an issue in UDP over IPv4 as
the UDP checksum field may be set to zero. However, this is a
problem for UDP over IPv6 as that protocol requires a valid,
non-zero checksum in UDP datagrams .
Messages sent over IPv6 with a UDP checksum of zero may fail to
reach the gateway. This is a well known issue for UDP-based
tunneling protocols that is described . A recommended solution is described in
.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 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 some number of complete samples can be
delivered even in the presence of packet loss. To prevent or
reduce undesirable fragmentation, the AMT protocol describes
specific procedures for handling multicast datagrams whose
encapsulation might exceed the path MTU. These procedures are
described in .This section provides a normative description of the AMT
protocol.The AMT protocol defines seven message types for control and
encapsulation. These messages are assigned the following names and
numeric identifiers:Message TypeMessage Name1Relay Discovery2Relay Advertisement3Request4Membership Query5Membership Update6Multicast Data7TeardownThese messages are exchanged as IPv4 or IPv6 UDP datagrams.A Relay Discovery message is used to solicit a response from a
relay in the form of a Relay Advertisement message.The UDP/IP datagram containing this message MUST carry a valid,
non-zero UDP checksum and carry the following IP address and UDP
port values: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.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.An anycast or unicast IP
address, i.e., the Relay Discovery Address advertised by a
relay.The IANA-assigned AMT port
number (See ).The protocol version number for this message is 0.The type number for this message is 1.Reserved bits that MUST be set to zero by the gateway and
ignored by the relay.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
.The Relay Advertisement message is used to supply a gateway with
a unicast IP address of a relay. A relay sends this message to a
gateway when it receives a Relay Discovery message from that
gateway.The UDP/IP datagram containing this message MUST carry a valid,
non-zero UDP checksum and carry the following IP address and UDP
port values:The destination IP address
carried by the Relay Discovery message (i.e., the Relay
Discovery Address advertised by the relay).The destination UDP port carried
by the Relay Discovery message (i.e., the IANA-assigned AMT port
number).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.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.The protocol version number for this message is 0.The type number for this message is 2.Reserved bits that MUST be set to zero by the relay and ignored
by the gateway.A 32-bit value copied from the Discovery Nonce field () contained in
the Relay Discovery message. The gateway uses this value to match
a Relay Advertisement to a Relay Discovery message.The unicast IPv4 or IPv6 address of the relay. A gateway uses
the length of the UDP datagram containing the Relay Advertisement
message to determine the address family; i.e., length - 8 = 4
(IPv4) or 16 (IPv6). The relay returns an IP address for the
protocol used to send the Relay Discovery message, i.e., an IPv4
relay address for an IPv4 discovery address or an IPv6 relay
address for an IPv6 discovery address.A gateway sends a Request message to a relay to solicit a
Membership Query response.The successful delivery of this message marks the start of the
first stage in the three-way handshake used to create or update
state within a relay.The UDP/IP datagram containing this message MUST carry a valid,
non-zero UDP checksum and carry the following IP address and UDP
port values: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.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.The unicast IP address of
the relay.The IANA-assigned AMT port
number.The protocol version number for this message is 0.The type number for this message is 3.Reserved bits that MUST be set to zero by the gateway and
ignored by the relay.The "P" flag is set to indicate which group membership protocol
the gateway wishes the relay to use in the Membership Query
response:ValueMeaning0The relay MUST respond with a Membership Query message that
contains an IPv4 packet carrying an IGMPv3 general query
message.1The relay MUST respond with a Membership Query message that
contains an IPv6 packet carrying an MLDv2 general query
message.A 32-bit random value generated by the gateway and echoed by
the relay in a Membership Query message. This value is used by the
relay to compute the Response MAC value and is used by the gateway
to correlate Membership Query messages with Request messages.
Request nonce generation is described in .A relay sends a Membership Query message to a gateway to solicit
a Membership Update response, but only after receiving a Request
message from the gateway.The successful delivery of this message to a gateway marks the
start of the second-stage in the three-way handshake used to create
or update tunnel state within a relay.The UDP/IP datagram containing this message MUST carry a valid,
non-zero UDP checksum and carry the following IP address and UDP
port values:The destination IP address
carried by the Request message (i.e., the unicast IP address of
the relay).The destination UDP port carried
by the Request message (i.e., the IANA-assigned AMT port
number).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.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.The protocol version number for this message is 0.The type number for this message is 4.Reserved bits that MUST be set to zero by the relay and ignored
by the gateway.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.A 1-bit flag set to 0 to indicate that the message does NOT
carry the Gateway Port and Gateway IP Address fields, and 1 to
indicate that it does. A relay implementation that supports the
optional teardown procedure (See ) SHOULD set this
flag and the Gateway Address field values. If a relay sets this
flag, it MUST also include the Gateway Address fields in the
message. A gateway implementation that does not support the
optional teardown procedure (See ) MAY ignore this flag
and the Gateway Address fields if they are present.A 48-bit source authentication hash generated by the relay as
described in .
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.A 32-bit value copied from the Request Nonce field () carried by a Request
message. The relay will have included this value in the Response
MAC hash 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.An IP-encapsulated IGMP or MLD message generated by the relay.
