tor/doc/tor-spec.txt

$Id$

                         Tor Protocol Specification

                              Roger Dingledine
                               Nick Mathewson

Note: This is an attempt to specify Tor as currently implemented.  Future
versions of Tor will implement improved protocols, and compatibility is not
guaranteed.

This is not a design document; most design criteria are not examined.  For
more information on why Tor acts as it does, see tor-design.pdf.

TODO: (very soon)
      - REASON_CONNECTFAILED should include an IP.
      - Copy prose from tor-design to make everything more readable.

0. Notation:

   PK -- a public key.
   SK -- a private key
   K  -- a key for a symmetric cypher

   a|b -- concatenation of 'a' and 'b'.

   [A0 B1 C2] -- a three-byte sequence, containing the bytes with
   hexadecimal values A0, B1, and C2, in that order.

   All numeric values are encoded in network (big-endian) order.

   Unless otherwise specified, all symmetric ciphers are AES in counter
   mode, with an IV of all 0 bytes.  Asymmetric ciphers are either RSA
   with 1024-bit keys and exponents of 65537, or DH where the generator
   is 2 and the modulus is the safe prime from rfc2409, section 6.2,
   whose hex representation is:

     "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E08"
     "8A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B"
     "302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9"
     "A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE6"
     "49286651ECE65381FFFFFFFFFFFFFFFF"

   All "hashes" are 20-byte SHA1 cryptographic digests.

   When we refer to "the hash of a public key", we mean the SHA1 hash of the
   DER encoding of an ASN.1 RSA public key (as specified in PKCS.1).

1. System overview

   Onion Routing is a distributed overlay network designed to anonymize
   low-latency TCP-based applications such as web browsing, secure shell,
   and instant messaging. Clients choose a path through the network and
   build a ``circuit'', in which each node (or ``onion router'' or ``OR'')
   in the path knows its predecessor and successor, but no other nodes in
   the circuit.  Traffic flowing down the circuit is sent in fixed-size
   ``cells'', which are unwrapped by a symmetric key at each node (like
   the layers of an onion) and relayed downstream.

2. Connections

   There are two ways to connect to an onion router (OR). The first is
   as an onion proxy (OP), which allows the OP to authenticate the OR
   without authenticating itself.  The second is as another OR, which
   allows mutual authentication.

   Tor uses TLS for link encryption.  All implementations MUST support
   the TLS ciphersuite "TLS_EDH_RSA_WITH_DES_192_CBC3_SHA", and SHOULD
   support "TLS_DHE_RSA_WITH_AES_128_CBC_SHA" if it is available.
   Implementations MAY support other ciphersuites, but MUST NOT
   support any suite without ephemeral keys, symmetric keys of at
   least 128 bits, and digests of at least 160 bits.

   An OP or OR always sends a two-certificate chain, consisting of a
   certificate using a short-term connection key and a second, self-
   signed certificate containing the OR's identity key. The commonName of the
   first certificate is the OR's nickname, and the commonName of the second
   certificate is the OR's nickname, followed by a space and the string
   "<identity>".

   All parties receiving certificates must confirm that the identity key is
   as expected.  (When initiating a connection, the expected identity key is
   the one given in the directory; when creating a connection because of an
   EXTEND cell, the expected identity key is the one given in the cell.)  If
   the key is not as expected, the party must close the connection.

   All parties SHOULD reject connections to or from ORs that have malformed
   or missing certificates.  ORs MAY accept or reject connections from OPs
   with malformed or missing certificates.

   Once a TLS connection is established, the two sides send cells
   (specified below) to one another.  Cells are sent serially.  All
   cells are 512 bytes long.  Cells may be sent embedded in TLS
   records of any size or divided across TLS records, but the framing
   of TLS records MUST NOT leak information about the type or contents
   of the cells.

   TLS connections are not permanent. An OP or an OR may close a
   connection to an OR if there are no circuits running over the
   connection, and an amount of time (KeepalivePeriod, defaults to 5
   minutes) has passed.

   (As an exception, directory servers may try to stay connected to all of
   the ORs.)

