Chapter 11 Multicast
What is multicasting?
Many new communication technologies are based on communication from one sender to
several recipients. Therefore, modern telecommunication systems such as voice over IP
or video and audio streaming (e.g. IPTV or Webradio) focus on reducing data traffic, e.g.
by offering TriplePlay (voice, video, data). Multicast is a cost-effective solution for effective
use of bandwidth because the sender of the data packet, which can be received by sever-
al recipients, only needs to send the packet once. The packet is sent to a virtual address
defined as a multicast group. Interested recipients log in to these groups.
Other areas of use
One classic area in which multicast is used is for conferences (audio/video) with several
recipients. The most well-known are probably the MBone Multimedia Audio Tool (VAT),
Video Conferencing Tool (VIC) and Whiteboard (WB). VAT can be used to hold audio
conferences. All subscribers are displayed in a window and the speaker(s) are indicated
by a black box. Other areas of use are of particular interest to companies. Here, multicast-
ing makes it possible to synchronise the databases of several servers, which is valuable
for multinationals or even companies with just a few locations.
Address range for multicast
For, IPv4 the IP addresses 224.0.0.0 to 239.255.255.255 (224.0.0.0/4) are reserved for
multicast in the class D network. An IP address from this range represents a multicast
group to which several recipients can log in. The multicast router then forwards the re-
quired packets to all subnets with logged in recipients.
Multicast basics
Multicast is connectionless, which means that any trouble-shooting or flow control needs
to be guaranteed at application level.
At transport level, UDP is used almost exclusively, as, in contrast to TCP, it is not based
on a point-to-point connection.
At IP level, the main difference is therefore that the destination address does not address
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11 Multicast
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