PIM – Source Specific Multicast (SSM) is ideal for one-to-many communication. A good use case is streaming audio/video from a single source to multiple groups. SSM only runs with MLDv2.

PIM-SSM optimizes bandwidth usage, solves multicast collision issues and denies unwanted Internet broadcast traffic by using source based trees.

The source sends the multicast directly to the receiver using the shortest path as indicated in the routers’ unicast routing table. Rendezvous Point (RP) mechanisms are not used.

Security is enhanced with SSM because receivers will only accept traffic from a sender (or senders) that it specifically requested. With Internet Standard Multicast (ISM), receivers can also specify a specific sender (or senders) they will not accept as the source. ISM and SSM can co-exist.

PIM-SSM is based on PIM-SM and is backward-compatible. All routers can use PIM-SSM or PIM-SM, except the last-hop router which must use SSM. If the network is already running SM, SSM can be deployed simply by configuring the last-hop router with SSM.

Pro Tip: Ensure the receiving host and the host’s application support SSM – it’s required.

  • MLDv2 uses a (S, G) source tree.
  • S = Unicast Source address
  • G = Multicast Group address.

The router’s unicast routing table is used to find the optimal path to the source.

Receivers subscribe to a channel by sending a Report message to ff02::16 to notify the last-hop router of the address of the specific desired source (S,G). The router, upon receipt of the Report message, will send a PIM Join message upstream towards the multicast source. The source then begins to deliver the multicast traffic to the layer 2 domain from which the Report message was sent.

When a receiver is finished with a multicast it can unsubscribe from channel. This is usually done by closing the multicast application on the receiver.

RFC 3560: An Overview of Source-Specific Multicast (SSM)
RFC 4607: Source-Specific Multicast for IP

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