PIM-SSM

This section describes the related concepts and the multicast data forwarding process in a PIM-SSM domain.

PIM-SM needs to maintain Rendezvous Points (RPs) to transmit multicast data. If receivers know the exact location of a multicast source and want to request multicast data directly from a multicast source, Protocol Independent Multicast-Source-Specific Multicast (PIM-SSM) can enable use hosts to rapidly join multicast groups. A shortest path tree (SPT) is set up between the multicast source and group members.

Unlike the Any-Source Multicast (ASM) model, the Source-Specific Multicast (SSM) model does not need to maintain an RP, construct a rendezvous point tree (RPT), or register a multicast source.

The SSM model is based on the PIM-SM technology and IGMPv3. The procedure for setting up a multicast forwarding tree on a PIM-SSM network is similar to the procedure for setting up an SPT on a PIM-SM network. The receiver's Designated router (DR), which knows the exact position of the multicast source, sends Join messages directly to the source so that multicast data streams can be sent to the receiver's DR.

Implementation

The process for forwarding multicast data in a PIM-SSM domain is as follows:

1. Neighbor Discovery

   Each PIM device in a PIM-SSM domain periodically sends Hello messages to all other PIM devices in the domain to discover PIM neighbors and maintain PIM neighbor relationships.

   

   By default, a PIM device permits other PIM control messages or multicast messages from a neighbor, irrespective of whether the PIM device has received Hello messages from the neighbor. However, if a PIM device has the neighbor check function, the PIM device permits other PIM control messages or multicast messages from a neighbor only after the PIM device has received Hello messages from the neighbor.

2. DR Election

   PIM devices exchange Hello messages to elect a DR on a shared network segment. The receiver's DR is the only multicast data forwarder on the segment.

3. SPT Setup

   Users on a PIM-SSM network can know the exact position of the multicast source in advance and can, therefore, specify the source when joining a multicast group. After receiving a Report message from a user, the receiver's DR sends a Join message towards the multicast source to establish an SPT between the source and the user. Multicast data is sent by the multicast source to the user along the SPT.

• The SPT establishment can be triggered by a user dynamically joining a multicast group, static join, or SSM-mapping.

• The DR in an SSM scenario is valid only in the shared network segment connected to group members. The DR on the group member side sends Join message to the multicast source, creates the (S, G) entry hop by hop, and then sets up an SPT.

• PIM-SSM supports a PIM DR switchover delay and PIM silent.