fff8 TOR RFC - Telecomix Crypto Munitions Bureau


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Fetched from the git repo at 2012-01-04.

                         Tor Protocol Specification

                              Roger Dingledine
                               Nick Mathewson

Note: This document aims to specify Tor as implemented in 0.2.1.x.  Future
versions of Tor may implement improved protocols, and compatibility is not
guaranteed.  Compatibility notes are given for versions and
later; earlier versions are not compatible with the Tor network as of this

This specification 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.

0. Preliminaries

      The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
      "OPTIONAL" in this document are to be interpreted as described in
      RFC 2119.

0.1.  Notation and encoding

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

   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.

   H(m) -- a cryptographic hash of m.

0.2. Security parameters

   Tor uses a stream cipher, a public-key cipher, the Diffie-Hellman
   protocol, and a hash function.

   KEY_LEN -- the length of the stream cipher's key, in bytes.

   PK_ENC_LEN -- the length of a public-key encrypted message, in bytes.
   PK_PAD_LEN -- the number of bytes added in padding for public-key
     encryption, in bytes. (The largest number of bytes that can be encrypted
     in a single public-key operation is therefore PK_ENC_LEN-PK_PAD_LEN.)

   DH_LEN -- the number of bytes used to represent a member of the
     Diffie-Hellman group.
   DH_SEC_LEN -- the number of bytes used in a Diffie-Hellman private key (x).

   HASH_LEN -- the length of the hash function's output, in bytes.

   PAYLOAD_LEN -- The longest allowable cell payload, in bytes. (509)

   CELL_LEN -- The length of a Tor cell, in bytes.

0.3. Ciphers

   For a stream cipher, we use 128-bit AES in counter mode, with an IV of all
   0 bytes.

   For a public-key cipher, we use RSA with 1024-bit keys and a fixed
   exponent of 65537.  We use OAEP-MGF1 padding, with SHA-1 as its digest
   function.  We leave the optional "Label" parameter unset. (For OAEP
   padding, see ftp://ftp.rsasecurity.com/pub/pkcs/pkcs-1/pkcs-1v2-1.pdf)

   For Diffie-Hellman, we use a generator (g) of 2.  For the modulus (p), we
   use the 1024-bit safe prime from rfc2409 section 6.2 whose hex
   representation is:


   As an optimization, implementations SHOULD choose DH private keys (x) of
   320 bits.  Implementations that do this MUST never use any DH key more
   than once.
   [May other implementations reuse their DH keys?? -RD]
   [Probably not. Conceivably, you could get away with changing DH keys once
   per second, but there are too many oddball attacks for me to be
   comfortable that this is safe. -NM]

   For a hash function, we use SHA-1.

   DH_LEN=128; DH_SEC_LEN=40.
   PK_ENC_LEN=128; PK_PAD_LEN=42.

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

   All "random" values MUST be generated with a cryptographically
   strong pseudorandom number generator seeded from a strong entropy
   source, unless otherwise noted.

   The "hybrid encryption" of a byte sequence M with a public key PK is
   computed as follows:
      1. If M is less than PK_ENC_LEN-PK_PAD_LEN, pad and encrypt M with PK.
      2. Otherwise, generate a KEY_LEN byte random key K.
         Let M1 = the first PK_ENC_LEN-PK_PAD_LEN-KEY_LEN bytes of M,
         and let M2 = the rest of M.
         Pad and encrypt K|M1 with PK.  Encrypt M2 with our stream cipher,
         using the key K.  Concatenate these encrypted values.
   [XXX Note that this "hybrid encryption" approach does not prevent
   an attacker from adding or removing bytes to the end of M. It also
   allows attackers to modify the bytes not covered by the OAEP --
   see Goldberg's PET2006 paper for details. We will add a MAC to this
   scheme one day. -RD]

0.4. Other parameter values


1. System overview

   Tor 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.

1.1. Keys and names

   Every Tor relay has multiple public/private keypairs:

    - A long-term signing-only "Identity key" used to sign documents and
      certificates, and used to establish relay identity.
    - A medium-term "Onion key" used to decrypt onion skins when accepting
      circuit extend attempts.  (See 5.1.)  Old keys MUST be accepted for at
      least one week after they are no longer advertised.  Because of this,
      relays MUST retain old keys for a while after they're rotated.
    - A short-term "Connection key" used to negotiate TLS connections.
      Tor implementations MAY rotate this key as often as they like, and
      SHOULD rotate this key at least once a day.

   Tor relays are also identified by "nicknames"; these are specified in

2. Connections

   Connections between two Tor relays, or between a client and a relay,
   use TLS/SSLv3 for link authentication and encryption.  All
   implementations MUST support the SSLv3 ciphersuite
   ciphersuite "TLS_DHE_RSA_WITH_AES_128_CBC_SHA" if it is available.

