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IP Multicast

IP Multicast. Fang Yan CSEE, UMBC 10/14/04. topics. Basics of multicasting MBone IPv4 routing protocols 6bone. What is multicasting. Application level one to many communication. A single packet is addressed to all intended recipients — host group

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IP Multicast

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  1. IP Multicast Fang Yan CSEE, UMBC 10/14/04

  2. topics • Basics of multicasting • MBone • IPv4 routing protocols • 6bone

  3. What is multicasting • Application level one to many communication. • A single packet is addressed to all intended recipients—host group • The network replicates packets only as needed—when paths diverge at a router

  4. Why Multicasting • It is efficient • better bandwidth utilization • less host/router processing • quicker participation • Many applications send data to multiple receivers. • Streaming multimedia • Content delivery (meeting, lecture, speech…) • Stock quotes, news • Database replication • Software distribution

  5. B Y A X C Z D E Flow of data in multiple unicasting

  6. B Y A X C Z D E Flow of data in multicasting

  7. Basic components of IP multicasting • Definition of a multicast host group • Protocols for establishing and controlling multicast groups • Router infrastructure for distribution of multicast traffic • Application protocols and APIs that enable desktop computers and workstations to put multicasting to good use

  8. multicast address • IPV4 4 28 bits class D • From 224.0.0.0 to 239.255.255.255 • From 224.0.0.0 to 224.0.0.255 is reserved • Totally 2^28-256=268 million addresses 1110 Multicast Group ID

  9. Reserved address examples • 224.0.0.1 all hosts • 224.0.0.1 all multicast routers • 224.0.0.4 DVMRP routers • 224.0.0.5 OSPF routers • 224.0.0.6 OSPF designated routers • 224.0.0.22 IGMP …

  10. multicast address (cont.) • IPV6 8 4 4 112 bits • Support 4millionbillionbillionbillion addresses • Flag • Transit/permanent • Scope • 1: node-local • 2: link-local • 5: site-local (set by administrators of routers at the site) • 8: organization local (set by administrators of routers of the organization) • 14: global 11111111 flags scope Multicast Group ID

  11. Ready to go? Unfortunately, there is a problem… At the early 90’s, the majority of the routers on the Internet don't know how to handle multicasting. Most routers are set up to move traditional IP unicast packets. Router manufacturers have been reluctant to create equipment that can do multicasting until there is a proven need for such equipment. But, as you might expect, it's difficult for users to try out a technology until they have a way to use it. Without the right routers, there's no multicasting. Without multicasting, there won't be the right routers. Solution? MBone !

  12. MBone—a little bit history • Stands for Multicast Backbone • In 1992, some bright fellows on the Internet Engineering Task Force (IETF) decided that no one would do in hardware, they could do in software. • Many scientific conferences, scientific events, concerts were broadcast over MBone since then. • In 1997, the first ISPs started rolling out commercial services utilizing multicasting on the Internet.

  13. MBone—architecture • A virtual overlay network on top of the Internet. • Consists of multicast-capable islands connected by tunnels • Each island (typically a LAN or group of interconnected LANs) supports hardware multicast to its hosts. • Each island contains one or more special routers called mrouters (multicast routers) • Mrouters are often workstations running multicast routing daemon (mrouted)

  14. mrouter C B LAN A D Multicast island

  15. tunneling • A tunnel is a connection between two mrouters using "IP over IP". • Tunnel parameters • Threshold — The minimum TTL required for a packet to be forwarded across this tunnel • Cost — A metric used to compute routing “distances” • encapsulate MBone packets within IP packet and send as regular unicast packets to the destination mrouter’s IP address

  16. Original packet

  17. Encapsulated packet

  18. Tunnel configure • Tunnel is configured manually • The administrator of the new island sends a message announcing its existence to the MBone mailing list. • The administrators of nearby sites then contact him to arrange to set up tunnels. • Existing tunnels may be reshuffled to take advantage of the new island to optimize the topology.

  19. MBone (cont.) • Initially used DVMRP routing algorithm • Migrated toward PIM • Uses UDP protocol • Problems • Scalability • Flow control • Congestion control • Future • Universe support of multicast?

