Multicast Communications

1. Multicast Communications Gruppo Reti TLC nome.cognome@polito.it http://www.tlc.polito.it/

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

3. 1. Multicasting • Unicast • 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., 130.192.255.255 • 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 Multiple Unicast: Multicast: Group address 1, 2and 3 Packet duplicated 1 1 Packet duplicated 2 3 2 3

5. Multicast advantages • Plus: • 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) 1 2 3 4 8 16 24 32 0 netid=7 bit hostid=24 bit Class A 1 0 netid=14 bit hostid=16 bit Class B 0 1 1 netid= 21 bit hostid=8 bit Class C 1 1 1 0 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 (46-1500 bytes) Pad Frame Check Seq. (4 bytes)

13. Ethernet adresses • Source and destination addresses: • 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 224.0.0.0 up to 239.255.255.255 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 0 Multicast group

15. Mapping: Ethernet to IP • Ethernet: 48-bit long addresses • A subset of addresses are multicast addresses • from 01.00.5e.00.00.00 to 01.00.5e.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. B A E C D Flooding A sends to group G: {B,C,D}

19. B A E C D Flooding

20. Plus and minus • Advantages: • 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 • 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. B A E C D Shared Spanning Tree A sends to group G: {B,C,D}

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

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

25. Plus and minus • Advantages: • 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. B A E C D Source Based Trees All edges has the same costs

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

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

30. Plus and minus • Advantages: • 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. B A E C D Reverse Path Forwarding All links have the same cost

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

34. Plus and minus • Advantages: • 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. B A E C D TRPB (Truncated RPB)

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

38. Plus and minus • Advantages: • 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. B A E C D Reverse Path Multicast All links have the same costs

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

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

43. B A E C D Reverse Path Multicast 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: • 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)