PLANET XGS3-24040 - Manual (Page 388)

Chapter 43 Ipv4 Multicast Protocol
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relation information of group members among multicast switches, this work is accomplished by each multicast
routing protocol. All hosts participating in multicast must implement IGMP protocol.
Hosts participating IP multicast can join in and exit multicast group at any location, any time and without limit
of member total. Multicast switch does not need and not likely to save all relationships of all hosts. It only gets
to know if there are receivers of some multicast group, i.e. group member, on the network segment each
interface connects to. And the host only needs to save which multicast groups it joined.
IGMP is asymmetric between host and router: the host needs to respond the IGMP query messages of
multicast switches, i.e. to report message response in membership; the switch sends out membership query
messages periodically, and then determine if there are hosts of some specific group joining in the sub-network
it belongs to based on the received response message, and send out query of specific group (IGMP version2)
when receiving the report of a host exiting the group to determine if there exists no member in some specific
group.
Up to now, there are three versions of IGMP: IGMP version1 (defined by RFC1112), IGMP version2 (defined
by RFC2236) and IGMP version3 (defined by RFC3376).
The main improvements of IGMP version2 over version1 are:
1. The election mechanism of multicast switches on the shared network segment
Shared network segment is the situation of there is more than one multicast switch on a network segment.
Under this kind of situation, since all switches which runs IGMP under this network segment can get
membership report message from the host, therefore, only one switch is required to transmit membership
query message, so an exchange election mechanism is required to determine a switch as query machine.
In IGMP version1, the selection of query machine is determined by Multicast Routing Protocol; IGMP
version2 made an improvement for it, it prescribed that when there are more than one multicast switches on
the same network segment, the multicast switch with the lowest IP address will be elected as the query
machine.
2. IGMP version2 added Leave Group Mechanism
In IGMP version 1, the host leaves the multicast group silently without sending any notification to any
multicast switch. This causes that the multicast switch can only determine the leave of multicast member by
multicast group response time-out. But in version2, when a host decides to leave a multicast group, if it is
the host which gives response to the latest membership query message, then it will send out a message
implying it is leaving.
3. IGMP version 2 added the query to specific group
In IGMP version1, a query of multicast switch is for all multicast groups on the network segment. This query
is called general group query. In IGMP version2, query of specific group is added besides general group
query. The destination IP address of this kind of query message is the IP address of the multicast group,
the group address field part of the message is also the IP address of the multicast group. Thus it is
prevented that hosts which are other multicast group members transmit response message.
4. IGMP version2 added the biggest response time field
IGMP version2 added the biggest response time field to dynamically adjust the response time of the host to
group query message.