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Multicast

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This article is about one-to-many communications in computer networks. For radio systems, seePoint-to-multipoint communication.For broadcasting multiple programs using a single channel, seeMulticast channel.
Routing schemes
Unicast

Broadcast

Multicast

Anycast

Incomputer networking,multicastis a type of group communication wheredata transmissionis addressed to a group of destination computers simultaneously.[1]Multicast can beone-to-manyormany-to-manydistribution.[2][3]Multicast differs from physical layerpoint-to-multipoint communication.

Group communication may either beapplication layermulticast[1]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 asrouters,switchesandcellular network base stations,but only to network segments that currently contain members of the group. Network assisted multicast may be implemented at thedata link layerusing one-to-many addressing and switching such asEthernet multicast addressing,Asynchronous Transfer Mode(ATM), point-to-multipoint virtual circuits (P2MP)[4]orInfiniBandmulticast. Network-assisted multicast may also be implemented at theInternet layerusingIP multicast.In IP multicast the implementation of the multicast concept occurs at the IP routing level, where routers create optimal distribution paths fordatagramssent to a multicast destination address.

Multicast is often employed inInternet Protocol(IP) applications ofstreaming media,such asIPTVandmultipoint videoconferencing.

Ethernet[edit]

Ethernet frameswith a value of 1 in the least-significant bit of the first octet of thedestination addressare treated as multicast frames and are flooded to all points on the network. This mechanism constitutes multicast at the data link layer. This mechanism is used by IP multicast to achieve one-to-many transmission for IP on Ethernet networks. Modern Ethernet controllers filter received packets to reduce CPU load, by looking up the hash of a multicast destination address in a table, initialized by software, which controls whether a multicast packet is dropped or fully received.

Ethernet multicast is available on all Ethernet networks. Multicasts span thebroadcast domainof the network.Multiple Registration Protocolcan be used to control Ethernet multicast delivery.

IP[edit]

The relationship between the multicast group management protocol family and the multicast routing protocols family based on the network topology terms.

IP multicastis a technique for one-to-many communication over an IP network. The destination nodes sendInternet Group Management Protocoljoinandleavemessages, for example in the case of IPTV 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 commontransport layerprotocol to usemulticast addressingisUser Datagram Protocol(UDP). By its nature, UDP is notreliable—messages may be lost or delivered out of order. By adding loss detection and retransmission mechanisms,reliable multicasthas been implemented on top of UDP or IP by variousmiddlewareproducts, e.g. those that implement theReal-Time Publish-Subscribe (RTPS) Protocolof theObject Management Group(OMG)Data Distribution Service(DDS) standard, as well as by special transport protocols such asPragmatic General Multicast(PGM).

IP multicast is always available within the local subnet. Achieving IP multicast service over a wider area requiresmulticast routing.Many networks, including the Internet, do not support multicast routing. Multicast routing functionality is available in enterprise-grade network equipment but is typically not available until configured by a network administrator. The Internet Group Management Protocol is used to control IP multicast delivery.

Application layer[edit]

Application layer multicast overlay services are not based on IP multicast ordata link layermulticast. Instead they use multiple unicast transmissions to simulate a multicast. These services are designed for application-level group communication.Internet Relay Chat(IRC) implements a singlespanning treeacross itsoverlay networkfor all conference groups.[5]The lesser-known PSYC technology uses custom multicast strategies per conference.[6]Somepeer-to-peertechnologies employ the multicast concept known aspeercastingwhen distributing content to multiple recipients.

Explicit multi-unicast(Xcast) is another multicast strategy that includes addresses of all intended destinations within each packet. As such, givenmaximum transmission unitlimitations, Xcast cannot be used for multicast groups with many destinations. The Xcast model generally assumes that 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.[7]

Wireless networks[edit]

Wireless communications (with exception topoint-to-point radio linksusingdirectional antennas) are inherently broadcasting media. However, the communication service provided may beunicast,multicast, orbroadcast,depending on if the data is addressed to an individual node, a specific group of nodes, or all nodes in the covered network, respectively.

Television[edit]

Indigital television,the concept of multicast service sometimes is used to refer tocontent protectionbybroadcast encryption,i.e. encryptedpay televisioncontent over a simplex broadcast channel only addressed to paying viewers. In this case, data is broadcast to all receivers but only addressed to a specific group.

The concept ofinteractive multicast,for example using IP multicast, may be used over TV broadcast networks to improve efficiency, offer more TV programs, or reduce the required spectrum. Interactive multicast implies that TV programs are sent only over transmitters where there are viewers and that only the most popular programs are transmitted. It relies on an additionalinteraction channel(a back-channel orreturn channel), where user equipment may send join and leave messages when the user changes TV channel. Interactive multicast has been suggested as an efficient transmission scheme inDVB-HandDVB-T2terrestrial digital television systems,[8]A similar concept isswitched broadcastover cable-TV networks, where only the currently most popular content is delivered in the cable-TV network.[9]Scalable video multicastin an application of interactive multicast, where a subset of the viewers receive additional data for high-resolution video.

TV gatewaysconvertssatellite(DVB-S,DVB-S2),cable(DVB-C,DVB-C2) andterrestrial television(DVB-T,DVB-T2) to IP for distribution using unicast and multicast in home, hospitality and enterprise applications

Another similar concept isCell-TV,and implies TV distribution over 3G cellular networks using the network-assisted multicasting offered by theMultimedia Broadcast Multicast Service(MBMS) service, or over 4G/LTEcellular networks with the eMBMS (enhanced MBMS) service.

See also[edit]

Wikiversity has learning resources aboutMulticast

References[edit]

  1. ^abMedia-communication based on Application-Layer Multicast
  2. ^Lawrence Harte,Introduction to Data Multicasting,Althos Publishing 2008.
  3. ^Li, Bing; Atwood, J. William (2016-06-19)."Secure receiver access control for IP multicast at the network level: Design and validation".Computer Networks.102:109–128.doi:10.1016/j net.2016.03.010.ISSN1389-1286.
  4. ^M. Noormohammadpour; et al. (July 10, 2017)."DCCast: Efficient Point to Multipoint Transfers Across Datacenters".USENIX.RetrievedJuly 26,2017.
  5. ^A Discussion on Computer Network Conferencing.sec. 2.5.1.RFC1324.
  6. ^White paper,EU: PSYC.
  7. ^Rick Boivie; Nancy Feldman; Yuji Imai; Wim Livens & Dirk Ooms (November 2007)."Explicit Multicast (Xcast) Concepts and Options".Internet Engineering Task Force.doi:10.17487/RFC5058.RetrievedMay 25,2013.{{cite journal}}:Cite journal requires|journal=(help)
  8. ^M. Eriksson, S.M. Hasibur Rahman, F. Fraille, M. Sjöström,”Efficient Interactive Multicast over DVB-T2 - Utilizing Dynamic SFNs and PARPS”,2013 IEEE International Conference on Computer and Information Technology (BMSB’13), London, UK, June 2013.
  9. ^N. Sinha, R. Oz and S. V. Vasudevan, “The statistics of switched broadcast”, Proceedings of the SCTE 2005 Conference on Emerging Technologies, Tampa, FL, USA, January 2005
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