Filename: 147-prevoting-opinions.txt
Title: Eliminate the need for v2 directories in generating v3 directories
Author: Nick Mathewson
Created: 2-Jul-2008
Status: Accepted
Target: 0.2.1.x

Overview

  We propose a new v3 vote document type to replace the role of v2
  networkstatus information in generating v3 consensuses.

Motivation

  When authorities vote on which descriptors are to be listed in the
  next consensus, it helps if they all know about the same descriptors
  as one another.  But a hostile, confused, or out-of-date server may
  upload a descriptor to only some authorities.  In the current v3
  directory design, the authorities don't have a good way to tell one
  another about the new descriptor until they exchange votes... but by
  the time this happens, they are already committed to their votes,
  and they can't add anybody they learn about from other authorities
  until the next voting cycle.  That's no good!

  The current Tor implementation avoids this problem by having
  authorities also look at v2 networkstatus documents, but we'd like
  in the long term to eliminate these, once 0.1.2.x is obsolete.

Design:

  We add a new value for vote-status in v3 consensus documents in
  addition to "consensus" and "vote": "opinion".  Authorities generate
  and sign an opinion document as if they were generating a vote,
  except that they generate opinions earlier than they generate votes.

  Authorities don't need to generate more than one opinion document
  per voting interval, but may.  They should send it to the other
  authorities they know about, at the regular vote upload URL, before
  the authorities begin voting, so that enough time remains for the
  authorities to fetch new descriptors.

  Additionally, authories make their opinions available at
     http://<hostname>/tor/status-vote/next/opinion.z
  and download opinions from authorities they haven't heard from in a
  while.

  Authorities MAY generate opinions on demand.

  Upon receiving an opinion document, authorities scan it for any
  descriptors that:
     - They might accept.
     - Are for routers they don't know about, or are published more
       recently than any descriptor they have for that router.
  Authorities then begin downloading such descriptors from authorities
  that claim to have them.

  Authorities MAY cache opinion documents, but don't need to.