  20. IGMPv1 • Stands for Internet Group Management Protocol • Described in RFC 1112. • Manages multicast group membership • Runs between hosts and their immediate neighboring mrouter • Only two kinds of packets: query and report • Packet format • 4 4 4 16 bits

  21. IGMPv1 How to join a host group • Mrouter initiated • Periodically, mrouter sends out a Query packet to its island asking who is interested in which channel. • Hosts wishing to (continue to) receive one or more channels send back Report packet. • Each mrouter keeps a table of which channels it must put out into its LAN • Receiver initiated • When a host first joins a group, it immediately transmits Report message rather than waiting from the next Query from the mrouter. • The request may need to be forwarded up to a router that’s already part of the host group • Reduce join latency

  22. More IGMP • IGMPv2 • Defined in RFC 2236 • Adds an explicit Leave message • Routers can more easily determine when a group has no interested listeners on a LAN • IGMPv3 • Defined in RFC 3376 • Optimizes support for a single source of content for a multicast group (SSM) • Backward compatible

  23. IPv4 multicast routing protocols • DVMRP (Distance Vector Multicast Routing Protocol) • MOSPF (Multicast Open Shortest Path First) • PIM-DM (Protocol Independent Multicast, Dense Mode) • PIM-SM (Protocol Independent Multicast, Sparse Mode) • CBT (Core-Based Tree)

  24. distance vector routing • Dynamic routing algorithm • Each router maintain a table giving the best known distance to each destination and which line to use to get there. • These tables are updated by exchanging information with the neighbors periodically. • Problem • Count-to-infinity

  25. link state shortest-path-first routing • Periodically, each router floods a link-state message, containing the router’s identity and its associated connectivity information to each of its immediate neighbors. • Each router runs the shortest-path-first algorithm to determine the shortest path from itself to all the other routers. • Problem: • Computation intensive

  26. RPF • Stands for reverse path forwarding • Simple algorithm developed to avoid duplicate packets on multi-access links • RPF algorithm takes advantage of the IP routing table to compute a multicast tree for each source. • RPF check • When a multicast packet is received, note its source (S) and interface (I) • If I belongs to the shortest path from S, forward to all interfaces except I • If test in step 2 is false, drop the packet • Packet is never forwarded back out the RPF interface!

  27. DVMRPexchange distance vectors • First multicast routing protocol ever deployed in the Internet • Each router maintains a ‘multicast routing table’ by exchanging distance vector information among routers • Constructs a source tree for each group using reverse path forwarding • There is a “designated forwarder” in each subnet • Multiple routers on the same LAN select designated forwarder by lower metric or lower IP address (discover when exchanging metric info.) • Once tree is created, it is used to forward messages from source to receivers

  28. DVMRPbroadcast & prune • Flood multicast packets based on RPF (Reverse path forwarding) rule to all interested routers. • Leaf routers check and sends prune message to upstream router when no group member is on their network • Upstream router prune the interface with no dependent downstream router. • Graft message to create a new branch for late participants • Restart forwarding after prune lifetime (standard : 720 minutes)

  29. DVMRP tables • Routing table • Source subnet: the source of multicast datagrams • From-Gateway: previous hop router leading back to source • Metric: cost • TTL • InPort: router’s interface for incoming traffic • OutPort: router’s interface for outgoing traffic • Forwarding table • Source subnet: the sub network sending multicast datagrams • Multicast group: class D IP address • TTL • InPort: parent interface, prune message • OutPort: child interface to forward, prune message

  30. DVMRP • Advantage • Relatively simple • Modest processing demands • Disadvantage • Convergence performance • Periodically flood multicast traffic to rebuild its trees – scalability

  31. MOSPF • Modified from OSPF • Suitable for multicasting within a single autonomous system (AS) • One router is selected as the designated router (DR) • DR floods Group-Membership Link State Advertisements (LSAs) • Each router maintains the up-to-date image of the topology of the entire network • Each router floods its local state through the AS • Source-rooted Shortest path tree for each [source network, destination group] pair • Source-rooted SPT calculated at each router • Based on Link-State database • Provides the best route to any destination in AS • Pruned SPT - Group membership LSAs

  32. MOSPF • Each router makes its forwarding decision based on the contents of its forwarding cache. • Forwarding cache is build from the source-based shortest-path tree for each (source, group) pair and the router’s local group database • Forwarding cache entry • Destination • Source • Upstream: receive from • Downstream: forward to • TTL • Advantage • Adapt rapidly to changes in group membership • Interoperate with OSPF—used to forward normal unicast IP traffic • Disadvantage • Computationally intensive • Not well-suited for handling sparse mode

  33. PIM-DM • Similar to DVMRP • Floods multicasts out of all interfaces except the source interface • Uses RPM • Prune message to eliminate unneeded branches • Protocol-independent • Needs to establish its own router-to-router dialogs

