1. Multicast Communications
Gruppo Reti TLC

2. Index Multicast advantages Addressing in multicast
Multicast Routing Protocols
Multicast in the Internet
IGMP

3. 1. Multicasting Unicast Broadcast Multicast
A single flow, from a source to a destination
IP packets have unicast or broadcast address
Broadcast
A single flow, from a source to all nodes
Multicast address hostid=all ones, e.g.,
Multicast
A single flow, from a source to a group of destinations
IP packets have multicast address, in class D

4. Multicast vs. Multiple Unicast
Group address
1, 2 and 3
Packet
duplicated 1 1
Packet
duplicated 2 3 2 3

5. Multicast advantages Plus: Requirements: Minor load on the source
The source sends a single copy of each packet
Better capacity utilization for the network
A single copy of a packet can travel on a link
Requirements:
Group management
Packet replication at network nodes
From a single input port to a set of output ports

6. Multicast applications
Push data
Video, audio streams
Resource discovery
Multicast queries can be used to find information if the destination is unknown, or includes multiple nodes (e.g., RIP/OSPF)
Multimedia collaboration
Video conference, e-learning

7. Multicast in Internet

8. IP Multicast: RFC 1112 Destination of messages are groups of hosts
The sender can be part or not of the destinations
A multicast address indentifies the destination group
Special subset of addresses
No limit on the number of hosts in a group
Group members can be anywhere in the network
Members are unknown to the sender
Members can join or leave at any moment

9. IP Multicast Packets are not sent outside the group
Multicast trees are used to route packets
On a LAN, a single copy of each packet is sent (LAN Multicast)
Packet must reach only interested members => group management is mandatory
The network should find the best multicast tree

10. Reliable Multicast What if you want reliable multicast:
Retransmission management are complex, and often it is easier to send n unicast flows….
Ack-implosion: members can overflow the sender with ACKs!
The source does not know the members!
“Crying baby”: the slowest receiver can limit the performance of all members
Heterogeneous Group size, link speed, members’ performance
Applications may have different requirements (not all need a la TCP reliability)
How to solve this problem?

11. Addressing (RFC 1466) Multicast addresses 1 2 3 4 8 16 24 32 Class A
Class A
netid=7 bit
hostid=24 bit
Class B 1
netid=14 bit
hostid=16 bit 1 1
Class C
netid= 21 bit
hostid=8 bit 1 1 1
Classe D
Multicast addresses
28 bit riservati 1 1 1 1
Classe E

12. Multicast: layer 2 and layer 3
Multicast bit
Destination
(6 bytes)
Source
(6 bytes)
Length (2 bytes)
Data
( bytes)
Pad
Frame Check Seq.
(4 bytes)

13. Ethernet adresses Source and destination addresses: Two types
6 bytes ciascuno
Two types
Physical address: identifies a single linecard
Multicast address: identifies a group of linecards
The first bit makes it clear
0 = physical address
1 = multicast address

14. Multicast addressing in IP
Class D address
From up to
host group: is the set of group members
multicast source: the sender in a host group
Only UDP at layer 4 is supported
28 bits 1 1 1
Multicast group

15. Mapping: Ethernet to IP
Ethernet: 48-bit long addresses
A subset of addresses are multicast addresses
from e to e.7f.ff.ff
23 bit to identify 2^23 multicast groups
IP has 28 bit to identify a multicast group
Only the least-significant 23 bits of the IP address can be mapped to the 23 bits of the ethernet address
Conflicts?

16. Multicast routing protocols
Flooding
Shared Spanning Tree
Source-Based Spanning Trees
Reverse Path Forwarding (RPF)
Truncated Reverse Path Broadcast (TRPB)
Reverse Path Multicasting (RPM)

17. Flooding Same algorithm that can be used for unicast
A router forwards a packet to all ports, but the one the it arrived from
Router have to track packet sequence numbers to identify duplicated
Packets with an old sequence number are discarded

18. Flooding
A sends to group G: {B,C,D} B A C D E

19. Flooding B A C D E

20. Plus and minus Advantages: Drawbacks: Simple to implement
No need to manage groups
Everybody gets everything
Drawbacks:
Routers have to store packet sequence number. Can be humongous
Several copies of the same packets are received
Packets go to everyone, even if not interested

