Multicast addressing is a
networkComputer networking is the engineering discipline concerned with communication between computer systems or devices. Networking, routers, routing protocols, and networking over the public Internet have their specifications defined in documents called RFCs...
technology for the delivery of
informationInformation as a concept has many meanings, from everyday usage to technical settings. The concept of information is closely related to notions of constraint, communication, control, data, form, instruction, knowledge, meaning, mental stimulus, pattern, perception, and representation.The English...
to a group of destinations simultaneously using the most efficient strategy to deliver the messages over each link of the network only once, creating copies only when the links to the multiple destinations split.
The word "multicast" is typically used to refer to
IP multicastIP multicast is a method of forwarding IP datagrams to a group of interested receivers. See the article on multicast for a general discussion of this subject - this article is specifically about IP multicast.- IP multicast implementations :...
which is often employed for
streaming mediaStreaming media are multimedia that are constantly received by, and normally presented to, an end-user while being delivered by a streaming provider . The name refers to the delivery method of the medium rather than to the medium itself...
and
Internet televisionInternet Protocol television is a system through which digital television service is delivered using the architecture and networking methods of the Internet Protocol Suite over a packet-switched network infrastructure, e.g., the Internet and broadband Internet access networks, instead of being...
applications.
Multicast addressing is a
networkComputer networking is the engineering discipline concerned with communication between computer systems or devices. Networking, routers, routing protocols, and networking over the public Internet have their specifications defined in documents called RFCs...
technology for the delivery of
informationInformation as a concept has many meanings, from everyday usage to technical settings. The concept of information is closely related to notions of constraint, communication, control, data, form, instruction, knowledge, meaning, mental stimulus, pattern, perception, and representation.The English...
to a group of destinations simultaneously using the most efficient strategy to deliver the messages over each link of the network only once, creating copies only when the links to the multiple destinations split.
The word "multicast" is typically used to refer to
IP multicastIP multicast is a method of forwarding IP datagrams to a group of interested receivers. See the article on multicast for a general discussion of this subject - this article is specifically about IP multicast.- IP multicast implementations :...
which is often employed for
streaming mediaStreaming media are multimedia that are constantly received by, and normally presented to, an end-user while being delivered by a streaming provider . The name refers to the delivery method of the medium rather than to the medium itself...
and
Internet televisionInternet Protocol television is a system through which digital television service is delivered using the architecture and networking methods of the Internet Protocol Suite over a packet-switched network infrastructure, e.g., the Internet and broadband Internet access networks, instead of being...
applications. In IP multicast the implementation of the multicast concept occurs at the IP routing level, where
routerA router is a networking device whose software and hardware are usually tailored to the tasks of routing and forwarding information. For example, on the Internet, information is directed to various paths by routers....
s create optimal distribution paths for datagrams sent to a multicast destination address
spanning treeThe Spanning Tree Protocol is a link layer network protocol that ensures a loop-free topology for any bridged LAN. It is based on an algorithm invented by Radia Perlman while working for Digital Equipment Corporation. In the OSI model for computer networking, STP falls under the OSI layer-2...
in real-time. At the
Data Link LayerThe Data Link Layer is Layer 2 of the seven-layer OSI model of computer networking. It corresponds to or is part of the link layer of the TCP/IP reference model....
,
multicast describes one-to-many distribution such as Ethernet multicast addressing,
Asynchronous Transfer ModeAsynchronous transfer mode is an electronic digital data transmission technology. ATM is implemented as a network protocol and was first developed in the mid 1980s. The goal was to design a single networking strategy that could transport real-time video conference and audio as well as image...
(ATM) point-to-multipoint virtual circuits or
InfinibandInfiniBand is a switched fabric communications link primarily used in high-performance computing. Its features include quality of service and failover, and it is designed to be scalable...
multicast.
Technical description
IP multicastIP multicast is a method of forwarding IP datagrams to a group of interested receivers. See the article on multicast for a general discussion of this subject - this article is specifically about IP multicast.- IP multicast implementations :...
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 low-level protocol to use multicast addressing is
User Datagram ProtocolThe User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...
(UDP). By its nature, UDP is not reliable—messages may be lost or delivered out of order.
Reliable multicastA reliable multicast protocol is a computer networking protocol that provides a reliable sequence of packets to multiple recipients simultaneously, making it suitable for applications like multi-receiver file transfer or streaming media.-Overview:...
protocols such as
Pragmatic General MulticastPragmatic General Multicast is a reliable multicast transport protocol. PGM provides a reliable sequence of packets to multiple recipients simultaneously, making it suitable for applications like multi-receiver file-transfer.-Overview:...
