Wrong TCp checksum on server side?

Hi Everyone,

I am having a strange problem. My TCP packets get dropped on the server
side. When i did debugging, this is what found out.

TCP checksum on server side is wrong? Well its really strange, as i did not
modified anything in the TCP header and TCP data. I made modification in the
IP header only. I think this should not make any difference in the TCP
checksum.

When i checked the packet on client, its exactly the same packet. There is
no change in the TCP header and Data. On the client side, etherreal says,
correct checksum i.e. 0x43e0.
While on the server side, i checked the TCP header. Its exactly same.
Etherreal says Incorrect checksum (0x43e0), should be 0x43e4.

I am not sure what is happening, i have only modified the destination
address 172.10.10.16 to 172.10.10.12.

Observation, the difference between correct checksum and incorrect checksum
is 4, its exactly the same difference between previous and modified IP.

So i am not sure, what is happening to the TCP checksum. I read the
documentation, IP checksum is the checksum of IP header, while TCp checksum
is TCP header + TCP data.
Thanks a lot in advance.
Raj

TCP checksum includes the psuedo-header (some fields of the IP
header - see the RFC for details). If the psuedo-header
changes, TCP checksum needs to be re-calculated or "adjusted".

"Rajesh Gupta" <(E-Mail Removed)> wrote in message
news:OLtwv$(E-Mail Removed)...
> Hi Everyone,
>
> I am having a strange problem. My TCP packets get dropped on the server
> side. When i did debugging, this is what found out.
>
> TCP checksum on server side is wrong? Well its really strange, as i did
not
> modified anything in the TCP header and TCP data. I made modification in
the
> IP header only. I think this should not make any difference in the TCP
> checksum.
>
> When i checked the packet on client, its exactly the same packet. There is
> no change in the TCP header and Data. On the client side, etherreal says,
> correct checksum i.e. 0x43e0.
> While on the server side, i checked the TCP header. Its exactly same.
> Etherreal says Incorrect checksum (0x43e0), should be 0x43e4.
>
> I am not sure what is happening, i have only modified the destination
> address 172.10.10.16 to 172.10.10.12.
>
> Observation, the difference between correct checksum and incorrect
checksum
> is 4, its exactly the same difference between previous and modified IP.
>
> So i am not sure, what is happening to the TCP checksum. I read the
> documentation, IP checksum is the checksum of IP header, while TCp
checksum
> is TCP header + TCP data.
> Thanks a lot in advance.
> Raj
>
>