must be created and maintained in routers that handle multicast packets sent among hosts in a so called multicast group: Internet multicast is not a connectionless service unlike the unicast case.
This, in turn requires a combination of signalling and routing protocols in order to set up, maintain and tear down connection state in the routers.
A single datagram transmitted by the sender is duplicated by routers within the networks. Only a single copy will ever traverse a link. On the other hand, considerable network layer support is needed to implement a multicast-aware network layer.
How to identify the receivers of a multicast datagram and how to address a datagram sent to these receivers are two problems to face with a multicast communication.
In the case of unicast communication, the IP address of the receiver is carried in each IP unicast datagram and identifies the single recipient, but in the case of multicast there are multiple receivers.
Including in a datagram the IP addresses of all receivers is not a feasible approach due to the amount of data to store in a datagram and because explicit identification of the receivers by the sender also requires that the sender knows the identities and addresses of all of the receivers.
For these reasons, in the Internet architecture a multicast datagram is addressed using address indirection. That is, a single identifier is used for the group of receivers and a copy of the datagram that is addressed to the group using this single identifier is delivered to all of the multicast receivers associated with that group.
The group of receivers associated with a multicast address is referred to as a multicast group .
Each host also has a unique IP unicast address that is completely independent of the multicast group in which it is participating.
There are two key aspects of multicast mechanisms in Internet: the first is a protocol to register hosts in a multicast group, it is IGMP [RFC 2236] and the second is a mechanism to coordinate the multicast routers thr oughout the Internet, this is accomplished by a network layer multicast routing algorithm such as PIM, DVMRP and MOSPF.
IGMP operates between a host and its directly attached router. It provides the means for a host to inform its attached router that an application running on the host wants to join
a specific multicast group. IGMP messages are carried in an IP datagram with an IP protocol number of 2.
Any host can join a multicast group at the network layer. A host simply issues a “membership_report” IGMP message to its attached router. That router, working in concert with other Internet routers will begin delivering multicast datagrams to the host. Joining a multicast group is thus receiver-driven. A sender need not be concerned with explicitly adding receivers to the multicast group but neither can it control who joins the group and therefore who receives datagrams sent to that group. Similarly there is no control over who sends to the multicast group. Network layer does not provide for filtering, ordering or privacy of multicast datagrams. This functionality should be provided by the upper layers.
In many ways, the current Internet multicast service model reflects the same philosophy as the Internet unicast service model: an extremely simple network layer with additional functionality being provided in the upper-layer protocols in the hosts at the edges of the networks.
The goal of a multicast routing algorithm is to find a tree that has attached hosts belonging to the multicast group. Multicast packets will then be routed along this tree from the sender to all of the hosts belonging to the multicast tree. The tree may contain routers that do not have attached hosts belonging to the multicast group.
In practice, two approaches have been adopted for determining the multicast routing tree. The two approaches differ according to whether a single tree is used to distribute the traffic for all senders in the group, or whether a source -specific routing tree is constructed for each individual sender:
• Group -shared tree. In the group-shared tree approach only a single routing tree is constructed for the entire multicast group.
• Source-based tree. In a source-based approach an individual routing tree is constructed for each sender in the multicast group. In a multicast group with N hosts, N different routing trees will be constructed for that single multicast group. Packets will be routed to multicast group members in a source-specific manner.
These types of trees have been shown to be the most scalable way of supporting reliable multicast transmissions [70].
DVMRP, MOSPF [48] , CBT and PIM are Internet multicast routing protocols [60]. DVMRP, Distance Vector Multicast Routing Algorithm, is the first routing protocol used in the Internet and the most widely supported multicast routing algorithm. It implements source-based tree with reverse path forwarding, pruning (in case a router receives multicast packets for a group to which no underlying hosts are subscribed) and
grafting (to “unprune” previously pruned multicast gr oups’ packets). DVRMP uses a distance vector algorithm that allows each router to compute the outgoing link (next hop) that is on its shortest path back to each possible source. This information is then used in the RPF algorithm [60].
MOSPF, Multicast Open Shortest Path First protocol [48], operates in an autonomous system (AS) that uses OSPF unicast protocol [60] for unicast routing. MOSPF extends OSPF by having routers add their multicast group membership to the link state advertisement that is broadcaste d by routers as part of the OSPF protocol. With this extension, all routers have not only complete topology information, but also know which edge routers have attached hosts belonging to various multicast groups. With this information, the routers within the AS can build source-specific, pre-pruned, shortest- path trees for each multicast group.