Filename: 196-transport-control-ports.txt
Title: Extended ORPort and TransportControlPort
Author: George Kadianakis, Nick Mathewson
Created: 14 Mar 2012
Status: Closed

1. Overview

  Proposal 180 defined Tor pluggable transports, a way to decouple
  protocol-level obfuscation from the core Tor protocol in order to
  better resist client-bridge censorship. This is achieved by
  introducing pluggable transport proxies, programs that obfuscate Tor
  traffic to resist DPI detection.

  Proposal 180 defined a way for pluggable transport proxies to
  communicate with local Tor clients and bridges, so as to exchange
  traffic. This document extends this communication protocol, so that
  pluggable transport proxies can exchange arbitrary operational
  information and metadata with Tor clients and bridges.

2. Motivation

  The communication protocol specified in Proposal 180 gives a way
  for transport proxies to announce the IP address of their clients
  to tor. Still, modern pluggable transports might have more (?)
  needs than this. For example:

  1. Tor might want to inform pluggable transport proxies on how to
     rate-limit incoming or outgoing connections.

  2. Server pluggable transport proxies might want to pass client
     information to an anti-active-probing system controlled by tor.

  3. Tor might want to temporarily stop a transport proxy from
     obfuscating traffic.

  To satisfy the above use cases, there must be real-time
  communication between the tor process and the pluggable transport
  proxy. To achieve this, this proposal refactors the Extended ORPort
  protocol specified in Proposal 180, and introduces a new port,
  TransportControlPort, whose sole role is the exchange of control
  information between transport proxies and tor.

  Specifically, transports proxies deliver each connection to the
  "Extended ORPort", where they provide metadata and agree on an
  identifier for each tunneled connection.  Once this handshake
  occurs, the OR protocol proceeds unchanged.

  Additionally, each transport maintains a single connection to Tor's
  "TransportControlPort", where it receives instructions from Tor
  about rate-limiting on individual connections.

3. The new port protocols

3.1. The new extended ORPort protocol

3.1.1. Protocol

  The extended server port protocol is as follows:

     COMMAND [2 bytes, big-endian]
     BODYLEN [2 bytes, big-endian]
     BODY [BODYLEN bytes]

     Commands sent from the transport proxy to the bridge are:

     [0x0000] DONE: There is no more information to give. The next
       bytes sent by the transport will be those tunneled over it.
       (body ignored)

     [0x0001] USERADDR: an address:port string that represents the
       client's address.

     [0x0002] TRANSPORT: a string of the name of the pluggable
       transport currently in effect on the connection.

     Replies sent from tor to the proxy are:

     [0x1000] OKAY: Send the user's traffic. (body ignored)

     [0x1001] DENY: Tor would prefer not to get more traffic from
       this address for a while. (body ignored)

     [0x1002] CONTROL: a NUL-terminated "identifier" string. The
       pluggable transport proxy must use the "identifier" to access
       the TransportControlPort. See the 'Association and identifier
       creation' section below.

  Parties MUST ignore command codes that they do not understand.

  If the server receives a recognized command that does not parse, it
  MUST close the connection to the client.

3.1.2. Command descriptions USERADDR

  An ASCII string holding the TCP/IP address of the client of the
  pluggable transport proxy. A Tor bridge SHOULD use that address to
  collect statistics about its clients.  Recognized formats are:

  (Current Tor versions may accept other formats, but this is a bug:
  transports MUST NOT send them.)

  The string MUST not be NUL-terminated. TRANSPORT

  An ASCII string holding the name of the pluggable transport used by
  the client of the pluggable transport proxy. A Tor bridge that
  supports multiple transports SHOULD use that information to collect
  statistics about the popularity of individual pluggable transports.

  The string MUST not be NUL-terminated.

  Pluggable transport names are C-identifiers and Tor MUST check them
  for correctness.

