[tor-dev] Some information about Tor relays

Rob Jansen rob.g.jansen at nrl.navy.mil
Fri Aug 26 18:39:38 UTC 2016

> On Aug 26, 2016, at 2:15 AM, grarpamp <grarpamp at gmail.com> wrote:
>> On Fri, Aug 26, 2016 at 01:42:38AM +0000, Liu, Zhuotao wrote:
>>> We hope to have an estimate about computation capacity of Tor relays. For
>>> instance, how many circuits a relay can maintain when its CPU is driven to
>>> about 100%? On average, how many circuits are maintained by a busy guard
>>> and
>>> what the CPU utilization is. These kinds of information would be really
>>> helpful.
> I used to report CPU exhaustion when pushing 15-25 high circuit
> flux application streams in parallel through a client and thus its guards.
> To gather and characterize current limitations in an operational context
> you might want to deploy a guard at your university and run some
> clients through it, instrumenting various things, until something saturates.
> I'd be interested in seeing estimates of what the net change in
> network usable CPU headroom [1] is when adding relays using certain
> fixed ratios of their own cpu/circuits and or cpu/clients and or
> cpu/bandwidth capacities.
> Perhaps in other words... we roughly know how a clients stream
> over 3 or 6 hops might consume an additional 1Gbps added to
> the network. But what does adding its CPU to the network
> get us... and effect of clients/net on that. And with each box
> added, are we adding the right ratio of CPU and bandwidth,
> do we need a knob there to ensure optimum balanced benefit
> to the net, or is it better to leave it float.
> [1] Left over for network meta purposes like circuit construction,
> directory services, consensus, parametric pathing computation, etc.

Hi Zhuotao,

We have performed some privacy-preserving measurements including the number of circuits and streams seen at exit relays, the amount of data transferred by exit relays, and the number of active/inactive clients connecting to entry relays. We only collected over small timeframes, and we didn't collect anything related to relay computation capacity, but you may be able to make some inferences based on our results.

The measurement system we developed, called PrivCount [1], uses differential privacy and secure aggregation protocols and is described in our upcoming paper "Safely Measuring Tor" [2] that will appear at the 23rd ACM Conference on Computer and Communication Security (CCS) in October. The measurement results are also presented in that paper.

Some highlights from the paper:

+ Tor has about 710,000 unique connected clients at any given time on average, of which about 550,000 (77%) are active (the remaining are connected but inactive). For comparison, Tor itself estimates about 1.75 million user *per day*, suggesting that the user population turns over about 2.5 times per day.

+ Data over ports 80 and 443 accounts for about 91% of the traffic exiting Tor, which is up from about 42% in 2010. This suggests that either there was a shift of file-sharing traffic onto standard web ports, or lower file-sharing usage overall, or both.

Hope this helps!


[1] https://github.com/privcount/privcount
[2] http://www.robgjansen.com/publications/privcount-ccs2016.pdf

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