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Multicast EECS 122: Lecture 16

Multicast EECS 122: Lecture 16. Department of Electrical Engineering and Computer Sciences University of California Berkeley. Broadcasting to Groups. Many applications are not one-one Broadcast Group collaboration Proxy/Cache updates Resource Discovery

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Multicast EECS 122: Lecture 16

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  1. MulticastEECS 122: Lecture 16 Department of Electrical Engineering and Computer Sciences University of California Berkeley

  2. Broadcasting to Groups • Many applications are not one-one • Broadcast • Group collaboration • Proxy/Cache updates • Resource Discovery • Packets must reach a Group rather than a single destination • Group membership may be dynamic • More than one group member might be a source • Idea: After a group is established • Interested receivers join the group • The network takes care of group management • Recall RSVP Webcasts Radio/TV Push/IE Channels Chats Video Conferencing Audio Conferencing Caches and Proxies A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  3. Membership access control open group: anyone can join closed group: restrictions on joining Sender access control anyone can send to group anyone in group can send to group only one host can send to group Packet delivery is best effort R1 joins G [G, data] [G, data] [G, data] R0 joins G [G, data] Rn-1 joins G The Multicast service Model R0 R1 S Net . . . Rn-1 A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  4. Multicast and Layering • Multicast can be implemented at different layers • data link layer • e.g. Ethernet multicast • network layer • e.g. IP multicast • application layer • e.g. as an overlay network like Kazaa • Which layer is best? A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  5. Multicast Implementation Issues • How are multicast packets addressed? • How is join implemented? • How is send implemented? • How does multicast traffic get routed? • This is easy at the link layer and hardest at the network layer • How much state is kept and who keeps it? A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  6. Ethernet Multicast • Reserve some Ethernet MAC addresses for multicast • To join group G • network interface card (NIC) normally only listens for packets sent to unicast address A and broadcast address B • to join group G, NIC also listens for packets sent to multicast address G (NIC limits number of groups joined) • implemented in hardware, so efficient • To send to group G • packet is flooded on all LAN segments, like broadcast • can waste bandwidth, but LANs should not be very large • Only host NICs keep state about who has joined  scalable to large number of receivers, groups A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  7. Limitations of Data Link Layer Multicast • Single LAN • limited to small number of hosts • limited to low diameter latency • essentially all the limitations of LANs compared to internetworks • Broadcast doesn’t cut it in larger networks A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  8. Interconnected LANs LANs support link-level multicast Map globally unique multicast address to LAN-based multicast address (LAN-specific algorithm) IP Group addresses are class D addresses 1110/28 or 224.0.0.0 to 239.255.255.255 IP Multicast: Interconnecting LANS A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  9. Internet Group Management ProtocolIGMP • Operates between Router and local Hosts, typically attached via a LAN (e.g., Ethernet) • Query response architecture • Router periodically queries the local Hosts for group membership information • Can be specific or general • Hosts receiving query set a random timer before responding • First host to respond sends membership reports • All the other hosts observe the query and suppress their own repots. • To Join send a group send an unsolicited Join • Start a group by joining it • To leave don’t have to do anything • Soft state Query to 224.0.0.1 Report Suppresses Report A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  10. [R0, data] [R0, data] [R1, data] [Rn-1, data] [R1, data] [Rn-1, data] Naïve Routing Option: Don’t change anything Point-to point routing R0 R1 S Net . . . Rn-1 Group abstraction not implemented in the network A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  11. Backbone ISP This approach does not scale… Broadcast Center A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  12. Backbone ISP Instead build trees Copy data at routers At most one copy of a data packet per link Broadcast Center • Routers keep track of groups in real-time • “Path” computation is Tree computation • LANs implement layer 2 multicast by broadcasting A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  13. Routing: Approaches • Kinds of Trees • Shared Tree • Source Specific Trees • Tree Computation Methods • Intradomain Update methods • Build on unicast Link State: MOSPF • Build on unicast Distance Vector: DVMRP • Protocol Independent: PIM • Interdomain routing: BGMP • This is still evolving… A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  14. Source Specific Trees 5 7 Each source is the route of its own tree. 4 8 6 11 2 10 3 1 13 12 A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  15. Source Specific Trees 5 7 Each source is the route of its own tree. One tree for each source 4 8 6 11 2 10 3 1 13 12 Can pick “good” trees but lots of state at the routers! A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  16. Shared Tree 5 7 One tree used by all 4 8 6 11 2 10 3 1 13 12 Can’t pick “good” trees but minimal state at the routers A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  17. 