[tor-dev] Proposal 293: Other ways for relays to know when to publish

[Nick Mathewson]

Filename: 293-know-when-to-publish.txt
Title: Other ways for relays to know when to publish
Author: Nick Mathewson
Created: 30-May-2018
Status: Open
Target: 0.3.5


1. Motivation

   In proposal 275, we give reasons for dropping the published-on
   field from consensus documents, to improve the performance of
   consensus diffs.  We've already changed Tor (as of 0.2.9.11) to
   allow us to set those fields far in the future -- but
   unfortunately, there is still one use case that requires them:
   relays use the published-on field to tell if they are about to fall
   out of the consensus and need to make new descriptors.

   Here we propose two alternative mechanisms for relays to know that
   they should publish descriptors, so we can enact proposal 275 and
   set the published-on field to some time in the distant future.


2. Mechanism One: The StaleDesc flag

   Authorities should begin voting aon a new StaleDesc flag.

   When authorities vote, if the most recent published_on date for
   a descriptor has over DESC_IS_STALE_INTERVAL in the past, the
   authorities should vote to give the StaleDesc flag to that relay.

   If any relay sees that it has the StaleDesc flag, it should upload
   some time in the first half of the voting interval.  (Implementors
   should take care not to re-upload over and over, though: Relays won't
   lose the flag until the next voting interval is reached.)

   (Define DESC_IS_STALE_INTERVAL as equal to
   FORCE_REGENERATE_DESCRIPTOR_INTERVAL.)


3. Mechanism Two: Uploading more frequently when rejected.

   Tor relays should remember the last time at which they uploaded a
   descriptor that was accepted by a majority of dirauths.  If this
   time is more than FAST_RETRY_DESCRIPTOR_INTERVAL in the past, we
   mark our descriptor as dirty from
   mark_my_descriptor_dirty_if_too_old().


4. Implications for proposal 275

   Once most relays are running verions that support the features
   above, and once authorities are generating consensuses with the
   StaleDesc flag, there will no longer be a need to keep the
   published time in consensus documents accurate -- we can start
   setting it to some time in the distant future, per proposal 275.