Bidirectional Protocol Independent Multicast (BIDIR-PIM) is a variant of PIM Sparse-Mode (PIM-SM) that builds bidirectional shared trees connecting multicast sources and receivers. 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 Dense Mode, 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.

In unidirectional PIM-SM, there are two possible methods for distributing data packets on the shared tree. These differ in the way packets are forwardesd from a source to the RP:

• Initially, when a source starts transmitting, its first hop router encapsulates data packets in special control messages (Registers) that are unicast to the RP. After reaching the RP, the packets are decapsulated and distributed on the shared tree.
• A transition from the above distribution mode can be made at a later stage. This is achieved by building source-specific state on all routers along the path between the source and the RP. This state is then used to natively forward packets from that source.

Both of these mechanisms suffer from problems. Encapsulation results in significant processing, bandwidth, and delay overheads. Forwarding using source-specific state has additional protocol and memory requirements.

Bidirectional trees are built using a fail-safe Designated Forwarder (DF) election mechanism operating on each link of a multicast topology. With the assistance of the DF, multicast data is natively forwarded from sources to the Rendezvous-Point (RP) and hence along the shared tree to receivers without requiring source-specific state. The DF election takes place at RP discovery time and provides the route to the RP, thus eliminating the requirement for data-driven protocol events.

Bidirectional PIM dispenses with both encapsulation and source state by allowing packets to be natively forwarded from a source to the RP using shared tree state. In contrast to PIM-SM, this mode of forwarding does not require any data-driven events.

Protocol Structure

The protocol format of BIDIR-PIM:

• PIM Ver - PIM Version number is 2.
• Type - All DF-Election PIM control messages share the PIM message Type of 10.
• Subtype - Subtypes for DF election messages are:
  • 1 = Offer
  • 2 = Winner
  • 3 = Backoff
  • 4 = Pass
• Rsvd - Set to zero on transmission.
• Checksum - A standard checksum IP checksum is used, i.e., the 16-bit one's complement of the one's complement sum of the entire PIM message. For computing the checksum, the checksum field is zeroed.
• RP Address - The bidirectional RPA for which the election is taking place.The format is described in [4], Section Sender Metric Preference - Preference value assigned to the unicast routing protocol that the message sender used to obtain the route to the RPA.
• Sender Metric - The unicast routing table metric used by the message sender to reach the RPA. The metric is in units applicable to the unicast routing protocol used.

Related Protocols: PIM-SM, BIDIR-PIM, ICMP, RIP, OSPF, DVMRP, IS-IS, BGP, IGRP, EIGRP

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

Reference: http://www.javvin.com/protocol/rfc5015.pdf: Bidirectional Protocol Independent Multicast (BIDIR-PIM)