tor/doc/spec/dir-spec.txt

$Id$

                      Tor directory protocol, version 3

0. Scope and preliminaries

   This directory protocol is used by Tor version 0.2.0.x-alpha and later.
   See dir-spec-v1.txt for information on the protocol used up to the
   0.1.0.x series, and dir-spec-v2.txt for information on the protocol
   used by the 0.1.1.x and 0.1.2.x series.

   Caches and authorities must still support older versions of the
   directory protocols, until the versions of Tor that require them are
   finally out of commission.  See Section XXXX on backward compatibility.

   This document merges and supersedes the following proposals:

       101  Voting on the Tor Directory System
       103  Splitting identity key from regularly used signing key
       104  Long and Short Router Descriptors

   AS OF 14 JUNE 2007, THIS SPECIFICATION HAS NOT YET BEEN COMPLETELY
   IMPLEMENTED, OR COMPLETELY COMPLETED.

   XXX when to download certificates.
   XXX timeline
   XXX fill in XXXXs

0.1. History

   The earliest versions of Onion Routing shipped with a list of known
   routers and their keys.  When the set of routers changed, users needed to
   fetch a new list.

   The Version 1 Directory protocol
   --------------------------------

   Early versions of Tor (0.0.2) introduced "Directory authorities": servers
   that served signed "directory" documents containing a list of signed
   "router descriptors", along with short summary of the status of each
   router.  Thus, clients could get up-to-date information on the state of
   the network automatically, and be certain that the list they were getting
   was attested by a trusted directory authority.

   Later versions (0.0.8) added directory caches, which download
   directories from the authorities and serve them to clients.  Non-caches
   fetch from the caches in preference to fetching from the authorities, thus
   distributing bandwidth requirements.

   Also added during the version 1 directory protocol were "router status"
   documents: short documents that listed only the up/down status of the
   routers on the network, rather than a complete list of all the
   descriptors.  Clients and caches would fetch these documents far more
   frequently than they would fetch full directories.

   The Version 2 Directory Protocol
   --------------------------------

   During the Tor 0.1.1.x series, Tor revised its handling of directory
   documents in order to address two major problems:

      * Directories had grown quite large (over 1MB), and most directory
        downloads consisted mainly of router descriptors that clients
        already had.

      * Every directory authority was a trust bottleneck: if a single
        directory authority lied, it could make clients believe for a time
        an arbitrarily distorted view of the Tor network.  (Clients
        trusted the most recent signed document they downloaded.) Thus,
        adding more authorities would make the system less secure, not
        more.

   To address these, we extended the directory protocol so that
   authorities now published signed "network status" documents.  Each
   network status listed, for every router in the network: a hash of its
   identity key, a hash of its most recent descriptor, and a summary of
   what the authority believed about its status.  Clients would download
   the authorities' network status documents in turn, and believe
   statements about routers iff they were attested to by more than half of
   the authorities.

   Instead of downloading all router descriptors at once, clients
   downloaded only the descriptors that they did not have.  Descriptors
   were indexed by their digests, in order to prevent malicious caches
   from giving different versions of a router descriptor to different
   clients.

   Routers began working harder to upload new descriptors only when their
   contents were substantially changed.


0.2. Goals of the version 3 protocol

   Version 3 of the Tor directory protocol tries to solve the following
   issues:

      * A great deal of bandwidth used to transmit router descriptors was
        used by two fields that are not actually used by Tor routers
        (namely read-history and write-history).  We save about 60% by
        moving them into a separate document that most clients do not
        fetch or use.

      * It was possible under certain perverse circumstances for clients
        to download an unusual set of network status documents, thus
        partitioning themselves from clients who have a more recent and/or
        typical set of documents.  Even under the best of circumstances,
        clients were sensitive to the ages of the network status documents
        they downloaded.  Therefore, instead of having the clients
        correlate multiple network status documents, we have the
        authorities collectively vote on a single consensus network status
        document.

      * The most sensitive data in the entire network (the identity keys
        of the directory authorities) needed to be stored unencrypted so
        that the authorities can sign network-status documents on the fly.
        Now, the authorities' identity keys are stored offline, and used
        to certify medium-term signing keys that can be rotated.

0.3. Some Remaining questions

   Things we could solve on a v3 timeframe:

     The SHA-1 hash is showing its age.  We should do something about our
     dependency on it.  We could probably future-proof ourselves here in
     this revision, at least so far as documents from the authorities are
     concerned.

     Too many things about the authorities are hardcoded by IP.

     Perhaps we should start accepting longer identity keys for routers
     too.

   Things to solve eventually:

     Requiring every client to know about every router won't scale forever.

     Requiring every directory cache to know every router won't scale
     forever.


1. Outline

   There is a small set (say, around 5-10) of semi-trusted directory
   authorities.  A default list of authorities is shipped with the Tor
   software.  Users can change this list, but are encouraged not to do so,
   in order to avoid partitioning attacks.

   Every authority has a very-secret, long-term "Authority Identity Key".
   This is stored encrypted and/or offline, and is used to sign "key
   certificate" documents.  Every key certificate contains a medium-term
   (3-12 months) "authority signing key", that is used by the authority to
   sign other directory information.  (Note that the authority identity
   key is distinct from the router identity key that the authority uses
   in its role as an ordinary router.)

   Routers periodically upload signed "routers descriptors" to the
   directory authorities describing their keys, capabilities, and other
   information.  Routers may also upload signed "extra info documents"
   containing information that is not required for the Tor protocol.
   Directory authorities serve router descriptors indexed by router
   identity, or by hash of the descriptor.

   Routers may act as directory caches to reduce load on the directory
   authorities.  They announce this in their descriptors.

   Periodically, each directory authority generates a view of
   the current descriptors and status for known routers.  They send a
   signed summary of this view (a "status vote") to the other
   authorities.  The authorities compute the result of this vote, and sign
   a "consensus status" document containing the result of the vote.

   Directory caches download, cache, and re-serve consensus documents.

   Clients, directory caches, and directory authorities all use consensus
   documents to find out when their list of routers is out-of-date.
   (Directory authorities also use vote statuses.) If it is, they download
   any missing router descriptors.  Clients download missing descriptors
   from caches; caches and authorities download from authorities.
   Descriptors are downloaded by the hash of the descriptor, not by the
   server's identity key: this prevents servers from attacking clients by
   giving them descriptors nobody else uses.

   All directory information is uploaded and downloaded with HTTP.

   [Authorities also generate and caches also cache documents produced and
   used by earlier versions of this protocol; see section XXX for notes.]

1.1. What's different from version 2?

   Clients used to download multiple network status documents,
   corresponding roughly to "status votes" above.  They would compute the
   result of the vote on the client side.

   Authorities used to sign documents using the same private keys they used
   for their roles as routers.  This forced them to keep these extremely
   sensitive keys in memory unencrypted.

   All of the information in extra-info documents used to be kept in the
   main descriptors.

1.2. Document meta-format

  Router descriptors, directories, and running-routers documents all obey the
  following lightweight extensible information format.

