Filename: 101-dir-voting.txt
Title: Voting on the Tor Directory System
Author: Nick Mathewson
Created: Nov 2006
Status: Closed
Implemented-In: 0.2.0.x
Overview
This document describes a consensus voting scheme for Tor directories;
instead of publishing different network statuses, directories would vote on
and publish a single "consensus" network status document.
This is an open proposal.
Proposal:
0. Scope and preliminaries
This document describes a consensus voting scheme for Tor directories.
Once it's accepted, it should be merged with dir-spec.txt. Some
preliminaries for authority and caching support should be done during
the 0.1.2.x series; the main deployment should come during the 0.2.0.x
series.
0.1. Goals and motivation: voting.
The current directory system relies on clients downloading separate
network status statements from the caches signed by each directory.
Clients download a new statement every 30 minutes or so, choosing to
replace the oldest statement they currently have.
This creates a partitioning problem: different clients have different
"most recent" networkstatus sources, and different versions of each
(since authorities change their statements often).
It also creates a scaling problem: most of the downloaded networkstatus
are probably quite similar, and the redundancy grows as we add more
authorities.
So if we have clients only download a single multiply signed consensus
network status statement, we can:
- Save bandwidth.
- Reduce client partitioning
- Reduce client-side and cache-side storage
- Simplify client-side voting code (by moving voting away from the
client)
We should try to do this without:
- Assuming that client-side or cache-side clocks are more correct
than we assume now.
- Assuming that authority clocks are perfectly correct.
- Degrading badly if a few authorities die or are offline for a bit.
We do not have to perform well if:
- No clique of more than half the authorities can agree about who
the authorities are.
1. The idea.
Instead of publishing a network status whenever something changes,
each authority instead publishes a fresh network status only once per
"period" (say, 60 minutes). Authorities either upload this network
status (or "vote") to every other authority, or download every other
authority's "vote" (see 3.1 below for discussion on push vs pull).
After an authority has (or has become convinced that it won't be able to
get) every other authority's vote, it deterministically computes a
consensus networkstatus, and signs it. Authorities download (or are
uploaded; see 3.1) one another's signatures, and form a multiply signed
consensus. This multiply-signed consensus is what caches cache and what
clients download.
If an authority is down, authorities vote based on what they *can*
download/get uploaded.
If an authority is "a little" down and only some authorities can reach
it, authorities try to get its info from other authorities.
If an authority computes the vote wrong, its signature isn't included on
the consensus.
Clients use a consensus if it is "trusted": signed by more than half the
authorities they recognize. If clients can't find any such consensus,
they use the most recent trusted consensus they have. If they don't
have any trusted consensus, they warn the user and refuse to operate
(and if DirServers is not the default, beg the user to adapt the list
of authorities).
2. Details.
2.0. Versioning
All documents generated here have version "3" given in their
network-status-version entries.
2.1. Vote specifications
Votes in v3 are similar to v2 network status documents. We add these
fields to the preamble:
"vote-status" -- the word "vote".
"valid-until" -- the time when this authority expects to publish its
next vote.
"known-flags" -- a space-separated list of flags that will sometimes
be included on "s" lines later in the vote.
"dir-source" -- as before, except the "hostname" part MUST be the
authority's nickname, which MUST be unique among authorities, and
MUST match the nickname in the "directory-signature" entry.
Authorities SHOULD cache their most recently generated votes so they
can persist them across restarts. Authorities SHOULD NOT generate
another document until valid-until has passed.
Router entries in the vote MUST be sorted in ascending order by router
identity digest. The flags in "s" lines MUST appear in alphabetical
order.
Votes SHOULD be synchronized to half-hour publication intervals (one
hour? XXX say more; be more precise.)
XXXX some way to request older networkstatus docs?
2.2. Consensus directory specifications
Consensuses are like v3 votes, except for the following fields:
"vote-status" -- the word "consensus".
"published" is the latest of all the published times on the votes.
"valid-until" is the earliest of all the valid-until times on the
votes.
"dir-source" and "fingerprint" and "dir-signing-key" and "contact"
are included for each authority that contributed to the vote.
"vote-digest" for each authority that contributed to the vote,
calculated as for the digest in the signature on the vote. [XXX
re-English this sentence]
"client-versions" and "server-versions" are sorted in ascending
order based on version-spec.txt.
"dir-options" and "known-flags" are not included.
[XXX really? why not list the ones that are used in the consensus?
