2007-01-26 18:47:18 +00:00
|
|
|
Filename: 101-dir-voting.txt
|
|
|
|
Title: Voting on the Tor Directory System
|
|
|
|
Version: $Revision$
|
|
|
|
Last-Modified: $Date$
|
|
|
|
Author: Nick Mathewson
|
|
|
|
Created:
|
2006-11-17 03:35:19 +00:00
|
|
|
|
|
|
|
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 0.1.3.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
|
2006-12-24 05:28:52 +00:00
|
|
|
(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.
|
2006-11-17 03:35:19 +00:00
|
|
|
|
|
|
|
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.
|
2006-12-12 06:08:07 +00:00
|
|
|
- Degrading badly if a few authorities die or are offline for a bit.
|
2006-11-17 03:35:19 +00:00
|
|
|
|
|
|
|
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.
|
|
|
|
|
2006-12-24 05:28:52 +00:00
|
|
|
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).
|
2006-11-17 03:35:19 +00:00
|
|
|
|
|
|
|
2. Details.
|
|
|
|
|
|
|
|
2.1. Vote specifications
|
|
|
|
|
2007-01-03 03:45:53 +00:00
|
|
|
Votes in v2.1 are similar to v2 network status documents. We add these
|
2006-11-17 03:35:19 +00:00
|
|
|
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 v2.1 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
|
2006-12-12 06:08:07 +00:00
|
|
|
order based on version-spec.txt.
|
2006-11-17 03:35:19 +00:00
|
|
|
|
|
|
|
"dir-options" and "known-flags" are not included.
|
2006-12-12 06:08:07 +00:00
|
|
|
[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]
|
2006-11-17 03:35:19 +00:00
|
|
|
|
|
|
|
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
|
2006-12-12 06:08:07 +00:00
|
|
|
of directory-signature, sorted in ascending order by nickname,
|
2006-11-17 03:35:19 +00:00
|
|
|
case-insensitively.
|
|
|
|
|
2006-12-07 18:57:22 +00:00
|
|
|
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.
|
2006-12-12 06:08:07 +00:00
|
|
|
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]
|
2006-12-07 18:57:22 +00:00
|
|
|
|
|
|
|
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
|
|
|
|
|
2006-12-21 02:59:15 +00:00
|
|
|
An authority SHOULD publish its vote immediately at the start of each voting
|
2006-12-07 18:57:22 +00:00
|
|
|
period. It does this by making it available at
|
|
|
|
http://<hostname>/tor/status-vote/current/authority.z
|
2006-12-21 02:59:15 +00:00
|
|
|
and sending it in an HTTP POST request to each other authority at the URL
|
2006-12-07 18:57:22 +00:00
|
|
|
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.
|
2006-11-17 03:35:19 +00:00
|
|
|
|
2006-12-07 18:57:22 +00:00
|
|
|
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
|
2006-12-21 02:59:15 +00:00
|
|
|
should send its detached signature to each other authority in an HTTP POST
|
|
|
|
request to the URL:
|
2006-12-07 18:57:22 +00:00
|
|
|
http://<hostname>/tor/post/consensus-signature
|
2006-11-17 03:35:19 +00:00
|
|
|
|
|
|
|
|
|
|
|
[XXXX Store votes to disk.]
|
|
|
|
|
2006-12-07 18:57:22 +00:00
|
|
|
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]
|
|
|
|
|
2006-11-17 03:35:19 +00:00
|
|
|
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.
|
|
|
|
|
2006-12-12 06:08:07 +00:00
|
|
|
[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]
|
|
|
|
|
2006-11-17 03:35:19 +00:00
|
|
|
3. Questions and concerns
|
|
|
|
|
|
|
|
3.1. Push or pull?
|
|
|
|
|
2006-12-21 02:59:15 +00:00
|
|
|
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.
|
2006-11-17 03:35:19 +00:00
|
|
|
|
|
|
|
4. Migration
|
|
|
|
|
2007-01-02 20:08:07 +00:00
|
|
|
* 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.
|
2006-12-07 18:57:22 +00:00
|
|
|
|