Date: Thu, 27 Feb 2003 22:48:31 +0100 Subject: Re: tcptrace [OFFTOPIC?] Looking for cheap timestamping... Message-ID: <20030227214831.GA32661@pusa.informat.uv.es> From: uaca@alumni.uv.es
Hi Jeremy
Thanks for your reply
On Thu, Feb 27, 2003 at 12:32:28PM -0800, Jeremy Elson wrote:
> uaca@alumni.uv.es writes:
> >I'm looking for inexpensive Gigabit Ethernet hardware that could attach to
> >Linux with basic packet timestamping capabilities.
>
> I've never used Gigabit ethernet, but using the libpcap library to
> capture packets gives us excellent timestamps for 100mbit ethernet.
> Most ethernet drivers timestamp packets in the interrupt handler so it
> has fairly low jitter. This timestamp is available if you use libpcap
> to capture the packets, as opposed to the normal socket interface in
> which case it's not available through any mechanism I know.
>
> Such in-kernel timestamping uses the kernel's internal gettimeofday()
> which has 1 usec resolution.
I'm aware of this
> Because of the low jitter,
What about interrupt latency mostly caused by interrupts being disabled?
Ulisses
> we've been
> able to use this feature to get order-1 usec precision timesync
> between machines, as described in our OSDI 2002 paper at
> http://lecs.cs.ucla.edu/Publications/papers/broadcast-osdi.pdf
>
> Our code is available online at http://cvs.cens.ucla.edu. In
> particular, the file
> http://cvs.cens.ucla.edu/lxr/source//cens/timesync/syncd/driver_udp.c
> shows how to read UDP datagrams using libpcap and extract the
> kernel-generated timestamps.
>
> Regards,
> Jeremy
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