Path-spec - fast circuits

[Scott Bennett]

     On Fri, 12 Feb 2010 22:46:18 -0500 Nick Mathewson <nickm at freehaven.net>
wrote:
>2010/2/12 ilter y=FCksel <ilteryuksel at gmail.com>:
>> Hello,
>>
>> For exit router selection path-spec says that;
>>
>> "For circuits that do not need to be "fast", when choosing among multiple
>> candidates for a path element, we choose randomly. For "fast" circuits, w=
>e
>> pick a given router as an exit with probability proportional to its
>> bandwidth."
>>
>> Could anybody explain why Tor pick exit router with probability proportio=
>nal
>> to its bandwidth only for fast circuits? As far as i know Tor uses this
>> technique for load-balance. But why it uses this technique only for fast
>> circuits?
>
>First of all, "Fast" circuits are a bit misnamed as used in
>path-spec.txt.  Basically, "fast" means "bandwidth-sensitive".  The
>only ones that aren't don't need to be "fast" in this sense are ones
>that are going to be used only for a tiny amount of traffic.
>
>That said, I think the statement in path-spec.txt may be poor.  It
>probably makes sense to weight all choices by bandwidth, now that
>bandwidth is measured rather than just being self-advertised.
>
     I've withheld comment on the above for a long time, mainly because
I had intended to include it in a write-up that I still haven't found
the time to do, but I really think it cannot be avoided any longer.
I would greatly appreciate a justification for the presumption that
any process other than the tor node in question can possibly provide
a more accurate measurement of its data rate capacities.  Any other
process, *even on the same computer*--much less anywhere else, can only
measure the performance of the TCP connections between itself and the
tor node in question, whereas the tor node in question has a complete
picture of all of its simultaneous connections to all processes, wherever
they may exist around the planet.
     Further, there has been an implied claim in previous postings by
tor developers that inaccurate reporting of measured capacity has been
a significant problem to date and has adversely affected client selection
of routes from the perspective of optimal distribution of data flow
throughout the tor network.  It is therefore incumbent upon those
claimants to provide evidence in support of their claims.


                                  Scott Bennett, Comm. ASMELG, CFIAG
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