RFC 7450
Automatic Multicast Tunneling
5. Protocol Description
This section provides a normative description of the AMT protocol.5.1. Protocol Messages
The AMT protocol defines seven message types for control and encapsulation. These messages are assigned the following names and numeric identifiers: +--------------+---------------------+ | Message Type | Message Name | +--------------+---------------------+ | 1 | Relay Discovery | | 2 | Relay Advertisement | | 3 | Request | | 4 | Membership Query | | 5 | Membership Update | | 6 | Multicast Data | | 7 | Teardown | +--------------+---------------------+ These messages are exchanged as IPv4 or IPv6 UDP datagrams.5.1.1. Relay Discovery
A Relay Discovery message is used to solicit a response from a relay in the form of a Relay Advertisement message. The UDP/IP datagram containing this message MUST carry a valid, non-zero UDP checksum and carry the following IP address and UDP port values: Source IP Address - The IP address of the gateway interface on which the gateway will listen for a relay response. Note: The value of this field may be changed as a result of network address translation before arriving at the relay. Source UDP Port - The UDP port number on which the gateway will listen for a relay response. Note: The value of this field may be changed as a result of network address translation before arriving at the relay. Destination IP Address - An anycast or unicast IP address, i.e., the Relay Discovery Address advertised by a relay. Destination UDP Port - The AMT port number (see Section 7.2).
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | V=0 |Type=1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Discovery Nonce | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 11: Relay Discovery Message Format5.1.1.1. Version (V)
The protocol version number for this message is 0.5.1.1.2. Type
The type number for this message is 1.5.1.1.3. Reserved
Reserved bits that MUST be set to zero by the gateway and ignored by the relay.5.1.1.4. Discovery Nonce
A 32-bit random value generated by the gateway and echoed by the relay in a Relay Advertisement message. This value is used by the gateway to correlate Relay Advertisement messages with Relay Discovery messages. Discovery nonce generation is described in Section 5.2.3.4.5.5.1.2. Relay Advertisement
The Relay Advertisement message is used to supply a gateway with a unicast IP address of a relay. A relay sends this message to a gateway when it receives a Relay Discovery message from that gateway. The UDP/IP datagram containing this message MUST carry a valid, non-zero UDP checksum and carry the following IP address and UDP port values: Source IP Address - The destination IP address carried by the Relay Discovery message (i.e., the Relay Discovery Address advertised by the relay). Source UDP Port - The destination UDP port carried by the Relay Discovery message (i.e., the AMT port number).
Destination IP Address - The source IP address carried by the Relay
Discovery message. Note: The value of this field may be changed
as a result of network address translation before arriving at the
gateway.
Destination UDP Port - The source UDP port carried by the Relay
Discovery message. Note: The value of this field may be changed
as a result of network address translation before arriving at the
gateway.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| V=0 |Type=2 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Discovery Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Relay Address (IPv4 or IPv6) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Relay Advertisement Message Format
5.1.2.1. Version (V)
The protocol version number for this message is 0.
5.1.2.2. Type
The type number for this message is 2.
5.1.2.3. Reserved
Reserved bits that MUST be set to zero by the relay and ignored by
the gateway.
5.1.2.4. Discovery Nonce
A 32-bit value copied from the Discovery Nonce field
(Section 5.1.1.4) contained in the Relay Discovery message. The
gateway uses this value to match a Relay Advertisement to a Relay
Discovery message.
5.1.2.5. Relay Address
The unicast IPv4 or IPv6 address of the relay. A gateway uses the length of the UDP datagram containing the Relay Advertisement message to determine the address family, i.e., length - 8 = 4 (IPv4) or 16 (IPv6). The relay returns an IP address for the protocol used to send the Relay Discovery message, i.e., an IPv4 address for an IPv4 Relay Discovery Address or an IPv6 address for an IPv6 Relay Discovery Address.5.1.3. Request
A gateway sends a Request message to a relay to solicit a Membership Query response. The successful delivery of this message marks the start of the first stage in the three-way handshake used to create or update state within a relay. The UDP/IP datagram containing this message MUST carry a valid, non-zero UDP checksum and carry the following IP address and UDP port values: Source IP Address - The IP address of the gateway interface on which the gateway will listen for a response from the relay. Note: The value of this field may be changed as a result of network address translation before arriving at the relay. Source UDP Port - The UDP port number on which the gateway will listen for a response from the relay. Note: The value of this field may be changed as a result of network address translation before arriving at the relay. Destination IP Address - The unicast IP address of the relay. Destination UDP Port - The AMT port number. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | V=0 |Type=3 | Reserved |P| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Request Nonce | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 13: Request Message Format
5.1.3.1. Version (V)
The protocol version number for this message is 0.5.1.3.2. Type
The type number for this message is 3.5.1.3.3. Reserved
Reserved bits that MUST be set to zero by the gateway and ignored by the relay.5.1.3.4. P Flag
The P flag is set to indicate which group membership protocol the gateway wishes the relay to use in the Membership Query response: Value Meaning 0 The relay MUST respond with a Membership Query message that contains an IPv4 packet carrying an IGMPv3 General Query message. 1 The relay MUST respond with a Membership Query message that contains an IPv6 packet carrying an MLDv2 General Query message.5.1.3.5. Request Nonce
A 32-bit random value generated by the gateway and echoed by the relay in a Membership Query message. This value is used by the relay to compute the Response MAC value and is used by the gateway to correlate Membership Query messages with Request messages. Request Nonce generation is described in Section 5.2.3.5.6.5.1.4. Membership Query
A relay sends a Membership Query message to a gateway to solicit a Membership Update response, but only after receiving a Request message from the gateway. The successful delivery of this message to a gateway marks the start of the second stage in the three-way handshake used to create or update tunnel state within a relay.
