1. Methods and Needs for Multicast Applications
Siavash Samadian Barzoki
University of Tehran

2. Multicast Addressing
multicast address  a set of Internet hosts comprising a multicast group.
Senders : Dest. Address = multicast address
IP multicast group Class D address
Class D address : bit multicast group ID  to
bits
Multicast Group ID
is reserved.
to  reserved for permanent assignments to various uses, including routing protocols and other protocols that require a well-known permanent address.

3. Multicast Addressing Some of the well-known groups include :
“all systems on this subnet”
“all routers on this subnet”
“all DVMRP routers”
“all OSPF routers”
“all OSPF designated routers”
“all RIP2 routers”
“all PIM routers”
“all CBT routers”
The remaining groups are either permanently assigned to various multicast applications or are available for dynamic assignment ( to for “Administratively scoped” applications)

4. Mapping a Class D Address to an IEEE-802 MAC Address
The block begin with E (hex) of the IEEE-802 MAC-layer multicast address space is used for IP multicast groups.
The low order 23 bits of class D address is replaced by the low order 23 bits of MAC-layer multicast address block.
(EA-8A-08-05) :
(E0-0A-08-05) :
(E1-8A-08-05) :
IEEE-802 MAC-layer multicast address :
Hex: E A

5. IP Multicast MAC Address Mapping (FDDI & Ethernet)
Be Aware of the 32:1 Address Overlap
32 - IP Multicast Addresses .
1 - Multicast MAC Address
(FDDI and Ethernet)
0x0100.5E

6. Internet Group Management Protocol (IGMP)
Hosts
Host-to-Router Protocols (IGMP)
Routers
Multicast Routing Protocols (PIM)

7. Internet Group Management Protocol (IGMP)
How hosts tell routers about group membership
RFC 1112 and RFC 2236 specify IGMPv1 and IGMPv2
IGMPv3 work-in-progress
More than 1 router on a LAN  a router is elected as a querier
The querier is responsible for periodically querying the LAN for the presence of any group members
Group Specific Query Message, TTL=1 is sent
The members if any  set a random timer
On timer expiration  the host sends a Report message and the other members suppress their same Reports

8. Internet Group Management Protocol (IGMP)
A host first joins a group  sends an IGMP Report rather than waiting for a router’s IGMP Query to reduce Join Latency
Join Latency : the interval from a host’s first IGMP Report sent until the first packet for that group arrives on that host’s subnetwork.
A host leaves a group  sends an Leave Group message
Inclusion Group-Source Report message to select the specific sources and Exclusion Group-Source Report message to block some sources’ traffic

9. Internet Group Management Protocol (IGMP)
Joining a Group H3 H2
Report H3 H1
Host sends IGMP Report to join group

10. Internet Group Management Protocol (IGMP)
Source =
Group =
Source =
Group = R1 R2
H1 wants to receive from S = but not from S =
With IGMP, specific sources can be pruned back - S = in this case R3
IGMPv3:
Join ,
Leave ,
H1 - Member of

11. Multicast Routing in Extended LANs
Broadcast based :
Single-Spanning Tree Multicast Routing
RPF (Reverse Path Flooding)
RPB (Reverse Path Broadcasting)
Multicast based :
TRPB (Truncated Reverse Path Broadcasting)
RPM (Reverse Path Multicasting)

12. Single-Spanning Tree Multicast Routing
Compute a spanning tree among the bridges using a distributed algorithm
A packet can be sent on the spanning tree
There can be a membership learning algorithm
A member of group G sends a membership-report to all-bridges group B periodically (Src. Adr.= G, Dest. Adr. = B)
The bridge adds G to a table entry (address, (outgoing-branch, age), (outgoing-branch, age), …) where address is set to G and outgoing-branch to the interface it received the report message
When a packet received by the bridge with Dest. Adr. = G, it will send it on the outgoing-branches which are in the table

13. Single-Spanning Tree Multicast Routingd G G G a b c G G

14. Single-Spanning Tree Multicast Routing
Advantages :
Relatively easy to implement  a great deal of experience with spanning tree protocols in Internet community
Disadvantages :
Centralize traffic on a small number of links
Computationally difficult to compute a spanning tree in large, complex topologies

15. Reverse Path Flooding (RPF)
A router forwards a broadcast packet originating at source S if and only if it arrives via the shortest path from the router back to S (i.e., the “reverse path”). Otherwise the packet will be discarded
The router forwards the packet out all incident links except the one on which the packet arrived.
Disadvantage :
Any single broadcast packet may be transmitted more than once across any link, up to the number of routers that share the link.