  34. PIM-DM Assert Mechanism • Multiple routers are active on the same link • Routers sends “PIM assert” messages • Compare distance and metric values • Router with best route to source wins • If metric & distance equal, highest IP address wins • Losing router stops sending (prunes interface)

  35. PIM-SM • Designed to provide efficient communication between members of sparsely distributed groups • Rendezvous point (RP) are used by senders to announce their existence and by receivers to learn about new senders of a group • Requires host group members explicitly join a delivery tree by transmitting Join message • One set of RPs per sparse-mode domain, not per group. • Each group has precisely one RP at any given time. • DR sends Join/Prune messages toward the RP and maintain the active RP

  36. PIM-SM (cont.) • Allows routers to create a source-based SPT on behalf of their attacked receivers • Reduce delay • Reduce concentration of traffic on RP-tree • Advantages • Less traffic • disadvantages • Bottle-neck at an RP router • Single point of failure

  37. PIM-SMjoining • DR receives IGMP Report message from receiver • DR creates a multicast forwarding entry for the (*,G) pair and transmits a unicast Join message toward the primary RP for this group • Intermediate router forwards the Join message, creating a forwarding entry for the (*,F) pair if not exists. Source (S) Designated Router Rendezvous Point (RP) for group G Host (receiver) Join Join

  38. Source (S) PIM Router DR Rendezvous Point PIM-Register PIM-Join Host (receiver) RP PIM-SMsending • DR encapsulates the initial multicast packets in a Register packet and sends them toward the primary RP • The active RP transmits Join messages back toward the source’s DR. • Intermediate routers create a new (S, G) pair. • When source gets a Join, it sends further packets without encapsulation

  39. CBT • Construct a single tree shared by a Group • Protocol independent • Core router equivalent to RP • Receiver cannot switch from RP-tree to SPT • CBT state bi-directional • data flows in either direction along the branch • Advantage • Less traffic • Better scalability • Disadvantage • Bottleneck at CR • Single point failure

  40. CBT • Host sends IGMP report to Join a group • JOIN-Request sent towards the core router • JOIN-Request is explicitly acked using JOIN-ACK by core or on-tree router • Intermediate routers set up Transient state • <Group, Incoming interface, Outgoing interface> • Transient state converted to Active state by JOIN-ACK • Bi-directional state • Keep-alive mechanism—CBT Echo protocol • Echo-Request • Each on-tree router is responsible for maintaining its Upstream link • Sent to the Upstream router • Carries a list of groups for which the upstream router is the parent • Echo-Reply • From the parent with the list of groups attached to the child interface

  41. Comparison

  42. Inter-Domain Routing • Hierarchical routing • HDVMPR • HPIM • M-BGP (Multicast Border Gateway Protocol) • MSDP (Multicast Source Discovery Protocol) • Connect multiple PIM-SM domains together

  43. 6bone • Stands fro IPv6 backbone • an IPv6 test bed to assist in the evolution and deployment of IPv6 • The network became a reality in March 1996 • started as a virtual network (using IPv6 over IPv4 tunneling/encapsulation) operating over the IPv4-based Internet to support IPv6 transport • slowly migrating to native links for IPv6 transport • Now 1478 sites from 60 countries

  44. 6bone hierarchy • The 6Bone network is organized in three hierarchical layers • the core backbone layer • made up of a mesh of IPv6 over IPv4 tunnels which connect only the backbone nodes • routing is based on BGP4+ (Border Gateway Protocol version 4 extended to support IPv6) (RFC 2858) • the transit node layer • connected to one or more backbone nodes • provide transit service to the leaf nodes • Routing outside of the backbone is mainly static • the peripheral or leaf node layer.

  45. 6bone addressing • based on the format established for IPv6 unicast addresses (RFC 2073) • backbone nodes act as experimental TLAs (Top Level Aggregators) called pseudo-TLA • responsible for assigning IPv6 addresses to nodes belonging to lower hierarchical levels which are configured as their "clients". • The entire 6Bone network is identified by a 16-bit prefix assigned directly by IANA • each backbone node is assigned a 24- or 28-bit long prefix

  46. 6bone future • Will stay in place until on longer needed • excellent place for an ISP to get early experience before going into production

  47. Thank you ! question?

  48. References: • IP Multicasting by Dave Kosiur • Multimedia communications by Fred Halsall • Computer Networks by Andrew S. Tanenbaum • http://www.6bone.net • http://www.savetz.com/mbone/ • http://www.juniper.net/techpubs/software/junos/junos60/swconfig60-multicast/html/swconfig60-multicastTOC.html

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