Protocol Independent Multicast – Sparse Mode (PIM-SM) is a multicast routing protocol that can use the underlying unicast routing information base or a separate multicast-capable routing information base. It builds unidirectional shared trees rooted at a Rendezvous Point (RP) per group, and optionally creates shortest-path trees per source.

Protocol Independent Multicast (PIM) refers to a group of multicast routing protocols, each optimized for a different environment. There are three PIM protocols, PIM Sparse Mode (PIM-SM), and PIM Dense Mode (PIM-DM), and Bi-directional PIM (BIDIR-PIM). All PIM protocols share a common control message format. PIM control messages are sent as raw IP datagrams, either multicast to the link-local ALL PIM ROUTERS multicast group, or unicast to a specific destination.

Protocol Independent Multicast-Sparse Mode (PIM-SM) is a protocol for efficiently routing to multicast groups that may span wide-area (WAN and inter-domain) internets. PIM-SM is not dependent on any particular unicast routing protocol, and is designed to support sparse groups. PIM-SM uses the traditional IP multicast model of receiver-initiated membership, supports both shared and shortest-path trees, and uses soft-state mechanisms to adapt to changing network conditions. PIM-SM can use the route information that any routing protocol enters into the multicast Routing Information Base (RIB). Examples of these routing protocols include unicast protocols such as the Routing Information Protocol (RIP) and Open Shortest Path First (OSPF), but multicast protocols that populate the routing tables-such as the Distance Vector Multicast Routing Protocol (DVMRP)-can also be used.

PIM-SM was designed to support the following goals:

• Maintain the traditional IP multicast service model of receiver-initiated multicast group membership.
• Leave the host model unchanged. PIM-SM is a router-to-router protocol, which means that the hosts don't have to be upgraded, but that PIM-SM-enabled routers must be deployed in the network.
• Support both shared and source distribution trees.
• Maintain independence from any specific unicast routing protocol (see above).
• Use soft-state mechanisms to adapt to changing network conditions and multicast group dynamics. Soft-state means that, unless it is refreshed, the router's state configuration is short-term and expires after a certain amount of time.

Currently, there are two versions of PIM-SM. We focus on version 2, which is the one deployed in real network.

Protocol Structure

PIM-SM version C The current PIM-SM version is 2.

Type -- Types for specific PIM messages.

Address length -- Address length in bytes. The length of the address field throughout, in the specific message. Reserved (the value of this field is set to 0, ignore on receipt)

Checksum - The 16-bit one's complement, of the one's complement sum of the entire PIM message.

Related Terms:IP, PIM-SM, ICMP, OSPF, RIP, DVMRP, IS-IS, BGP, IGRP, EIGRP

Sponsor Source:PIM-SM is defined by IETF (http://www.ietf.org).

Reference:
http://www.javvin.com/protocol/rfc4601.pdf: Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)
http://www.javvin.com/protocol/rfc5059.pdf: Bootstrap Router (BSR) Mechanism for Protocol Independent Multicast (PIM)