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

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

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., 130.192.255.255 Multicast A single flow, from a source to a group of destinations IP packets have multicast address, in class D

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

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

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

Multicast in Internet

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

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

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?

Addressing (RFC 1466) Multicast addresses 1 2 3 4 8 16 24 32 Class A 1 2 3 4 8 16 24 32 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

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)

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

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 Multicast group

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?

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

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

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

Flooding B A C D E

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

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

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

Shared Spanning Tree B A C D E Spanning Tree Path

Shared Spanning Tree B A C D E Spanning Tree Path

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

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

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

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

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...

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

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

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

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

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

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

TRPB (Truncated RPB) B A C D E

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

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

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

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

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

Reverse Path Multicast B 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

Reverse Path Multicast B A C D E Will be discarded These branches are chopped C

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

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

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)