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Multicast Configuration

Multicast Configuration. Don Salvin National Center for Network Engineering Pittsburgh Supercomputing Center salvin@psc.edu. The NCNE GigaPOP (the commercial!). Originally PSCnet The network providing regional access to Pittsburgh Supercomputing Center resources.

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Multicast Configuration

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  1. Multicast Configuration Don Salvin National Center for Network Engineering Pittsburgh Supercomputing Center salvin@psc.edu

  2. The NCNE GigaPOP(the commercial!) • Originally PSCnet • The network providing regional access to Pittsburgh Supercomputing Center resources. • Heavy participant in the Pennsylvania regional network (PrepNET). • After the demise of regional nets, became aggregator for Carnegie Mellon Unversity, University of Pittsburgh.

  3. The NCNE GigaPOP • Now includes: • Carnegie Mellon University • Pennsylvania State University • University of Pittsburgh • West Virginia University (I2 only) • Also part of NLANR • National Laboratory for Applied Network Research

  4. Housekeeping • Are we Cisco-centric? • For routers, probably • For switches, probably not • Pass around attendees signup sheet!

  5. And now for the good stuff!

  6. VERY broad overview! • Concepts • Multicast addressing • Protocols • Campus issues

  7. Multicast Concept - RPF • Reverse path forwarding • The basic forwarding mechanism for Multicast • Different paradigm than Unicast • Forward away from the source, rather than to a destination.

  8. Multicast Concept - RPF • Forward to interfaces which aren’t a path to the source address • If packet received on an interface that doesn’t point to the source, DROP IT. • Cause of confusion if Multicast and Unicast routing topologies aren’t congruent.

  9. Multicast state notation • (*, G) denotes state of a group. • Also denotes Shared tree. • (S, G) indicates a traffic source within a group. • Shortest path tree

  10. Multicast state notation • Specific Example: • (128.182.61.159, 224.2.246.13) • Group is 224.2.246.13 • This source is 128.182.61.159

  11. Multicast state notation • Shared Tree • Conserves network resources if many members sending low data rates. • But, leads to congestion if too much traffic. • Shortest Path Tree, Also known as Source Tree. • Allows individual receivers to get optimal delivery. • May reduce congestion.

  12. Multicast addresses • Class D address space • high order bits of address are “1110” • Therefore IP group addresses are: • 224.0.0.0 thru 239.255.255.255

  13. Multicast addresses • Special reserved groups • 224.0.0.0 224.0.0.255 • 224.0.0.1 All systems this subnet • 224.0.0.2 All routers this subnet • 224.0.0.4 DVMRP routers • Definitive list of reserved addresses is • ftp://ftp.isi.edu/in-notes/iana/assignments/multicast-addresses

  14. Multicast addresses - local • 239.0.0.0 through 239.255.255.255 • Locally scoped addresses. • Not unique across domains. • defined in Best Current Practices - RFC2365

  15. Layer 2 multicast addresses • Bottom 23 bits of IP address translated to bottom 24 bits of ethernet MAC address with prefix of 01-00-5E (includes MC bit!). • There are 28 bits of unique multicast address, so the mapping potentially produces 32 (2^(28-23)) IP addresses per MAC address!

  16. Multicast addressing • Token ring a special case, because no “multicast destination” bit.

  17. Multicast Protocols - IGMP • Provides communication between hosts and first/last hop multicast routers. • Establishes host multicast group membership. • Senders don’t need to be members of a group.

  18. Multicast Protocols - IGMP • Two versions - • IGMP v2 mostly upwardly compatible with v1 • RFC1112 - IGMP v1 • RFC2236 - IGMP v2 • IGMP v3 future…

  19. Multicast Protocols - IGMP • IGMP is the protocol that hosts use to inform routers that a receiver is listening to a group. • Routers prune multicast traffic from interfaces without receivers for a group.

  20. Multicast Protocols - IGMP • Since IP multicast traffic has the MAC layer “multicast” bit set, multicast traffic goes everywhere on a particular LAN, even if it is switched! • So there are ramifications for the LAN environment.

  21. Multicast Constraints - IGMP snooping • Switches look at IP layer 3 to discern prunes and leaves • Stop multicast traffic going to networks without receivers

  22. Multicast Constraints - CGMP • A Cisco’ism • IGMP aware routers and CGMP switches form client-server team to block multicast traffic from networks without any receivers

  23. Multicast Routing Protocols - DVMRP • The “original” Mbone multicast routing protocol • Hop count based • Generates it own routing database • might be different than the underlying Unicast topolog, given: • tunnels • multicast only networks

  24. Multicast Routing Protocols - DVMRP • Must retain (S, G) state for all multicast networks! Currently more than 7000 routes. • Doesn’t scale because: • Hop count only metrics • Imagine building a 7000 route network out of RIP!

  25. Multicast Routing Protocols - PIM • Rely on Unicast topology

  26. Multicast Routing Protocols - PIM - Dense Mode • Uses flood and prune • flood traffic until downstream neighbors issue a prune. • do it all over again 3 minutes later. • Good when groups are dense! • Poor over limited capacity links, such as the wide area.

  27. Multicast Routing Protocols - PIM Sparse mode • The concept of the Rendevous Point • Receivers join to the RP • Senders register with the RP • RPF check depends on tree type • For shared trees, use RP address • For source trees, use source address • One RP per domain, mostly.

  28. Multicast Protocols - MSDP • Multicast Source Discovery Protocol • allows RP’s in different domains to exchange information about sources. • Facilities for scoping

  29. Multicast Routing Protocols - mBGP • BGP4 extensions to allow independent routing topologies for multicast and unicast traffic • neighbor 1.1.1.1 remote-as 1 nlri multicast • neighbor 1.1.1.1 remote-as 1 nlri unicast • neighbor 1.1.1.1 remote-as 1 nlri multicast unicast

  30. Campus Issues • Scoping • ways of controlling the extent of distribution of multicast traffic. • by TTL • by explicit group or source addresses

  31. Campus Issues • Unknown (to you) DVMRP based routes can cause: • RPF failures. • Your network to become a transit network for external traffic! • Ciscos automatically listen! Beware! • Ip dvmrp accept 1 • access-list 1 deny any

  32. Campus Issues • The dangers of multiple high bandwidth streams flowing into single 10Mb nets • Some control with IGMP snooping, CGMP

  33. How can NCNE help you? Ncne@nlanr.net

  34. Some multicast resources • http://www.ncne.nlanr.net/faq/index.html • ftp://ftpeng.cisco.com/ipmulticast • ftp://ftpeng.cisco.com/ipmulticast/multicast_training.html

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