#34257: Analyze unusual distribution of time to extend to first hop in circuit
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Reporter: karsten | Owner: metrics-team
Type: defect | Status: new
Priority: Medium | Milestone:
Component: Metrics/Onionperf | Version:
Severity: Normal | Resolution:
Keywords: | Actual Points:
Parent ID: | Points:
Reviewer: | Sponsor: Sponsor59-must
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Comment (by dennis.jackson):
I think arma's 1st hypothesis is exactly right. I ran Arthur's Exit
Scanning [https://github.com/arthuredelstein/tor_dns_survey script] over
the weekend which measures relay latency with two strategies:
* Fix a guard and build a 2-hop circuit for each exit.
* Fix an exit and build a 2-hop circuit for each guard.
The distribution of round trip times is roughly equivalent for both,
however the circuit construction times are much longer in the latter case
and roughly inline with what !OnionPerf reports. This seems to confirm the
theory as the first case we can reuse the existing connection, but the
latter case cannot.
I thee 'hidden' round trips prior to the first extension does seem to
account for all of the skew and the scaling effect. It also leads to the
question of what the right mode for !OnionPerf is? A different sampling
strategy would report much better first hop latency.
--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/34257#comment:16>
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