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

Chapter 271
Distance Vector Multicast Routing Protocol (DVMRP)

The following text about the DVMRP routing protocol is aligned with the DVMRP version 3 specification, which as of this writing is still a draft RFC. DVMRP uses the Reverse Path Multicasting (RPM) algorithm to dynamically build multicast delivery trees and determine the router’s position in the multicast tree in reference to the source subnet of a multicast datagram. DMVRP is a “broadcast and prune” multicast routing protocol. It uses Reverse Path Forwarding to see if a multicast datagram should be forwarded downstream. A multicast forwarding tree is built between a source and all members of the group (receivers). For those familiar with this protocol, it should be noted up front that the latest version of this protocol for Unix, mrouted 3.5, is also based on RPM, which is significantly different from previous versions in the areas of packet format, tunneling, and so forth.

In order for DVMRP to work, two tables must be built: a unicast route table and a forwarding route table. The unicast route table is used to determine if a multicast datagram was received on the correct port (the upstream interface). The forwarding table is used to determine on which interfaces of a router, a router should forward a multicast datagram (the downstream interface).

To build the unicast routing table, DVMRPs pass route reports to each other containing entries for source subnets. This table is processed like RIP and the shortest distance back to a source is computed and placed in the table. The forwarding table is built by broadcasting the first multicast datagram received and then waiting for other routers to send back Prune and Graft messages to indicate who and who does not want the datagram. Why not simply use the unicast routing table? This is accomplished to allow multicast traffic to follow a different path than the unicast traffic and for the support of a tunnel interface, which unicast traffic does not understand.

DVMRP routers support two types of interfaces: router and tunnel. The multicast router interface is obvious, but the tunnel interface is not so obvious. To allow for nonmulticast routers to exist in a multicast network, the concept of tunnels is used. Multicast datagrams are encapsulated in unicast IP packets (using IP in IP) and these are send over the unicast routers. Contained in the IP header is a route list that the unicast routers should use. The last entry in this list is the end of the tunnel and is a router that again supports IP multicast. The last router strips off the unicast information and sends the datagram on.

DVMRP is a protocol that uses a distance-vector distributed routing algorithm in order for each router to determine the distance from itself to any IP multicast traffic source. When DVMRP determines this, it creates IP multicast delivery trees between a source and its distributed group hosts.