>> 1) The cost of IPs vs. bandwidth is definitely a function of market offers. 
>> Your $500/Gbps/month seems quite expensive compared to what can be found on 
>> OVH (which is hosting a large number of relays): they ask ~3 euros/IP/month, 
>> including unlimited 100 Mbps traffic. If we assume that wgg = 2/3 and a 
>> water level at 10Mbps, this means that, if you want to have 1Gbps of guard 
>> bandwidth,
>> - the current Tor mechanisms would cost you 3 * 10 * 3/2 = 45 euros/month
>> - the waterfilling mechanism would cost you 3 * 100 = 300 euros/month
> 
> The question of what the cheapest attack is can indeed be estimated by 
> looking at market prices for the required resources. Your cost estimate of 
> 3.72 USD/Gbps/month for bandwidth seems off by two orders of magnitude.

I see that I misread your cost calculation, and that you estimated 
$37.20/Gbps/month instead of $3.72/Gbps/month. This still seems low by an order 
of magnitude. Thus, my argument stands: waterfilling would appear to decrease 
the cost to an adversary of getting guard probability compared to Tor’s current 
weighting scheme.

Best,
Aaron
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