  The highest level object is a Document, which consists of one or more
  Items.  Every Item begins with a KeywordLine, followed by zero or more
  Objects. A KeywordLine begins with a Keyword, optionally followed by
  whitespace and more non-newline characters, and ends with a newline.  A
  Keyword is a sequence of one or more characters in the set [A-Za-z0-9-].
  An Object is a block of encoded data in pseudo-Open-PGP-style
  armor. (cf. RFC 2440)

  More formally:

    Document ::= (Item | NL)+
    Item ::= KeywordLine Object*
    KeywordLine ::= Keyword NL | Keyword WS ArgumentChar+ NL
    Keyword = KeywordChar+
    KeywordChar ::= 'A' ... 'Z' | 'a' ... 'z' | '0' ... '9' | '-'
    ArgumentChar ::= any printing ASCII character except NL.
    WS = (SP | TAB)+
    Object ::= BeginLine Base-64-encoded-data EndLine
    BeginLine ::= "-----BEGIN " Keyword "-----" NL
    EndLine ::= "-----END " Keyword "-----" NL

    The BeginLine and EndLine of an Object must use the same keyword.

  When interpreting a Document, software MUST ignore any KeywordLine that
  starts with a keyword it doesn't recognize; future implementations MUST NOT
  require current clients to understand any KeywordLine not currently
  described.

  The "opt" keyword was used until Tor 0.1.2.5-alpha for non-critical future
  extensions.  All implementations MUST ignore any item of the form "opt
  keyword ....." when they would not recognize "keyword ....."; and MUST
  treat "opt keyword ....."  as synonymous with "keyword ......" when keyword
  is recognized.

  Implementations before 0.1.2.5-alpha rejected any document with a
  KeywordLine that started with a keyword that they didn't recognize.
  When generating documents that need to be read by older versions of Tor,
  implementations MUST prefix items not recognized by older versions of
  Tor with an "opt" until those versions of Tor are obsolete.  [Note that
  key certificates, status vote documents, extra info documents, and
  status consensus documents will never be read by older versions of Tor.]

  Other implementations that want to extend Tor's directory format MAY
  introduce their own items.  The keywords for extension items SHOULD start
  with the characters "x-" or "X-", to guarantee that they will not conflict
  with keywords used by future versions of Tor.

  In our document descriptions below, we tag Items with a multiplicity in
  brackets.  Possible tags are:

    "At start, exactly once": These items MUST occur in every instance of
      the document type, and MUST appear exactly once, and MUST be the
      first item in their documents.

    "Exactly once": These items MUST occur exactly one time in every
      instance of the document type.

    "At end, exactly once": These items MUST occur in every instance of
      the document type, and MUST appear exactly once, and MUST be the
      last item in their documents.

    "At most once": These items MAY occur zero or one times in any
      instance of the document type, but MUST NOT occur more than once.

    "Any number": These items MAY occur zero, one, or more times in any
      instance of the document type.

    "Once or more": These items MUST occur at least once in any instance
      of the document type, and MAY occur more.

1.3. Signing documents

   Every signable document below is signed in a similar manner, using a
   given "Initial Item", a final "Signature Item", a digest algorithm, and
   a signing key.

   The Initial Item must be the first item in the document.

   The Signature Item has the following format:

     <signature item keyword> [arguments] NL SIGNATURE NL

   The "SIGNATURE" Object contains a signature (using the signing key) of
   the PKCS1-padded digest of the entire document, taken from the
   beginning of the Initial item, through the newline after the Signature
   Item's keyword and its arguments.

   Unless otherwise, the digest algorithm is SHA-1.

   All documents are invalid unless signed with the correct signing key.

   The "Digest" of a document, unless stated otherwise, is its digest *as
   signed by this signature scheme*.

1.4. Voting timeline

   Every consensus document has a "valid-after" (VA) time, a "fresh-until"
   (FU) time and a "valid-until" (VU) time.  VA MUST precede FU, which MUST
   in turn precede VU.  Times are chosen so that every consensus will be
   "fresh" until the next consensus becomes valid, and "valid" for a while
   after.  At least 3 consensuses should be valid at any given time.

   The timeline for a given consensus is as follows:

   VA-DistSeconds-VoteSeconds: The authorities exchange votes.

   VA-DistSeconds-VoteSeconds/2: The authorities try to download any
   votes they don't have.

   VA-DistSeconds: The authorities calculate the consensus and exchange
   signatures.

   VA-DistSeconds/2: The authorities try to download any signatures
   they don't have.

   VA: All authorities have a multiply signed consensus.

   VA ... FU: Caches download the consensus.  (Note that since caches have
        no way of telling what VA and FU are until they have downloaded
        the consensus, they assume that the present consensus's VA is
        equal to the previous one's FU, and that its FU is one interval after
        that.)

   FU: The consensus is no longer the freshest consensus.

   FU ... (the current consensus's VU): Clients download the consensus.
        (See note above: clients guess that the next consensus's FU will be
        two intervals after the current VA.)

   VU: The consensus is no longer valid.

   VoteSeconds and DistSeconds MUST each be at least 20 seconds; FU-VA and
   VU-FU MUST each be at least 5 minutes.

2. Router operation and formats

   ORs SHOULD generate a new router descriptor and a new extra-info
   document whenever any of the following events have occurred:

      - A period of time (18 hrs by default) has passed since the last
        time a descriptor was generated.

      - A descriptor field other than bandwidth or uptime has changed.

      - Bandwidth has changed by a factor of 2 from the last time a
        descriptor was generated, and at least a given interval of time
        (20 mins by default) has passed since then.

      - Its uptime has been reset (by restarting).

      [XXX this list is incomplete; see router_differences_are_cosmetic()
       in routerlist.c for others]

   ORs SHOULD NOT publish a new router descriptor or extra-info document
   if none of the above events have occurred and not much time has passed
   (12 hours by default).

   After generating a descriptor, ORs upload them to every directory
   authority they know, by posting them (in order) to the URL

      http://<hostname:port>/tor/

2.1. Router descriptor format

   Router descriptors consist of the following items.  For backward
   compatibility, there should be an extra NL at the end of each router
   descriptor.

   In lines that take multiple arguments, extra arguments SHOULD be
   accepted and ignored.  Many of the nonterminals below are defined in
   section 2.3.

     "router" nickname address ORPort SOCKSPort DirPort NL

       [At start, exactly once.]

       Indicates the beginning of a router descriptor.  "nickname" must be a
       valid router nickname as specified in 2.3.  "address" must be an IPv4
       address in dotted-quad format.  The last three numbers indicate the
       TCP ports at which this OR exposes functionality. ORPort is a port at
       which this OR accepts TLS connections for the main OR protocol;
       SOCKSPort is deprecated and should always be 0; and DirPort is the
       port at which this OR accepts directory-related HTTP connections.  If
       any port is not supported, the value 0 is given instead of a port
       number.  (At least one of DirPort and ORPort SHOULD be set;
       authorities MAY reject any descriptor with both DirPort and ORPort of
       0.)

    "bandwidth" bandwidth-avg bandwidth-burst bandwidth-observed NL

       [Exactly once]

       Estimated bandwidth for this router, in bytes per second.  The
       "average" bandwidth is the volume per second that the OR is willing to
       sustain over long periods; the "burst" bandwidth is the volume that
       the OR is willing to sustain in very short intervals.  The "observed"
       value is an estimate of the capacity this server can handle.  The
       server remembers the max bandwidth sustained output over any ten
       second period in the past day, and another sustained input.  The
       "observed" value is the lesser of these two numbers.

