The PIM Architecture for Wide-Area Multicast Routing

Deering, Estrin, Farinacci, Jacobson, Liu, Wei

network routing multicast

@article{deering:ton-1996,
  title={The {PIM} Architecture for Wide-Area Multicast Routing},
  author={Deering, S. and Estrin, D.L. and Farinacci, D. and
          Jacobson, V. and Liu, C.G. and Wei, L.},
  journal={{IEEE}/{ACM} Transactions on Networking ({ToN})},
  volume={4},
  number={2},
  pages={153--162},
  year={1996},
  publisher={{IEEE}}
}

Previous schemes assumed much bandwidth, or many consumers

Efficiency determined by router state, control messages processing, and data packet processing requirements over the entire network

Receiver initiated membership

Shared and source-specific trees

Independence of specific unicast routing protocol

Soft state mechanisms to adapt network and group conditions

Link state has trouble scaling because of all the state it has to keep

• Every router must store information about every group

CBT concentrates traffic, perhaps needlessly so if the core is chosen poorly

Maximum delay of optimal core based tree is twice the shortest path

Shared trees may be appropriate for large numbers of low data rate sources

Shortest path trees more appropriate for high data rate, fewer sources

In PIM, group can choose to use shortest path or group-shared trees

• First hop routers of receivers can choose independently
• Can choose different types for different sources

Multicast packets need to be checked against interface similar to DVMRP, as they can fan out as they loop

Sparse groups aren't necessarily small, just spread out

Efficiency of DVMRP and CBT relies on characteristics of the group and network topology. DVMRP's flooding and continual pruning can consume significant resources, but may make sense when many group members exist throughout the network. Conversely, CBT reduces network wide resource consumption when group members are dispersed, but concentrates group traffic on particular paths among the shared tree. Further, those paths may not efficiently connect current group members, involving otherwise unengaged nodes and incurring unnecessary latency.

Protocol Independent Multicast (PIM)~\cite{deering:ton-1996} addresses these tradeoffs by supporting Dense and Sparse modes. Each respectively operates similarly to DVMRP and CBT. PIM Dense Mode pushes traffic from sources to all routers, which use prune messages to cease unwanted traffic. PIM Sparse Mode operates similarly to CBT, with shared trees constructed via routing to rendezvous points. However, individual PIM routers in the shared tree may also seamlessly construct and switch to source specific shortest path trees. This is typically done to minimize delay or concentration of traffic from high volume sources. The combination of these three aproaches enables PIM to support administrative and online adaptation to varying traffic and group profiles.