Very Computer

by **Vlad Klimo** » Sun, 28 Feb 1999 04:00:00

Help ME!
I am trying to write nice program.
I need to generate 3 packets of tcp ip and sent it myself.
And i do it.
But CRS of header is incorrect.
Please help me.
I nee correct function to calculate CRS of TCP IP packet.
Language - C.
OS - Debian Linux

by **Duane Elmer Smecker** » Sun, 28 Feb 1999 04:00:00

CRC-16 uses the polynomial generator 11000000000000101
(That is x**16+x**15+x**2+x**0)

The algorithm is simple enough:
1) Take the original frame with the checksum set to 0 (augmented frame)
2) Divide the extended frame by the generator polynomial
11000000000000101 using MOD(2) division to get the remainder.
3) Subtract the remainder from the extended frame from step 1.

It is a simple shift and subtract algorithm.

I lifted it from:<cite>
"Computer Networks" by Tanenbaum, Prentice Hall, 1996, pp187-190
</cite>

>Help ME!
>I am trying to write nice program.
>I need to generate 3 packets of tcp ip and sent it myself.
>And i do it.
>But CRS of header is incorrect.
>Please help me.
>I nee correct function to calculate CRS of TCP IP packet.
>Language - C.
>OS - Debian Linux

by **Malwar** » Tue, 02 Mar 1999 04:00:00

Hi Vlad,

> I nee correct function to calculate CRS of TCP IP packet.

See following RFCs for detailed information and some code for
calculation the checksum: 1071, 1141, 1146, 1624.

Malware

by **Malwar** » Tue, 02 Mar 1999 04:00:00

Hi Duane,

> CRC-16 uses the polynomial generator 11000000000000101
> (That is x**16+x**15+x**2+x**0)

But IP, TCP and UDP do not use the CRC-16.

Malware

Very Computer

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