[tor-bugs] #16559 [Core Tor/sbws]: bwauth code needs to be smarter about failed circuits

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#16559: bwauth code needs to be smarter about failed circuits
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 Reporter:  TvdW              |          Owner:  juga
     Type:  defect            |         Status:  assigned
 Priority:  Medium            |      Milestone:  sbws: unspecified
Component:  Core Tor/sbws     |        Version:
 Severity:  Normal            |     Resolution:
 Keywords:  tor-bwauth, sbws  |  Actual Points:
Parent ID:                    |         Points:
 Reviewer:                    |        Sponsor:
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Comment (by juga):

 In `sbws`, when a relay is going to be measured, it selects randomly other
 relay that has double or equal bandwidth than the relay to measure, so it
 will likely not fail because of the other relay.
 The next time it will be measured, it will likely not be measured with the
 same other relay.

 However, the fastest relay will be restricted to the be measured with
 slower relays and small set of possible relays. There's an scaling process
 after this, but maybe it's a good idea that gets restricted anyway.

 In version 1.0.2, `sbws` was even prioritizing to measure relays with
 higher number of failures, but it was observed that then it'll continuosly
 try to measure unstable relays that will probably fail again.
 This has been removed in the last version and it only prioritizes relays
 to measure based on how long ago they were measured before.

 Regarding the exit policies, it only affects to choose whether the relay
 to measure will be the first or the second hop and it only checks that
 policy allows to exit to port 443.
 A reason why an exit might always fail to be measured is when it retrieves
 the data from a CDN, the local resolver returns an IPv6 address, and the
 exit can exit to an IPv6 address. Maybe this is something to be monitered,
 but it'd not happen when #28463 is implemented.

 I think this ticket can be closed, but it'd be great to get opinions on
 whether `sbws` design solves this.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/16559#comment:22>
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