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

Wed Jun 7 19:35:09 UTC 2006

On Tue, Jun 06, 2006 at 05:19:33PM +0200, Gunther Stammwitz wrote:
> 
> 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? 

I can't tell you what is going on.  But I can ask, (a) why are you doing
asymmetrical routing in the first place?  and, (b) is it possible that
the MicroSoft versions of these tools are reporting errors BECAUSE of
the asynchronous routing?

-- 
Joe Yao
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