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Distance Vector Multicast Routing Protocol

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The Distance Vector Multicast Routing Protocol (DVMRP) is used to share information between routers to transport IP Multicast packets among networks.

It is based on the RIP protocol to forward packets: the router generates a routing table with the multicast group that it has knowledge with its corresponding distance (number of devices -routers- in the middle to reach it). When a Multicast packet is received by a router, it is forwarded by router's interfaces specified in the routing table.

The DVMRP protocol uses IGMP messages to exchange information with other routers.

DVMRP uses IGMP to exchange routing datagrams. IGMP Header is:
4 bits version
4 bits Type
8 bits Subtype,
where the subtype is one of: 

  1 = Response;
  2 = Request;
  3 = Non-membership report;
  4 = Non-membership cancellation;

16 bits Checksum


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Distance-Vector Multicast Routing Protocol (DVMRP) The Distance-Vector Multicast Routing Protocol (DVMRP) uses a reverse path-flooding technique and is used as the basis for the Internet's multicast backbone (MBONE). DVMRP is defined in RFC 1075 and has certain some shortcomings. In particular, DVMRP is notorious for poor network scaling, resulting from reflooding, particularly with versions that do not implement pruning. DVMRP's flat unicast routing mechanism also affects its capability to scale. The reverse path-flooding operation involves a router sending a copy of a packet out to all paths (except the path back to the origin) upon the packet's receipt. Routers then send a prune message back to the source to stop a data stream if the router is attached to a LAN that does not want to receive a particular multicast group. Reflooding and DVMRP unicast are used in DVMRP path-flooding operations. In reflooding, DVMRP routers periodically reflood an attached network to reach new hosts. The flooding mechanism uses an algorithm that takes into account the frequency of flooding and the time required for a new multicast group member to receive the data stream. DVMRP unicast is used to determine which interface leads back to the source of a data stream. It is unique to DVMRP but is similar to RIP in that it is based on hop count. The DVMRP unicast environment permits the use of a different path than the path used for multicast traffic.

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