Topic: Multicast routing What you will learn Broadcast routing algorithms Multicasting IGMP Multicast routing algorithms Multicast routing in the Internet Multicasting 1/21 Unicasting One source node and one destination node Each router in the path of the packet tries to forward it on one and only oneof its interfaces Network to which the destination node is attached Multicasting 2/21 1
Multicasting One source and a group of destinations The destination address is a group address (a multicast address) A multicast router may have to send out copies of the same packet through more than one interface Multicasting 3/21 Need for multicast addresses A host, which is a member on ngroups, actually has (n+ 1) addresses Multicasting 4/21 2
Multicasting versus Multiple Unicasting Multicasting could be emulated with multiple unicasting, but multicasting requires less bandwidth than multiple unicasting in multiple unicasting, the packets are created by the source with a relative delay between packets; in multicasting there is no delay because only one packet is created Multicasting 5/21 Why are we interested in multicasting? Multicasting has many applications today Access to distributed databases A user s request is multicast to all the database locations and the location that has the information responds Information dissemination Businesses can easily disseminate news to their customers through multicasting Teleconferencing The individuals attending a teleconference all need to receive the same information at the same time Distance learning Lessons taught by one professor can be received by a specific group of students Multicasting 6/21 3
Broadcasting Broadcasting means one-to-all communication: a host sends a packet to all hosts in an internet Broadcasting in this sense is not provided at the Internet level it might create a huge volume of traffic and use a huge amount of bandwidth Partial broadcasting is done in the Internet Gnutella uses application-levelbroadcast in order to broadcast queries among peers A form of sequence-number controlled flooding (see later) is used to broadcast link-state advertisements (LSAs) in the OSPF routing protocol Multicasting 7/21 Broadcasting: uncontrolled flooding Broadcasting can be achieved by a flooding approach: the source sends a copy of the broadcast packet to all of its neighbours when a router receives a broadcast packet, it forwards a copy of the packet to all of its neighbours (except the neighbour from which it received the packet) Problems with uncontrolled flooding in case the graph has cycles: Endless cycling Broadcast storm The flooding should be controlled A router chooses when to flood a packet and when not to flood it Multicasting 8/21 4
Broadcasting: controlled flooding Sequence-number controlled flooding (e.g., in Gnutella) A source node puts its address (or other unique identifier) as well as a sequence number into a broadcast packet A router keeps track of packets already broadcast and broadcasts a packet only if the packet has not been previously forwarded Reverse Path Forwarding A router forwards a packet on all of its outgoing links (except the link on which it was received) only if the packet arrived on the link that is on its own shortest unicast path back to the source S: source R2 R1 R4 Packet that will be forwarded Packet that will not be forwarded R5 R3 R6 R7 Multicasting 9/21 Spanning-Tree broadcast Ideally, every node should receive only one copy of the broadcast packet If a broadcast packet were forwarded only along links within a tree, each and every node would receive exactly one copyof the packet A spanning tree can be used by any node to begin a broadcast A source node sends the broadcast packet out on all of the incident links that belong to the spanning tree A node has to forward a packet to all its neighbours in the spanning tree A A c B c B F E D F E D (a) broadcast initiated at A G G (b) broadcast initiated at D Multicasting 10/21 5
Broadcasting: Spanning-Tree creation Center-based approach to build a spanning tree A center node(a.k.a. rendezvous pointor core) is defined Nodes unicast join messages addressed to the center node A join message is forwarded using unicast routinguntil it either arrives at a node already belonging to the spanning tree or arrives at the center node The path that the join message has followed is grafted onto the existing tree F 1 c A E B (a) stepwise construction of spanning tree (center: E) 4 3 2 D 5 G F c A E B (b) constructed spanning tree D G Multicasting 11/21 Multicasting (in the Internet) Components: Multicast addressing A protocol for collecting information about groups Internet Group Management Protocol (IGMP) Multicast routing protocols Multicasting 12/21 6
Multicast addresses in IPv4 5 bits unused Mapping a multicast IPv4 address to an Ethernet physical address 23 bits IP multicast address MAC multicast address Multicasting 13/21 IGMP Queryand reportmessages in IGMP version 3: A query message is periodically sent by a router to all hosts attached to it to collect information about their membership A report message is sent by a host as a response to a query message If a host needs to join a group, it waits until it receives a query message and then sends a report message If a given host intends to leave a group, simply it does not respond to a query message Then the router infers that the host is no longer a member of that group and removes the related state information IGMP messages are encapsulated within IP datagrams with the destination address 224.0.0.1 for a query message or 224.0.0.22 for a report message, the protocol field set to 2and the TTL field set to 1 ICMP IGMP IP ROUTING PROTOCOLS Network ARP RARP Multicasting 14/21 7
Multicast routing algorithms Goal of multicast routing: to find a tree of links (multicast tree) that connects all the routers that have attached hosts belonging to a given multicast group Two approaches: Group-shared Tree Source-based Tree Multicast hosts, their attached routers, and other routers Multicasting 15/21 Group-shared Tree approach Only a single shared routing tree is constructed for the entire multicast group (regardless of the sender) A center-based approach (see broadcasting) is used to build the multicast routing tree. A router (center router) is designed to act as the representative for the group Multicasting 16/21 8
Constructing a center-based multicast tree Routers with attached hosts belonging to the multicast group send join messages addressed to the center router A join message is forwarded using unicast routing towards the center router until it either arrives at a router that already belongs to the multicast tree or arrives at the center router The path that the join message has followed is grafted onto the existing multicast tree Core router Multicasting 17/21 Source-Based Tree approach It provides for constructing a multicast routing tree for each individual source in the multicast group In practice, an RPF algorithm (with source S) is used to construct a multicast tree for multicast datagrams originating at source S Problem: D will forward packets to router G, even though router Ghas no attached hosts belonging to the multicast group Multicasting 18/21 9
Solution: Pruning A multicast router that receives multicast packets and has no attached hosts joined to that group will send a prune message to its upstream router If a router receives prune messages from each of its downstream routers, it can forward a prune message upstream. Multicasting 19/21 Multicast routing in the Internet Distance-Vector Multicast Routing Protocol (DVMRP) Source-based tree with RPF and pruning Protocol-Independent Multicast (PIM) routing protocol PIM-Dense Mode(PIM-DM) the number of routers with attached group members is large with respect to the total number of routers Source-based tree approach similar to DVRMP It is reasonable to assume that many or most of the multicast routers need to be involved in routing multicast packets PIM-Sparse Mode(PIM-SM) the number of routers with attached group members is small with respect to the total number of routers Group-shared tree approach A router should not be involved in multicast distribution unless it wants to join a multicast group Multicasting 20/21 10
Tunneling How can multicast routing be employed in the Internet? Multicast routers can be connected by a tunnel In the outer IP header: Source IP address: IP address of the sending multicast router (at one end of the tunnel) Destination IP address: IP address of the multicast router at the other end of the tunnel multicast router unicast router Multicasting 21/21 11