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+
+                      Tor directory protocol, version 2
+
+0. Scope and preliminaries
+
+   This directory protocol is used by Tor version 0.1.1.x and 0.1.2.x.  See
+   dir-spec-v1.txt for information on earlier versions, and dir-spec.txt
+   for information on later versions.
+
+0.1. Goals and motivation
+
+   There were several problems with the way Tor handles directory information
+   in version 0.1.0.x and earlier.  Here are the problems we try to fix with
+   this new design, already implemented in 0.1.1.x:
+      1. Directories were very large and use up a lot of bandwidth: clients
+         downloaded descriptors for all router several times an hour.
+      2. 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.
+      3. Our current "verified server" system is kind of nonsensical.
+
+      4. Getting more directory authorities would add more points of failure
+         and worsen possible partitioning attacks.
+
+   There are two problems that remain unaddressed by this design.
+      5. Requiring every client to know about every router won't scale.
+      6. Requiring every directory cache to know every router won't scale.
+
+   We attempt to fix 1-4 here, and to build a solution that will work when we
+   figure out an answer for 5.  We haven't thought at all about what to do
+   about 6.
+
+1. Outline
+
+   There is a small set (say, around 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.
+
+   Routers periodically upload signed "descriptors" to the directory
+   authorities describing their keys, capabilities, and other information.
+   Routers may act as directory mirrors (also called "caches"), to reduce
+   load on the directory authorities.  They announce this in their
+   descriptors.
+
+   Each directory authority periodically generates and signs a compact
+   "network status" document that lists that authority's view of the current
+   descriptors and status for known routers, but which does not include the
+   descriptors themselves.
+
+   Directory mirrors download, cache, and re-serve network-status documents
+   to clients.
+
+   Clients, directory mirrors, and directory authorities all use
+   network-status documents to find out when their list of routers is
+   out-of-date.  If it is, they download any missing router descriptors.
+   Clients download missing descriptors from mirrors; mirrors 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.
+
+   Coordination among directory authorities is done client-side: clients
+   compute a vote-like algorithm among the network-status documents they
+   have, and base their decisions on the result.
+
+1.1. What's different from 0.1.0.x?
+
+   Clients used to download a signed concatenated set of router descriptors
+   (called a "directory") from directory mirrors, regardless of which
+   descriptors had changed.
+
+   Between downloading directories, clients would download "network-status"
+   documents that would list which servers were supposed to running.
+
+   Clients would always believe the most recently published network-status
+   document they were served.
+
+   Routers used to upload fresh descriptors all the time, whether their keys
+   and other information had changed or not.
+
+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 one 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 ArgumentsChar+ NL
+    Keyword = KeywordChar+
+    KeywordChar ::= 'A' ... 'Z' | 'a' ... 'z' | '0' ... '9' | '-'
+    ArgumentChar ::= any printing ASCII character except NL.
+    WS = (SP | TAB)+
+    Object ::= BeginLine Base64-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.
+
+  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.
+
+2. Router operation
+
+   ORs SHOULD generate a new router descriptor 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 at least 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).
+
+   After generating a descriptor, ORs upload it to every directory
+   authority they know, by posting it to the URL
+
+      http://<hostname:port>/tor/
+
+2.1. Router descriptor format
+
+   Every router descriptor MUST start with a "router" Item; MUST end with a
+   "router-signature" Item and an extra NL; and MUST contain exactly one
+   instance of each of the following Items: "published" "onion-key"
+   "signing-key" "bandwidth".
+
+   A router descriptor MAY have zero or one of each of the following Items,
+   but MUST NOT have more than one: "contact", "uptime", "fingerprint",
+   "hibernating", "read-history", "write-history", "eventdns", "platform",
+   "family".
+
+   For historical reasons some options are prefixed with "opt ", for instance
+   "opt fingerprint". This "opt " prefix should be ignored with the second word
+   used as the keyword instead.
+
+   Additionally, a router descriptor MAY contain any number of "accept",
+   "reject", and "opt" Items.  Other than "router" and "router-signature",
+   the items may appear in any order.
