Multicast traffic is routed by efficiently delivering a single stream of data from a source to a group of destination hosts simultaneously, primarily utilizing a core concept called Reverse Path Forwarding (RPF).
Unlike unicast routing, which sends data to a single destination, or broadcast, which sends data to all hosts on a network segment, multicast is designed for targeted delivery to multiple interested receivers across an IP network. The source sends the traffic to a group of hosts, identified by a specific multicast IP address. Routers along the path replicate the traffic only when necessary to forward it down branches of a distribution tree leading to group members.
The Core Mechanism: Reverse Path Forwarding (RPF)
Reverse Path Forwarding (RPF) is a core concept in multicast forwarding. Its primary function is to ensure that multicast packets are received on the interface that lies on the shortest path back to the source. This mechanism prevents routing loops and ensures the efficient construction of the multicast distribution tree.
As stated in the reference, Reverse Path Forwarding (RPF) is forwarding the traffic away from the source. When a router receives a multicast packet, it performs an RPF check:
- It examines the source IP address of the packet.
- It looks up the routing table to determine the shortest path back to that source IP address.
- It checks if the packet arrived on the interface that corresponds to this shortest path back to the source.
If the packet arrived on the correct interface (the RPF interface), the RPF check passes, and the router proceeds to forward the packet out interfaces leading to downstream receivers. If it arrived on any other interface, the RPF check fails, and the packet is discarded.
Router's Role in Managing Multicast
Routers play a crucial role in multicast forwarding. The router keeps track of which interfaces are incoming and which are outgoing for specific multicast groups and sources. This information is used in conjunction with RPF to build and maintain the multicast distribution tree.
- Incoming Interface: The interface on which a multicast packet from a specific source is expected to arrive, based on the RPF check.
- Outgoing Interfaces: The interfaces connected to networks or other routers that have members of the multicast group or lead to paths towards members.
Routers build and maintain state information about active multicast groups, sources sending to those groups, and which interfaces need to forward traffic for specific (Source, Group) pairs.
Building the Distribution Tree
Multicast routing protocols are used by routers to dynamically build and maintain the distribution trees (or paths) from sources to receivers. These trees ensure that multicast traffic flows efficiently through the network, reaching only the segments that have interested listeners.
Common multicast routing protocols include:
- Protocol Independent Multicast (PIM): The most widely deployed family of multicast routing protocols. PIM can operate in Dense Mode (PIM-DM) or Sparse Mode (PIM-SM), depending on how widely distributed group members are.
- Distance Vector Multicast Routing Protocol (DVMRP): An older protocol that builds source-based trees using a truncated broadcast approach.
These protocols leverage the RPF check and router state information to determine which interfaces should forward multicast traffic, effectively pruning branches of the distribution tree that do not lead to group members.
Why Use Multicast Routing?
Multicast routing offers significant advantages for sending the same data stream to multiple recipients:
- Bandwidth Efficiency: Data is sent only once from the source onto the network, reducing bandwidth consumption on the source network and upstream links. Replication happens only when paths diverge towards receivers.
- Reduced Source/Router Load: The source only needs to send the data once, regardless of the number of receivers. Intermediate routers handle replication, distributing the load.
- Optimized Network Resource Usage: Minimizes unnecessary traffic duplication across the network core.
In summary, multicast traffic routing relies heavily on routers performing RPF checks to validate the packet path and then forwarding the data out specific interfaces that lead to multicast group members, efficiently delivering the source's traffic to the group of hosts.