   There are three ways to perform TLS handshakes with a Tor server.  In
   the first way, "certificates-up-front", both the initiator and
   responder send a two-certificate chain as part of their initial
   handshake.  (This is supported in all Tor versions.)  In the second
   way, "renegotiation", the responder provides a single certificate,
   and the initiator immediately performs a TLS renegotiation.  (This is
   supported in Tor and later.)  And in the third way,
   "in-protocol", the initial TLS renegotiation completes, and the
   parties bootstrap themselves to mutual authentication via use of the
   Tor protocol without further TLS handshaking.  (This is supported in and later.)

   Each of these options provides a way for the parties to learn it is
   available: a client does not need to know the version of the Tor
   server in order to connect to it properly.

   In "certificates up-front" (a.k.a "the v1 handshake"),
   the connection initiator always sends a
   two-certificate chain, consisting of an X.509 certificate using a
   short-term connection public key and a second, self-signed X.509
   certificate containing its identity key.  The other party sends a similar
   certificate chain.  The initiator's ClientHello MUST NOT include any
   ciphersuites other than:

   In "renegotiation" (a.k.a. "the v2 handshake"),
   the connection initiator sends no certificates, and
   the responder sends a single connection certificate.  Once the TLS
   handshake is complete, the initiator renegotiates the handshake, with each
   party sending a two-certificate chain as in "certificates up-front".
   The initiator's ClientHello MUST include at least one ciphersuite not in
   the list above -- that's how the initiator indicates that it can
   handle this handshake.  The responder SHOULD NOT select any
   ciphersuite besides those in the list above.
     [The above "should not" is because some of the ciphers that
     clients list may be fake.]

   In "in-protocol" (a.k.a. "the v3 handshake"), the initiator sends no
   certificates, and the
   responder sends a single connection certificate.  The choice of
   ciphersuites must be as in a "renegotiation" handshake.  There are
   additionally a set of constraints on the connection certificate,
   which the initiator can use to learn that the in-protocol handshake
   is in use.  Specifically, at least one of these properties must be
   true of the certificate:
      * The certificate is self-signed
      * Some component other than "commonName" is set in the subject or
        issuer DN of the certificate.
      * The commonName of the subject or issuer of the certificate ends
        with a suffix other than ".net".
      * The certificate's public key modulus is longer than 1024 bits.
   The initiator then sends a VERSIONS cell to the responder, which then
   replies with a VERSIONS cell; they have then negotiated a Tor
   protocol version.  Assuming that the version they negotiate is 3 (the
   only one specified for use with this handshake right now), the
   responder sends a CERTS cell, an AUTH_CHALLENGE cell, and a NETINFO
   cell to the initiator, which may send either CERTS, AUTHENTICATE,
   NETINFO if it wants to authenticate, or just NETINFO if it does not.

   For backward compatibility between later handshakes and "certificates
   up-front", the ClientHello of an initiator that supports a later
   handshake MUST include at least one ciphersuite other than those listed
   above. The connection responder examines the initiator's ciphersuite list
   to see whether it includes any ciphers other than those included in the
   list above.  If extra ciphers are included, the responder proceeds as in
   "renegotiation" and "in-protocol": it sends a single certificate and
   does not request
   client certificates.  Otherwise (in the case that no extra ciphersuites
   are included in the ClientHello) the responder proceeds as in
   "certificates up-front": it requests client certificates, and sends a
   two-certificate chain.  In either case, once the responder has sent its
   certificate or certificates, the initiator counts them.  If two
   certificates have been sent, it proceeds as in "certificates up-front";
   otherwise, it proceeds as in "renegotiation" or "in-protocol".

   To decide whether to do "renegotiation" or "in-protocol", the
   initiator checks whether the responder's initial certificate matches
   the criteria listed above.

   All new relay implementations of the Tor protocol MUST support
   backwards-compatible renegotiation; clients SHOULD do this too.  If
   this is not possible, new client implementations MUST support both
   "renegotiation" and "in-protocol" and use the router's
   published link protocols list (see dir-spec.txt on the "protocols" entry)
   to decide which to use.

   In all of the above handshake variants, certificates sent in the clear
   SHOULD NOT include any strings to identify the host as a Tor relay. In
   the "renegotiation" and "backwards-compatible renegotiation" steps, the
   initiator SHOULD choose a list of ciphersuites and TLS extensions
   to mimic one used by a popular web browser.

   Responders MUST NOT select any TLS ciphersuite that lacks ephemeral keys,
   or whose symmetric keys are less then KEY_LEN bits, or whose digests are
   less than HASH_LEN bits.  Responders SHOULD NOT select any SSLv3
   ciphersuite other than those listed above.

