Source-Specific Multicast Groups Overview
Source-specific multicast (SSM) is a service model that identifies session traffic by both source and group address. SSM implemented in the Junos OS has the efficient explicit join procedures of Protocol Independent Multicast (PIM) sparse mode but eliminates the immediate shared tree and rendezvous point (RP) procedures using (*,G) pairs. The (*) is a wildcard referring to any source sending to group G, and “G” refers to the IP multicast group. SSM builds shortest-path trees (SPTs) directly represented by (S,G) pairs. The “S” refers to the source's unicast IP address, and the “G” refers to the specific multicast group address. The SSM (S,G) pairs are called channels to differentiate them from any-source multicast (ASM) groups. Although ASM supports both one-to-many and many-to-many communications, ASM's complexity is in its method of source discovery. For example, if you click on a link in a browser, the receiver is notified about the group information, but not the source information. With SSM, the client receives both source and group information.
SSM is ideal for one-to-many multicast services such as network entertainment channels. However, many-to-many multicast services might require ASM.
To deploy SSM successfully, you need an end-to-end multicast-enabled network and applications that use an Internet Group Management Protocol version 3 (IGMPv3) or Multicast Listener Discovery version 2 (MLDv2) stack or configure SSM mapping from IGMPv1/IGMPv2 to IGMPv3. An IGMPv3 stack provides the capability of a host operating system to use the IGMPv3 protocol. IGMPv3 is available for Windows XP, Windows Vista, and most UNIX operating systems.
SSM mapping allows operators to support an SSM network without requiring all hosts to support IGMPv3. This support exists in static (S,G) configurations, but SSM mapping also supports dynamic per-source group state information, which changes as hosts join and leave the group using IGMP.
SSM is typically supported with a subset of IGMPv3 and PIM sparse mode known as PIM SSM. Using SSM, a client can receive multicast traffic directly from the source. PIM SSM uses the PIM sparse-mode functionality to create an SPT between the client and the source, but builds the SPT without the help of an RP.
An SSM-configured network has distinct advantages over a traditionally configured PIM sparse-mode network. There is no need for shared trees or RP mapping (no RP is required), or for RP-to-RP source discovery through the Multicast Source Discovery Protocol (MSDP).
For information about standards supported for source-specific multicast, see IP Multicast Specifications.
