[atlas] Questions about paris traceroute

On 2014/07/17 11:28 , Randy Bush wrote:
>> While technically it would be possible to add some feature to the
>> system to do multiple paris variations at (or close to) the same time,
>> this would add a lot of complications to the code base and to result
>> parsing (which is already not simple). Instead, it is already possible
>> to schedule multiple measurements to run at roughly the same time,
>> which provides an approximation to what you describe here.
> 
> how close an approximation?  chasing ecmp and lag is bad enough without
> other changes.  folk need reproducible results.

As far as I know, the actual hashing algorithms used by load balancers
are in general not known.

So the only thing paris-traceroute can do is to try to provide
consistent results for each variation and try to create enough
difference between variations that it likely that all paths are tried.

In this context, it doesn't really matter if you try different
variations in one measurement or run different measurements. In both
cases some bits will be different. But it not possible to predict in
advance what is going to happen.

Worth pointing out in this context (and it is also documented somewhere)
is that if a probe reboots, the bits it uses for paris-traceroute are
likely to be different. So consistent results with a single measurement
are only achieved for as long as a probe doesn't reboot.

Philip