Hi all –
We’re seeing some slightly strange behaviours out of
Wireshark in certain situations; the system in question has both a Realtek
network card & a DekTec card.
- Capturing under Windows on either
card, there are negative (absolute, not relative) timestamps crop up periodically
throughout the trace. Reading around, there seemed to be an
incompatibility between an older version of WinPcap and a newer version of
Wireshark; this does not appear to be the problem here (bundled install).
Running a Linux version (from a LiveCD), the problem disappears with the
Realtek card; I have not yet built a LiveCD with the DekTec drivers to be
able to test that case. I’ve seen some suggestions that the
packet forwarding to the WinPcap driver under Windows are a little
indeterminate, is this the likely culprit?
- The packets being transmitted
have a defined period between them (about 1ms), yet at the start of every
capture, the delta is in the order of 1us instead. My best guess is
that the Realtek card buffer is being flushed to the pcap driver initially…?
Since the interface is in promiscuous mode before the capture starts, I’m
not sure that the assumption is valid.
My suspicion is that for absolute timing tests of this
nature, we really ought to be using a lower-level network analyser to keep the
vagaries of the OS out of the way.
All comments gratefully received.
Peter Bond
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