(PGM) have been developed to add loss detection and retransmission on top of IP multicast.
Key concepts in IP multicast include an IP multicast group address, a multicast distribution tree and receiver driven tree creation.
An IP multicast group address is used by sources and the receivers to send and receive content. Sources use the group address as the IP destination address in their data packets. Receivers use this group address to inform the network that they are interested in receiving packets sent to that group. For example, if some content is associated with group 239.1.1.1, the source will send data packets destined to 239.1.1.1. Receivers for that content will inform the network that they are interested in receiving data packets sent to the group 239.1.1.1. The receiver "joins" 239.1.1.1. The protocol used by receivers to join a group is called the
Internet Group Management ProtocolThe Internet Group Management Protocol is a communications protocol used to manage the membership of Internet Protocol multicast groups.IGMP is used by IP hosts and adjacent multicast routers to establish multicast group memberships....
(IGMP).
Once the receivers join a particular IP multicast group, a multicast distribution tree is constructed for that group. The protocol most widely used for this is
Protocol Independent MulticastProtocol-Independent Multicast is a family of multicast routing protocols for Internet Protocol networks that provide one-to-many and many-to-many distribution of data over a LAN, WAN or the Internet...
(PIM). It sets up multicast distribution trees such that data packets from senders to a multicast group reach all receivers which have joined the group. For example, all data packets sent to the group 239.1.1.1 are received by receivers who joined 239.1.1.1. There are many different variations of PIM implementations:
Sparse ModeProtocol Independent Multicast - Sparse-Mode is a protocol for efficiently routing Internet Protocol packets to multicast groups that may span wide-area and inter-domain internets...
(SM),
Dense ModeDense mode multicast is one mode that multicast can use to construct a tree for sending packets to the multicast subscribers. It is the opposite of sparse multicast....
(DM),
Source Specific ModeSource-specific multicast is a method of delivering multicast packets in which the only packets that are delivered to a receiver are those originating from a specific source address requested by the receiver...
(SSM) and Bidirectional Mode (Bidir, or Sparse-Dense Mode, SDM). Of these, PIM-SM is the most widely deployed ; SSM and Bidir are simpler and scalable variations developed more recently are gaining in popularity.
IP multicast operation does not require a source sending to a given group to know about the receivers of the group. The multicast tree construction is initiated by network nodes which are close to the receivers or is receiver driven. This allows it to scale to a large receiver population. The IP multicast model has been described by Internet architect
Dave ClarkDavid Dana Clark is an American computer scientist. He graduated from Swarthmore College in 1966. In 1968, he received his Master's and Engineer's degrees in Electrical Engineering from the Massachusetts Institute of Technology, where he worked on the I/O architecture of Multics under Jerry...
as follows:
You put packets in at one end, and the network conspires to deliver them to anyone who asks.
IP multicast creates state information ("state") per multicast distribution tree in the network, i.e., current IP multicast routing protocols do not aggregate state corresponding to multiple distribution trees. So if a router is part of 1000 multicast trees, it has 1000 multicast routing and forwarding entries. As a result there are worries about scaling multicast to large numbers of distribution trees. However, because multicast state exists only along the distribution tree it is unlikely that any single router in the Internet maintains state for all multicast trees. This is a common misunderstanding compared to unicast. A unicast router needs to know how to reach all other unicast addresses in the Internet, even if it does this using just a default route. For this reason, aggregation is key to scaling unicast routing. Also, there are core routers that carry routes in the hundreds of thousands because they contain the Internet routing table. On the other hand, a multicast router does
NOT need to know how to reach all other multicast trees in the Internet. It only needs to know about multicast trees for which it has receivers downstream of it. This is key to scaling multicast. It is very unlikely that core Internet routers would need to keep state for all multicast distribution trees, they only need to keep state for trees with downstream membership. When this type of router joins a shared forwarding tree it is referred to as a
graft and when it is removed it is called a
prune.
IP multicast is widely deployed in enterprises, commercial
stock exchangeA stock exchange is a corporation or mutual organization which provides "trading" facilities for stock brokers and traders, to trade stocks and other securities. Stock exchanges also provide facilities for the issue and redemption of securities as well as other financial instruments and capital...
s, and multimedia content delivery networks. A common enterprise use of IP multicast is for
IPTVInternet Protocol television is a system through which digital television service is delivered using the architecture and networking methods of the Internet Protocol Suite over a packet-switched network infrastructure, e.g., the Internet and broadband Internet access networks, instead of being...
applications such as distance learning and televised company meetings.
