TOR protocol simulation

[fiskio]

Hi everybody,
i am working on a simulation project on TOR.
I read all the documentation but there are still some thechnical question i
would like to ask. 

1 - Tor uses cells of 512 bytes and they are encrypted.
   I am wondering how much this encryption takes on an average machine.
   If the time involved is relevant compared to the time of transmission
   i 'll have to deal with that in order to get significant results. 

2 - How does it decide the circuit and how long is it?
   It seems a totally random decision but it must have some policy enforced
   like the number of nodes or which ones? am i right? 

3 - The circuit is changed every minute right? how often do the OP start a
   new circuit construction? And if there is no traffic does it stick with
   the old
   circuit or does it keep on changing it? 

4 - Since i want it to be accurate i am intrested in your policies for rate
   limiting and fairness. I am talking about the bucket approach and the
   euristich to distinguish an interactive stream. 

5 - The same is about congestion control. I could simulate the protocol
   without it but it seems very important in order to decrerase latency. 

Thanks to everybody who'll answer my questions. 

Marco Fiscato.