Path-spec - fast circuits

[Nick Mathewson]

2010/2/12 ilter yüksel <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, we
> pick a given router as an exit with probability proportional to its
> bandwidth."
>
> Could anybody explain why Tor pick exit router with probability proportional
> 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.

To see what the code is actually doing, the string to search for is
need_capacity or NEED_CAPACITY.  The most interesting layer to look
for this is at is where it's passed as a flag to
circuit_launch_by_router() or circuit_launch_by_extend_info().

-- 
Nick
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