Multicast for IP Networks 6th April 2000

1. Multicast for IP Networks 6th April 2000 John A. Clark- Technical Account Manager

2. Agenda • Overview of IP Multicast • IP Multicast Addressing • IGMP • IP Multicast Routing Protocols • DVMRP • MOSPF • PIM-DM • PIM-SM • PGM • Protocol Comparison • Summary

3. IP Multicast Overview … What is it & Why do we need it?

4. What is Multicast? Multicast is a method for sending identical data streams to groups of multiple end-stations • Delivers data only to interested users • Enables mass data distribution without specialised processors • Eases administration for data sources • Efficient use of bandwidth for transport of common data

5. Why use Multicasting? • Multicasting fits many of today’s business activities • Multicast support on Operating Systems — Windows 98, NT, Solaris, Linux, etc. • Nortel customers are using multicast applications in production environments — Real-time data push – SIAC, Stock Ticker – NASA for Mission Critical satellite telemetry, command and control and shuttle mission data. — Multimedia broadcast distribution – Marshall Space Center, IP TV, – UC Davis, distance learning

6. An Endstations View • To Send Multicast Data • Host Endstations simply transmit to a Class D Multicast Address • To Receive Multicast Data • Receiving Endstations register interest in data stream associated with a Class D Multicast address (group membership)

7. Multicast Addressing • Class D Addresses (starting 1110) • 224.0.0.0 - 239.255.255.255 • Multicast Addresses are also mapped to Ethernet addresses ……...

8. Multicast Ethernet Addresses • IANA (InterNet Assigned Numbers Authority) owns the reserved block 01-00-5E • The low order 23 bits of the IP Address are mapped to the low order 23 bits of the Ethernet address

9. IGMP IGMP End-to-end Protocol Involvement • Routers at both the source host LAN and receiving end-station LANs use IGMP to learn the existence of host group members on their directly attached subnets • Routers use a Multicast routing protocol (e.g. DVMRP, MOSPF etc.) to establish DVMRP MOSPF • Routers know there is a receiver - not how many • Receivers don’t know who the host sender is • Senders don’t know who the receivers are

10. Host Membership Query 224.1.200.1 224.100.1.1 224.1.90.5 224.1.2.1 224.1.1.1 228.1.1.1 IGMP - Internet Group Management Protocol Host Membership Reports Memberships 224.1.1.1 224.100.1.1 Memberships 228.1.1.1 224.1.2.1 224.1.90.5 Memberships 224.1.200.1 224.100.1.1 Memberships 224.1.90.5 224.1.1.1 224.100.1.1 Memberships 228.1.1.1 224.1.2.1 224.1.90.5 • Multicast router periodically sends a data link layer IGMP Host Membership Query to all nodes on its LAN • query is sent to the all-hosts group (network address 224.0.0.1) • TTL of 1 - queries are not propagated outside of the LAN • Each host sends back one IGMP Host Membership Report message per host group • sent to the group address i.e. only one member reports membership

11. IGMP Protocol Format • Sends IGMP queries and IP hosts report their host group memberships. • IGMP is loosely analogous to ICMP (rfc 1112) • IGMP messages are encapsulated in IP datagrams. • IGMP has only two kinds of packets: Host Membership Query and Host Membership Report, • Simple fixed format - payload 1st word: control information 2nd word: class D address

12. Routers and Multicast Multicast routers: • Translate multicast addresses into host addresses (Class D addresses identify a multicast stream not a specific destination) • Make copies of multicast frames to forward throughout the forwarding tree • Use a choice of routing protocols to provide forwarding throughout the network (e.g. DVMRP, MOSPF etc.) • Exchange information about neighboring routers • Keep ‘state’ for all group memberships • Elect a Designated Router for each LAN (via IGMP) to avoid duplication of multicast tables

13. Host sends single packet to Multicast Group Router table has members of this Group Router forwards one copy of packet Downstream Router table has members of this Group Router forwards one copy of packet on each downstream interface Router forwards a single copy of the packet onto the attached LAN where a group member resides The Forwarding Tree

14. Data flows in the direction of the destination endstation Routers consult the routing table to the source to make forwarding decision Reverse Path Forwarding Source of Multicast Data Destination Destination

15. Two Types of Multicast Routing Protocolsusing Two Types of Multicast Trees • Dense Mode • Multicast group members are densely distributed • Assumes many of the subnets contain at least one group member • Uses Source Tree • Shortest path tree from source to all receivers • Sparse Mode • Multicast group members are sparsely distributed • May be many members - but assumes they are widely dispersed • Uses Shared Tree • Core delivers data to receivers on shared tree