   Even though the connection protocol is identical, we will think of the
   initiator as either an onion router (OR) if it is willing to relay
   traffic for other Tor users, or an onion proxy (OP) if it only handles
   local requests. Onion proxies SHOULD NOT provide long-term-trackable
   identifiers in their handshakes.

   In all handshake variants, once all certificates are exchanged, 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.

   When connecting to an OR, all parties SHOULD reject the connection if that
   OR has a malformed or missing certificate.  When accepting an incoming
   connection, an OR SHOULD NOT reject incoming connections from parties with
   malformed or missing certificates.  (However, an OR should not believe
   that an incoming connection is from another OR unless the certificates
   are present and well-formed.)

   [Before version, ORs rejected incoming connections from ORs and
   OPs alike if their certificates were missing or malformed.]

   Once a TLS connection is established, the two sides send cells
   (specified below) to one another.  Cells are sent serially.  Standard
   cells are CELL_LEN bytes long, but variable-length cells also exist; see
   Section 3.  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. Either side MAY close a connection
   if there are no circuits running over it and an amount of time
   (KeepalivePeriod, defaults to 5 minutes) has passed since the last time
   any traffic was transmitted over the TLS connection.  Clients SHOULD
   also hold a TLS connection with no circuits open, if it is likely that a
   circuit will be built soon using that connection.

   Client-only Tor instances are encouraged to avoid using handshake
   variants that include certificates, if those certificates provide
   any persistent tags to the relays they contact. If clients do use
   certificates, they SHOULD NOT keep using the same certificates when
   their IP address changes.  Clients MAY send certificates using any
   of the above handshake variants.

3. Cell Packet format

   The basic unit of communication for onion routers and onion
   proxies is a fixed-width "cell".

   On a version 1 connection, each cell contains the following

        CircID                                [2 bytes]
        Command                               [1 byte]
        Payload (padded with 0 bytes)         [PAYLOAD_LEN bytes]

   On a version 2 or 3 connection, all cells are as in version 1 connections,
   except for variable-length cells, whose format is:

        CircID                                [2 octets]
        Command                               [1 octet]
        Length                                [2 octets; big-endian integer]
        Payload                               [Length bytes]

   On a version 2 connection, variable-length cells are indicated by a
   command byte equal to 7 ("VERSIONS").  On a version 3 or
   higher connection, variable-length cells are indicated by a command
   byte equal to 7 ("VERSIONS"), or greater than or equal to 128.

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

   The 'Command' field of a fixed-length cell holds one of the following
         0 -- PADDING     (Padding)                 (See Sec 7.2)
         1 -- CREATE      (Create a circuit)        (See Sec 5.1)
         2 -- CREATED     (Acknowledge create)      (See Sec 5.1)
         3 -- RELAY       (End-to-end data)         (See Sec 5.5 and 6)
         4 -- DESTROY     (Stop using a circuit)    (See Sec 5.4)
         5 -- CREATE_FAST (Create a circuit, no PK) (See Sec 5.1)
         6 -- CREATED_FAST (Circuit created, no PK) (See Sec 5.1)
         8 -- NETINFO     (Time and address info)   (See Sec 4.5)
         9 -- RELAY_EARLY (End-to-end data; limited)(See Sec 5.6)

    Variable-length command values are:
         7 -- VERSIONS    (Negotiate proto version) (See Sec 4)
         128 -- VPADDING  (Variable-length padding) (See Sec 7.2)
         129 -- CERTS     (Certificates)            (See Sec 4.2)
         130 -- AUTH_CHALLENGE (Challenge value)    (See Sec 4.3)
         131 -- AUTHENTICATE (Client authentication)(See Sec 4.5)

   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 contains a reason for closing the circuit.
               (see 5.4)
   Upon receiving any other value for the command field, an OR must
   drop the cell.  Since more cell types may be added in the future, ORs
   should generally not warn when encountering unrecognized commands.

   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 5 below.

   RELAY cells are used to send commands and data along a circuit; see
   section 6 below.

   VERSIONS and NETINFO cells are used to set up connections in link
   protocols v2 and higher; in link protocol v3 and higher, CERTS,
   AUTH_CHALLENGE, and AUTHENTICATE may also be used.  See section 4

4. Negotiating and initializing connections

   After Tor instances negotiate handshake with either the "renegotiation" or
   "in-protocol" handshakes, they must exchange a set of cells to set up
   the Tor connection and make it "open" and usable for circuits.

   When the renegotiation handshake is used, both parties immediately
   send a VERSIONS cell (4.1 below), and after negotiating a link
   protocol version (which will be 2), each send a NETINFO cell (4.5
   below) to confirm their addresses and timestamps.  No other intervening
   cell types are allowed.