16. Reverse Path Flooding (RPF)
What is RPF?
A router forwards a multicast datagram if received on the interface used to send unicast datagrams to the source
Unicast B C
Receiver
Source A F D E
Multicast

17. Reverse Path Flooding (RPF)
The RPF Check
The routing table used for multicasting is checked against the “source” IP address in the packet.
If the datagram arrived on the interface specified in the routing table for the source address; then the RPF check succeeds.
Otherwise, the RPF Check fails.

18. Reverse Path Flooding (RPF)
A closer look: RPF Check Fails
Multicast Packet from
Source X
Discard Packet! S0
RPF Check Fails! S1 S2
Unicast Route Table
Network Interface
/16 S1
/24 S0
/24 E0 E0 S1
Packet Arrived on Wrong Interface!

19. Reverse Path Flooding (RPF)
A closer look: RPF Check Succeeds
Multicast Packet from
Source S0 S1 S2
RPF Check Succeeds! E0
Unicast Route Table
Network Interface
/16 S1
/24 S0
/24 E0 S1
Packet Arrived on Correct Interface!
Forward out all outgoing interfaces. (i. e. down the distribution tree)

20. Reverse Path Broadcasting (RPB)
Eliminates the duplicate broadcast packets generated by Reverse Path Forwarding
It is necessary for each router to identify which of its links are “child” links in the shortest reverse path tree rooted at any given source S.
When a broadcast packet originating at S arrives via the shortest path back to S, the router can forward it out only the child links for S.

21. Reverse Path Broadcasting (RPB)5 6 X Y
Child link
D=5
D=6 a
D=6 Z b

22. Reverse Path Broadcasting (RPB)
Advantages :
Easy to implement
Saving of link bandwidth and of host and router processing time achieved by eliminating duplicate broadcast packets.
Disadvantages :
Extra storage consumed by the children bit-map in each routing table entry
It does not take into account multicast group membership  extraneous datagrams may be forwarded onto subnetworks that have no group members

23. Truncated Reverse Path Broadcasting (TRPB)
The previous algorithms used broadcasting and so were wasting the bandwidth on the links that had no group members.
In TRPB only non-member leaf networks are deleted from each broadcast tree.
Leaf networks are the networks which have no downstream router.
So the leaf networks must be found out and it must be determined if there is any member on the leaf network or not then.

24. Reverse Path Multicasting (RPM)
Provides on-demand pruning of shortest-path multicast trees.
When a source first sends a multicast packet to a group, it uses TRPB algorithm.
When the packet reaches a router for whom all of the child links are leaves and none of them have members of the destination group, a non-membership report (NMR) for that (source, group) pair is generated and sent back to the router that is one hop towards the source.
If the one-hop-back router receives NMRs from all of its child routers, and if its child links also have no members, it in turn sends an NMR back to its predecessor.
Subsequent multicast packets from the same source to the same group are blocked from traveling down the unnecessary branches by the NMRs sitting in intermediate routers.
A non-membership report includes an age field that is used to cancel the NMR effect after some time.

