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:
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:
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 220.127.116.11 to 18.104.22.168 inclusive. Addresses within 22.214.171.124/24 are considered link-local and should not be forwarded between subnets. Addresses within 126.96.36.199/8 are reserved for SSM usage. Addresses in 188.8.131.52/8 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 | +--------+----+----+---------------------------------------------+
+-+-+-+-+ |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.
1 node-local scope 2 link-local scope 5 site-local scope 8 organization-local scope E global scope
RFC 2373 gives more details about IPv6 multicast addresses.
XORP supports the following multicast protocols: