IGMP Packet Format

IP DATAGRAM WITH IGMP HEADER

---

VERSION	IHL	ToS	Total Length
Identification		Flags |   Fragment offset
TTL	Protocol	Header Checksum
Source Address
Destination Address
Options			Padding
TYPE <- 8 bits -> MaxResp Time <-8 bits-> CHECKSUM <- 16 bits -> GROUP ADDRESS <- 32 bits -> < IGMP Payload or future expansion >

 

FIELD DESCRIPTIONS

---

Type

Type of IGMP message. There are three types: Membership Query, Membership Report and Leave Group.

Maximum Response Time

This field is used only in Membership Query messages. This field is the maximum time a host is allowed to produce and send a Membership Report message after receiving a Membership Query message.

Checksum

This is the one's compliment of the one's complement sum of the entire IGMP message, which basically works out to be the entire payload of the IP datagram the IGMP datagram is encapsulated within.

Group Address

Behavior of this field varies by the type of message sent:
Membership Query: (set to)
    General Query:  All zeroes
    Group Specific Query:  multicast group address
Membership Report:  multicast group address
Leave Group:  multicast group address

 

---