Manual

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Москва, Новозаводская ул., 18, стр. 1
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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.
The main features of version3 is allowing the host to choose receiving from or rejecting a
certain source, which is the basis of SSM (Source-Specific Multicast) multicast. For example,
when a host is sending a report of INCLUDE{10.1.1.1, 10.1.1.2} to some group G, that means
the host needs the router to forward the flux from 10.1.1.1 and 10.1.1.2; when a host is
sending a report of EXCLUDE{192.168.1.1} to some group G, that means the host needs the
flux from all sources of group G except 192.168.1.1. This makes a great difference from the
previous IGMP.
The main improvements of IGMP Version3 over IGMP Version1 and Version2 are:
The status to be maintained is group and source list, not only the groups in IGMPv2.
The interoperations with IGMPv1 and IGMPv2 are defined in IGMPv3 status.
IP service interface is modified to allow specific source list thereby.
The queried includes his/her Robustness Variable and Query Interval in query group to allow