   When the in-protocol handshake is used, the initiator sends a
   VERSIONS cell to indicate that it will not be renegotiating.  The
   responder sends a VERSIONS cell, a CERTS cell (4.2 below) to give the
   initiator the certificates it needs to learn the responder's
   identity, an AUTH_CHALLENGE cell (4.3) that the initiator must include
   as part of its answer if it chooses to authenticate, and a NETINFO
   cell (4.5).  The initiator can use the CERTS cell to confirm whether
   the responder is correctly authenticated. If the initiator does not wish
   to authenticate, it can send a NETINFO cell once it has received the
   VERSIONS cell from the responder. If the initiator does wish to
   authenticate, it waits until it gets the AUTH_CHALLENGE cell, and then
   sends a CERTS cell, an AUTHENTICATE cell (4.4), and a NETINFO
   cell.  When this handshake is in use, the first cell must
   still be VERSIONS, and no other cell type is allowed to intervene
   besides those specified, except for PADDING and VPADDING cells.

4.1. Negotiating versions with VERSIONS cells

   There are multiple instances of the Tor link connection protocol.  Any
   connection negotiated using the "certificates up front" handshake (see
   section 2 above) is "version 1".  In any connection where both parties
   have behaved as in the "renegotiation" handshake, the link protocol
   version must be 2.  In any connection where both parties have behaved
   as in the "in-protocol" handshake, the link protocol must be 3 or higher.

   To determine the version, in any connection where the "renegotiation"
   or "in-protocol" handshake was used (that is, where the responder
   sent only one certificate at first and where the initiator did not
   send any certificates in the first negotiation), both parties MUST
   send a VERSIONS cell.  In "renegotiation", they send a VERSIONS cell
   right after the renegotiation is finished, before any other cells are
   sent.  In "in-protocol", the initiator sends a VERSIONS cell
   immediately after the initial TLS handshake, and the responder
   replies immediately with a VERSIONS cell.  Parties MUST NOT send any
   other cells on a connection until they have received a VERSIONS cell.

   The payload in a VERSIONS cell is a series of big-endian two-byte
   integers.  Both parties MUST select as the link protocol version the
   highest number contained both in the VERSIONS cell they sent and in the
   versions cell they received.  If they have no such version in common,
   they cannot communicate and MUST close the connection.

   Since the version 1 link protocol does not use the "renegotiation"
   handshake, implementations MUST NOT list version 1 in their VERSIONS
   cell.  When the "renegotiation" handshake is used, implementations
   MUST list only the version 2.  When the "in-protocol" handshake is
   used, implementations MUST NOT list any version before 3, and SHOULD
   list at least version 3.

4.2. CERTS cells

   The CERTS cell describes the keys that a Tor instance is claiming
   to have.  It is a variable-length cell.  Its payload format is:

        N: Number of certs in cell            [1 octet]
        N times:
           CertType                           [1 octet]
           CLEN                               [2 octets]
           Certificate                        [CLEN octets]

   Any extra octets at the end of a CERTS cell MUST be ignored.

     CertType values are:
        1: Link key certificate certified by RSA1024 identity
        2: RSA1024 Identity certificate
        3: RSA1024 AUTHENTICATE cell link certificate

   The certificate format for the above certificate types is X509.

   A CERTS cell may have no more than one certificate of each CertType.

   To authenticate the responder, the initiator MUST check the following:
     * The CERTS cell contains exactly one CertType 1 "Link" certificate.
     * The CERTS cell contains exactly one CertType 2 "ID" certificate.
     * Both certificates have validAfter and validUntil dates that
       are not expired.
     * The certified key in the Link certificate matches the
       link key that was used to negotiate the TLS connection.
     * The certified key in the ID certificate is a 1024-bit RSA key.
     * The certified key in the ID certificate was used to sign both
     * The link certificate is correctly signed with the key in the
       ID certificate
     * The ID certificate is correctly self-signed.
   Checking these conditions is sufficient to authenticate that the
   initiator is talking to the Tor node with the expected identity,
   as certified in the ID certificate.

   To authenticate the initiator, the responder MUST check the
     * The CERTS cell contains exactly one CertType 3 "AUTH" certificate.
     * The CERTS cell contains exactly one CertType 2 "ID" certificate.
     * Both certificates have validAfter and validUntil dates that
       are not expired.
     * The certified key in the AUTH certificate is a 1024-bit RSA key.
     * The certified key in the ID certificate is a 1024-bit RSA key.
     * The certified key in the ID certificate was used to sign both
     * The auth certificate is correctly signed with the key in the
       ID certificate.
     * The ID certificate is correctly self-signed.
   Checking these conditions is NOT sufficient to authenticate that the
   initiator has the ID it claims; to do so, the cells in 4.3 and 4.4
   below must be exchanged.


