From: Alan M Chambers To: p8021@nic.hep.net Subject: IP multicast, Floyd Backes's presentation, Denver Date: Wed, 01 Nov 1995 02:07:22 +0100 =================================================================== IP Multicast -- a "quickie" overview * Why IP Multicast * Brief overview * More detail of IGMP Discussion of how it works now, not how it may work in bridged LANs in the future (e.g., EGMP or other tricky stuff) IEEE 802 VLAN INTERIM MEETING 11 October 1995 Floyd Backes floyd_backes@3mail.3com.com =================================================================== Why IP Multicast? * Multimedia applications appearing in marketplace * Multimedia add-ons easy and inexpensive * MBone tools and applications freely available * Remote conferencing applications * Windows 95 multicast support in IP stack * Routing protocol development and implementation (DVMRP, MOSPF, PiM, CBT) =================================================================== Purpose of IP Multicast * To allow end systems to request which multicasts they want * To allow networks to deliver multicasts only where they need to go =================================================================== IP Multicast Description 2 Parts: * ES - IS Protocol -- IGMP * IS - IS Protocol (Routing Protocols) - DVMRP - MOSPF - PIM - CBT =================================================================== IP Multicast - The Distribution Tree << Tree diagram: a tree! >> * Reverse Path multicasting * Exactly one packet, only on segments with receiver =================================================================== DVMRP -- MBONE Routing Protocol * Based on an expired experimental RFC * Sources freely available * Distance-vector algorithm to route to source * Continuous improvements by Steve Deering, Steve Casner, Bill Femer, Ajit Thyagarajan and others =================================================================== MOSPF -- Multicast Extensions to OSPF * Fully specified in two RFCs: - RFC 1483 - OSPF Version II - RFC 1584 - Multicast Extensions to OSPF * Well written and stable * Implemented by John Moy @ Proteon (field tested) * Link State Protocol * Dijkstra algorithm computes route to source only through multicast-capable routers =================================================================== PIM -- Multicast "Forwarding" Protocol * Distribution Tree begins as static tree of selected routers, called Rendezvous Points (RPs) * Algorithm of Prunes and Graphs to move traffic from RP tree to shortest path tree * Protocol issues not fully known, practice is restricted to Cisco environments * Several incidents of MBONE meltdown related to PIM clouds =================================================================== CBT -- Core Based Trees * Alternative to PIM for inter-domain routing of multicast IP packets * Theoretical work is promising - no direct experience yet =================================================================== IGMP -- Internet Group Management Protocol * ES - IS Protocol, ES lets IS know which multicasts it wants * IP multicast addresses map algorithmically to MAC multicast addresses * IP-capable routers periodically send Host Membership Queries to "allhosts" IP Group Address * End Systems respond with IGNP Host Membership Reports, addressed to group address being requested (Separate report sent for each multicast) * Routers receive reports and feed the information into whatever IS - IS protocol is used to build multicast trees =================================================================== IGMP -- More Details * If a router sees a Query from another router with a lower IP address, it shuts up * ESs also monitor other reports, and don't send duplicates - use timers and random backoofs - only one report per multicast address per subnet =================================================================== "Host Leaves Group" * Optional message sent by "newer" host implementations * May be sent when an ES is no longer interested in a particular multicast * Causes router to issue Host Membership Query to that multicast * If that times out, router can stop sending that multicast and prune itself from the tree * Router must ignore Host Leaves Group Reports on any links that have "old" ESs, because it cannot rely on lack of response to a "group specific" query * Simple timeouts used on links with "old" hosts =================================================================== Acknowledgements to Cyndi Jung ! References / Further Reading T Lin, "Design specification for Multicast Router", 3 March 1995 S Deering, "Host Extensions for IP Multicasting", RFC 1112, August 1989 D Waizman, C Partridge, BBN STC and S Deering, "Distance Vector Multicast Routing Protocol", RFC 1075, November 1988 J Moy, "Multicast Extensions to OSPF", RFC 1584, March 1994 A J Ballardie, P F Tsuchiya and J Crowcroft, "Core Based Trees (CBT)", Proceedings of the ACM SIGCOMM, March 1993 S Deering, D Estrin, D Farinacci, V Jacobson, C Liu and L Wei, "Protocol Independent Multicast (PIM): Protocol Specification", Internet Draft, January 1995 S Deering, "Multicast Routing in Internetworks and Extended LANs", Proceedings of the ACM SIGCOMM, August 1988 Y K Dalal and R M Metcalf, "Reverse Path Forwarding of Broadcast Packets", Communications of the ACM, December 1978 ===================================================================