How to compute an IP Checksum? 1

[bardley]

We need to verify checksums to check for bad packets. Can anyone explain this process to me, or point me toward some code that can?

Very much appreciated!
Brad Gunsalus
Cymtec Systems, Inc.
bgunsalus@cymtec.com

 

[MikeLacey]

from

http://www.ee.siue.edu/~rwalden/networking/iphead.html

Header Checksum: 16 bits

A checksum on the header only. Since some header fields change (e.g., time to live), this is recomputed and verified at each point that the internet header is processed.

The checksum algorithm is:

The checksum field is the 16 bit one's complement of the one's complement sum of all 16 bit words in the header. For purposes of computing the checksum, the value of the checksum field is zero.

This is a simple to compute checksum and experimental evidence indicates it is adequate, but it is provisional and may be replaced by a CRC procedure, depending on further experience.

There you go <wry smile> it even says it's &quot;simple&quot;.
Mike
michael.j.lacey@ntlworld.com
Email welcome if you're in a hurry or something -- but post in tek-tips as well please, and I will post my reply here as well.