Re: constrained socket buffers patch

[Juliusz Chroboczek <Juliusz.Chroboczek@xxxxxxxxxxxxxx>]

>> Now the throughput (``bandwidth'') of a TCP connection is limited by
>> window/rtt.  What this means is that with ConstrainedSockets enabled,
>> your tor server will have basically unlimited throughput on a local
>> connection, but be limited to roughly 40 kB/s per connection (that's
>> bytes, not bits) over a transatlantic link[1].

> I'm interested to see your [1] where you do the numbers. :)

You asked for it ;-)

The distance between Paris to New York is 5851 km, which is pretty
close to 20 ms * c.  The speed of light in fiber is almost exactly
2/3 * c, which means that just the signal propagation gives a round
trip time of 60 ms; make it 70 ms to allow for router latency.

A 4 kB window contains 2 Ethernet-size packets (I'm assuming that your
TCP is sending full-size packets, i.e. that's it's doing SWS-avoidance),
so you send 3 kB of data every 70 ms, or 43 kB per second.

> Tor servers advertise aggregate throughput.

Good to hear -- I guess this solves the issue.

There is another issue, which doesn't appear in the above.  TCP is
extremely sensitive to packet loss when the window is smaller than
4 packets (fast retransmit doesn't work in that case).  So could
I suggest a default value for ConstrainedSockSize of 8 kB?

                                        Juliusz