Multicast (point-to-multipoint) is a communication pattern in which a source host sends a message to a group of destination hosts. Although, this can be done by sending different unicast (point-to-point) messages to each of the destination hosts, there are many reasons, which make having the multicasting capability desirable.

Most high-level network protocols only provide a unicast transmission service. That is, nodes of the network only have the ability to send to one other node at a time: All transmission with a unicast service is inherently point-to-point. If a node wants to send the same information to many destinations using a unicast transport service, it must perform a replicated unicast and send N copies of the data to each destination in turn.

A better way to transmit data from one source to many destinations is to provide a multicast transport service. With a multicast transport service, a single node can send data to many destinations by making just a single call on the transport service: For those applications which involve a single node sending to many recipients, a multicast facility is clearly a more natural programming paradigm than unicast. However, the benefits of multicast are more than just logical: Many underlying transmission media provide support for multicast and broadcast at the hardware and media-access level. When a multicast service is implemented over such a network, there is a huge improvement in performance: If the hardware supports multicast, a packet, which is destined for N recipients, can be sent as just a single packet!

Multicast is useful because it allows the construction of truly distributed applications, and provides important performance optimizations over unicast transmission

IP Multicast is a protocol for transmitting IP datagrams from one source to many destinations in a local or wide-area network of hosts, which run the TCP/IP suite of protocols. The basic facility provided by the IP protocol is a unicast transmission service: That is, the current standard for IP provides only unreliable transmission of datagrams from a single source host to a single destination host. The resulting IP Multicast routing protocol provides efficient delivery of datagrams from one source to an arbitrary number of destinations throughout a large, heterogeneous network such as the Internet.

Many new multi-point applications are emerging as demand for multicasting content grows. We can divide these applications into two major areas:

Real-time applications such as live broadcasts, financial data delivery, whiteboard collaboration and videoconferencing.
Non-real-time application such as file transfer, data and file replication, and on-demand broadcasting.
The IP Multicast protocol is a standard extension to the IP protocol. The IETF-recommended standard, RFC 1112, defines extensions to the IP protocol. This standard specifies the extensions required of a host implementation of the Internet Protocol (IP) to support multicasting. It also specifies the format, addressing and routing of messages on the Internet.

IP Multicast allows a sender to send a single message to a group address. Any receiver that joins the network under this group address receives the message. Routers within the network handle the replication and forwarding of the message to the appropriate receivers intelligently. All major router vendors support IP Multicast.

An IP multicast-enabled network requires two essential protocol components:

An IP host-based protocol to allow a receiver application to notify a local router that it has joined the group.

An IP router-based protocol to allow any routers with multicast group members on their local networks to communicate with other routers to ensure that all data sent to the group address are forwarded to all receivers. Currently, such routers exist and are standardized.
Advantages of IP Multicasting:

Businesses that rely on subscription-based pricing would benefit from having control over their customer base on virtual private networks.

Virtual private networks make software updating and software distribution possible from one-to-many over the Internet.

Content providers can implement discriminatory pricing strategies and versioning enabling various services mixes to be offered at various prices to different market segments.
The hope for successful deployment of multi-media applications over the Internet lies in the popularity of IP multicasting and high bandwidth Internet connectivity to the home.

IP multicasting solves the problems presented by both bandwidth-consuming point-to point communication and by broadcasting content from one-to-all. IP multicasting reduces the number of hops a packet uses to get to its destination by approximately 45% and reaches only those clients who have requested the multicast.

Ashish Agarwal
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Astek Networking & Solutions
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Sanjay Place, Agra 282 002
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