[tor-dev] Why the seeming anticorrelation between obfs3 and vanilla bridges in metrics graphs?

Arlo Breault arlo at torproject.org
Sat Oct 25 21:03:53 UTC 2014

On Thursday, October 23, 2014 at 10:32 AM, David Fifield wrote:
> In the past few months of bridge user graphs, there is an apparent
> negative correlation between obfs3 users and vanilla users: when one
> goes up, the other goes down. If you draw a horizontal line at about
> 5500, they are almost mirror images of each other. I don't see it with
> any other transport pairs. Any idea why it might be?
> I can see what could cause a simultaneous decrease in vanilla and
> increase in obfs3: Tor gets blocked somewhere and users switch to obfs3.
> But I wouldn't expect blocking events to look so smooth or happen so
> frequently, and it doesn't explain why the reverse change happens later
> (obfs3 being blocked while Tor is unblocked is less plausible). I can
> also understand the overall long-term trend of obfs3 increasing and
> vanilla decreasing. But I don't see why they should mirror each other so
> closely over short time periods.
> Some hypotheses:
> 1. There are lots of users who have a mix of vanilla and obfs3 bridges
> configured. Their tor (randomly?) chooses one of them, which usually
> works. The number of such users is constant over the short term;
> i.e. the sum of obfs3+vanilla is constant, but the proportion of
> obfs3 and vanilla fluctuates randomly.
> 2. Maybe vanilla-down/obfs3-up is caused by blocking events, and
> vanilla-up/obfs3-down is caused by natural new-user churn and/or
> coincidence.

My guess is that there’s an order

0. vanilla
1. obfs3
2. else

and it *always* starts from the top
and tries going down the list until it finds one that works.

So when vanilla is unreachable it tries obfs3.
At a later date, when vanilla is no longer blocked,
it moves back.
> 3. There is something about the way BridgeDB hands out bridges, or the
> way in which users use it, that causes it to give out obfs3 bridges
> at the expense of vanilla and vice versa.
> 4. Some kind of feedback loop: obfs3 bridges get used and get
> congested, so users switch to vanilla, which then get used and
> congested, etc.
> David Fifield
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> - userstats-bridge-transport-<OR>-obfs3-2014-08-01-2014-10-23.png

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