This field will contain one of the following IP datagrams:IPv4:IGMPv3 Membership QueryIPv6:MLDv2 Listener QueryThe source address carried by the query message should be
set as described in .The Querier's Query Interval Code (QQIC) field in the general
query is used by a relay to specify the time offset a gateway
should use to schedule a new three-way handshake to refresh the
group membership state within the relay (current time + Query
Interval). The QQIC field is defined in Section 4.1.7 in and Section 5.1.9 in.The Querier's Robustness Variable (QRV) field in the general
query is used by a relay to specify the number of times a gateway
should retransmit unsolicited membership reports, encapsulated
within Membership Update messages, and optionally, the number of
times to send a Teardown message. The QRV field is defined in
Section 4.1.6 in and Section 5.1.8 in.The Gateway Port Number and Gateway Address fields are present
in the Membership Query message if, and only if, the "G" flag is
set.A gateway need not parse the encapsulated IP datagram to
determine the position of these fields within the UDP datagram
containing the Membership Query message - if the G-flag is set,
the gateway may simply subtract the total length of the fields (18
bytes) from the total length of the UDP datagram to obtain the
offset.A 16-bit UDP port containing a UDP port value.The Relay sets this field to the value of the UDP source port
of the Request message that triggered the Query message.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 ). This address must match that used by
the relay to compute the value stored in the Response MAC
field.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
receives a Membership Query from a relay because the gateway must
copy the Request Nonce and Response MAC values carried by a
Membership Query into any subsequent Membership Update messages it
sends back to that relay. These values are used by the relay to
verify that the sender of the Membership Update message was the
recipient of the Membership Query message from which these values
were copied.The successful delivery of this message to the relay marks the
start of the final stage in the three-way handshake. This stage
concludes when the relay successfully verifies that 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 UDP checksum and carry the following IP address and UDP
port values:The IP address of the gateway
interface on which the gateway will listen for Multicast Data
messages from the relay. The address must be the same address
used to send the initial Request message or the message will be
ignored. Note: The value of this field may be changed as a
result of network address translation before arriving at the
relay.The UDP port number on which the
gateway will listen for Multicast Data messages from the relay.
This port must be the same port used to send the initial Request
message or the message will be ignored. Note: The value of this
field may be changed as a result of network address translation
before arriving at the relay.The unicast IP address of
the relay.The IANA-assigned AMT port
number.The protocol version number for this message is 0.The type number for this message is 5.Reserved bits that MUST be set to zero by the gateway and
ignored by the relay.A 48-bit value copied from the Response MAC field () in a Membership
Query message. Used by the relay to perform source
authentication.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.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 ReportIPv4:IGMPv2 Leave GroupIPv4:IGMPv3 Membership ReportIPv6:MLDv1 Multicast Listener ReportIPv6:MLDv1 Multicast Listener DoneIPv6:MLDv2 Multicast Listener ReportThe source address carried by the message should be set as
described in .A relay sends a Multicast Data message to deliver an multicast IP
datagram or datagram fragment to a gateway.The checksum field in the UDP header of this message MAY contain
a value of zero when sent over IPv4 but SHOULD, if possible, contain
a valid, non-zero value when sent over IPv6 (See ).The UDP/IP datagram containing this message MUST carry the
following IP address and UDP port values:The unicast IP address of the
relay.The IANA-assigned AMT port
number.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.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.The protocol version number for this message is 0.The type number for this message is 6.Bits that MUST be set to zero by the relay and ignored by the
gateway.A complete IPv4 or IPv6 multicast datagram or datagram
fragment.A gateway sends a Teardown message to a relay to request that it
stop sending Multicast Data messages to a tunnel endpoint created by
an earlier Membership Update message. A gateway sends this message
when it detects that a Request message sent to the relay carries an
address that differs from that carried by a previous Request
message. The gateway uses the Gateway IP Address and Gateway Port
Number Fields in the Membership Query message to detect these
address changes.To provide backwards compatibility with early implementations of
the AMT protocol, support for this message and associated procedures
is considered OPTIONAL - gateways are not required to send this
message and relays are not required to act upon it.The UDP/IP datagram containing this message MUST carry a valid,
non-zero UDP checksum and carry the following IP address and UDP
port values: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.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.The unicast IP address of
the relay.The IANA-assigned AMT port
number.The protocol version number for this message is 0.The type number for this message is 7.Reserved bits that MUST be set to zero by the gateway and
ignored by the relay.A 48-bit value copied from the Response MAC field () 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, Gateway Port Number, Request
Nonce fields and a private secret (just as it does in the
Membership Update message).A 32-bit value copied from the Request Nonce field () 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.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 () 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.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 to identify the tunnel
instance to tear down. The relay provides this value to the
gateway using the Gateway IP Address field () 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
). This address must match that used
by the relay to compute the value stored in the Response MAC
field.The following sections describe gateway implementation
requirements. A non-normative discussion of gateway operation may be
found in .Gateway operation requires a subset of 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) 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
and Section 6.1 of. 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 . These datagrams MUST possess the IP headers, header
options and header values called for in , with the following exception; a gateway
MAY use any source address value in an IGMP report datagram
including the "unspecified" address (all octets are zero ). This
exception is made because a gateway pseudo-interface might not
possess a valid IPv4 address, and even if an address has been
assigned to the interface, that address might not be a valid
link-local source address on any relay interface. It is for this
reason that a relay must accept encapsulated IGMP reports regardless
of the source address they carry. See .The encapsulated MLD messages generated by a gateway MUST conform
to the description found in Section 5 of
. These datagrams MUST possess the IP headers, header options
and header values called for in , with
the following exception; a gateway MAY use any source address value
in an MLD report datagram including the "unspecified" address (all
octets are zero ). This exception is made because a gateway
pseudo-interface might not possess a valid IPv6 address, and even if
an address has been assigned to the interface, that address might
not be a valid link-local source address on any relay interface. As
with IGMP, it is for this reason that a relay must accept
encapsulated MLD reports regardless of the source address they
carry. See .The gateway IGMP/MLD implementation SHOULD retransmit unsolicited
membership state-change reports and merge new state change reports
with pending reports as described in Section
5.1 of and Section 6.1 of. The
number of retransmissions is specified by the relay in the Querier's
Robustness Variable (QRV) field in the last general query forwarded
by the pseudo-interface. See Section 4.1.6
in and Section 5.1.8 in.The gateway IGMP/MLD implementation SHOULD handle general query
messages as described in Section 5.2
of and Section 6.2 of, but MAY
