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IP Multicasting

IP Multicasting. Part of this presentation based on: Cisco, Forouzan TCP/IP textbook, Anthony Chung, Dale Buchholz. Topics. IP Class D Group Addressing IGMP (Internet Group Management Protocol) Multicast Strategies: Broadcast/Flooding Multiple Unicast Distribution trees

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IP Multicasting

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  1. IP Multicasting Part of this presentation based on: Cisco, Forouzan TCP/IP textbook, Anthony Chung, Dale Buchholz.

  2. Topics • IP Class D Group Addressing • IGMP (Internet Group Management Protocol) • Multicast Strategies: • Broadcast/Flooding • Multiple Unicast • Distribution trees • Multicast forwarding (based on Reverse Path Forwarding) • Multicast Routing Protocols • DVMRP (Distance Vector Multicast Routing Protocol) • MOSPF (Multicast OSPF) • PIM (Protocol Independent Multicast) • MBONE (Multicast Backbone) • Implementation and Configuration (for Cisco)

  3. Practical Applications of IP Multicast • Multimedia: A number of users “tunes” to a video or audio transmission from a multimedia source station. • Teleconferencing: A group of workstations form a multicast group such that a transmission from any member is received by all other group members. • Database: All copies of a replicated file or database are updated at the same time. • Distributed computation: Intermediate results are sent to all participants. • Real-time workgroup: Files, graphics, and message are exchanged among active group members in real time. • MOST prevalent usage right now: trading – Used to multicast market data. • Can be VERY large; OPRA (Options Price Reporting Authority) feed is over 80 Mbps SUSTAINED • Very latency sensitive. Trading platforms are trying to optimize at the micro-second levels.

  4. Multicasting within single LAN • In 802.3, MAC address can specify a broadcast/multicast • 0xFFFF.FFFF.FFFF is for broadcast • Bit 0 of octet 0 indicates Broadcast/Multicast • Bit 1 of octet 0 indicates Locally Administer Address (LAA) vs use of Universally Administered Addresses (UAA) • IANA was given MAC address block 01:00:5E IANA defined range available: • 0100.5E00.0000 to 0100.5E7F.FFFF – That is 23 bits • Multicast IP address is defined by 28 bits (1110 defines class D) • The lowest 23 bits of the IP multicast address maps to a MAC address • All stations that recognise the MAC address of the multicast address accept the packet – Done at L2 admission instead of L3 like it would be in broadcast. (a broadcast is accepted by all stations, then sent to L3 for processing where it will be dropped if no process uses it) • Works because of broadcast nature of LAN • Packet only sent once • Much harder on Internet or routed infrastructure

  5. IP Class D Group Addressing (multicast address or group ID) Mapping IP Multicast Address to Ethernet Address Figure 10.10 This prefix is given for multicast MAC addresses

  6. MAC address Ambiguities • Because upper 5 bits in IP multicast address are dropped, result MAC address is not unique: 32 multicast group IDs map to a single MAC address. • Example all groups below maps to 0x0100.5E01.0101 • 224.1.1.1 • 224.129.1.1 • 225.1.1.1 • 225.129.1.1 • … • 238.1.1.1 • 238.129.1.1 • 239.1.1.1 • 239.129.1.1

  7. Multicast Strategies across routers

  8. Multicast Strategies across routers • Broadcast • Multiple Unicasts • True Multicast

  9. Broadcast • Assume location of recipients not know • Send packet to every network • Packet addressed to N3 traverses N1, link L3, N3 • Router B translates IP multicast address to MAC multicast address • Repeat for each network • Generates lots of packets • Delivery of packets done via Routing Protocols

  10. S Multicast Strategies Broadcast: send one copy to each network (not knowing which network has members)

  11. Multiple Unicast • Location of each member of multicast group known to source • Table maps multicast address to list of networks • Only need to send to networks containing members of multicast group • Reduced traffic (a bit)

  12. S Multicast Strategies Multiple Unicast: send one copy to each network where there is a member.

  13. True Multicast • Least cost path from source to each network containing member of group is determined • Gives spanning tree configuration • For networks containing group members only • Source transmits packet along spanning tree • Packet replicated by routers at branch points of spanning tree • Reduced traffic

  14. Multicast Strategies Multicast along a spanning tree (A sub-graph that reaches all nodes without cycles) to networks with members only S

  15. Multicast Transmission Example

  16. Requirements for Multicasting • Router must forward one or more copies of incoming packet • Addressing • IPv4 uses class D • Start 1110 plus 28 bit group id • IPv6 uses 8 bit prefix of all 1s, 4 bit flags field, 4 bit scope field 112 bit group id • Router must translate between IP multicast address and list of networks containing members of group – router must keep track of membership subscriptions and tree. • Multicast addresses may be permanent or dynamic

