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CSE 401N:Computer Network

CSE 401N:Computer Network

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CSE 401N:Computer Network

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  1. CSE 401N:Computer Network LECTURE-14 MULTICAST ROUTING

  2. Chapter 4 roadmap 4.1 Introduction and Network Service Models 4.2 Routing Principles 4.3 Hierarchical Routing 4.4 The Internet (IP) Protocol 4.5 Routing in the Internet 4.6 What’s Inside a Router? 4.7 IPv6 4.8 Multicast Routing 4.9 Mobility

  3. Multicast: act of sending a datagram to multiple receivers with single “transmit” operation Question: how to achieve multicast Multicast via unicast • source sends N unicast datagrams, one addressed to each of N receivers routers forward unicast datagrams multicast receiver (red) not a multicast receiver (grey) Multicast: one sender to many receivers

  4. Question: how to achieve multicast Multicast: one sender to many receivers Application-layer multicast • receivers involved in multicast copy and forward unicast datagrams among themselves

  5. Question: how to achieve multicast Multicast: one sender to many receivers Network multicast • Router actively participate in multicast, making copies of packets as needed and forwarding towards multicast receivers • Only a single copy of a datagram will ever traverse a link Multicast routers (red) duplicate and forward multicast datagrams

  6. Multicast Groups How to identify the receivers of a multicast datagram? • Let datagram carry the IP addresses of all the recipients • Only workable with a small number of recipients • Sender needs to know the addresses of all receivers • Not a good solution! • Internet solution: use multicast group • A multicast group identifies a set of receivers and has a single identifier • Packet sent to a multicast group are received by all members in the group

  7. Internet Multicast Service Model • Multicast group in the Internet has its own Class D multicast address • IP addresses to reserved for multicast (IP addresses begin with 1110) multicast group

  8. Internet Multicast Service Model • Host group semantics: • anyone can join/leave a multicast group • anyone can send to a multicast group • no network-layer identification of hosts that have joined a multicast group • Questions to answer: • How are hosts join/leave a group? A: Internet Group Management Protocol (IGMP) • How do the routers deliver a multicast datagram to all group members? A: multicast routing protocols

  9. IP multicasting: two-step process • local: host informs local router of desire to join a multicast group: IGMP • wide area: local router interacts with other routers to receive multicast datagrams • many multicast routing protocols (e.g., DVMRP, MOSPF, PIM) IGMP IGMP wide-area multicast routing IGMP

  10. Internet Group Management Protocol (IGMP) • RFC 2236 • Operate between a host and its directly attached router • Provide the means for: • A host to inform its attached router that it wants to join/leave a multicast group • A router to detect if a LAN has any members for a particular multicast group

  11. IGMP Messages Three message types: membership query, membership report, leave group • General membership query • Sent by a router to all nodes on the LAN • Query multicast groups joined by attached hosts • Multicast group address field set to 0 • Specific membership query • Sent by a router to all nodes on the LAN • Query if a specific multicast group is joined by attached hosts • Multicast group address field set to the multicast group address being queried Max. Resp. Time Type Checksum Multicast group address

  12. IGMP Messages (cont’d) • Membership report message • Sent by host to all nodes on the LAN in response to a membership query message or when first joins a multicast group • Contains the address of a group the host has joined • A router only cares about whether one or more hosts belong to a given group want to hear from only one of the hosts that belongs to each group • Each membership query message includes a “maximum response time” field • A host waits a random time between 0 and maximum response time before sending a membership report message • Feedback suppression: if the host observes a membership report message from some other host for the multicast group, discard its own pending membership report message

  13. IGMP Messages (cont’d) • Leave group message: • sent by host to report leaving a group • leave group message is optional • Router periodically sends a membership querymessage • A router infers that no hosts are joined to a group when no host responds to the query message • Soft state protocol: state is removed via a timeout event if it is not explicitly refreshed • IGMP messages are encapsulated within an IP datagram • There is no control over who sends to the group and who joins the group

  14. Source-based trees Multicast Routing: Problem Statement • Goal:Given a multicast group,find a tree connecting routers having local multicast group members • shared tree: a single tree used by all group members • source-based tree: different tree for each sender Shared tree

  15. Approaches for building mcast trees • group-shared tree: group uses one tree • minimal cost (Steiner) tree • center-based tree • source-based tree: one tree per source • shortest path tree • reverse path forwarding …we first look at basic approaches, then specific protocols adopting these approaches

  16. Group-Shared Tree: Steiner Tree • All multicast packets are routed along the same tree, regardless of the sender • Steiner Tree: minimum cost tree connecting all routers with attached group members • problem is NP-complete • good heuristics exist • not used in practice: • information about all links needed • rerun whenever link costs change • not able to use unicast routing info 4 3 2 1 2 2 1 1 Router having attached group members Router having no attached group members

  17. Group-Shared Tree: Center-based tree R1 R4 • single delivery tree shared by all • one router identified as centerof tree • to join: • router with attached group members sends unicast join message addressed to the center • join message“processed” by intermediate routers and forwarded towards center • join messageeither hits existing tree branch for this center, or arrives at center • path taken by join message becomes new branch of tree for this router 3 R2 2 R5 R3 1 R7 R6 center router with attached group member router with no attached group member 1 path order in which join messages generated