ignore the Max Resp Code field value and generate a current state
report without any delay.An IPv4 gateway implementation MUST accept IPv4 datagrams that
carry the general query variant of the IGMPv3 Membership Query
message, as described in Section 4 of . The gateway MUST accept the IGMP datagram
regardless of the IP source address carried by that datagram.An IPv6 gateway implementation MUST accept IPv6 datagrams that
carry the general query variant of the MLDv2 Multicast Listener
Query message, as described in Section 5 of . The gateway MUST accept the MLD datagram
regardless of the IP source address carried by that datagram.A gateway host may possess or create multiple gateway
pseudo-interfaces, each with a unique configuration that describes a
binding to a specific IP protocol, relay address, relay discovery
address or upstream network interface.If a gateway implementation uses AMT relay discovery to obtain
a relay address, it must first be supplied with a relay discovery
address. The relay discovery address may be an anycast or unicast
address. A gateway implementation may rely on a static address
assignment or some form of dynamic address discovery. This
specification does not require that a gateway implementation use
any particular method to obtain a relay discovery address - an
implementation may employ any method that returns a suitable relay
discovery address.Before a gateway implementation can execute the AMT protocol to
request and receive multicast traffic, it must be supplied with a
unicast relay address. A gateway implementation may rely on static
address assignment or support some form of dynamic address
discovery. This specification does not require the use of any
particular method to obtain a relay address - an implementation
may employ any method that returns a suitable relay address.A gateway host that possesses multiple network interfaces or
addresses may allow for an explicit selection of the interface to
use when communicating with a relay. The selection might be made
to satisfy connectivity, tunneling or IP protocol
requirements.A gateway implementation that supports retransmission MAY
require the following information:Initial time to wait for a response to a
Relay Discovery message.Maximum number of Relay Discovery
retransmissions to allow before terminating relay discovery
and reporting an error.Initial
time to wait for a response to a Request message.Maximum number of Request retransmissions to
allow before abandoning a relay and restarting relay discovery
or reporting an error.The maximum number of times a gateway should
attempt to send the same Request or Membership Update message
after receiving an ICMP "Destination Unreachable".In the following descriptions, a gateway pseudo interface is
treated as a passive entity managed by a gateway service. The
gateway pseudo-interface provides the state and the gateway service
provides the processing. The term "gateway" is used when describing
service behavior with respect to a single pseudo-interface.When a gateway pseudo-interface is started, the gateway service
begins listening for AMT messages sent to the UDP endpoint(s)
associated with the pseudo-interface and for any locally-generated
IGMP/MLD messages passed to the pseudo-interface. The handling of
these messages is described below.When the pseudo-interface is enabled, the gateway service
MAY:Optionally execute the relay discovery procedure described
in .Optionally execute the membership query procedure described
in
to start the periodic membership update cycle.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 .A gateway may receive Multicast Data messages after it sends a
Membership Update message to a relay that adds a group
subscription. The gateway may continue to receive Multicast Data
messages long after the gateway sends a Membership Update message
that deletes existing group subscriptions. The gateway MUST be
prepared to receive these messages at any time, but MAY ignore
them or discard their contents if the gateway no longer has any
interest in receiving the multicast datagrams contained within
them.A gateway MUST ignore a Multicast Data message if it fails to
satisfy any of the following requirements: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).The destination address carried by the encapsulated IP
datagram MUST fall within the multicast address allocation
assigned to the relevant IP protocol, i.e., 224.0.0.0/4 for
IPv4 and FF00::/8 for IPv6.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 . If supported, a gateway SHOULD employ
the solution described in 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.This section describes gateway requirements related to the
relay discovery message sequence described in .A gateway may start or restart the relay discovery procedure
in response to the following events:When a gateway pseudo-interface is started (enabled).When the gateway wishes to report a group subscription
when none currently exist.Before sending the next Request message in a membership
update cycle, i.e., each time the query timer expires (see
below).After the gateway fails to receive a response to a
Request message.After the gateway receives a Membership Query message
with the L-flag set to 1.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 .The gateway MUST save a copy of 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 .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
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 ).When a gateway receives a Relay Advertisement message it must
first determine whether it should accept or ignore the message.
A gateway MUST ignore a Relay Advertisement message if it fails
to satisfy any of the following requirements:The gateway MUST be waiting for a Relay Advertisement
message.The Discovery Nonce value contained in the Relay
Advertisement message MUST equal to the Discovery Nonce
value contained in the Relay Discovery message.The source IP address and UDP port of the Relay
Advertisement message MUST equal to the destination IP
address and UDP port of the matching Relay Discovery
message.Once a gateway receives a Relay Advertisement response
to a Relay Discovery message, it SHOULD ignore any other Relay
Advertisements that arrive on the AMT interface until it sends a
new Relay Discovery message.If a gateway executes the relay discovery procedure at the
start of each membership update cycle and the relay address
returned in the latest Relay Advertisement message differs from
the address returned in a previous Relay Advertisement message,
then the gateway SHOULD send a Teardown message (if supported)
to the old relay address, using information from the last
Membership Query message received from that relay, as described
in . This
behavior is illustrated in the following diagram.The discovery nonce MUST be a random, non-zero, 32-bit value,
and if possible, SHOULD be computed using a cryptographically
secure pseudo random 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.This section describes gateway requirements related to the
membership update message sequence described in .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:When the gateway pseudo-interface is activated.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:Better performance by allowing state-change reports to be
sent as they are generated, thus minimizing the time to
join.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.Simplified implementation by eliminating any need to
queue IGMP/MLD messages for delivery after a Membership
Query is received, since the IGMP/MLD state-change messages
may be sent as they are generated.However, this approach places an additional load on
relays as a gateway will send periodic requests even when it has
no multicast subscriptions. To reduce load on a relay, a gateway
SHOULD only send a Membership Update message while it has active
group subscriptions. A relay will still need to compute a
Response MAC for each Request, but will not be required to
recompute it a second time to authenticate a Membership Update
message that contains no subscriptions.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 . The gateway
MUST set the "P" flag in the Request message to identify the
protocol the gateway wishes the relay to use for the general
query response.A gateway MUST send a Request message using the current Relay
Address and 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 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 .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 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 ).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.When a gateway receives a Membership Query message it must
first determine whether it should accept or ignore the message.
