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

Tue Jun 6 16:31:09 UTC 2006

On 6-Jun-2006, at 08:19, Gunther Stammwitz wrote:

> 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)

(if it comes with win xp, then that sounds interesting-yet-surprising  
-- it's more usually found at <http://www.bitwizard.nl/mtr/>).

> [...]
>
> 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 starting point for any investigation like this is to compare the  
traceroute that apparently shows loss or other problems with  
traceroutes from strategic points in the path back to the source.

If there's a congestion problem which is the cause of the concern  
then comparing traceroutes in both directions will usually help find it.

If there's no congestion problem, or the apparent problem is unusual  
latency or loss in the numbers mtr displays for particular routers in  
the path, then mtr's ICMP echo requests towards the control elements  
of particular routers are probably being deliberately rate-limited by  
the operators of those routers.

Joe