Make a new directory for specification proposals, and move some proposals there. Also, move dir-spec-v1.txt to spec.

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+$Id: /tor/branches/eventdns/doc/dir-spec.txt 9469 2006-11-01T23:56:30.179423Z nickm  $
+
+                     Voting on the Tor Directory System
+
+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
+  (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.
+
+2.1. Vote specifications
+
+  Votes in v2.1 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 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
+       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.
+
+3.2. Dropping "opt".
+
+  The "opt" keyword in Tor's directory formats was originally intended to
+  mean, "it is okay to ignore this entry if you don't understand it"; the
+  default behavior has been "discard a routerdesc if it contains entries you
+  don't recognize."
+
+  But so far, every new flag we have added has been marked 'opt'.  It would
+  probably make sense to change the default behavior to "ignore unrecognized
+  fields", and add the statement that clients SHOULD ignore fields they don't
+  recognize.  As a meta-principle, we should say that clients and servers
+  MUST NOT have to understand new fields in order to use directory documents
+  correctly.
+
+  Of course, this will make it impossible to say, "The format has changed a
+  lot; discard this quietly if you don't understand it." We could do that by
+  adding a version field.
+
+3.3. Multilevel keys.
+
+  Replacing a directory authority's identity key in the event of a compromise
+  would be tremendously annoying.  We'd need to tell every client to switch
+  their configuration, or update to a new version with an uploaded list.  So
+  long as some weren't upgraded, they'd be at risk from whoever had
+  compromised the key.
+
+  With this in mind, it's a shame that our current protocol forces us to
+  store identity keys unencrypted in RAM.  We need some kind of signing key
+  stored unencrypted, since we need to generate new descriptors/directories
+  and rotate link and onion keys regularly.  (And since, of course, we can't
+  ask server operators to be on-hand to enter a passphrase every time we
+  want to rotate keys or sign a descriptor.)
+
+  The obvious solution seems to be to have a signing-only key that lives
+  indefinitely (months or longer) and signs descriptors and link keys, and a
+  separate identity key that's used to sign the signing key.  Tor servers
+  could run in one of several modes:
+    1. Identity key stored encrypted.  You need to pick a passphrase when
+       you enable this mode, and re-enter this passphrase every time you
+       rotate the signing key.
+    1'. Identity key stored separate.  You save your identity key to a
+       floppy, and use the floppy when you need to rotate the signing key.
+    2. All keys stored unencrypted.  In this case, we might not want to even
+       *have* a separate signing key.  (We'll need to support no-separate-
+       signing-key mode anyway to keep old servers working.)
+    3. All keys stored encrypted. You need to enter a passphrase to start
+       Tor.
+  (Of course, we might not want to implement all of these.)
+
+  Case 1 is probably most usable and secure, if we assume that people don't
+  forget their passphrases or lose their floppies.  We could mitigate this a
+  bit by encouraging people to PGP-encrypt their passphrases to themselves,
+  or keep a cleartext copy of their secret key secret-split into a few
+  pieces, or something like that.
+
+  Migration presents another difficulty, especially with the authorities.  If
+  we use the current set of identity keys as the new identity keys, we're in
+  the position of having sensitive keys that have been stored on
+  media-of-dubious-encryption up to now.  Also, we need to keep old clients
+  (who will expect descriptors to be signed by the identity keys they know
+  and love, and who will not understand signing keys) happy.
+
+  I'd enumerate designs here, but I'm hoping that somebody will come up with
+  a better one, so I'll try not to prejudice them with more ideas yet.
+
+  Oh, and of course, we'll want to make sure that the keys are
+  cross-certified. :)
+
+  Ideas? -NM
+
+3.4. Long and short descriptors
+
+  Some of the costliest fields in the current directory protocol are ones
+  that no client actually uses.  In particular, the "read-history" and
+  "write-history" fields are used only by the authorities for monitoring the
+  status of the network.  If we took them out, the size of a compressed list
+  of all the routers would fall by about 60%.  (No other disposable field
+  would save more than 2%.)
+
+  One possible solution here is that routers should generate and upload a
+  short-form and long-form descriptor.  Only the short-form descriptor should
+  ever be used by anybody for routing.  The long-form descriptor should be
+  used only for analytics and other tools.  (If we allowed people to route with
+  long descriptors, we'd have to ensure that they stayed in sync with the
+  short ones somehow.)  We can ensure that the short descriptors are used by
+  only recommending those in the network statuses.
+
+  Another possible solution would be to drop these fields from descriptors,
+  and have them uploaded as a part of a separate "bandwidth report" to the
+  authorities.  This could help prevent the mistake of using long descriptors
+  in the place of short ones.
+
+  Thoughts? -NM
+
+3.5. Compression
+
+  Gzip would be easier to work with than zlib; bzip2 would result in smaller
+  data lengths.  [Concretely, we're looking at about 10-15% space savings at
+  the expense of 3-5x longer compression time for using bzip2.]  Doing
+  on-the-fly gzip requires zlib 1.2 or later; doing bzip2 requires bzlib.
+  Pre-compressing status documents in multiple formats would force us to use
+  more memory to hold them.
+
+4. Migration
+
+  For directory voting:
+     * 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.
+
+  For dropping the "opt" requirement:
+     * stopped requiring it as of 0.1.2.5-alpha.  Stop generating it once
+       earlier formats are obsolete.
+
+  For multilevel keys:
+     * no idea
+
+  For long/short descriptors:
+     * In 0.1.2.x:
+       * Authorities should accept both, now, and silently drop short
+         descriptors.
+       * Routers should upload both once authorities accept them.
+       * There should be a "long descriptor" url and the current "normal" URL.
+         Authorities should serve long descriptors from both URLs.
+     * Once tools that want long descriptors support fetching them from the
+       "long descriptor" URL:
+       * Have authorities remember short descriptors, and serve them from the
+         'normal' URL.
+