A gateway MUST ignore a Membership Query message, or the
encapsulated IP datagram within it, if the message fails to
satisfy any of the following requirements:The gateway MUST be waiting for a Membership Query
message.The Request Nonce value contained in the Membership Query
MUST equal the Request Nonce value contained in the Request
message.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).The encapsulated IP datagram MUST carry an IGMPv3 or
MLDv2 message. The protocol MUST match the protocol
identified by the "P" flag in the Request message.The IGMPv3 or MLDv2 message MUST be a general query
message.The total length of the encapsulated IP datagram as
computed from the lengths contained in the datagram
header(s) MUST NOT exceed the available field length within
the Membership Query message.Once a gateway receives a Membership Query response to a
Request message, it SHOULD ignore any other Membership Query
messages that arrive on the AMT interface until it sends a new
Request message.The gateway MUST save the Membership Query message, or the
Request Nonce, Response MAC, Gateway IP Address and Gateway Port
Number fields for use in sending subsequent Membership Update
and Teardown messages.The gateway extracts the encapsulated IP datagram and
forwards it to the local IP protocol implementation for checksum
verification and dispatching to the IGMP or MLD implementation
running on the pseudo-interface. The gateway MUST NOT forward
any octets that might exist between the encapsulated IP datagram
and the end of the message or Gateway Address fields.The MLD protocol specification indicates that senders should
use a link-local source IP address in message datagrams. This
requirement must be relaxed for AMT because gateways and relays
do not normally share a common subnet. For this reason, a
gateway implementation MUST accept MLD (and IGMP) query message
datagrams regardless of the source IP address they carry. This
may require additional processing on the part of the gateway
that might be avoided if the relay and gateway use the IPv4 and
IPv6 addresses allocated for use in AMT encapsulated control
packets as described in .The gateway MUST start a timer that will trigger the next
iteration of the membership update cycle by executing the
membership query procedure. The gateway SHOULD compute the timer
duration from the Querier's Query Interval Code carried by the
general-query. A gateway MAY use a smaller timer duration if
required to refresh a NAT mapping that would otherwise timeout.
A gateway MAY use a larger timer duration if it has no group
subscriptions to report.If the gateway supports the Teardown message and the G-flag
is set in the Membership Query message, the gateway MUST compare
the Gateway IP Address and Gateway Port Number on the new
Membership Query message with the values carried by the previous
Membership Query message. If either value has changed the
gateway MUST send a Teardown message to the relay as described
in .If the L-flag is set in the Membership Query message, the
relay is reporting that it is NOT accepting Membership Update
messages that create new tunnel endpoints and will simply ignore
any that do. If the L-flag is set and the gateway is not
currently reporting any group subscriptions to the relay, the
gateway SHOULD stop sending periodic Request messages and
restart the relay discovery procedure (if discovery is enabled)
to find a new relay with which to communicate. 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 and the gateway endpoint address
has not changed since the last Membership Query was received
(see previous paragraph).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.The request nonce MUST be a random value, and if possible,
SHOULD be computed using a cryptographically secure pseudo
random 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.This section describes gateway requirements related to the
membership update message sequence described in .The membership update process is primarily driven by the
host-mode IGMP or MLD protocol implementation running on the
gateway pseudo-interface. The IGMP and MLD protocols produce
current-state reports in response to general queries 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.The gateway pseudo-interface MUST accept the following IP
datagrams from the IPv4/IGMP and IPv6/MLD protocols running on
the pseudo-interface:IPv4 datagrams that carry an IGMPv2, or IGMPv3 Membership
Report or an IGMPv2 Leave Group message as described in
Section 4 of .IPv6 datagrams that carry an MLDv1 or MLDv2 Multicast
Listener Report or an MLDv1 Multicast Listener Done message
as described in Section 5 of .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:Discard IGMP or MLD datagrams until it receives a
Membership Query message, at which time it processes the
Membership Query message as normal to eventually produce a
current-state report on the pseudo-interface which describes
the end state (RECOMMENDED).Insert IGMP or MLD datagrams into a queue for
transmission after it receives a Membership Query
message.If and when a gateway receives a Membership Query
message (for IGMP or MLD) it sends any queued or incoming IGMP
or MLD datagrams to the relay as described in the next
section.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 to obtain a
relay address. If the gateway has a relay address, but has not
yet received a Membership Query message, it MUST first execute
the membership query procedure described in to obtain a
Request Nonce and Response MAC that can be used to send a
Membership Update message.Once a gateway possesses a valid Relay Address, Request Nonce
and Response MAC, it may encapsulate the IP datagram containing
the IGMP/MLD message into a Membership Update message. The
gateway MUST copy the Request Nonce and Response MAC values from
the last Membership Query received from the relay into the
corresponding fields in the Membership Update. The gateway MUST
send the Membership Update message using the Relay Address and
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 .This section describes gateway requirements related to the
teardown message sequence described in .Gateway support for the Teardown message is RECOMMENDED.A gateway that supports Teardown SHOULD make use of Teardown
functionality if it receives a Membership Query message from a
relay that has the "G" flag set to indicate that it contains valid
gateway address fields.As described in , if a
gateway supports the Teardown message, has reported active group
subscriptions, and receives a Membership Query message with the
"G" flag set, the gateway MUST compare the Gateway IP Address
and Gateway Port Number on the new Membership Query message with
the values carried by the previous Membership Query message. If
either value has changed the gateway MUST send a Teardown
message as described in the next section.A gateway sends a Teardown message to a relay to request that
it stop delivering Multicast Data messages to the gateway and
delete any group memberships created by the gateway.When a gateway constructs a Teardown message, it MUST copy
the Request Nonce, Response MAC, Gateway IP Address and Gateway
Port Number fields from the Membership Query message that
provided the Response MAC for the last Membership Update message
sent, into the corresponding fields of the Teardown message.A gateway MUST send the Teardown message using the Relay
Address and 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 Section 4.1.6
in and Section 5.1.7 in).
