In computer networking, multicast is group communication 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 networks segments that currently resides members of the group.
Network assisted multicast may be implemented at the Internet layer using IP multicast,
which is often employed in Internet Protocol applications of streaming media, such as Internet
television scheduled content 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 point-to-multipoint
virtual circuits or Infiniband multicast.
IP multicast
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 TV channel. 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. By its nature, UDP is not reliable—messages
may be lost or delivered out of order. Reliable multicast protocols such as Pragmatic General
Multicast have been developed to add loss detection and retransmission on top of IP
multicast. 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. As of 2006, 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. Application layer multicast
Application layer multicast-over-unicast overlay services for application level group communication
are widely used. Notably the Internet Relay Chat, which is more pragmatic and scales better
for large numbers of small groups. IRC implements a single spanning tree across its overlay
network for all conference groups. However, this leads to suboptimal routing for some
of these groups. Additionally, IRC keeps a large amount of distributed states that limit
growth of an IRC network, leading to fractioning into several non-interconnected networks.
The lesser known PSYC technology uses custom multicast strategies per conference. Also
some peer-to-peer technologies employ the multicast concept when distributing content
to multiple recipients, known as peercasting. Explicit Multi-Unicast is an alternate multicast
strategy 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
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.
Multicast over wireless networks and cable-TV Wireless communications as well as cable TV
bus networks are inherently broadcasting media, i.e. multipoint channels, especially if the
antennas are omni-directional and radio/TV transmitters covering a region form a broadcasting
network that send the same content. However, the communication service provided may be
unicast, multicast as well as broadcast, depending on if the data is addressed to one, to a group
or to all receivers in the covered network, respectively.
In digital-TV, the concept of multicast service sometimes is used to refer to content protection
by broadcast encryption, i.e. encrypted content 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 of interactive multicast, for
example using IP multicast, may be used over TV broadcast networks in view the improve
the efficiency and 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 additional interaction
channel, where user equipment may send join an leave messages when the user changes TV
channel. Interactive multicast has been suggested as an efficient transmission scheme in DVB-H
and DVB-T2 terrestrial digital television systems, A similar concept is switched broadcast
over cable-TV networks, where only the currently most popular content is delivered in the cable-TV
network. Another similar concept is Cell-TV, i.e. implies
TV distribution over 3G cellular networks using the network-assisted multicasting offered
by the Multimedia Broadcast Multicast Service service, or over 4G/LTE cellular networks
with the eMBMS service. Scalable video multicast in an application
of interactive multicast, where a subset of the viewers receive additional data for high-resolution
video. Other multicast technologies
In an optical mesh network, protecting multicast lightpaths is one of the key concerns. The
most straight forward approach to protect a multicast tree is to establish a link-disjoint
backup tree which establishes dedicated protection. It is much easier to find an arc-disjoint
path for each leaf node in a light tree. The essence of protecting a multicast session
is to find a backup path for each destination node when a link on the working path to that
node fails. See also
References ^ Lawrence Harte, Introduction to Data Multicasting,
Althos Publishing 2008. ^ a b Media-communication based on Application-Layer
Multicast ^ RFC 1324 2.5.1 Size, IETF.
^ White paper, EU: PSYC . ^ Rick Boivie, Nancy Feldman, Yuji Imai, Wim
Livens, and Dirk Ooms. "Explicit Multicast Concepts and Options". Retrieved May 25, 2013.
^ 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, London, UK, June 2013.
^ 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
External links "IP multicast", iOS, Cisco .
MBONE Deployment, IETF . Multicast & Anycast Group Membership, IETF .
Multicast Mobility, IETF , Reliable Multicast Transport, IETF .
Protocol Independent Multicast, IETF . Source-Specific Multicast, IETF .
Multicast Security, IETF . IP-Ethernet multicast, CX: Firewall .
Scalable Adaptive Multicast, IRTF . Multicast, Sockets . IP details.
Multicast on Packet Radio, US packet . Multicast over TCP/IP, The GNU/Linux documentation
project, Mar 1998 . Multicast in Linux. Some sections are outdated and do not cover recent
software. qpimd – PIM Daemon for Quagga, GNU — PIM
module for the Quagga Routing Suite. RFC 1112: Host Extensions for IP Multicasting,
incl. mapping of IPv4-Multicast addresses to MAC-addresses
RFC 3170: IP Multicast Applications: Challenges and Solutions
RFC 3180: GLOP Addressing in 233/8 RFC 4608: Source-Specific Protocol Independent
Multicast in 232/8 RFC 5757: Multicast Mobility in Mobile IP
Version 6: Problem Statement and Brief Survey RFC 6224: Base Deployment for Multicast Listener
Support in Proxy Mobile IPv6 Domains RFC 7046: A Common API for Transparent Hybrid
Multicast Mobile Multicast Fast Handover, IETF . Multicast
Listener Extensions for MIPv6 and PMIPv6 Fast Handovers.
Mobile Multicast Sender Support, IETF . Mobile Multicast Sender Support in Proxy Mobile IPv6
Domains. "JSSM", Java, Oracle . Source-specific multicasts.
Multicast, UK: Southampton University . XCAST, JP , a commercial MediaWiki-powered
website on explicit multicast technology. Xorp – a free software router with multicast
support. Simlo, Kernen, AMT: Automatic IP Multicast
without explicit Tunnels, CH: EBU .