This document defines the Protocol Independent Multicast-Sparse Mode (PIM-SM), an experimental protocol for efficiently routing to multicast groups that may span wide-area and inter-domain internets. PIM-SM is designed to support sparse groups and is not dependent on any particular unicast routing protocol. The protocol operates by creating and maintaining a shared, RP-centered distribution tree that reaches all group members. Key components include: 1. **Join/Prune Messages**: Routers send Join/Prune messages to join or prune branches off the multicast distribution tree. These messages are multicast to all routers on multi-access networks and can be triggered by various events, such as changes in group membership or receipt of data packets. 2. **Designated Router (DR)**: The DR is responsible for sending triggered Join/Prune and Register messages toward the Rendezvous Point (RP). It is elected based on the highest IP address among neighboring routers. 3. **Rendezvous Point (RP)**: The RP is the central point for distributing data packets to all group members. It receives Register messages from sources and forwards data packets to the distribution tree. 4. **Bootstrap Messages**: All routers within a PIM domain collect Bootstrap messages to obtain RP information. These messages are used to distribute RP information and perform dynamic BSR elections. 5. **Security**: All PIM control messages can use IPSec to address security concerns. 6. **Interoperability with Dense-Mode Protocols**: PIM-SM can interoperate with dense-mode protocols like DVMRP by pulling out packets to PIM Multicast Border Routers (PMBRs) and injecting them into the DVMRP network. 7. **Multi-access Networks**: Additional mechanisms are described for operating PIM over multi-access networks, including Designated Router election, Assert messages to resolve parallel paths, and Join/Prune suppression to reduce duplicate control message overhead. 8. **Unicast Routing Changes**: When unicast routing changes, RPF checks are performed on active entries, and affected interfaces are updated. 9. **Inter-domain Multicast**: Future documents will address the use of PIM-SM as a backbone interdomain multicast routing protocol. The document provides a detailed specification of the protocol operations, including the handling of various message types and the processing of Join/Prune messages.This document defines the Protocol Independent Multicast-Sparse Mode (PIM-SM), an experimental protocol for efficiently routing to multicast groups that may span wide-area and inter-domain internets. PIM-SM is designed to support sparse groups and is not dependent on any particular unicast routing protocol. The protocol operates by creating and maintaining a shared, RP-centered distribution tree that reaches all group members. Key components include: 1. **Join/Prune Messages**: Routers send Join/Prune messages to join or prune branches off the multicast distribution tree. These messages are multicast to all routers on multi-access networks and can be triggered by various events, such as changes in group membership or receipt of data packets. 2. **Designated Router (DR)**: The DR is responsible for sending triggered Join/Prune and Register messages toward the Rendezvous Point (RP). It is elected based on the highest IP address among neighboring routers. 3. **Rendezvous Point (RP)**: The RP is the central point for distributing data packets to all group members. It receives Register messages from sources and forwards data packets to the distribution tree. 4. **Bootstrap Messages**: All routers within a PIM domain collect Bootstrap messages to obtain RP information. These messages are used to distribute RP information and perform dynamic BSR elections. 5. **Security**: All PIM control messages can use IPSec to address security concerns. 6. **Interoperability with Dense-Mode Protocols**: PIM-SM can interoperate with dense-mode protocols like DVMRP by pulling out packets to PIM Multicast Border Routers (PMBRs) and injecting them into the DVMRP network. 7. **Multi-access Networks**: Additional mechanisms are described for operating PIM over multi-access networks, including Designated Router election, Assert messages to resolve parallel paths, and Join/Prune suppression to reduce duplicate control message overhead. 8. **Unicast Routing Changes**: When unicast routing changes, RPF checks are performed on active entries, and affected interfaces are updated. 9. **Inter-domain Multicast**: Future documents will address the use of PIM-SM as a backbone interdomain multicast routing protocol. The document provides a detailed specification of the protocol operations, including the handling of various message types and the processing of Join/Prune messages.