   An AUTH_CHALLENGE cell is a variable-length cell with the following
       Challenge [32 octets]
       N_Methods [2 octets]
       Methods   [2 * N_Methods octets]

   It is sent from the responder to the initiator. Initiators MUST
   ignore unexpected bytes at the end of the cell.  Responders MUST
   generate every challenge independently using a strong RNG or PRNG.

   The Challenge field is a randomly generated string that the
   initiator must sign (a hash of) as part of authenticating.  The
   methods are the authentication methods that the responder will
   accept.  Only one authentication method is defined right now:
   see 4.4 below.


   If an initiator wants to authenticate, it responds to the
   AUTH_CHALLENGE cell with a CERTS cell and an AUTHENTICATE cell.
   The CERTS cell is as a server would send, except that instead of
   sending a CertType 1 cert for an arbitrary link certificate, the
   client sends a CertType 3 cert for an RSA AUTHENTICATE key.
   (This difference is because we allow any link key type on a TLS
   link, but the protocol described here will only work for 1024-bit
   RSA keys.  A later protocol version should extend the protocol
   here to work with non-1024-bit, non-RSA keys.)

   An AUTHENTICATE cell contains the following:

        AuthType                              [2 octets]
        AuthLen                               [2 octets]
        Authentication                        [AuthLen octets]

   Responders MUST ignore extra bytes at the end of an AUTHENTICATE
   cell.  If AuthType is 1 (meaning "RSA-SHA256-TLSSecret"), then the
   Authentication contains the following:

       TYPE: The characters "AUTH0001" [8 octets]
       CID: A SHA256 hash of the initiator's RSA1024 identity key [32 octets]
       SID: A SHA256 hash of the responder's RSA1024 identity key [32 octets]
       SLOG: A SHA256 hash of all bytes sent from the responder to the
         initiator as part of the negotiation up to and including the
         AUTH_CHALLENGE cell; that is, the VERSIONS cell, the CERTS cell,
         the AUTH_CHALLENGE cell, and any padding cells.  [32 octets]
       CLOG: A SHA256 hash of all bytes sent from the initiator to the
         responder as part of the negotiation so far; that is, the
         VERSIONS cell and the CERTS cell and any padding cells. [32
       SCERT: A SHA256 hash of the responder's TLS link certificate. [32
       TLSSECRETS: A SHA256 HMAC, using the TLS master secret as the
         secret key, of the following:
           - client_random, as sent in the TLS Client Hello
           - server_random, as sent in the TLS Server Hello
           - the NUL terminated ASCII string:
             "Tor V3 handshake TLS cross-certification"
          [32 octets]
       TIME: The time of day in seconds since the POSIX epoch. [8 octets]
       RAND: A 16 byte value, randomly chosen by the initiator [16 octets]
       SIG: A signature of a SHA256 hash of all the previous fields
         using the initiator's "Authenticate" key as presented.  (As
         always in Tor, we use OAEP-MGF1 padding; see tor-spec.txt
         section 0.3.)
          [variable length]

   To check the AUTHENTICATE cell, a responder checks that all fields
   from TYPE through TLSSECRETS contain their unique
   correct values as described above, and then verifies the signature.
   signature.  The server MUST ignore any extra bytes in the signed
   data after the SHA256 hash.

4.5. NETINFO cells

   If version 2 or higher is negotiated, each party sends the other a
   NETINFO cell.  The cell's payload is:

         Timestamp              [4 bytes]
         Other OR's address     [variable]
         Number of addresses    [1 byte]
         This OR's addresses    [variable]

   The address format is a type/length/value sequence as given in section
   6.4 below.  The timestamp is a big-endian unsigned integer number of
   seconds since the Unix epoch.

   Implementations MAY use the timestamp value to help decide if their
   clocks are skewed.  Initiators MAY use "other OR's address" to help
   learn which address their connections are originating from, if they do
   not know it.  [As of, nodes use neither of these values.]

   Initiators SHOULD use "this OR's address" to make sure
   that they have connected to another OR at its canonical address.
   (See 5.3.1 below.)

