Re: [tor-talk] understanding metrics bw graphs

[Karsten Loesing <karsten@xxxxxxxxxxxxxx>]

On 05/08/14 01:33, Nusenu wrote:
>>> Which would bring me to the next question (which would be 
>>> required to actually do the required accounting for (b)): Is a 
>>> relay able to tell whether it is being used as first or second 
>>> hop solely by looking at packets (not their source)?
> 
>> Looking at the packet source and comparing it to the list of
>> known relays would probably be the most reliable way for counting
>> bytes as first or second hop.  But this is not implemented, and
>> there are plans to implement this, to the best of my knowledge.
> 
> If relays are unable to tell in which position they are used, how
> does AllowSingleHopExits work? I guess by simple checking if the
> client IP matches an IP in the consensus?

From a quick look at the code: yes.

https://gitweb.torproject.org/tor.git/blob/HEAD:/src/or/connection_edge.c#l2494

Relays also distinguish whether a circuit was created using a
CREATE_FAST or a CREATE cell, but that distinction is going away with
CREATE_FAST cells being phased out.  See proposal 221 for details.

https://gitweb.torproject.org/torspec.git/blob/HEAD:/proposals/221-stop-using-create-fast.txt

So, yes, by comparing the identity presented by the previous hop with
the identities contained in the known consensus.

All the best,
Karsten
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