3. Cell Packet format

   The basic unit of communication for onion routers and onion
   proxies is a fixed-width "cell".  Each cell contains the following
   fields:

        CircID                                [2 bytes]
        Command                               [1 byte]
        Payload (padded with 0 bytes)         [509 bytes]
                                         [Total size: 512 bytes]

   The CircID field determines which circuit, if any, the cell is
   associated with.

   The 'Command' field holds one of the following values:
         0 -- PADDING     (Padding)                 (See Sec 6.2)
         1 -- CREATE      (Create a circuit)        (See Sec 4)
         2 -- CREATED     (Acknowledge create)      (See Sec 4)
         3 -- RELAY       (End-to-end data)         (See Sec 5)
         4 -- DESTROY     (Stop using a circuit)    (See Sec 4)
         5 -- CREATE_FAST (Create a circuit, no PK) (See sec 4)
         6 -- CREATED_FAST (Circtuit created, no PK) (See Sec 4)

   The interpretation of 'Payload' depends on the type of the cell.
      PADDING: Payload is unused.
      CREATE:  Payload contains the handshake challenge.
      CREATED: Payload contains the handshake response.
      RELAY:   Payload contains the relay header and relay body.
      DESTROY: Payload is unused.
   Upon receiving any other value for the command field, an OR must
   drop the cell.

   The payload is padded with 0 bytes.

   PADDING cells are currently used to implement connection keepalive.
   If there is no other traffic, ORs and OPs send one another a PADDING
   cell every few minutes.

   CREATE, CREATED, and DESTROY cells are used to manage circuits;
   see section 4 below.

   RELAY cells are used to send commands and data along a circuit; see
   section 5 below.

4. Circuit management

4.1. CREATE and CREATED cells

   Users set up circuits incrementally, one hop at a time. To create a
   new circuit, OPs send a CREATE cell to the first node, with the
   first half of the DH handshake; that node responds with a CREATED
   cell with the second half of the DH handshake plus the first 20 bytes
   of derivative key data (see section 4.2). To extend a circuit past
   the first hop, the OP sends an EXTEND relay cell (see section 5)
   which instructs the last node in the circuit to send a CREATE cell
   to extend the circuit.

   The payload for a CREATE cell is an 'onion skin', which consists
   of the first step of the DH handshake data (also known as g^x).

   The data is encrypted to Bob's PK as follows: Suppose Bob's PK
   modulus is L octets long. If the data to be encrypted is shorter
   than L-42, then it is encrypted directly (with OAEP padding: see
   ftp://ftp.rsasecurity.com/pub/pkcs/pkcs-1/pkcs-1v2-1.pdf). If the
   data is at least as long as L-42, then a randomly generated 16-byte
   symmetric key is prepended to the data, after which the first L-16-42
   bytes of the data are encrypted with Bob's PK; and the rest of the
   data is encrypted with the symmetric key.

   So in this case, the onion skin on the wire looks like:
       RSA-encrypted:
         OAEP padding                  [42 bytes]
         Symmetric key                 [16 bytes]
         First part of g^x             [70 bytes]
       Symmetrically encrypted:
         Second part of g^x            [58 bytes]

   The relay payload for an EXTEND relay cell consists of:
         Address                       [4 bytes]
         Port                          [2 bytes]
         Onion skin                    [186 bytes]
         Public key hash               [20 bytes]

   The port and address field denote the IPV4 address and port of the next
   onion router in the circuit; the public key hash is the SHA1 hash of the
   PKCS#1 ASN1 encoding of the next onion router's identity (signing) key.

   The payload for a CREATED cell, or the relay payload for an
   EXTENDED cell, contains:
         DH data (g^y)                 [128 bytes]
         Derivative key data (KH)      [20 bytes]   <see 4.2 below>

   The CircID for a CREATE cell is an arbitrarily chosen 2-byte integer,
   selected by the node (OP or OR) that sends the CREATE cell.  To prevent
   CircID collisions, when one OR sends a CREATE cell to another, it chooses
   from only one half of the possible values based on the ORs' public
   identity keys: if the sending OR has a lower key, it chooses a CircID with
   an MSB of 0; otherwise, it chooses a CircID with an MSB of 1.