+
+   The items' formats are as follows:
+    "router" nickname address ORPort SocksPort DirPort
+
+       Indicates the beginning of a router descriptor.  "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.
+
+    "bandwidth" bandwidth-avg bandwidth-burst bandwidth-observed
+
+       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
+
+       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
+
+       The time, in GMT, when this descriptor was generated.
+
+    "fingerprint"
+
+       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.
+
+    "hibernating" 0|1
+
+       If the value is 1, then the Tor server was hibernating when the
+       descriptor was published, and shouldn't be used to build circuits.
+
+    "uptime"
+
+       The number of seconds that this OR process has been running.
+
+    "onion-key" NL a public key in PEM format
+
+       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.
+
+    "signing-key" NL a public key in PEM format
+
+       The OR's long-term identity key.
+
+    "accept" exitpattern
+    "reject" exitpattern
+
+       These lines describe 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
+
+       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
+
+        Describes a way to contact the server's administrator, preferably
+        including an email address and a PGP key fingerprint.
+
+    "family" names NL
+
+        '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
+    "write-history" YYYY-MM-DD HH:MM:SS (NSEC s) NUM,NUM,NUM,NUM,NUM... NL
+
+        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.
+
+    "eventdns" bool NL
+
+        Declare whether this version of Tor is using the newer enhanced
+        dns logic.  Versions of Tor without eventdns 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 eventdns was in use.  With 0.2.0.1-alpha, the
+         old 'dnsworker' logic has been removed, rendering this option of
+         historical interest only.]
+
+2.2. 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.]
+
+   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"
+
+   Ports are required; if they are not included in the router
+   line, they must appear in the "ports" lines.
+
+3. Network status format
+
+   Directory authorities generate, sign, and compress network-status
+   documents.  Directory servers SHOULD generate a fresh network-status
+   document when the contents of such a document would be different from the
+   last one generated, and some time (at least one second, possibly longer)
+   has passed since the last one was generated.
+
+   The network status document contains a preamble, a set of router status
+   entries, and a signature, in that order.
+
+   We use the same meta-format as used for directories and router descriptors
+   in "tor-spec.txt".  Implementations MAY insert blank lines
+   for clarity between sections; these blank lines are ignored.
+   Implementations MUST NOT depend on blank lines in any particular location.
+
+   As used here, "whitespace" is a sequence of 1 or more tab or space
+   characters.
+
+   The preamble contains:
+
+      "network-status-version" -- A document format version.  For this
+         specification, the version is "2".
+      "dir-source" -- The authority's hostname, current IP address, and
+         directory port, all separated by whitespace.
+      "fingerprint" -- A base16-encoded hash of the signing key's
+         fingerprint, with no additional spaces added.
+      "contact" -- An arbitrary string describing how to contact the
+         directory server's administrator.  Administrators should include at
+         least an email address and a PGP fingerprint.
+      "dir-signing-key" -- The directory server's public signing key.
+      "client-versions" -- A comma-separated list of recommended client
+        versions.
+      "server-versions" -- A comma-separated list of recommended server
+        versions.
+      "published" -- The publication time for this network-status object.
+      "dir-options" -- A set of flags, in any order, separated by whitespace:
+          "Names" if this directory authority performs name bindings.
+          "Versions" if this directory authority recommends software versions.
+          "BadExits" if the directory authority flags nodes that it believes
+              are performing incorrectly as exit nodes.
+          "BadDirectories" if the directory authority flags nodes that it
+              believes are performing incorrectly as directory caches.
+
+   The dir-options entry is optional.  The "-versions" entries are required if
+   the "Versions" flag is present.  The other entries are required and must
+   appear exactly once. The "network-status-version" entry must appear first;
+   the others may appear in any order.  Implementations MUST ignore
+   additional arguments to the items above, and MUST ignore unrecognized
+   flags.
+
+   For each router, the router entry contains:  (This format is designed for
+   conciseness.)
+
+      "r" -- followed by the following elements, in order, separated by
+          whitespace:
+          - The OR's nickname,
+          - A hash of its identity key, encoded in base64, with trailing =
+            signs removed.
