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Internetworking with TCP/IP- P38: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | Sec. 17.14 IGMP Implementation 329 send general IGMP queries to the all hosts address hosts send some IGMP messages to the all routers address and both hosts and routers send IGMP messages that are specific to a group to the group s address. Thus datagrams carrying IGMP messages are transmitted using hardware multicast if it is available. As a result on networks that support hardware multicast hosts not participating in IP multicast never receive IGMP messages. Second when polling to determine group membership a multicast router sends a single query to request information about all groups instead of sending a separate message to eachf. The default polling rate is 125 seconds which means that IGMP does not generate much traffic. Third if multiple multicast routers attach to the same network they quickly and efficiently choose a single router to poll host membership. Thus the amount of IGMP traffic on a network does not increase as additional multicast routers are attached to the net. Fourth hosts do not respond to a router s IGMP query at the same time. Instead each query contains a value N that specifies a maximum response time the default is 10 seconds . When a query arrives a host chooses a random delay between 0 and N which it waits before sending a response. In fact if a given host is a member of multiple groups the host chooses a different random number for each. Thus a host s response to a router s query will be spaced randomly over 10 seconds. Fifth each host listens for responses from other hosts in the group and suppresses unnecessary response traffic. To understand why extra responses from group members can be suppressed recall that a multicast router does not need to keep an exact record of group membership. Transmissions to the group are sent using hardware multicast. Thus a router only needs to know whether at least one host on the network remains a member of the group. Because a query sent to the all systems address reaches every member of a group .
