Hi,

On 2013-11-29, at 10:42, Toke Høiland-Jørgensen <toke@toke.dk> wrote:
> Well, what the LINCS people have done (the link in my previous mail) is
> basically this: Sniff TCP packets that have timestamps on them (i.e.
> with the TCP timestamp option enabled), and compute the delta between
> the timestamps as a latency measure.

we tried this too. The TCP timestamps are too coarse-grained for datacenter latency measurements, I think under at least Linux and FreeBSD they get rounded up to 1ms or something. (Midori, do you remember the exact value?)

> Putting timestamps into the TCP stream and reading them out at the other
> end might work; but is there a way to force each timestamp to be in a
> separate packet?

No, but the sender and receiver can agree to embed them every X bytes in the stream. Yeah, sometimes that timestamp may be transmitted in two segments, but I guess that should be OK?

> Do you know how that worked more specifically and/or do you have a link
> to the source code?

http://e2epi.internet2.edu/thrulay/ is the original. There are several variants, but I think they also have been abandoned:

http://thrulay-hd.sourceforge.net/
http://thrulay-ng.sourceforge.net/

Lars