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TITLE: LIVE Webcast From MacWorld expo XW York 99'

START: 19 Jul 99 12: OB END: 2* Jul 99 12:00 iiOJJEs fir«s*nt«4 By Mac 30 pot Com Contact: . cam

: 45th iBXf Unter&et Engineering Task Force Meeting) Q»l®, Norway

URL; http://www.ietf.org/meetxngs Contact:_

There are also tools that utilize such protocols as Session Description Protocol (SDP) and Session Advertisement Protocol (SAP) to describe multicast events and advertise those descriptions. Figure 5-8 shows an example of an application that uses these protocols. A user also may learn of a multicast session by invitation, such as via a simple e-mail.

f iffy re 5-8 Applications. Such as Multikit Listen for SDP and SAP and Display the Multicast Sessions Advertised by Those Protocols

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A detailed discussion of these mechanisms is beyond the scope of this book. This section presumes that hosts have somehow learned of a multicast group, and it examines the issues around joining and leaving the group. After examining these issues, you will see how they are handled by the Internet Group Management Protocol (IGMP), the de facto protocol for managing IP multicast groups on individual subnets.

Joining and Leaving a Group

Interestingly, the source of a multicast session does not have to be a member of the multicast group to which it is sending traffic. In fact, the source typically does not even know what hosts, if any, are members of the group. Receivers are free to jom and leave groups at any time. This again fits the earlier analogy of a radio or television signal; audience members can tune in or tune out at any time, and the originating station has no direct way of knowing who is listening.

If the source and all group members share a common LAN, no other protocols are required The source sends packets to a multicast IP (and MAC) address, and the group members "tune in" to this address But sending multicast traffic over a routed internetwork becomes more complicated Every router could merely forward all multicast packets onto every LAN, in case there are group members on the LAN, but this partially circumvents the goal of multicasting, which is to conserve network resources If no group members are on the LAN, bandwidth and processing is wasted not oi\ly on that subnet, but also on all data links and routers leading to it

Therefore, a router must have some means to learn whether a connected network includes group members, and if so, members of what group When a router becomes aware of a multicast session, it can query all of its attached subnets for hosts that want to join the receiving group The query might be addressed to the "all systems on this subnet" address of 224 0 0 1, or it might be addressed to the specific address of the group for which it is querying If one or more hosts respond, the router can then forward the session's packets onto the appropriate subnet, as illustrated in Figure 5-9

Figure 5-9 Multicast Group Member Discovery

The router can periodically resend queries to the subnet If there are still group members on the subnet, they will respond to all queries to let the router know they are still active in the group If no hosts respond, the router assumes that all hosts on the subnet have left the group, and it ceases forwarding the group's packets onto the subnet

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