1 Computer Communications - CSCI 551 Copyright © William C. ChengCS551Multicast RoutingBill Chenghttp://merlot.usc.edu/cs551-f12allows you to send one packet and let the network makecopies to everyoneWant to send information to a group of people2Why Study Multicast? Computer Communications - CSCI 551 Copyright © William C. Chengdon’t have to keep track of individual usersAnonymous addressingdon’t worry about changes in group membershipbut:some applications want to know how many or who arein the groupnot all users need all the information all the time (e.g., inretransmissions, some users need not see theretransmissions)Efficient data distributionUnicast: one source to one destination3Multicast Routing Computer Communications - CSCI 551 Copyright © William C. ChengMulticast: one source to many destinationsTwo main functions:Logical naming of a group (anonymous group addressing)4Unicast Computer Communications - CSCI 551 Copyright © William C. ChengSrc5Unicast (Cont...) Computer Communications - CSCI 551 Copyright © William C. ChengSrc6Unicast (Cont...) Computer Communications - CSCI 551 Copyright © William C. ChengSrc7Unicast (Cont...) Computer Communications - CSCI 551 Copyright © William C. ChengSrc8Unicast (Cont...) Computer Communications - CSCI 551 Copyright © William C. ChengSrc9Unicast (Cont...) Computer Communications - CSCI 551 Copyright © William C. ChengSrc10Multicast Computer Communications - CSCI 551 Copyright © William C. ChengSrc11Multicast (Cont...) Computer Communications - CSCI 551 Copyright © William C. ChengSrc12Multicast (Cont...) Computer Communications - CSCI 551 Copyright © William C. ChengSrc13Multicast (Cont...) Computer Communications - CSCI 551 Copyright © William C. ChengSrc14Multicast (Cont...) Computer Communications - CSCI 551 Copyright © William C. ChengSrc15Multicast (Cont...) Computer Communications - CSCI 551 Copyright © William C. ChengSrclearn of the existence of multicast groups(advertisement)Router:16Multicast State Computer Communications - CSCI 551 Copyright © William C. Chengidentify links with group membersestablish state to route packetsreplicate packets on appropriate interfaces Srcbroadcast, teleconference, etc.Applications:17Bandwidth Reduction Computer Communications - CSCI 551 Copyright © William C. Chengreliability? how do we do failure/loss recovery inmulticast?Caveatsdifferent users with different start times?solutions include starting people in the middle (like TV),caching (like Tivo), multiple versions of streamApplication level multicast: mailing lists18Logical Naming / Anonymous Addressing Computer Communications - CSCI 551 Copyright © William C. ChengSingle address maps to logically related set of destinationsConvenienceScaling: single name/address as group grows, changesSpecial case: anycastfind me any member of a group, don’t care which one(e.g., mailing a letter, any post office will do)number of sourcesScalability19Common Problems in Multicast Computer Communications - CSCI 551 Copyright © William C. Chengnumber of receiversgeographic/network distance (sparse vs. dense groups)message (NACK) implosionadaption to many receivers (some receivers are slower thanothers, some have higher lost rates, etc.)rather than reliable send and ACK, send periodicallySoft state20Common Techniques in Multicast Computer Communications - CSCI 551 Copyright © William C. Chengdelay may be biased to favor certain hosts respondingResponse after randomized delaylisten to others responses: if they say the same as whatyou are planning to say, you don’t need to say itSuppression of duplicate responsesThese techniques are used in many placesglobally known portion of address spaceMembers are the intended receivers21Multicast groups Computer Communications - CSCI 551 Copyright © William C. ChengSenders may or may not be membersDestination address is class D IP addressHosts may belong to many groupsHosts may send to many groupsSupport dynamic creation of groups, dynamic membership,dynamic sourcesGroups can have different scope22Scope Computer Communications - CSCI 551 Copyright © William C. ChengLAN (local scope)Campus/admin scopingTTL scoping must be used with cautionConcept of scope important to multipoint protocols andapplications (later...)Broadcast audio/video23Example applications Computer Communications - CSCI 551 Copyright © William C. ChengPush-based systemsSoftware distributionWeb-cache updatesTeleconferencing (audio, video, shared whiteboard, texteditor)Multi-player gamesServer/service locationDistributed applicationsSensor networks?Mcast address must be unique in space and timeMulticast address allocation (later)24Other Parts Of The Architecture Computer Communications - CSCI 551 Copyright © William C. ChengAssume address is advertisedAvoid collisions as much as possibleUse randomizationCan’t have highly used address spaceMultiple multicast groups per conference...different appstreams, different layers...more laterApplication level multicast25Some Concepts Computer Communications - CSCI 551 Copyright © William C. ChengNetwork level multicastAside: active networks26Application-level Multicast Computer Communications - CSCI 551 Copyright © William C. ChengSrcan overlay linka underlay link27Application-level Multicast Computer Communications - CSCI 551 Copyright © William C. ChengSrc28Application-level Multicast (Cont...) Computer Communications - CSCI 551 Copyright © William C. ChengSrc29Application-level Multicast (Cont...) Computer Communications - CSCI 551 Copyright © William C. ChengSrccannot use link-level multicastCons of application-level multicast:30Why Not Just Use Application-level Multicast? Computer Communications - CSCI 551 Copyright © William C. Chengdoes not have direct access to unicast routing anddoes not know about network topologyPros of application-level multicastcan deploy applications today and does not need helpfrom ISPduplicate data on links31Components of the IP Multicast Architecture Computer Communications - CSCI 551 Copyright © William C. Chenghostsroutersservice modelhost-to-routerprotocol (IGMP)multicast routingprotocols (various)Each group identified by a single IP address32IP Multicast Service Model (RFC-1112) Computer Communications - CSCI 551 Copyright © William C. ChengGroups may be of any sizeMembers of groups may be located anywhere in the InternetMembers of groups can join and leave at willSenders need not be memberseach multicast address is like a radio
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