[Winpcap-users] Where does the Winpcap timestamp come from?andothers

[John Wang]

Hi,

2009/5/5 Gianluca Varenni <gianluca.varenni at cacetech.com>

>
>
> ----- Original Message -----
> *From:* John Wang <locationdev at gmail.com>
> *To:* winpcap-users at winpcap.org
> *Sent:* Saturday, May 02, 2009 8:45 PM
> *Subject:* Re: [Winpcap-users] Where does the Winpcap timestamp come
> from?andothers
>
> Hi ,
>
>
>   >>2. Which function in the NPF or Packet.dll can be used as a trigger to
>> request CPU timer, like queryperformancecounter (), to >>timestamp the
>> arrival packet to obatin higher precision?
>>
>> >>--GV--
>> >>There is no way to obtain higher precision than we are currently using
>> in WinPcap, apart from using custom hardware that >>timestamps the packets
>> in the hardware itself.
>> >>--GV--
>>
>
> Is there any possible to know that which function inside WinPcap is used to
> timestamp the packets?
>
> The functions used to timestamp packets are in the driver code, in the file
> time_calls.h. Packets are timestamped in function "NPF_Tap", look for
> "GET_TIME"
>
>
>
>
>> >>4. Can I reduce the size of the user buffer inside the NPF to set it
>> small enough to triger the CPU timer to timestamp when >>every packet is
>> arrival?
>> >>-GV--
>> >>Timestamps are generated before the packet is stored in the buffers. So
>> changing the buffer size doesn't help.
>> >>--GV--
>>
>>
> Because I try to timestamp the arrival packets again by using my timing
> functions, I'm looking for a trigger. As I know, the frequency of NPF
> copying the arrival packets from kernel space to user space depends on the
> size of user buffer. If I can set the size of user buffer as same as the
> size of one arrival packet, then, the NPF would copy every single packet
> from kernel space to user space
> once the packet arrives. And I can use this operation as a trigger to
> timestamp the arrival packet again with my timing functions, am I right?
>
> You don't want to do that. Unless you want to kill your application
> performance. What exactly are you trying to achieve with your timing
> functions vs. the timestamps generated by WinPcap?
>
>

I'm trying to record the packets arrival timestamp in nanosecond precision.
Then I will compare the arrival timestamp with a microsecond precision
transmitting timesamp from the transmitter to explore the performance of
wireless network. If my method can't work, do you have any suggestion how
can I get the nanosecond packets arrival timestamp from the wireless adapter
or Winpcap?

Cheers

John
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