21. Shared Spanning Tree Build a spanning tree Forwarding:
A tree that spans through all the network
May be inside into a single Autonomous System
Forwarding:
A router forwards a multicast packet to all ports in the spanning tree, but the one it was received from
No loops: no duplicate packets

22. Shared Spanning Tree
A sends to group G: {B,C,D} B A C D E

23. Shared Spanning Tree B A C D E
Spanning Tree Path

24. Shared Spanning Tree B A C D E
Spanning Tree Path

25. Plus and minus Advantages: Drawbacks:
Traffic goes on a subset of links
No packet duplicate
Drawbacks:
The network has to manage the spanning tree
Shared spanning tree: everybody receives everything even if not interested
Link of the spanning tree may be congested

26. Source Based Trees Setup a single spanning tree rooted at each source
Each source-based spanning tree is explicitily built using the shortest paths from the source to each destination

27. Source Based Trees
All edges has
the same costs B A C D E

28. Source Based Trees etc... Spanning Tree Path for (A, G)
Spanning Tree Path for (C, G)
etc...

29. Source Based Trees etc... Spanning Tree Path for (A, G)
A sends to group G: {B,C,D} B A C D E
Spanning Tree Path for (A, G)
Spanning Tree Path for (C, G)
etc...

30. Plus and minus Advantages: Drawbacks:
Packets are routed using minimum cost paths
No duplicate packet
Drawbacks:
A single Source Based Tree is build for each sender
Multicast routing tables grow large (one entry for each source)
Everybody receive everything

31. Reverse Path Forwarding
Also known as Reverse Path Broadcast
RPF is a simple algorithm to setup source-based spanning tree
It uses the same unicast routing tables!
Forwarding:
When a packet is received,
If the packet is received from a port that would be used to forward packets to the source, forwad the packet to all the ports (but the one you received it from)
If the packets arrives from any other port, drop it

32. Reverse Path Forwarding
All links have
the same cost B A C D E

33. Reverse Path Forwarding
A sends to group G: {B,C,D} B A C D E
Will be discarded
Will be discarded C

34. Plus and minus Advantages: Disadvantages:
Efficient and simple to be implemented
No need to store packet sequence numbers
Use the same information of the unicast routing tables
Disadvantages:
Some duplicates packets are generated
Everybody keeps receiving everything, even if not interested

35. TRPB (Truncated RPB) Extension of RPF
Signaling is used to identify interested nodes
Internet Group Management Protocol (IGMP)
If the router does not get any messages from interested nodes, it drops forwarding packet on that port

36. TRPB (Truncated RPB) B A C D E

37. TRPB (Truncated RPB) All links have the same costs
A sends to group G: {B,C,D}
All links have
the same costs B A C D E
Will be discarder
Will be discarded
The router knows that E is not interested
and stops forwarding packets C

38. Plus and minus Advantages: Disadvantages:
Same as RPB, but only interested nodes get the packets
Disadvantages:
Packets get to all routers in the network

39. Reverse Path Multicast (RPM)
Generalization of the idea of dropping links going to uninterested hosts
If a multicast packet arrives to a router that has no interested member, it sends a “prune” message to the sending router (explicit leave)
Prune messages allow the parent router to know if there is no one interested in the multicast packets

40. Reverse Path Multicast
All links have
the same costs B A C D E

41. Reverse Path Multicast
A sends to group G: {B,C,D} B A C D E
Will be discarded
Will be discarded
The router knows that E is not interested
and stops forwarding packets C

42. Reverse Path MulticastB A C D E
Will be discarded
Will be discarded
Prune messages
The router knows that E is not interested
and stops forwarding packets C

43. Reverse Path MulticastB A C D E
Will be discarded
These branches are
chopped C

44. Reverse Path Multicast
Pruned branches will automatically “grow” again after a timeout
This allows new members to be reached again by the information
Router may use explicit signaling messages (graft messages) to request parents to start forwarding packets to it again (explicit join)
Graft messages speed-up the multicast group join procedure

45. Plus and Minus Advantages: Drawbacks: Same as TRPB
Plus only interested routers gets the packets
Drawbacks:
Complexity grows: prune (and graft) messages

46. Multicast in Internet Multicast protocols
Distance Vector Multicast Routing Protocol (DVMRP – RFC 1075)
Multicast OSPF (MOSPF – RFC 1584)
Core Based Trees (CBT – RFC 2201)
Protocol Independent Multicast (PIM – RFC 2362, RFC 3873)