The UDP/IP datagram containing this message MUST carry a valid,
non-zero UDP checksum and carry the following IP address and UDP port
values:
Source IP Address - The destination IP address carried by the Request
message (i.e., the unicast IP address of the relay).
Source UDP Port - The destination UDP port carried by the Request
message (i.e., the AMT port number).
Destination IP Address - The source IP address carried by the Request
message. Note: The value of this field may be changed as a result
of network address translation before arriving at the gateway.
Destination UDP Port - The source UDP port carried by the Request
message. Note: The value of this field may be changed as a result
of network address translation before arriving at the gateway.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| V=0 |Type=4 | Reserved |L|G| Response MAC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Encapsulated General Query Message |
~ IPv4:IGMPv3(Membership Query) ~
| IPv6:MLDv2(Listener Query) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gateway Port Number | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+ +
| Gateway IP Address (IPv4 or IPv6) |
+ +
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: Membership Query Message Format
5.1.4.1. Version (V)
The protocol version number for this message is 0.5.1.4.2. Type
The type number for this message is 4.5.1.4.3. Reserved
Reserved bits that MUST be set to zero by the relay and ignored by the gateway.5.1.4.4. Limit (L) Flag
A 1-bit flag set to 1 to indicate that the relay is NOT accepting Membership Update messages from new gateway tunnel endpoints and that it will ignore any that are. A value of 0 has no special significance -- the relay may or may not be accepting Membership Update messages from new gateway tunnel endpoints. A gateway checks this flag before attempting to create new group subscription state on the relay to determine whether it should restart relay discovery. A gateway that has already created group subscriptions on the relay may ignore this flag. Support for this flag is RECOMMENDED.5.1.4.5. Gateway Address (G) Flag
A 1-bit flag set to 0 to indicate that the message does NOT carry the Gateway Port Number and Gateway IP Address fields, and 1 to indicate that it does. A relay implementation that supports the optional teardown procedure (see Section 5.3.3.5) SHOULD set this flag as well as the Gateway Port Number and Gateway IP Address field values. If a relay sets this flag, it MUST also include the Gateway Port Number and Gateway IP Address fields in the message. A gateway implementation that does not support the optional teardown procedure (see Section 5.2.3.7) MAY ignore this flag and the Gateway Address fields if they are present.5.1.4.6. Response MAC
A 48-bit source authentication value generated by the relay as described in Section 5.3.5. The gateway echoes this value in subsequent Membership Update messages to allow the relay to verify that the sender of a Membership Update message was the intended receiver of a Membership Query sent by the relay.
5.1.4.7. Request Nonce
A 32-bit value copied from the Request Nonce field (Section 5.1.3.5) carried by a Request message. The relay will have included this value in the Response MAC computation. The gateway echoes this value in subsequent Membership Update messages. The gateway also uses this value to match a Membership Query to a Request message.5.1.4.8. Encapsulated General Query Message
An IP-encapsulated IGMP or MLD message generated by the relay. This field will contain one of the following IP datagrams: IPv4:IGMPv3 Membership Query IPv6:MLDv2 Listener Query The source address carried by the query message should be set as described in Section 5.3.3.3. The Querier's Query Interval Code (QQIC) field in the General Query is used by a relay to specify the time offset a gateway should use to schedule a new three-way handshake to refresh the group membership state within the relay (current time + Query Interval). The QQIC field is defined in Section 4.1.7 of [RFC3376] and Section 5.1.9 of [RFC3810]. The Querier's Robustness Variable (QRV) field in the General Query is used by a relay to specify the number of times a gateway should retransmit unsolicited membership reports, encapsulated within Membership Update messages, and, optionally, the number of times to send a Teardown message. The QRV field is defined in Section 4.1.6 of [RFC3376] and Section 5.1.8 of [RFC3810].5.1.4.9. Gateway Address Fields
The Gateway Port Number and Gateway Address fields are present in the Membership Query message if, and only if, the G flag is set. A gateway need not parse the encapsulated IP datagram to determine the position of these fields within the UDP datagram containing the Membership Query message -- if the G flag is set, the gateway may simply subtract the total length of the fields (18 bytes) from the total length of the UDP datagram to obtain the offset.
5.1.4.9.1. Gateway Port Number
A 16-bit UDP port number containing a UDP port value. The relay sets this field to the value of the UDP source port of the Request message that triggered the Query message.5.1.4.9.2. Gateway IP Address
A 16-byte IP address that, when combined with the value contained in the Gateway Port Number field, forms the gateway endpoint address that the relay will use to identify the tunnel instance, if any, created by a subsequent Membership Update message. This field may contain an IPv6 address or an IPv4 address stored as an IPv4-compatible IPv6 address, where the IPv4 address is prefixed with 96 bits set to zero (see [RFC4291]). This address must match that used by the relay to compute the value stored in the Response MAC field.5.1.5. Membership Update
A gateway sends a Membership Update message to a relay to report a change in group membership state, or to report the current group membership state in response to receiving a Membership Query message. The gateway encapsulates the IGMP or MLD message as an IP datagram within a Membership Update message and sends it to the relay, where it may (see below) be decapsulated and processed by the relay to update group membership and forwarding state. A gateway cannot send a Membership Update message until it receives a Membership Query from a relay, because the gateway must copy the Request Nonce and Response MAC values carried by a Membership Query into any subsequent Membership Update messages it sends back to that relay. These values are used by the relay to verify that the sender of the Membership Update message was the recipient of the Membership Query message from which these values were copied. The successful delivery of this message to the relay marks the start of the final stage in the three-way handshake. This stage concludes when the relay successfully verifies that the sender of the Membership Update message was the recipient of a Membership Query message sent earlier. At this point, the relay may proceed to process the encapsulated IGMP or MLD message to create or update group membership and forwarding state on behalf of the gateway.
