Multicast - PIM Control Messages
Protocol Independent Multicast or PIM is a multicast routing protocol used in Multicast topologies. PIM uses specific messages to allow multicast-enabled routers to exchange information.
According to the RFC that defines PIM Sparse Mode:
- All PIM control messages have IP protocol number 103.
- PIM messages are either unicast or multicast with TTL 1 to the ALL-PIM-ROUTERS group.
- The source address used for unicast messages is a domain-wide reachable address.
- The source address used for multicast messages is the link-local address of the interface from which the message is being sent.
The PIM header that is common to all PIM messages is the following:
- The PIM version is 2.
- The Type field contains a value that indicates the control message type that the PIM message is carrying. The values of the PIM control message Types are as follows:
| Type Value | Message Type | Destination |
|---|---|---|
| 0 | Hello | Multicast to ALL-PIM-ROUTERS |
| 1 | Register | Unicast to RP |
| 2 | Register-Stop | Unicast to source of Register Message |
| 3 | Join/Prune | Multicast to ALL-PIM-ROUTERS |
| 4 | Bootstrap | Multicast to ALL-PIM-ROUTERS |
| 5 | Assert | Multicast to ALL-PIM-ROUTERS |
| 6 | Graft (used in PIM-DM only) | Unicast to RPF'(S) |
| 7 | Graft-Ack (used in PIM-DM only) | Unicast to source of Graft Message |
| 8 | Candidate-RP-Advertisement | Unicast to Domain's BSR |
- The Reserved field is reserved for future use and is set to all zeros and is ignored.
- The Checksum is a standard IP checksum.