15-441 Computer NetworkingGroup Communication ApplicationsMulticast – Efficient Data DistributionOverviewIP Multicast ArchitectureIP Multicast Service Model (rfc1112)IP Multicast AddressesIP Multicast ServiceMulticast Scope Control – Small TTLsMulticast Scope Control – Large TTLsSlide 11Multicast Router ResponsibilitiesSlide 13Internet Group Management ProtocolHow IGMP WorksHow IGMP Works (cont.)Slide 17Slide 18Multicast RoutingRouting TechniquesSource-based TreesDistance-Vector Multicast RoutingExample TopologyBroadcast with TruncationPruneGraftSteady StateSlide 28ImplosionRetransmissionExposureIdeal Recovery ModelMulticast Congestion ControlSlide 34Supporting Multicast on the InternetSlide 36IP MulticastEnd System MulticastPotential Benefits Over IP MulticastConcerns with End System Multicast15-441 Computer NetworkingLecture 24 – MulticastLecture 24: 11-20-2002 2Group Communication Applications•Broadcast audio/video•Software distribution•Web-cache updates •Teleconferencing (audio, video, shared whiteboard, text editor)•Multi-player games•Server/service location•Other distributed applicationsLecture 24: 11-20-2002 3Multicast – Efficient Data DistributionSrc SrcLecture 24: 11-20-2002 4Overview•IP multicast service basics•Multicast routing•Multicast transport•Overlay multicastLecture 24: 11-20-2002 5IP Multicast ArchitectureHostsRoutersService modelHost-to-router protocol(IGMP)Multicast routing protocols(various)Lecture 24: 11-20-2002 6IP Multicast Service Model (rfc1112)•Each group identified by a single IP address•Groups may be of any size•Members of groups may be located anywhere in the Internet•Members of groups can join and leave at will•Senders need not be members•Group membership not known explicitly •Analogy:•Each multicast address is like a radio frequency, on which anyone can transmit, and to which anyone can tune-in.Lecture 24: 11-20-2002 7IP Multicast Addresses•Class D IP addresses•224.0.0.0 – 239.255.255.255•How to allocated these addresses?•Well-known multicast addresses, assigned by IANA•Transient multicast addresses, assigned and reclaimed dynamically, e.g., by “sdr” program1 1 1 0 Group IDLecture 24: 11-20-2002 8IP Multicast Service•Sending – same as before•Receiving – two new operations•Join-IP-Multicast-Group(group-address, interface)•Leave-IP-Multicast-Group(group-address, interface)•Receive multicast packets for joined groups via normal IP-Receive operationLecture 24: 11-20-2002 9Multicast Scope Control – Small TTLs•TTL expanding-ring search to reach or find a nearby subset of a groups123Lecture 24: 11-20-2002 10Multicast Scope Control – Large TTLs•Administrative TTL Boundaries to keep multicast traffic within an administrative domain, e.g., for privacy or resource reasonsAn administrative domainTTL threshold set oninterfaces to these links,greater than the diameterof the admin. domainThe rest of the InternetLecture 24: 11-20-2002 11Overview•IP multicast service basics•Multicast routing•Multicast transport•Overlay multicastLecture 24: 11-20-2002 12Multicast Router Responsibilities•Learn of the existence of multicast groups (through advertisement)•Identify links with group members•Establish state to route packets•Replicate packets on appropriate interfaces•Routing entry:Src, incoming interface List of outgoing interfacesSrcLecture 24: 11-20-2002 13IP Multicast ArchitectureHostsRoutersService modelHost-to-router protocol(IGMP)Multicast routing protocols(various)Lecture 24: 11-20-2002 14Internet Group Management Protocol•End system to router protocol is IGMP•Each host keeps track of which mcast groups are subscribed to•Socket API informs IGMP process of all joins•Objective is to keep router up-to-date with group membership of entire LAN•Routers need not know who all the members are, only that members existLecture 24: 11-20-2002 15How IGMP Works•On each link, one router is elected the “querier”•Querier periodically sends a Membership Query message to the all-systems group (224.0.0.1), with TTL = 1•On receipt, hosts start random timers (between 0 and 10 seconds) for each multicast group to which they belong QRouters:Hosts:Lecture 24: 11-20-2002 16How IGMP Works (cont.)•When a host’s timer for group G expires, it sends a Membership Report to group G, with TTL = 1•Other members of G hear the report and stop their timers•Routers hear all reports, and time out non-responding groupsQG G G GRouters:Hosts:Lecture 24: 11-20-2002 17How IGMP Works (cont.)•Note that, in normal case, only one report message per group present is sent in response to a query•Query interval is typically 60-90 seconds•When a host first joins a group, it sends one or two immediate reports, instead of waiting for a queryLecture 24: 11-20-2002 18IP Multicast ArchitectureHostsRoutersService modelHost-to-router protocol(IGMP)Multicast routing protocols(various)Lecture 24: 11-20-2002 19Multicast Routing•Basic objective – build distribution tree for multicast packets•Multicast service model makes it hard•Anonymity•Dynamic join/leaveLecture 24: 11-20-2002 20Routing Techniques•Flood and prune•Begin by flooding traffic to entire network•Prune branches with no receivers•Examples: DVMRP, PIM-DM•Unwanted state where there are no receivers•Link-state multicast protocols•Routers advertise groups for which they have receivers to entire network•Compute trees on demand•Example: MOSPF•Unwanted state where there are no sendersLecture 24: 11-20-2002 21Source-based TreesRouterSourceReceiverSRRRRRSSLecture 24: 11-20-2002 22Distance-Vector Multicast Routing•DVMRP consists of two major components:•A conventional distance-vector routing protocol (like RIP) •A protocol for determining how to forward multicast packets, based on the routing table•DVMRP router forwards a packet if•The packet arrived from the link used to reach the source of the packet (reverse path forwarding check – RPF)•If downstream links have not pruned the treeLecture 24: 11-20-2002 23Example TopologyG GSGLecture 24: 11-20-2002 24Broadcast with TruncationG GSGLecture 24: 11-20-2002 25PruneG GSPrune (s,g)Prune (s,g)GLecture 24: 11-20-2002 26Graft (s,g)Graft (s,g)GraftG GSGGReport (g)Lecture 24: 11-20-2002 27Steady StateG GSGGLecture 24: 11-20-2002 28Overview•IP multicast service basics•Multicast routing•Multicast transport•Overlay multicastLecture 24: 11-20-2002
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