    "platform" string NL

       [At most once]

       A human-readable string describing the system on which this OR is
       running.  This MAY include the operating system, and SHOULD include
       the name and version of the software implementing the Tor protocol.

    "published" YYYY-MM-DD HH:MM:SS NL

       [Exactly once]

       The time, in GMT, when this descriptor (and its corresponding
       extra-info document if any)  was generated.

    "fingerprint" fingerprint NL

       [At most once]

       A fingerprint (a HASH_LEN-byte of asn1 encoded public key, encoded in
       hex, with a single space after every 4 characters) for this router's
       identity key. A descriptor is considered invalid (and MUST be
       rejected) if the fingerprint line does not match the public key.

       [We didn't start parsing this line until Tor 0.1.0.6-rc; it should
        be marked with "opt" until earlier versions of Tor are obsolete.]

    "hibernating" bool NL

       [At most once]

       If the value is 1, then the Tor server was hibernating when the
       descriptor was published, and shouldn't be used to build circuits.

       [We didn't start parsing this line until Tor 0.1.0.6-rc; it should be
        marked with "opt" until earlier versions of Tor are obsolete.]

    "uptime" number NL

       [At most once]

       The number of seconds that this OR process has been running.

    "onion-key" NL a public key in PEM format

       [Exactly once]

       This key is used to encrypt EXTEND cells for this OR.  The key MUST be
       accepted for at least 1 week after any new key is published in a
       subsequent descriptor. It MUST be 1024 bits.

    "signing-key" NL a public key in PEM format

       [Exactly once]

       The OR's long-term identity key.  It MUST be 1024 bits.

    "accept" exitpattern NL
    "reject" exitpattern NL

       [Any number]

       These lines describe an "exit policy": the rules that an OR follows when
       deciding whether to allow a new stream to a given address.  The
       'exitpattern' syntax is described below.  The rules are considered in
       order; if no rule matches, the address will be accepted.  For clarity,
       the last such entry SHOULD be accept *:* or reject *:*.

    "router-signature" NL Signature NL

       [At end, exactly once]

       The "SIGNATURE" object contains a signature of the PKCS1-padded
       hash of the entire router descriptor, taken from the beginning of the
       "router" line, through the newline after the "router-signature" line.
       The router descriptor is invalid unless the signature is performed
       with the router's identity key.

    "contact" info NL

       [At most once]

       Describes a way to contact the server's administrator, preferably
       including an email address and a PGP key fingerprint.

    "family" names NL

        [At most once]

        'Names' is a space-separated list of server nicknames or
        hexdigests. If two ORs list one another in their "family" entries,
        then OPs should treat them as a single OR for the purpose of path
        selection.

        For example, if node A's descriptor contains "family B", and node B's
        descriptor contains "family A", then node A and node B should never
        be used on the same circuit.

    "read-history" YYYY-MM-DD HH:MM:SS (NSEC s) NUM,NUM,NUM,NUM,NUM... NL
        [At most once]
    "write-history" YYYY-MM-DD HH:MM:SS (NSEC s) NUM,NUM,NUM,NUM,NUM... NL
        [At most once]

        Declare how much bandwidth the OR has used recently. Usage is divided
        into intervals of NSEC seconds.  The YYYY-MM-DD HH:MM:SS field
        defines the end of the most recent interval.  The numbers are the
        number of bytes used in the most recent intervals, ordered from
        oldest to newest.

        [We didn't start parsing these lines until Tor 0.1.0.6-rc; they should
         be marked with "opt" until earlier versions of Tor are obsolete.]

        [See also migration notes in section 2.2.1.]

    "eventdns" bool NL

        [At most once]

        Declare whether this version of Tor is using the newer enhanced
        dns logic.  Versions of Tor with this field set to false SHOULD NOT
        be used for reverse hostname lookups.

        [All versions of Tor before 0.1.2.2-alpha should be assumed to have
         this option set to 0 if it is not present.  All Tor versions at
         0.1.2.2-alpha or later should be assumed to have this option set to
         1 if it is not present.  Until 0.1.2.1-alpha-dev, this option was
         not generated, even when the new DNS code was in use.  Versions of Tor
         before 0.1.2.1-alpha-dev did not parse this option, so it should be
         marked "opt".  The dnsworker logic has been removed, so this option
         should not be used by new server code.  However, it can still be
         used, and should still be recognized by new code until Tor 0.1.2.x
         is obsolete.]

   "caches-extra-info" NL

       [At most once.]

       Present only if this router is a directory cache that provides
       extra-info documents.

       [Versions before 0.2.0.1-alpha don't recognize this, and versions
        before 0.1.2.5-alpha will reject descriptors containing it unless
        it is prefixed with "opt"; it should be so prefixed until these
        versions are obsolete.]

   "extra-info-digest" digest NL

       [At most once]

       "Digest" is a hex-encoded digest (using upper-case characters) of the
       router's extra-info document, as signed in the router's extra-info
       (that is, not including the signature).  (If this field is absent, the
       router is not uploading a corresponding extra-info document.)

       [Versions before 0.2.0.1-alpha don't recognize this, and versions
        before 0.1.2.5-alpha will reject descriptors containing it unless
        it is prefixed with "opt"; it should be so prefixed until these
        versions are obsolete.]

   "hidden-service-dir" *(SP VersionNum) NL

       [At most once.]

       Present only if this router stores and serves hidden service
       descriptors. If any VersionNum(s) are specified, this router
       supports those descriptor versions. If none are specified, it
       defaults to version 2 descriptors.

2.2. Extra-info documents

   Extra-info documents consist of the following items:

    "extra-info" Nickname Fingerprint NL
        [At start, exactly once.]

        Identifies what router this is an extra info descriptor for.
        Fingerprint is encoded in hex (using upper-case letters), with
        no spaces.

    "published"

       [Exactly once.]

       The time, in GMT, when this document (and its corresponding router
       descriptor if any) was generated.  It MUST match the published time
       in the corresponding router descriptor.

    "read-history" YYYY-MM-DD HH:MM:SS (NSEC s) NUM,NUM,NUM,NUM,NUM... NL
        [At most once.]
    "write-history" YYYY-MM-DD HH:MM:SS (NSEC s) NUM,NUM,NUM,NUM,NUM... NL
        [At most once.]

        As documented in 2.1 above.  See migration notes in section 2.2.1.

    "router-signature" NL Signature NL
        [At end, exactly once.]

        A document signature as documented in section 1.3, using the
        initial item "extra-info" and the final item "router-signature",
        signed with the router's identity key.

2.2.1. Moving history fields to extra-info documents.

   Tools that want to use the read-history and write-history values SHOULD
   download extra-info documents as well as router descriptors.  Such
   tools SHOULD accept history values from both sources; if they appear in
   both documents, the values in the extra-info documents are authoritative.

   At some future time, to save space, new versions of Tor will no longer
   generate router descriptors containing read-history or write-history.
   Tools should continue to accept read-history and write-history values
   in router descriptors produced by older versions of Tor.

2.3. Nonterminals in router descriptors

   nickname ::= between 1 and 19 alphanumeric characters, case-insensitive.
   hexdigest ::= a '$', followed by 20 hexadecimal characters.
      [Represents a server by the digest of its identity key.]

   exitpattern ::= addrspec ":" portspec
   portspec ::= "*" | port | port "-" port
   port ::= an integer between 1 and 65535, inclusive.

