Multicast Routing

IP Multicast is a technology that allows one-to-many and many-to-many distribution of data on the Internet. Senders send their data to a multicast IP destination address, and receives express an interest in receiving traffic destined for such an address. The network then figures out how to get the data from senders to receivers.

If both the sender and receiver for a multicast group are on the same local broadcast subnet, then the routers do not need to be involved in the process, and communication can take place directly. If, however, the sender and receiver are on different subnets, then a multicast routing protocol needs to be involved in setting up multicast forwarding state on the tree between the sender and the receivers.

Broadly speaking, there are two different types of multicast routing protocols:

• Dense-mode protocols, where traffic from a new multicast source is delivered to all possible receivers, and then subnets where there are no members request to be pruned from the distribution tree.
• Sparse-mode protocols, where explicit control messages are used to ensure that traffic is only delivered to the subnets where there are receivers that requested to receive it.

Examples of dense-mode protocols are DVMRP and PIM Dense Mode. Examples of sparse-mode protocols are PIM Sparse Mode, CBT, and MOSPF. Most of these protocols are largely historic at this time, with the exception of PIM Sparse Mode (PIM-SM) and PIM Dense Mode (PIM-DM), and even PIM-DM is not very widely used.

In addition to the routing protocols used to set up forwarding state between subnets, a way is needed for the routers to discover that there are local receivers on a directly attached subnet. For IPv4 this role is served by the Internet Group Management Protocol (IGMP) and for IPv6 this role is served by the Multicast Listener Discovery protocol (MLD).

There are two different models for IP multicast:

• Any Source Multicast (ASM), in which a receiver joins a multicast group, and receives traffic from any senders that send to that group.
• Source-Specific Multicast (SSM), in which a receiver explicitly joins to a (source, group) pairing.

Traditionally IP multicast used the ASM model, but problems deploying inter-domain IP multicast resulted in the much simpler SSM model being proposed. In the future it is likely that ASM will continue to be used within intranets and enterprises, but SSM will be used when multicast is used inter-domain. The two models are compatible, and PIM-SM can be used as a multicast routing protocol for both. The principal difference is that ASM only requires IGMPv2 or MLDv1, whereas SSM requires IGMPv3 or MLDv2 to permit the receivers to specify the address of the sending host.

For IPv4, multicast addresses are in the range 224.0.0.0 to 239.255.255.255 inclusive. Addresses within 224.0.0.0/24 are considered link-local and should not be forwarded between subnets. Addresses within 232.0.0.0/8 are reserved for SSM usage. Addresses in 239.0.0.0/8 are ASM addresses defined for varying sizes of limited scope.

IPv6 multicast addresses are a little more complex. IPv6 multicast addresses start with the prefix ff, and have the following format:

     |      8      | 4 | 4 |        112 bits                          |
     +------ -+----+----+---------------------------------------------+
     |11111111|flgs|scop|           group ID                          |
     +--------+----+----+---------------------------------------------+

• 11111111 (ff in hexadecimal) at the start of the address identifies the address as being a multicast address.
• flgs is a set of 4 flags:

                +-+-+-+-+
                |0|0|0|T|
                +-+-+-+-+
         

  The high-order 3 flags are reserved, and must be initialized to 0.
  T = 0 indicates a permanently-assigned (“well-known”) multicast address, assigned by the global internet numbering authority.
  T = 1 indicates a non-permanently-assigned (“transient”) multicast address.

• scop is a 4-bit multicast scope value used to limit the scope of the multicast group. The values in hex are:

          1 node-local scope
          2 link-local scope
          5 site-local scope
          8 organization-local scope
          E global scope

• group ID identifies the multicast group, either permanent or transient, within the given scope.

RFC 2373 gives more details about IPv6 multicast addresses.

XORP supports the following multicast protocols:

• PIM Sparse Mode for both ASM and SSM multicast routing for IPv4.
• PIM Sparse Mode for both ASM and SSM multicast routing for IPv6.
• IGMPv1, IGMPv2, and IGMPv3 for IPv4 local multicast membership.
• MLDv1 and MLDv2 for IPv6 local multicast membership.