[tor-dev] Proposal 215: Let the minimum consensus method change with time

[Nick Mathewson]

Filename: 215-update-min-consensus-ver.txt
Title: Let the minimum consensus method change with time
Author: Nick Mathewson
Created: 15 Nov 2012
Status: Open


0. Overview

   This proposal suggests that we drop the requirement that
   authorities support the very old consensus method "1", and instead
   move to a wider window of recognized consensus methods as Tor
   evolves.

1. Background and Motivation

   When we designed the directory voting system, we added the notion
   of "consensus method" so that we could smoothly upgrade the voting
   process over time.  We also said that all authorities must support
   the consensus method '1', and must fall back to it if they don't
   support the method that the supermajority of authorities will
   choose.

   Consensus method 1 is no longer viable for the Tor network.  It
   doesn't result in a microdescriptor consensus, and omits other
   fields that clients need in order to work well.  Consensus methods
   under 12 have security issues, since they let a single authority
   set a consensus parameter.

   In the future, new consensus methods will be needed so that
   microdescriptor-using clients can use IPv6 exits and ECC
   onion-keys.  Rolling back from those would degrade functionality.

   We need a way to change the minimum consensus method over time.

2. Design

   I propose that we change the minimum consensus method about once
   per release cycle, or once per ever other release cycle.

   As a rule of thumb, let the minimum consensus method in Tor series
   X be the highest method supported by the oldest version that
   "anybody reasonable" would use for running an authority.
   Typically, that's the stable version of the previous release
   series.

   For flexibility, it might make sense to choose a slightly older
   method, if falling back to that method wouldn't cause security
   problems.


   For example, while Tor 0.2.4.x is under development, authorities
   should really not be running anything before Tor 0.2.3.x.  Tor
   0.2.3.x has supported consensus method 13 since 0.2.3.21-rc, so
   it's okay for 0.2.4.x to require 13 as the minimum method.  We even
   might go back to method 12, since the worst outcome of not using 13
   would be some warnings in client logs.  Consensus method 12 was a
   security improvement, so we don't want to roll back before that.

2.1. Behavior when the method used is one we don't know

   The spec currently says that if an authority sees that a method
   will be used that it doesn't support, it should act as if the
   consensus method will be "1".  This attempt will be doomed, since
   the other authorities will be computing the consensus with a more
   recent method, and any attempt to use method "1" won't get enough
   signatures.

   Instead, let's say that authorities fall back to the most recent
   method that they *do* support.  This isn't any likelier to reach
   consensus, but it is less likely to result in anybody signing
   something they don't like.


3. Likely outcomes

   If a bunch of authorities were to downgrade to a much older
   version, all at once, then newer authorities would not be able to
   sign the consensus they made.  That's probably for the best: if a
   bunch of authorities were to suddenly start running 0.2.0.x,
   consensing along with them would be a poor idea.

4. Alternatives

   We might choose a less narrow window of allowable method, when we
   can do so securely.  Maybe two release series, rather than one,
   would be a good interval to do when the consensus format isn't
   changing rapidly.

   We might want to have the behavior when we see that everybody else
   will be using a method we don't support be "Don't make a consensus
   at all."  That's harder to program, though.