      [Some implementations incorrectly generate ports with value 0.
       Implementations SHOULD accept this, and SHOULD NOT generate it.
       Connections to port 0 are never permitted.]

   addrspec ::= "*" | ip4spec | ip6spec
   ipv4spec ::= ip4 | ip4 "/" num_ip4_bits | ip4 "/" ip4mask
   ip4 ::= an IPv4 address in dotted-quad format
   ip4mask ::= an IPv4 mask in dotted-quad format
   num_ip4_bits ::= an integer between 0 and 32
   ip6spec ::= ip6 | ip6 "/" num_ip6_bits
   ip6 ::= an IPv6 address, surrounded by square brackets.
   num_ip6_bits ::= an integer between 0 and 128

   bool ::= "0" | "1"

3. Formats produced by directory authorities.

   Every authority has two keys used in this protocol: a signing key, and
   an authority identity key.  (Authorities also have a router identity
   key used in their role as a router and by earlier versions of the
   directory protocol.)  The identity key is used from time to time to
   sign new key certificates using new signing keys; it is very sensitive.
   The signing key is used to sign key certificates and status documents.

   There are three kinds of documents generated by directory authorities:

     Key certificates
     Status votes
     Status consensuses

   Each is discussed below.

3.1. Key certificates

   Key certificates consist of the following items:

    "dir-key-certificate-version" version NL

        [At start, exactly once.]

        Determines the version of the key certificate.  MUST be "3" for
        the protocol described in this document.  Implementations MUST
        reject formats they don't understand.

    "dir-address" IP NL
        [Once or more]

        An IP:Port for this authority's directory port.

    "fingerprint" fingerprint NL

        [Exactly once.]

        Hexadecimal encoding without spaces based on the authority's
        identity key.

    "dir-identity-key" NL a public key in PEM format

        [Exactly once.]

        The long-term authority identity key for this authority.  This key
        SHOULD be at least 2048 bits long; it MUST NOT be shorter than
        1024 bits.

    "dir-key-published" YYYY-MM-DD HH:MM:SS NL

        [Exactly once.]

        The time (in GMT) when this document and corresponding key were
        last generated.

    "dir-key-expires" YYYY-MM-DD HH:MM:SS NL

        [Exactly once.]

        A time (in GMT) after which this key is no longer valid.

    "dir-signing-key" NL a key in PEM format

        [Exactly once.]

        The directory server's public signing key.  This key MUST be at
        least 1024 bits, and MAY be longer.

    "dir-key-certification" NL Signature NL

        [At end, exactly once.]

        A document signature as documented in section 1.3, using the
        initial item "dir-key-certificate-version" and the final item
        "dir-key-certification", signed with the authority identity key.

   Authorities MUST generate a new signing key and corresponding
   certificate before the key expires.

3.2. Vote and consensus status documents

   Votes and consensuses are more strictly formatted then other documents
   in this specification, since different authorities must be able to
   generate exactly the same consensus given the same set of votes.

   The procedure for deciding when to generate vote and consensus status
   documents are described in section XXX below.

   Status documents contain a preamble, an authority section, a list of
   router status entries, and one more footers signature, in that order.

   Unlike other formats described above, a SP in these documents must be a
   single space character (hex 20).

   Some items appear only in votes, and some items appear only in
   consensuses.  Unless specified, items occur in both.

   The preamble contains the following items.  They MUST occur in the
   order given here:

    "network-status-version" SP version NL.

        [At start, exactly once.]

        A document format version.  For this specification, the version is
        "3".

    "vote-status" SP type NL

        [Exactly once.]

        The status MUST be "vote" or "consensus", depending on the type of
        the document.

    "consensus-methods" SP IntegerList NL

        [Exactly once for votes; does not occur in consensuses.]

        A space-separated list of supported methods for generating
        consensuses from votes.  See section 3.4.1 for details.  Method "1"
        MUST be included.

    "consensus-method" SP Integer NL

        [Exactly once for consensuses; does not occur in votes.]

        See section 3.4.1 for details.

        (Only included when the vote is generated with consensus-method 2 or
        later.)

    "published" SP YYYY-MM-DD SP HH:MM:SS NL

        [Exactly once for votes; does not occur in consensuses.]

        The publication time for this status document (if a vote).

    "valid-after" SP YYYY-MM-DD SP HH:MM:SS NL

        [Exactly once.]

        The start of the Interval for this vote.  Before this time, the
        consensus document produced from this vote should not be used.
        See 1.4 for voting timeline information.

    "fresh-until" SP YYYY-MM-DD SP HH:MM:SS NL

        [Exactly once.]

        The time at which the next consensus should be produced; before this
        time, there is no point in downloading another consensus, since there
        won't be a new one.  See 1.4 for voting timeline information.

    "valid-until" SP YYYY-MM-DD SP HH:MM:SS NL

        [Exactly once.]

        The end of the Interval for this vote.  After this time, the
        consensus produced by this vote should not be used.  See 1.4 for
        voting timeline information.

    "voting-delay" SP VoteSeconds SP DistSeconds NL

        [Exactly once.]

        VoteSeconds is the number of seconds that we will allow to collect
        votes from all authorities; DistSeconds is the number of seconds
        we'll allow to collect signatures from all authorities. See 1.4 for
        voting timeline information.

    "client-versions" SP VersionList NL

        [At most once.]

        A comma-separated list of recommended client versions, in
        ascending order.  If absent, no opinion is held about client
        versions.

    "server-versions" SP VersionList NL

        [At most once.]

        A comma-separated list of recommended server versions, in
        ascending order.  If absent, no opinion is held about server
        versions.

    "known-flags" SP FlagList NL

        [Exactly once.]

        A space-separated list of all of the flags that this document
        might contain.  A flag is "known" either because the authority
        knows about them and might set them (if in a vote), or because
        enough votes were counted for the consensus for an authoritative
        opinion to have been formed about their status.


   The authority section of a vote contains the following items, followed
   in turn by the authority's current key certificate:

    "dir-source" SP nickname SP identity SP address SP IP SP dirport SP
       orport NL

        [Exactly once, at start]

        Describes this authority.  The nickname is a convenient identifier
        for the authority.  The identity is an uppercase hex fingerprint of
        the authority's current (v3 authority) identity key.  The address is
        the server's hostname.  The IP is the server's current IP address,
        and dirport is its current directory port. XXXXorport

    "contact" SP string NL

        [At most once.]

        An arbitrary string describing how to contact the directory
        server's administrator.  Administrators should include at least an
        email address and a PGP fingerprint.

   The authority section of a consensus contains groups the following items,
   in the order given, with one group for each authority that contributed to
   the consensus, with groups sorted by authority identity digest:

    "dir-source" SP nickname SP identity SP address SP IP SP dirport SP
       orport NL

        [Exactly once, at start]

        As in the authority section of a vote.

    "contact" SP string NL

        [At most once.]

        As in the authority section of a vote.

    "vote-digest" SP digest NL

        [Exactly once.]

        A digest of the vote from the authority that contributed to this
        consensus, as signed (that is, not including the signature).
        (Hex, upper-case.)

