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In computer networking, multicast (one-to-many or many-to-many distribution〔Lawrence Harte, ''(Introduction to Data Multicasting )'', Althos Publishing 2008.〕) is group communication〔(Media-communication based on Application-Layer Multicast )〕 where information is addressed to a group of destination computers simultaneously. Multicast should not be confused with physical layer point-to-multipoint communication. Group communication may either be ''application layer multicast''〔 or ''network assisted multicast'', where the latter makes it possible for the source to efficiently send to the group in a single transmission. Copies are automatically created in other network elements, such as routers, switches and cellular network base stations, but only to network segments that currently contain members of the group. Network assisted multicast may be implemented at the Internet layer using IP multicast, which is often employed in Internet Protocol (IP) applications of streaming media, such as Internet television scheduled content (but not media-on-demand) and multipoint videoconferencing, but also for ghost distribution of backup disk images to multiple computers simultaneously. 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. Network assisted multicast may also be implemented at the Data Link Layer using one-to-many addressing and switching such as Ethernet multicast addressing, Asynchronous Transfer Mode (ATM) point-to-multipoint virtual circuits (P2MP) or Infiniband multicast. ==IP multicast== (詳細はIP multicast is a technique for one-to-many communication over an IP infrastructure in a network. The destination nodes send join and leave messages, for example in the case of Internet television when the user changes from one TV channel to another. IP multicast 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. By adding loss detection and retransmission mechanisms, reliable multicast has been implemented on top of UDP or IP by various middleware products, e.g. those that implement the Real-Time Publish-Subscribe (RTPS) Protocol of the Object Management Group (OMG) Data Distribution Service (DDS) standard, as well as by special transport protocols such as Pragmatic General Multicast (PGM). 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. , most effort at scaling multicast up to large networks have concentrated on the simpler case of single-source multicast, which seems to be more computationally tractable. Still, the large state requirements in routers make applications using a large number of trees unable to work while using IP multicast. Take presence information as an example where each person needs to keep at least one tree of its subscribers, if not several. 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. For these reasons, and also reasons of economics, IP multicast is not, in general, used in commercial Internet backbones. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Multicast」の詳細全文を読む スポンサード リンク
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