[tor-bugs] #2794 [Analysis]: Find a good metric for bridge churn

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#2794: Find a good metric for bridge churn
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 Reporter:  karsten   |          Owner:  karsten 
     Type:  task      |         Status:  assigned
 Priority:  normal    |      Milestone:          
Component:  Analysis  |        Version:          
 Keywords:            |         Parent:          
   Points:            |   Actualpoints:          
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Changes (by karsten):

 * cc: arma, Sebastian (added)


Comment:

 Screw the metric from 7 months ago.  Here's the new plan:

 While working on the Guard/Stable flag analysis (#2911), I came up with a
 new notion of bridge stability: "A stable bridge relay should be running
 on the same IP address a few days after a client learns about the bridge
 and should be available most of the time in the upcoming weeks."

 The idea is that there are two cases when a bridge that BridgeDB told us
 becomes useless: a) when the bridge isn't around much or b) when the
 bridge changes its IP address or port.

 I suggest to make Tonga calculate two metrics for all bridges, similar to
 how the directory authorities calculate them for relays: a) Weighted
 Fractional Uptime (WFU) and b) Mean Time Between Address Change (MTBAC).
 Tonga then assigns a StableBridge flag to bridges with a WFU at least as
 high as the median WFU and a MTBAC at least as high as the median MTBAC.
 BridgeDB would give out at least one bridge with the StableBridge flag
 (instead of doing so for the Stable flag).

 I'm going to evaluate this new flag by looking at the sanitized bridge
 descriptor archive of, say, the first half of 2011.  I should be able to
 reconstruct when a bridge failed (dropped out of the bridge network
 status) and when it changed its address or port.  The latter part may be
 difficult, because we're changing the key of the keyed hash function once
 per month.  The result is going to be sets of bridges that would have
 gotten the StableBridge flag at any given time in the analysis interval.
 Then I'm going to look up the future WFU and the time until next address
 change (TUNAC).  Ideally, most of the bridges with the StableBridge would
 have a WFU of 90% or more and a TUNAC of a few days or more.

 Does that approach make sense?  I'm asking now, because this analysis is
 going to keep me and Mr. CPU busy for a few days.  If there's something
 else I should consider in the analysis, please let me know before I start.

-- 
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/2794#comment:3>
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