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 .When a gateway pseudo-interface is stopped and the gateway has
existing group subscriptions, the gateway SHOULD either:Send a Teardown message to the relay as described in , but only if the
gateway supports the Teardown message, and the current relay
is returning gateway address fields in Membership Query
messages, orSend a Membership Update message to the relay that will
delete existing group subscriptions.A gateway may receive an ICMP "Destination Unreachable" message
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:The gateway MAY simply abandon the current relay and
restart relay discovery (if used). This is the least desirable
approach as it does not allow for transient network
changes.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.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:The gateway MAY simply abandon the current relay and
restart relay discovery (if used). This is the least desirable
approach as it does not allow for transient network
changes.If the last message sent was a Relay Discovery, Request or
Teardown message, the gateway MAY attempt to transmit the new
message. If the gateway is configured to retransmit Relay
Discovery, Request or Teardown messages, the normal
retransmission behavior for those messages is preserved to
prevent the gateway from prematurely abandoning a relay.If the last message sent was a Membership Update message,
the gateway SHOULD start a new membership update and
associated Request retransmission cycle.The following sections describe relay implementation requirements.
A non-normative discussion of relay operation may be found in .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 and Section 7.3 of.A relay MUST handle incoming reports as described in Section 6.4 of and Section 7.4 of 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.If a relay is deployed for anycast discovery, the relay MUST
advertise an anycast Relay Discovery Address Prefix into the unicast
routing system of the anycast domain. An address within that prefix,
i.e., a Relay Discovery Address, MUST be assigned to a relay
interface.A unicast IPv4 and/or IPv6 address MUST be assigned to the relay
interface that will be used to send and receive AMT control and data
messages. This address or addresses are returned in Relay
Advertisement messages.The remaining details of relay "startup" are highly
implementation-dependent and are not addressed in this document.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.A relay MUST ignore any message other than a Relay Discovery,
Request, Membership Update or Teardown message. The handling of
Relay Discovery, Request, Membership Update, and Teardown messages
is addressed in the sections that follow.Support for the Teardown message is OPTIONAL. If a relay does
not support the Teardown message, it MUST also ignore this
message.A relay that conforms to this specification MUST ignore any
message with a Version field value other than zero.This section describes relay requirements related to the relay
discovery message sequence described in .A relay MUST accept and respond to Relay Discovery messages
sent to an anycast relay discovery address or the unicast relay
address. If a relay receives a Relay Discovery message sent to its
unicast address, it MUST respond just as it would if the message
had been sent to its anycast discovery address.When a relay receives a Relay Discovery message it responds by
sending a Relay Advertisement message back to the source of the
Relay Discovery message. The relay MUST use the source IP address
and UDP port of the Relay Discovery message as the destination IP
address and UDP port. The relay MUST use the destination IP
address and UDP port of the Relay Discovery as the source IP
address and UDP port 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.This section describes relay requirements related to the
membership query portion of the message sequence described in
.When a relay receives a Request message it responds by sending
a Membership Query message back to the source of the Request
message.The relay MUST use the source IP address and UDP port of the
Request message as the destination IP address and UDP port for the
Membership Query message. The source IP address and UDP port
carried by the Membership Query MUST match the destination IP
address and UDP port of the Request to ensure successful NAT
traversal.The relay MUST return the value contained in the Request Nonce
field of the Request message in the Request Nonce field of the
Membership Query message. The relay MUST compute a MAC value, as
described in ,
and return that value in the Response MAC field of the Membership
Query message.If a relay supports the Teardown message, it MUST set the
G-flag in the Membership Query message and return the source IP
address and UDP port carried by the Request message in the
corresponding Gateway IP Address and Gateway Port Number fields.