The UDP/IP datagram containing this message MUST carry a valid,
non-zero UDP checksum and carry the following IP address and UDP port
values:
Source IP Address - The IP address of the gateway interface on which
the gateway will listen for Multicast Data messages from the
relay. The address must be the same address used to send the
initial Request message, or the message will be ignored. Note:
The value of this field may be changed as a result of network
address translation before arriving at the relay.
Source UDP Port - The UDP port number on which the gateway will
listen for Multicast Data messages from the relay. This port must
be the same port used to send the initial Request message, or the
message will be ignored. Note: The value of this field may be
changed as a result of network address translation before arriving
at the relay.
Destination IP Address - The unicast IP address of the relay.
Destination UDP Port - The AMT port number.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| V=0 |Type=5 | Reserved | Response MAC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Encapsulated Group Membership Update Message |
~ IPv4:IGMP(Membership Report|Leave Group) ~
| IPv6:MLD(Listener Report|Listener Done) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15: Membership Update Message Format
5.1.5.1. Version (V)
The protocol version number for this message is 0.
5.1.5.2. Type
The type number for this message is 5.
5.1.5.3. Reserved
Reserved bits that MUST be set to zero by the gateway and ignored by the relay.5.1.5.4. Response MAC
A 48-bit value copied from the Response MAC field (Section 5.1.4.6) in a Membership Query message. Used by the relay to perform source authentication.5.1.5.5. Request Nonce
A 32-bit value copied from the Request Nonce field in a Request or Membership Query message. Used by the relay to perform source authentication.5.1.5.6. Encapsulated Group Membership Update Message
An IP-encapsulated IGMP or MLD message produced by the host-mode IGMP or MLD protocol running on a gateway pseudo-interface. This field will contain one of the following IP datagrams: IPv4:IGMPv2 Membership Report IPv4:IGMPv2 Leave Group IPv4:IGMPv3 Membership Report IPv6:MLDv1 Multicast Listener Report IPv6:MLDv1 Multicast Listener Done IPv6:MLDv2 Multicast Listener Report The source address carried by the message should be set as described in Section 5.2.1.5.1.6. Multicast Data
A relay sends a Multicast Data message to deliver a multicast IP datagram or datagram fragment to a gateway. The Checksum field in the UDP header of this message MAY contain a value of zero when sent over IPv4 but SHOULD, if possible, contain a valid, non-zero value when sent over IPv6 (see Section 4.2.2.3).
The UDP/IP datagram containing this message MUST carry the following
IP address and UDP port values:
Source IP Address - The unicast IP address of the relay.
Source UDP Port - The AMT port number.
Destination IP Address - A tunnel endpoint IP address, i.e., the
source IP address carried by the Membership Update message sent by
a gateway to indicate an interest in receiving the multicast
packet. Note: The value of this field may be changed as a result
of network address translation before arriving at the gateway.
Destination UDP Port - A tunnel endpoint UDP port, i.e., the source
UDP port carried by the Membership Update message sent by a
gateway to indicate an interest in receiving the multicast packet.
Note: The value of this field may be changed as a result of
network address translation before arriving at the gateway.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| V=0 |Type=6 | Reserved | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
~ IP Multicast Packet ~
| |
+ - - - - - - - - - - - - - - - - - - - - - - - -+
| : : : :
+-+-+-+-+-+-+-+-+- - - - - - - - - - - - - - - - - - - - - - - -
Figure 16: Multicast Data Message Format
5.1.6.1. Version (V)
The protocol version number for this message is 0.
5.1.6.2. Type
The type number for this message is 6.
5.1.6.3. Reserved
Reserved bits that MUST be set to zero by the relay and ignored by
the gateway.
5.1.6.4. IP Multicast Data
A complete IPv4 or IPv6 multicast datagram or datagram fragment.5.1.7. Teardown
A gateway sends a Teardown message to a relay to request that it stop sending Multicast Data messages to a tunnel endpoint created by an earlier Membership Update message. A gateway sends this message when it detects that a Request message sent to the relay carries an address that differs from that carried by a previous Request message. The gateway uses the Gateway IP Address and Gateway Port Number fields in the Membership Query message to detect these address changes. To provide backwards compatibility with early implementations of the AMT protocol, support for this message and associated procedures is considered OPTIONAL -- gateways are not required to send this message, and relays are not required to act upon it. The UDP/IP datagram containing this message MUST carry a valid, non-zero UDP checksum and carry the following IP address and UDP port values: Source IP Address - The IP address of the gateway interface used to send the message. This address may differ from that used to send earlier messages. Note: The value of this field may be changed as a result of network address translation before arriving at the relay. Source UDP Port - The UDP port number. This port number may differ from that used to send earlier messages. Note: The value of this field may be changed as a result of network address translation before arriving at the relay. Destination IP Address - The unicast IP address of the relay. Destination UDP Port - The AMT port number.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | V=0 |Type=7 | Reserved | Response MAC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Request Nonce | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Gateway Port Number | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | + + | Gateway IP Address (IPv4 or IPv6) | + + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 17: Membership Teardown Message Format5.1.7.1. Version (V)
The protocol version number for this message is 0.5.1.7.2. Type
The type number for this message is 7.5.1.7.3. Reserved
Reserved bits that MUST be set to zero by the gateway and ignored by the relay.5.1.7.4. Response MAC
A 48-bit value copied from the Response MAC field (Section 5.1.4.6) in the last Membership Query message the relay sent to the gateway endpoint address of the tunnel to be torn down. The gateway endpoint address is provided by the Gateway IP Address and Gateway Port Number fields carried by the Membership Query message. The relay validates the Teardown message by comparing this value with one computed from the Gateway IP Address field, Gateway Port Number field, Request Nonce field, and a private secret (just as it does in the Membership Update message).
