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147-prevoting-opinions.txt
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147-prevoting-opinions.txt
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Filename: 147-prevoting-opinions.txt
Title: Eliminate the need for v2 directories in generating v3 directories
Author: Nick Mathewson
Created: 2-Jul-2008
Status: Rejected
Target: 0.2.4.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 But a hostile, confused, or out-of-date server may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in the long term to eliminate these, once 0.1.2.x is obsolete.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.
[This proposal doesn't say what lines must be contained in opinion
documents. It seems that an authority that parses an opinion
document is only interested in a) relay fingerprint, b) descriptor
publication time, and c) descriptor digest; unless there's more
information that helps authorities decide whether "they might
accept" a descriptor. If not, opinion documents only need to
contain a small subset of headers and all the "r" lines that would
be contained in a later vote. -KL]
[This seems okay. It would however mean that we can't use the same
parsing logic as we use for regular votes. -NM]
[Authorities should use the same "valid-after", "fresh-until",
and "valid-until" lines in opinion documents as they are going to
use in their next vote. -KL]
[Maybe these lines should just get ignored on opinions. Or
omitted. -NM]
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
http://<hostname>/tor/post/opinion ,
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 SHOULD send their opinion document to all other
authorities OpinionSeconds seconds before voting and request
missing opinion documents OpinionSeconds/2 seconds before voting.
OpinionSeconds SHOULD be defined as part of "voting-delay" lines
and otherwise default to the same number of seconds as VoteSeconds.
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 also download corresponding extra-info descriptors for
any router descriptor they learned from parsing an opinion document.
Authorities MAY cache opinion documents, but don't need to.
Reasons for rejection:
1. Authorities learn about new relays from each others' vote documents.
See git commits 2e692bd8 and eaf5487d, which went into 0.2.2.12-alpha:
o Major bugfixes:
- Many relays have been falling out of the consensus lately because
not enough authorities know about their descriptor for them to get
a majority of votes. When we deprecated the v2 directory protocol,
we got rid of the only way that v3 authorities can hear from each
other about other descriptors. Now authorities examine every v3
vote for new descriptors, and fetch them from that authority. Bugfix
on 0.2.1.23.
2. Authorities don't serve version 2 statuses anymore.
Since January 2013, there was only a single version 3 directory
authority left that served version 2 statuses: dizum. moria1 and tor26
have been rejecting version 2 requests for a long time, and it was
mostly an oversight that dizum still served them. As of January 2014,
dizum does not serve version 2 statuses anymore. The other six
authorities have never generated version 2 statuses for others to be
used as pre-voting opinions.
3. Vote documents indicate that pre-voting opinions wouldn't help much.
From January 1 to 7, 2014, only 0.4 relays on average were not included
in a consensus because they were listed in less than 5 votes. These 0.4
relays could probably have been included with pre-voting opinions.
(Here's how to find out: extract the votes-2014-01.tar.bz2 tarball, run
`grep -R "^r " 0[1-7] | cut -c 4-22,112- | cut -d" " -f1,3 | sort | uniq
-c | sort | grep " [1-4] " | wc -l`, result is 63, divide by 7*24
published consensuses, obtain 0.375 as end result.)