   Public keys are compared numerically by modulus.

   (Older versions of Tor compared OR nicknames, and did it in a broken and
   unreliable way.  To support versions of Tor earlier than 0.0.9pre6,
   implementations should notice when the other side of a connection is
   sending CREATE cells with the "wrong" MSG, and switch accordingly.)

4.1.1. CREATE_FAST/CREATED_FAST cells

   When initializing the first hop of a circuit, the OP has already
   established the OR's identity and negotiated a secret key using TLS.
   Because of this, it is not always necessary for the OP to perform the
   public key operations to create a circuit.  In this case, the
   OP SHOULD send a CREATE_FAST cell instead of a CREATE cell for the first
   hop only.  The OR responds with a CREATED_FAST cell, and the circuit is
   created.

   A CREATE_FAST cell contains:

       Key material (X)    [20 bytes]

   A CREATED_FAST cell contains:

       Key material (Y)    [20 bytes]
       Derivative key data [20 bytes]

   [Versions of Tor before 0.1.0.6-rc did not support these cell types;
    clients should not send CREATE_FAST cells to older Tor servers.]

4.2. Setting circuit keys

   Once the handshake between the OP and an OR is completed, both servers can
   now calculate g^xy with ordinary DH.  Before computing g^xy, both client
   and server MUST verify that the received g^x/g^y value is not degenerate;
   that is, it must be strictly greater than 1 and strictly less than p-1
   where p is the DH modulus.  Implementations MUST NOT complete a handshake
   with degenerate keys.  Implementions MAY discard other "weak" g^x values.

   (Discarding degenerate keys is critical for security; if bad keys are not
   discarded, an attacker can substitute the server's CREATED cell's g^y with
   0 or 1, thus creating a known g^xy and impersonating the server.)

   (The mainline Tor implementation discards all g^x values that are less
   than 2^24, that are greater than p-2^24, or that have more than 1024-16
   identical bits.  This constitutes a negligible portion of the keyspace;
   the chances of stumbling on such a key at random are astronomically
   small.  Nevertheless, implementors may wish to make their implementations
   discard such keys.)

   From the base key material g^xy, they compute derivative key material as
   follows.  First, the server represents g^xy as a big-endian unsigned
   integer.  Next, the server computes 100 bytes of key data as K = SHA1(g^xy
   | [00]) | SHA1(g^xy | [01]) | ... SHA1(g^xy | [04]) where "00" is a single
   octet whose value is zero, [01] is a single octet whose value is one, etc.
   The first 20 bytes of K form KH, bytes 21-40 form the forward digest Df,
   41-60 form the backward digest Db, 61-76 form Kf, and 77-92 form Kb.

   KH is used in the handshake response to demonstrate knowledge of the
   computed shared key. Df is used to seed the integrity-checking hash
   for the stream of data going from the OP to the OR, and Db seeds the
   integrity-checking hash for the data stream from the OR to the OP. Kf
   is used to encrypt the stream of data going from the OP to the OR, and
   Kb is used to encrypt the stream of data going from the OR to the OP.

   The fast-setup case uses the same formula, except that X|Y is used
   in place of g^xy in determining K.  That is,
      K = SHA1(X|Y | [00]) | SHA1(X|Y | [01]) | ... SHA1(X|Y| | [04])
   The values KH, Kf, Kb, Df, and Db are established and used as before.

4.3. Creating circuits

   When creating a circuit through the network, the circuit creator
   (OP) performs the following steps:

      1. Choose an onion router as an exit node (R_N), such that the onion
         router's exit policy does not exclude all pending streams
         that need a circuit.

      2. Choose a chain of (N-1) onion routers
         (R_1...R_N-1) to constitute the path, such that no router
         appears in the path twice.

      3. If not already connected to the first router in the chain,
         open a new connection to that router.

      4. Choose a circID not already in use on the connection with the
         first router in the chain; send a CREATE cell along the
         connection, to be received by the first onion router.