   Each router status entry contains the following items.  Router status
   entries are sorted in ascending order by identity digest.

    "r" SP nickname SP identity SP digest SP publication SP IP SP ORPort
        SP DirPort NL

        [At start, exactly once.]

        "Nickname" is the OR's nickname.  "Identity" is a hash of its
        identity key, encoded in base64, with trailing equals sign(s)
        removed.  "Digest" is a hash of its most recent descriptor as
        signed (that is, not including the signature), encoded in base64 as
        "identity".  "Publication" is the
        publication time of its most recent descriptor, in the form
        YYYY-MM-DD HH:MM:SS, in GMT.  "IP" is its current IP address;
        ORPort is its current OR port, "DirPort" is it's current directory
        port, or "0" for "none".

    "s" SP Flags NL

        [At most once.]

        A series of space-separated status flags, in alphabetical order.
        Currently documented flags are:

          "Authority" if the router is a directory authority.
          "BadExit" if the router is believed to be useless as an exit node
             (because its ISP censors it, because it is behind a restrictive
             proxy, or for some similar reason).
          "BadDirectory" if the router is believed to be useless as a
             directory cache (because its directory port isn't working,
             its bandwidth is always throttled, or for some similar
             reason).
          "Exit" if the router is useful for building general-purpose exit
             circuits.
          "Fast" if the router is suitable for high-bandwidth circuits.
          "Guard" if the router is suitable for use as an entry guard.
          "HSDir" if the router is considered a v2 hidden service directory.
          "Named" if the router's identity-nickname mapping is canonical,
             and this authority binds names.
          "Stable" if the router is suitable for long-lived circuits.
          "Running" if the router is currently usable.
          "Valid" if the router has been 'validated'.
          "V2Dir" if the router implements the v2 directory protocol.
          "V3Dir" if the router implements this protocol.

    "v" SP version NL

        [At most once.]

        The version of the Tor protocol that this server is running.  If
        the value begins with "Tor" SP, the rest of the string is a Tor
        version number, and the protocol is "The Tor protocol as supported
        by the given version of Tor."  Otherwise, if the value begins with
        some other string, Tor has upgraded to a more sophisticated
        protocol versioning system, and the protocol is "a version of the
        Tor protocol more recent than any we recognize."

   The signature section contains the following item, which appears
   Exactly Once for a vote, and At Least Once for a consensus.

    "directory-signature" SP identity SP signing-key-digest NL Signature

        This is a signature of the status document, with the initial item
        "network-status-version", and the signature item
        "directory-signature", using the signing key.  (In this case, we take
        the hash through the _space_ after directory-signature, not the
        newline: this ensures that all authorities sign the same thing.)
        "identity" is the hex-encoded digest of the authority identity key of
        the signing authority, and "signing-key-digest" is the hex-encoded
        digest of the current authority signing key of the signing authority.

3.3. Deciding how to vote.

   (This section describes how directory authorities choose which status
   flags to apply to routers, as of Tor 0.2.0.0-alpha-dev. Later directory
   authorities MAY do things differently, so long as clients keep working
   well.  Clients MUST NOT depend on the exact behaviors in this section.)

   In the below definitions, a router is considered "active" if it is
   running, valid, and not hibernating.

   "Valid" -- a router is 'Valid' if it is running a version of Tor not
   known to be broken, and the directory authority has not blacklisted
   it as suspicious.

   "Named" -- Directory authority administrators may decide to support name
   binding.  If they do, then they must maintain a file of
   nickname-to-identity-key mappings, and try to keep this file consistent
   with other directory authorities.  If they don't, they act as clients, and
   report bindings made by other directory authorities (name X is bound to
   identity Y if at least one binding directory lists it, and no directory
   binds X to some other Y'.)  A router is called 'Named' if the router
   believes the given name should be bound to the given key.

   "Unnamed" -- Directory authorities that support naming should vote for a
   router to be 'Unnamed' if its given nickname is mapped to a different
   identity.

   "Running" -- A router is 'Running' if the authority managed to connect to
   it successfully within the last 30 minutes.

   "Stable" -- A router is 'Stable' if it is active, and either its Weighted
   MTBF is at least the median for known active routers or its Weighted MTBF
   corresponds to at least 7 days. Routers are never called Stable if they are
   running a version of Tor known to drop circuits stupidly.  (0.1.1.10-alpha
   through 0.1.1.16-rc are stupid this way.)

        To calculate weighted MTBF, compute the weighted mean of the lengths
        of all intervals when the router was observed to be up, weighting
        intervals by $\alpha^n$, where $n$ is the amount of time that has
        passed since the interval ended, and $\alpha$ is chosen so that
        measurements over approximately one month old no longer influence the
        weighted MTBF much.

        [XXXX what happens when we have less than 4 days of MTBF info.]

   "Fast" -- A router is 'Fast' if it is active, and its bandwidth is
   either in the top 7/8ths for known active routers or at least 100KB/s.

   "Guard" -- A router is a possible 'Guard' if its Weighted Fractional
   Uptime is at least the median for "familiar" active routers, and if
   its bandwidth is at least median or at least 250KB/s.
   If the total bandwidth of active non-BadExit Exit servers is less
   than one third of the total bandwidth of all active servers, no Exit is
   listed as a Guard.

        To calculate weighted fractional uptime, compute the fraction
        of time that the router is up in any given day, weighting so that
        downtime and uptime in the past counts less.

        A node is 'familiar' if 1/8 of all active nodes have appeared more
        recently than it, OR it has been around for a few weeks.

   "Authority" -- A router is called an 'Authority' if the authority
   generating the network-status document believes it is an authority.

   "V2Dir" -- A router supports the v2 directory protocol if it has an open
   directory port, and it is running a version of the directory protocol that
   supports the functionality clients need.  (Currently, this is
   0.1.1.9-alpha or later.)

   "V3Dir" -- A router supports the v3 directory protocol if it has an open
   directory port, and it is running a version of the directory protocol that
   supports the functionality clients need.  (Currently, this is
   0.2.0.?????-alpha or later.)

   "HSDir" -- A router is a v2 hidden service directory if it stores and
   serves v2 hidden service descriptors and the authority managed to connect
   to it successfully within the last 24 hours.

   Directory server administrators may label some servers or IPs as
   blacklisted, and elect not to include them in their network-status lists.

   Authorities SHOULD 'disable' any servers in excess of 3 on any single IP.
   When there are more than 3 to choose from, authorities should first prefer
   authorities to non-authorities, then prefer Running to non-Running, and
   then prefer high-bandwidth to low-bandwidth.  To 'disable' a server, the
   authority *should* advertise it without the Running or Valid flag.

   Thus, the network-status vote includes all non-blacklisted,
   non-expired, non-superseded descriptors.

3.4. Computing a consensus from a set of votes

   Given a set of votes, authorities compute the contents of the consensus
   document as follows:

     The "valid-after", "valid-until", and "fresh-until" times are taken as
     the median of the respective values from all the votes.

     The times in the "voting-delay" line are taken as the median of the
     VoteSeconds and DistSeconds times in the votes.

     Known-flags is the union of all flags known by any voter.

    "client-versions" and "server-versions" are sorted in ascending
     order; A version is recommended in the consensus if it is recommended
     by more than half of the voting authorities that included a
     client-versions or server-versions lines in their votes.

