IP Multicast Support

IP Multicasting Overview

In addition to unicast and broadcast support, IP also provides a mechanism to send and receive IP multicast traffic. IP multicast traffic is sent to a single destination IP address but is received and processed by multiple IP hosts, regardless of their location on an IP internetwork. A host listens for a specific IP multicast address and receives all packets to that IP address.

IP multicast is more efficient than IP unicast or broadcast for one-to-many delivery of data. Unlike unicast, only one copy of the data is sent. Unlike broadcast, the traffic is only received and processed by computers that are listening for it.

The additional elements of IP multicast include the following:

The set of hosts listening on a specific IP multicast address is called a host group.
Host group membership is dynamic, and hosts can join and leave the group at any time.
There are no limitations to the size of a host group.
A host group can span IP routers across multiple network segments. This configuration requires IP multicast support on IP routers and the ability for hosts to register themselves with the router. Host registration is accomplished using the Internet Group Management Protocol (IGMP).
A host can send traffic to an IP multicast address without belonging to the corresponding host group.

IP multicast addresses, also known as group addresses, are in the class D range of 224.0.0.0 to 239.255.255.255 as defined by setting the first four high order bits to 1110. In network prefix or Classless Inter-Domain Routing (CIDR) notation, IP multicast addresses are summarized as 224.0.0.0/4. Multicast addresses in the range 224.0.0.0 to 224.0.0.255 (224.0.0.0/24) are reserved for the local subnet and are not forwarded by IP routers regardless of the Time to Live (TTL) in the IP header.

The IP multicast addresses from 224.0.1.0 to 238.255.255.255 are either reserved or assigned to a multicasting application. The addresses from 239.0.0.0 to 239.255.255.255 (239.0.0.0/8) are reserved for applications that can be administratively scoped. For more information about these addresses, see "Multicast Boundaries" later in this chapter.

The following are examples of reserved IP multicast addresses:

224.0.0.1 - all hosts on this subnet.
224.0.0.2 - all routers on this subnet.
224.0.0.5 - Open Shortest Path First (OSPF) Version 2, designed to reach all OSPF routers on a network.
224.0.0.6 - OSPF Version 2, designed to reach all OSPF designated routers on a network.
224.0.0.9 - Routing Information Protocol (RIP) Version 2.
224.0.1.1 - Network Time Protocol.

For the latest list of reserved multicast addresses, see the Information Sciences Institute link at http://windows.microsoft.com/windows2000/reskit/webresources.

For more information about IP multicast support, see Internet Engineering Task Force (IETF) Request for Comments (RFC) 1112.