Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 263
HostJoin

When a host joins a multicast group, it immediately transmits a version 2 Membership Report two or three times (remember, IGMP does not use ICMP or TCP). This is done in case that host is the first member of the group and it is repeated in case the first report gets lost or clobbered.

When a host leaves a group (IGMPv2), it transmits a Leave Group message. A host may or may not be able to determine if it is the last member of a group—this is a storage and processing decision on the part of the implementer. A host that can determine if it is the last host in the group will transmit the Leave Group message. Other implementations that cannot determine this may or may not send this message. The multicast router will determine if any hosts exist for a group by the query message anyway. When a multicast router receives this message, it will send group-specific queries to the group being left. If no reports are received, then the multicast router will assume there are no members left in that group and it will not forward any multicast datagrams for that group out that interface.

IGMPv2 is a preliminary draft specification as of this writing and can be found at the InterNIC (ds.internic.net) under Draft RFCs. It mostly contains the ability to conserve bandwidth by allowing a host to elect to receive traffic from specified sources (IP addresses) of a multicast group. Alternatively, it allows a host to specify which sources it does not want to receive information from. What is a source? It is simply a host that originated a multicast datagram. There may be many sources in any one group. With IGMP versions 1 and 2, a host is required to receive all information for a group of which it is a member, no matter which source transmitted it. Also, the Leave Group message is enhanced to allow a host to specify which sources it no longer wishes to receive information from. The multicast router will receive this and possibly stop sending information to that group from that source.

Now that we understand how the host operates with IP multicast and how a host interacts with a multicast router, we need to learn how multicast actually operates. First, we will study the algorithms and then we will take an in-depth look at one multicast protocol: the Distance Vector Multicast Routing Protocol, or DVMRP. The draft RFC used is version 3 of DVMRP.

HostJoin