5. Router Requirements

5.1. Packet Forwarding

A router that receives an IP datagram with a source-specific destination address MUST silently drop it unless a neighboring host or router has communicated a desire to receive packets sent from the source and to the destination address of the received packet.

5.2. Protocols

Certain IP multicast routing protocols already have the ability to communicate source-specific joins to neighboring routers (in particular, PIM-SM [PIM-SM]), and these protocols can, with slight modifications, be used to provide source-specific semantics. A router that supports the SSM service model MUST implement the PIM-SSM subset of the PIM-SM protocol from [PIM-SM] and MUST implement the router portion of [IGMPv3] for IPv4 and [MLDv2] for IPv6. An SSM router MUST also conform to the IGMPv3/MLDv2 behavior described in [GMP-SSM].

With PIM-SSM, successful establishment of an (S,G) forwarding path from the source S to any receiver depends on hop-by-hop forwarding of the explicit join request from the receiver toward the source. The protocol(s) and algorithms that are used to select the forwarding path for this explicit join must provide a loop-free path. When using PIM-SSM, the PIM-SSM implementation MUST (at least) support the ability to use the unicast topology database for this purpose.

A network can concurrently support SSM in the SSM address range and any-source multicast in the rest of the multicast address space, and it is expected that this will be commonplace. In such a network, a router may receive a non-source-specific, or "(,G)" in conventional terminology, request for delivery of traffic in the SSM range from a neighbor that does not implement source-specific multicast in a manner compliant with this document. A router that receives such a non-source-specific request for data in the SSM range MUST NOT use the request to establish forwarding state and MUST NOT propagate the request to other neighboring routers. A router MAY log an error in such a case. This applies both to any request received from a host (e.g., an IGMPv1 or IGMPv2 [IGMPv2] host report) and to any request received from a routing protocol (e.g., a PIM-SM (,G) join). The inter-router case is further discussed in Section 8, "Transition Considerations".

It is essential that all routers in the network give source-specific semantics to the same range of addresses in order to achieve the full benefit of SSM. To comply with this specification, a router MUST treat ALL IANA-allocated SSM addresses with source-specific semantics.