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.

PIM is used for efficient routing to multicast groups that might span wide-area and interdomain internetworks. It is called “protocol independent” because it does not depend on a particular unicast routing protocol. The Junos OS supports sparse mode, dense mode, and sparse-dense mode.

For information about standards supported for PIM, see IP Multicast Specifications.

Because the PIM mode you choose determines the PIM configuration properties, you first must decide whether PIM operates in sparse, dense, or sparse-dense mode in your network. Each mode has distinct operating advantages in different network environments.

In sparse mode, routers must join and leave multicast groups explicitly. Upstream routers do not forward multicast traffic to a router unless it has sent an explicit request (by means of a join message) to the rendezvous point (RP) router to receive this traffic. The RP serves as the root of the shared multicast delivery tree and is responsible for forwarding multicast data from different sources to the receivers.

Sparse mode is well suited to the Internet, where frequent interdomain joins and prunes are common.

Unlike sparse mode, in which data is forwarded only to routers sending an explicit PIM join request, dense mode implements a flood-and-prune mechanism, similar to the Distance Vector Multicast Routing Protocol (DVMRP). In dense mode, a router receives the multicast data on the incoming interface, then forwards the traffic to the outgoing interface list. Flooding occurs periodically, and is used to refresh state information, such as the source IP address and multicast group pair. If the router has no interested receivers for the data, and the outgoing interface list becomes empty, the router sends a PIM prune message upstream.

Dense mode works best in networks where few or no prunes occur. In such instances, dense mode is actually more efficient than sparse mode.

Sparse-dense mode, as the name implies, allows the interface to operate on a per-group basis in either sparse or dense mode. A group specified as “dense” is not mapped to an RP. Instead, data packets destined for that group are forwarded by means of PIM dense mode rules. A group specified as “sparse” is mapped to an RP, and data packets are forwarded by means of PIM sparse-mode rules. Sparse-dense mode is useful in networks implementing auto-RP for PIM sparse mode.

Basic PIM Network Components

PIM dense mode requires only a multicast source and series of multicast-enabled routers running PIM dense mode to allow receivers to obtain multicast content. Dense mode makes sure that everything gets everywhere by periodically flooding the network with multicast traffic, and relies on prune messages to make sure that subnets where all receivers are uninterested in that particular multicast group stop receiving packets.

PIM sparse mode is more complicated, and requires the establishment of special routers called rendezvous points (RPs) in the network core. These routers are where upstream join messages from interested receivers meet downstream traffic from the source of the multicast group content. A network can have many RPs, but PIM sparse mode allows only one RP to be active for any multicast group.

If there is only one RP in a routing domain, the RP and adjacent links might become congested and form a single point of failure for all multicast traffic. So multiple RPs are the rule, but the issue then becomes how other multicast routers find the RP that is the source of the multicast group the receiver is trying to join. This RP-to-group mapping is controlled by a special bootstrap router (BSR) running the PIM BSR mechanism. There can be more than one bootstrap router as well, also for single-point-of-failure reasons.

The bootstrap router does not have to be an RP itself, although this is a common implementation. The bootstrap router's main function is to manage the collection of RPs and allow interested receivers to find the source of their group's multicast traffic.

PIM SSM can be seen as a subset of a special case of PIM sparse mode and requires no specialized equipment other than that used for PIM sparse mode (and IGMP version 3).

PIM Modes of Operation

PIM operates in two basic modes: sparse mode and dense mode. In addition, PIM can operate in sparse-dense mode, with some multicast groups configured as dense mode (flood-and-prune, [S,G] state) and others configured as sparse mode (explicit join to rendezvous point [RP], [*,G] state).

PIM drafts also establish a mode known as PIM source-specific mode, or PIM SSM. In PIM SSM there is only one specific source for the content of a multicast group within a given domain.