      5. Wait until a CREATED cell is received; finish the handshake
         and extract the forward key Kf_1 and the backward key Kb_1.

      6. For each subsequent onion router R (R_2 through R_N), extend
         the circuit to R.

   To extend the circuit by a single onion router R_M, the OP performs
   these steps:

      1. Create an onion skin, encrypted to R_M's public key.

      2. Send the onion skin in a relay EXTEND cell along
         the circuit (see section 5).

      3. When a relay EXTENDED cell is received, verify KH, and
         calculate the shared keys.  The circuit is now extended.

   When an onion router receives an EXTEND relay cell, it sends a CREATE
   cell to the next onion router, with the enclosed onion skin as its
   payload.  The initiating onion router chooses some circID not yet
   used on the connection between the two onion routers.  (But see
   section 4.1. above, concerning choosing circIDs based on
   lexicographic order of nicknames.)

   As an extension (called router twins), if the desired next onion
   router R in the circuit is down, and some other onion router R'
   has the same public keys as R, then it's ok to extend to R' rather than R.

   When an onion router receives a CREATE cell, if it already has a
   circuit on the given connection with the given circID, it drops the
   cell.  Otherwise, after receiving the CREATE cell, it completes the
   DH handshake, and replies with a CREATED cell.  Upon receiving a
   CREATED cell, an onion router packs it payload into an EXTENDED relay
   cell (see section 5), and sends that cell up the circuit.  Upon
   receiving the EXTENDED relay cell, the OP can retrieve g^y.

   (As an optimization, OR implementations may delay processing onions
   until a break in traffic allows time to do so without harming
   network latency too greatly.)

4.4. Tearing down circuits

   Circuits are torn down when an unrecoverable error occurs along
   the circuit, or when all streams on a circuit are closed and the
   circuit's intended lifetime is over.  Circuits may be torn down
   either completely or hop-by-hop.

   To tear down a circuit completely, an OR or OP sends a DESTROY
   cell to the adjacent nodes on that circuit, using the appropriate
   direction's circID.

   Upon receiving an outgoing DESTROY cell, an OR frees resources
   associated with the corresponding circuit. If it's not the end of
   the circuit, it sends a DESTROY cell for that circuit to the next OR
   in the circuit. If the node is the end of the circuit, then it tears
   down any associated edge connections (see section 5.1).

   After a DESTROY cell has been processed, an OR ignores all data or
   destroy cells for the corresponding circuit.

   (The rest of this section is not currently used; on errors, circuits
   are destroyed, not truncated.)

   To tear down part of a circuit, the OP may send a RELAY_TRUNCATE cell
   signaling a given OR (Stream ID zero).  That OR sends a DESTROY
   cell to the next node in the circuit, and replies to the OP with a
   RELAY_TRUNCATED cell.

   When an unrecoverable error occurs along one connection in a
   circuit, the nodes on either side of the connection should, if they
   are able, act as follows:  the node closer to the OP should send a
   RELAY_TRUNCATED cell towards the OP; the node farther from the OP
   should send a DESTROY cell down the circuit.

4.5. Routing relay cells

   When an OR receives a RELAY cell, it checks the cell's circID and
   determines whether it has a corresponding circuit along that
   connection.  If not, the OR drops the RELAY cell.

   Otherwise, if the OR is not at the OP edge of the circuit (that is,
   either an 'exit node' or a non-edge node), it de/encrypts the payload
   with AES/CTR, as follows:
        'Forward' relay cell (same direction as CREATE):
            Use Kf as key; decrypt.
        'Back' relay cell (opposite direction from CREATE):
            Use Kb as key; encrypt.

   The OR then decides whether it recognizes the relay cell, by
   inspecting the payload as described in section 5.1 below.  If the OR
   recognizes the cell, it processes the contents of the relay cell.
   Otherwise, it passes the decrypted relay cell along the circuit if
   the circuit continues.  If the OR at the end of the circuit
   encounters an unrecognized relay cell, an error has occurred: the OR
   sends a DESTROY cell to tear down the circuit.

