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USC CSCI 551 - 16a_mcastrouting

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1 Computer Communications - CSCI 551 Copyright © William C. ChengCS551Multicast Routing: IGMPBill Chenghttp://merlot.usc.edu/cs551-f122Components of the IP Multicast Architecture Computer Communications - CSCI 551 Copyright © William C. Chenghostsroutersservice modelhost-to-routerprotocol (IGMP)multicast routingprotocols (various)the protocol by which hosts report their multicast groupmemberships to neighboring routers3Internet Group Management Protocol (IGMP) Computer Communications - CSCI 551 Copyright © William C. Chengversion 1, the current Internet Standard, is specified inRFC-1112operates over broadcast LANs and point-to-point linksoccupies similar position and role as ICMP in the TCP/IPprotocol stackversion 2: RFC 2236an IP multicast packet is transmitted as a link-layermulticast, on those links that support multicastTransmission:4Link-layer Transmission/reception Computer Communications - CSCI 551 Copyright © William C. Chengthe link-layer destination address is determined by analgorithm specific to the type of link (next slide)the necessary steps are taken to receive desiredmulticasts on a particular link, such as modifying addressreception filters on LAN interfacesReception:multicast routers must be able to receive all IP multicastson a link, without knowing in advance which groups willbe sent tofor Ethernet and other LANs using 802 addresses:5Mapping to Link-layer Multicast Addresses Computer Communications - CSCI 551 Copyright © William C. Chengfor point-to-point links: no mapping neededLAN multicast address0 0 0 0 23 bits0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 1 1 1 1 0 01 1 1 0 28 bitsIP multicast addressgroup bit6IGMP Version 1 Message Format Computer Communications - CSCI 551 Copyright © William C. ChengChecksumGroup AddressReservedTypeVersVersionTypeChecksumGroup Address::::11 = Membership Query2 = Membership Reportstandard IP-style checksum ofthe IGMP Messagegroup beingreported(zero in Queries)just care about some vs. zero receivers, not how manyDetermine what IP multicast groups have receivers presenton the LAN7IGMP Goal Computer Communications - CSCI 551 Copyright © William C. Chengdesignate one router as IGMP "querier"Approachit asks all hostsget at least one response per active groupexample of soft state (periodically query), so occasionallosses are okayon each link, one router is elected the "querier"8How IGMP Works Computer Communications - CSCI 551 Copyright © William C. Chengquerier periodically sends a Membership Query messageto the all-systems group (224.0.0.1), with TTL = 1on receipt, hosts start random timers (between 0 and10 seconds) for each multicast group to which they belongQrouters:hosts:9How IGMP Works (Cont...) Computer Communications - CSCI 551 Copyright © William C. Chengwhen a host’s timer for group G expires, it sends aMembership Report to group G, with TTL = 1Qrouters:hosts: G G G Gother members of G hear the report and stop their timersrouters hear all reports, and time out nonrespondinggroups10IGMP Implications Computer Communications - CSCI 551 Copyright © William C. ChengIn normal case, only one report message per group present issent in response to a query (routers need not know who allthe members are, only that members exist)Query interval is typically 60 -- 90 secondsTo reduce join latency, when a host first joins a group, itsends one or two immediate reports (unsolicited responses),instead of waiting for a queryIGMPv2 adds explicit leave messagesnew message and procedures to reduce "leave latency"changes from version 1:11IGMP Version 2 Computer Communications - CSCI 551 Copyright © William C. Chengstandard querier election method specifiedversion and type fields merged into a single fieldbackward-compatible with version 1soon to appear as a Proposed Standard RFCwidely implemented already12 Computer Communications - CSCI 551 Copyright © William C. ChengCS551Multicast RoutingBill Chenghttp://merlot.usc.edu/cs551-f1213Components of the IP Multicast Architecture Computer Communications - CSCI 551 Copyright © William C. Chenghostsroutersservice modelhost-to-routerprotocol (IGMP)multicast routingprotocols (various)anonymityMulticast service model makes it hard to locate receivers14Multicast Routing Computer Communications - CSCI 551 Copyright © William C. Chengdynamic join/leaveflood data packets to entire network, orOptions so far (not very efficient)tell routers about all possible groups and receivers sothey can create routes (trees)begin by flooding traffic to entire networkFlood and prune15Early Routing Techniques Computer Communications - CSCI 551 Copyright © William C. Chengprune branches with no receiversunwanted state where there are no receiversrouters advertise groups for which they have receiversto entire networkLink-state multicast protocolscompute trees on demandunwanted state where there are no sendersexamples: DVMRP, PIM-DMexamples: MOSPF16Rendezvous Options Computer Communications - CSCI 551 Copyright © William C. Chengexamples: CBT, PIM-SMSpecify rendezvous (or meeting place) to which sources sendinitial packets, and receivers join; requires mapping betweenmulticast group address and meeting placeMulticast routing can build different types of distributiontrees17Multicast Tree Taxonomy Computer Communications - CSCI 551 Copyright © William C. Chengseparate shortest path tree (SPT) for each senderSource-based treessingle tree shared by all membersShared treesshared tree rooted at group core/rendezvous pointexamples: DVMRP, MOSPF, PIM-DM, PIM-SMexamples: CBT, PIM-SMcan have multiple senders per group18Source-based Trees Computer Communications - CSCI 551 Copyright © William C. Chengoutput link determined from input link, multicastaddress, and source address19A Shared Tree Computer Communications - CSCI 551 Copyright © William C. Chengoutput link determined from input link & multicastaddressshortest path trees - low delay, better load distributionSource-based trees20Shared v.s. Source-Based Trees Computer Communications - CSCI 551 Copyright © William C. Chengmore state at routers (per-source state)efficient for dense-area multicasthigher delay (bounded by factor of 2), traffic concentrationShared treesper-group state at routersefficient for sparse-area multicastDVMRP - source-based trees21Protocol Taxonomy Computer Communications - CSCI 551 Copyright © William C. ChengMOSPF - source-based treesPIM - shared and source-based treesmodel used by most multicast applicationsAnyone (Deering’s service model)22Who


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