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Filename: 177-flag-abstention.txt
Title: Abstaining from votes on individual flags
Author: Nick Mathewson
Created: 14 Feb 2011
Status: Reserve
Target: 0.2.4.x

Overview:

   We should have a way for authorities to vote on flags in
   particular instances, without having to vote on that flag for all
   servers.

Motivation:

   Suppose that the status of some router becomes controversial, and
   an authority wants to vote for or against the BadExit status of
   that router.  Suppose also that the authority is not currently
   voting on the BadExit flag.  If the authority wants to say that
   the router is or is not "BadExit", it cannot currently do so
   without voting yea or nay on the BadExit status of all other
   routers.

   Suppose that an authority wants to vote "Valid" or "Invalid" on a
   large number of routers, but does not have an opinion on some of
   them.  Currently, it cannot do so: if it votes for the Valid flag
   anywhere, it votes for it everywhere.

Design:

   We add a new line "extra-flags" in directory votes, to appear
   after "known-flags".  It lists zero or more flags that an
   authority has occasional opinions on, but for which the authority
   will usually abstain.  No flag may appear in both extra-flags and
   known-flags.

   In the router-status section for each directory vote, we allow an
   optional "s2" line to appear after the "s" line.  It contains
   zero or more flag votes.  A flag vote is of the form of one of
   "+", "-", or "/" followed by the name of a flag.  "+" denotes a
   yea vote, and "-" denotes a nay vote, and "/" notes an
   abstention.  Authorities may omit most abstentions, except as
   noted below.  No flag may appear in an s2 line unless it appears
   in the known-flags or extra-flags line.We retain the rule that no
   flag may appear in an s line unless it appears in the known-flags
   line.

   When using an appropriate consensus method to vote, we use these
   new rules to determine flags:

   A flag is listed in the consensus if it is in the known-flags
   section of at least one voter, and in the known-flags or
   extra-flags section of at least three voters (or half the
   authorities, whichever set is smaller).

   A single authority's vote for a given flag on a given router is
   interpreted as follows:

      - If the authority votes +Flag or -Flag or /Flag in the s2 line for
        that router, the vote is "yea" or "nay" or "abstain" respectively.
      - Otherwise, if the flag is listed on the "s" line for the
        router, then the vote is "yea".
      - Otherwise, if the flag is listed in the known-flags line,
        then the vote is "nay".
      - Otherwise, the vote is "abstain".

   A router is assigned a flag in the consensus iff the total "yeas"
   outnumber the total "nays".

   As an exception, this proposal does not affect the behavior of
   the "Named" and "Unnamed" flags; these are still treated as
   before.  (An authority can already abstain from a single naming
   decision by not voting Named on any router with a given name.)

Examples:

   Suppose that it becomes important to know which Tor servers are
   operated by burrowing marsupials.  Some authority operators
   diligently research this question; others want to vote about
   individual routers on an ad hoc basis when they learn about a
   particular router's being e.g. located underground in New South
   Wales.

   If an authority usually has no opinions on the RunByWombats flag,
   it should list it in the "extra-flags" of its votes.  If it
   occasionally wants to vote that a router is (or is not) run by
   wombats, it should list "s2 +RunByWombats" or "s2 -RunByWombats"
   for the routers in question.  Otherwise it can omit the flag from
   its s and s2 lines entirely.

   If an authority usually has an opinion on the RunByWombats flag,
   but wants to abstain in some cases, it should list "RunByWombats"
   in the "known-flags" part of its votes, and include
   "RunByWombats" in the s line for every router that it believes is
   run by wombats. When it wants to vote that a router is not run
   by wombats, it should list the RunByWombats flag in neither the s
   nor the s2 line.  When it wants to abstain, it should list "s2
   /RunByWombats".

   In both cases, when the new consensus method is used, a router
   will get listed as "RunByWombats" if there are more authorities
   that say it is run by wombats than there are authorities saying
   it is not run by wombats.  (As now, "no" votes win ties.)


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