25. Reverse Path Multicasting (RPM)
Send based on TRPB
Send Prune message R G
Prune the branches R G R R R G R G

26. Intra-Domain Multicast vs. Inter-Domain Multicast
1992 to 1997  standardization and deployment focused on a single flat topology
Beginning in 1997  the need for a hierarchical multicast infrastructure and inter-domain routing
The existing protocols then categorized as intra-domain protocol

27. Multicast protocols – Distribution Trees
Shortest Path or Source Distribution Tree
Source 1
Notation: (S, G)
S = Source
G = Group
Source 2 B A D F C E
Receiver 1
Receiver 2

28. Multicast protocols – Distribution Trees
Shared Distribution Tree
Source 1
Notation: (*, G)
* = All Sources
G = Group
Source 2 A B
D (Shared Root) F C E
Receiver 1
Receiver 2

29. Multicast Protocols - Characteristics
Distribution trees
Source tree
Uses more memory O(S x G) but you get optimal paths from source to all receivers, minimizes delay
Shared tree
Uses less memory O(G) but you may get suboptimal paths from source to all receivers, may introduce extra delay
Protocols
PIM, DVMRP, MOSPF, CBT

30. Multicast Protocols - Characteristics
Types of multicast protocols
Dense-mode : The receivers are densely populated
Broadcast and prune (flood & prune) behavior
Similar to radio broadcast
Sparse-mode : The receivers are sparsely populated
Explicit join behavior
Similar to pay-per-view

31. Multicast Protocols - DVMRP
Stands for Distance Vector Multicast Routing Protocol
First protocol implemented on MBone
Broadcast and prune
Source Distribution Trees
Dense Mode Protocol
Uses own routing table (Distance Vector similar to RIP)
Many implementations
Disadvantages :
Slow Convergence — RIP-like behavior
Significant amount of multicast routing state information stored in routers — (S,G) everywhere
Protocol Dependent

32. Multicast Protocols - MOSPF
Stands for Multicast Extensions to OSPF (Open Shortest Path First)
Information about group receivers are included in the attached Link State (LS)
Flood the area with LS  same view of group membership in the area
Source Tree  Each router uses Dijkstra algorithm to compute shortest-path tree for each source and group
Dense Mode or Sparse Mode ?!  Membership information is broadcasted so dense mode , but data is sent to those receivers that specifically request it so sparse mode

33. MOSPF - Membership LSA’s
Area 0
MABR1
MABR2
Membership LSA’s
Membership LSA’s
Area 1
Area 2 MB MA

34. MOSPF - Intra-Area Traffic
Not receiving (S2 , A) traffic
MABR1
MABR2
Area 1
Area 2 MB
(S1 , B)
(S2 , A) MA

35. MOSPF - Inter-Area Traffic
MABR routers inject Summary Membership LSAs into Area 0.
Wildcard Receivers “pull” traffic from all sources in the area.
Summarized
Membership LSA
(GA , GB)
(GA )
Wildcard Receiver Flag
(*, *)
MABR1
MABR2
Membership LSA’s
Membership LSA’s
Area 1
Area 2 MB
(S1 , B)
(S2 , A) MA

36. MOSPF - Inter-Area Traffic
Unnecessary traffic still flowing to the MABR Routers!!
Wildcard Receiver Flag
(*, *)
MABR1
MABR2
Area 1
Area 2
(S1 , B)
(S2 , A)

37. MOSPF - Inter-Area Traffic
MASBR
External AS
(S1 , A)
Summarized
Membership LSA
(S2 , B)
(GA , GB)
(GA )
MABR1
MABR2
Membership LSA’s
Membership LSA’s
Area 1
Area 2 MB MA MA MA

38. MOSPF - Inter-Area Traffic
Unnecessary traffic may flow all the way to the MASBR Router!!
MASBR
External AS
Wildcard Receiver Flag
(*, *)
MABR1
MABR2
Area 1
Area 2
(S1 , B)
(S2 , A)

39. MOSPF - Disadvantages
Protocol dependent - works only in OSPF-based networks
Significant scaling problems
Dijkstra algorithm run for EVERY multicast (SNet, G) pair!
Dijkstra algorithm rerun when:
Group Membership changes
Line-flaps

40. PIM-DM Stands for Protocol Independent Multicast – Dense Mode
Broadcast and prune
Source Distribution Trees
Fewer implementations than DVMRP
Branches that don't care for data are pruned
Grafts to join existing source tree
Uses asserts to determine forwarder for multi-access LAN
PIM-DM vs. DVMRP : DVMRP tries to avoid sending unnecessary packets to neighbors who will then generate prune messages based on a failed RPF check. PIM-DM avoids this complexity, but the tradeoff is that packets are forwarded on all outgoing interfaces. Unnecessary packets are often forwarded to routers which must then generate prune messages because of the resulting RPF failure.