If the relay does not support the Teardown message it SHOULD NOT
set these fields as this may cause the gateway to generate
unnecessary Teardown messages.If the P-flag in the Request message is 0, the relay MUST
return an IPv4-encapsulated IGMPv3 general query in the Membership
Query message. If the P-flag is 1, the relay MUST return an
IPv6-encapsulated MLDv2 general query in the Membership Query
message.If the relay is not accepting Membership Update messages that
create new tunnel endpoints due to resource limitations, it SHOULD
set the L-flag in the Membership Query message to notify the
gateway of this state. Support for the L-flag is OPTIONAL. See
.The encapsulated IGMPv3 general query datagrams generated by a
relay MUST conform to the descriptions found in Section 4.1 of . These datagrams MUST
possess the IP headers, header options and header values called
for in , with the following
exception; a relay MAY use any source IP address for an IGMP
general query datagram including the "unspecified" address (all
octets are zero). This exception is made because any source
address that a relay might normally send may not be a valid
link-local address on any gateway interface. It is for this reason
that a gateway must accept encapsulated IGMP queries regardless of
the source address they carry. See .The encapsulated MLDv2 general query datagrams generated by a
relay MUST conform to the descriptions found in Section 5.1 of . These datagrams MUST
possess the IP headers, header options and header values called
for in , with the following
exception; a relay MAY use any source IP address for an MLD
general query datagram including the "unspecified" address (all
octets are zero). This exception is made because any source
address that a relay might normally send may not be a valid
link-local address on any gateway interface. As with IGMP, it is
for this reason that a gateway must accept encapsulated MLD
queries regardless of the source address they carry. See .A relay MUST set the Querier's Query Interval Code (QQIC) field
in the general query to supply the gateway with a suggested time
duration to use for the membership query timer. The QQIC field is
defined in Section 4.1.7 in and
Section 5.1.9 in . 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 specified in the QQIC field, so a relay may
be limited in its ability to spread out Requests coming from a
gateway.A relay MUST set the Querier's Robustness Variable (QRV) field
in the general query to a non-zero value. This value SHOULD be
greater than one. If a gateway retransmits membership state change
messages, it will retransmit them (robustness variable - 1) times.
The QRV field is defined in Section 4.1.6
in and Section 5.1.8 in.A relay SHOULD set the Maximum Response Code field in the
general query to a value of 1 to trigger an immediate response
from the gateway (some host IGMP/MLD implementations may not
accept a value of zero). A relay SHOULD NOT use the IGMPv3/MLDv2
Query Response Interval variable, if available, to generate the
Maximum Response Code field value as the Query Response Interval
variable is used in setting the duration of group state timers and
must not be set to such a small value. The Maximum Response Code
field is defined in Section 4.1.1 in
and Section 5.1.3 in. See .This section describes relay requirements related to the
membership update portion of the message sequence described in
.When a relay receives a Membership Update message it must first
determine whether it should accept or ignore the message. A relay
MUST NOT make any changes to group membership and forwarding state
if the message fails to satisfy any of the following
requirements: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.The encapsulated IP datagram MUST satisfy the IP header
requirements for the IGMP or MLD message type as described in
Section 4 of , Section 2 of
, Section 5 of , and Section 3 of , with the following exception - a
relay MUST accept an IGMP or MLD message regardless of the IP
source address carried by the datagram.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.The computed checksums for the encapsulated IP datagram and
its payload MUST match the values contained therein. Checksum
computation and verification varies by protocol; See for IPv4, for IGMPv3, and for MLD (ICMPv6).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. Because the
private secret used to compute Response MAC values may change
over time, the relay MUST retain the previous version of the
private secret to use in authenticating Membership Updates
sent during the subsequent query interval. If the first
attempt at Response MAC authentication fails, the relay MUST
attempt to authenticate the Response MAC using the previous
private secret value unless 2*query_interval time has elapsed
since the private secret change. See . An alternative
approach to Response MAC generation that avoids repeated
Response MAC computations may be found in .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: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.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 , a
relay uses the source IP address and source UDP port carried by a
Membership Update messages 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
.A relay MUST maintain some form of group membership database
for each endpoint. The per-endpoint databases are used update a
forwarding table containing entries that map an (*,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.This section describes relay requirements related to the
teardown message sequence described in .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 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 . A
relay MUST ignore a Teardown message If the computed MAC does not
equal the value of the Response MAC field.If a relay determines that a Teardown message is authentic, it
MUST immediately stop transmitting Multicast Data messages to the
endpoint identified by the Gateway IP Address and Gateway Port
Number fields in the message. The relay MUST eventually delete any
group membership and forwarding state associated with the
endpoint, but MAY delay doing so to allow a gateway to recreate
group membership state on a new endpoint and thereby avoid making
unnecessary (temporary) changes in upstream routing/forwarding
state.The state changes made by a relay when processing a Teardown
message MUST be identical to those that would be made 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.When a multicast IP datagram is forwarded to the relay
pseudo-interface, the relay MUST, for each gateway that has
expressed an interest in receiving the datagram, encapsulate the
IP datagram into a Multicast Data message or messages and send
that message or messages to the gateway. This process is highly
implementation dependent, but conceptually requires the following
steps: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.Possibly replicate the datagram for each gateway endpoint
listed for that (*,G) or (S,G) entry.If the multicast IP datagram size exceeds the Tunnel MTU as
determined according to the procedure described in , the relay must
execute the procedure described in .Encapsulate and transmit the IP datagram according to the
procedure described in .The relay pseudo-interface MUST ignore any other IP datagrams
forwarded to the pseudo-interface.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:The relay MAY provide a configuration option that
establishes a fixed PMTU that will be applied to all AMT
tunnels originating at the relay.The relay MAY dynamically adjust PMTU value(s) in
response to receipt of ICMP/ICMPv6 "Datagram Too Big"
messages as described in and
.If a relay supports dynamic adjustment of per-tunnel or
per-relay PMTU values in response to ICMP messages, the relay
MUST provide a configuration option that disables this feature
and also provide a configuration option that establishes a
minimum PMTU for all tunnels. These configuration options may be
used to mitigate certain types of denial of service attacks
(See). When
dynamic PMTU adjustments are disabled, the PMTU for all tunnels
MUST default to the Link MTU (first-hop) on the downstream
interface.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.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
Destination Unreachable message
to the source, with type equal to 4 (fragmentation needed and
DF set). The ICMP Destination Unreachable message MUST contain
an next-hop MTU (as specified by ) and the relay MUST set the next-hop
MTU to the TMTU associated with the tunnel or tunnels. If the
DF bit in the multicast datagram header is set to 0 (May
Fragment), the relay MUST fragment the datagram and
encapsulate each fragment within Multicast Data messages for
transmission through the tunnel or tunnels. This ensures that
gateways will receive complete, non-fragmented Multicast Data
messages, containing fragmented multicast datagram payloads.