2 2 1 2 2 12 15 2 2 2 3 2 12 7 11 1 12 2 Tree Computation • A tree which connects all the group nodes is a Steiner Tree • Finding the min cost Steiner Tree is NP hard 5 7 4 8 6 11 2 10 3 1 13 12 A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  18. 2 2 1 2 2 12 15 2 2 2 3 2 12 7 11 1 12 2 Tree Computation • A tree which connects all the group nodes is a Steiner Tree • Finding the min cost Steiner Tree is NP hard 5 7 4 8 6 11 2 10 3 1 13 12 A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  19. 2 2 1 2 2 12 15 2 2 2 3 2 12 7 11 1 12 2 Tree Computation • A tree which connects all the group nodes is a Steiner Tree • Finding the min cost Steiner Tree is NP hard • The tree does not span the network • Heuristics are known 5 7 4 8 6 11 2 10 3 1 13 12 A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  20. 2 2 1 2 2 12 15 2 2 2 3 2 12 7 11 1 12 2 Tree Computation • A tree that connects all of the nodes in the graph is a spanning tree • Finding a minimum spanning tree is much easier 5 7 4 8 6 11 2 10 3 1 13 12 A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  21. 2 2 1 2 2 12 15 2 2 2 3 2 12 7 11 1 12 2 Tree Computation • A tree that connects all of the nodes in the graph is a spanning tree • Finding a minimum spanning tree is much easier 5 7 4 8 6 11 2 10 3 1 13 12 A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  22. Tree Computation • A tree that connects all of the nodes in the graph is a spanning tree • Finding a minimum spanning tree is much easier • Prune back to get a multicast tree 2 2 5 7 4 1 2 2 8 12 6 15 2 2 11 2 2 10 3 2 12 7 11 1 3 1 13 12 12 2 A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  23. 2 2 1 2 2 12 15 2 2 2 3 2 12 7 11 1 12 2 Tree Computation • A tree that connects all of the nodes in the graph is a spanning tree • Finding a minimum spanning tree is much easier • Prune back to get a multicast tree 5 7 4 8 6 11 2 10 3 1 13 12 A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  24. Link State Protocols e.g. MOSPF • Use in conjunction with a link state protocol for unicast • Enhance the LSP updates with group membership • Compute best tree from source • Flood Membership in link state advertisements • Dynamics are a problem A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  25. Distance Vector Multicast Routing • An elegant extension to DV routing • Use shortest path DV routes to determine if link is on the source-rooted spanning tree A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  26. r Distance Vector Multicast • Extension to DV unicast routing • Packet forwarding • iff incoming link is shortest path to source • out all links except incoming • Reverse Path Flooding (RPF) • packets always take shortest path • assuming delay is symmetric • Issues • Every link receives each multicast packet, even if no interested hosts: Pruning • Some links (LANs) may receive multiple copies: Reverse Path Broadcasting s:3 s:2 s:3 s:1 s:2 s A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  27. Example • Flooding can cause a given packet to be sent multiple times over the same link • Solution: Reverse Path Broadcasting S x y a duplicate packet z b A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  28. r Reverse Path Broadcasting (RPB) • Extend DV to eliminate duplicate packets • Choose parent router for each link • router with shortest path to source • lowest address breaks ties • each router can compute independently from already known information • each router keeps a bitmap with one bit for each of its links • Only parent forwards onto link s:3 C s:2 s:3 P s:1 s:2 s A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  29. Identify Child Links • Routing updates identify parent • Since distances are known, each router can easily figure out if it's the parent for a given link • In case of tie, lower address wins A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  30. forward only to child link Reverse Path Broadcasting (RPB) S 5 6 x y a child link of x for S z b A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  31. Don’t really want to flood! • This is still a broadcast algorithm – the traffic goes everywhere • Need to “Prune” the tree when there are subtrees with no group members • Strategy • Identify leaf networks with no members • IGMP • Propagate this information up the subtree A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  32. How much to Prune? • Truncated Reverse Path Broadcasting: Prunes to prevent flooding of all packets • Reverse Path Multicasting: More aggressive. Scale router state with the number of active groups • Use on-demand pruning so that router group state scales with number of active groups (not all groups) A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  33. Pruning Details • Prune (Source,Group) at leaf if no members • Send Non-Membership Report (NMR) up tree • If all children of router R prune (S,G) • Propagate prune for (S,G) to parent R • On timeout: • Prune dropped • Flow is reinstated • Down stream routers re-prune • Note: again a soft-state approach A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  34. Pruning Details • How to pick prune timers? • Too long  large join time • Too short  high control overhead • What do you do when a member of a group (re)joins? • Issue prune-cancellation message (grafts) A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  35. IPM IPM MBONE • What to do if most of the routers in the internet are not multicast enabled? • Tunnel