406 | Internet Group Management Protocolw w w . d e l l . c o m | s u p p o r t . d e l l . c o m Figure 19-3. IGMP version 3 Membership Report Packet FormatJoining and Filtering Groups and SourcesFigure 19-4 shows how multicast routers maintain the group and source information from unsolicitedreports.1. The first unsolicited report from the host indicates that it wants to receive traffic for group 224.1.1.1.2. The host’s second report indicates that it is only interested in traffic from group 224.1.1.1, source10.11.1.1. Include messages prevent traffic from all other sources in the group from reaching thesubnet, so before recording this request, the querier sends a group-and-source query to verify that thereare no hosts interested in any other sources. The multicast router must satisfy all hosts if they haveconflicting requests. For example, if another host on the subnet is interested in traffic from 10.11.1.3,then the router cannot record the include request. There are no other interested hosts, so the request isrecorded. At this point, the multicast routing protocol prunes the tree to all but the specified sources.3. The host’s third message indicates that it is only interested in traffic from sources 10.11.1.1 and10.11.1.2. Since this request again prevents all other sources from reaching the subnet, the router sendsanother group-and-source query so that it can satisfy all other hosts. There are no other interested hostsso the request is recorded.fnC0071mpType Reserved ReservedChecksum Number of GroupRecordsGroup Record 1Value used by IGMP to calculatemulticast reception stateGroup Record 2 Group Record NRecord Type Auxiliary DataLength(0)Multicast AddressNumber ofSourcesSourceAddressesLength of AuxiliaryData fieldNumber of source addressesto be filteredGroup address to whichthe group record pertainsAuxiliary DataSource addressesto be filteredNone defined in RFC 3376Range: 1-6Code: 1: Current state is Include2: Current state is Exclude3: State change to Include4: State change to Exclude5: Allow new sources and no state change6: Block old sources and no state changeVersion(4)IHL TOS(0xc0)Total Length Flags Frag Offset HeaderChecksumSrc IP AddrTTL(1)Protocol(2)Dest IP Addr(224.0.0.22)Options(Router Alert)Padding IGMP Packet0x12: IGMP version 1 Membership Report0x16: IGMP version 2 Membership Report0x17: IGMP Leave Group0x22: IGMP version 3 Membership Report
