Distance Vector Multicast Routing Protocol is a multicast routing protocol. When a router receives a multicast packet, DVMRP builds a distribution tree by flooding the packet out to all interfaces except for the one that received the incoming packet. This results in the packet reaching all subnets in the internetwork. Subsequently, routers that have no need to receive a particular multicast group (ie: no systems on their subnets have requested to recieve the multicast) can send a prune message back upstream to cut off traffic where it is not required. Determining which routers are located upstream is handled via a distance-vector protocol based on hop counts, similar to RIP, and DVMRP shares several of RIP's shortcomings