Other multicast technologies
, 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.
Still, the large state requirements in routers make applications using a large number of trees unworkable using IP multicast. Take
presence informationIn computer and telecommunications networks, presence information is a status indicator that conveys ability and willingness of a potential communication partner--for example a user--to communicate...
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 use in the commercial Internet.
Explicit Multi-Unicast (XCAST) is an alternate multicast strategy to IP multicast that provides reception addresses of all destinations with each packet. As such, since the IP packet size is limited in general, XCAST cannot be used for multicast groups of large number of destinations. The XCAST 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.
Other multicast technologies not based on IP multicast are more widely used. Notably the
Internet Relay ChatInternet Relay Chat is a form of real-time Internet text messaging or synchronous conferencing. It is mainly designed for group communication in discussion forums, called channels, but also allows one-to-one communication via private message as well as chat and data transfers via Direct...
(IRC), which is more pragmatic and scales better for large numbers of small groups. IRC implements a single
spanning treeThe Spanning Tree Protocol is a link layer network protocol that ensures a loop-free topology for any bridged LAN. It is based on an algorithm invented by Radia Perlman while working for Digital Equipment Corporation. In the OSI model for computer networking, STP falls under the OSI layer-2...
across its
overlay networkAn overlay network is a computer network which is built on top of another network. Nodes in the overlay can be thought of as being connected by virtual or logical links, each of which corresponds to a path, perhaps through many physical links, in the underlying network. For example, many...
for all conference groups. This leads to suboptimal routing for some of these groups however. Additionally IRC keeps a large amount of distributed state, which limits growth of an IRC network, leading to fractioning into several non-interconnected networks. The lesser known
PSYCPSYC is a flexible text-based protocol for delivery of data to a large number of recipients or people. It is used for text chat, instant messaging, presence display, newscasting and social networking.- History :...
technology uses custom multicast strategies per conference. Also some
peer-to-peerA peer-to-peer, commonly abbreviated to P2P, distributed network architecture is composed of participants that make a portion of their resources directly available to other network participants, without the need for central coordination instances...
technologies employ the multicast concept when distributing content to multiple recipients.
Commercial deployment
Recently the
BBCThe British Broadcasting Corporation, usually referred to by its abbreviation as the "BBC", is the longest established and largest broadcaster in the world...
has begun encouraging UK-based ISPs to adopt Multicast onto their networks by
providing BBC Radio at higher quality than is available via their
Unicastright|250pxIn computer networking, unicast transmission is the sending of messages to a single network destination host on a packet switching network....
delivered services. This has also been supported by a variety of commercial radio networks, including
Virgin RadioAbsolute Radio is one of the UK's three Independent National Radio stations. The station rebranded to its current name at 7.45am on 29 September 2008.The station is based in London and plays popular rock music...
,
GCAPGCAP may refer to:* GCAP Campaign - Global Call to Action Against Poverty is a global anti-poverty movement/coalition, considered by many to be the largest in the world* GCap Media - British commercial radio company...
and
EMAPEMAP is a British media company, specialising in the production of business-to-business magazines, and the organisation of business events and conferences...
.
The German public-service broadcaster
ARDARD , is a joint organization of Germany's regional public-service broadcasters...
and
ZDFZweites Deutsches Fernsehen , ZDF, is a public-service German television channel based in Mainz. It is run as an independent non-profit agency established by joint contract between the German federal states...
and the Franco-German network
ArteArte is a Franco-German TV network. It describes itself as a European culture channel and aims to promote quality programming especially in areas of culture and the arts...
offer their TV program via Multicast on
several networks.
AustriaAustria , officially the Republic of Austria , is a landlocked country of roughly 8.3 million people in Central Europe. It borders both Germany and the Czech Republic to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the west...
n internet provider
Telekom AustriaTelekom Austria is a provider of a range of fixed line, mobile, data, and Internet communications services.In addition to its operations in Austria, Telekom Austria also has a wireline segment represented in the Czech Republic with Telekom Austria Czech Republic, a leading Internet service provider...
offers its
DSLDSL or xDSL is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop, but the term digital subscriber line has been widely adopted as a more marketing-friendly term for Asymmetric Digital...
customers a TV set-top box that uses Multicast for receiving TV and radio broadcasts. In
GermanyGermany , officially the Federal Republic of Germany , is a country in Central Europe. It is bordered to the north by the North Sea, Denmark, and the Baltic Sea; to the east by Poland and the Czech Republic; to the south by Austria and Switzerland; and to the west by France, Luxembourg, Belgium,...
