[tor-relays] Bandwidth Measurement

[Julien ROBIN]

Hi,

I think that in this case, every existing connection and transfers act 
as "best effort" (it means that used bandwidth for some connection 
aren't absolutely reserved),  the new transfer is making a place by 
reducing just a little bit the others connection's available speed.

If 50 active connection are using 1 MB/s each at the same time, with a 
total amount of 50 MB/s which is the maximum of a given server for 
example, then a new connection trying to get 1 MB/s will make the 50 
Megabits divided by 51, then the usable bandwidth for all connection 
becomes 0,98 MB/s in the place of 1 MB/s

When your relay is very overused and severe congestion occurs, then the 
bandwidth measurement will be very poor, this will reduce the weight of 
your relay in the consensus in order to reduce the congestion. The 
algorithm is designed to give the ideal consensus weight for your relay 
in order to be as close as possible of your maximum available bandwidth, 
without placing too much users on it (with too much users on it, the 
total used bandwidth will be at the maximum, but each user get a painful 
browsing experience because the available bandwidth per user becomes too 
weak)

Bye !

Julien

PS : if the advertised bandwidth is incorrect then congestion will occur 
and the consensus weight of your relay will be decreased because of poor 
performance during measurements, that's how possible 
cheating/disturbance is avoided in this case, I think.

I think there is some documentation on this subject, by searching if you 
have some time and want to know more on this subject.


Le 20/02/2017 à 18:39, Marcobr a écrit :
> Hey everyone,
>
> as far as I know, the DAs running the bwauth scripts conduct hourly 
> bandwidth measurements. Depending on the advertised bandwidth of the 
> relay node, a smaller/bigger file gets transmitted on a custom 
> 2-hop-circuit and the transmission time determines the actual bandwidth.
>
> Now, here is my problem: Assuming that my relay node advertised 
> 50MB/s, and is currently at 98% load (49 MB/s). To conduct the 
> measurement, the bwauth script will send a 8MB file over my relay, but 
> since it can only serve another 1MB/s, how will the bwauth script 
> determine that my advertised 50MB/s is correct?
>
>
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