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Phantom packet loss is being shown when using pathping in connection with asynchronous routing - although there is no real loss.

  • From: Gunther Stammwitz
  • Date: Tue Jun 06 11:20:43 2006

Hallo colleagues,

Maybe someone of you can help me to understand the phenomenon of pack loss
when using asynchronous routing?

I have customers who are complaining about packet loss and they are
providing me with MTRs and pathpings (that's some sort of traceroute that
pings every hop it sees several times - comes with windows xp) that show the
loss starting at my routers and ending at their server (=the last hop). All
users are coming from a (dialup-)network where the way from them to our
servers are going via a carrier different than the carrier we are using to
route the traffic back to the dial user.
The interesting thing is that there is no loss at all when the users either
use a ping instead of this pathping/mtr-stuff or when I perform a ping or
even an mtr on my server in direction of the dialup customer. 

The nasty thing is that there is de facto NO LOSS on the line but the users
is seeing some sort of phantom loss.

The problem immediately disappears when I change to way back to the same
carrier as the way to us so that we have synchronous routing again.

My assumption is that pathping and mtr somehow get irritated by the icmp
messages due to a wrong timing or something like that. Any ideas?