5.1.7.5. Request Nonce
A 32-bit value copied from the Request Nonce field (Section 5.1.4.7) in the last Membership Query message the relay sent to the gateway endpoint address of the tunnel to be torn down. The gateway endpoint address is provided by the Gateway IP Address and Gateway Port Number fields carried by the Membership Query message. This value must match that used by the relay to compute the value stored in the Response MAC field.5.1.7.6. Gateway Port Number
A 16-bit UDP port number that, when combined with the value contained in the Gateway IP Address field, forms the tunnel endpoint address that the relay will use to identify the tunnel instance to tear down. The relay provides this value to the gateway using the Gateway Port Number field (Section 5.1.4.9.1) in a Membership Query message. This port number must match that used by the relay to compute the value stored in the Response MAC field.5.1.7.7. Gateway IP Address
A 16-byte IP address that, when combined with the value contained in the Gateway Port Number field, forms the tunnel endpoint address that the relay will use to identify the tunnel instance to tear down. The relay provides this value to the gateway using the Gateway IP Address field (Section 5.1.4.9.2) in a Membership Query message. This field may contain an IPv6 address or an IPv4 address stored as an IPv4-compatible IPv6 address, where the IPv4 address is prefixed with 96 bits set to zero (see [RFC4291]). This address must match that used by the relay to compute the value stored in the Response MAC field.5.2. Gateway Operation
The following sections describe gateway implementation requirements. A non-normative discussion of gateway operation may be found in Section 4.2.5.2.1. IP/IGMP/MLD Protocol Requirements
Gateway operation requires a subset of host-mode IPv4/IGMP and IPv6/ MLD functionality to provide group membership tracking, query processing, and report generation. A gateway MAY use IGMPv2 (ASM), IGMPv3 (ASM and SSM), MLDv1 (ASM), or MLDv2 (ASM and SSM).
An application with embedded gateway functionality must provide its own implementation of this subset of the IPv4/IGMP and IPv6/MLD protocols. The service interface used to manipulate group membership state need not match that described in the IGMP and MLD specifications, but the actions taken as a result SHOULD be similar to those described in Section 5.1 of [RFC3376] and Section 6.1 of [RFC3810]. The gateway application will likely need to implement many of the same functions as a host IP stack, including checksum verification, dispatching, datagram filtering and forwarding, and IP encapsulation/decapsulation. The encapsulated IGMP datagrams generated by a gateway MUST conform to the descriptions found in Section 4 of [RFC3376]. These datagrams MUST possess the IP headers, header options, and header values called for in [RFC3376], with the following exception: a gateway MAY use any source address value in an IGMP report datagram, including the "unspecified" address (all octets are zero). This exception is made because a gateway pseudo-interface might not possess a valid IPv4 address, and even if an address has been assigned to the interface, that address might not be a valid link-local source address on any relay interface. It is for this reason that a relay must accept encapsulated IGMP reports regardless of the source address they carry. See Section 5.3.1. The encapsulated MLD messages generated by a gateway MUST conform to the description found in Section 5 of [RFC3810]. These datagrams MUST possess the IP headers, header options, and header values called for in [RFC3810], with the following exception: a gateway MAY use any source address value in an MLD report datagram, including the "unspecified" address (all octets are zero). This exception is made because a gateway pseudo-interface might not possess a valid IPv6 address, and even if an address has been assigned to the interface, that address might not be a valid link-local source address on any relay interface. As with IGMP, it is for this reason that a relay must accept encapsulated MLD reports regardless of the source address they carry. See Section 5.3.1. The gateway IGMP/MLD implementation SHOULD retransmit unsolicited membership state-change reports and merge new state-change reports with pending reports as described in Section 5.1 of [RFC3376] and Section 6.1 of [RFC3810]. The number of retransmissions is specified by the relay in the Querier's Robustness Variable (QRV) field in the last General Query forwarded by the pseudo-interface. See Section 4.1.6 of [RFC3376] and Section 5.1.8 of [RFC3810].
The gateway IGMP/MLD implementation SHOULD handle General Query messages as described in Section 5.2 of [RFC3376] and Section 6.2 of [RFC3810] but MAY ignore the Max Resp Code (Maximum Response Code) field value and generate a current-state report without any delay. An IPv4 gateway implementation MUST accept IPv4 datagrams that carry the General Query variant of the IGMPv3 Membership Query message, as described in Section 4 of [RFC3376]. The gateway MUST accept the IGMP datagram regardless of the IP source address carried by that datagram. An IPv6 gateway implementation MUST accept IPv6 datagrams that carry the General Query variant of the MLDv2 Multicast Listener Query message, as described in Section 5 of [RFC3810]. The gateway MUST accept the MLD datagram regardless of the IP source address carried by that datagram.5.2.2. Pseudo-Interface Configuration
A gateway host may possess or create multiple gateway pseudo-interfaces, each with a unique configuration that describes a binding to a specific IP protocol, Relay Address, Relay Discovery Address, or upstream network interface.5.2.2.1. Relay Discovery Address
If a gateway implementation uses AMT relay discovery to obtain a Relay Address, it must first be supplied with a Relay Discovery Address. The Relay Discovery Address may be an anycast or unicast address. A gateway implementation may rely on a static address assignment or some form of dynamic address discovery. This specification does not require that a gateway implementation use any particular method to obtain a Relay Discovery Address -- an implementation may employ any method that returns a suitable Relay Discovery Address.5.2.2.2. Relay Address
Before a gateway implementation can execute the AMT protocol to request and receive multicast traffic, it must be supplied with a unicast Relay Address. A gateway implementation may rely on static address assignment or support some form of dynamic address discovery. This specification does not require the use of any particular method to obtain a Relay Address -- an implementation may employ any method that returns a suitable Relay Address.
