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IP Multicast

IP Multicast. Richard Yang 11/26/2001. Review. New security issues distributed denial of service multicast security incentive-compatible others?. Why Multicast?. Many applications require multicast communications any examples? Why not use unicast?. IP Multicast.

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IP Multicast

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  1. IP Multicast Richard Yang 11/26/2001

  2. Review • New security issues • distributed denial of service • multicast security • incentive-compatible • others?

  3. Why Multicast? • Many applications require multicast communications • any examples? • Why not use unicast?

  4. IP Multicast • Service model developed in [Deering & Cheriton 1990] • Many years of effort to implement the service model • Implemented in almost every OS and routers

  5. IP Multicast Service Model • A group is identified by a location-independent address (class D IP address: prefix 1110) • how many IP address? how to get an address? • Anyone can send packets to a “logical” group address • why? • Anyone can join/leave a group and receive packets • why? • Normal, best-effort delivery semantics of IP

  6. Implementation: Network Model • Interconnected LANs • LANs support link-level multicast • Map globally unique multicast address to LAN-based multicast address (LAN-specific algorithm)

  7. Implementation: LAN • Multicast capability widely available on individual LANs • example: Ethernet multicast (prefix 00:00:5e) • Use IGMP to maintain group membership on a LAN • when an application subscribes a group, the host will • turn on Ethernet multicast, and • send IGMP join message to the first hop router

  8. Implementation: Across LANs • Design goals • Approaches • source tree: • Distance-vector multicast routing protocol (DVMRP) • shared tree: • Core-Based Tree

  9. Distance Vector Multicast Routing • An elegant extension to DV routing • Use shortest path DV routes to determine if a link is on the source-rooted spanning tree • Reverse Path Flooding (RPF) • Reverse Path Broadcasting (RPB) • Reverse Path Multicasting (RPM)

  10. Reverse Path Flooding (RPF) • A router forwards a broadcast packet from source (S) iff it arrives via the shortest path from the router back to S • A packet is replicated to all but the incoming interface • Discussion • what are the problems? S y x z forward packets that arrive on the shortest path from “t” to “S” t a

  11. Another Example S x y a duplicate packet z b

  12. Reverse Path Broadcasting (RPB) • Basic idea: forward a packet from S only on child links for S • A child link of router x for source S: a link that has x as parent on the shortest path from the link to S S 5 6 forward only to child links x y a a child link of x for S z b

  13. Reverse Path Multicast (RPM) • Prune back transmission so that only absolutely necessary links carry traffic • Use on-demand pruning so that the group state at a router scales with the number of active groups (not all groups)

  14. Basic RPM Idea • Prune (Source, Group) at a leaf router if no members • send No-Membership Report (NMR) up tree • If all children of router R prune (S,G) • propagate prune for (S,G) to its parent • What do you do when a member of a group (re)joins? • Graft • Both NRM and graft messages are positively acknowledged • On timeout: • prune dropped • flow is reinstated • down stream routers re-prune • Note: again a soft-state approach

  15. RMP Scaling • What is the state requirements of RPM? • which routers maintain states? • what are the state requirement at a router? • How to get better scaling? • Hierarchical multicast • Core-based trees (CBT)

  16. Core Based Trees (CBT) • Ballardie, Francis, and Crowcroft, • “Core Based Trees (CBT): An Architecture for Scalable Inter-Domain Multicast Routing”, SIGCOMM 93 • Tree construction is receiver-based • a receiver sends a join message to the core • one tree per group • only nodes on tree maintain states • A sender unicasts a packet to core and bounces it back to multicast group • Reduce routing table state from O(S x G) to O(G)

  17. Example: M3 Joins • Group members: M1, M2 core M1 M2 M3 control (join) messages S1

  18. Example: M1 Sends Data • Group members: M1, M2, M3 • M1 sends data core M1 M2 M3 control (join) messages data S1

  19. Discussion • What are the problems of CBT?

  20. IP Multicast Discussion • Many years of research and many compelling applications • Many of today’s routers implement IP multicast • Protocol Independent Multicast (PIM) • Sparse mode similar to CBT • Dense mode similar to DV-RMP • IP multicast is still not widely deployed • why?

  21. EXPRESS [Holbrook & Cheriton ’99] • What are the modifications to the IP multicast service model?

  22. Channel & Addressing

  23. Discussion • What are the advantages of the Channel model? • for sender • for receiver • for ISP

  24. Added API • Sender • channelKey( channel, K(S, E) ) • count = CountQuery( channel, countID, timeout ) • Subscriber • result = newSubscription( channel, [, K(S, E)] ) • Count( channel, countId, count )

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