tor/doc/tor-spec.txt

$Id$

Tor Spec

Note: This is an attempt to specify Tor as it exists as implemented in
early March, 2004.  It is not recommended that others implement this
design as it stands; future versions of Tor will implement improved
protocols.

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 with the safe prime
   from rfc2409, section 6.2, whose hex representation is:

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

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 OR always sends a two-certificate chain, consisting of a self-signed
   certificate containing the OR's identity key, and a second certificate
   using a short-term connection key.  The commonName of the second
   certificate is the OR's nickname, and the commonName of the first
   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.

   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.

   OR-to-OR connections are never deliberately closed.  When an OR
   starts or receives a new directory, it tries to open new
   connections to any OR it is not already connected to.

   OR-to-OP connections are not permanent. An OP should 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.

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)

   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 is
   L octets long.  If the data to be encrypted is shorter than L-42,
   then it is encrypted directly (with OAEP padding).  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 ASN1 encoding of the next onion router's identity key.

   [XXXX Before 0.0.8, EXTEND cells did not include the public key hash.
   Servers running 0.0.8 distinguish the old-style cells based on the
   length of payloads. (Servers running 0.0.7 blindly pass on the extend
   cell regardless of length.) In a future release, old-style EXTEND
   cells will not be supported.]

   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' nicknames: if the sending OR has a
   lexicographically earlier nickname, it chooses a CircID with a high
   bit of 0; otherwise, it chooses a CircID with a high bit of 1.

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.  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.

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) chain of N 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 is be a DNS hostname, or an IPv4 address in
   dotted-quad format; 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 the 4-byte IP address to which the connection was made.

   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_CONNECTFAILED  (couldn't connect to host/port)
       4 -- REASON_EXITPOLICY     (OR refuses to connect to host or port)
       5 -- REASON_DESTROY        (circuit is being destroyed [???-NM])
       6 -- REASON_DONE           (anonymized TCP connection was closed)
       7 -- REASON_TIMEOUT        (OR timed out while connecting [???-NM])

   (With REASON_EXITPOLICY, the 4-byte IP address forms the optional
   data; no other reason currently has extra data.)


   *** [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.  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)
   "Length" is the length of the Value field.
   "Type" is one of:
      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.

7.1. 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" "link-key"
"signing-key".  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)?
   "ports" ORPort SocksPort DirPort
   "bandwidth" bandwidth-avg bandwidth-burst
   "platform" string
   "published" YYYY-MM-DD HH:MM:SS
   "onion-key" NL a public key in PEM format
   "signing-key" NL a public key in PEM format
   "accept" string
   "reject" string
   "router-signature" NL "-----BEGIN SIGNATURE-----" NL Signature NL
                      "-----END SIGNATURE-----"
   "opt" SP keyword string? NL,Object?

ORport ::= port where the router listens for routers/proxies (speaking cells)
SocksPort ::=  where the router listens for applications (speaking socks)
DirPort ::= where the router listens for directory download requests
bandwidth-avg ::= maximum average bandwidth, in bytes/s
bandwidth-burst ::= maximum bandwidth spike, in bytes/s
nickname ::= between 1 and 19 alphanumeric characters, case-insensitive.

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

"opt" is reserved for non-critical future extensions.

7.2. Directory format

A Directory begins with a "signed-directory" item, followed by one each of
the following, in any order: "recommended-software", "published",
"running-routers".  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"
    "published" YYYY-MM-DD HH:MM:SS
    "recommended-software"  comma-separated-version-list
    "running-routers" comma-separated-nickname-list
    "directory-signature" nickname-of-dirserver NL Signature

Note:  The router descriptor for the directory server must appear first.
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 should reject only that router descriptor, and continue using the
others.  If it encounters an unrecognized keyword in the directory header,
it should reject the entire directory.

7.3. Behavior of a directory server

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

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