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3. Router Requirements for Source-Specific Multicast

Routers must be aware of the SSM address range in order to provide the SSM service model. A router that knows the SSM address range and is capable of applying SSM semantics to it as described in this section is described as an "SSM-aware" router. An SSM-aware router MAY have a configuration option to apply SSM semantics to addresses other than the IANA-allocated range, but if such an option exists, it MUST default to the IANA-allocated range.

This section documents the behavior of routers with respect to the following types of SFGMP messages for source-specific destination addresses:

  • IGMPv3 and MLDv2 Reports (3.1)
  • IGMPv3 and MLDv2 General Query (3.2)
  • IGMPv3 and MLDv2 Group-Specific Query (3.3)
  • IGMPv3 and MLDv2 Group-and-Source Specific Query (3.4)
  • IGMPv1/v2 and MLDv1 Reports (3.5)
  • IGMPv1/v2 and MLDv1 Queries (3.6)
  • IGMPv2 Leave and MLDv1 Done (3.7)

3.1. IGMPv3 and MLDv2 Reports​

SFGMP Reports are used to report source-specific subscriptions in the SSM address range. A router SHOULD ignore a group record of either of the following types if it refers to an SSM destination address:

  • MODE_IS_EXCLUDE Current-State Record

  • CHANGE_TO_EXCLUDE_MODE Filter-Mode-Change Record

A router MAY choose to log an error in either case. It MUST process any other group records within the same report. These behaviors are MODIFICATIONS to [IGMPv3, MLDv2] to prevent non-source-specific semantics from being applied to SSM addresses, and to avoid reverting to older-version compatibility mode.

A CHANGE_TO_INCLUDE_MODE Filter-Mode-Change Record is processed per the normal SFGMP rules; Section 2.2.2 describes a legitimate scenario when this could occur.

3.2. IGMPv3 and MLDv2 General Queries​

An SSM router sends periodic SFGMP General Queries as per the IGMPv3 and MLDv2 specifications. No change in behavior is required for SSM.

3.3. IGMPv3 and MLDv2 Group-Specific Queries​

SFGMP routers that support source-specific multicast may send group-specific queries for addresses in the source-specific range. This specification does not explicitly prohibit such a message, although, at the time of this writing, a router conformant to [IGMPv3, MLDv2] would not send one.

3.4. IGMPv3 and MLDv2 Group-and-Source-Specific Queries​

SFGMP Group-and-Source-Specific Queries are used when a receiver has indicated that it is no longer interested in receiving traffic from a particular (S,G) pair to determine if there are any remaining directly-attached hosts with interest in that (S,G) pair. Group-and-Source-Specific Queries are used within the source-specific address range when a router receives a BLOCK_OLD_SOURCES Record for one or more source-specific groups. These queries are sent normally, as per [IGMPv3, MLDv2].

3.5. IGMPv1/v2 and MLDv1 Reports​

An IGMPv1/v2 or MLDv1 report for an address in the source-specific range could be sent by a non-SSM-aware host. A router SHOULD ignore all such reports and specifically SHOULD NOT use them to establish IP forwarding state. This is a MODIFICATION to [IGMPv3, MLDv2]. A router MAY log an error if it receives such a report (also a MODIFICATION).

3.6. IGMPv1/v2 and MLDv1 Queries​

An SFGMP router that loses the querier election to a lower version router must log an error, as specified by [IGMPv3, MLDv2].

3.7. IGMPv2 Leave and MLDv1 Done​

An IGMPv2 Leave or MLDv1 Done message may be sent by a non-SSM-aware host. A router SHOULD ignore all such messages in the source-specific address range and MAY log an error (MODIFICATION).

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