This document describes the Protocol Independent Multicast-Sparse Mode (PIM-SM) protocol, an experimental protocol for efficiently routing multicast traffic across wide-area and inter-domain internets. PIM-SM is not dependent on any specific unicast routing protocol and supports sparse multicast groups. The protocol defines how routers interact to create and maintain multicast distribution trees, with a focus on efficient routing and state management. PIM-SM operates by using a designated router (DR) to manage the distribution of multicast traffic. The DR sends Join/Prune messages to a group-specific rendezvous point (RP) to establish a shared, RP-centered distribution tree. When a data source sends packets to a group, the DR encapsulates the packets in Register messages and unicasts them to the RP. If the data rate is high, the RP can send source-specific Join/Prune messages to establish a shortest path tree (SPT) for the source. Routers can switch between the shared RP-tree and the SPT based on data rate and membership changes. The protocol includes mechanisms for handling multi-access networks, such as designated router election, assert messages to resolve parallel paths, and join/prune suppression timers to reduce redundant control messages. PIM-SM also supports interoperability with dense mode protocols like DVMRP through special entry types and mechanisms for handling RP information. The document outlines the detailed protocol operations, including the processing of join/prune messages, register messages, and the handling of multicast data packets. It describes the state management for different types of multicast entries, including shared trees, source-specific trees, and RP-based trees. The protocol also includes mechanisms for handling unicast routing changes and ensuring the reliability of multicast distribution trees. PIM-SM is designed to be efficient and scalable, with mechanisms to handle dynamic changes in network topology and membership. The protocol is structured to allow for flexibility in routing decisions and to ensure that multicast traffic is delivered efficiently to the intended receivers. The document provides a comprehensive overview of the protocol, including its operation, state management, and mechanisms for handling various network scenarios.This document describes the Protocol Independent Multicast-Sparse Mode (PIM-SM) protocol, an experimental protocol for efficiently routing multicast traffic across wide-area and inter-domain internets. PIM-SM is not dependent on any specific unicast routing protocol and supports sparse multicast groups. The protocol defines how routers interact to create and maintain multicast distribution trees, with a focus on efficient routing and state management. PIM-SM operates by using a designated router (DR) to manage the distribution of multicast traffic. The DR sends Join/Prune messages to a group-specific rendezvous point (RP) to establish a shared, RP-centered distribution tree. When a data source sends packets to a group, the DR encapsulates the packets in Register messages and unicasts them to the RP. If the data rate is high, the RP can send source-specific Join/Prune messages to establish a shortest path tree (SPT) for the source. Routers can switch between the shared RP-tree and the SPT based on data rate and membership changes. The protocol includes mechanisms for handling multi-access networks, such as designated router election, assert messages to resolve parallel paths, and join/prune suppression timers to reduce redundant control messages. PIM-SM also supports interoperability with dense mode protocols like DVMRP through special entry types and mechanisms for handling RP information. The document outlines the detailed protocol operations, including the processing of join/prune messages, register messages, and the handling of multicast data packets. It describes the state management for different types of multicast entries, including shared trees, source-specific trees, and RP-based trees. The protocol also includes mechanisms for handling unicast routing changes and ensuring the reliability of multicast distribution trees. PIM-SM is designed to be efficient and scalable, with mechanisms to handle dynamic changes in network topology and membership. The protocol is structured to allow for flexibility in routing decisions and to ensure that multicast traffic is delivered efficiently to the intended receivers. The document provides a comprehensive overview of the protocol, including its operation, state management, and mechanisms for handling various network scenarios.