Filename: 290-deprecate-consensus-methods.txt
Title: Continuously update consensus methods
Author: Nick Mathewson
Created: 2018-02-21
Status: Open
1. Background
Directory authorities use the "consensus method" mechanism to achieve
forward compatibility during voting. When each authority publishes
its vote, it includes a list of numbered consensus methods that it
supports. Each authority chooses to calculate the consensus
according to the highest consensus method it knows supported by more
than 2/3 of the voting authorities. So long as all the authorities
have a method in common, they will all reach the same consensus.
Consensus method 1 was first introduced in the Tor 0.2.0 series
around 2008. But by 2012, we realized that we had a problem: we were
stuck documenting and supporting old consensus methods indefinitely.
With proposal 215, we deprecated and removed support for all
consensus methods before method 13. That was good as far as it went,
but it didn't solve the problem going forward: the latest consensus
methods is now 28.
This proposal describes a policy for removing older consensus methods
going forward, so we won't have to keep supporting them forever.
2. Proposal
I propose that from time to time, old consensus methods should be
deprecated.
Specifically, I propose that we deprecate all methods older than the
highest method supported in first stable release of the oldest LTS
(long-term support) release series.
For example, the current oldest LTS series is 0.2.5.x. The first
stable release in that series was 0.2.5.10. The highest consensus
method listed by 0.2.5.10 is 18. Therefore, we should currently
consider ourselves free to deprecate all methods before 18.
Once 0.2.5.x is deprecated, 0.2.9.x will become the oldest LTS
series. The first stable release in that series was 0.2.9.8. The
highest consensus method listed by 0.2.9.8 is 25. Therefore, once
0.2.5.x is deprecated (in May 2018), we may deprecate all methods
before 25.
When a consensus method is deprecated, it should no longer be listed
or implemented by the latest Tor releases. (It's okay for older
authorities to keep advertising it.)
Most consensus methods add a feature that is used in "method M or
later". Deprecating method M-1 means that the feature is used in all
supported consensus methods. Therefore, we can remove any code that
makes the feature conditional on a consensus method, and any code for
previous implementations of the feature.
Some consensus methods remove a feature that was used up to method
N. Deprecating method M means that the feature is no longer used by
any supported consensus methods. Therefore, we can remove any code
that implements the feature.
A. Acknowledgments
Thanks to isis and teor for the discussion that led to this proposal.
I believe that teor first suggested the policy described in section 2
above.
B. Client and relay compatibility notes
Dear reader: you may be worrying that this proposal will cause old
clients or relays to stop working prematurely. That is not the case.
Consensus methods determine how the authorities behave, but they do
not represent backward-incompatible changes in how they generate
their consensuses.
