[tor-dev] Proposal Waterfilling

Florentin Rochet florentin.rochet at uclouvain.be
Wed Mar 7 08:28:04 UTC 2018

Hi Aaron,

Thanks for your comments, you are definitely touching interesting aspects.

Here are thoughts regarding your objections:

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

We do not believe that this is conclusive, as the market changes, and 
there certainly are dozens of other providers.

The same applies for 0-day attacks: if you need to buy them just for 
attacking Tor, then they are expensive. If you are an organization in 
the business of handling 0-day attacks for various other reasons, then 
the costs are very different. And it may be unclear to determine if it 
is easier/cheaper to compromise 1 top relay or 20 mid-level relays.

And we are not sure that the picture is so clear about botnets either: 
bots that can become guards need to have high availability (in order to 
pass the guard stability requirements), and such high availability bots 
are also likely to have a bandwidth that is higher than the water level 
(abandoned machines in university networks, ...). As a result, 
waterfilling would increase the number of high availability bots that 
are needed, which is likely to be hard.

2) Waterfilling makes it necessary for an adversary to run a larger 
number of relays. Apart from the costs of service providers, this large 
number of relays need to be managed in an apparently independent way, 
otherwise they would become suspicious to community  members, like 
nusenu who is doing a great job spotting all anomalies. It seems 
plausible that running 100 relays in such a way that they look 
independent is at least as difficult as doing that with 10 relays.

3) The question of the protection from relays, ASes or IXPs is puzzling, 
and we do not have a strong opinion about it. We focused on relays 
because they are what is available to any attacker, compared to ASes or 
IXPs which are more specific adversaries. But, if there is a consensus 
that ASes or IXPs should rather be considered as the main target, it is 
easy to implement waterfilling at the AS or IXP level rather than at the 
IP level: just aggregate the bandwidth relayed per AS or IXP, and apply 
the waterfilling level computation method to them. Or we could mix the 
weights obtained for all these adversaries, in order to get some 
improvement against all of them instead of an improvement against only 
one and being agnostic about the others.

4) More fundamentally, since the fundamental idea of Tor is to mix 
traffic through a large number of relays, it seems to be a sound design 
principle to make the choice of the critical relays as uniform as 
possible, as Waterfilling aims to do. A casual Tor user may be concerned 
to see that his traffic is very likely to be routed through a very small 
number of top relays, and this effect is likely to increase as soon as 
a  multi-cores compliant implementation of Tor rises (rust dev). Current 
top relays which suffer from the main CPU bottleneck will probably be 
free to relay even more bandwidth than they already do, and gain an even 
more disproportionate consensus weight. Waterfilling might prevent that, 
and keep those useful relays doing their job at the middle position of 

We hope those thoughts can help, and thanks again for sharing yours.


