Multicast EECS 122 Lecture 16 Department of Electrical

Multicast EECS 122: Lecture 16 Department of Electrical Engineering and Computer Sciences University of California Berkeley March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures

Broadcasting to Groups n Many applications are not one-one q q n Webcasts Radio/TV Push/IE Channels Packets must reach a Group rather than a single destination q q n Broadcast Group collaboration Proxy/Cache updates Resource Discovery Chats Video Conferencing Audio Conferencing Group membership may be dynamic More than one group member might be a source Idea: After a group is established q q q Caches and Proxies Interested receivers join the group The network takes care of group management Recall RSVP March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 2

The Multicast service Model n Membership access control q S [G, data] Net G djoaitnas] R 0 , [G R 1 [G, data]G s R 1 R 0 join [RG, n-1 da joitna] s. G q n q Rn-1 q n March 18, 2003 Sender access control q . . . open group: anyone can join closed group: restrictions on joining anyone can send to group anyone in group can send to group only one host can send to group Packet delivery is best effort A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 3

Multicast and Layering n Multicast can be implemented at different layers q data link layer n q network layer n q e. g. IP multicast application layer n n e. g. Ethernet multicast e. g. as an overlay network like Kazaa Which layer is best? March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 4

Multicast Implementation Issues How are multicast packets addressed? n How is join implemented? n How is send implemented? n q How does multicast traffic get routed? n n This is easy at the link layer and hardest at the network layer How much state is kept and who keeps it? March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 5

Ethernet Multicast n n Reserve some Ethernet MAC addresses for multicast To join group G q q q n To send to group G q q n 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 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 March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 6

Limitations of Data Link Layer Multicast n Single LAN limited to small number of hosts q limited to low diameter latency q essentially all the limitations of LANs compared to internetworks q n Broadcast doesn’t cut it in larger networks March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 7

IP Multicast: Interconnecting LANS n n Interconnected LANs support link-level multicast Map globally unique multicast address to LANbased multicast address (LAN-specific algorithm) IP Group addresses are class D addresses q 1110/28 or 224. 0. 0. 0 to 239. 255 March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 8

Internet Group Management Protocol IGMP n Operates between Router and local Hosts, typically attached via a LAN (e. g. , Ethernet) q 1. Router periodically queries the local Hosts for group membership information 1. 2. 3. 4. n 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 q n Query response architecture Start a group by joining it To leave don’t have to do anything q Report Query to 224. 0. 0. 1 Suppresses Report Soft state March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 9

Naïve Routing Option: Don’t change anything Point-to point routing ata] d , R [ 0 [R 1, data] [R 0, data] S [R 1, data] [Rn-1, data] Net [R n-1 , dat a] R 0 R 1. . . Rn-1 Group abstraction not implemented in the network March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 10

This approach does not scale… Broadcast Center March 18, 2003 Backbone ISP A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 11

Instead build trees Copy data at routers At most one copy of a data packet per link Broadcast Center Backbone ISP • Routers keep track of groups in real-time • “Path” computation is Tree computation March 18, 2003 • LANs implement layer 2 multicast by broadcasting A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 12

Routing: Approaches n Kinds of Trees q q n n Tree Computation Methods Intradomain Update methods q q q n Shared Tree Source Specific Trees Build on unicast Link State: MOSPF Build on unicast Distance Vector: DVMRP Protocol Independent: PIM Interdomain routing: BGMP q This is still evolving… March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 13

Source Specific Trees 5 4 7 Each source is the route of its own tree. 8 6 11 2 1 March 18, 2003 10 3 13 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 12 14

Source Specific Trees 5 4 7 Each source is the route of its own tree. One tree for each source 8 6 11 2 1 10 3 13 12 Can pick “good” trees but lots of state at the routers! March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 15

Shared Tree 5 4 7 One tree used by all 8 6 11 2 1 10 3 13 12 Can’t pick “good” trees but minimal state at the routers March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 16

