can anyone tell me about the bandwidth weights?

Question

I have read about Computing Bandwidth weights in Tor directory protocol, version 3. But I could not understand about how bandwidth weights are calculated. And since they appear only once in consensus, how they help in path selection by network balancing?

Answer 1

Maybe it's easiest to explain this by example. The following comes from the current consensus:

[...]
r TorNinurtaName AA8YrCza5McQugiY3J4h5y4BF9g TsD6f/8KF1h7Npm/w2vgkzDumjY 2014-05-08 23:15:06 151.236.6.198 9001 9030
s Fast Guard HSDir Named Running Stable V2Dir Valid
v Tor 0.2.3.25
w Bandwidth=238
p reject 1-65535
[...]
bandwidth-weights Wbd=849 Wbe=0 Wbg=4145 Wbm=10000 Wdb=10000 Web=10000 Wed=8301 Wee=10000 Weg=8301 Wem=10000 Wgb=10000 Wgd=849 Wgg=5855 Wgm=5855 Wmb=10000 Wmd=849 Wme=0 Wmg=4145 Wmm=10000
[...]

The W?? numbers are the bandwidth weights. For example, Wgg is the "Weight for Guard-flagged nodes in the guard position".

If your client were to consider that relay as a possible guard, it would multiply its consensus weight 238 with Wgg, so 238*5855=1393490. The reason is that this relay has the Guard flag, but not the Exit flag. Your client does that for all relays and then selects one of them.