+          - A hash of its most recent descriptor, encoded in base64, with
+            trailing = signs removed.  (The hash is calculated as for
+            computing the signature of a descriptor.)
+          - The publication time of its most recent descriptor, in the form
+            YYYY-MM-DD HH:MM:SS, in GMT.
+          - An IP address
+          - An OR port
+          - A directory port (or "0" for none")
+      "s" -- A series of whitespace-separated status flags, in any order:
+          "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.
+          "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 this protocol.
+      "v" -- 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 "r" entry for each router must appear first and is required. The
+      "s" entry is optional (see Section 3.1 below for how the flags are
+      decided). Unrecognized flags on the "s" line and extra elements
+      on the "r" line must be ignored.  The "v" line is optional.
+
+   The signature section contains:
+
+      "directory-signature" nickname-of-dirserver NL Signature
+
+      Signature is a signature of this network-status document
+      (the document up until the signature, including the line
+      "directory-signature <nick>\n"), using the directory authority's
+      signing key.
+
+   We compress the network status list with zlib before transmitting it.
+
+3.1. Establishing server status
+
+   (This section describes how directory authorities choose which status
+   flags to apply to routers, as of Tor 0.1.1.18-rc. 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.
+
+   "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
+   uptime is at least the median uptime for known active routers, or
+   its uptime is at least 30 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.)
+
+   "Fast" -- A router is 'Fast' if it is active, and its bandwidth is
+   in the top 7/8ths for known active routers.
+
+   "Guard" -- A router is a possible 'Guard' if it is 'Stable' and its
+   bandwidth is above median for known active routers. 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.
+
+   "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.)
+
+   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 list includes all non-blacklisted,
+   non-expired, non-superseded descriptors.
+
+4. Directory server operation
+
+   All directory authorities and directory mirrors ("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.
+
+4.2. Downloading network-status documents (authorities and caches)
+
+   All directory servers (authorities and mirrors) try to keep a fresh
+   set of network-status documents from every authority.  To do so,
+   every 5 minutes, each authority asks every other authority for its
+   most recent network-status document.  Every 15 minutes, each mirror
+   picks a random authority and asks it for the most recent network-status
+   documents for all the authorities the authority knows about (including
+   the chosen authority itself).
+
+   Directory servers and mirrors remember and serve the most recent
+   network-status document they have from each authority.  Other
+   network-status documents don't need to be stored.  If the most recent
+   network-status document is over 10 days old, it is discarded anyway.
+   Mirrors SHOULD store and serve network-status documents from authorities
+   they don't recognize, but SHOULD NOT use such documents for any other
+   purpose.  Mirrors SHOULD discard network-status documents older than 48
+   hours.
+
+4.3. Downloading and storing router descriptors (authorities and caches)
+
+   Periodically (currently, every 10 seconds), directory servers check
+   whether there are any specific descriptors (as identified by descriptor
+   hash in a network-status document) that they do not have and that they
+   are not currently trying to download.
+
+   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 network-status.  When 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 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 current
+   network-status document from any authority.  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.)
+
+   Authorities SHOULD NOT download descriptors for routers that they would
+   immediately reject for reasons listed in 3.1.
+
+4.4. HTTP URLs
+
+   "Fingerprints" in these URLs are base16-encoded SHA1 hashes.
+
+   The authoritative network-status published by a host should be available at:
+      http://<hostname>/tor/status/authority.z
+
+   The network-status published by a host with fingerprint
+   <F> should be available at:
+      http://<hostname>/tor/status/fp/<F>.z
+
+   The network-status documents published by hosts with fingerprints
+   <F1>,<F2>,<F3> should be available at:
+      http://<hostname>/tor/status/fp/<F1>+<F2>+<F3>.z
+
+   The most recent network-status documents from all known authorities,
+   concatenated, should be available at:
+      http://<hostname>/tor/status/all.z
+
+   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
+
+   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 an ordered list of directory authorities.
+   Insofar as possible, clients SHOULD all use the same ordered list.
+
+   For each network-status document a client has, it keeps track of its
+   publication time *and* the time when the client retrieved it.  Clients
+   consider a network-status document "live" if it was published within the
+   last 24 hours.
