http://wiki.ethereal.com/TCP_20Checksum_20Verification
On Thu, 23 Dec 2004 21:26:17 -0800, Vitaly Kruglikov
<Vitaly.Kruglikov@xxxxxxxxxxx> wrote:
>
>
>
> Recently, I started paying attention to the TCP frames on PPPDump logs that
> I collected on a GPRS smart-phone device. I noticed that on a particular
> HTTP transaction, Ethereal identified every other packet coming from the
> HTTP server as having an *incorrect* TCP packet checksum. At the same time,
> Ethereal identified *all* "data-carrying" packets going from the smart-phone
> to the HTTP server as having an "incorrect" TCP packet checksum. In the
> phone-to-server direction, only the Ack packets were identified as having a
> correct TCP packet checksum. Also, none of the packets appeared to be
> retries.
>
>
>
> Could someone please help me understand how the HTTP data exchange is able
> to take place between the two machines with so much of the traffic being
> identified by Ethereal as having *incorrect* TCP packet checksums?
>
>
>
> I am using Ethereal 0.10.8 on Windows XP. I had the same results with
> 0.10.6.
>
>
>
> I am attaching the PPPDump trace (BadTCPChecksum.eth) to this posting,
> however this being my first posting, I am not sure whether the attachment
> will actually show up on the thread. I collected the PPP trace in the
> phone's PPP driver, so the server's possible offloading of TCP checksums
> should not have been a factor here. Also, there is no TCP check-sum
> offloading on the phone, so the packets sent from the phone should have all
> the fields filled in before I trace them. Please note that 4.23.88.219 is
> the HTTP server and 10.120.26.127 is the phone in this PPPDump. The server
> and client both make use of VJ compression and Selective ACKs.
>
>
>
> Many thanks,
>
>
>
> Vitaly
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