Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!apple!julius.cs.uiuc.edu!zaphod.mps.ohio-state.edu!think.com!mintaka!mit-eddie!uw-beaver!milton!sumax!ole!thebes!amc-gw!brian
From: brian@amc-gw.amc.com (Brian Crowley)
Newsgroups: comp.protocols.tcp-ip
Subject: IP Header Checksum Calculation
Message-ID: <5212@amc-gw.amc.com>
Date: 29 Jan 91 22:11:49 GMT
Organization: Applied Microsystems Corp.; Redmond, WA
Lines: 27



I wonder if someone can't give me some pointers on what I feel should
be a simple operation, but one that I just can't seem to get a handle on.

I am building an IP packet for transmission over an Ethernet Network.
No problem building the packet, big problems getting the IP header
checksum to come out correctly!

According to the RFC in front of me, the IP header checksum is
calculated by treating the IP header as a sequence of 16-bit values,
which are added together using one's complement arithmetic, and then
the one's complement of the result is used as the checksum.

To test my algorithm, I pulled some IP packets off our local network and 
ran the IP headers through.  Alas, the results of my program do not 
match the checksums stored in the packets!

Would some kind soul please send me their algorithm to accomplish this
feat?  A "C" code fragment, a written algorithm, any help you can give
would be appreciated.

---------------------------------------------------------------------------
|Brian Crowley              |          DNS: brian@amc.com                 |
|Applied Microsystems Corp. |         UUCP: uunet!amc-gw!brian            |
|Redmond, WA                |          ATT: 206-882-2000  Ext. 328        |
---------------------------------------------------------------------------