5.2.2.3. Upstream Interface Selection
A gateway host that possesses multiple network interfaces or addresses may allow for an explicit selection of the interface to use when communicating with a relay. The selection might be made to satisfy connectivity, tunneling, or IP protocol requirements.5.2.2.4. Optional Retransmission Parameters
A gateway implementation that supports retransmission MAY require the following information: Discovery Timeout Initial time to wait for a response to a Relay Discovery message. Maximum Relay Discovery Retransmission Count Maximum number of Relay Discovery retransmissions to allow before terminating relay discovery and reporting an error. Request Timeout Initial time to wait for a response to a Request message. Maximum Request Retransmission Count Maximum number of Request retransmissions to allow before abandoning a relay and restarting relay discovery or reporting an error. Maximum Retries Count for "Destination Unreachable" The maximum number of times a gateway should attempt to send the same Request or Membership Update message after receiving an ICMP Destination Unreachable message.5.2.3. Gateway Service
In the following descriptions, a gateway pseudo-interface is treated as a passive entity managed by a gateway service. The gateway pseudo-interface provides the state, and the gateway service provides the processing. The term "gateway" is used when describing service behavior with respect to a single pseudo-interface.5.2.3.1. Startup
When a gateway pseudo-interface is started, the gateway service begins listening for AMT messages sent to the UDP endpoint(s) associated with the pseudo-interface and for any locally generated IGMP/MLD messages passed to the pseudo-interface. The handling of these messages is described below.
When the pseudo-interface is enabled, the gateway service MAY:
o Optionally execute the relay discovery procedure described in
Section 5.2.3.4.
o Optionally execute the membership query procedure described in
Section 5.2.3.5 to start the periodic membership update cycle.
5.2.3.2. Handling AMT Messages
A gateway MUST ignore any datagram it receives that cannot be
interpreted as a Relay Advertisement, Membership Query, or Multicast
Data message. The handling of Relay Advertisement, Membership Query,
and Multicast Data messages is addressed in the sections that follow.
A gateway that conforms to this specification MUST ignore any message
with a Version field value other than zero.
While listening for AMT messages, a gateway may be notified that an
ICMP Destination Unreachable message was received as a result of an
AMT message transmission. Handling of ICMP Destination Unreachable
messages is described in Section 5.2.3.9.
5.2.3.3. Handling Multicast Data Messages
A gateway may receive Multicast Data messages after it sends a
Membership Update message to a relay that adds a group subscription.
The gateway may continue to receive Multicast Data messages long
after the gateway sends a Membership Update message that deletes
existing group subscriptions. The gateway MUST be prepared to
receive these messages at any time but MAY ignore them or discard
their contents if the gateway no longer has any interest in receiving
the multicast datagrams contained within them.
A gateway MUST ignore a Multicast Data message if it fails to satisfy
any of the following requirements:
o The source IP address and UDP port carried by the Multicast Data
message MUST be equal to the destination IP address and UDP port
carried by the matching Membership Update message (i.e., the
current Relay Address).
o The destination address carried by the encapsulated IP datagram
MUST fall within the multicast address allocation assigned to the
relevant IP protocol, i.e., 224.0.0.0/4 for IPv4 and ff00::/8
for IPv6.
The gateway extracts the encapsulated IP datagram and forwards it to the local IP protocol implementation for checksum verification, fragmented datagram reassembly, source and group filtering, and transport-layer protocol processing. Because AMT uses UDP encapsulation to deliver multicast datagrams to gateways, it qualifies as a tunneling protocol subject to the limitations described in [RFC6936]. If supported, a gateway SHOULD employ the solution described in [RFC6936] to ensure that the local IP stack does not discard IPv6 datagrams with zero checksums. If Multicast Data message datagrams are processed directly within the gateway (instead of the host IP stack), the gateway MUST NOT discard any of these datagrams because they carry a UDP checksum of zero.5.2.3.4. Relay Discovery Procedure
This section describes gateway requirements related to the relay discovery message sequence described in Section 4.2.1.1.5.2.3.4.1. Starting Relay Discovery
A gateway may start or restart the relay discovery procedure in response to the following events: o When a gateway pseudo-interface is started (enabled). o When the gateway wishes to report a group subscription when none currently exist. o Before sending the next Request message in a membership update cycle, i.e., each time the query timer expires (see below). o After the gateway fails to receive a response to a Request message. o After the gateway receives a Membership Query message with the L flag set to 1.5.2.3.4.2. Sending a Relay Discovery Message
A gateway sends a Relay Discovery message to a relay to start the relay discovery process. The gateway MUST send the Relay Discovery message using the current Relay Discovery Address and AMT port number as the destination. The Discovery Nonce value in the Relay Discovery message MUST be computed as described in Section 5.2.3.4.5.
