Internet Core Protocols: The Definitive Guide

Name

Checksum

Synopsis

Used to store a checksum of the IGMP message, allowing destination systems to validate the contents of the message and test for possible corruption.

Size

Sixteen bits.

Default Value

N/A

Defined In

RFCs 1112 and 2236.

Notes

The checksum field provides a simple validation scheme for IGMP messages. When calculating the checksum, the sending system looks at the entire message (with the Checksum field set to 0) and performs a simple checksum operation. This procedure is reversed by all recipients before the contents of the message are processed.

Note that the checksum applies to the entire message, even if the message is more than eight bytes long (longer messages may be a result of future versions or external protocols). As long as the Message Type is recognized as an IGMP “core” message, then the segment should be accepted and only the first eight bytes should be processed. However, the checksum value will apply to the entire message, and it is not limited to the first eight bytes of the message.

Also note that the Multicast Group field provides some additional sanity checking, allowing the recipient to verify that the IP datagram’s destination multicast address is the same as that used in the Multicast Group field.

Capture Sample

In the capture shown in Figure 4.10, the Checksum is shown as “ee 9b”.

Figure 4.10. The Checksum field

See Also

Message Type

Multicast Group