The relay SHOULD avoid generating a separate ICMP message for
each tunnel, but instead send a single ICMP message with a
Next-hop MTU equal to the smallest TMTU of all tunnels to
which the datagram was to be forwarded.The relay MUST discard the packet and, if the datagram
originated from an SSM source, send an ICMPv6 Packet Too Big message to the payload
source. The MTU specified in the Packet Too Big message MUST
be equal to the TMTU associated with the tunnel or tunnels.
The relay SHOULD avoid generating a separate ICMPv6 message
for each tunnel, but instead send a single ICMPv6 message with
a Next-hop MTU equal to the smallest TMTU of all tunnels to
which the datagram was to be forwarded.A relay encapsulates a multicast IP datagram in a UDP/IP
Membership Data message, using the tunnel endpoint UDP/IP
address as the destination address and the unicast relay address
and 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 .The following sections describe additional requirements
related to the IP protocol of the tunnel and that of the
multicast IP datagram.When a relay delivers an IPv4 payload over an IPv4 tunnel,
and the DF Bit in the payload header is set to 1 (Don't
Fragment), the relay MUST set the DF bit in the Multicast Data
IP header to 1. When a relay delivers an IPv4 payload over an
IPv4 tunnel, and the DF Bit in the payload header is set to 0
(May Fragment), by default, the relay MUST set the DF bit in
the Multicast Data IP header to 1. However, a relay MAY
provide a configuration option that allows the DF bit to be
copied from the payload header to the Multicast Data IP header
to allow downstream fragmentation of the Multicast Data
message. When a relay delivers an IPv6 payload over an IPv4
tunnel, the relay MUST set the DF bit in the Multicast Data IP
header to 1. The relay MUST NOT transmit a Multicast Data
message with an IP header in which the MF (More Fragments) bit
is set to 1.When a tunneling over IPv6, a relay MUST NOT emit a
Multicast Data message datagram containing an IPv6 fragment
header.If a relay receives a sequence of ICMP or ICMPv6 messages of
type "Destination Unreachable" 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
the tunnel for that gateway (Handling of ICMP "Destination
Unreachable" messages with code 4, "fragmentation required" is
covered in ). If
a relay provides this capability, it MUST provide a
configuration option that indicates what number of sequential
"Destination Unreachable" messages can be received and ignored
before the relay will automatically shutdown a tunnel.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
or Section 7 of, 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_IntervalIf IGMP/MLD default values are used for these variables, the
gateway will timeout 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.A relay may be configured with various service limits to ensure
a minimum level of performance for gateways that connect to
it.If a relay has determined that it has reached or exceeded
maximum allowable capacity or has otherwise exhausted resources
required to support additional gateways, it SHOULD withdraw any
Relay Discovery Address Prefix it has advertised into the unicast
internetwork and SHOULD set the L-flag in any Membership Query
messages it returns to gateways while in this state.If the relay receives an update from a gateway that adds group
membership or forwarding state for an endpoint that has already
reached maximum allowable state entries, the relay SHOULD continue
to accept updates from the gateway but ignore any group
membership/forwarding state additions requested by that
gateway.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.The following steps should be treated as an abstract description
of the shutdown procedure for a relay:Withdraw the Relay Discovery Address Prefix advertisement (if
used).Stop listening for Relay Discovery messages.Stop listening for control messages from gateways.Stop sending data messages to gateways.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.A Response MAC is produced by a hash digest computation. A
Response MAC computation is required in the following
situations:To generate a Response MAC value from a Request message for
inclusion in a Membership Query message.To generate a Response MAC value from a Membership Update
message for use in authenticating the Response MAC carried
within that message.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 hash function RECOMMENDED for use in computing the
Response MAC is the MD5 hash digest ,
though hash functions or keyed-hash functions of greater
cryptographic strength may be used.The digest MUST be computed over the following values:The Source IP address of the message (or Teardown Gateway IP
Address field)The Source UDP port of the message (or Teardown Gateway Port
Number field)The Request Nonce contained in the message.A private secret known only to the relayAn Response MAC generation solution that satisfies these
requirements is described in .The private secret, or hash-key, is a random value that the relay
includes in the Response MAC hash digest computation. A 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.The private secret SHOULD be computed using a
cryptographically-secure pseudo-random number generator. The private
secret width SHOULD equal that of the hash function used to compute
the Response MAC, e.g., 128-bits for an MD5 hash.AMT is not intended to be a strongly secured protocol. In general,
the protocol provides the same level of security and robustness as is
provided by the UDP, IGMP and MLD protocols on which it relies. The lack
of strong security features can largely be attributed to the desire to
make the protocol light-weight 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 may be employed against the protocol to launch
various types of denial-of-service attacks that can affect the
functioning of gateways or their ability to locate and communicate with
a relay. These scenarios are described below.As is the case for UDP, IGMP and MLD, the AMT protocol provides no
mechanisms for ensuring message delivery or integrity. The protocol does
not provide confidentiality - multicast groups, sources and streams
requested by a gateway are sent in the clear.The protocol does use a three-way handshake to provide trivial source
authentication for state allocation and updates (see below). The
protocol also requires gateways and relays to ignore malformed messages
and those messages that do not carry expected address values or protocol
payload types or content.The three-way handshake provided by the membership update message
sequence (See) provides a
defense against source-spoofing-based resource-exhaustion attacks on a
relay by requiring source authentication before state allocation.
