Protocol Independent Multicast (PIM) is the name given to a common protocol that needs a unicast routing protocol for its operation, but the unicast protocol can be either a distance-vector protocol or a link-state protocol.

In other words, PIM needs to use the forwarding table of a unicast routing protocol to find the next router in a path to the destination, but it does not matter how the forwarding table is created.

PIM has another interesting feature: it can work in two different modes: dense and sparse. The term dense here means that the number of active members of a group in the internet is large; the probability that a router has a member in a group is high. The term sparse, on the other hand, means that only a few routers in the internet have active members in the group; the probability that a router has a member of the group is low.

When the protocol is working in the dense mode, it is referred to as PIM-DM; when it is working in the sparse mode, it is referred to as PIM-SM.

Protocol Independent Multicast-Dense Mode (PIM-DM)

When the number of routers with attached members is large relative to the number of routers in the internet, PIM works in the dense mode and is called PIM-DM.

In this mode, the protocol uses a source-based tree approach and is similar to DVMRP, but simpler.

PIM-DM uses only two strategies described in DVMRP: RPF and RPM. But unlike DVMRP, forwarding of a packet is not suspended awaiting pruning of the first subtree. There are two steps used in PIM-DM,

1. A router that has received a multicast packet from the source S destined for the group G first uses the RPF strategy to avoid receiving a duplicate of the packet. It consults the forwarding table of the underlying unicast protocol to find the next router if it wants to send a message to the source S (in the reverse direction). If the packet has not arrived from the next router in the reverse direction, it drops the packet and sends a prune message in that direction to prevent receiving future packets related to (S, G).
2. If the packet in the first step has arrived from the next router in the reverse direction, the receiving router forwards the packet from all its interfaces except the one from which the packet has arrived and the interface from which it has already received a prune message related to (S, G). Note that this is actually a broadcasting instead of a multicasting if the packet is the first packet from the source S to group G. However, each router downstream that receives an unwanted packet sends a prune message to the router upstream, and eventually the broadcasting is changed to multicasting. PIM-DM does not care about this precaution because it assumes that most routers have an interest in the group.

Figure 1 shows the idea behind PIM-DM. The first packet is broadcast to all networks, which have or do not have members. After a prune message arrives from a router with no member, the second packet is only multicast.

Protocol Independent Multicast-Sparse Mode (PIM-SM)

When the number of routers with attached members is small relative to the number of routers in the internet, PIM works in the sparse mode and is called PIM-SM.

In this environment, the use of a protocol that broadcasts the packets until the tree is pruned is not justified; PIM-SM uses a group-shared tree approach to multicasting.

The core router in PIM-SM is called the rendezvous point (RP). Multicast communication is achieved in two steps. Any router that has a multicast packet to send to a group of destinations first encapsulates the multicast packet in a unicast packet (tunneling) and sends it to the RP.

The RP then decapsulates the unicast packet and sends the multicast packet to its destination.

PIM-SM uses a complex algorithm to select one router among all routers in the internet as the RP for a specific group. This means that if we have m active groups, we need m RPs, although a router may serve more than one group. After the RP for each group is selected, each router creates a database and stores the group identifier and the IP address of the RP for tunneling multicast packets to it.

PIM-SM uses a spanning multicast tree rooted at the RP with leaves pointing to designated routers connected to each network with an active member.

To create a multicast tree rooted at the RP, PIM-SM uses join and prune messages. Figure 2 shows the operation of join and prune messages in PIM-SM. First, three networks join group G1 and form a multicast tree. Later, one of the networks leaves the group and the tree is pruned.