, T-Home (a brand of
Deutsche TelekomDeutsche Telekom AG is a telecommunications company headquartered in Bonn, Germany. It is the largest telecommunications company in Europe....
), offers a similar service.
See also
- Ad hoc routing protocol list
An ad hoc routing protocol is a convention, or standard, that controls how nodes decide which way to route packets between computing devices in a mobile ad-hoc network ....
for more multicast protocols
- Spanning tree protocol
The Spanning Tree Protocol is a link layer network protocol that ensures a loop-free topology for any bridged LAN. It is based on an algorithm invented by Radia Perlman while working for Digital Equipment Corporation. In the OSI model for computer networking, STP falls under the OSI layer-2...
- Flooding algorithm
A flooding algorithm is an algorithm for distributing material to every part of a connected network. The name derives from the concept of inundation by a flood....
- Packet forwarding
- Overlay network
An overlay network is a computer network which is built on top of another network. Nodes in the overlay can be thought of as being connected by virtual or logical links, each of which corresponds to a path, perhaps through many physical links, in the underlying network. For example, many...
- Multicast address
In computer networking a multicast address is an identifier for a group of hosts that have joined a multicast group. Multicast addressing can be used in the Link Layer , such as Ethernet Multicast, as well as at the Internet Layer as IPv4 or IPv6 Multicast.- IPv4 multicast addresses :IPv4...
- Broadcast address
In computer networking, a broadcast address is a network address that allows information to be sent to all nodes on a network, rather than to a specific network host.-IP network broadcasting:...
- Anycast
Anycast is a network addressing and routing scheme whereby data is routed to the "nearest" or "best" destination as viewed by the routing topology.The term is intended to echo the terms unicast, broadcast and multicast....
- Unicast
right|250pxIn computer networking, unicast transmission is the sending of messages to a single network destination host on a packet switching network....
- IGMP
- NBMA Nonbroadcast Multiple Access Network
- VideoLAN
VideoLAN is a project that develops software for playing video and other media formats. It originally developed two programs for media streaming—VideoLAN Client and VideoLAN Server —but most of the features of VLS have been incorporated into VLC, with the result renamed VLC media player.The...
, a free softwareFree software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with minimal restrictions only to ensure that further recipients can also...
multicasted video streaming applicationApplication software is a computer program that functions and is operated by means of a computer, with the purpose of supporting or improving the software user's work. In other words, it is the subclass of computer software that employs the capabilities of a computer directly and thoroughly to a...
- Session Announcement Protocol
Session Announcement Protocol is a protocol for broadcasting multicast session information.A SAP listening application can listen to the well-known SAP multicast address and construct a guide of all advertised multicast sessions...
- CastGate
CastGate is an attempt from the ETRO-TELE research group at the Vrije Universiteit Brussel to break the chicken and egg problem of multicast adoption on the internet....
, an attempt at providing connectivity to the multicast network for hosts which have none
External links
- RFC 3170 (Sep. 2001)
- Multicast over TCP/IP HOWTO (Mar. 1998) describes Multicast in the Linux kernel, although some sections (specially multicast programs) are outdated and do not cover recent software.
- IETF Reliable Multicast Transport (rmt) Working Group (Aug. 2008)
- IETF Multicast & Anycast Group Membership (magma) Working Group
- IETF Protocol Independent Multicast (pim) Working Group
- IETF Source-Specific Multicast (ssm) Working Group
- IETF Multicast Security (msec) Working Group
- Multicast IP details at sockets.com
- Multicast on Packet Radio
- qpimd - PIM Daemon for Quagga: Multicast PIM
Protocol-Independent Multicast is a family of multicast routing protocols for Internet Protocol networks that provide one-to-many and many-to-many distribution of data over a LAN, WAN or the Internet...
module for the Quagga Routing SuiteQuagga is a network routing suite providing implementations of OSPF , RIP , BGP and IS-IS for Unix-like platforms, particularly FreeBSD, Linux, Solaris and NetBSD....
- Xorp, a free software
Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with minimal restrictions only to ensure that further recipients can also...
multicast router
- XCAST, a commercial MediaWiki-powered website based in Japan (English-language), devoted to Explicit Multicast technology.
- IP-Ethernet multicast tutorial.
- Southampton University multicast commentary
- Good illustrative video about IP multicast at Cisco.