On Wed, Jun 20, 2012 at 11:48 AM, Philip Homburg <philip.homburg@ripe.net> wrote:

On 6/20/12 9:11 , Randy Bush wrote:

we want to use atlas probes in an experiment. being prudent (you can
tell it was not i), we decided to try to get some basic calibration.
one run was just on a local LAN.

three hosts on the same gige switch
o probe 2285
o psg.com, a not very fancy or fast freebsd 9 box with intel/pro1000
gige ports
o bbgp.psg.com, a funky older freebsd 9 box with bge gige

probe 2285 pinging bbgp.psg.com, average RTT: 1.5606994382 [0], number
of pings: 356*3

psg.com pinging bbgp.psg.com, average RTT: 0.253424332344 [0], number of
pings: 674

has anyone done similar probe calibration experiments? does anyone have
any clue as to why the difference?

Just to confirm my suspicion, I tried to other way around:

This is an old AMD64 running FreeBSD pinging an Atlas probe on the same LAN:
$ ping 130.37.15.50
PING 130.37.15.50 (130.37.15.50): 56 data bytes
64 bytes from 130.37.15.50: icmp_seq=0 ttl=64 time=2.515 ms
64 bytes from 130.37.15.50: icmp_seq=1 ttl=64 time=0.913 ms
64 bytes from 130.37.15.50: icmp_seq=2 ttl=64 time=0.915 ms
64 bytes from 130.37.15.50: icmp_seq=3 ttl=64 time=0.929 ms

And this is the same FreeBSD box pinging a Celeron 766 MHz, running a micro kernel operating system, also on the same LAN:
$ ping prism
PING prism.hq.phicoh.net (130.37.15.36): 56 data bytes
64 bytes from 130.37.15.36: icmp_seq=0 ttl=96 time=0.364 ms
64 bytes from 130.37.15.36: icmp_seq=1 ttl=96 time=0.210 ms
64 bytes from 130.37.15.36: icmp_seq=2 ttl=96 time=0.211 ms
64 bytes from 130.37.15.36: icmp_seq=3 ttl=96 time=0.214 ms

This does not involve any of the Atlas software, just the ucLinux kernel running on the probe.

My conclusion is: probes are just very slow.

They are fine for measuring multi millisecond delays on WAN links but not for sub-millisecond delays on local links.