  17. Requirements for Multicasting • Individual hosts may join or leave dynamically • Need mechanism to inform routers • Routers exchange information on which subnets contain members of groups • Routers exchange information to calculate shortest path to each network • Need routing protocol and algorithm • Routes determined based on source and destination addresses • Avoids unnecessary duplication of packets

  18. IGMP Operation • Host uses IGMP (Internet Group Management Protocol) to make itself know as member of group to other hosts and routers • To join, send IGMP membership report message • Send to multicast destination of group being joined • Routers periodically issue IGMP query • To all-hosts multicast address • Hosts respond with report message for each group to which it belongs • Only one host in group needs to respond to keep group alive • Host keeps timer and responds if no other reply heard in time • Host sends leave group message • Group specific query from router determines if any members remain • Note IGMP v1 did not include “leave”: Multicast would be sent until a timer expires

  19. IGMP – a protocol designed to help a multicast router identify the hosts in a LAN that are the members of a group. Figure 10.2IGMP (v2) message types

  20. IGMP – a protocol designed to help a multicast router identify the hosts in a LAN that are the members of a group. From routers to hosts. Asking for group membership info From hosts to routers. Reporting group membership info

  21. Reserved Link Local Addresses • IANA reserved 224.0.0.0 through 224.0.0.255 for routing protocols on local networks. These packets should NEVER be forwarded and should have TTL of 1.

  22. Glop Addressing (RFC 2770) • RFC 2770 (revised 3180) proposes that 233.0.0.0/8 be reserved for static Multicast groups for organizations that have a reserved, registered public AS number. • That AS # form the 2nd and 3rd Octet of the IP range. • Gives an AS owner 255 unique static and global Multicast groups. • Example: AS #62010 • Decimal 62010 = 0xF23A • F2 is 2nd octet = 242 • 3A is 3rd octet = 58 • 233.242.58.0 is globally reserved for AS 62010 to use • Question: what does “GLOP” stands for?

  23. Using IGMP, R learns about the list of groups on this LAN. When R receives a packet with a destination multicast address, it checks against the list and determine if the packet should be propagated to the LAN.

  24. Four situations of IGMP operations VERSION 1!! (initially) (periodic) (in response to a query) (time out) Router erase after no responses to 3 queries

  25. (A host waits for a random period of time before sending out a report) (they hear that the group has been reported)

  26. IGMP v2 improvements • IGMPv2 • Defines a procedure for the election of multicast queriers for each LAN (the router with the lowest IP address). • Group-specific query message • Leave group message (to lower IGMP’s “leave latency”) • IGMPv3 • -Enables hosts to listen only to a specified subset of the hosts sending to the group

  27. IGMP v2 and V3 improvements

  28. Multicast Distribution Trees • Multicast Distribution Trees • Control the path which IP Multicast traffic takes through the network in order to deliver traffic to all receivers. • Source Trees • A source tree with its root at the source and branches forming a spanning tree through the network to the receivers. • Also referred to as a shortest path tree (SPT). • Remind you of anything??? 

  29. Multicast Source Tree • Advantage of creating the optimal path between the source and the receivers. • The routers must maintain path information for each source. • Memory consumption: O(S * G)

  30. Shared Trees • A single common root placed at some chosen point in the network. This shared root is called a Rendezvous Point (RP). • Advantage of requiring the minimum amount of state in each router. Memory requirements: O(G) • The disadvantage of shared trees is that under certain circumstances the paths between the source and receivers might not be the optimal paths. • The placement of the RP must be carefully considered. • THIS IS THE PREFERED MANNER to implement Multicasts

  31. Multicast Forwarding Multicast Forwarding In unicast routing, a router does not really care about the source address. In multicast routing, the multicast router must determine which direction is upstream (towards the source) and which direction (or directions) is downstream.

  32. Multicast Forwarding • Reverse Path Forwarding (RPF) • An approximation of the source tree. • Make use of the existing unicast routing table to determine the upstream and downstream neighbors. A router will only forward a multicast packet if it is received on the upstream interface. This RPF check helps to guarantee that the distribution tree will be loop free. • Step 1.   Router looks up the source address in the unicast routing table to determine if it has arrived on the interface that is on the reverse path back to the source. • Step 2.   If packet has arrived on the interface leading back to the source, the RPF check is successful and the packet will be forwarded. • Step 3.   If the RPF check in 2 fails, the packet is dropped.

