Multicast routing protocols

Explain Multicast routing protocols in detail.

A host places a multicast address in the destination address field to send packets to a set of hosts belonging to a group.

Internet multicast can be implemented on physical networks that support broadcasting by extending forwarding functions. The extended ones are:

o  Link-State Multicast

o  Distance-Vector Multicast

o  Protocol Independent Multicast (PIM)

Link-State Multicast

Multicasting is added to the existing link-state routing.

o  Each router knows entire topology by way of update messages.

o   Dijkstra's algorithm is used to compute shortest path spanning tree to reach all destinations.

Each router determines which groups have members on which LAN by monitoring the periodical announcements.

o   If a host does not advertise periodically, then it has left the group.

Equipped with group and membership knowledge, each router computes shortest path multicast tree from any source to any group using Dijkstra's algorithm.

 Link-state routing is expensive as each router must store a multicast tree from every source to every group.

Distance-Vector Multicast

Multicasting is added to existing distance-vector routing in two stages.

o   Each router maintains a table of (Destination, Cost, NextHop) for all destination through exchange of distance vectors.

o  Reverse Path Broadcast mechanism that floods packets to other networks

o   Reverse Path Multicasting that prunes end networks that do not have hosts belonging to a multicast group.

Reverse-Path Broadcasting

A router when it receives a multicast packet from source S to a Destination from NextHop, then it forwards the packet on all out-going links.

The drawbacks are:

o  It floods a network, even if it has no members for that group

o   Duplicate flooding, i.e., packets are forwarded over the LAN by each router connected to that LAN.

Duplicate flooding is avoided by

o   Designating a router on the shortest path as parent router.

o   Only parent router is allowed to forward multicast packets from source S to that LAN.

Reverse-Path Multicasting

Multicasting is achieved by pruning networks that do not have members for a group G. Pruning is achieved by identifying a leaf network, which has only one router (parent). The leaf network is monitored to determine if it has any members for group G.

The router then decides whether or not to forward packets addressed to G over that LAN. The information "no members of G here" is propagated up the shortest path tree.

Thus routers can come to know for which groups it should forward multicast packets. Including all this information in a routing update is expensive.

Protocol Independent Multicast (PIM)

The above two multicast routing did not scale well.

PIM divides multicast routing into sparse and dense mode.

In PIM sparse mode (PIM-SM), routers leave and join multicast group using PIM Join and Prune messages.

PIM designates a rendezvous point (RP) for each group in a domain to receive PIM messages.

All routers in the domain know the IP address of RP for each group.

A multicast forwarding tree is built as a result of routers sending Join messages to the RP. The tree may be either shared by multiple senders or source-specific to a sender.

Shared Tree

When a router sends Join message for group G to RP, it goes through a sequence of routers.

Each router along the path creates an entry (*, G) in its forwarding table for the shared tree before forwarding the Join message.

Eventually, the message arrives at RP. Thus a shared tree with RP as root is formed.