PIM-SM (SSM Model)

Implementation

The SSM model uses IGMPv3/MLDv2 and PIM-SM technology. There is no need to maintain an RP, set up an RPT, or register a multicast source. An SPT can be built directly between the source and group members.

In the SSM model, user hosts know the positions of multicast sources in advance of requesting multicast services. When user hosts join multicast groups, they can specify the sources from which they want to receive data. After receiving requests from user hosts, the receiver DR directly forwards Join messages to the source DR. The Join message is then transmitted upstream hop by hop to set up an SPT between the source and group members.

In the SSM model, PIM-SM uses the following mechanisms: neighbor discovery, DR election, and SPT setup. For details about all of these three mechanisms, see the sections below.

Neighbor Discovery

Neighbor discovery in PIM-SM is similar to that in PIM-DM. For details, see "PIM-DM Neighbor Discovery".

DR Election

DR election in PIM-SM (SSM model) is similar to that in PIM-SM (ASM model). For details, see "PIM-SM (ASM model) DR Election".

SPT Setup

Figure 4-14 SPT setup

Figure 4-14 shows the SPT setup process:

1. Using IGMPv3/MLDv2, RouterD and RouterE learn that packets from user hosts have the same multicast group address but are requesting multicast data from different source addresses. They send Join messages to sources hop by hop.
2. PIM routers create (S1, G) and (S2, G) entries based on the Join messages and set up SPTs from S1 to HostA and from S2 to HostB.
3. After SPTs are set up, the sources forward multicast packets along the SPTs to group members.

Comparison with the ASM Model

The major difference between the SSM and ASM models is that the SSM model allows hosts to specify desired multicast sources and the ASM does not. Table 4-1 describes differences of the two models.