TURN TCP: Traversal Using Relays around NAT (TURN) Extensions for TCP Allocations

Internet Engineering Task Force (IETF)                 S. Perreault, Ed.
Request for Comments: 6062                                      Viagenie
Category: Standards Track                                   J. Rosenberg
ISSN: 2070-1721                                              jdrosen.net
                                                           November 2010


Traversal Using Relays around NAT (TURN) Extensions for TCP Allocations

Abstract

   This specification defines an extension of Traversal Using Relays
   around NAT (TURN), a relay protocol for Network Address Translator
   (NAT) traversal.  This extension allows a TURN client to request TCP
   allocations, and defines new requests and indications for the TURN
   server to open and accept TCP connections with the client's peers.
   TURN and this extension both purposefully restrict the ways in which
   the relayed address can be used.  In particular, it prevents users
   from running general-purpose servers from ports obtained from the
   TURN server.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6062.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  4
   4.  Client Processing  . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Creating an Allocation . . . . . . . . . . . . . . . . . .  6
     4.2.  Refreshing an Allocation . . . . . . . . . . . . . . . . .  7
     4.3.  Initiating a Connection  . . . . . . . . . . . . . . . . .  7
     4.4.  Receiving a Connection . . . . . . . . . . . . . . . . . .  7
     4.5.  Sending and Receiving Data . . . . . . . . . . . . . . . .  8
     4.6.  Data Connection Maintenance  . . . . . . . . . . . . . . .  8
   5.  TURN Server Behavior . . . . . . . . . . . . . . . . . . . . .  8
     5.1.  Receiving a TCP Allocate Request . . . . . . . . . . . . .  8
     5.2.  Receiving a Connect Request  . . . . . . . . . . . . . . .  9
     5.3.  Receiving a TCP Connection on a Relayed Transport
           Address  . . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.4.  Receiving a ConnectionBind Request . . . . . . . . . . . . 11
     5.5.  Data Connection Maintenance  . . . . . . . . . . . . . . . 11
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  New STUN Methods . . . . . . . . . . . . . . . . . . . . . 11
     6.2.  New STUN Attributes  . . . . . . . . . . . . . . . . . . . 12
       6.2.1.  CONNECTION-ID  . . . . . . . . . . . . . . . . . . . . 12
     6.3.  New STUN Error Codes . . . . . . . . . . . . . . . . . . . 12
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 13

1.  Introduction

   Traversal Using Relays around NAT (TURN) [RFC5766] is an extension to
   the Session Traversal Utilities for NAT [RFC5389] protocol.  TURN
   allows for clients to communicate with a TURN server and ask it to
   allocate ports on one of its host interfaces, and then relay traffic
   between that port and the client itself.  TURN, when used in concert
   with STUN and Interactive Connectivity Establishment (ICE) [RFC5245],
   forms a solution for NAT traversal for UDP-based media sessions.

   However, TURN itself does not provide a way for a client to allocate
   a TCP-based port on a TURN server.  Such an allocation is needed for
   cases where a TCP-based session is desired with a peer, and NATs
   prevent a direct TCP connection.  Examples include application
   sharing between desktop softphones, or transmission of pictures
   during a voice communications session.

   This document defines an extension to TURN that allows a client to
   obtain a TCP allocation.  It also allows the client to initiate
   outgoing TCP connections from that allocation to peers and to accept
   incoming TCP connection requests from peers made towards that
   allocation.

   The term "TCP allocation" means a TURN allocation where TCP is used
   as the transport protocol instead of UDP.  Such an allocation is
   uniquely identified by its relayed transport address, which consists
   of an IP address and TCP port (defined in [RFC5766]).

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Overview of Operation

                                                      +--------+
                                                      |        |
                                                      | Peer1  |
                                                   /  |        |
                                                  /   |        |
                                                 /    +--------+
                                                /
                                               /
                                              / Peer Data 1
                                             /
      +--------+  Control       +--------+  /
      |        | -------------- |        | /
      | Client | Client Data 1  | TURN   |
      |        | -------------- | Server | \
      |        | -------------- |        |  \
      +--------+ Client Data 2  +--------+   \
                                              \
                                               \
                                                \     +--------+
                                                 \    |        |
                                      Peer Data 2 \   | Peer2  |
                                                   \  |        |
                                                      |        |
                                                      +--------+

                         Figure 1: TURN TCP Model

   The overall model for TURN-TCP is shown in Figure 1.  The client will
   have two different types of connections to its TURN server.  For each
   allocated relayed transport address, it will have a single control
   connection.  Control connections are used to obtain allocations and
   open up new connections.  Furthermore, for each connection to a peer,
   the client will have a single connection to its TURN server.  These
   connections are called data connections.  Consequently, there is a
   data connection from the client to its TURN server (the client data
   connection) and one from the TURN server to a peer (the peer data
   connection).  Actual application data is sent on these connections.
   Indeed, after an initial TURN message that binds the client data
   connection to a peer data connection, only application data can be
   sent -- no TURN messaging.  This is in contrast to the control
   connection, which only allows TURN messages and not application data.

