Multicast

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Multicast is sometimes used to refer to a multiplexed broadcast

Image:Multicast vs broadcast illustrated.png

Multicast is the delivery of information to a group of destinations simultaneously using the most efficient strategy to deliver the messages over each link of the network only once and only create copies when the links to the destinations split.

1 Multicast technologies on the Internet
2 IP multicast
3 IP Multicast protocols
4 See also
5 External links

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Multicast technologies on the Internet

The word "Multicast" is typically used to refer to IP Multicast, which is a protocol for efficiently sending to multiple receivers at the same time on TCP/IP networks, by use of a multicast address. It is also commonly associated with audio/video protocols such as RTP.

But there are also other protocols that implement the multicast concept. As examples of some connection-oriented link layers, such as Asynchronous Transfer Mode, have built-in mechanisms for "point-to-multipoint" or "multipoint-to-multipoint" connections. This model generally assumes that the stations participating in the communication are known ahead of time, so that distribution trees can be generated and resources allocated by network elements in advance of actual data traffic. By contrast, the IP Multicast model has been described by Internet architect Dave Clark as "You put packets in at one end, and the network conspires to deliver them to anyone who asks."

There are other kinds of multicast technologies used in the Internet. While IP Multicast uses the multicast address class described below in detail, the Explicit multicast (as known as Xcast) uses the unicast addresses of all the destinations, instead of the designated multicast address. Since the IP packet size is limited in general, the Explicit multicast cannot be used for multicast groups of large number of destinations.

The IP Multicast model requires a great deal more state inside the network than the IP unicast model of best-effort delivery does, and this has been the cause of some criticism. Also, no mechanism has yet been demonstrated that would allow the IP Multicast model to scale to millions of senders and millions of multicast groups and, thus, it is not yet possible to make fully-general multicast applications practical in the commercial Internet. As of 2003, most efforts at scaling multicast up to large networks have concentrated on the simpler case of single-source multicast, which seems to be more computationally tractable.

For both these reasons, and also reasons of economics, IP Multicast is not in general use in the commercial Internet. Other multicast technologies not based on IP Multicast are more widely used. These include Internet Relay Chat and PSYC, which are more pragmatic and scale better for large numbers of small groups.

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IP multicast

Template:Mergeto However, some communities within the public Internet make regular use of IP Multicast (see the Mbone for an example), and IP Multicast is used for special applications within private IP networks such as Internet2's Abilene Network.

Link local multicast, where IP Multicast packets are sent to groups of hosts on the same physical or virtual data link layer, does not require complex routing, and is therefore much more widely deployed. It is used in IPv6 for address resolution, and in zeroconf networks for service discovery, name resolution, and address conflict resolution, replacing inefficient broadcast protocols.

IP multicast conferencing was first demonstrated on a wide scale when it was used to broadcast several sessions from the 23rd IETF in March, 1992 to researchers and interested observers around the world. Selected IETF sessions have continued to be multicast over the MBONE and private multicast networks since then.

Multicast security is a major issue. Standard, practical, communications security solutions normally employ symmetric cryptography. But applying that to IP Multicast traffic would enable any of the receivers to pose as the sender. This is clearly unacceptable. The IETF MSEC workgroup is developing security protocols to solve this problem, mostly within the architectural framework of the IPsec protocol suite.

IPsec cannot be used in the multicast scenario because IPsec security associations are bound to two hosts and not many. IETF proposed a new protocol TESLA, which is quite convincing and flexible for multicast security.

Concerning IP Multicast, there are three basic kinds of delivery:

Modern multicast algorithms are often capable of combining and managing the first two.

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