1 CS 268: Computer Networking L-21 Multicast 2 Multicast Routing • Unicast: one source to one destination • Multicast: one source to many destinations • Two main functions: • Efficient data distribution • Logical naming of a group2 3 Example 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 applications 4 Overview • IP Multicast Service Basics • Multicast Routing Basics • Overlay Multicast • Reliability • Congestion Control3 5 IP Multicast Architecture Hosts Routers Service model Host-to-router protocol (IGMP) Multicast routing protocols (various) 6 Multicast – Efficient Data Distribution Src Src4 7 Multicast 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 interfaces 8 IP 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.5 9 IP Multicast Addresses • Class D IP addresses • 224.0.0.0 – 239.255.255.255 • How to allocate these addresses? • Well-known multicast addresses, assigned by IANA • Transient multicast addresses, assigned and reclaimed dynamically, e.g., by “sdr” program 1 1 1 0 Group ID 10 Multicast 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 sources6 11 Multicast Scope Control – Small TTLs • TTL expanding-ring search to reach or find a nearby subset of a group s 1 2 3 12 Multicast Scope Control – Large TTLs • Administrative TTL Boundaries to keep multicast traffic within an administrative domain, e.g., for privacy or resource reasons An administrative domain TTL threshold set on interfaces to these links, greater than the diameter of the admin. domain The rest of the Internet7 13 Overview • IP Multicast Service Basics • Multicast Routing Basics • Overlay Multicast • Reliability • Congestion Control 14 IP Multicast Architecture Hosts Routers Service model Host-to-router protocol (IGMP) Multicast routing protocols (various)8 15 Multicast Routing • Basic objective – build distribution tree for multicast packets • Multicast service model makes it hard • Anonymity • Dynamic join/leave 16 Shared 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-SM9 17 Source-based Trees Router Source Receiver S R R R R R S S 18 Shared Tree RP Router Source Receiver S S S R R R R R10 19 Shared vs. Source-Based Trees • Source-based trees • Shortest path trees – low delay, better load distribution • More state at routers (per-source state) • Efficient for in dense-area multicast • Shared trees • Higher delay (bounded by factor of 2), traffic concentration • Choice of core affects efficiency • Per-group state at routers • Efficient for sparse-area multicast • Which is better? extra state in routers is bad! 20 Routing 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 senders11 21 Routing 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-SM 22 Distance-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 tree12 23 Example Topology G G S G 24 Broadcast with Truncation G G S G13 25 Prune G G S Prune (s,g) Prune (s,g) G 26 Graft (s,g) Graft (s,g) Graft G G S G G Report (g)14 27 Steady State G G S G G 28 Overview • IP Multicast Service Basics • Multicast Routing Basics • Overlay Multicast • Reliability • Congestion Control15 29 Supporting Multicast on the Internet IP Application Internet architecture Network ? ? At which layer should multicast be implemented? 30 IP Multicast CMU Berkeley MIT UCSD routers end systems multicast flow • Highly efficient • Good delay16 31 End System Multicast MIT1 MIT2 CMU1 CMU2 UCSD MIT1 MIT2 CMU2 Overlay Tree Berkeley CMU1 CMU Berkeley MIT UCSD 32 • Quick deployment • All multicast state in end systems • Computation at forwarding points simplifies support for higher level functionality Potential Benefits Over IP Multicast MIT1 MIT2 CMU1 CMU2 CMU Berkeley MIT UCSD17 33 Concerns with End System Multicast • Self-organize recipients into multicast delivery overlay tree • Must be closely matched to real network topology to be efficient • Performance concerns compared to IP Multicast • Increase in delay • Bandwidth waste (packet duplication) MIT2 Berkeley MIT1 UCSD CMU2 CMU1 IP
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