Protocol Independent Multicast{Sparse Mode (PIM-SM): Motivation and Architecture

Traditional multicast routing mechanisms (e.g. DVMRP and MOSPF [1, 2]) were intended for use within regions where groups are widely represented or bandwidth is universally plentiful. When group members, and senders to those group members, are distributed sparsely across a wide area, these schemes are not e cient; data packets or membership report information are periodically sent over many links that do not lead to receivers or senders, respectively. This characteristic lead the Internet community to investigate multicast routing architectures that e ciently establish distribution trees across wide-area internets, where many groups are sparsely represented and where bandwidth is not uniformly plentiful due to the distances and multiple administrations traversed. E ciency is evaluated in terms of the state, control message processing, and data packet processing required across the entire network in order to deliver data packets to the members of the group. The Protocol Independent Multicast{Sparse Mode (PIM-SM) architecture: (a) maintains the traditional IP multicast service model of receiver-initiated membership; (b) uses explicit joins that propagate hop-by-hop from members' directly connected routers toward the distribution tree. (c) builds a shared multicast distribution tree centered at a Rendezvous Point, and then builds source-speci c trees for those sources whose data tra c warrants it. (d) is not dependent on a speci c unicast routing protocol; and (e) uses soft-state mechanisms to adapt to underlying network conditions and group dynamics. The robustness, exibility, and scaling properties of this architecture make it well suited to large heterogeneous inter-networks. This document motivates and describes the PIM-SM architecture. Companion documents describe the detailed protocol mechanisms for PIM-SM and PIM-DM, respectively [3, 4]. draft-ietf-idmr-pim-arch-04.ps 3