  18. Multicast Routing Using a Source-Based Tree A • Construct a multicast routing tree for each source in the multicast group • Router maintains state for each (group, sender) pair • Shortest path tree: union of the shortest paths from source to each receiver • Obtained by Dijkstra’s algorithm B C D F E Router having attached group members Router having no attached group members

  19. Steiner Tree v.s Shortest Path Tree A A (source) 4 4 3 3 2 2 1 1 2 2 2 2 1 1 1 1 Steiner tree: minimize the sum of the link costs in the tree Shortest path tree: minimize the cost from source to each receiver

  20. Compute Shortest Path Tree • Use link state algorithm: each router needs to know the state of each link in the network • Can we compute the tree without link state information? A: use reverse path forwarding algorithm

  21. Reverse Path Forwarding (RPF) • Forward packet from source S to all interfaces (except the incoming one) iff the packet arrived on the link that is on its shortest path to S • A router need only know the next hop on its shortest path to the sender • result is a source-specific reverse shortest path tree A B C D F E G Packet that will be forwarded Packet not forwarded beyond receiving router

  22. RPF: Pruning • forwarding tree contains subtrees with no mcast group members • no need to forward datagrams down subtree • “prune” msgs sent upstream by router with no downstream group members • Router tells parent in the tree to stop forwarding • Prune message propagate upstream A B C D F E G Prune message

  23. Rejoining If a router sends a prune message upstream, what should happen if it later needs to join the group? • Send a graftmessage to upstream router • Or, allow pruned branches to time-out and be added again to the tree

  24. Issues in Wide-Area Multicast • Hosts may join and leave a multicast group dynamicallyneed to dynamically update routing tree • State information for a multicast connection must be established and maintained in routers • Group-shared tree: per group state • Source-based tree: per (group, source) state

  25. Internet Multicasting Routing: DVMRP • DVMRP: distance vector multicast routing protocol, RFC1075 • flood and prune: source-based tree, reverse path forwarding • each router uses a distance vector algorithm to compute the next hop on its shortest path to each source • initial datagram to mcast group flooded everywhere via RPF • routers not wanting multicast datagram: send upstream prune messages • routers store state for each (source, group) pair

  26. DVMRP: continued… • soft state: DVMRP router periodically “forgets” branches are pruned: • multicast data again flows down unpruned branch • router: reprune or else continue to receive data • routers can quickly regraft to tree using graft message • odds and ends • commonly implemented in commercial routers • MBONE (Multicast Backbone) routing done using DVMRP

  27. Tunneling • some Internet routers are not multicast-capable • Internet MBONE: a virtual network of multicast-capable routers on top of a physical network containing a mix of unicast and multicast routers A A B B C C logical topology physical topology • mcast datagram encapsulated inside unicast datagram • unicast IP datagram sent thru “tunnel” via regular IP unicast to receiving mcast router • receiving mcast router extracts mcast datagram

  28. not dependent on any particular underlying unicast routing protocol (works with all) two different multicast distribution scenarios: PIM: Protocol Independent Multicast • Dense mode: • group members densely located, many routers need to be involved in routing multicast datagrams • Sparse mode: • # routers with attached group members small wrt total # routers

  29. Dense mode group membership by routers assumeduntil routers explicitly prune data-driven construction of mcast tree (e.g., RPF) Sparse mode routers not involved in multicast distribution until explicitly join a group receiver- driven construction of mcast tree (e.g., center-based) Consequences of Sparse-Dense Dichotomy

  30. PIM - Dense Mode • flood-and-prune reverse path forwarding • similar to DVMRP, but can interoperate with any underlying unicast routing protocol

  31. center-based approach router sends JOIN msg to rendezvous point (RP) to join the tree intermediate routers update state and forward JOIN message toward RP PRUNE message is sent when a router leaves a group PIM - Sparse Mode R1 R4 join R2 join R5 join R3 R7 R6 rendezvous point

  32. sender unicast data to RP, which multicasts down RP-rooted tree RP can send STOP message to source if no routers are jointed to the tree “no one is listening!” after joining RP, router can switch to source-specific tree PIM - Sparse Mode R1 R4 join R2 join R5 join R3 R7 R6 all data multicast from rendezvous point rendezvous point

  33. Switch from group-shared tree to a source-specific tree • In group-shared tree, E sends to RP, then RP sends to A • when data rate from E exceeds a threshold • A sends join message directly to E • A does not send prune message towards RP so it can continue to receive from other sources • increased performance: less traffic concentration, shorter paths source Before switch E D After switch A B C RP receiver

  34. MOSPF: Multicast Open Shortest Path First • Multicast extension to OSPF • Routers flood group membership information with Link State Advertisements • Each router independently computes source-specific, shortest-path trees for each multicast group

  35. Scalability: the amount of state required in the routers Group-shared tree: per group state Source-based tree: per group, per source state Excess traffic received? Yes: DVMRP, PIM Dense Mode No: MOSPF, PIM Sparse Mode Traffic Concentration Group-shared tree tends to concentrate traffic on a smaller number of links Source-based trees tend to distribute multicast traffic more evenly Optimality of forwarding paths Minimum cost multicast tree not used in practice Shortest path trees and center based trees are used Evaluating a Multicast Routing Protocol

  36. THANX.