Illustrated TCP/IP Illustrated TCP/IP
by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568   Pub Date: 11/01/98  

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Chapter 281
PIM—Dense Mode (PIM-DM)

PIM–Dense Mode (PIM-DM)

•  Similar to DVMRP but does not build its own unicast routing table.
•  Less complex than DVMRP.
•  Broadcast and Prune.
•  It will continue to broadcast packets until Prune messages are received
•  Accepts duplicate packets in a trade off for efficiency.
•  Assumes all downstream interfaces want to receive all multicast packets.
•  Three mechanisms used to build a multicast tree:
•  Prune
•  Graft
•  Leaf Network detection

Dense Mode is the easiest to explain, especially if you have read the previous section on DVMRP. It functions similar to DVMRP in that it uses RPM to build source-routed multicast trees. However, unlike DVMRP, PIM does not rely on a independent unicast routing protocol.

When a multicast packet arrives on a PIM-DM interface, it is forwarded to all interfaces until the branches are specifically pruned. Unlike DVMRP, PIM-DM will continue to forward multicast packets until specific Prune messages are received. No tables are build from these prune messages. DVMRP uses a routing table to determine if there are downstream routers that want to receive the multicast datagrams for a specific group. DVMRP, relying on a routing table that is sent to all multicast routers, is more selective when it forwards messages during the construction of a source-rooted multicast tree. The reasoning behind this is that simplicity and protocol independence are considered a higher priority than additional overhead caused by packet duplication. Building a unicast routing table virtually eliminates duplicate packets. PIM-DM accepts duplicate packets as an alternative to not become dependent on a unicast routing protocol, and therefore avoids building yet another routing database. PIM-DM assumes that all downstream interfaces want to receive multicast datagrams. PIM was actually built for sparse-mode multicast networks and DM was added for simple functionality. PIM-DM does not contain the concept of rendezvous points and there are no periodic joins (however, the Join message is still used). There currently is a draft RFC to allow for “border routers,” which allow PIM and DVMRP interoperability. (The RFC can be found at netweb.usc.edu/pim/.)

PIM-DM is less complex than DVMRP. There are three mechanisms that PIM-DM uses to build a multicast tree: Prune, Graft, and Leaf network detection.


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