16. Dense Mode Protocols • Protocols that use a “Flood & Prune” mechanism for packet delivery • Useful in campus and environments where bandwidth is plentiful • Source Tree - Shortest Path campus

17. WAN WAN WAN WAN Sparse Mode Protocols • Protocols that use Explicit routing and forwarding • Useful in WANs and environments where bandwidth is not plentiful • Shared Tree from Core (Rendezvous Point)

18. Source Tree Red group source host • Shortest path tree from source to all receivers • Data flooded to all end-stations where not pruned • Dense Mode Protocols • DVMRP, MOSPF, PIM-DM Blue group source host

19. – Receivers build shared trees to Core – Source sends to Core – Core delivers data to receiver on shared tree Shared Tree • Data sent and received via Core (Rendezvous Point) • Data explicitly forwarded to endstations • Sparse Mode Protocols • CBT, PIM-SM Red group source host Rendezvous Point (Core) Shared tree Core to receivers Blue group source host Source to core

20. no receivers pruning Grafting no receivers pruning Flooding, Pruning and Grafting leaf • Flooding • Sending multicast packets to all router interfaces - except that on which the packet arrived • Pruning • Explicitly removing router interfaces for which there are no multicast group members • Grafting • Reconnecting router interfaces to a pruned multicast group leaf leaf leaf Source tree source host leaf new receiver

21. DVMRPDistance Vector Multicast Routing Protocol

22. Distance Vector Multicast Routing Protocol • “RIP for IP Multicast” - defined in RFC 1075 • Separate Multicast routing protocol, in addition to unicast RIP • Collects information about multicast group membership via IGMP • State maintained on all routers • Can tunnel multicast data through non-multicast networks using IP-in-IP • Uses Reverse Path Multicasting

23. DVMRP Forwarding • Assumes initially every host is part of the multicast group • Designated router on source subnet transmits a multicast message to all adjacent routers • Each router selectively forwards the message downstream, until it is passed to all multicast group members • Receiving routers check unicast routing tables to determine shortest path back to the source • Forwards multicast message to all adjacent routers, other than the one that sent the message • Ensures loop-free tree with shortest paths from the source to all recipients

24. DVMRP Protocol Operation ... • Routers send prune messages resulting in a source specific tree • Graft messages are used to reconnect to the pruned tree • DVMRP routing table shows reverse path tree from the router • Leaf detection relies upon “poison reverse” • Multicast source network advertised at infinity • Advertisements not at infinity are for leaf networks • Leaf timeout 200 seconds (default) - flooding and pruning follows • Neighbor timeout 240 seconds (default) - DVMRP route flushing follows

25. MOSPFMulticast Extensions to OSPF

26. Multicast Extensions to OSPF • Link state Multicast routing protocol defined in rfc1584 • Utilizes the unicast OSPF link state database • Can mix MOSPF and OSPF routers in the same network • Routers indicate MOSPF capabilities by setting MC bit in Router-LSA • Group membership LSAs are distributed throughout the OSPF area • When data for a group arrives the forwarding tree is calculated by running the Dijkstra algorithm

27. MOSPF Protocol Operation • MOSPF routers collects information about multicast group membership via IGMP • Routers update their internal link-state information based on information flooded by adjacent routers • Each router can independently calculate a least-cost tree with the multicast source as the root and the group members as leaves • All routers will calculate exactly the same tree, since they share link-state information • A Dijkstra calculation is required to compute a shortest-path tree for each (source, destination group) pair.

28. MOSPF Protocol Operation ... • Wildcard multicast forwarders (ABRs) ensure that all multicast data is flooded into the backbone area • All ABRs advertise group membership into the backbone area ensuring multicast data flows across areas • Designated router selection is as per OSPF • Designated routers solicit and listen to IGMP group membership messages. Backup Designated routers listen only • In a mixed OSPF/MOSPF the designated must be an MOSPF router

29. PIMProtocol Independent Multicast

30. Protocol Independent Multicast/Dense Mode • Similar to DVMRP - also employs Reverse Path Multicasting (RPM) to construct source trees. • Uses pruning to create the Shortest Path tree • Designate router is elected using PIM Router-Query messages transmitted every 30 seconds • Introduces new message type call “assert” to resolve looping …..