Tree Computation 2 4 • A tree which connects all the group nodes is a Steiner Tree • Finding the min cost Steiner Tree is NP hard 2 5 1 7 2 8 2 12 6 15 2 2 1 1 13 12 March 18, 2003 12 7 11 3 11 10 2 3 2 2 12 2 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 17

Tree Computation 2 4 • A tree which connects all the group nodes is a Steiner Tree • Finding the min cost Steiner Tree is NP hard 2 5 1 7 2 8 2 12 6 15 2 2 1 1 13 12 March 18, 2003 12 7 11 3 11 10 2 3 2 2 12 2 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 18

Tree Computation 2 4 • 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 2 5 1 7 2 8 2 12 6 15 2 2 1 1 13 12 March 18, 2003 12 7 11 3 11 10 2 3 2 2 12 2 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 19

Tree Computation 2 4 • A tree that connects all of the nodes in the graph is a spanning tree • Finding a minimum spanning tree is much easier 2 5 1 7 2 8 2 12 6 15 2 2 1 1 13 12 March 18, 2003 12 7 11 3 11 10 2 3 2 2 12 2 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 20

Tree Computation 2 4 • A tree that connects all of the nodes in the graph is a spanning tree • Finding a minimum spanning tree is much easier 2 5 1 7 2 8 2 12 6 15 2 2 1 1 13 12 March 18, 2003 12 7 11 3 11 10 2 3 2 2 12 2 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 21

Tree Computation 2 4 • 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 5 1 7 2 8 2 12 6 15 2 2 1 1 13 12 March 18, 2003 12 7 11 3 11 10 2 3 2 2 12 2 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 22

Tree Computation 2 4 • 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 5 1 7 2 8 2 12 6 15 2 2 1 1 13 12 March 18, 2003 12 7 11 3 11 10 2 3 2 2 12 2 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 23

Link State Protocols e. g. MOSPF n n n 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 March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 24

Distance Vector Multicast Routing An elegant extension to DV routing n Use shortest path DV routes to determine if link is on the source-rooted spanning tree n March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 25

Distance Vector Multicast n n Extension to DV unicast routing Packet forwarding q iff incoming link is shortest path to source q out all links except incoming q Reverse Path Flooding (RPF) q packets always take shortest path n n s: 2 s: 3 s: 1 s: 2 s r assuming delay is symmetric Issues q Every link receives each multicast packet, even if no interested hosts: Pruning q Some links (LANs) may receive multiple copies: Reverse Path Broadcasting March 18, 2003 s: 3 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 26

Example n Flooding can cause a given packet to be sent multiple times over the same link S x y a duplicate packet z b n Solution: Reverse Path Broadcasting March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 27

Reverse Path Broadcasting (RPB) n n Extend DV to eliminate duplicate packets Choose parent router for each link router with shortest path to source q lowest address breaks ties q each router can compute independently from already known information q each router keeps a bitmap with one bit for each of its links s: 3 C q n s: 2 s: 3 s: 1 s: 2 s r P Only parent forwards onto link March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 28

Identify Child Links Routing updates identify parent n Since distances are known, each router can easily figure out if it's the parent for a given link n In case of tie, lower address wins n March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 29

Reverse Path Broadcasting (RPB) S 5 x forward only to child link of x for S 6 y a z b March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 30

Don’t really want to flood! n n n 1. 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 1. 2. IGMP Propagate this information up the subtree March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 31

How much to Prune? n n 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 q Use on-demand pruning so that router group state scales with number of active groups (not all groups) March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 32

Pruning Details n Prune (Source, Group) at leaf if no members q n If all children of router R prune (S, G) q n Propagate prune for (S, G) to parent R On timeout: q q q n Send Non-Membership Report (NMR) up tree Prune dropped Flow is reinstated Down stream routers re-prune Note: again a soft-state approach March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 33