   When a relay cell arrives at an OP, the OP decrypts the payload
   with AES/CTR as follows:
         OP receives data cell:
            For I=N...1,
                Decrypt with Kb_I.  If the payload is recognized (see
                section 5.1), then stop and process the payload.

   For more information, see section 5 below.

5. Application connections and stream management

5.1. Relay cells

   Within a circuit, the OP and the exit node use the contents of
   RELAY packets to tunnel end-to-end commands and TCP connections
   ("Streams") across circuits.  End-to-end commands can be initiated
   by either edge; streams are initiated by the OP.

   The payload of each unencrypted RELAY cell consists of:
         Relay command           [1 byte]
         'Recognized'            [2 bytes]
         StreamID                [2 bytes]
         Digest                  [4 bytes]
         Length                  [2 bytes]
         Data                    [498 bytes]

   The relay commands are:
         1 -- RELAY_BEGIN
         2 -- RELAY_DATA
         3 -- RELAY_END
         4 -- RELAY_CONNECTED
         5 -- RELAY_SENDME
         6 -- RELAY_EXTEND
         7 -- RELAY_EXTENDED
         8 -- RELAY_TRUNCATE
         9 -- RELAY_TRUNCATED
        10 -- RELAY_DROP
        11 -- RELAY_RESOLVE
        12 -- RELAY_RESOLVED

   The 'Recognized' field in any unencrypted relay payload is always
   set to zero; the 'digest' field is computed as the first four bytes
   of the running SHA-1 digest of all the bytes that have travelled
   over this circuit, seeded from Df or Db respectively (obtained in
   section 4.2 above), and including this RELAY cell's entire payload
   (taken with the digest field set to zero).

   When the 'recognized' field of a RELAY cell is zero, and the digest
   is correct, the cell is considered "recognized" for the purposes of
   decryption (see section 4.5 above).

   All RELAY cells pertaining to the same tunneled stream have the
   same stream ID.  StreamIDs are chosen randomly by the OP.  RELAY
   cells that affect the entire circuit rather than a particular
   stream use a StreamID of zero.

   The 'Length' field of a relay cell contains the number of bytes in
   the relay payload which contain real payload data. The remainder of
   the payload is padded with NUL bytes.

5.2. Opening streams and transferring data

   To open a new anonymized TCP connection, the OP chooses an open
   circuit to an exit that may be able to connect to the destination
   address, selects an arbitrary StreamID not yet used on that circuit,
   and constructs a RELAY_BEGIN cell with a payload encoding the address
   and port of the destination host.  The payload format is:

         ADDRESS | ':' | PORT | [00]

   where  ADDRESS can be a DNS hostname, or an IPv4 address in
   dotted-quad format, or an IPv6 address surrounded by square brackets;
   and where PORT is encoded in decimal.

   [What is the [00] for? -NM]
   [It's so the payload is easy to parse out with string funcs -RD]

   Upon receiving this cell, the exit node resolves the address as
   necessary, and opens a new TCP connection to the target port.  If the
   address cannot be resolved, or a connection can't be established, the
   exit node replies with a RELAY_END cell.  (See 5.4 below.)
   Otherwise, the exit node replies with a RELAY_CONNECTED cell, whose
   payload is in one of the following formats:
       The IPv4 address to which the connection was made [4 octets]
       A number of seconds (TTL) for which the address may be cached [4 octets]
    or
       Four zero-valued octets [4 octets]
       An address type (6)     [1 octet]
       The IPv6 address to which the connection was made [16 octets]
       A number of seconds (TTL) for which the address may be cached [4 octets]
   [XXXX Versions of Tor before 0.1.1.6 ignore and do not generate the TTL
   field.  No version of Tor currently generates the IPv6 format.]

   The OP waits for a RELAY_CONNECTED cell before sending any data.
   Once a connection has been established, the OP and exit node
   package stream data in RELAY_DATA cells, and upon receiving such
   cells, echo their contents to the corresponding TCP stream.
   RELAY_DATA cells sent to unrecognized streams are dropped.

