IGMPv3 packet structure

IGMP letters are agitated in bald IP packets with IP agreement amount 2.7 There is no carriage band acclimated with IGMP messaging, agnate to ICMP for example.

editMembership Query Message

Membership Queries are beatific by multicast routers to actuate which multicast addresses are of absorption to systems absorbed to its network. Routers periodically forward General Queries to brace the accumulation associates accompaniment for all systems on its network. Group-Specific Queries are acclimated for free the accession accompaniment for a accurate multicast address. Group-and-Source-Specific Queries acquiesce the router to actuate if any systems admiration accession of letters beatific to a multicast accumulation from a antecedent abode defined in a account of unicast addresses.

IGMPv3 packet structure

bit offset 0–3 4 5–7 8–15 16–31

0 Type = 0x11 Max Resp Code Checksum

32 Group Address

64 Resv S QRV QQIC Number of Sources (N)

96 Source Abode 1

128 Source Abode 2

. . .

Source Abode N

Max Resp Cipher

This acreage specifies the best time (in 1/10 second) accustomed afore sending a responding report. If the amount is beneath 128, the amount is acclimated directly. If the amount is 128 or more, it is interpreted as an backer and mantissa.

Checksum

This is the 16-bit one's accompaniment of the one's accompaniment sum of the absolute IGMP message.

Group Abode

This is the multicast abode getting queried if sending a Group-Specific or Group-and-Source-Specific Query. The acreage is zeroed if sending a General Query.

Resv

This acreage is reserved. It should be zeroed if beatific and abandoned if received.

S (Suppress Router-side Processing) Banderole

When this banderole is set, it indicates to accepting routers that they are to abolish the accustomed timer updates.

QRV (Querier's Robustness Variable)

If this is non-zero, it contains the Robustness Variable amount acclimated by the sender of the Query. Routers should amend their Robustness Variable to bout the a lot of afresh accustomed Query unless the amount is zero.

QQIC (Querier's Query Interval Code)

This cipher is acclimated for specify the Query Interval amount (in seconds) acclimated by the querier. If the amount is beneath 128, the amount is acclimated directly. If the amount is 128 or more, it is interpreted as an backer and mantissa.

Number of Sources (N)

This acreage specifies the amount of antecedent addresses present in the Query. For General and Group-Specific Queries, this amount is zero. For Group-and-Source-Specific Queries, this amount is non-zero, but bound by the network's MTU.

Source Abode i

The Antecedent Abode i fields are a agent of n IP unicast addresses, area n is the amount in the Amount of Sources (N) field.