The gateway MUST save a copy of the Relay Discovery message or save the Discovery Nonce value for possible retransmission and verification of a Relay Advertisement response. When a gateway sends a Relay Discovery message, it may be notified that an ICMP Destination Unreachable message was received as a result of an earlier AMT message transmission. Handling of ICMP Destination Unreachable messages is described in Section 5.2.3.9.5.2.3.4.3. Waiting for a Relay Advertisement Message
A gateway MAY retransmit a Relay Discovery message if it does not receive a matching Relay Advertisement message within some timeout period. If the gateway retransmits the message multiple times, the timeout period SHOULD be adjusted to provide a random exponential back-off. The RECOMMENDED timeout is a random value in the range [initial_timeout, MIN(initial_timeout * 2^retry_count, maximum_timeout)], with a RECOMMENDED initial_timeout of 1 second and a RECOMMENDED maximum_timeout of 120 seconds (which is the recommended minimum NAT mapping timeout described in [RFC4787]).5.2.3.4.4. Handling a Relay Advertisement Message
When a gateway receives a Relay Advertisement message, it must first determine whether it should accept or ignore the message. A gateway MUST ignore a Relay Advertisement message if it fails to satisfy any of the following requirements: o The gateway MUST be waiting for a Relay Advertisement message. o The Discovery Nonce value contained in the Relay Advertisement message MUST be equal to the Discovery Nonce value contained in the Relay Discovery message. o The source IP address and UDP port of the Relay Advertisement message MUST be equal to the destination IP address and UDP port of the matching Relay Discovery message. Once a gateway receives a Relay Advertisement response to a Relay Discovery message, it SHOULD ignore any other Relay Advertisements that arrive on the AMT interface until it sends a new Relay Discovery message. If a gateway executes the relay discovery procedure at the start of each membership update cycle and the Relay Address returned in the latest Relay Advertisement message differs from the address returned in a previous Relay Advertisement message, then the gateway SHOULD send a Teardown message (if supported) to the old Relay Address,
using information from the last Membership Query message received from that relay, as described in Section 5.2.3.7. This behavior is illustrated in the following diagram. Gateway Relay-1 ------- ------- : : Query Expired | | Timer (QT)-------->| | | Relay Discovery | |------------------->| | | | Relay Advertisement| |<-------------------| | | | Request | |------------------->| | | | Membership Query | |<===================| Start | | (QT)<--------| Membership Update | |===================>| | | ~ ~ Relay-2 Expired | | ------- (QT)-------->| | : | Relay Discovery | | |------------------------------------>| | | | | Relay Advertisement| | |<------------------------------------| | | | | Teardown | | |------------------->| | | | | | Request | | |------------------------------------>| | | | | Membership Query | | |<====================================| Start | | | (QT)<--------| Membership Update | | |====================================>| | | | : : : Figure 18: Teardown after Relay Address Change
5.2.3.4.5. Discovery Nonce Generation
The discovery nonce MUST be a random, non-zero 32-bit value and, if possible, SHOULD be computed using a cryptographically secure pseudorandom number generator. A new nonce SHOULD be generated each time the gateway restarts the relay discovery process. The same nonce SHOULD be used when retransmitting a Relay Discovery message.5.2.3.5. Membership Query Procedure
This section describes gateway requirements related to the membership update message sequence described in Section 4.2.1.2.5.2.3.5.1. Starting the Membership Update Cycle
A gateway may send a Request message to start a membership update cycle (following the optional relay discovery procedure) in response to the following events: o When the gateway pseudo-interface is activated. o When the gateway wishes to report a group subscription when none currently exist. Starting the membership update cycle when a gateway pseudo-interface is started provides several benefits: o Better performance by allowing state-change reports to be sent as they are generated, thus minimizing the time to join. o More robustness by relying on unsolicited state-change reports to update group membership state rather than the current-state reports generated by the membership update cycle. Unsolicited state-change reports are typically retransmitted multiple times while current-state reports are not. o Simplified implementation by eliminating any need to queue IGMP/ MLD messages for delivery after a Membership Query is received, since the IGMP/MLD state-change messages may be sent as they are generated. However, this approach places an additional load on relays, as a gateway will send periodic requests even when it has no multicast subscriptions. To reduce load on a relay, a gateway SHOULD only send a Membership Update message while it has active group subscriptions. A relay will still need to compute a Response MAC for each Request
but will not be required to recompute it a second time to authenticate a Membership Update message that contains no subscriptions.5.2.3.5.2. Sending a Request Message
A gateway sends a Request message to a relay to solicit a Membership Query response and start the membership update cycle. A gateway constructs a Request message containing a Request Nonce value computed as described in Section 5.2.3.5.6. The gateway MUST set the P flag in the Request message to identify the protocol the gateway wishes the relay to use for the General Query response. A gateway MUST send a Request message using the current Relay Address and AMT port number as the destination. A gateway MUST save a copy of the Request message or save the Request Nonce and P flag values for possible retransmission and verification of a Membership Query response. When a gateway sends a Request message, it may be notified that an ICMP Destination Unreachable message was received as a result of an earlier AMT message transmission. Handling of ICMP Destination Unreachable messages is described in Section 5.2.3.9.5.2.3.5.3. Waiting for a Membership Query Message
A gateway MAY retransmit a Request message if it does not receive a matching Membership Query message within some timeout period. If the gateway retransmits the message multiple times, the timeout period SHOULD be adjusted to provide a random exponential back-off. The RECOMMENDED timeout is a random value in the range [initial_timeout, MIN(initial_timeout * 2^retry_count, maximum_timeout)], with a RECOMMENDED initial_timeout of 1 second and a RECOMMENDED maximum_timeout of 120 seconds (which is the recommended minimum NAT mapping timeout described in [RFC4787]). If a gateway that uses relay discovery does not receive a Membership Query within a specified time period or after a specified number of retries, the gateway SHOULD stop waiting for a Membership Query message and restart relay discovery to locate another relay.
