PIM Background
The predominant multicast routing protocol in use on the Internet today is Protocol Independent Multicast, or PIM. The type of PIM used on the Internet is PIM sparse mode. PIM sparse mode is so accepted that when the simple term “PIM” is used in an Internet context, some form of sparse mode operation is assumed.
PIM emerged as an algorithm to overcome the limitations of dense-mode protocols such as the Distance Vector Multicast Routing Protocol (DVMRP), which was efficient for dense clusters of multicast receivers, but did not scale well for the larger, sparser, groups encountered on the Internet. The Core Based Trees (CBT) protocol was intended to support sparse mode as well, but CBT, with its all-powerful core approach, made placement of the core critical, and large conference-type applications (many-to-many) resulted in bottlenecks in the core. PIM was designed to avoid the dense-mode scaling issues of DVMRP and the potential performance problems of CBT at the same time.
PIM is one of the most rapidly evolving specifications on the Internet today. Since its introduction in 1995, PIM has already seen two major revisions to its packet structure (PIM version 1 [PIMv1] and PIM version 2 [PIMv2]), two major RFCs (RFC 2362 obsoleted RFC 2117), and numerous drafts describing major components of PIM, such as many-to-many trees and source-specific multicast (SSM). Long-lasting RFCs are not a feature of PIM, and virtually all of PIM must be researched, understood, and implemented directly from Internet drafts. In fact, no current RFC describes PIMv1 at all: The drafts have all expired, and PIMv1 was never issued as an official RFC.
PIM itself is not nonstandard or unstable, however. PIM has been a promising multicast routing protocol since its inception, especially PIM sparse mode, the first real sparse-mode multicast routing protocol. Work continues on PIM in a number of areas, from bidirectional trees to network management, and the rapid pace of development makes drafts essential for PIM.
PIMv1 and PIMv2 can coexist on the same router or even on the same interface. The main difference between PIMv1 and PIMv2 is the packet format. PIMv1 messages use Internet Group Management Protocol (IGMP) packets, whereas PIMv2 has its own IP protocol number (103) and packet structure. All routers connecting to an IP subnet such as a LAN must use the same PIM version. Some PIM implementations can recognize PIMv1 packets and automatically switch the router interface to PIMv1. Because the difference between PIMv1 and PIMv2 involves the message format, but not the meaning of the message or how the router processes the PIM message, a router can easily mix PIMv1 and PIMv2 interfaces.
