The Internet Group Management Protocol (IGMP) is used by IP
hosts to report their host group memberships to any immediately neighboring
multicast routers. IGMP is a integral part of IP. It must be implemented by all
hosts conforming to level 2 of the IP multicasting specification. IGMP messages
are encapsulated in IP datagrams, with an IP protocol number of 2.Version 3 of
IGMP adds support for source filtering. This indicates the ability for a system
to report interest in receiving packets;only from specific source addresses, or
from all but specific source addresses, sent to a particular multicast address.
The format of the IGMP packet is shown in the following illustration:
|
8
|
16
|
32 bits
|
|
Type
|
Max response
time
|
Checksum
|
|
Group address
|
|
IGMP packet structure
|
Version
The protocol version.
Type
The message type:
| 0x11 |
Membership query. There are 2 sub-types of
membership queries, general query and group-specific query. |
| 0x16 |
Version 2 membership report. |
| 0x17 |
Leave group. |
| 0x12 |
Version 1 membership report. Provides compatibility
with IGMPv1. |
| 0x22 |
Version 3 membership report. |
Max Response Time
Used only in Membership query messages. Specifies the maximum time allowed
before sending a responding report in units of 1/10 second. In all other
messages, it is set to 0 by the sender and ignored by the receiver.
Checksum
The checksum.
Group Address
In a Membership query message, The Group address is set to 0 when sending a
general query. It is set to the group address being queried, when sending a
group specific query or group-and-source-specific query. In a membership report
of a leave group message, it holds the IP multicast group address of the group
being reported or left.
IGMP Version Membership Report
Messages
IGMP Version Membership Report messages have the following format:
| 8 |
16 |
32 bits |
| Type |
Reserved |
Checksum |
|
| Reserved |
Number of group record |
| Group record |
| Group record |
| Group record |
| Group record |
Type
The Type field has a value of 21.
Reserved.
Set to zero on transmission, and ignored on reception.
Checksum
The Checksum is the 16-bit one's complement of the one's complement sum of the
whole IGMP message.
Number of Group Records (M)
The number of Group records present in this report.
Group Record
A block of fields containing information about the sender's membership in a
single multicast group, on the interface from which the report is sent.
Group Record Type
There are a number of different types of Group Records that may be included in a
Report message:
Current-State Record, which has the following record types:
1. Include mode
2. Exclude mode
Filter-Mode-Change Record, which has the following two record types:
3. Change to Include Mode
4. Change to Exclude Mode
Source-List-Change Record, which has the following two record types:
5. Allow New Sources
6. Block Old Sources |
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