Thanks Graig for your reply.
I suspected it. I disabled TCP offload using ethtool but nothing changed. below is the ethtool output:

sudo ethtool -k eth0
Offload parameters for eth0:
rx-checksumming: off
tx-checksumming: off
scatter-gather: off
tcp-segmentation-offload: off
udp-fragmentation-offload: off
generic-segmentation-offload: off
generic-receive-offload: on
large-receive-offload: off
rx-vlan-offload: on
tx-vlan-offload: on
ntuple-filters: off
receive-hashing: off

Thanks,

> Subject: RE: TCP checksum not transmited to the wire!
> Date: Mon, 28 Jan 2013 11:22:22 -0500
> From: cjackson@ebscohost.com
> To: thisiskd@hotmail.com
> CC: kernelnewbies@kernelnewbies.org
>
> Many server-grade systems these days have TCP checksum offloads. Could
> this be what you're seeing?
>
> Craig Jackson
>
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