PIM-SSM

Protocol Independent Multicast-Source-Specific Multicast (PIM-SSM) enables a user host to rapidly join a multicast group if the user knows a multicast source address. PIM-SSM sets up a shortest path tree (SPT) from a multicast source to a multicast group, while PIM-SM uses rendezvous points (RPs) to set rendezvous point trees (RPTs). Therefore, PIM-SSM implements a more rapid join function than PIM-SM.

Different from the any-source multicast (ASM) model, the SSM model does not need to maintain an RP, construct an RPT, or register a multicast source.

The SSM model is based on PIM-SM and IGMPv3/Multicast Listener Discovery version 2 (MLDv2). 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 DR, which knows the multicast source address, sends Join messages directly to the source so that multicast data streams can be sent to the receiver's designated router (DR).

In SSM mode, multicast traffic forwarding is based on (S, G) channels. To receive the multicast traffic of a channel, a multicast user must join the channel. A multicast user can join or leave a multicast channel by subscribing to or unsubscribing from the channel. Currently, only IGMPv3 can be used for channel subscription or unsubscription.

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 multicast source address 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 then sent by the multicast source to the user along the SPT.

• SPT establishment can be triggered by user join requests (both dynamic and static) and 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 messages to the multicast source, creates the (S, G) entry hop by hop, and then sets up an SPT.

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