3.2. The new TransportControlPort protocol

  The TransportControlPort protocol is as follows:

     CONNECTIONID[16 bytes, big-endian]
     COMMAND [2 bytes, big-endian]
     BODYLEN [2 bytes, big-endian]
     BODY [BODYLEN bytes]

     Commands sent from the transport proxy to the bridge:

     [0x0001] RATE_LIMITED: Message confirming that the rate limiting
       request of the bridge was carried out successfully (body
       ignored). See the 'Rate Limiting' section below.

     [0x0002] NOT_RATE_LIMITED: Message notifying that the transport
       proxy failed to carry out the rate limiting request of the
       bridge (body ignored). See the 'Rate Limiting' section below.

     Configuration commands sent from the bridge to the transport
     proxy are:

     [0x1001] NOT_ALLOWED: Message notifying that the CONNECTIONID
       could not be matched with an authorized connection ID. The
       bridge SHOULD shutdown the connection.

     [0x1001] RATE_LIMIT: Carries information on how the pluggable
       transport proxy should rate-limit its traffic. See the 'Rate
       Limiting' section below.

  CONNECTIONID should carry the connection identifier described in the
  'Association and identifier creation' section.

  Parties should ignore command codes that they do not understand.

3.3. Association and identifier creation

  For Tor and a transport proxy to communicate using the
  TransportControlPort, an identifier must have already been negotiated
  using the 'CONTROL' command of Extended ORPort.

  The TransportControlPort identifier should not be predictable by a
  user who hasn't received a 'CONTROL' command from the Extended
  ORPort. For this reason, the TransportControlPort identifier should
  not be cryptographically-weak or deterministically created.

  Tor MUST create its identifiers by generating 16 bytes of random

4. Configuration commands

4.1. Rate Limiting

  A Tor relay should be able to inform a transport proxy in real-time
  about its rate-limiting needs.

  This can be achieved by using the TransportControlPort and sending a
  'RATE_LIMIT' command to the transport proxy.

  The body of the 'RATE_LIMIT' command should contain two integers,
  4 bytes each, in big-endian format. The two numbers should represent
  the bandwidth rate and bandwidth burst respectively in 'bytes per
  second' which the transport proxy must set as its overall
  rate-limiting setting.

  When the transport proxy sets the appropriate rate limiting, it
  should send back a 'RATE_LIMITED' command. If it fails while setting
  up rate limiting, it should send back a 'NOT_RATE_LIMITED' command.

  After sending a 'RATE_LIMIT' command. the tor bridge MAY want to
  stop pushing data to the transport proxy, till it receives a
  'RATE_LIMITED' command. If, instead, it receives a 'NOT_RATE_LIMITED'
  command it MAY want to shutdown its connections to the transport

5. Authentication

  To defend against cross-protocol attacks on the Extended ORPort,
  proposal 213 defines an authentication scheme that should be used to
  protect it.

  If the Extended ORPort is enabled, Tor should regenerate the cookie
  file of proposal 213 on startup and store it in

  The location of the cookie can be overriden by using the
  configuration file parameter ExtORPortCookieAuthFile, which is
  defined as:

    ExtORPortCookieAuthFile <path>

  where <path> is a filesystem path.

  XXX should we also add an ExtORPortCookieFileGroupReadable torrc option?

6. Security Considerations

  Extended ORPort or TransportControlPort do _not_ provide link
  confidentiality, authentication or integrity. Sensitive data, like
  cryptographic material, should not be transferred through them.

  An attacker with superuser access, is able to sniff network traffic,
  and capture TransportControlPort identifiers and any data passed
  through those ports.

  Tor SHOULD issue a warning if the bridge operator tries to bind
  Extended ORPort or TransportControlPort to a non-localhost address.

  Pluggable transport proxies SHOULD issue a warning if they are
  instructed to connect to a non-localhost Extended ORPort or

7. Future

  In the future, we might have pluggable transports which require the
  _client_ transport proxy to use the TransportControlPort and exchange
  control information with the Tor client. The current proposal doesn't
  yet support this, but we should not add functionality that will
  prevent it from being possible.