Multicast Routing

1. Multicast Routing

2. Multicast Routing Introduction (uses, definitions, etc.) Group Management Routing Algorithms Routing Protocols Other Considerations (ATM, scalability, security)

3. Terminology Unicast: Traditional point-to-point network communication Multicast: Node(s) can send to members of a group (one-to-many or many-to-many) Broadcast: Blast the packet to everybody on the network

4. The Basic Idea IP Multicast traffic for a (source, destination group) pair is transmitted from the source to the receivers via a spanning tree that connects all hosts in the group. Different routing protocols use different techniques to build the trees. Once constructed all multicast traffic is distributed through the tree

5. Communication Tiers Locally: host informs local multicast router of desire to join the group (using IGMP) Globally: local routers interact with other routers to receive multicast packets (using DVMRP, MOSPF, PIM)

6. Introduction: Uses Bulk data transfer (software updates, mailing list distribution, stock updates) Streamed Continuous Media (audio/visual conferences) Shared Application Data (shared whiteboard) Interactive Gaming or Simulations (very intensive)

7. The Mbone Collection of multicast subnetworks(islands) interconnected (tunneled) through the Internet for faster multicast deployment Started in 1992 with 40 subnets; up to 2800 subnets in 1996 Based primarily on UDP for end-to-end transmission, IGMP for group management, and DVMRP for routing Use sd (session directory) to get a listing of active multicast sessions

8. Multicast Propertiess Must service a variety of applications (some have specific timing needs) The overall group should not be effected by individual join/leave operations Connectionless Current Best-effort service

9. Two Scenarios Dense Mode Group members are densely distributed throughout network (i.e. Many subnets contain at least one group member) Bandwidth is plentiful Uses flooding to propogate information Protocols include: DVMRP, MOSPF, PIM-DM Sparse Mode Group members are sparsely distributed throughout network Bandwidth is restricted (such as ISDN, home users) Uses selective techniques to set up and maintain multicast trees Protocols include: CBT, PIM-SM

10. What defines a group? Network Group: logical grouping to control membership in local group (group address, group identifier, group properties, and group management) Social Group: participants in the communication Group management procedures advertise groups to potential members, broadcast routing information to multicast nodes, and control various group properties

11. IGMP The host sends IGMP report when an application joins the multicast group IP_ADD_MEMBERSHIP socket option host not required to explicitly "unjoin" group when leaving The router sends IGMP query at regular intervals which hosts belonging to the multicast group are obligated to reply

12. IGMP Evolution IGMPv1: Router sends Host Membership Query msg broadcast on LAN Host replies to indicate group membership Randomized delay before responding Implicit leave via no reply to Query(timeout) RFC 1112 IGMPv2: Added Group-Specific Query Elected router queries to see if any hosts left in group Leave Group Message Last host replying to query can send explicit Leave Group msg RFC 2236

13. IGMP Evolution (cont.) IGMPv3: Lets source know if nobody is listening Receivers can selectively choose sources to listen to (source pruning) Still being designed

14. Virtual Circuit Group Mgmt Group of endpoints named at cirtuit setup time Master may be able to later add new receivers or senders to multicast circuit Challenges in reducing state information required when adding new receivers

15. Routing Algorithms: The Requirements Minimize network load. Need to optimize network resources while avoiding loops and concentrated traffic Reliable transmission: data delivery shouldn't be affected by routing changes, link failures, etc. Use optimal routes as determined by resource availability, bandwidth, node connectivity, price paid Minimize router state (especially for larger groups)

16. The Building Blocks Most routing protocols use these basic tree algorithms Reverse Path Forwarding (one tree per source) Flood and prune Steiner Trees (a minimal spanning tree, which is shared for entire group) Core-based Trees (one tree per group)

17. Desirable Characteristics Flooding can be danagerous in heterogeneous networks. Have to periodically flood if new receiver appears Recall the challenges for routing algorithms: Evolve with group membership. Algorithm should uniquely identify group members Minimize state information in the nodes Optimize routing given cost considerations Avoid traffic concentration

18. Reverse Path Forwarding Rely on router's existing knowledge of unicast routing table (shortest path from self to sender) Best for densely distributed receivers Result of algorithm is are directed graphs: source-rooted delivery trees emanating from the subnet directly connected to source Efficient and easy to implement but doesn't use group membership information to build the tree so packets may hit subnets without members

