Re: Timestamp Skew

["Gianluca Varenni" <gianluca.varenni () cacetech com>]

--------------------------------------------------
From: "Guy Harris" <guy () alum mit edu>
Sent: Friday, January 15, 2010 2:40 PM
To: "Community support list for Wireshark" <wireshark-users () wireshark org>
Subject: Re: [Wireshark-users] Timestamp Skew

> On Jan 14, 2010, at 12:33 PM, Gianluca Varenni wrote:
>
> > Well, you already got an answer from the WinPcap team... I work in the
> > WinPcap team.
> >
> > If a timestamp precision in the order of some milliseconds is ok for you,
> > then you can switch the timestamping mode to a less precise one that is
> > sync'ed with the system time. You can find details on how to change the
> > timestamping mode in this email:
> >
> > http://www.winpcap.org/pipermail/winpcap-bugs/2010-January/001153.html
>
> That should perhaps be in the WinPcap FAQ.  Using performance counters 
> could cause not only the failure to get NTP-synced timestamps but also, at 
> least in mode 3, failures on machines where the CPU isn't running at full 
> speed, such as
>
> https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=4249
>
> Does KeQueryPerformanceCounter(), on x86, use RDTSC but compensate for CPU 
> clock speed changes?

It's not documented, and depends on the version of Windows, the service 
pack, and the HAL type... Most of the times it's a non-compensated RDTSC, 
however.

GV


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