Pruning Details n How to pick prune timers? Too long large join time q Too short high control overhead q n What do you do when a member of a group (re)joins? q Issue prune-cancellation message (grafts) March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 34

MBONE n n What to do if most of the routers in the internet are not multicast enabled? Tunnel between multicast enabled routers IPM IP n n IPM Creates an “overlay” network but both operate at Level 3… This is how multicast was first deployed March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 35

RMP Scaling n State requirements: q n O(Sources Groups) active state How to get better scaling? Hierarchical Multicast q Core-based Trees q March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 36

Core Based Trees (CBT) n Pick a “rendevouz point” for the group called the core. q n n Shared tree Unicast packet to core and bounce it back to multicast group Tree construction is receiver-based Joins can be tunneled if required q Only nodes on One tree per group tree involved q n Reduce routing table state from O(S x G) to O(G) March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 37

Example Group members: M 1, M 2, M 3 n M 1 sends data n root M 1 M 2 M 3 control (join) messages data March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 38

Disadvantages Sub-optimal delay n Single point of failure n q n Core goes out and everything lost until error recovery elects a new core Small, local groups with non-local core Need good core selection q Optimal choice (computing topological center) is NP complete q March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 39

PIM n Popular intradomain method q n Recognizes that most groups are very sparse q n UUNET streaming using this Why have all of the routers participate in keeping state? Two modes q q Dense mode: flood and prune Sparse mode: Core-based shared tree approach with a twist March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 40

PIM Sparse Mode n n n Routers explicitly issue JOIN and Prune messages to the Core Recievers typically send a Join message of the form (*, G) q 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 q This can lead to bad performance To optimize sending from S, the core can send Join message of the form (S, G) to S. q 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 March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 41

Problems with Network Layer Multicast n Scales poorly with number of groups A router must maintain state for every group that traverses it q many groups traverse core routers q n Supporting higher level functionality is difficult NLM: best-effort multi-point delivery service q Reliability and congestion control for NLM complicated q n Deployment is difficult and slow q Difficult to debug problems given the service model March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 42

NLM Reliability n n Assume reliability through retransmission Sender can not keep state about each receiver e. g. , what receivers have received q number of receivers unknown and possibly very large q n Sender can not retransmit every lost packet q n even if only one receiver misses packet, sender must retransmit, lowering throughput N(ACK) implosion q described next March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 43

(N)ACK Implosion n n (Positive) acknowledgements q ack every n received packets q what happens for multicast? Negative acknowledgements q only ack when data is lost q assume packet 2 is lost R 1 S 3 2 1 R 2 R 3 March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 44

NACK Implosion n When a packet is lost all receivers in the subtree originated at the link where the packet is lost send NACKs 3 R 1 2? S 3 R 2 2? 2? 3 March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures R 3 45

Scalable Reliable Multicast (SRM) n n n 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 March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 46

Avoiding NACK Implosions n Every node estimates distance (in time) from every other node q n Information is carried in session reports (< 5% of bandwidth) Nodes use randomized function of distance to decide when to Send a request repair q Reply to a request repair q March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 47

ISPs charge by bandwidth Broadcast Center Backbone ISP Remember what interdomain protocols optimize for…. March 18, 2003 They make more money without multicast A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 48

Application Layer Multicast n n n 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 q Performance can be optimized to application q n Loss, priorities etc. More control over the topology of the tree q Easier to monitor and control groups q n Disadvantages Scale q Performance if just implemented on the hosts (not gateways) q March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 49

Summary n n Large amount of work on multicast routing Major problems preventing flooding q minimizing state in routers q denial-of-service attacks q deployment q n Multicast can be implemented at different layers lower layers optimize performance q higher layers provide more functionality q n IP Multicast still not widely deployed Ethernet multicast is deployed q application layer multicast systems are promising q March 18, 2003 A. Parekh, EE 122 S 2003. Revised and enhanced F'02 Lectures 50