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

Chapter 262
Router Functions of IGMP

Multicast routers use IGMP to learn which groups have members on each of the routers’ interfaces. This information will in turn be used to build multicast trees for forwarding multicast data. It will also have a timer for each of those group memberships. A router that runs IGMP is also a member of any host group that has members on one or more of its interfaces. The multicast router keeps a list of the memberships on its interfaces and not the individual hosts that belong to that group. There really is no need to keep track of the hosts. Simple enough: If only one host on a router interface wishes to join a group, the router has to forward multicast datagrams on that interface. It does not matter if there are 100 hosts or 1 on that interface; the router must be a member of that group as well and forward multicast datagrams out that interface. The multicast router will know if there are any members of a group left on its interface by sending a query packet out that interface.

For IGMP version 2, a multicast router may assume one of two roles: querier or nonquerier. All multicast routers, upon initializing, assume they are the querier router. Since multicast routers periodically transmit a query to find hosts for groups, the new router will eventually receive a query if there is another router providing this function. If the router receives another query message from another router and only if that router has a lower IP address, the new router will assume the role as a nonquerier. If the new router has a lower IP address, it will assume the role of querier and the other router will assume the role of nonquerier.

Router Functions of IGMP

When a host receives a query, it will set delay timers for each group to which it belongs (set between 0 and the timer indicated in the Max Response Time field of the received query). For a host with more than one interface, each interface maintains its own timers. When the time is up, the host responds with a version 2 Membership Report. The TTL field of the IP header will be set to 1. This ensures that the packet will not be forwarded beyond that local network on which it was transmitted. If the host receives a report from another host in the same group, the host will stop its timer and will not send a report. This is to conserve bandwidth and processing time, and avoids having duplicate reports on the network. All of this is accomplished using multicast addressing. 224.0.0.2 is the all-routers multicast address and 224.0.0.1 is the all-hosts address.

If the router receives a report or reports, it will add the group to its internal list, noting the interface on which it received the report. It then sets a timer for the next query message. If it receives more reports for a group, then the timer will be reset to the max value and restarted. If no reports are received before this timer expires, then the router assumes there are no members for that group and it will not forward remotely received multicast datagrams on the interface.