15-441 Computer NetworkingA Virtual ClassroomSlide 3The emerging InternetIP MulticastSlide 6Standard Questions for Any New Network FunctionalitiesIP Multicast AddressesMulticast Router Data PlaneAddress or Name?IP Multicast Service Model (rfc1112)IP Multicast APIMulticast Scope Control – Small TTLsMulticast Scope Control – Large TTLsIP Multicast Control PlaneInternet Group Management Protocol (Part 1 of Control Plane)How IGMP WorksHow IGMP Works (cont.)Slide 19IP Multicast Control PlaneMulticast Routing Protocols (Part 2 of Control Plane)Multicast OSPF (MOSPF)Impact on Route ComputationExampleLink Failure/Topology ChangeMembership ChangeShared vs. Source-based TreesSource-based TreesShared TreeShared vs. Source-Based TreesDistance-Vector Multicast RoutingExample TopologyBroadcast with TruncationPruneGraftSteady StateFailure of IP MulticastError Control: Reliable MulticastAck ImplosionRouters Collect AcksCongestion/Flow ControlSupporting Multicast on the InternetEnd System MulticastEnd System Multicast: BenefitsPerformance ChallengesImportant Concepts15-441 Computer NetworkingLecture 11 – MulticastLecture 11: 10-3-2006 2A Virtual Classroom Prof. Harry BovikCMUStanfordBerkeleyGatechLecture 11: 10-3-2006 3A Virtual Classroom•Poor performance scalability•delay, throughput•sender, networkStanfordBerkeleyGatechLecture 11: 10-3-2006 4The emerging Internet•A plethora of multi-party applications...•Audio/video conferencing•Multi-party games•Software distribution •Internet Television•And now consider a world with ...•Millions of groups•Each group with tens to several thousand membersLecture 11: 10-3-2006 5IP Multicast•Router duplicates multicast packets•One packet on each link•Good performance scaling propertyBerkeleyGatechStanfordCMULecture 11: 10-3-2006 6IP Multicast•How to tell a packet is multicast? •How to decide where to branch?Lecture 11: 10-3-2006 7Standard Questions for Any New Network Functionalities •What does the data plane look like?•What is format of the forwarding table entry? •What is the key to the lookup table? •What does the control plane look like?•How is the forwarding table constructed? •What is the service interface?Lecture 11: 10-3-2006 8IP 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 11: 10-3-2006 9Multicast Router Data Plane•Replicate packets on appropriate interfacesSrc IP Address, IP Multicast AddressList of outgoing interfacesLecture 11: 10-3-2006 10Address or Name? •Single name/address maps to logically related set of destinations•Destination set = multicast group •Key challenge: scalability•Single name/address independent of group growth or changesLecture 11: 10-3-2006 11IP 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 11: 10-3-2006 12IP Multicast API•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 operation•Implemented using socket optionsLecture 11: 10-3-2006 13Multicast Scope Control – Small TTLs•TTL expanding-ring search to reach or find a nearby subset of a groups123Lecture 11: 10-3-2006 14Multicast 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 11: 10-3-2006 15IP Multicast Control PlaneHostsRoutersService modelHost-to-router protocol(IGMP)Multicast routing protocols(various)Lecture 11: 10-3-2006 16Internet Group Management Protocol(Part 1 of Control Plane)•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 11: 10-3-2006 17How 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 11: 10-3-2006 18How 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 11: 10-3-2006 19How IGMP Works (cont.)•Note that, in normal case, only one report message per group present is sent in response to a query• Power of randomization + suppression•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 11: 10-3-2006 20IP Multicast Control Plane HostsRoutersService modelHost-to-router protocol(IGMP)Multicast routing protocols(various)Lecture 11: 10-3-2006 21Multicast Routing Protocols(Part 2 of Control Plane)•Basic objective – build distribution tree for multicast packets•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 11: 10-3-2006 22Multicast OSPF (MOSPF)•Add-on to OSPF (Open Shortest-Path First,a link-state, intra-domain routing protocol)•Multicast-capable routers flag link state routing advertisements•Link-state packets
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