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Chapter 9
Multicast
That was a simple overview; now let's look at how this is accomplished
in more detail. We will start by discussing three major host subscription
protocols:
IGMPv1
IGMPv2
CGMP
The differences among them will become apparent as we get further into the
discussion.
Internet Group Management Protocol Version 1 (IGMPv1)
As the name indicates,
Internet Group Management Protocol version 1
(IGMPv1)
was the first version of the protocol. It was a result of RFC 1112.
The purpose of this protocol is to allow hosts to subscribe to or join specified
multicast groups. By subscribing to groups, the hosts are thereby enabled to
receive multicast data forwarded from the router.
IGMP has several processes that it executes to manage multicast group
subscription and maintenance. We will discuss them in greater detail so you
can get an understanding of what happens.
IGMPv1 Processes
Three processes are employed by version 1 of IGMP:
Query
Joining
Leaving
These processes are the means by which multicast group membership is
maintained. The first two processes are functional processes, whereas the
Leaving process is more of a time-out than a formal request. Each process is
defined in detail in the following sections.
Membership Query Process
One important process is the
IGMP Query process
, which is kindred to a
keepalive procedure. Because the router needs to keep tabs on which multi-
cast groups need to remain active or be made active or inactive, it sends a
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