Some applications require data to be delivered from a sender to multiple receivers. Examples of such applications include audio and video broadcasts, real-time delivery of stock quotes, and teleconferencing applications. A service where data is delivered from a sender to multiple receivers is called multipoint communication or multicast, and applications that involve a multicast delivery service are called multicast applications.

Bellow Figure compares multicast to other communication paradigms. In unicast or point-to-point communication, data is sent to a single host. In broadcast or one-to-all communications, data is transmitted to all hosts with respect to a given scope, for example, all hosts in a LAN. Multicast can be thought of as a generalization of unicast and broadcast. In multicast, data is transmitted to a set of hosts that have indicated interest in receiving the data, referred to as a multicast group or host group.

In principle, it is feasible to implement multicast in a network using either unicast or broadcast. However, both solutions have shortcomings. In a unicast solution to multicast, the sender transmits one copy of the data separately to each host in the multicast group. This is viable for small multicast groups, but when the number of hosts is large, transmitting the same data multiple times wastes a lot of resources. In a broadcast solution to multicast, data is delivered to all hosts in a network; for example, and hosts drop the data if they are not members of the multicast group. This solutions works when the hosts of a multicast group are all located on the same LAN and the LAN supports broadcast transmissions. Otherwise, sending data to a large number of hosts just to have it dropped by most hosts is not an economical use of network capacity.

Making multicast delivery efficient in a packet switching network requires a whole set of new protocols and mechanisms at the network layer. First, multicast addresses must be available that can designate a multicast group as the destination of a datagram. Second, there must be mechanisms that allow a host to join and leave a multicast group. Third, there is a need for multicast routing protocols that set up paths, called distribution tree, from the sender to the members of a multicast group. The issues related to setting up multicast distribution trees are referred to as multicast routing.

The network-layer mechanisms in the Internet that support multicast are referred to as IP multicast. Figure depicts an example of multicast in an IP network. The figure shows four hosts and eight routers. Routers are connected by point-to-point links and hosts connect to routers via Ethernet LANs. Host H1 is a source of multicast data, and hosts H2, H3 and H4 are Multicast receivers. The distribution tree, indicated with arrows, is established by the routers using a multicast routing protocol. Data is delivered downstream in the distribution tree from the source to the receivers.

IP multicast involves both hosts and routers. In IPv4, support of IP mult icast is optional, but almost all hosts and most routers support multicast. Hosts that are members of a multicast group exchange Internet Group Management Protocol (IGMP) messages with routers. Routers perform two main processes in IP multicast: multicast routing and multicast forwarding. Multicast routing sets up the distribution tree for a multicast group by setting the content of multicast routing tables. In multicast, a routing table may list multiple next hop addresses for a routing table entry. As in unicast, forwarding refers to the processing of an incoming datagram, the routing table lookup, and the transmission on an outgoing interface. When a multicast packet arrives at a router, the router performs a lookup in the multicast routing table for a matching entry. The router forwards one copy of the packet to each next hop address in the matching routing table entry.


 Multicast routing is concerned with setting up distribution trees that provide a path from the multicast sources to multicast group members. This section discusses the objectives of multicast routing, and the protocol mechanisms used in routing protocols.

 a) Multicast Routing Algorithms in a Graph

In this graph, the task of multicast routing is the embedding of a tree into the graph such that all multicast group members are connected by the tree. So, how would an ideal embedding look like? One can think immediately of two objectives to build a good tree, which are shown. In the figures, the thick lines indicate the distribution trees. The first objective is to build a tree that minimizes the path cost from the source to each receiver. Such a tree is called a shortest-path tree or source-based tree. A shortest-path tree can be relatively quickly computed using a shortest-path algorithm, but has the drawback that the tree is dependent on the source of the multicast tree. Note in figure, that the shortest-path tree is different when a different node is the source in the multicast group. Therefore, a distribution tree must be computed separately for each source. The second objective is to build a tree that minimizes the total cost of the edges, called a minimum-cost tree.

