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(Page 2 of 2)
Multicast Group Management
Group management encompasses all
of the activities required to set up groups of devices. They must be
able to dynamically join groups and leave groups, and information about
groups must be propagated around the IP internetwork. To support these
activities, additional techniques are required. The Internet Group
Management Protocol (IGMP) is the chief tool used for this purpose.
It defines a message format to allow information about groups and group
membership to be sent between devices and routers on the internet.
Multicast Datagram Processing and Routing
This is probably the most complicated:
handling and routing datagrams in a multicast environment. There are
several issues here:
- Since we are sending from one device to many
devices, we need to actually create multiple copies of the datagram
for delivery, in contrast to the single datagram used in the unicast
case. Routers must be able to tell when they need to create these copies.
- Routers must use special algorithms to determine
how to forward multicast datagrams. Since each one can lead to many
copies being sent various places, efficiency is important to avoid creating
unnecessary volumes of traffic.
- Routers must be able to handle datagrams sent
to a multicast group even if the source is not a group member.
Routing in a multicast environment
requires significantly more intelligence on the part of router hardware.
Several special protocols, such as the Distance Vector Multicast
Routing Protocol (DVMRP), and the multicast version of OSPF,
are used to enable routers to forward multicast traffic effectively.
These algorithms must balance the need to ensure that every device in
a group receives a copy of all datagrams intended for that group, with
the need to prevent unnecessary traffic from moving across the internetwork.
Key Concept: IP multicasting allows special applications to be developed where one device sends information to more than one other, across a private internet or the global Internet. It is more complex than conventional unicast IP and requires special attention particularly in the areas of addressing and routing.
This overview has only scratched
the surface of IP multicasting. The complexity involved in handling
groups and forwarding messages to multicast groups is one reason why
support for the feature has been quite uneven and as a consequence,
it is not used widely. Another issue is the demanding nature of multicasting;
it uses a great deal of network bandwidth for copies of messages, and
also requires more work of already-busy routers.
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Version 3.0 - Version Date: September 20, 2005
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