Chapter
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6.2.9 Multicast
6.2.9.1 Overview
Multicast, a network technology, allows one or multiple senders (multicast source) to
send a single packet to multiple receivers (at one time and simultaneously). Multicast
sources send packets to a specific multicast group; only those addresses in this
multicast group can receive packets. Multicast saves network bandwidth greatly
because a single packet is transmitted over any link of the whole network no matter
how many destination addresses there are. Multicast improves efficiency of data
transmission and reduces possible backbone congestion. Hosts in a multicast group can
be located in the same physical network or come from different physical networks (if
supported by a multicast router).
1. Prerequisite for the multicast technology:
To implement IP multicast transmission, the multicast source, receiver, and
lower-layer network between them must support multicast. This covers the
following:
1) The TCP/IP implementation of the host supports the sending and receiving of IP
multicast;
2) The host network interface supports multicast;
3) A set of group management protocols are available for addition, leave and query,
that is, the IGMP (v1, v2) must be provided;
4) A set of IP address allocation strategies must be provided to map L3 IP multicast
addresses to L2 MAC addresses;
5) The IP multicast application software must be supported;
6) The multicast technology is supported by all routers, hubs, switches, TCP/IP
stacks, and firewalls between the multicast sources and receivers.
2. Definition of the multicast address
In multicast communication, we need two kinds of addresses: One IP multicast
address and one Ethernet multicast address. The IP multicast address identifies a
multicast group. As all IP packets are encapsulated in an Ethernet frame, a
multicast Ethernet address is also needed. To make multicast run normally, the
host must be able to receive unicast and multicast data simultaneously. This
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