1. CMPE 252A: Computer Networks
Lecture 13
CMPE252A Fall 2004

2. Multicast Routing
CMPE252A Fall 2004

3. Unicast versus Multicast
Unicast: one to one.
Multicast: many to many.
General case.
nXm.
Two main functions:
Efficient data distribution.
Logical naming of a group.
CMPE252A Fall 2004

4. Network-Supported Multicast
As opposed to application-layer multicast.
What’s the difference?
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5. Multicast through unicast
Src
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6. Multicast through unicast
Src
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7. Multicast through unicast
Src
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8. Multicast through unicast
Src
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9. Multicast through unicast
Src
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10. Multicast
Src
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11. Multicast
Src
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12. Multicast
Src
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13. Multicast
Src
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14. Multicast
Src
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15. Multicast state
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16. Multicast state Router:
Learns of existence of multicast groups through advertisements (IGMP).
Identify links with group members.
Establish state to route packets.
Replicate packets on appropriate interfaces.
Routing entry:
Destination (mcast group)
List of outgoing interfaces
CMPE252A Fall 2004

17. Multicast groups Members are the intended receivers.
Senders may or may not be members.
Destination address is class D IP address.
Globally known portion of address space.
Hosts may belong to many groups.
Hosts may send to many groups.
Support dynamic group membership.
CMPE252A Fall 2004

18. Example applications Broadcast audio/video. Push-based systems.
Software distribution.
Web-cache updates.
Teleconferencing (audio, video, shared whiteboard, text editor).
Multi-player games.
Server/service location.
CMPE252A Fall 2004

19. Components of the IP Multicast Architecture
service model
hosts
host-to-router protocol (IGMP)
routers
multicast routing protocols (various)
CMPE252A Fall 2004

20. IP Multicast Service Model (RFC-1112)
Each group identified by single IP address.
Groups may be of any size.
Members of groups may be located anywhere in the Internet.
Members of groups can join and leave at will.
Senders need not be members.
CMPE252A Fall 2004

21. Service model (cont’d)
Group membership not known explicitly.
Analogy:
Each multicast address is like a radio frequency, on which anyone can transmit, and to which anyone can tune-in.
CMPE252A Fall 2004

22. IP Multicast Addresses
Class D IP addresses:
in “dotted decimal” notation: —
Two administrative categories:
“Well-known” multicast addresses, assigned by ICANN.
“Transient” multicast addresses, assigned and reclaimed dynamically, e.g., by “sdr” program. 1
group ID
CMPE252A Fall 2004

23. IP Multicast Service — Sending
Uses normal IP-Send operation, with an IP multicast address specified as the destination.
Application can:
Specify IP time-to-live (TTL) on outgoing packet.
Scope.
Enable/disable loop-back if the sending host is/not a member of the destination group.
CMPE252A Fall 2004

24. IP Multicast Service — Membership and Receiving
Two new operations:
Join-IP-Multicast-Group ( group-address, interface )
Leave-IP-Multicast-Group ( group-address, interface )
Receive multicast packets for joined groups via normal IP-Receive operation.
CMPE252A Fall 2004

25. IGMP
CMPE252A Fall 2004

26. Components of the IP Multicast Architecture
service model
hosts
host-to-router protocol (IGMP)
routers
multicast routing protocols (various)
CMPE252A Fall 2004

27. IGMP
Protocol used by hosts to report their multicast group memberships to neighboring routers.
Version 1, the current Internet Standard, is specified in RFC-1112.
Operates over broadcast LANs and point-to-point links.
Occupies similar position and role as ICMP in the TCP/IP protocol stack.
CMPE252A Fall 2004

28. Relevant protocol layers
Upper-Layer Protocol Modules
IP Service Interface
ICMP
IGMP
IP Module
Link-Layer Service Interface
IP to link-layer address
mapping (e.g., ARP)
Link-Layer Modules
(e.g., Ethernet)
CMPE252A Fall 2004

29. IGMP messages
Mcast routers send periodic IGMP query messages with TTL=1.
One router per subnet is designated IGMP querier.
Group address= 0.
Hosts send 1 reply message per group.
Random response timers.
Suppression.
Do routers need to know group membership?
CMPE252A Fall 2004

30. How IGMP Works On each subnet, 1 router is elected the “querier”.
routers:
hosts:
On each subnet, 1 router is elected the “querier”.
Querier periodically sends a query message to the all-systems group ( ), with TTL=1.
On receipt, hosts start random timers (between 0 and 10 seconds) for each multicast group to which they belong.
CMPE252A Fall 2004

31. How IGMP Works (cont’d)Q G G G G
When host’s timer for group G expires, it sends a Membership Report to group G, with TTL = 1.
Other members of G hear the report and stop their timers.
Routers hear all reports, and time out non-responding groups.
CMPE252A Fall 2004

32. How IGMP Works (cont’d)
Ideally, only one report message per group sent in response to a query.
(routers need not know who all the members are, only that members exist)
Query interval is typically 60—90 seconds.
When a host first joins a group, it sends one or two immediate reports, instead of waiting for a query.
CMPE252A Fall 2004

33. Multicast routing
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34. Multicast routing
Multicast service model doesn’t require knowledge about group membership.
Scalability.
Anonymity.
Dynamic join/leave.
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35. Multicast routing trees
Span all receivers in a group.
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36. Shared & source-based trees
Separate shortest path tree for each sender.
Shared trees:
Single tree shared by all members.
Data flows on same tree regardless of sender.
CMPE252A Fall 2004