     The authority item groups (dir-source, contact, fingerprint,
     vote-digest) are taken from the votes of the voting
     authorities. These groups are sorted by the digests of the
     authorities identity keys, in ascending order.

     A router status entry:
        * is included in the result if some router status entry with the same
          identity is included by more than half of the authorities (total
          authorities, not just those whose votes we have).

        * For any given identity, we include at most one router status entry.

        * A router entry has a flag set if that is included by more than half
          of the authorities who care about that flag.

        * Two router entries are "the same" if they have the same
          <descriptor digest, published time, nickname, IP, ports> tuple.
          We choose the tuple for a given router as whichever tuple appears
          for that router in the most votes.  We break ties in favor of
          the more recently published.

        * The Named flag appears if it is included for this routerstatus by
          _any_ authority, and if all authorities that list it list the same
          nickname. However, if consensus-method 2 or later is in use, and
          any authority calls this identity/nickname pair Unnamed, then
          this routerstatus does not get the Named flag.

        * If consensus-method 2 or later is in use, the Unnamed flag is
          set for a routerstatus if any authorities have voted for a different
          identities to be Named with that nickname, or if any authority
          lists that nickname/ID pair as Unnamed.

          (With consensus-method 1, Unnamed is set like any other flag.)

        * The version is given as whichever version is listed by the most
          voters, with ties decided in favor of more recent versions.

     The signatures at the end of a consensus document are sorted in
     ascending order by identity digest.

   All ties in computing medians are broken in favor of the smaller or
   earlier item.

3.4.1. Forward compatibility

   Future versions of Tor will need to include new information in the
   consensus documents, but it is important that all authorities (or at least
   half) generate and sign the same signed consensus.

   To achieve this, authorities list in their votes their supported methods
   for generating consensuses from votes.  Later methods will be assigned
   higher numbers.  Currently recognized methods:
     "1" -- The first implemented version.
     "2" -- Added support for the Unnamed flag.

   Before generating a consensus, an authority must decide which consensus
   method to use.  To do this, it looks for the highest version number
   supported by more than 2/3 of the authorities voting.  If it supports this
   method, then it uses it.  Otherwise, it falls back to method 1.

   (The consensuses generated by new methods must be parsable by
   implementations that only understand the old methods, and must not cause
   those implementations to compromise their anonymity.  This is a means for
   making changes in the contents of consensus; not for making
   backward-incompatible changes in their format.)

3.5. 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 items as follows:

    "consensus-digest" SP Digest NL

        [At start, at most once.]

        The digest of the consensus being signed.

    "valid-after" SP YYYY-MM-DD SP HH:MM:SS NL
    "fresh-until" SP YYYY-MM-DD SP HH:MM:SS NL
    "valid-until" SP YYYY-MM-DD SP HH:MM:SS NL

        [As in the consensus]

    "directory-signature"

        [As in the consensus; the signature object is the same as in the
        consensus document.]


4. Directory server operation

   All directory authorities and directory caches ("directory servers")
   implement this section, except as noted.

4.1. Accepting uploads (authorities only)

   When a router posts a signed descriptor to a directory authority, the
   authority first checks whether it is well-formed and correctly
   self-signed.  If it is, the authority next verifies that the nickname
   in question is not already assigned to a router with a different
   public key.
   Finally, the authority MAY check that the router is not blacklisted
   because of its key, IP, or another reason.

   If the descriptor passes these tests, and the authority does not already
   have a descriptor for a router with this public key, it accepts the
   descriptor and remembers it.

   If the authority _does_ have a descriptor with the same public key, the
   newly uploaded descriptor is remembered if its publication time is more
   recent than the most recent old descriptor for that router, and either:
      - There are non-cosmetic differences between the old descriptor and the
        new one.
      - Enough time has passed between the descriptors' publication times.
        (Currently, 12 hours.)

   Differences between router descriptors are "non-cosmetic" if they would be
   sufficient to force an upload as described in section 2 above.

   Note that the "cosmetic difference" test only applies to uploaded
   descriptors, not to descriptors that the authority downloads from other
   authorities.

   When a router posts a signed extra-info document to a directory authority,
   the authority again checks it for well-formedness and correct signature,
   and checks that its matches the extra-info-digest in some router
   descriptor that it believes is currently useful.  If so, it accepts it and
   stores it and serves it as requested.  If not, it drops it.

4.2. Voting (authorities only)

   Authorities divide time into Intervals.  Authority administrators SHOULD
   try to all pick the same interval length, and SHOULD pick intervals that
   are commonly used divisions of time (e.g., 5 minutes, 15 minutes, 30
   minutes, 60 minutes, 90 minutes).  Voting intervals SHOULD be chosen to
   divide evenly into a 24-hour day.

   Authorities SHOULD act according to interval and delays in the
   latest consensus.  Lacking a latest consensus, they SHOULD default to a
   30-minute Interval, a 5 minute VotingDelay, and a 5 minute DistDelay.

   Authorities MUST take pains to ensure that their clocks remain accurate
   within a few seconds.  (Running NTP is usually sufficient.)

   The first voting period of each day begins at 00:00 (midnight) GMT.  If
   the last period of the day would be truncated by one-half or more, it is
   merged with the second-to-last period.

   An authority SHOULD publish its vote immediately at the start of each voting
   period (minus VoteSeconds+DistSeconds).  It does this by making it
   available at
     http://<hostname>/tor/status-vote/next/authority.z
   and sending it in an HTTP POST request to each other authority at the URL
     http://<hostname>/tor/post/vote

   If, at the start of the voting period, minus DistSeconds, an authority
   does not have a current statement from another authority, the first
   authority downloads the other's statement.

   Once an authority has a vote from another authority, it makes it available
   at
      http://<hostname>/tor/status-vote/next/<fp>.z
   where <fp> is the fingerprint of the other authority's identity key.
   And at
      http://<hostname>/tor/status-vote/next/d/<d>.z
   where <d> is the digest of the vote document.

   The consensus status, along with as many signatures as the server
   currently knows, should be available at
      http://<hostname>/tor/status-vote/next/consensus.z
   All of the detached signatures it knows for consensus status should be
   available at:
      http://<hostname>/tor/status-vote/next/consensus-signatures.z

   Once there are enough signatures, or once the voting period starts,
   these documents are available at
      http://<hostname>/tor/status-vote/current/consensus.z
   and
      http://<hostname>/tor/status-vote/current/consensus-signatures.z

   The other vote documents are analogously made available under
     http://<hostname>/tor/status-vote/current/authority.z
     http://<hostname>/tor/status-vote/current/<fp>.z
     http://<hostname>/tor/status-vote/current/d/<d>.z
   once the consensus is complete.

   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

   [XXX Note why we support push-and-then-pull.]

   [XXX possible future features include support for downloading old
    consensuses.]

4.3. Downloading consensus status documents (caches only)

   All directory servers (authorities and caches) try to keep a recent
   network-status consensus document to serve to clients.  A cache ALWAYS
   downloads a network-status consensus if any of the following are true:
     - The cache has no consensus document.
     - The cache's consensus document is no longer valid.
   Otherwise, the cache downloads a new consensus document at a randomly
   chosen time after its current consensus stops being fresh.  (This time is
   chosen at random to avoid swarming the authorities at the start of each
   period.)