   Relay RELAY_DROP cells are long-range dummies; upon receiving such
   a cell, the OR or OP must drop it.

5.3. Closing streams

   When an anonymized TCP connection is closed, or an edge node
   encounters error on any stream, it sends a 'RELAY_END' cell along the
   circuit (if possible) and closes the TCP connection immediately.  If
   an edge node receives a 'RELAY_END' cell for any stream, it closes
   the TCP connection completely, and sends nothing more along the
   circuit for that stream.

   The payload of a RELAY_END cell begins with a single 'reason' byte to
   describe why the stream is closing, plus optional data (depending on
   the reason.)  The values are:

       1 -- REASON_MISC           (catch-all for unlisted reasons)
       2 -- REASON_RESOLVEFAILED  (couldn't look up hostname)
       3 -- REASON_CONNECTREFUSED (remote host refused connection) [*]
       4 -- REASON_EXITPOLICY     (OR refuses to connect to host or port)
       5 -- REASON_DESTROY        (Circuit is being destroyed)
       6 -- REASON_DONE           (Anonymized TCP connection was closed)
       7 -- REASON_TIMEOUT        (Connection timed out, or OR timed out
                                   while connecting)
       8 -- (unallocated) [**]
       9 -- REASON_HIBERNATING    (OR is temporarily hibernating)
      10 -- REASON_INTERNAL       (Internal error at the OR)
      11 -- REASON_RESOURCELIMIT  (OR has no resources to fulfill request)
      12 -- REASON_CONNRESET      (Connection was unexpectedly reset)
      13 -- REASON_TORPROTOCOL    (Sent when closing connection because of
                                   Tor protocol violations.)

   (With REASON_EXITPOLICY, the 4-byte IPv4 address or 16-byte IPv6 address
   forms the optional data; no other reason currently has extra data.
   As of 0.1.1.6, the body also contains a 4-byte TTL.)

   OPs and ORs MUST accept reasons not on the above list, since future
   versions of Tor may provide more fine-grained reasons.

   [*] Older versions of Tor also send this reason when connections are
       reset.
   [**] Due to a bug in versions of Tor through 0095, error reason 8 must
        remain allocated until that version is obsolete.

   --- [The rest of this section describes unimplemented functionality.]

   Because TCP connections can be half-open, we follow an equivalent
   to TCP's FIN/FIN-ACK/ACK protocol to close streams.

   An exit connection can have a TCP stream in one of three states:
   'OPEN', 'DONE_PACKAGING', and 'DONE_DELIVERING'.  For the purposes
   of modeling transitions, we treat 'CLOSED' as a fourth state,
   although connections in this state are not, in fact, tracked by the
   onion router.

   A stream begins in the 'OPEN' state.  Upon receiving a 'FIN' from
   the corresponding TCP connection, the edge node sends a 'RELAY_FIN'
   cell along the circuit and changes its state to 'DONE_PACKAGING'.
   Upon receiving a 'RELAY_FIN' cell, an edge node sends a 'FIN' to
   the corresponding TCP connection (e.g., by calling
   shutdown(SHUT_WR)) and changing its state to 'DONE_DELIVERING'.

   When a stream in already in 'DONE_DELIVERING' receives a 'FIN', it
   also sends a 'RELAY_FIN' along the circuit, and changes its state
   to 'CLOSED'.  When a stream already in 'DONE_PACKAGING' receives a
   'RELAY_FIN' cell, it sends a 'FIN' and changes its state to
   'CLOSED'.

   If an edge node encounters an error on any stream, it sends a
   'RELAY_END' cell (if possible) and closes the stream immediately.

