[atlas] Questions about paris traceroute
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Pelsser Cristel
cristel at iij.ad.jp
Fri Jul 18 03:07:58 CEST 2014
On Jul 17, 2014, at 6:49 PM, Randy Bush <randy at psg.com> wrote: >>> 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. > > a multi-id measurement needs to be closely clusetered in time or changes > in routing or other network events can create ugly and unreproducible > (in the someone else tries the same experiment) results. > > don't know about others, but i don't really care about load balancers. > it's ecmp and lag on big pipes. With ecmp and lag, I hope that a routeur's hash stays consistent as long as the number of interfaces stays constant. The fact that a probe reboot may lead to a change in flow id is an issue. Cristel
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