5.2.3.5.4. Handling a Membership Query Message
When a gateway receives a Membership Query message, it must first determine whether it should accept or ignore the message. A gateway MUST ignore a Membership Query message, or the encapsulated IP datagram within it, if the message fails to satisfy any of the following requirements: o The gateway MUST be waiting for a Membership Query message. o The Request Nonce value contained in the Membership Query MUST equal the Request Nonce value contained in the Request message. o The source IP address and UDP port of the Membership Query MUST equal the destination IP address and UDP port of the matching Request message (i.e., the current Relay Address). o The encapsulated IP datagram MUST carry an IGMPv3 or MLDv2 message. The protocol MUST match the protocol identified by the P flag in the Request message. o The IGMPv3 or MLDv2 message MUST be a General Query message. o The total length of the encapsulated IP datagram as computed from the lengths contained in the datagram header(s) MUST NOT exceed the available field length within the Membership Query message. Once a gateway receives a Membership Query response to a Request message, it SHOULD ignore any other Membership Query messages that arrive on the AMT interface until it sends a new Request message. The gateway MUST save the Membership Query message, or the Request Nonce, Response MAC, Gateway IP Address, and Gateway Port Number fields for use in sending subsequent Membership Update and Teardown messages. The gateway extracts the encapsulated IP datagram and forwards it to the local IP protocol implementation for checksum verification and dispatching to the IGMP or MLD implementation running on the pseudo-interface. The gateway MUST NOT forward any octets that might exist between the encapsulated IP datagram and the end of the message or Gateway Address fields. The MLD protocol specification indicates that senders should use a link-local source IP address in message datagrams. This requirement must be relaxed for AMT because gateways and relays do not normally share a common subnet. For this reason, a gateway implementation MUST accept MLD (and IGMP) query message datagrams regardless of the
source IP address they carry. This may require additional processing on the part of the gateway that might be avoided if the relay and gateway use the IPv4 and IPv6 addresses allocated for use in AMT-encapsulated control packets as described in Section 5.2.1. The gateway MUST start a timer that will trigger the next iteration of the membership update cycle by executing the membership query procedure. The gateway SHOULD compute the timer duration from the Querier's Query Interval Code carried by the General Query. A gateway MAY use a smaller timer duration if required to refresh a NAT mapping that would otherwise time out. A gateway MAY use a larger timer duration if it has no group subscriptions to report. If the gateway supports the Teardown message and the G flag is set in the Membership Query message, the gateway MUST compare the Gateway IP Address and Gateway Port Number on the new Membership Query message with the values carried by the previous Membership Query message. If either value has changed, the gateway MUST send a Teardown message to the relay as described in Section 5.2.3.7. If the L flag is set in the Membership Query message, the relay is reporting that it is NOT accepting Membership Update messages that create new tunnel endpoints and will simply ignore any that do. If the L flag is set and the gateway is not currently reporting any group subscriptions to the relay, the gateway SHOULD stop sending periodic Request messages and restart the relay discovery procedure (if discovery is enabled) to find a new relay with which to communicate. Even if the L flag is set, the gateway MAY continue to send updates if it has previously reported group subscriptions to the relay, one or more subscriptions still exist, and the gateway endpoint address has not changed since the last Membership Query was received (see previous paragraph).5.2.3.5.5. Handling Query Timer Expiration
When the query timer (started in the previous step) expires, the gateway should execute the membership query procedure again to continue the membership update cycle.5.2.3.5.6. Request Nonce Generation
The Request Nonce MUST be a random value and, if possible, SHOULD be computed using a cryptographically secure pseudorandom number generator. A new nonce MUST be generated each time the gateway starts the membership query process. The same nonce SHOULD be used when retransmitting a Request message.
5.2.3.6. Membership Update Procedure
This section describes gateway requirements related to the membership update message sequence described in Section 4.2.1.2. The membership update process is primarily driven by the host-mode IGMP or MLD protocol implementation running on the gateway pseudo-interface. The IGMP and MLD protocols produce current-state reports in response to General Query messages generated by the pseudo-interface via AMT and produce state-change reports in response to receiver requests made using the IGMP or MLD service interface.5.2.3.6.1. Handling an IGMP/MLD IP Datagram
The gateway pseudo-interface MUST accept the following IP datagrams from the IPv4/IGMP and IPv6/MLD protocols running on the pseudo-interface: o IPv4 datagrams that carry an IGMPv2 or IGMPv3 Membership Report or an IGMPv2 Leave Group message as described in Section 4 of [RFC3376]. o IPv6 datagrams that carry an MLDv1 or MLDv2 Multicast Listener Report or an MLDv1 Multicast Listener Done message as described in Section 5 of [RFC3810]. The gateway must be prepared to receive these messages any time the pseudo-interface is running. The gateway MUST ignore any datagrams not listed above. A gateway that waits to start a membership update cycle until after it receives a datagram containing an IGMP/MLD state-change message MAY: o Discard IGMP or MLD datagrams until it receives a Membership Query message, at which time it processes the Membership Query message as normal to eventually produce a current-state report on the pseudo-interface, which describes the end state (RECOMMENDED). o Insert IGMP or MLD datagrams into a queue for transmission after it receives a Membership Query message. If and when a gateway receives a Membership Query message (for IGMP or MLD), it sends any queued or incoming IGMP or MLD datagrams to the relay as described in the next section.
