Message-Id: <200302250411.UAA04101@postal.lbl.gov> From: brdraney@nersc.gov Subject: Re: tcptrace [OFFTOPIC?] Looking for cheap timestamping... Date: Thu, 27 Feb 2003 15:42:42 -0800
We also use tcpdump/libpcap with our Gigabit border taps. It turns
out that you can't really tcpdump at more than about 500Mbps without
doing some tricks. The best trick is interrupt coalescing but it
totally destroys the kernel timing of the packets.
It turns out that the syskonnect nic's have a fast timer and we've
(ok, Craig Leres) hacked the driver and got it to pass the timing
info up to the kernel.
This seems to have solved all of our timing issues.
Brent Draney
NERSC Network and Security
> 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. Because of the low jitter, 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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