5. Circuit management

5.1. 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 5.2). 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).
   This value is hybrid-encrypted (see 0.3) to Bob's onion key, giving
   an onion-skin of:
         Padding                       [PK_PAD_LEN bytes]
         Symmetric key                 [KEY_LEN bytes]
         First part of g^x             [PK_ENC_LEN-PK_PAD_LEN-KEY_LEN bytes]
       Symmetrically encrypted:
         Second part of g^x            [DH_LEN-(PK_ENC_LEN-PK_PAD_LEN-KEY_LEN)

   The relay payload for an EXTEND relay cell consists of:
         Address                       [4 bytes]
         Port                          [2 bytes]
         Onion skin                    [DH_LEN+KEY_LEN+PK_PAD_LEN bytes]
         Identity fingerprint          [HASH_LEN 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 hash of the PKCS#1
   ASN1 encoding of the next onion router's identity (signing) key.  (See 0.3
   above.)  Including this hash allows the extending OR verify that it is
   indeed connected to the correct target OR, and prevents certain
   man-in-the-middle attacks.

   The payload for a CREATED cell, or the relay payload for an
   EXTENDED cell, contains:
         DH data (g^y)                 [DH_LEN bytes]
         Derivative key data (KH)      [HASH_LEN bytes]   <see 5.2 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 node 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 node has a lower key, it chooses a CircID with
   an MSB of 0; otherwise, it chooses a CircID with an MSB of 1.

   (An OP with no public key MAY choose any CircID it wishes, since an OP
   never needs to process a CREATE cell.)

   Public keys are compared numerically by modulus.

   As usual with DH, x and y MUST be generated randomly.


   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 MAY 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

   A CREATE_FAST cell contains:

       Key material (X)    [HASH_LEN bytes]

   A CREATED_FAST cell contains:

       Key material (Y)    [HASH_LEN bytes]
       Derivative key data [HASH_LEN bytes] (See 5.2 below)

   The values of X and Y must be generated randomly.

   If an OR sees a circuit created with CREATE_FAST, the OR is sure to be the
   first hop of a circuit.  ORs SHOULD reject attempts to create streams with
   RELAY_BEGIN exiting the circuit at the first hop: letting Tor be used as a
   single hop proxy makes exit nodes a more attractive target for compromise.

5.2. Setting circuit keys

   Once the handshake between the OP and an OR is completed, both can
   now calculate g^xy with ordinary DH.  Before computing g^xy, both parties
   MUST verify that the received g^x or 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.  Implementations MUST NOT discard other "weak"
   g^x values.

   (Discarding degenerate keys is critical for security; if bad keys
   are not discarded, an attacker can substitute the OR's CREATED
   cell's g^y with 0 or 1, thus creating a known g^xy and impersonating
   the OR. Discarding other keys may allow attacks to learn bits of
   the private key.)

   If CREATE or EXTEND is used to extend a circuit, both parties
   base their key material on K0=g^xy, represented as a big-endian unsigned

   If CREATE_FAST is used, both parties base their key material on

   From the base key material K0, they compute KEY_LEN*2+HASH_LEN*3 bytes of
   derivative key data as
       K = H(K0 | [00]) | H(K0 | [01]) | H(K0 | [02]) | ...

   The first HASH_LEN bytes of K form KH; the next HASH_LEN form the forward
   digest Df; the next HASH_LEN 41-60 form the backward digest Db; the next
   KEY_LEN 61-76 form Kf, and the final KEY_LEN form Kb.  Excess bytes from K
   are discarded.

   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.

5.3. 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 includes at least one pending stream that
         needs a circuit (if there are any).

      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 onion 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.  As special cases, if the extend cell includes a digest of
   all zeroes, or asks to extend back to the relay that sent the extend
   cell, the circuit will fail and be torn down. The initiating onion
   router chooses some circID not yet used on the connection between the
   two onion routers.  (But see section 5.1. above, concerning choosing
   circIDs based on lexicographic order of nicknames.)

   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.)

5.3.1. Canonical connections

   It is possible for an attacker to launch a man-in-the-middle attack
   against a connection by telling OR Alice to extend to OR Bob at some
   address X controlled by the attacker.  The attacker cannot read the
   encrypted traffic, but the attacker is now in a position to count all
   bytes sent between Alice and Bob (assuming Alice was not already
   connected to Bob.)

   To prevent this, when an OR we gets an extend request, it SHOULD use an
   existing OR connection if the ID matches, and ANY of the following
   conditions hold:
       - The IP matches the requested IP.
       - The OR knows that the IP of the connection it's using is canonical
         because it was listed in the NETINFO cell.
       - The OR knows that the IP of the connection it's using is canonical
         because it was listed in the server descriptor.

   [This is not implemented in Tor]

5.4. 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 6.1).

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

   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

   [Note: If an OR receives a TRUNCATE cell and it has any RELAY cells
   still queued on the circuit for the next node it will drop them
   without sending them.  This is not considered conformant behavior,
   but it probably won't get fixed until a later version of Tor.  Thus,
   clients SHOULD NOT send a TRUNCATE cell to a node running any current
   version of Tor if a) they have sent relay cells through that node,
   and b) they aren't sure whether those cells have been sent on yes.]