19. RPF with Pruning Use RPF technique plus record group membership. Timer controlled; have to periodically flood and re-prune to capture new members Requires state information per source and per group to be kept at each node (leads to scalability problems)

20. Steiner Trees Minimum cost tree connecting all routers with attached group members; network resources are globally optimized NP-complete problem; but good heuristics exist Forms an undirected tree ==> symmetric links Generally not used in practice: computational complexity requires knowledge of entire network topology must be rerun when a router needs to leave/join

21. Core-Based Trees Single tree shared by everybody  (multiple sender / multiple receiver) One router is labeled as center of tree (but finding center is NP-complete) Receiver based approach; packets naturally limited to group members Best for sparse receiver distribution State only kept per group (not for source/group pairs) Can lead to traffic congestion

22. Algorithms Summary Most algorithms include challenging parameters (network topology, group dynamics, member location, other routing algorithms) Requirements for the optimal algorithm: Should be transparent to the members that remain in a group Should maintain properties of the original route Should not perturb ongoing data transfers Should be receiver driven (so it adapts well to changing group membership)

23. Routing Protocols Distance-vector multicast routing protocol (DVMRP): flood and prune Multicast Open Shortest Path First (MOSPF): source-based trees (RPF) Protocol-Independent Multicast - Dense Mode (PIM-DM): flood and prune Protocol-Independent Multicast - Sparse Mode (PIM-SM): core-based trees Inter-domain Multicast Routing (IDMR): border gateway multicast protocol

24. DVMRP RFC 1075 (original RFC) An extension to RIP: multicast routers share reverse path distances to build the (source-based) delivery tree for each group then use pruning Use TTL to define distance packet will travel Hierarchical DVMRP in the works: define regions within which any protocol can be used and DVMRP is used between regions (not yet deployed)

25. MOSPF RFC 1584 Designed for a single routing domain; depends on OSPF for unicast routing Routers build efficient shortest path source-based trees without initially flooding Higher link effieiency (routers can discard packets immediately based on TTL values Computation on demand (only when the first packet from source to group is received)

26. PIM-DM Used when group membership is plentiful within a region of an internet Like DVMRP but imports unicast routes instead of calculating for self (thus the independence)

27. PIM-SM Used when group members are spread thinly across regions of an internet Join messages are sent to rendezvous points to meet new sources Uses shared trees

28. Mobile Hosts When mobile unit wants to send to group, it uses RPF-based approach from the mobile support router When mobile unit wants to receive a multicast packet, the packet is sent to the wired address of the mobile unit then forwarded to the mobile address Main Problems: 1. Mobile source packets may not reach all group members; 2. Mobile host may see delays as enters a new cell; 3. TTL can limit cell reachability

29. ATM Environments Introduced point-to-multipoint in UNI for audio conferences Multigroup address concept doesn't exist in ATM; sender has to know all members of the group Only VC root node may add or remove leaf nodes Various Approaches: MPOA(multi-protocol over ATM), VC mesh (sender to all members), MCS per cluster, SMART, MARS, Lan Emulation

30. Traffic Control Problems introduced by multicast: Retaining capabilities for heterogeneity Defining fairness and relative fairness between unicast and multicast Timescales for congestion control Scaling of control traffic and techniques Design Issues include: Scalable feedback mechanism Congestion detection by receivers not senders Probabilistic query/reply schemes, random delay responses Number control packets fixed WRT data packets RTP, RLM (receiver-driven layered multicast)

31. End to End Concerns Reliability: don't want every receiver to ACK the sender; better solution is to use NACK Reliability protocols include: Virtual token ring-based protocol, XTP, SRM QoS options for group communication: Parameters defined by sender with no negociation Parameters equal to minimum of each group member Senders send with higher QoS and each receiver controls for self Resource reservation very complex QoS overall not an easy problem

32. Scalability Need to make join and leave operations as efficient as possible Hierarchical group addressing (and routing) schemes are being developed

33. Finally, security! Efficient key distribution scheme for multiple recipients Group communication leads to greater opportunity for traffic analyzers, denial of service attacks, covert channel signaling May become too complex to understand link usage (for filtering rules)

34. Final Thoughts.... Lots of new applications are now becoming a reality with multicast Further research is needed in many areas: Multicast in different types of networks (satellite, SMDS) Lots to work out with congestion control issues Fairness and billing issues...everybody needs to make $$$ Still need better tests for really large groups (scalability!)