Multicast Routing

Multicasting

• Source S sends packets to multicast group G1 (and minimize the number of copies)

Reverse-Path Broadcasting (RPB)

• Reverse-Path Broadcasting(反向路径广播)
• Fact: Set of shortest paths to the source node S forms a shortest path tree that spans the network
  • Approach: Follow paths in reverse direction
• Assume each router knows current shortest path to S
  • Upon receipt of a multicast packet, router records the packet’s source address and the port it arrives on
  • If shortest path to source is through same port (“parent port”), router forwards the packet to all other ports
  • Else, router drops the packet
• Loops are suppressed; each packet forwarded a router exactly once
• Implicitly assume shortest path to source S is same as shortest path from source

Example: Shortest Paths from S

• Spanning tree of shortest paths to node S and parent ports are shown in read

Example: S sends a packet

• Node 1 forwards it to all other ports (links shown in blue)

Example: Hop 1 nodes broadcast

• Nodes 2, 3, 4, and 5 broadcast, except on parent ports

Example: Broadcast continues

• All nodes, not only G1, receive packets

Truncated RPB (TRPB)

• Truncated RPB (TRPB): Leaf routers do not broadcast if none of its attached hosts belong to packet’s multicast group

Internet Group Management Protocol

• Internet Group Management Protocol (IGMP): allows a host to signal its multicast group membership to its attached router
• Each multicast router periodically sends an IGMP query message to check whether there are hosts belonging to multicast groups
• Routers determine which multicast groups are associated with a certain port
• Routers only forward packets on ports that have hosts belonging to the multicast group

Reverse-Path Multicasting

• Reverse Path Multicasting (RPM) relies on IGMP to identify multicast group membership
• RPM is an enhancement of TRPB, but unlike TRPB, RMP forwards a multicast packet only to a router that will lead to a leaf router with group members.