41. PIM-DM I Gets Pruned E’s Prune is Ignored G’s Prune is Overridden
C and D Assert to Determine Forwarder for the LAN, C Wins
Initial Flood of Data and Creation of State
New Receiver, I Sends Graft
Prune to Non-RPF Neighbor
Source
Link
Data
Control
Prune D F I B C A E G H
Prune
Asserts
Join Override
Prune
Graft
Receiver 3
Receiver 1
Receiver 2

42. PIM-SM (Overview) Explicit join model RPF check depends on tree type
Receivers join to the Rendezvous Point (RP)
Senders register with the RP
Data flows down the shared tree and goes only to places that need the data from the sources
Last hop routers can join source tree if the data rate warrants by sending joins to the source
RPF check depends on tree type
For shared trees, uses RP address
For source trees, uses Source address

43. PIM-SM (Overview) Only one RP is chosen for a particular group
RP statically configured or dynamically learned (Auto-RP, PIM v2 candidate RP advertisements)
Data forwarded based on the source state (S, G) if it exists, otherwise use the shared state (*, G)
RFC “PIM Sparse Mode Protocol Spec”

44. PIM-SM (Shared Tree Join)RP
(*, G) State created only
along the Shared Tree.
(*, G) Join
Shared Tree
Receiver

45. PIM-SM (Sender Registration)RP
Source
(S, G) State created only
along the Source Tree.
Traffic Flow
Shared Tree
(S, G) Join
(S, G) Register
(unicast)
Source Tree
Receiver

46. PIM-SM (Sender Registration)RP
Source
(S, G) traffic begins arriving at the RP via the Source tree.
Shared Tree
Traffic Flow
RP sends a Register-Stop back to the first-hop router to stop the Register process.
Source Tree
(S,G) Register-Stop
(unicast)
(S, G) Register
Receiver

47. PIM-SM (Sender Registration)RP
Source
Source traffic flows natively along SPT to RP.
Shared Tree
Traffic Flow
From RP, traffic flows down the Shared Tree to Receivers.
Source Tree
Receiver

48. PIM-SM (SPT Switchover)RP
Source
Last-hop router joins the Source Tree.
Shared Tree
Traffic Flow
Additional (S, G) State is created along new part of the Source Tree.
(S, G) Join
Source Tree
Receiver

49. PIM-SM (SPT Switchover)RP
Source
Traffic begins flowing down the new branch of the Source Tree.
Shared Tree
Traffic Flow
Additional (S, G) State is created along the Shared Tree to prune off (S, G) traffic.
Source Tree
(S, G)RP-bit Prune
Receiver

50. PIM-SM (SPT Switchover)RP
Source
(S, G) Traffic flow is now pruned off of the Shared Tree and is flowing to the Receiver via the Source Tree.
Traffic Flow
Shared Tree
Source Tree
Receiver

51. PIM-SM (SPT Switchover)RP
Source
(S, G) traffic flow is no longer needed by the RP so it Prunes the flow of (S, G) traffic.
Traffic Flow
Shared Tree
Source Tree
(S, G) Prune
Receiver

52. PIM-SM (SPT Switchover)RP
Source
(S, G) Traffic flow is now only flowing to the Receiver via a single branch of the Source Tree.
Traffic Flow
Shared Tree
Source Tree
Receiver

53. PIM-SM Advantages: Traffic only sent down “joined” branches
Can switch to optimal source-trees for high traffic sources dynamically
Unicast routing protocol-independent
Basis for inter-domain multicast routing

54. IP Multicast Protocol Characteristics
Dense
Distribution
Sparse
Distribution
Extensible to IDMR
Protocol-
Independent
Standards
Status
Industry
Usage
Efficient
PIM-DM X X X X X X
DVMRP X X X X
MOSPF X X X X X X X
CBT X
PIM-SM X X X X X X

55. Any Questions ?
Multicasting