31. PIM Dense Mode Assert Messages • An assert message is sent indicating the metric to the source • Upon receiving an assert metric value is compared - if lower the interface is pruned. Designate router wins equal cost • Downstream routers must listen to asserts to ensure knowledge of the correct upstream router

32. Protocol Independent Multicast/Sparse Mode • Uses unicast routing table • Utilizes Rendezvous Points (RP) to build the forwarding tree • Provides both native and encapsulated data transport • Allows routers to bypass the RP when high data volumes are present

33. PIM Sparse Protocol Operation • Transmitted data is forwarded to the RP for distribution to the RP-Tree • If the data rate is warrants, the RP or end-point router can switch to the SPF tree • Timers are used for tree maintenance • Designate router election is identical to PIM/Dense (PIM Router-Query messages transmitted every 30 seconds) • Assert messages are used for loop detection

34. PGMPragmatic General Multicast

35. Introduction to PGM • Reliable Multicast transport protocol • Original title, Pretty Good Multicast • Re-labeled Pragmatic General Multicast • Provides ordered, duplicate free, multicast data delivery from multiple sources to multiple receivers. • Guarantees that a receiver in a multicast group either receives all data from transmissions and retransmissions, or is capable of detecting unrecoverable data packet loss. • Standard is in Internet-Draft status - future??

36. PGM Basic Operation Receiver Receiver Receiver Source SPM Frame Group members & sources periodicallyinterleave Source Path Messages (SPM’s) with multicast data Numbered data frames are used to detect missing data and issue No Acknowledgement (NAK) messages if data is lost Hosts will continue to send NAK messagesuntil they receive a NAK Confirmation (NCF) The source will then retransmit the requested frame Multicast Data Frame 1 Multicast Data Frame 2 Multicast Data Frame 3 NAK Frame 2 NCF Frame Multicast Data Frame 2

37. PGM Protocol Messages • Source Path Messages (SPMs) • Destination Address; Multicast Group Address. • Transmitted by Sources to establish source-path state • Must send SPM, before sending Multicast Data Stream • Negative Acknowledgement (NAK) • Destination Address; Unicast, PGM-hop by PGM-hop, back to the source • Negative Acknowledgement Confirmation (NCF) • Destination Address; Multicast Group Address • Transmitted by Network Elements and sources in response to NAKs • Designated Local Transmitters (DLRs) may respond to a NCF with their own NCF making themselves available as a DLR (Redirection NCF)

38. PGM Network Element Source Receiver Receiver SPM Frame Mcast Data Frame 2 Mcast Data NCF Frame NAK Frame 2 ModifiedSPMFRAME Mcast Data Mcast Data Frame 2 Receiver Receiver NCF Frame NAK Frame 2 Mcast Data ModifiedSPMFRAME Mcast Data Frame 2 NCF Frame NAK Frame 2 Receiver Receiver

39. Designated Local Retransmitter Source Receiver Receiver SPM Frame Mcast Data Null NAK ModifiedSPMFRAME Mcast Data Null NAK Receiver Receiver DLR NAK Frame 2 Mcast Data ModifiedSPMFRAME NCF Frame Rdata Receiver Redirecting NCF Frame

40. Multicast Routing Protocol Comparison

41. Making the Protocol Decision • DVMRP is the most widely implement Multicast routing protocol available today • DVMRP tunnels can provide the connection between MOSPF networks • MOSPF was the first multicast routing protocol to achieve full standard • Multicast extensions to OSPF have the convergence and reliability characteristics of OSPF- proven OSPF technology for multicast forwarding • MOSPF provides state for both Unicast and Multicast forwarding in a single database • PIM-SM supports shared tree & source path trees • PIM-SM can switch from shared tree to source tree

42. Multicasting and the Internet • Multicasting is carried through the internet overlaid on the unicast network using DVMRP tunnels • MBONE consist of around 2750 routes • Because of the MBONE’s size, it suffers from reliability problems • Public domain software is used in the internet for video, voice, shared text and whiteboard session • A recommended set of TTL’s exist for use in the internet

43. Summary - IP Multicast in One Page • IP Multicast is receiver oriented • Receivers & hosts “join” multicast groups • Group is defined by a multicast address • Uses Internet Group Mgmt Protocol (IGMP) to communicate group interest to serving router • Routers know there is a receiver, not how many • Receivers don’t know who the host sender is • Senders don’t know who the receivers are • IP Multicast routing is the “glue” • Transparently forwards data from sender to receivers

44. Thank you Questions ?