   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.

   The payload of a RELAY_TRUNCATED or DESTROY cell contains a single octet,
   describing why the circuit is being closed or truncated.  When sending a
   TRUNCATED or DESTROY cell because of another TRUNCATED or DESTROY cell,
   the error code should be propagated.  The origin of a circuit always sets
   this error code to 0, to avoid leaking its version.

   The error codes are:
     0 -- NONE            (No reason given.)
     1 -- PROTOCOL        (Tor protocol violation.)
     2 -- INTERNAL        (Internal error.)
     3 -- REQUESTED       (A client sent a TRUNCATE command.)
     4 -- HIBERNATING     (Not currently operating; trying to save bandwidth.)
     5 -- RESOURCELIMIT   (Out of memory, sockets, or circuit IDs.)
     6 -- CONNECTFAILED   (Unable to reach relay.)
     7 -- OR_IDENTITY     (Connected to relay, but its OR identity was not
                           as expected.)
     8 -- OR_CONN_CLOSED  (The OR connection that was carrying this circuit
     9 -- FINISHED        (The circuit has expired for being dirty or old.)
    10 -- TIMEOUT         (Circuit construction took too long)
    11 -- DESTROYED       (The circuit was destroyed w/o client TRUNCATE)
    12 -- NOSUCHSERVICE   (Request for unknown hidden service)

5.5. Routing relay cells

   When an OR receives a RELAY or RELAY_EARLY cell, it checks the cell's
   circID and determines whether it has a corresponding circuit along that
   connection.  If not, the OR drops the 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 the stream cipher, 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.
   Note that in counter mode, decrypt and encrypt are the same operation.

   The OR then decides whether it recognizes the relay cell, by
   inspecting the payload as described in section 6.1 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 the stream cipher as follows:
         OP receives data cell:
            For I=N...1,
                Decrypt with Kb_I.  If the payload is recognized (see
                section 6..1), then stop and process the payload.

   For more information, see section 6 below.

5.6. Handling relay_early cells

   A RELAY_EARLY cell is designed to limit the length any circuit can reach.
   When an OR receives a RELAY_EARLY cell, and the next node in the circuit
   is speaking v2 of the link protocol or later, the OR relays the cell as a
   RELAY_EARLY cell.  Otherwise, it relays it as a RELAY cell.

   If a node ever receives more than 8 RELAY_EARLY cells on a given
   outbound circuit, it SHOULD close the circuit. (For historical reasons,
   we don't limit the number of inbound RELAY_EARLY cells; they should
   be harmless anyway because clients won't accept extend requests. See
   bug 1038.)

   When speaking v2 of the link protocol or later, clients MUST only send
   EXTEND cells inside RELAY_EARLY cells.  Clients SHOULD send the first ~8
   RELAY cells that are not targeted at the first hop of any circuit as
   RELAY_EARLY cells too, in order to partially conceal the circuit length.

   [In a future version of Tor, relays will reject any EXTEND cell not
   received in a RELAY_EARLY cell.  See proposal 110.]

6. Application connections and stream management

6.1. 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                    [CELL_LEN-14 bytes]

   The relay commands are:
         1 -- RELAY_BEGIN     [forward]
         2 -- RELAY_DATA      [forward or backward]
         3 -- RELAY_END       [forward or backward]
         4 -- RELAY_CONNECTED [backward]
         5 -- RELAY_SENDME    [forward or backward] [sometimes control]
         6 -- RELAY_EXTEND    [forward]             [control]
         7 -- RELAY_EXTENDED  [backward]            [control]
         8 -- RELAY_TRUNCATE  [forward]             [control]
         9 -- RELAY_TRUNCATED [backward]            [control]
        10 -- RELAY_DROP      [forward or backward] [control]
        11 -- RELAY_RESOLVE   [forward]
        12 -- RELAY_RESOLVED  [backward]
        13 -- RELAY_BEGIN_DIR [forward]

        32..40 -- Used for hidden services; see rend-spec.txt.

   Commands labelled as "forward" must only be sent by the originator
   of the circuit. Commands labelled as "backward" must only be sent by
   other nodes in the circuit back to the originator. Commands marked
   as either can be sent either by the originator or other nodes.

   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 digest of all the bytes that have been destined for
   this hop of the circuit or originated from this hop of the circuit,
   seeded from Df or Db respectively (obtained in section 5.2 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 5.5 above).

   (The digest does not include any bytes from relay cells that do
   not start or end at this hop of the circuit. That is, it does not
   include forwarded data. Therefore if 'recognized' is zero but the
   digest does not match, the running digest at that node should
   not be updated, and the cell should be forwarded on.)

