IP multicast is an extension to the Internet Protocol suite which
provides a logical addressing capability, using the class D
Internet address space
, and a bandwidth efficient way of
sending a message from one host to any number of other hosts. The
host extensions for multicasting are specified in detail
in[46], and multicast routing is discussed
in[5,47].
The two key concepts for building application protocols on top of IP multicast are:
Multicast groups and the operations which are possible on them via the Internet Group Management Protocol (IGMP) are defined in Appendix I of [46].
Essentially, a multicast group consists of a class D IP address - such as 224.2.0.1.
The host extensions for multicasting define a means for individual machines to join and leave these groups, together with a control mechanism which is used in routing to determine which groups are active on a given network.
Applications may choose to join or leave multicast groups dynamically, and it is possible to send a message to a multicast group without having to join it.
Since the MBONE connects multicast aware networks all over the world together, it is conceivable that a multicast group may have members distributed over a very wide area.
Of course, it will not be desirable in all cases to propagate a message to all of the hosts on the MBONE which have registered an interest in a particular multicast group. Multicast routing technology uses the time-to-live (TTL) value in the traffic to limit the scope of messages which are sent to a group.
For example, MBONE traffic in the UK is asked to observe the following guidelines:
Each of the routers which a multicast packet passes through normally decrements the packet's TTL by a threshold before forwarding it. The packet will not be forwarded if this amount exceeds the TTL value supplied.
It should be noted that there is no way to know whether someone else is using a particular multicast group, other than to join that group and examine any traffic received. There are, however, several groups which have been declared permanent. These are noted in the list of Internet Assigned Numbers[48].
Furthermore, it should not be assumed that a particular TTL setting will always cause multicast packets to reach a particular host or hosts. The TTL setting necessary to reach a particular host is dependent on the MBONE topology between it and the host which is sending - usually this mirrors the physical network topology.