+
+   Clients try to have a live network-status document hours from *every*
+   authority, and try to periodically get new network-status documents from
+   each authority in rotation as follows:
+
+   If a client is missing a live network-status document for any
+   authority, it tries to fetch it from a directory cache.  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 live network-status documents from more than half the authorities
+   it trusts, and it has descriptors for more than 1/4 of the routers
+   that it believes are running.
+
+   If the most recently _retrieved_ network-status document is over 30
+   minutes old, the client attempts to download a network-status document.
+   When choosing which documents to download, clients treat their list of
+   directory authorities as a circular ring, and begin with the authority
+   appearing immediately after the authority for their most recently
+   retrieved network-status document.  If this attempt fails (either it
+   fails to download at all, or the one it gets is not as good as the
+   one it has), the client retries at other caches several times, before
+   moving on to the next network-status document in sequence.
+
+   Clients discard all network-status documents over 24 hours old.
+
+   If enough mirrors (currently 4) claim not to have a given network status,
+   we stop trying to download that authority's network-status, until we
+   download a new network-status that makes us believe that the authority in
+   question is running.  Clients should wait a little longer after each
+   failure.
+
+   Clients SHOULD try to batch as many network-status requests as possible
+   into each HTTP GET.
+
+   (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 the most recently published descriptor listed for that router
+        by at least two network-status documents.
+        OR,
+      * No descriptor for that router is listed by two or more
+        network-status documents, and it is the most recently published
+        descriptor listed by any 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.
+
+   No descriptors are downloaded until the client has downloaded more than
+   half of the network-status documents.
+
+   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
+   it is recommended by at least one networkstatus AND no "better"
+   descriptor has been downloaded.  [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 live network-status documents, it downloads
+   network-status documents from a randomly chosen authority.  In all other
+   cases, the client downloads from mirrors 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.
+
+   Tor implementations only pay attention to "live" network-status documents.
+   A network status is "live" if it is the most recently downloaded network
+   status document for a given directory server, and the server is a
+   directory server trusted by the client, and the network-status document is
+   no more than 1 day old.
+
+   For time-sensitive information, Tor implementations focus on "recent"
+   network-status documents.  A network status is "recent" if it is live, and
+   if it was published in the last 60 minutes.  If there are fewer
+   than 3 such documents, the most recently published 3 are "recent."  If
+   there are fewer than 3 in all, all are "recent.")
+
+   Circuits SHOULD NOT be built until the client has enough directory
+   information: network-statuses (or failed attempts to download
+   network-statuses) for all authorities, network-statuses for at more than
+   half of the authorities, and descriptors for at least 1/4 of the servers
+   believed to be running.
+
+   A server is "listed" if it is included by more than half of the live
+   network status documents.  Clients SHOULD NOT use unlisted servers.
+
+   Clients believe the flags "Valid", "Exit", "Fast", "Guard", "Stable", and
+   "V2Dir" about a given router when they are asserted by more than half of
+   the live network-status documents.  Clients believe the flag "Running" if
+   it is listed by more than half of the recent network-status documents.
+
+   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.
+
+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 all the live "Naming" network-status documents the client has
+      claim that the name binds to some identity ID, and the client has at
+      least three live network-status documents, the client maps the name to
+      ID.
+
+   When a user tries to refer to a router with a name that does not have a
+   mapping under the above rules, the implementation SHOULD warn the user.
+   After giving the warning, the implementation MAY use a router that at
+   least one Naming authority maps the name to, so long as no other naming
+   authority maps that name to a different router.  If no Naming authority
+   maps the name to a router, 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 (or has
+   attempted to fetch and failed four consecutive times) a network-status
+   for each authority, and it is running a software version
+   not listed on more than half of the live "Versioning" network-status
+   documents.
+
+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 (or attempted to fetch and failed four
+   consecutive times) a network-status for every authority, and at
+   least one of the authorities is "Naming", and no live "Naming"
+   authorities 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 networkstatus's "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.
+
+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 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
+
+  XXX We should write down what return codes dirservers send in what
+  situations.
+