[tor-project] Proxy Blocking Latency (preprint)

[David Fifield]

On Wed, Jun 01, 2016 at 10:51:43AM -0400, Philipp Winter wrote:
> On Mon, May 30, 2016 at 05:53:10PM -0700, David Fifield wrote:
> > Lynn Tsai and I, with the help of others, have been measuring how long
> > it takes for Tor Browser's default bridges to be blocked.
> 
> As always, great work!
> 
> I find it odd that the diurnal blocking pattern seems to change over
> time.  For example, ndnop3:24215 in Figure 1 seems completely
> unaccessible at first.  A couple of days later, the diurnal pattern
> seems to kick in.

We don't know what's causing it. It might be changing routes, for
example. We unfortunately don't have traceroutes. Or perhaps one of
their load balancers is dodgy, and they re-hashed everything on those
days.

I found it reminiscent of your paper,
https://petsymposium.org/2015/papers/01_Ensafi.pdf
particularly Fig. 7 and Table 1, which show bursty failures of the
firewall ("This shows that No-packets-dropped cases generally happen in
bursts of hours"). Though Roya said that particular measurement only
lasted a day so you wouldn't have been able to see any patterns.

> On what looks like February 20 and March 15, all bridges seemed
> accessible for a short amount of time from within China -- but also
> LeifEricson:41213 for your control machine.  I am curious how these two
> events relate?

The LeifEricson:41213 line in the figure is being scanned from China.
It's under "controls" not because it depicts the results from our U.S.
probe site, but because it was not added as a new bridge during the
experiment (it was already blocked when we started). Perhaps "controls"
is a bad way to label it on the figure. Everything in Figure 1 is
reachability from China.

We don't know what caused those brief periods of reachability. I can
only guess that they are temporary failures of the GFW.

Curiously, the brief span on reachability on March 27, visible in the
graph, coincides with a publicly reported GFW failure.
http://www.scmp.com/tech/china-tech/article/1931301/google-breaks-through-chinas-great-firewall-only-just-over-hour
The news article says Google was reachable from 15:30 to 17:15 (UTC).
The bridges were reachable from 10:00 to 18:00 (UTC).