Re: How to handle TCP checksum, if adapter support TCP checksum offloading?

From: Stephan Wolf [MVP] (stewo68_at_hotmail.com)
Date: 08/03/04 

Date: Tue, 03 Aug 2004 23:19:05 +0200

See

http://www.google.com/groups?threadm=e006f0hduqmhs0tklt7b7opucnmmf3rpm1%404ax.com

 [Read the whole thread]

Stephan 

---
On Mon, 2 Aug 2004 16:20:08 -0700, "Rajesh Gupta" <guptar@gmail.com>
wrote:
>
>
>Hi Everyone,
>I am writting a IM driver, where i am modifing the Source Ip and Destonation
>IP in the outgoing packets.
>
>I am not sure what to do with the TCP checksum, if adapter supports TCP
>checksum offloading.
>
>I have tested my driver with adapter, which does not support TCP checksum
>offloading. It works fine. My checksum calculations are fine. When i am
>testing my IM driver on adapter which supports TCP checksum offloading, its
>not working.
>
>I am not sure, ho to handle TCp checksum offloading in IM driver. Here are
>the strange things i have noticed.
>
>1. If i do not change anything, then it does not work. Understood.
>
>2. If i calculate the TCP checksum (using TCp psuedo header and TCP segment)
>and insert this checksum in the TCP header. It works for the first TCp
>packet. For rest of the packets, its incorrect. I have written simple server
>and client socket application. So this checksum works for first packet i.e.
>SYN packet. Thats strange to me, why it worked for SYN and after that its
>incorrect for all other packets.
>
>NOTE: I am checking my packets on another machine. So i am seeing the
>correct data send on wire using etherreal.
>
>3. I read somewhere, that sometimes TCP calculate the checksum of
>psudoheader and fill this in the TCP header. So hardware only calculate the
>TCP segment checksum and does not have to touch the IP header. I tried that.
>Nothing works after that.
>
>I am not sure, why SYN worked and how should i handle this in my IM driver?
>
>Thanks a lot in advance.
>
>Rajesh

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