5.4. Remote hostname lookup

   To find the address associated with a hostname, the OP sends a
   RELAY_RESOLVE cell containing the hostname to be resolved.  (For a reverse
   lookup, the OP sends a RELAY_RESOLVE cell containing an in-addr.arpa
   address.)  The OR replies with a RELAY_RESOLVED cell containing a status
   byte, and any number of answers.  Each answer is of the form:
       Type   (1 octet)
       Length (1 octet)
       Value  (variable-width)
       TTL    (4 octets)
   "Length" is the length of the Value field.
   "Type" is one of:
      0x00 -- Hostname
      0x04 -- IPv4 address
      0x06 -- IPv6 address
      0xF0 -- Error, transient
      0xF1 -- Error, nontransient

    If any answer has a type of 'Error', then no other answer may be given.

    The RELAY_RESOLVE cell must use a nonzero, distinct streamID; the
    corresponding RELAY_RESOLVED cell must use the same streamID.  No stream
    is actually created by the OR when resolving the name.

6. Flow control

6.1. Link throttling

   Each node should do appropriate bandwidth throttling to keep its
   user happy.

   Communicants rely on TCP's default flow control to push back when they
   stop reading.

6.2. Link padding

   Currently nodes are not required to do any sort of link padding or
   dummy traffic. Because strong attacks exist even with link padding,
   and because link padding greatly increases the bandwidth requirements
   for running a node, we plan to leave out link padding until this
   tradeoff is better understood.

6.3. Circuit-level flow control

   To control a circuit's bandwidth usage, each OR keeps track of
   two 'windows', consisting of how many RELAY_DATA cells it is
   allowed to package for transmission, and how many RELAY_DATA cells
   it is willing to deliver to streams outside the network.
   Each 'window' value is initially set to 1000 data cells
   in each direction (cells that are not data cells do not affect
   the window).  When an OR is willing to deliver more cells, it sends a
   RELAY_SENDME cell towards the OP, with Stream ID zero.  When an OR
   receives a RELAY_SENDME cell with stream ID zero, it increments its
   packaging window.

   Each of these cells increments the corresponding window by 100.

   The OP behaves identically, except that it must track a packaging
   window and a delivery window for every OR in the circuit.

   An OR or OP sends cells to increment its delivery window when the
   corresponding window value falls under some threshold (900).

   If a packaging window reaches 0, the OR or OP stops reading from
   TCP connections for all streams on the corresponding circuit, and
   sends no more RELAY_DATA cells until receiving a RELAY_SENDME cell.
[this stuff is badly worded; copy in the tor-design section -RD]

6.4. Stream-level flow control

   Edge nodes use RELAY_SENDME cells to implement end-to-end flow
   control for individual connections across circuits. Similarly to
   circuit-level flow control, edge nodes begin with a window of cells
   (500) per stream, and increment the window by a fixed value (50)
   upon receiving a RELAY_SENDME cell. Edge nodes initiate RELAY_SENDME
   cells when both a) the window is <= 450, and b) there are less than
   ten cell payloads remaining to be flushed at that edge.

7. Directories and routers

7.1. Extensible information format

Router descriptors and directories both 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 a space 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 SP ArgumentsChar+ NL
    Keyword = KeywordChar+
    KeywordChar ::= 'A' ... 'Z' | 'a' ... 'z' | '0' ... '9' | '-'
    ArgumentChar ::= any printing ASCII character except NL.
    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 reject any document containing a
KeywordLine that starts with a keyword it doesn't recognize.

The "opt" keyword is reserved 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.

7.2. 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" "link-key"
"signing-key" "bandwidth".  Additionally, a router descriptor MAY contain any
number of "accept", "reject", "fingerprint", "uptime", 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 Port values will soon be
      deprecated; using them here is equivalent to using them in a "ports"
      item.

   "ports" ORPort SocksPort DirPort

      Indicates 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 the port at which this OR accepts SOCKS
      connections; 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 (20 byte SHA1 hash of asn1 encoded public key, encoded
      in hex, with spaces after every 4 characters) for this router's
      identity 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" 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.

      [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"

      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 XXXX hours 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, in order, 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.

   "router-signature" NL Signature NL

       The "SIGNATURE" object contains a signature of the PKCS1-padded SHA1
       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. 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.

       [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.]

nickname ::= between 1 and 19 alphanumeric characters, case-insensitive.

