In computer networking, multicast is the delivery of a message or information to a group of destination computers simutaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topoloty of the network requires it.
Multicast is most commonly implemented in IP multicast, which is often employed in Internet Protocol(IP) applications of streaming media and Internet television. In IP multicast the implementation of the multicast concept occurs at the IP routing level, where routers create optimal distribution paths for datagrams sent to a multicast destination address.
At the Data link Layer, multicast describes one-to-many distribution such as Ethernet multicast addresssing ,Asynchronous Transfer Mode(ATM) point-to-multipoint virtual circuits or Infiniband multicast.
IP multicast is a technique for one-to-many communication over an IP infrastructure in a network. It scales to a larger receiver population by not requiring prior knowledge of who or how many receivers there are. Multicast uses network infrastructure efficiently by requiring the source to send a packet only once, even if it needs to be delivered to a large number of receivers. The nodes in the network take care of replicating the packet to reach multiple receivers only when necessary.
The most common transport layer protocol to use multicast addressing is User Datagram Protocol (UDP). By its nature, UDP is not reliable—messages may be lost or delivered out of order. Reliable multicast protocols such as Pragmatic General Multicast (PGM) have been developed to add loss detection and retransmission on top of IP multicast.
IP multicast is widely deployed in enterprises, commercial stock exchanges, and multimedia content delivery networks. A common enterprise use of IP multicast is for IPTV applications such as distance learning and televised company meetings.
Multicast in DTN:
Multicast group delivery in a DTN presents an unfamiliar isssue with respect to group membership.
In relatively low delay networks, such as the Internet, nodes may be considered to be part of the group if they have expressed interest to join it “recently”.In a DTN, however, nodes may wish to receive data sent to a group during an interval of time earlier than when they are actually able to receive it. More precisely, an application expresses its desire to receive data sent to EID e at time t. Prior to this ,during the interval [t0,t1],t > t1, data may have been generated for group e. For the application to receive any of this data, the data must be available a potentially long time after senders have ceased sending to the group. Thus, the data may need to be stored within the network in order to support temporal group semantics of this kind. How to design and implement this remains a research issure, as it is likely to be at least as hard as problems related to reliable multicast.