timing problem in captured packets

I'm playng with the ping-5.click script tutorial made available by the
PATS group.

looking at the packets exchanged between the two router is quite strange

http://imageshack.us/photo/my-images/41/timealigned.png/
there two arp replies, then a ping reply, then two arp queries, and at
last a ping request. of course that's wrong!


sorting the packets by time anyway gives me this
http://imageshack.us/photo/my-images/684/numberaligned.png/
that is quite correct, except for the timings... some time is even negative!

why is that?
luca

-- 
Chiave pubblica http://luca.costantino.googlepages.com/luca.costantino.asc

Prima di tutto vennero a prendere gli zingari e fui contento, perché
rubacchiavano.
Poi vennero a prendere gli ebrei e stetti zitto, perché mi stavano antipatici.
Poi vennero a prendere gli omosessuali, e fui sollevato, perché mi
erano fastidiosi.
Poi vennero a prendere i comunisti, e io non dissi niente, perché non
ero comunista.
Un giorno vennero a prendere me, e non c’era rimasto nessuno a protestare.
(Martin Niemöller)