5.2.3.6.2. Sending a Membership Update Message
A gateway cannot send a Membership Update message to a relay until it has received a Membership Query message from a relay. If the gateway has not yet located a relay with which to communicate, it MUST first execute the relay discovery procedure described in Section 5.2.3.4 to obtain a Relay Address. If the gateway has a Relay Address but has not yet received a Membership Query message, it MUST first execute the membership query procedure described in Section 5.2.3.5 to obtain a Request Nonce and Response MAC that can be used to send a Membership Update message. Once a gateway possesses a valid Relay Address, Request Nonce, and Response MAC, it may encapsulate the IP datagram containing the IGMP/ MLD message into a Membership Update message. The gateway MUST copy the Request Nonce and Response MAC values from the last Membership Query received from the relay into the corresponding fields in the Membership Update. The gateway MUST send the Membership Update message using the Relay Address and AMT port number as the destination. When a gateway sends a Membership Update message, it may be notified that an ICMP Destination Unreachable message was received as a result of an earlier AMT message transmission. Handling of ICMP Destination Unreachable messages is described in Section 5.2.3.9.5.2.3.7. Teardown Procedure
This section describes gateway requirements related to the teardown message sequence described in Section 4.2.1.3. Gateway support for the Teardown message is RECOMMENDED. A gateway that supports Teardown SHOULD make use of Teardown functionality if it receives a Membership Query message from a relay that has the G flag set to indicate that it contains valid Gateway Address fields.5.2.3.7.1. Handling a Membership Query Message
As described in Section 5.2.3.5.4, if a gateway supports the Teardown message, has reported active group subscriptions, and receives a Membership Query message with the G flag set, the gateway MUST compare the Gateway IP Address and Gateway Port Number on the new Membership Query message with the values carried by the previous Membership Query message. If either value has changed, the gateway MUST send a Teardown message as described in the next section.
5.2.3.7.2. Sending a Teardown Message
A gateway sends a Teardown message to a relay to request that it stop delivering Multicast Data messages to the gateway and delete any group memberships created by the gateway. When a gateway constructs a Teardown message, it MUST copy the Request Nonce, Response MAC, Gateway IP Address, and Gateway Port Number fields from the Membership Query message that provided the Response MAC for the last Membership Update message sent, into the corresponding fields of the Teardown message. A gateway MUST send the Teardown message using the Relay Address and AMT port number as the destination. A gateway MAY send the Teardown message multiple times for robustness. The gateway SHOULD use the Querier's Robustness Variable (QRV) field contained in the query encapsulated within the last Membership Query to set the limit on the number of retransmissions (see Section 4.1.6 of [RFC3376] and Section 5.1.8 of [RFC3810]). If the gateway sends the Teardown message multiple times, it SHOULD insert a delay between each transmission using the timing algorithm employed in IGMP/MLD for transmitting unsolicited state-change reports. The RECOMMENDED default delay value is 1 second. When a gateway sends a Teardown message, it may be notified that an ICMP Destination Unreachable message was received as a result of an earlier AMT message transmission. Handling of ICMP Destination Unreachable messages is described in Section 5.2.3.9.5.2.3.8. Shutdown
When a gateway pseudo-interface is stopped and the gateway has existing group subscriptions, the gateway SHOULD either: o Send a Teardown message to the relay as described in Section 5.2.3.7, but only if the gateway supports the Teardown message and the current relay is returning Gateway Address fields in Membership Query messages, or o Send a Membership Update message to the relay that will delete existing group subscriptions.5.2.3.9. Handling ICMP Destination Unreachable Responses
A gateway may receive an ICMP Destination Unreachable message [RFC0792] after sending an AMT message. Whether the gateway is notified that an ICMP message was received is highly dependent on firewall and gateway IP stack behavior and gateway implementation.
If the reception of an ICMP Destination Unreachable message is
reported to the gateway while waiting to receive an AMT message, the
gateway may respond as follows, depending on platform capabilities
and which outgoing message triggered the ICMP response:
1. The gateway MAY simply abandon the current relay and restart
relay discovery (if used). This is the least desirable approach,
as it does not allow for transient network changes.
2. If the last message sent was a Relay Discovery or Request
message, the gateway MAY simply ignore the ICMP response and
continue waiting for incoming AMT messages. If the gateway is
configured to retransmit Relay Discovery or Request messages, the
normal retransmission behavior for those messages is preserved to
prevent the gateway from prematurely abandoning a relay.
3. If the last message sent was a Membership Update message, the
gateway MAY start a new membership update and associated Request
retransmission cycle.
If the reception of an ICMP Destination Unreachable message is
reported to the gateway when attempting to transmit a new AMT
message, the gateway may respond as follows, depending on platform
capabilities and which outgoing message triggered the ICMP response:
1. The gateway MAY simply abandon the current relay and restart
relay discovery (if used). This is the least desirable approach,
as it does not allow for transient network changes.
2. If the last message sent was a Relay Discovery, Request, or
Teardown message, the gateway MAY attempt to transmit the new
message. If the gateway is configured to retransmit Relay
Discovery, Request, or Teardown messages, the normal
retransmission behavior for those messages is preserved to
prevent the gateway from prematurely abandoning a relay.
3. If the last message sent was a Membership Update message, the
gateway SHOULD start a new membership update and associated
Request retransmission cycle.
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