For example, right now BadExit is in use, but no servers would be
labelled BadExit, and it's still worth knowing that it was considered
by the authorities. -RD]
The fields MUST occur in the following order:
"network-status-version"
"vote-status"
"published"
"valid-until"
For each authority, sorted in ascending order of nickname, case-
insensitively:
"dir-source", "fingerprint", "contact", "dir-signing-key",
"vote-digest".
"client-versions"
"server-versions"
The signatures at the end of the document appear as multiple instances
of directory-signature, sorted in ascending order by nickname,
case-insensitively.
A router entry should be included in the result if it is included by more
than half of the authorities (total authorities, not just those whose votes
we have). A router entry has a flag set if it is included by more than
half of the authorities who care about that flag. [XXXX this creates an
incentive for attackers to DOS authorities whose votes they don't like.
Can we remember what flags people set the last time we saw them? -NM]
[Which 'we' are we talking here? The end-users never learn which
authority sets which flags. So you're thinking the authorities
should record the last vote they saw from each authority and if it's
within a week or so, count all the flags that it advertised as 'no'
votes? Plausible. -RD]
The signature hash covers from the "network-status-version" line through
the characters "directory-signature" in the first "directory-signature"
line.
Consensus directories SHOULD be rejected if they are not signed by more
than half of the known authorities.
2.2.1. Detached signatures
Assuming full connectivity, every authority should compute and sign the
same consensus directory in each period. Therefore, it isn't necessary to
download the consensus computed by each authority; instead, the authorities
only push/fetch each others' signatures. A "detached signature" document
contains a single "consensus-digest" entry and one or more
directory-signature entries. [XXXX specify more.]
2.3. URLs and timelines
2.3.1. URLs and timeline used for agreement
An authority SHOULD publish its vote immediately at the start of each voting
period. It does this by making it available at
http://<hostname>/tor/status-vote/current/authority.z
and sending it in an HTTP POST request to each other authority at the URL
http://<hostname>/tor/post/vote
If, N minutes after the voting period has begun, an authority does not have
a current statement from another authority, the first authority retrieves
the other's statement.
Once an authority has a vote from another authority, it makes it available
at
http://<hostname>/tor/status-vote/current/<fp>.z
where <fp> is the fingerprint of the other authority's identity key.
The consensus network status, along with as many signatures as the server
currently knows, should be available at
http://<hostname>/tor/status-vote/current/consensus.z
All of the detached signatures it knows for consensus status should be
available at:
http://<hostname>/tor/status-vote/current/consensus-signatures.z
Once an authority has computed and signed a consensus network status, it
should send its detached signature to each other authority in an HTTP POST
request to the URL:
http://<hostname>/tor/post/consensus-signature
[XXXX Store votes to disk.]
2.3.2. Serving a consensus directory
Once the authority is done getting signatures on the consensus directory,
it should serve it from:
http://<hostname>/tor/status/consensus.z
Caches SHOULD download consensus directories from an authority and serve
them from the same URL.
2.3.3. Timeline and synchronization
[XXXX]
2.4. Distributing routerdescs between authorities
Consensus will be more meaningful if authorities take steps to make sure
that they all have the same set of descriptors _before_ the voting
starts. This is safe, since all descriptors are self-certified and
timestamped: it's always okay to replace a signed descriptor with a more
recent one signed by the same identity.
In the long run, we might want some kind of sophisticated process here.
For now, since authorities already download one another's networkstatus
documents and use them to determine what descriptors to download from one
another, we can rely on this existing mechanism to keep authorities up to
date.
[We should do a thorough read-through of dir-spec again to make sure
that the authorities converge on which descriptor to "prefer" for
each router. Right now the decision happens at the client, which is
no longer the right place for it. -RD]
3. Questions and concerns
3.1. Push or pull?
The URLs above define a push mechanism for publishing votes and consensus
signatures via HTTP POST requests, and a pull mechanism for downloading
these documents via HTTP GET requests. As specified, every authority will
post to every other. The "download if no copy has been received" mechanism
exists only as a fallback.
4. Migration
* It would be cool if caches could get ready to download consensus
status docs, verify enough signatures, and serve them now. That way
once stuff works all we need to do is upgrade the authorities. Caches
don't need to verify the correctness of the format so long as it's
signed (or maybe multisigned?). We need to make sure that caches back
off very quickly from downloading consensus docs until they're
actually implemented.