PIM Source Discovery for ASM Deployments

defines Protocol Independent Multicast - Sparse Mode (PIM-SM). PIM-SM supports the Any-Source Multicast (ASM) service model by building shared trees rooted at a Rendezvous Point (RP) for multicast groups, and by optionally switching to source trees after sources are discovered. ASM has been widely deployed in operational networks. In many deployments, especially IPTV deployments, the ASM service model is not only a network design choice but also part of the host and application behavior. A network operator cannot always remove ASM simply by changing the network. Moving away from ASM may require changes on receivers, applications, middleware, or customer premises devices. In IPTV networks, the number of set-top boxes can be in the millions, so requiring all end hosts to be upgraded or replaced is often not operationally practical. The PIM architecture defines the RP as the point where receivers and sources meet in the ASM model. Multiple mechanisms have been specified or deployed to configure or discover the RP, including static configuration and dynamic discovery mechanisms. In addition, MSDP has been defined to support source discovery across PIM-SM domains, allowing RPs in different domains to advertise active sources to one another. These mechanisms solve important parts of the ASM problem space, but they also add operational dependencies around RP placement, RP discovery, inter-domain source discovery, filtering policy, scaling, and troubleshooting. As a result, the overall ASM deployment experience can become difficult for operators. The network may need to maintain RP state, source registration behavior, RP discovery behavior, MSDP peering and policy, and multicast forwarding state across many groups, sources, domains, and administrative boundaries. When service behavior is incorrect, the operator often has to reason across several control-plane mechanisms before determining whether the issue is host signaling, RP discovery, source registration, MSDP source advertisement, PIM Join/Prune state, or multicast forwarding. There is therefore a need to explore whether the ASM deployment experience can be simplified without requiring a disruptive migration of existing end hosts away from ASM. This document is intended to examine possible PIM extensions that can improve source discovery and operational behavior while preserving compatibility with deployed ASM host behavior.