between multicast enabled routers • Creates an “overlay” network but both operate at Level 3… • This is how multicast was first deployed IP A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  36. RMP Scaling • State requirements: • O(Sources  Groups) active state • How to get better scaling? • Hierarchical Multicast • Core-based Trees A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  37. Core Based Trees (CBT) • Pick a “rendevouz point” for the group called the core. • Shared tree • Unicast packet to core and bounce it back to multicast group • Tree construction is receiver-based • Joins can be tunneled if required • Only nodes on One tree per group tree involved • Reduce routing table state from O(S x G) to O(G) A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  38. Example • Group members: M1, M2, M3 • M1 sends data root M1 M2 M3 control (join) messages data A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  39. Disadvantages • Sub-optimal delay • Single point of failure • Core goes out and everything lost until error recovery elects a new core • Small, local groups with non-local core • Need good core selection • Optimal choice (computing topological center) is NP complete A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  40. PIM • Popular intradomain method • UUNET streaming using this • Recognizes that most groups are very sparse • Why have all of the routers participate in keeping state? • Two modes • Dense mode: flood and prune • Sparse mode: Core-based shared tree approach with a twist A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  41. PIM Sparse Mode • Routers explicitly issue JOIN and Prune messages to the Core • Recievers typically send a Join message of the form (*,G) • As it propagates towards the core it establishes a new branch of the shared tree • To send on the tree, tunnel to the core and then traverse the shared tree • This can lead to bad performance • To optimize sending from S, the core can send Join message of the form (S,G) to S. • Creates a specific path from S to the core • Receivers can send (S,G) messages as well to S and gradually replace the shared tree with a source specific tree A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  42. Problems with Network Layer Multicast • Scales poorly with number of groups • A router must maintain state for every group that traverses it • many groups traverse core routers • Supporting higher level functionality is difficult • NLM: best-effort multi-point deliveryservice • Reliability and congestion control for NLM complicated • Deployment is difficult and slow • Difficult to debug problems given the service model A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  43. NLM Reliability • Assume reliability through retransmission • Sender can not keep state about each receiver • e.g., what receivers have received • number of receivers unknown and possibly very large • Sender can not retransmit every lost packet • even if only one receiver misses packet, sender must retransmit, lowering throughput • N(ACK) implosion • described next A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  44. (N)ACK Implosion • (Positive) acknowledgements • ack every n received packets • what happens for multicast? • Negative acknowledgements • only ack when data is lost • assume packet 2 is lost R1 3 2 1 S R2 R3 A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  45. 2? 2? 2? NACK Implosion • When a packet is lost all receivers in the sub-tree originated at the link where the packet is lost send NACKs R1 3 S 3 R2 R3 3 A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  46. Scalable Reliable Multicast (SRM) • Randomize NACKs (request repairs) • All traffic including request repairs and repairs are multicast • A repair can be sent by any node that heard the request • A node suppresses its request repair if another node has just sent a request repair for the same data item • A node suppresses a repair if another node has just sent the repair A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  47. Avoiding NACK Implosions • Every node estimates distance (in time) from every other node • Information is carried in session reports (< 5% of bandwidth) • Nodes use randomized function of distance to decide when to • Send a request repair • Reply to a request repair A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  48. Backbone ISP ISPs charge by bandwidth Broadcast Center Remember what interdomain protocols optimize for…. They make more money without multicast A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  49. Application Layer Multicast • Provide multicast functionality above the IP unicast • Gateway nodes could be the hosts or multicast gateways in the network • Advantages • No multicast dial-tone needed • Performance can be optimized to application • Loss, priorities etc. • More control over the topology of the tree • Easier to monitor and control groups • Disadvantages • Scale • Performance if just implemented on the hosts (not gateways) A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

  50. Summary • Large amount of work on multicast routing • Major problems • preventing flooding • minimizing state in routers • denial-of-service attacks • deployment • Multicast can be implemented at different layers • lower layers optimize performance • higher layers provide more functionality • IP Multicast still not widely deployed • Ethernet multicast is deployed • application layer multicast systems are promising A. Parekh, EE122 S2003. Revised and enhanced F'02 Lectures

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