[tor-dev] Proposal 287: Reduce circuit lifetime without overloading the network.

teor teor2345 at gmail.com
Tue Dec 12 22:34:36 UTC 2017

> On 13 Dec 2017, at 06:33, Fernando Fernández Mancera <ffernandezmancera at gmail.com> wrote:
> Hi all,
> I have updated the proposal 287 taking into account the advice and
> comments that you gave me. So please take a look and if you have
> something to improve, don't hesitate to let me know.
> Thanks everyone for the help. :-)
> ffmancera.
> -----------------------------------
> Filename: 287-reduce-lifetime.txt
> Title: Reduce circuit lifetime without overloading the network
> Author: Fernando Fernandez Mancera
> Created: 30-Nov-2017
> Updated: 11-Dec-2017
> Status: Open
> Motivation:
> Currently Tor users are reusing a given circuit for ten minutes (by default)
> after it's first used. This time is too long because a malicious Exit relay can
> trace a user's pseudonymous profile, especially if connections from multiple
> protocols are put on the same circuit.
> This time is established on MaxCircuitDirtiness parameter and by default its
> value is ten minutes.
> I have been thinking in a way to fix this. The first idea that came to my mind
> was to use StreamIsolationByHost and StreamIsolationByPort on it, but I wasn't
> able to sort it out.
> One day, I thought "Why is time so important?" and later on I realized that
> maybe focusing on the amount of bytes running through the circuit could end up
> being a better approach on this problem.
> Design:
> I propose two options to reduce this problem, both based on taking into account
> the amount of bytes running through a circuit.
> MaxCircuitSizeDirtiness (temporal parameter name) will take an integer field
> that is contained on an interval and represents the maximum amount of bytes
> that can be written/read (we need to discuss about the use of one for both) by
> the circuit. If the circuit exceeds that amount, new streams won't use this
> circuit anymore.
> MaxCircuitSizeDirtinessByPort (temporal parameter name) will take an array of
> integers that are contained on an interval and represents the maximum amount of
> bytes that can be written/read (we need to discuss about the use of one for
> both) by the circuit per port (StreamIsolationByPort). This array is parallel
> to the array of ports from StreamIsolationByPort. If the circuit exceeds that
> amount, new streams won't use this circuit anymore.
> Regarding default values it would be useful to set up one a bit lower than the
> average amount of bytes per circuit. On MaxCircuitSizeDirtinessByPort after
> discussing it we shouldn't set up a default value because someone can identify
> the port used. About MaxCircuitDirtiness, if the others are set up by default
> it could be bigger, like thirty minutes, so if the user doesn't send/receive a
> significant amount of data the circuit will be changed anyway.
> In order to make it more difficult for an attacker to identify user by focusing
> on the established amount of bytes, we can introduce a range of
> MaxCircuitSizeDirtiness * [0.8, 1.1] per circuit.

Is there any reason you picked these values?

Typically, when we randomise something, we make sure the parameter
values is the average.

> Security Implications:
> It is believed that the proposed changes will improve the anonymity for end
> users. The end user won't reuse a given circuit if they have sent a
> considerable amount of bytes, thus making it more difficult for malicious Exit
> relays to be able to trace a user's pseudonymous profile.

Is there any research supporting this?
Is it compelling enough to offset the usability issues?
(See below.)

> About the security on Onion services, the proposed changes could improve the
> anonymity because if too much data gets sent through a single circuit, it makes
> it easy to identify the service's guard. It is not ensured that the guard
> changes but it increases the probability.

This we believe to be true, and we think we've seen attacks using it.
But changing circuits doesn't help those attacks.

Also, are you proposing that services mark circuits dirty?
Because Tor doesn't work like that right now.
Only clients mark circuits dirty.

> Obviously this is a probability, of course it's possible that sensitive data
> will leak in a little amount of data but it's even more possible that sensitive
> data will leak in a large amount.
> Specification:
> In order to implement this feature we will need to add some new
> functionalities. We need to parse MaxCircuitSizeDirtiness and
> MaxCircuitSizeDirtinessByPort from the torrc config file. We need to create a
> function or improve one to check the amount of bytes that are running through
> the circuit and if this amount is higher than the established value, consider
> the circuit dirty.
> Compatibility:
> The proposed changes should not create any compatibility issues. New Tor
> clients will be able to take advantage of this without any modification to the
> network.
> Implementation:
> It is proposed that MaxCircuitSizeDirtiness will be enabled by default and also
> increase MaxCircuitDirtiness to thirty minutes.
> It is proposed that MaxCircuitSizeDirtinessByPort won't be enabled by default
> for port 22, 53, and port 80 as StreamIsolationByPort.
> How it works with KeepAliveIsolateSOCKSAuth:
>  - If KeepAliveIsolateSOCKSAuth option is active, then if on
>    (MaxCircuitDirtiness * 0.5) minutes the amount of bytes sent/received
>    through the circuit doesn't surpass half of the established amount,
>    this amount will be reset.

This will break usability on many websites that depend on requests
coming from the same IP address. This is why Tor Browser manages its
own circuit lifetimes, and this change would break that.

>  - If KeepAliveIsolateSOCKSAuth option isn't active, then
>    MaxCircuitSizeDirtiness will work as normal.
> Performance and scalability notes:
> The proposed changes will reduce Tor network stress as users who do not exceed
> the set amount will reduce circuit generation by three (if default
> MaxCircuitDirtinesss value is thirty minutes).
> I want to work on demonstrate that by a research but first it's nice to get the
> idea accepted.
> References:
> Tor project research ideas [https://research.torproject.org/ideas.html]
> Enhancing Tor's Performance using Real-time Traffic Classification
> [https://www.cypherpunks.ca/~iang/pubs/difftor-ccs.pdf] (It's not exactly about
> that, but they talk about circuit lifetime and the ten minutes problem a few
> times. Also it is an interesting paper.)

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