2. Semantics of Source-Specific Multicast Addresses

The source-specific multicast service is defined as follows:

A datagram sent with source IP address S and destination IP address G in the SSM range is delivered to each host socket that has specifically requested delivery of datagrams sent by S to G, and only to those sockets.

Where, using the terminology of [IGMPv3],

"socket" is an implementation-specific parameter used to distinguish among different requesting entities (e.g., programs or processes or communication end-points within a program or process) within the requesting host; the socket parameter of BSD Unix system calls is a specific example.

Any host may send a datagram to any SSM address, and delivery is provided according to the above semantics.

The IP module interface to upper-layer protocols is extended to allow a socket to "Subscribe" to or "Unsubscribe" from a particular channel identified by an SSM destination address and a source IP address. The extended interface is defined in Section 4.1. It is meaningless for an application or host to request reception of datagrams sent to an SSM destination address G, as is supported in the any-source multicast model, without also specifying a corresponding source address, and routers MUST ignore any such request.

Multiple source applications on different hosts can use the same SSM destination address G without conflict because datagrams sent by each source host Si are delivered only to those sockets that requested delivery of datagrams sent to G specifically by Si.

The key distinguishing property of the model is that a channel is identified (addressed) by the combination of a unicast source address and a multicast destination address in the SSM range. So, for example, the channel

S,G = (192.0.2.1, 232.7.8.9)

differs from

S,G = (192.0.2.2, 232.7.8.9),

even though they have the same destination address portion. Similarly, for IPv6,

S,G = (2001:3618::1, FF33::1234)

and

S,G = (2001:3618::2, FF33::1234)

are different channels.