Different from a shortest-path tree, a minimum-cost tree does not change when a different node becomes the source. Thus, the same tree can be shared by all sources. The main drawback of a minimum-cost tree is that its computation is prohibitively expensive in most cases. In fact, the problem of calculating a minimum-cost tree is known to be an NP-complete problem, meaning that the computation of the tree is intractable unless the network is small. The two objectives for multicast routing, shortest-path tree and minimum-cost tree, represent a set of trade-off for multicast routing. Shortest-path trees minimize the cost of each receiver, whereas the minimum-cost tree minimizes the cost of the total tree. A single minimum-cost tree can be shared by all senders, whereas, a shortest-path trees must be built for each source. Lastly, a shortest-path tree can be computed relatively quickly, whereas the computation of a minimum cost-tree is not tractable in large networks. Some of the above trade-offs of multicast routing are reflected in the multicast routing protocols for the Internet. However, routing protocols have to satisfy additional constraints, which are not expressed in the formulation of a graph problem. For example, routing protocols must be able to adapt the distribution tree quickly when hosts join and leave a multicast group. Additionally, most multicast routing protocols require computing the distribution tree in a distributed fashion without any central coordination. Finally, most multicast routing protocols try to leverage off unicast routing protocols, by constructing the distribution tree using information from the unicast routing tables. This imposes additional constraints on a multicast routing protocol.

b) Reverse Path Forwarding

 The majority of routing protocols in the Internet built either source-based trees or core-based trees. A source-based tree is essentially a shortest-path tree, with a multicast source at the root of the distributions tree. With source-based trees, one distribution tree must be built for each multicast source. Protocols that built core-based trees avoid the need for multiple distribution trees by building a single distribution tree that is shared by all sources. Core-based tree routing protocols do not attempt to construct a minimum-cost tree. Instead, one router is selected as core or rendezvous-point, and a shortest-path tree is constructed with the core router as the root of the tree. Thus, both source-based trees and core-based trees build shortest path trees. For reasons that will become clear in a moment, routing protocols generally minimize the paths from the receivers to the source, as opposed to minimizing the path from the source to the receiver.

 A reverse shortest path tree can be built by applying a concept called reverse path forwarding (RPF). RPF is a mechanism to build a shortest path tree in a distributed fashion by taking of the unicast routing tables. The idea of RPF is simple. Given the address of the root of the tree, a router selects as its upstream neighbor in the tree of the router which is the next-hop neighbor for forwarding unicast packets to the root. The network interface which is used to reach this upstream neighbor is called the RPF interface and the neighbor router is called the RPF neighbor. An illustration of the basic concept of reverse path forwarding is given in Figure 10.12. Here, host H1 is the root of the distribution tree. Router R3 determines that R2 is its RPF neighbor for H1 from a lookup in its routing table. The interface that connects to R2 is the RPF interface.

If all routers determine their upstream neighbor using reverse path forwarding, the resulting distribution tree is such that packets from the root to a receiver are forwarded on the reverse path taken by unicast packets sent from the receiver to the root. This is where the name reverse path forwarding comes from. Since unicast routing tables are set so that the path from the receiver to the root is minimal, the tree generated by reverse path forwarding is a reverse shortest path. Reverse path forwarding is applied to construct source-based, as well as core-based trees. In the former case, the root of the tree is a multicast source. In the latter case, the root of the tree is the core.

The general structure of a multicast routing table is shown in Figure 10.13. There are columns for a source IP address, a multicast address, an incoming interface and a list of outgoing interfaces. Routing table entries for source-based trees and for core-based trees are different. In a source based tree, a routing table entry contains a source address as well as a group address. This is often called a (Source, Group) or (S, G) entry. Since a source-based tree has one distribution tree for each source there may be multiple routing entries for the same multicast group. Routing table entries for a core-based tree do not list a source IP address, which is indicated with a ‗*‘

(star) in the source IP address column. The corresponding entry is called a (*, G) entry. In an (S, G) entry, the incoming interface entry is the RPF interface for address S. In an (*, G) entry, the incoming interface is set to the RPF interface with respect to the core.

An arriving multicast packet must match the source address and the group address in an (S, G) entry, and the group address in an (*, G) entry. If there are matches for both (S, G) and (*, G) entries, the (S, G) entries take preference. When a match is found in the routing table, the router verifies that the packet arrived on the incoming interface listed in the incoming interface column of the routing table. This is called an RPF check. If an RPF check is successful, one copy of the datagram is forwarded on each interface listed in the outgoing interface list. If there is no match in the routing table or if the RPF check failed, the packet is discarded.

Source-based trees

We illustrate the construction of a source-based tree for the network shown in Figure. Each host is connected to a router via an Ethernet network and routers are connected by point to point links. H1 is the source of a multicast group and hosts H3 and H4 are receivers that have joined the multicast group. The arrows in the figure indicate the reverse shortest paths for all routers in the network. Data transmissions in the source-based distribution tree for H1 follow the opposite direction of the reverse shortest paths. The paths for H3 and H4 are H1à R1à R2 à R5 à H3 and H1à R1à R3 à R7 à H4, respectively.

