[Winpcap-users] Out-going TCP/IP checksums

[Hadriel Kaplan]

You mean you see them for packets you're transmitting?  If it's a 3Com
mini-port driver-based one with tcp-offload capabilities, windows will
calculate the one's complement of the pseudo-header in the TCP checksum
field, to give the driver/NIC a starting point. (the driver/NIC then adds
onto it the TCP header and payload checksum, and complements it).  So you
will see a checksum of the pseudo-header only which the NIC/driver will
overwrite.  But on the receive side you should see the real one I think.
-hadriel

----- Original Message ----- 
From: "Quinton Tormanen" <quintont at deltacompsys.com>
To: <winpcap-users at winpcap.org>
Sent: Friday, June 03, 2005 7:54 AM
Subject: [Winpcap-users] Out-going TCP/IP checksums


I am using the direct Packet interface (e.g. PacketReceivePacket), and I
have found that the vast majority of TCP/IP packets being sent out by
the PC running packet show up with invalid TCP header checksums.  I am
running on Windows 2K, SP4 and the interface I am monitoring is a 3COM
Ethernet card.

I am assuming that the invalid checksums are not actually getting out
onto the wire, but instead has something to do with when the checksums
are computed.  Can anyone confirm that this is the case?

Quinton Tormanen
Software Engineer
Delta Computer Systems, Inc.
http://www.deltacompsys.com




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