However, attackers may still attempt to flood a relay with Request and
Membership Update messages to force the relay to make the hash
computations in an effort to consume computational resources.
Implementations may choose to limit the frequency with which a relay
responds to Request messages sent from a single IP address or IP
address and UDP port pair, but support for this functionality is not
required. The three-way handshake provides no defense against an
eavesdropping or man-in-the-middle attacker.Attackers that execute the gateway protocol may consume relay
resources by instantiating a large number of tunnels or joining a
large number of multicast streams. A relay implementation should
provide a mechanism for limiting the number of tunnels (Multicast Data
message destinations) that can be created for a single gateway source
address. Relays should also provide a means for limiting the number of
joins per tunnel instance as a defense against these attacks.Relays may withdraw their AMT anycast prefix advertisement when
they reach configured maximum capacity or exhaust required resources.
This behavior allows gateways to use the relay discovery process to
find the next topologically-nearest relay that has advertised the
prefix. This behavior also allows a successful resource exhaustion
attack to propagate from one relay to the next until all relays
reachable using the anycast address have effectively been taken
offline. This behavior may also be used to acquire the unicast
addresses for individual relays which 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, 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 is vulnerable to
two denial of service attacks (see Section 8 of 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.A passive eavesdropper may launch a denial-of-service attack on a
gateway by capturing a Membership Query or Membership Update message
and using the request nonce and message authentication code carried by
the captured message to send a spoofed 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 disrupt or supplant existing
traffic flows. A properly implemented gateway will filter Multicast
Data messages that do not originate from the expected relay address
and should filter non-multicast packets and multicast IP packets whose
group or source addresses are not included in the current reception
state for the gateway pseudo-interface.An active eavesdropper may launch a man-in-the-middle attack in
which messages normally exchanged between a gateway and relay are
intercepted, modified, spoofed or discarded by the attacker. The
attacker may deny access to, modify or replace requested multicast
traffic. The AMT protocol provides no means for detecting or defending
against a man-in-the-middle attack - any such functionality must be
provided by multicast receiver applications through independent
detection and validation of incoming multicast datagrams.The anycast discovery technique for finding relays (see ) introduces a risk that a rogue
router or a rogue AS could introduce a bogus route to a specific Relay
Discovery Address prefix, and thus divert or absorb Relay Discovery
messages sent by gateways. Network managers must guarantee the
integrity of their routing to a particular Relay Discovery Address
prefix in much the same way that they guarantee the integrity of all
other routes.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 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.The following unicast prefixes have been assigned to provide
anycast routing of relay discovery messages to public AMT Relays as
described in .We suggest that IANA assign an x.x.x.x/24 from the IPv4 Recovered
Address Space Registry, but any /24 which has been unassigned and
unadvertised for at least twelve months is acceptable. The block
should be registered as follows:IANA should register the following special-purpose address block
for IPv6 anycast AMT relay discovery.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 this port number to AMT upon
acceptance of this I-D.The following people provided significant contributions to the design
of the protocol and earlier versions of this specification:The authors would like to thank the following individuals for their
suggestions, comments, and corrections: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 as well as an open source implementation.This specification does not require relays to use any particular
method to compute the Response MAC field value - only that it contain
a hash of the source IP address, source UDP port, request nonce, and a
private secret known only to the relay. This allows the relay
implementor a significant amount of leeway in the computation and
structure of the value stored in the Response MAC field.Section
states that a relay should periodically compute a new private secret
(or hash-key) for MAC generation. To prevent the relay from rejecting
Membership Update messages that contain Response MAC values computed
from an old secret, the relay is required to retain the previous
secret so that it can re-attempt authentication using the old secret,
should authentication fail after recomputing the MAC using the new
secret. However, this approach requires a relay to do at least two
hash computations for every Membership Update message that carries an
old or a invalid MAC. A better approach would be to include
information within the message that the relay could use to choose a
single secret for authentication rather relying on sequential
authentication failures to test all possible secrets.The solution proposed here is to compute and exchange an
"authentication cookie" rather than a simple hash value in the
Response MAC field. The authentication cookie would combine a
timestamp with a hash value. The timestamp is used to calculate the
age of the cookie, allowing the relay to reject a message if the
cookie's age is greater than some maximum allowable value. If the
cookie has not expired, the relay uses the timestamp to lookup the
secret that was in use at that time and then compute and compare the
hash portion of the cookie to authenticate the message source.A second purpose served by including the timestamp in the MAC field
is that it allows the relay to contribute an unpredictable value to
the authentication hash. This contribution provides a defense against
attempts to use a hash reversal algorithm to determine the relay's
private secret as the hash result will change over time even if the
nonce carried by the Request message does not.The timestamp is an unsigned integer measured relative to the start
time of relay. The age of the MAC is computed by subtracting the MAC
timestamp from the current system timestamp. The operands must be
unsigned 16-bit integers and the subtraction must use unsigned
arithmetic to allow for timestamp wrap-around. The timestamp
resolution must provide range sufficient to handle the maximum
allowable age for a MAC, e.g., a resolution of 1 second allows a
maximum age of 18 hours. The timestamp should start at a random value
by adding a random offset, computed at startup, to the current system
time.The timestamp is not only used to compute the age of the MAC, but
is also used to lookup the private secret used to generate the MAC.
Each time a new private secret is computed, the value and the time at
which the value was computed is pushed into a fixed-length queue of
recent values (typically only 2-deep). The relay uses the timestamp
contained in the MAC field to lookup the appropriate secret. The relay
iterates over the list of secrets, starting with the newest entry,
until it finds the first secret with a timestamp that is older than
that contained in the MAC field. The relay then uses that secret to
compute the MAC that will be compared with that carried by the
message.