Distance Vector Multicast Routing Protocol

Description

The Distance Vector Multicast Routing Protocol (DVMRP) is an Internet protocol used for routing multicast datagrams within an autonomous system. Its architecture is based on routers exchanging routing information with directly connected neighbors using a distance-vector algorithm, similar to RIP but extended for multicast. Key components include DVMRP routers that maintain routing tables with metrics (hop counts) to multicast sources, and the protocol mechanism for building source-rooted multicast distribution trees using a technique called Reverse Path Forwarding (RPF).

DVMRP operates in several phases. First, DVMRP neighbors discover each other and exchange route advertisements. These advertisements inform routers of the shortest path back to the source of a multicast group. When a multicast packet arrives, a router performs an RPF check: it verifies if the packet arrived on the interface that is the shortest path back to the packet's source address (as per its unicast routing table derived from DVMRP advertisements). If the check passes, the packet is forwarded to all downstream interfaces except the one it arrived on; otherwise, it is discarded. To prune unwanted traffic, DVMRP uses graft and prune messages to dynamically modify the distribution tree, cutting off branches where no group members exist.

In the 3GPP architecture, DVMRP is referenced in the context of interworking between the Gateway GPRS Support Node (GGSN) or Packet Data Network Gateway (PGW) and external IP networks (e.g., the Internet or a content provider's network). When a UE subscribes to a multicast service (like MBMS), the network may need to route multicast traffic from an external source. DVMRP is one of the protocols that could be used in that external routing domain. The 3GPP specs define how the GGSN/PGW interacts with such external multicast routers, including IGMP/MLD proxying and tunnel establishment, but the internal mechanics of DVMRP itself are defined by the IETF.

Purpose & Motivation

The purpose of referencing DVMRP in 3GPP standards is to acknowledge and provide for interworking with legacy multicast routing infrastructures that may exist in external IP networks connected to the mobile core. It solves the problem of delivering IP multicast traffic from sources located in traditional, router-based IP networks to mobile subscribers via the 3GPP packet core. Without support for such interworking protocols, multicast services would be limited to sources within the operator's own network.

Historically, DVMRP was one of the first widely deployed multicast routing protocols for the Internet (Mbone). While more advanced protocols like Protocol Independent Multicast (PIM) are now prevalent, DVMRP existed in many networks. 3GPP's inclusion of DVMRP, starting from Rel-8, ensured backward compatibility and broad interoperability for MBMS and other IP multicast services. It addressed the limitation of the mobile core being a multicast 'island' by specifying how the gateway node participates in or interfaces with external multicast routing domains, regardless of the specific protocol (DVMRP, PIM-SM, etc.) used therein, enabling a wider ecosystem for multicast content delivery.