[dns-operations] Question for fellow users of RIPE ATLAS (broken/saturated probes or what?)

Moritz Muller moritz.muller at sidn.nl
Wed Dec 19 09:56:44 UTC 2018


Thanks for digging into the problem.
Definitely something we have to take into account when monitoring our systems.

Moritz

> On 18 Dec 2018, at 13:25, Philip Homburg <philip.homburg at ripe.net> wrote:
> 
> Hi,
> 
> I've been looking into what could cause the big difference in reported
> reponse times between V1/V2 probes and more modern probes.
> 
> My preliminary conclusion is that it caused by recording the start time
> of the measurement, for the purpose of computing the response time,
> before all initialisation is complete.
> 
> This initialisation code involves creating the DNS request to send,
> opening the socket, etc. It is just a bit of C code. However the CPUs in
> those probes is slow enough that a bit of code quickly adds up to quite
> a bit of time. There is also a bit of debug output left in this path,
> which doesn't help.
> 
> Hard to measure, but quite plausible is that oneoff measurement are
> delayed more because they run concurrent with periodic measurements.
> I.e., oneoff measurements experience interference from periodic
> measurements of which there are many. On average, periodic measurements
> would not experience interference from oneoff measurements because
> oneoff measurements are very infrequent.
> 
> Other effects, such as a delay in processing response packets when
> another measurement is using CPU time will also increase the recorded
> response time.
> 
> V1 and V2 probes can be excluded from measurements using probe tags. V1
> and V2 probes have tags system-v1 and system-v2. To exclude hose probes
> from a measurement add the tags to the exclude list in the probe
> specification of a measurement.
> 
> A few years ago, the Atlas team decided that deployment of new firmware
> for V1 and V2 probes would be limited to fixing security issues. We are
> currently evaluating whether or not to issue a new firmware release to
> fix this issue.
> 
> Philip
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