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

Ports are required; if they are not included in the router
line, they must appear in the "ports" lines.

7.3. Directory format

A Directory begins with a "signed-directory" item, followed by one each of
the following, in any order: "recommended-software", "published",
"router-status", "dir-signing-key".  It may include any number of "opt"
items.  After these items, a directory includes any number of router
descriptors, and a single "directory-signature" item.

    "signed-directory"

        Indicates the start of a directory.

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

        The time at which this directory was generated and signed, in GMT.

    "dir-signing-key"

        The key used to sign this directory; see "signing-key" for format.

    "recommended-software"  comma-separated-version-list

        A list of which versions of which implementations are currently
        believed to be secure and compatible with the network.

    "running-routers" space-separated-list

        A description of which routers are currently believed to be up or
        down.  Every entry consists of an optional "!", followed by either an
        OR's nickname, or "$" followed by a hexadecimal encoding of the hash
        of an OR's identity key.  If the "!" is included, the router is
        believed not to be running; otherwise, it is believed to be running.
        If a router's nickname is given, exactly one router of that nickname
        will appear in the directory, and that router is "approved" by the
        directory server.  If a hashed identity key is given, that OR is not
        "approved".  [XXXX The 'running-routers' line is only provided for
        backward compatibility.  New code should parse 'router-status'
        instead.]

    "router-status" space-separated-list

        A description of which routers are currently believed to be up or
        down, and which are verified or unverified.  Contains one entry for
        every router that the directory server knows.  Each entry is of the
        format:

              !name=$digest  [Verified router, currently not live.]
              name=$digest   [Verified router, currently live.]
              !$digest       [Unverified router, currently not live.]
          or  $digest        [Unverified router, currently live.]

        (where 'name' is the router's nickname and 'digest' is a hexadecimal
        encoding of the hash of the routers' identity key).

        When parsing this line, clients should only mark a router as
        'verified' if its nickname AND digest match the one provided.

    "directory-signature" nickname-of-dirserver NL Signature

The signature is computed by computing the SHA-1 hash of the
directory, from the characters "signed-directory", through the newline
after "directory-signature".  This digest is then padded with PKCS.1,
and signed with the directory server's signing key.

If software encounters an unrecognized keyword in a single router descriptor,
it MUST reject only that router descriptor, and continue using the
others.  Because this mechanism is used to add 'critical' extensions to
future versions of the router descriptor format, implementation should treat
it as a normal occurrence and not, for example, report it to the user as an
error.  [Versions of Tor prior to 0.1.1 did this.]

If software encounters an unrecognized keyword in the directory header,
it SHOULD reject the entire directory.

7.4. Network-status descriptor

A "network-status" (a.k.a "running-routers") document is a truncated
directory that contains only the current status of a list of nodes, not
their actual descriptors.  It contains exactly one of each of the following
entries.

     "network-status"

        Must appear first.

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

        (see 7.3 above)

     "router-status" list

        (see 7.3 above)

     "directory-signature" NL signature

        (see 7.3 above)

7.5. Behavior of a directory server

lists nodes that are connected currently
speaks HTTP on a socket, spits out directory on request

Directory servers listen on a certain port (the DirPort), and speak a
limited version of HTTP 1.0. Clients send either GET or POST commands.
The basic interactions are:
  "%s %s HTTP/1.0\r\nContent-Length: %lu\r\nHost: %s\r\n\r\n",
    command, url, content-length, host.
  Get "/tor/" to fetch a full directory.
  Get "/tor/dir.z" to fetch a compressed full directory.
  Get "/tor/running-routers" to fetch a network-status descriptor.
  Post "/tor/" to post a server descriptor, with the body of the
    request containing the descriptor.

  "host" is used to specify the address:port of the dirserver, so
  the request can survive going through HTTP proxies.

A.1. Differences between spec and implementation

- The current specification requires all ORs to have IPv4 addresses, but
  allows servers to exit and resolve to IPv6 addresses, and to declare IPv6
  addresses in their exit policies.  The current codebase has no IPv6
  support at all.