   All RELAY cells pertaining to the same tunneled stream have the
   same stream ID.  StreamIDs are chosen arbitrarily by the OP.  RELAY
   cells that affect the entire circuit rather than a particular
   stream use a StreamID of zero -- they are marked in the table above
   as "[control]" style cells. (Sendme cells are marked as "sometimes
   control" because they can take include a StreamID or not depending
   on their purpose -- see Section 7.)

   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.

   If the RELAY cell is recognized but the relay command is not
   understood, the cell must be dropped and ignored. Its contents
   still count with respect to the digests, though.

6.2. 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 a decimal integer between 1 and 65535, inclusive.

   [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 6.4 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]
       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 No version of Tor currently generates the IPv6 format.]

   [Tor exit nodes before set the TTL field to a fixed value.  Later
   versions set the TTL to the last value seen from a DNS server, and expire
   their own cached entries after a fixed interval.  This prevents certain

   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.

   If the exit node does not support optimistic data (i.e. its
   version number is before, then the OP MUST wait
   for a RELAY_CONNECTED cell before sending any data.  If the exit
   node supports optimistic data (i.e. its version number is or later), then the OP MAY send RELAY_DATA cells
   immediately after sending the RELAY_BEGIN cell (and before
   receiving either a RELAY_CONNECTED or RELAY_END cell).

   RELAY_DATA cells sent to unrecognized streams are dropped.  If
   the exit node supports optimistic data, then RELAY_DATA cells it
   receives on streams which have seen RELAY_BEGIN but have not yet
   been replied to with a RELAY_CONNECTED or RELAY_END are queued.
   If the stream creation succeeds with a RELAY_CONNECTED, the queue
   is processed immediately afterwards; if the stream creation fails
   with a RELAY_END, the contents of the queue are deleted.

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

6.2.1. Opening a directory stream

   If a Tor relay is a directory server, it should respond to a
   RELAY_BEGIN_DIR cell as if it had received a BEGIN cell requesting a
   connection to its directory port.  RELAY_BEGIN_DIR cells ignore exit
   policy, since the stream is local to the Tor process.

   If the Tor relay is not running a directory service, it should respond

   Clients MUST generate an all-zero payload for RELAY_BEGIN_DIR cells,
   and relays MUST ignore the payload.

   [RELAY_BEGIN_DIR was not supported before Tor; clients
   SHOULD NOT send it to routers running earlier versions of Tor.]

6.3. 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 -- REASON_NOROUTE        (Routing error while attempting to
                                   contact destination)
       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.)
      14 -- REASON_NOTDIRECTORY   (Client sent RELAY_BEGIN_DIR to a
                                   non-directory relay.)

   (With REASON_EXITPOLICY, the 4-byte IPv4 address or 16-byte IPv6 address
   forms the optional data, along with a 4-byte TTL; no other reason
   currently has extra data.)

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

   Tors SHOULD NOT send any reason except REASON_MISC for a stream that they
   have originated.

   [*] Older versions of Tor also send this reason when connections are

   --- [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

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

6.4. Remote hostname lookup

   To find the address associated with a hostname, the OP sends a
   RELAY_RESOLVE cell containing the hostname to be resolved with a NUL
   terminating byte. (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.

7. Flow control

7.1. Link throttling

   Each client or relay 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.

   The mainline Tor implementation uses token buckets (one for reads,
   one for writes) for the rate limiting.

   Since 0.2.0.x, Tor has let the user specify an additional pair of
   token buckets for "relayed" traffic, so people can deploy a Tor relay
   with strict rate limiting, but also use the same Tor as a client. To
   avoid partitioning concerns we combine both classes of traffic over a
   given OR connection, and keep track of the last time we read or wrote
   a high-priority (non-relayed) cell. If it's been less than N seconds
   (currently N=30), we give the whole connection high priority, else we
   give the whole connection low priority. We also give low priority
   to reads and writes for connections that are serving directory
   information. See proposal 111 for details.

7.2. Link padding

   Link padding can be created by sending PADDING or VPADDING cells
   along the connection; relay cells of type "DROP" can be used for
   long-range padding.  The contents of a PADDING, VPADDING, or DROP
   cell SHOULD be chosen randomly, and MUST be ignored.

   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.

7.3. 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
   originate (package for transmission), and how many RELAY_DATA cells
   it is willing to consume (receive for local streams).  These limits
   do not apply to cells that the OR receives from one host and relays
   to another.

   Each 'window' value is initially set based on the consensus parameter
   'circwindow' in the directory (see dir-spec.txt), or to 1000 data cells
   if no 'circwindow' value is given,
   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]

7.4. 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.

A.1. Differences between spec and implementation

- The current specification requires all ORs to have IPv4 addresses, but
  allows relays 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.
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