[tor-bugs] #27338 [Core Tor/sbws]: How long should sbws keep measured and observed bandwidths?

Mon Aug 27 15:16:33 UTC 2018

#27338: How long should sbws keep measured and observed bandwidths?
---------------------------+-------------------------------------
 Reporter:  teor           |          Owner:  (none)
     Type:  task           |         Status:  new
 Priority:  Medium         |      Milestone:  sbws 1.0 (MVP must)
Component:  Core Tor/sbws  |        Version:
 Severity:  Normal         |     Resolution:
 Keywords:                 |  Actual Points:
Parent ID:  #27108         |         Points:
 Reviewer:                 |        Sponsor:
---------------------------+-------------------------------------

Comment (by teor):

 Replying to [comment:2 juga]:
 > > We need to decide which strategy to use, update the bandwidth file
 spec, and implement this feature in sbws.
 >
 > i'm not sure how we're going to decide this. Try with each of the
 methods for 1 week and graph results and/or calculate % differences with
 Torflow?

 No, we have a method that is good enough, because it is close enough to
 torflow.

 So we need to use what we know about the tor network to make sure we have
 a good design. Let's make some some rules for the minimum viable product.
 Then we can merge any design that fits those rules.

 Here's what I suggest:

 The minimum viable product must:
 * Use the latest descriptor bandwidth limit, because:
   - the latest descriptor contains the limit that the operator has asked
 for
 * Use at least 2 sbws measured bandwidths over at least 2 days, because:
   - tor relay usage varies on a daily cycle
   - each sbws measurement depends on the time of day
 * Use at least 2 descriptor observed bandwidths over at least 2 days,
 because:
   - a single download by a single client can increase the observed
 bandwidth
   - for security, we want results that don't depend on a single client's
 behaviour
 * Don't keep bandwidths for more than 1 week
   - old bandwidths do not help us work out current relay capacity

 What do you think?
 Can you implement something based on these suggestions?

 If you want, I can write or review patches, or write a detailed spec.

 I put some other suggestions in #27346. They are complicated. We don't
 need them for the MVP release.

 > In case it's useful, the descriptors' observed bandwidth collected at
 the time of doing measurements in the results, for 2 days:

 Since most relays only observe bandwidth once per day, a 2 day collection
 is not long enough to be useful.

 > - number of relays' descriptor observed bandwidth: 6462
 > - mean of all relays' descriptor observed bandwidth taking the last for
 each relay: 5621550
 > - mean of all relays' descriptor observed bandwidth taking the mean from
 the relay's results: 5609508
 > - median of all relays' descriptor observed bandwidth taking the last
 for each relay: 2065215
 > - mean of all relays' descriptor observed bandwidth taking the mean from
 the relay's results: 2060907
 > - number of relays for which it was collected 1 descriptor observed
 bandwidth: 5087 (79%)
 > - number of relays for which it was collected 1 descriptor observed
 bandwidth: 1368 (21%)
 > - number of relays for which it was collected 1 descriptor observed
 bandwidth: 7 (0.11%)

 Do you mean 1, 2, 3 on the last 3 lines?

 > I've also being collecting descriptors' observed bandwidth every hour
 (in a separated script). Would be useful to compare only the descriptors'
 observed bandwidth collected in these 3 different ways?.

 It might be useful, but it is not essential. Let's focus on getting a
 minimal viable product. Then we can make small improvements later.

 > I'm having a lot of new code because of all the changes, tests and
 graphs, i could:
 > 1. continue with the experiments and make PR only when we have decided
 this
 > 2. keep the experiments code so that we can reproduce them in a future
 and start creating PRs with it.
 > Is it 2 ok?.

 Please create a PR that fits the minimum viable product rules above.
 Prefer code that is simple, fast to write, and easy to read.

 > For instance, If we collect descriptors' observed bandwidth, that's new
 code. I think it's fine i keep the code to store descriptors' observed
 bandwidth only at the time of doing measurements?. I can configure it in a
 way that the method to be used can be passed as parameter.

 Please implement one simple method for MVP 1.0.
 We don't need alternative methods.

 If you want, you can make the number of measured and observed bandwidths
 configurable. I suggest 2 measurements over 2 days is a good default.

 Replying to [comment:3 juga]:
 > Replying to [comment:2 juga]:
 >
 > > > Oh, and I think we should always use the latest
 {Relay,}Bandwidth{Rate,Burst}.
 > >
 > > Do you mean descriptors' bandwidth burst [0]?. We have not used it yet
 for anything. How should we use them?.

 The bandwidth burst can be ignored.

 > > We have only used descriptors' bandwidth average [1] to cap the
 measurements.

 That is ok.

 > Bandwidth in consensus[2] is min(observed bandwidth, bandwidth rate
 limit, 10MB/s)
 > I guess bandwidth rate limit here is bandwidth burst, right?

 How does the consensus help us, when we are looking at relay descriptors?

 If the "Measured=" bandwidth is available in the consensus, clients use
 it:
 https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n2601

 The bandwidth in the "w" line in the consensus is only used if the network
 has less than 3 bandwidth authorities voting.

 > Should the torflow or sbws scaled bandwidth be limited to the the
 bandwidth burst?. AFAIK torflow is not doing that.

 The descriptor has:
 {{{
 "bandwidth" bandwidth-avg bandwidth-burst bandwidth-observed NL
 }}}

 Torflow does:
 {{{
 bw_observed = min(bandwidth-avg, bandwidth-burst, bandwidth-observed)
 }}}
 https://gitweb.torproject.org/pytorctl.git/tree/TorCtl.py#n459

 But that's redundant, because tor relays do:
 {{{
 "bandwidth" min(RelayBandwidthRate, RelayBandwidthBust, BandwidthRate,
 BandwidthBurst, MaxAdvertisedBandwidth) min(RelayBandwidthBust,
 BandwidthBurst) bandwidth-observed NL
 }}}
 See get_effective_bwrate() and get_effective_bwburst().

 So you can use min(bandwidth-avg, bandwidth-burst) or just bandwidth-avg.
 The results will be the same.

 > > [0] https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n427
 > > [1] https://github.com/pastly/simple-bw-
 scanner/blob/master/sbws/lib/v3bwfile.py#L314
 > [2] https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n2595

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/27338#comment:4>
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