115-744: Computer NetworkingL-9 MulticastL -9; 11-19-04© Srinivasan Seshan, 2004 2Multicast Routing• Multicast• routing, reliability, congestion control • Network measurements• Assigned reading• [DC90] Multicast Routing in Datagram Internetworks and Extended LANs • [MS97] Introduction to IP Multicast Routing • [CRSZ01] Enabling Conferencing Applications on the Internet using an Overlay Multicast Architecture• [LAWD04] A First-Principles Approach to Understanding the Internet's Router-level Topology • [Pax97] End-to-End Internet Packet Dynamics • [F+97] A Reliable Multicast Framework for Light-Weight Sessions and Application Level FramingL -9; 11-19-04© Srinivasan Seshan, 2004 3Multicast Routing• Unicast: one source to one destination• Multicast: one source to many destinations• Two main functions:• Efficient data distribution • Logical naming of a group L -9; 11-19-04© Srinivasan Seshan, 2004 4Overview• What/Why Multicast• IP Multicast Service Basics• Multicast Routing Basics•DVMRP• Overlay Multicast• Reliability• Congestion Control2L -9; 11-19-04© Srinivasan Seshan, 2004 5Multicast – Efficient Data DistributionSrc SrcL -9; 11-19-04© Srinivasan Seshan, 2004 6Multicast 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 interfacesL -9; 11-19-04© Srinivasan Seshan, 2004 7Logical Naming• Single name/address maps to logically related set of destinations• Destination set = multicast group • How to scale?• Single name/address independent of group growth or changesL -9; 11-19-04© Srinivasan Seshan, 2004 8Multicast Groups• Members are the intended receivers• Senders may or may not be members• Hosts may belong to many groups• Hosts may send to many groups• Support dynamic creation of groups, dynamic membership, dynamic sources3L -9; 11-19-04© Srinivasan Seshan, 2004 9Scope• Groups can have different scope• LAN (local scope)• Campus/admin scoping• TTL scoping• Concept of scope important to multipoint protocols and applicationsL -9; 11-19-04© Srinivasan Seshan, 2004 10Example Applications• Broadcast audio/video• Push-based systems• Software distribution• Web-cache updates • Teleconferencing (audio, video, shared whiteboard, text editor)• Multi-player games• Server/service location• Other distributed applicationsL -9; 11-19-04© Srinivasan Seshan, 2004 11Overview• What/Why Multicast• IP Multicast Service Basics• Multicast Routing Basics•DVMRP• Overlay Multicast• Reliability• Congestion ControlL -9; 11-19-04© Srinivasan Seshan, 2004 12IP Multicast ArchitectureHostsRoutersService modelHost-to-router protocol(IGMP)Multicast routing protocols(various)4L -9; 11-19-04© Srinivasan Seshan, 2004 13IP 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.L -9; 11-19-04© Srinivasan Seshan, 2004 14IP 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” program1110 Group IDL -9; 11-19-04© Srinivasan Seshan, 2004 15IP 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 operationL -9; 11-19-04© Srinivasan Seshan, 2004 16Multicast Scope Control – Small TTLs• TTL expanding-ring search to reach or find a nearby subset of a groups1235L -9; 11-19-04© Srinivasan Seshan, 2004 17Multicast 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 InternetL -9; 11-19-04© Srinivasan Seshan, 2004 18Overview• What/Why Multicast• IP Multicast Service Basics• Multicast Routing Basics•DVMRP• Overlay Multicast• Reliability• Congestion ControlL -9; 11-19-04© Srinivasan Seshan, 2004 19IP Multicast ArchitectureHostsRoutersService modelHost-to-router protocol(IGMP)Multicast routing protocols(various)L -9; 11-19-04© Srinivasan Seshan, 2004 20Multicast Routing• Basic objective – build distribution tree for multicast packets• Multicast service model makes it hard• Anonymity• Dynamic join/leave6L -9; 11-19-04© Srinivasan Seshan, 2004 21Routing 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 sendersL -9; 11-19-04© Srinivasan Seshan, 2004 22Routing Techniques• Core based protocols• Specify “meeting place” aka core• Sources send initial packets to core• Receivers join group at core• Requires mapping between multicast group address and “meeting place”• Examples: CBT, PIM-SML -9; 11-19-04© Srinivasan Seshan, 2004 23Shared vs. Source-based Trees• Source-based trees• Separate shortest path tree for each sender • DVMRP, MOSPF, PIM-DM, PIM-SM• Shared trees• Single tree shared by all members• Data flows on same tree regardless of sender• CBT, PIM-SML -9; 11-19-04© Srinivasan Seshan, 2004 24Source-based TreesRouterSourceReceiverSRRRRRSS7L -9; 11-19-04© Srinivasan Seshan, 2004 25Shared TreeRPRouterSourceReceiverSSSRRRRRL -9; 11-19-04© Srinivasan Seshan, 2004 26Shared vs. Source-Based Trees• Source-based trees• Shortest path trees – low delay, better load distribution• More state at routers (per-source state)•
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