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

[David Chang]

Quinton,

Any TCP checksum that doesn't make it to the wire is suspect because many
NICs compute the checksum in hardware (after the O.S., but before it gets to
the wire).  Thus, smart drivers don't bother to compute it.  If the packet
made it to the wire with a checksum error, TCP would reject the packet.

DC

----- 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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