4.4. Downloading and storing router descriptors (authorities and caches)

   Periodically (currently, every 10 seconds), directory servers check
   whether there are any specific descriptors that they do not have and that
   they are not currently trying to download.  Caches identify these
   descriptors by hash in the recent network-status consensus documents;
   authorities identify them by hash in vote (if publication date is more
   recent than the descriptor we currently have).

 [XXXX need a way to fetch descriptors ahead of the vote?  v2 status docs can
 do that for now.]

   If so, the directory server launches requests to the authorities for these
   descriptors, such that each authority is only asked for descriptors listed
   in its most recent vote (if the requester is an authority) or in the
   consensus (if the requester is a cache).  If we're an authority, and more
   than one authority lists the descriptor, we choose which to ask at random.

   If one of these downloads fails, we do not try to download that descriptor
   from the authority that failed to serve it again unless we receive a newer
   network-status (consensus or vote) from that authority that lists the same
   descriptor.

   Directory servers must potentially cache multiple descriptors for each
   router. Servers must not discard any descriptor listed by any recent
   consensus.  If there is enough space to store additional descriptors,
   servers SHOULD try to hold those which clients are likely to download the
   most.  (Currently, this is judged based on the interval for which each
   descriptor seemed newest.)
[XXXX define recent]

   Authorities SHOULD NOT download descriptors for routers that they would
   immediately reject for reasons listed in 3.1.

4.5. Downloading and storing extra-info documents

   All authorities, and any cache that chooses to cache extra-info documents,
   and any client that uses extra-info documents, should implement this
   section.

   Note that generally, clients don't need extra-info documents.

   Periodically, the Tor instance checks whether it is missing any extra-info
   documents: in other words, if it has any router descriptors with an
   extra-info-digest field that does not match any of the extra-info
   documents currently held.  If so, it downloads whatever extra-info
   documents are missing.  Caches download from authorities; non-caches try
   to download from caches.  We follow the same splitting and back-off rules
   as in 4.4 (if a cache) or 5.3 (if a client).

4.6. General-use HTTP URLs

   "Fingerprints" in these URLs are base-16-encoded SHA1 hashes.

   The most recent v3 consensus should be available at:
      http://<hostname>/tor/status-vote/current/consensus.z

   A concatenated set of all the current key certificates should be available
   at:
      http://<hostname>/tor/keys/all.z

   The key certificate for this server (if it is an authority) should be
   available at:
      http://<hostname>/tor/keys/authority.z

   The key certificate for an authority whose authority identity fingerprint
   is <F> should be available at:
      http://<hostname>/tor/keys/fp/<F>.z

   The key certificate whose signing key fingerprint is <F> should be
   available at:
      http://<hostname>/tor/keys/sk/<F>.z
   [XXX020 cross-certify?]

   The most recent descriptor for a server whose identity key has a
   fingerprint of <F> should be available at:
      http://<hostname>/tor/server/fp/<F>.z

   The most recent descriptors for servers with identity fingerprints
   <F1>,<F2>,<F3> should be available at:
      http://<hostname>/tor/server/fp/<F1>+<F2>+<F3>.z

   (NOTE: Implementations SHOULD NOT download descriptors by identity key
   fingerprint. This allows a corrupted server (in collusion with a cache) to
   provide a unique descriptor to a client, and thereby partition that client
   from the rest of the network.)

   The server descriptor with (descriptor) digest <D> (in hex) should be
   available at:
      http://<hostname>/tor/server/d/<D>.z

   The most recent descriptors with digests <D1>,<D2>,<D3> should be
   available at:
      http://<hostname>/tor/server/d/<D1>+<D2>+<D3>.z

   The most recent descriptor for this server should be at:
      http://<hostname>/tor/server/authority.z
    [Nothing in the Tor protocol uses this resource yet, but it is useful
     for debugging purposes. Also, the official Tor implementations
     (starting at 0.1.1.x) use this resource to test whether a server's
     own DirPort is reachable.]

   A concatenated set of the most recent descriptors for all known servers
   should be available at:
      http://<hostname>/tor/server/all.z

   Extra-info documents are available at the URLS
      http://<hostname>/tor/extra/d/...
      http://<hostname>/tor/extra/fp/...
      http://<hostname>/tor/extra/all[.z]
      http://<hostname>/tor/extra/authority[.z]
         (As for /tor/server/ URLs: supports fetching extra-info
         documents by their digest, by the fingerprint of their servers,
         or all at once. When serving by fingerprint, we serve the
         extra-info that corresponds to the descriptor we would serve by
         that fingerprint. Only directory authorities of version
         0.2.0.1-alpha or later are guaranteed to support the first
         three classes of URLs.  Caches may support them, and MUST
         support them if they have advertised "caches-extra-info".)

   For debugging, directories SHOULD expose non-compressed objects at URLs like
   the above, but without the final ".z".
   Clients MUST handle compressed concatenated information in two forms:
     - A concatenated list of zlib-compressed objects.
     - A zlib-compressed concatenated list of objects.
   Directory servers MAY generate either format: the former requires less
   CPU, but the latter requires less bandwidth.

   Clients SHOULD use upper case letters (A-F) when base16-encoding
   fingerprints.  Servers MUST accept both upper and lower case fingerprints
   in requests.

5. Client operation: downloading information

   Every Tor that is not a directory server (that is, those that do
   not have a DirPort set) implements this section.

5.1. Downloading network-status documents

   Each client maintains a list of directory authorities.  Insofar as
   possible, clients SHOULD all use the same list.

   Clients try to have a live consensus network-status document at all times.
   A network-status document is "live" if the time in its valid-until field
   has not passed.

   If a client is missing a live network-status document, it tries to fetch
   it from a directory cache (or from an authority if it knows no caches).
   On failure, the client waits briefly, then tries that network-status
   document again from another cache.  The client does not build circuits
   until it has a live network-status consensus document, and it has
   descriptors for more than 1/4 of the routers that it believes are running.

   (Note: clients can and should pick caches based on the network-status
   information they have: once they have first fetched network-status info
   from an authority, they should not need to go to the authority directly
   again.)


5.2. Downloading and storing router descriptors

   Clients try to have the best descriptor for each router.  A descriptor is
   "best" if:
      * It is listed in the consensus network-status document.

   Periodically (currently every 10 seconds) clients check whether there are
   any "downloadable" descriptors.  A descriptor is downloadable if:
      - It is the "best" descriptor for some router.
      - The descriptor was published at least 10 minutes in the past.
        (This prevents clients from trying to fetch descriptors that the
        mirrors have probably not yet retrieved and cached.)
      - The client does not currently have it.
      - The client is not currently trying to download it.
      - The client would not discard it immediately upon receiving it.
      - The client thinks it is running and valid (see 6.1 below).

   If at least 16 known routers have downloadable descriptors, or if
   enough time (currently 10 minutes) has passed since the last time the
   client tried to download descriptors, it launches requests for all
   downloadable descriptors, as described in 5.3 below.

   When a descriptor download fails, the client notes it, and does not
   consider the descriptor downloadable again until a certain amount of time
   has passed. (Currently 0 seconds for the first failure, 60 seconds for the
   second, 5 minutes for the third, 10 minutes for the fourth, and 1 day
   thereafter.)  Periodically (currently once an hour) clients reset the
   failure count.