Florentin and Olivier

On 2018-03-05 23:30, Aaron Johnson wrote:
> Hello,
> I recently took the time to read the waterfilling paper. I’m not sure its a good idea even for the goal of increasing the cost of traffic correlation attacks. It depends on whether it is easier for an adversary to run many small relays of total weight x or a few large relays of total weight y, where x = y*c with c the fraction of a Guard-flagged relay used in the guard position (I believe that c=2/3 currently, as Wgg=7268 and Wmg=2732). Just to emphasize it: waterfilling requires *less bandwidth* to achieve a given guard probability as is needed in Tor currently.
> Based on prices I’ve seen (~$2/IP/month vs. ~$500/Gbps/month), its significantly cheaper to add a new relay than it is to add bandwidth commensurate with the highest-bandwidth relays. If an adversary finds it easier to compromise machines, then waterfilling might help as it lowers the guard probability of high-bandwidth relays. However, for adversaries with the resources to posses zero-day vulnerabilities against the well-run high-bandwidth relays, it seems to me that those adversaries would easily also have the resources to run relays instead, and in fact it would probably be cheaper for them to run relays as zero-days are expensive. Adversaries with botnets, which have many IPs but generally low bandwidth, would benefit from waterfilling, as it would increase the number of clients choosing them as guards that they can then attack. Waterfilling doesn’t clearly make things better or worse against network-level adversaries.
> Thus, it doesn’t seem to me that waterfilling protects Tor’s users against their likely adversaries, and in fact is likely to make things less secure in a few important cases.
> Best,
> Aaron
>> On Jan 31, 2018, at 5:01 PM, teor <teor2345 at gmail.com> wrote:
>> On 1 Feb 2018, at 07:15, Florentin Rochet <florentin.rochet at uclouvain.be> wrote:
>>>> On 18/01/18 01:03, teor wrote:
>>>>> I've added this concern within the 'unanswered questions' section. This
>>>>> proposal assumes relay measurement are reliable (consensus weight).
>>>> How reliable?
>>>> Current variance is 30% - 40% between identical bandwidth authorities, and
>>>> 30% - 60% between all bandwidth authorities.
>>>> Sources:
>>>> https://tomrittervg.github.io/bwauth-tools/#apples-to-apples-comparison
>>>> https://tomrittervg.github.io/bwauth-tools/#updated-01
>>>> Is this sufficient?
>>> My apologies, I was not enough specific: we assume bandwidth
>>> measurements reliable as an hypothesis to make the claim that
>>> Waterfilling is not going to reduce or improve the performance. If these
>>> measurements are not reliable enough, then Waterfilling might make
>>> things better, worse or both compared to the current bandwidth-weights
>>> is some unpredictable way.
>> This variance is measurement error. In this case, discretization error is
>> less than 1%.
>> We need to know whether measurement inaccuracy makes the network
>> weights converge or diverge under your scheme.
>> It looks like they converge on the current network with the current
>> bandwidth authorities. This is an essential property we need to keep.
>>> All of this depends on the bandwidth
>>> authority. Anyway, I willingly agree that we need some kind of tools
>>> able to report on such modification. Besides, those tools could be
>>> reused for any new proposal impacting the path selection, such as
>>> research protecting against network adversaries or even some of the
>>> changes you already plan to do (such as Prop 276).
>> Yes, we are hoping to introduce better tools over time.
>>>> <skip>
>>>>> - The upper bound in (a) is huge, and would be appreciated for an
>>>>> adversary running relays. The adversary could manage to set relays with
>>>>> almost 2 times the consensus weight of the water level, and still being
>>>>> used at 100% in the entry position. This would reduce a lot the benefits
>>>>> of this proposal, right?
>>>> I do not know how much the benefits of the proposal depend on the exact
>>>> water level, and how close relays are to the water level.
>>>> How much variance will your proposal tolerate?
>>>> Because current variance is 30% - 60% anyway (see above).
>>> The variance is not a problem if the water level is adapted
>>> (re-computed) at each consensus.
>> I'm not sure we're talking about the same thing here.
>> The variance I am talking about here is measurement error and
>> discretization error. Re-computation doesn't change the error.
>> (And going from relay measurement to consensus bandwidth can take hours.)
>> See my comment above about convergence: we need to converge in
>> the presence of discretization error, too.
>>>>> With your explanations below (weight change on clients), and given that
>>>>> the consensus diff size is a thing, I am leaning to believe that the
>>>>> weight calculation should be done on clients. Anyway, I have added a
>>>>> remark about this possibility within the proposal.
>>>> Another alternative is to apply proposal 276 weight rounding to these
>>>> weights as well.
>>>> https://gitweb.torproject.org/torspec.git/tree/proposals/276-lower-bw-granularity.txt
>>>> I think this may be our best option. Because running all these divisions on
>>>> some mobile clients will be very slow and cost a lot of power.
>>> Added this to the proposal. We might also "divide" the algorithm: what
>>> about computing the weights on dirauths but broadcasting only the pivot
>>> (the index of the relay at the water level). Clients can then resume the
>>> computation and produce the weights themselves with a reduced cost.
>>> Strength:
>>>   - The weight calculation would be O(n) on clients (n being the size of
>>> the guard set) instead of O(n*log(n))
>>>   - No impact on the consensus diff (well, except 1 line, the pivot value).
>>> Weakness:
>>>   - We still have O(n) divisions on the client, each time we download a
>>> new consensus.
>> Why not list the waterfilling level on a single line in the consensus?
>> That way:
>> * authorities do the expensive calculation
>> * clients can re-weight relays using a simple calculation:
>> if it is less than or equal to the waterfilling level:
>>   use the relay's weight as its guard weight
>>   use 0 as its middle weight
>> otherwise:
>>   use the waterfilling level as the relay's guard weight
>>   use the relay's weight minus the waterfilling level as its middle weight
>> This is O(n) and requires one comparison and one subtraction in the worst case.
>> T
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