[Winpcap-users] timestamping and huge latency

[Fish" (David B. Trout]

Another thing: if the sending and receiving/capturing systems' clocks are
not synchronized with each other, then how are you measuring the supposed
latency between sending and receiving/capturing?

 

That is to say, if the two clocks are not synchronized with one another,
then normal clock drift would completely invalidate your measured latency in
my opinion.

 

Are the two system's clocks synchronized with one another?

 

If they're not then there's your answer.

 

 

 

p.s. if/when you do synchronize them with each other, remember to ensure
that either: a) neither or them synchronizes with an external clock, or: b)
only ONE of them does! (but not both!)  The same rule applies to the Windows
Time Service too: disable it on both or else ensure the one with it enabled
is configured as the "time server".  Otherwise your results would once again
be completely invalid/inconclusive.

-- 
  "Fish"  (David B. Trout) 
    fish at softdevlabs.com

 

From: winpcap-users-bounces at winpcap.org
[mailto:winpcap-users-bounces at winpcap.org] On Behalf Of Helmut Vaupotitsch
Sent: Tuesday, September 14, 2010 9:18 AM
To: winpcap-users at winpcap.org
Subject: [Winpcap-users] timestamping and huge latency
Importance: High

 

Hi Gianluca and all others,

I am facing a major latency problem on *long lasting* capture sessions which
maybe 
has to do with timestamping by the driver, every hint to solve it is
appreciated:

We developed a proprietary protocol to configure, manage and monitor 
our self-developed hardware, the config software uses WinPCap to capture and
send raw packets.

Everything is working fine, but after some days of continuous capturing i
face:
- On some machines, the latency between sending requests and receiving the
answer 
  increases to some seconds (can be up to >30 secs after capturing for a
week!)
Closing and re-opening the driver would solve the problem, but i definitely
need to capture
for months and longer without interrupt!

I know that the driver timestamp is drifting apart from the System
Time(which can be
synchronized by e.g. a NTP server), therefore i timestamp the frames my
myself(which 
is also important if a use timeouts)

My question is:
What could be the reason(s) for huge latency on long lasting captures?
I know that the latency increases on receiving packets
Currently i don´t know if sending´s latency also increases
Maybe it has something to do with the GetSystemTimeAdjustment setting?


Thanks for reading

Best regards from Austria
Helmut


Gianluca Varenni schrieb: 

The return value of QuerySystemTime and QueryPerformanceCounter is 
synchronized at the beginning of the capture (to compute the offset between 
epoch time and QueryPerformanceCounter), and then the counter and frequency 
returned by QPC are used to compute the number of seconds (and microseconds)

and added to the offset.
 
The timestamping code is available in the source code of WinPcap, 
winpcap\packetntx\driver\time_calls.h
 
Have a nice day
GV
 
 
 
--------------------------------------------------
From: "Jan Martinec"  <mailto:martij12 at fel.cvut.cz> <martij12 at fel.cvut.cz>
Sent: Tuesday, September 14, 2010 7:23 AM
To:  <mailto:winpcap-users at winpcap.org> <winpcap-users at winpcap.org>
Subject: [Winpcap-users] timestamp
 
  

Hello!
I've got a question about timestamping method. I know that a timestamp
is got using method QueryPerformanceCounter (resp.
keQueryPerformanceCounter), which is a number of ticks of Performance
counter. But timestamp is by Winpcap returned in "Seconds since Epoch"
format. So how is the recomputation done?
 
Thank you very much
 
Best regards,
Jan Martinec
 
 
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