   Clients retain the most recent descriptor they have downloaded for each
   router so long as it is not too old (currently, 48 hours), OR so long as
   no better descriptor has been downloaded for the same router.

   [Versions of Tor before 0.1.2.3-alpha would discard descriptors simply for
   being published too far in the past.]  [The code seems to discard
   descriptors in all cases after they're 5 days old. True? -RD]

5.3. Managing downloads

   When a client has no consensus network-status document, it downloads it
   from a randomly chosen authority.  In all other cases, the client
   downloads from caches randomly chosen from among those believed to be V2
   directory servers.  (This information comes from the network-status
   documents; see 6 below.)

   When downloading multiple router descriptors, the client chooses multiple
   mirrors so that:
     - At least 3 different mirrors are used, except when this would result
       in more than one request for under 4 descriptors.
     - No more than 128 descriptors are requested from a single mirror.
     - Otherwise, as few mirrors as possible are used.
   After choosing mirrors, the client divides the descriptors among them
   randomly.

   After receiving any response client MUST discard any network-status
   documents and descriptors that it did not request.

6. Using directory information

   Everyone besides directory authorities uses the approaches in this section
   to decide which servers to use and what their keys are likely to be.
   (Directory authorities just believe their own opinions, as in 3.1 above.)

6.1. Choosing routers for circuits.

   Circuits SHOULD NOT be built until the client has enough directory
   information: a live consensus network status [XXXX fallback?]  and
   descriptors for at least 1/4 of the servers believed to be running.

   A server is "listed" if it is included by the consensus network-status
   document.  Clients SHOULD NOT use unlisted servers.

   These flags are used as follows:

     - Clients SHOULD NOT use non-'Valid' or non-'Running' routers unless
       requested to do so.

     - Clients SHOULD NOT use non-'Fast' routers for any purpose other than
       very-low-bandwidth circuits (such as introduction circuits).

     - Clients SHOULD NOT use non-'Stable' routers for circuits that are
       likely to need to be open for a very long time (such as those used for
       IRC or SSH connections).

     - Clients SHOULD NOT choose non-'Guard' nodes when picking entry guard
       nodes.

     - Clients SHOULD NOT download directory information from non-'V2Dir'
       caches.

   See the "path-spec.txt" document for more details.

6.2. Managing naming

   In order to provide human-memorable names for individual server
   identities, some directory servers bind names to IDs.  Clients handle
   names in two ways:

   When a client encounters a name it has not mapped before:

      If the consensus lists any router with that name as "Named", or if
      consensus-method 2 or later is in use and the consensus lists any
      router with that name as having the "Unnamed" flag, then the name is
      bound.  (It's bound to the ID listed in the entry with the Named,
      or to an unknown ID if no name is found.)

   When the user refers to a bound name, the implementation SHOULD provide
   only the router with ID bound to that name, and no other router, even
   if the router with the right ID can't be found.

   When a user tries to refer to a non-bound name, the implementation SHOULD
   warn the user. After warning the user, the implementation MAY use any
   router that advertises the name.

   Not every router needs a nickname.  When a router doesn't configure a
   nickname, it publishes with the default nickname "Unnamed".  Authorities
   SHOULD NOT ever mark a router with this nickname as Named; client software
   SHOULD NOT ever use a router in response to a user request for a router
   called "Unnamed".

6.3. Software versions

   An implementation of Tor SHOULD warn when it has fetched a consensus
   network-status, and it is running a software version not listed.

6.4. Warning about a router's status.

   If a router tries to publish its descriptor to a Naming authority
   that has its nickname mapped to another key, the router SHOULD
   warn the operator that it is either using the wrong key or is using
   an already claimed nickname.

   If a router has fetched a consensus document,, and the
   authorities do not publish a binding for the router's nickname, the
   router MAY remind the operator that the chosen nickname is not
   bound to this key at the authorities, and suggest contacting the
   authority operators.

   ...

6.5. Router protocol versions

   A client should believe that a router supports a given feature if that
   feature is supported by the router or protocol versions in more than half
   of the live networkstatuses' "v" entries for that router.  In other words,
   if the "v" entries for some router are:
       v Tor 0.0.8pre1                (from authority 1)
       v Tor 0.1.2.11                 (from authority 2)
       v FutureProtocolDescription 99 (from authority 3)
   then the client should believe that the router supports any feature
   supported by 0.1.2.11.

   This is currently equivalent to believing the median declared version for
   a router in all live networkstatuses.

7. Standards compliance

   All clients and servers MUST support HTTP 1.0.  Clients and servers MAY
   support later versions of HTTP as well.

7.1. HTTP headers

  Servers MAY set the Content-Length: header.  Servers SHOULD set
  Content-Encoding to "deflate" or "identity".

  Servers MAY include an X-Your-Address-Is: header, whose value is the
  apparent IP address of the client connecting to them (as a dotted quad).
  For directory connections tunneled over a BEGIN_DIR stream, servers SHOULD
  report the IP from which the circuit carrying the BEGIN_DIR stream reached
  them.  [Servers before version 0.1.2.5-alpha reported 127.0.0.1 for all
  BEGIN_DIR-tunneled connections.]

  Servers SHOULD disable caching of multiple network statuses or multiple
  router descriptors.  Servers MAY enable caching of single descriptors,
  single network statuses, the list of all router descriptors, a v1
  directory, or a v1 running routers document.  XXX mention times.

7.2. HTTP status codes

  Tor delivers the following status codes.  Some were chosen without much
  thought; other code SHOULD NOT rely on specific status codes yet.

  200 -- the operation completed successfully
      -- the user requested statuses or serverdescs, and none of the ones we
         requested were found (0.2.0.4-alpha and earlier).

  304 -- the client specified an if-modified-since time, and none of the
         requested resources have changed since that time.

  400 -- the request is malformed, or
      -- the URL is for a malformed variation of one of the URLs we support,
          or
      -- the client tried to post to a non-authority, or
      -- the authority rejected a malformed posted document, or

  404 -- the requested document was not found.
      -- the user requested statuses or serverdescs, and none of the ones we
         requested were found (0.2.0.5-alpha and later).

  503 -- we are declining the request in order to save bandwidth
      -- user requested some items that we ordinarily generate or store,
         but we do not have any available.

9. Backward compatibility and migration plans

  Until Tor versions before 0.1.1.x are completely obsolete, directory
  authorities should generate, and mirrors should download and cache, v1
  directories and running-routers lists, and allow old clients to download
  them.  These documents and the rules for retrieving, serving, and caching
  them are described in dir-spec-v1.txt.

  Until Tor versions before 0.2.0.x are completely obsolete, directory
  authorities should generate, mirrors should download and cache, v2
  network-status documents, and allow old clients to download them.
  Additionally, all directory servers and caches should download, store, and
  serve any router descriptor that is required because of v2 network-status
  documents. These documents and the rules for retrieving, serving, and
  caching them are described in dir-spec-v1.txt.

A. Consensus-negotiation timeline.


   Period begins: this is the Published time.
     Everybody sends votes
   Reconciliation: everybody tries to fetch missing votes.
     consensus may exist at this point.
   End of voting period:
     everyone swaps signatures.
   Now it's okay for caches to download
     Now it's okay for clients to download.

   Valid-after/valid-until switchover