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

  • From: Joe Abley
  • Date: Tue Jun 06 12:36:28 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





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