37. Shared tree RP
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38. Source-based trees S1 S2
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39. Source- versus shared trees
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40. Shared & source-based trees
Shortest path trees - low delay, better load distribution.
More state at routers (per-source state).
Efficient for dense-area multicast.
Shared trees
Higher delay.
Traffic concentration.
Per-group state at routers.
Efficient for sparse-area multicast.
CMPE252A Fall 2004

41. Routing protocol taxonomy
DVMRP - source-based trees.
MOSPF - source-based trees.
CBT- shared trees.
PIM - shared (SM) and source-based (DM) trees
CMPE252A Fall 2004

42. Distance-vector multicast routing protocol (DVMRP)
CMPE252A Fall 2004

43. Distance-Vector Multicast Routing Protocol
DVMRP consists of two major components:
Routing: conventional distance-vector routing protocol.
Forwarding: how to forward multicast packets, based on the routing table.
CMPE252A Fall 2004

44. Multicast forwarding A DVMRP router forwards a packet if
The packet arrived from the link used to reach the source of the packet (reverse path forwarding - RPF).
If downstream links have not been pruned from tree.
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45. Example g g s g
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46. Phase 1: Flood g g s
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47. Phase 2: Prune g g
prune (s,g)
prune (s,g) s g
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48. Phase 3: Graft g g g s g report (g) graft (s,g) graft (s,g)
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49. Phase 4: Steady State g g g s g
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50. Multicast Routing: MOSPF
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51. Multicast OSPF (MOSPF)
Add-on to OSPF (Open Shortest-Path First).
Link-state, intra-domain routing protocol.
Multicast-capable routers flag link state routing advertisements.
Each router indicates groups for which there are directly-connected members.
CMPE252A Fall 2004

52. MOSPF (cont’d)
Link-state advertisements augmented with multicast group addresses to which local members have joined.
Link-state routing algorithm augmented to compute shortest-path distribution tree from any source to any set of destinations.
CMPE252A Fall 2004

53. Link state: each router floods link state advertisement.
Multicast: add membership information to “link state”.
Each router computes multicast tree for each active source,
builds forwarding entry with outgoing interface list. S1 R1 R2 X Y Z
CMPE252A Fall 2004

54. Z has network map, including membership at X and Y.
Z computes shortest path tree from S1 to X and Y.
Z builds multicast entry with one outgoing interface.
W, Q, R, each build multicast entries. S1 Z W Q X R R1 Y R2
CMPE252A Fall 2004

55. Link state advertisement with new topology may require
re-computation of tree and forwarding entry. S1 Z W Q X R R1 Y R2
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56. R3 T Link state advertisement (T) with new membership (R3) may
require incremental computation and addition of interface
to outgoing interface list (Z) S1 Z R3 W Q T X R R1 Y R2
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57. Impact on route computation
Can’t pre-compute all source multicast trees.
Compute on demand when first packet from a source S to a group G arrives.
CMPE252A Fall 2004

58. Processing link-state advertisements
New link-state advertisement may lead to addition or deletion of outgoing interfaces if it contains different group addresses.
New link-state advertisement may lead to re-computation of entire tree if routes between source and destinations are affected.
CMPE252A Fall 2004

59. Evaluation Graceful add-on to OSPF and computes shortest-path trees.
Shares scaling benefits and limitations of OSPF.
Other scaling questions re. dynamics of on-demand and re-computation for volatile groups.
CMPE252A Fall 2004

60. Cores, centers, and rendezvous points
CMPE252A Fall 2004

61. Rendezvous With source-based trees senders and receivers meet by:
Flooding and pruning.
LS distribution of group and receiver state.
How do we solve the problem with shared trees?
Establish a meeting place: center, core or rendezvous point.
CMPE252A Fall 2004

62. Core-based trees (CBT)
Trees are receiver-based.
Core: router from which branches emanate.
Branches made up of other non-core routers.
Branches are shortest-path between member hosts and core (reverse SPT).
CMPE252A Fall 2004

63. Building shared tree Join Request Join ACK Join Request forwarded to
Core
Join Request forwarded to
next router on the path
to core until it reaches core
or router that is already in the
tree.
Join Request
Join ACK
CMPE252A Fall 2004

64. Sending data: sender near core
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65. Sending data: sender away from core
Data is unicast toward
core; once it reaches core
or non-core router, data is
Multicast along shared tree.
CMPE252A Fall 2004

66. Observations Core placement affects efficiency.
Finding the optimal core location is an NP-hard problem - use heuristics.
For dynamic groups core location may have to be calculated frequently.
Need multiple cores for robustness.
CMPE252A Fall 2004

67. Protocol Independent Multicast (PIM)
CMPE252A Fall 2004

68. PIM
PIM’s goal: address wide-area multicast by establishing efficient distribution trees both for dense and sparse groups.
Router state.
Control and data message processing.
Unicast protocol independence:
Use of routing tables independent of how they are computed.
CMPE252A Fall 2004

69. Broadcast-and-Prune (DVMRP)
Simple, data-driven (dense-mode multicast).
Broadcast first packets throughout network.
Routers with no downstream members send prune messages and store prune state.
Does not scale well as a long-term, general solution for internet-wide multicast
Broadcast and prune has excessive bandwidth and state overhead for sparse groups in the wide area.
CMPE252A Fall 2004

70. Broadcast-and-prune overhead
multicast distrib. tree
broadcast-and-prune overhead R1
CMPE252A Fall 2004 R2