  33. Enhancement • Only forward to your neighbor on a link if the neighbor uses this link to reach the source. • How does the node know? • For link state routing: easy • For distance vector routing • Modify to include next hop information in messages; or • “Poison reverse”

  34. Figure 15.7Taxonomy of common multicast routing protocols JP note: Core Based Trees – Never seen live • MOSPF (RFC 1584) • Extension to OSPF unicast routing protocol • Includes multicast information in OSPF link state advertisements to construct multicast distribution trees • Group membership LSAs are flooded throughout the OSPF routing domain so MOSPF routers can compute outgoing interface lists • Uses Dijkstra algorithm to compute shortest-path tree • Separate calculation is required for each (S Net , G) pair (Source, Group address pair) • Data-driven – a router construct a tree the first time it sees a (S Net , G) pair.

  35. Multicast Routing Protocols • Dense-Mode (DM) • Uses “Push” Model • Traffic Flooded throughout network • Pruned back where it is unwanted • Flood & Prune behavior (typically every 3 minutes) • Sparse-Mode (SM) • Uses “Pull” Model • Traffic sent only to where it is requested • Explicit Join behavior

  36. DVRMPv3 (Internet Draft) • Dense Mode Protocol • Distance vector-based • Similar to RIP • Infinity = 32 hops • Subnet masks in route advertisements • DVMRP Routes used: • For RPF Check • To build Truncated Broadcast Trees (TBTs) • Uses special “Poison-Reverse” mechanism • Uses Flood and Prune operation • Traffic initially flooded down TBT’s • TBT branches are pruned where traffic is unwanted. • Prunes periodically time-out causing reflooding.

  37. Multicast Extension to OSPF (MOSPF) • Enables routing of IP multicast datagrams within single AS • Each router uses MOSPF to maintain local group membership information • Each router periodically floods this to all routers in area • Routers build shortest path spanning tree from a source network to all networks containing members of group (Dijkstra) • Takes time, so on demand only

  38. Forwarding Multicast Packets • If multicast address not recognised, discard • If router attaches to a network containing a member of group, transmit copy to that network • Consult spanning tree for this source-destination pair and forward to other routers if required

  39. Equal Cost Multipath Ambiguities • Dijkstra’ algorithm will include one of multiple equal cost paths • Which depends on order of processing nodes • For multicast, all routers must have same spanning tree for given source node • MOSPF has tiebreaker rule

  40. Inter-Area Multicasting • Multicast groups may contain members from more than one area • Routers only know about multicast groups with members in its area • Subset of area’s border routers forward group membership information and multicast datagrams between areas • Interarea multicast forwarders

  41. Inter-AS Multicasting • Certain boundary routers act as inter-AS multicast forwarders • Run an inter-AS multicast routing protocol as well as MOSPF and OSPF • MOSPF makes sure they receive all multicast datagrams from within AS • Each such router forwards if required • Use reverse path routing to determine source • Assume datagram from X enters AS at point advertising shortest route back to X • Use this to determine path of datagram through MOSPF AS

  42. MOSPF (RFC 1584) • Extension to OSPF unicast routing protocol • Includes multicast information in OSPF LSAs to construct multicast distribution trees • Group membership LSAs are flooded throughout the OSPF routing domain so MOSPF routers can compute outgoing interface lists. • Uses Dijkstra’s algorithm to compute shortest-path tree for each multicast group.

  43. Protocol Independent Multicast (PIM) • Independent of unicast routing protocols • Extract required routing information from any unicast routing protocol • Work across multiple AS with different unicast routing protocols • JP Note: this is usually how we do Multicasting in real life 

  44. PIM Strategy • Flooding is inefficient over large sparse internet • Little opportunity for shared spanning trees • Focus on providing multiple shortest path unicast routes • Two operation modes • Dense mode • For intra-AS • Alternative to MOSPF • Sparse mode • Inter-AS multicast routing

  45. PIM-DM (Internet Draft) • Protocol Independent • Supports all underlying unicast routing protocols including: static, RIP, IGRP, EIGRP, IS-IS, BGP, and OSPF • Uses reverse path forwarding • Floods network and prunes back based on multicast group membership • Assert mechanism used to prune off redundant flows • Appropriate for... • Smaller implementations and pilot networks

  46. PIM-SM (RFC 2362) • Supports both source and shared trees • Assumes no host wants multicast traffic unless it specifically ask for it • Uses a Rendezvous Point (RP) • Senders and Receivers “rendezvous” at this point to learn of each others existence. • Senders are “registered” with RP by their first-hop router. • Receivers are “joined” to the Shared Tree (rooted at the RP) by their local Designated Router (DR). • Appropriate for… • Wide scale deployment for both densely and sparsely populated groups in the enterprise • Optimal choice for all production networks regardless of size and membership density.

  47. Sparse Mode • A sparse group: • Number of networks/domains with group members present significantly small than number of networks/domains in internet • Internet spanned by group not sufficiently resource rich to ignore overhead of current multicast schemes

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