Re: same first hops

[Scott Bennett <bennett@xxxxxxxxxx>]

     On Thu, 9 Oct 2008 19:23:48 +0100 Geoff Down <downie@xxxxxxxxxxxxxx>
wrote:
>On 9 Oct 2008, at 13:33, Scott Bennett wrote:
>>      While we're on this subject, I'd like to point out a problem with 
>> tor's
>> current data rate capacity testing during server initialization.  In 
>> order
>> to get some initial observations of the available data rates over a 
>> server's
>> network connections, tor builds a few (3?) test circuits that make a 
>> loop
>> from itself into the tor network and then back to itself.  At present 
>> it
>> uses the normal route length to do this, which can give a drastically 
>> low
>> measurement.  A better way would seem to be to use a single hop, i.e., 
>> a
>> circuit that goes to one other relay and the back to its source.  That 
>> may
>> still provide a low estimate however, so the value obtained from a 
>> single
>> hop test probably ought to be doubled for use as an estimate of the 
>> data rate
>> capacity of the server that is being initialized.
>
>Interestingly, I had about 6 single nodes showing on the Vidalia 
>network map yesterday, whilst my traffic was going via a normal 3-node 
>circuit and another 3-node circuit was in preparation.
>The single nodes disappeared after 20 minutes or so.
>
     Were those nodes your entry guards by any chance?  Although tor initially
tries to build a few (3?) circuits, once they have expired and no longer have
any active streams in them, they get torn down *except* for the links between
your client and the entry guard nodes.  This not only improves security, but
also means that a new circuit already has the first hop connected when tor
goes to build that new circuit.
     Of course, that doesn't explain why those links disappeared after about
20 minutes, and right offhand, no other explanation comes to mind.


                                  Scott Bennett, Comm. ASMELG, CFIAG
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