Since the RPF interface tells each router the upstream neighbor in the distribution tree, but not the downstream neighbors in a distribution tree, additional protocol mechanisms are needed to determine the outgoing interfaces in the multicast routing tables. One method to achieve this is flood-and-prune, which starts out by forwarding multicast packets on all its interfaces, and then deletes interfaces which are not part of the distribution tree. Another method, called explicit join, requires that multicast receivers initiate the process of getting connected to the distribution tree. We will describe both methods.

In flood-and-prune, the outgoing interface list in a routing table entry initially lists all interfaces other than the RPF interface. When a router receives a multicast packet from source address S for a multicast group G, on the RPF interface for source S, it forwards the packet on all interfaces, with exception of the RPF interface. When a router receives a packet on an interface that is different from the RPF interface for S, it discards the packet. Figure 10.15 shows how multicast packets from H1 are forwarded with this method. Arrows indicate the transmission of packets and crosses show where packets are discarded. Figure 10.15 show that due the flooding of messages, many routers receive multiple copies of the same packet. All but the copy arriving on the RPF interface are discarded. A router connected to a LAN, forwards a datagram to the LAN only if some hosts on the LAN are multicast group members. In Figure 10.15, these are the LANs where the multicast group members H3 and H4 reside. A router knows about group members on a LAN from IGMP messages.

In Figure when hosts H3 and H4 want to receive multicast packets from source S for group G, the connected routers, R5 and R7, send a join message to the router on their RPF interfaces. When a router receives a join message, it adds the interface where the join message was received to the list of outgoing interfaces of the (S, G) routing table entry. If an (S, G) routing entry does not exist, it will be created when the join message arrives. In Figure 10.18, when R2 receives a join message from R5, it adds the interface that connects to R5 to the outgoing interface list. As with graft messages, a router forwards a join message on its RPF interface only if the router is not part of the distribution tree.

When a router in the distribution tree receives a join message, it does not forward the join message. This method of explicit join messages avoids the transmission of duplicate packets as in flood-and-prune. On the other hand, building source-based trees with explicit join messages assume that routers know the identity of hosts that transmit to a multicast group.

Routing protocols that built core-based trees designate a router, called the core, and built a reverse shortest path tree with the core as the root of the tree. The core becomes the central hub for disseminating multicast packets sent to the group. When a source transmits a packet to a multicast group, the packet is sent to the core. When the packet reaches the core, it is forwarded using the reverse shortest path tree.

The construction of a core-based tree for a multicast group G is illustrated in Figure. Each receiver that joins a multicast group sends a join message to the core router of the group. The message is sent on the RPF interface with respect to the core. If the join message reaches a router that is not part of the tree, the router adds an (*, G) entry to the multicast routing table, adds the interface where the join message arrived to the outgoing interface list, and sets the incoming interface to the RPF interface. Then, the router forwards the join message on its RPF interface in the direction of the core. If the router is already part of the shared tree for group G, the router, upon receiving a join message, adds the interface where the join message arrived to the outgoing interface list of the corresponding (*, G) entry, but does not forward the join message. Since join messages are sent on the reverse shortest path, the resulting distribution tree is a reverse shortest-path tree rooted at the core. The Figure illustrates the transmission of join messages with router R4 as core, and hosts H3, H4, and H5 has members of the multicast group.

Forwarding of multicast packets in core-based trees is illustrated in Figure. When H1 transmits a multicast packet, the packet is forwarded on the unicast shortest path to the core. A possible route is H1à R2 à R4. When the packet reaches the tree at the core, it is sent downstream in the core-based tree. Sending packets always to the core and inserting packets into the distribution tree only at the core can be inefficient. For example, suppose the path from R1 to R4 is through R3. Then, a packet from H1 to H5 takes the route H1à R1 à R3 à R4 à R3 à à R6 à R8. Here, it would be better to have R3, instead of forwarding the packet to the core, forward the datagram using the core-based tree. R3 should forward the datagram downstream the tree to R6, as well as upstream the tree to R4. However, such bidirectional trees, where packets are forwarded both upstream and downstream a core-based tree, are prone to routing loops, and special precautions are necessary to avoid such loops.

 The main advantage of using core-based trees is that only one distribution tree is required for each multicast group. This reduces the complexity of the routing tables, as well as the volume of multicast routing protocol messages. The main disadvantage of core-based trees is that, dependent on the placement of the core, the paths from the sender to the receivers through the core can be much longer than the direct path between a source and a receiver.

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