   To obtain a TCP-based allocation, a client first opens a TCP or TLS
   connection to its TURN server.  The client then sends an Allocate
   request over that control connection.  That request contains a
   REQUESTED-TRANSPORT attribute, which indicates a TCP-based allocation
   is desired.  A server that supports this extension will allocate a
   TCP relayed transport address and begin listening for connection
   requests on it.  It then returns the allocated relayed transport
   address to the client in the response to the Allocate request.  The
   connection on which the Allocate request was sent is the control
   connection.

   If a client wishes to establish a TCP connection to a peer from that
   relayed transport address, it issues a Connect request to the TURN
   server over the control connection.  That request contains an XOR-
   PEER-ADDRESS attribute identifying the peer IP address and port
   (i.e., its "transport address") to which a connection is to be made.
   The TURN server attempts to open the TCP connection, and assuming it
   succeeds, then responds to the Connect request with a success
   response.  The server also creates a connection identifier associated
   with this connection and passes that connection identifier back to
   the client in the success response.  Note that a maximum of one
   connection to a given peer transport address can be established per
   allocation.

      Note: Establishing a relayed connection from the client to a peer
      is done in two steps.  First, the allocation is created, and
      second, the connection is established.  Combining the two is not
      desirable for NAT traversal.  It is expected that, between the
      first and second steps, the client will communicate off-band with
      the peer (e.g., using ICE [RFC5245]) and tell it the relayed
      transport address that the TURN server allocated and from which it
      is about to initiate a connection.  The peer can then "get ready":
      open holes in its firewall, try to poke holes in a NAT, attempt a
      TCP simultaneous open, etc.

   In order to actually send data on the new connection or otherwise
   utilize it in any way, the client establishes a new TCP connection to
   its TURN server.  Once established, it issues a ConnectionBind
   request to the server over this new connection.  That request echoes
   back the connection identifier to the TURN server.  The TURN server
   uses it to correlate the two connections.  As a consequence, the TCP
   connection to the peer is associated with a TCP connection to the
   client one-to-one.  The two connections are now data connections.  At
   this point, if the server receives data from the peer, it forwards
   that data towards the client, without any kind of encapsulation.  Any
   data received by the TURN server from the client over the client data
   connection is forwarded to the peer, again without encapsulation or
   framing of any kind.  Once a connection has been bound using the
   ConnectionBind request, TURN messaging is no longer permitted on the
   connection.

   In a similar way, when a peer opens a TCP connection towards the
   relayed transport address, the server checks if there is a permission
   in place for that peer.  If there is none, the connection is closed.
   Permissions are created with the CreatePermission request sent over
   the control connection, just as for UDP TURN.  If there is a
   permission in place, the TURN server sends to the client a
   ConnectionAttempt Indication over the control connection.  That
   indication contains a connection identifier.  Once again, the client
   initiates a separate TCP connection to its TURN server, and over that
   connection, issues a ConnectionBind request.  Once received, the TURN
   server will begin relaying data back and forth.  The server closes
   the peer data connection if no ConnectionBind request is received
   after a timeout.

   If the client closes a client data connection, the corresponding peer
   data connection is closed.  If the peer closes a peer data
   connection, the corresponding client data connection is closed.  In
   this way, the status of the connection is directly known to the
   client.

   The TURN server will relay the data between the client and peer data
   connections.  End-to-end flow control is maintained by the relay
   process: if the relay process is no longer able to write data to the
   destination of the relayed data, the relay process stops reading data
   from the source.

4.  Client Processing

4.1.  Creating an Allocation

   To create a TCP allocation, a client MUST initiate a new TCP or TLS
   connection to its TURN server, identical to the TCP or TLS procedures
   defined in [RFC5766].  TCP allocations cannot be obtained using a UDP
   association between client and server.

   Once set up, a client MUST send a TURN Allocate request.  That
   request MUST contain a REQUESTED-TRANSPORT attribute whose value is
   6, corresponding to TCP.

   The request MUST NOT include a DONT-FRAGMENT, RESERVATION-TOKEN, or
   EVEN-PORT attribute.  The corresponding features are specific to UDP-
   based capabilities and are not utilized by TURN-TCP.  However, a
   LIFETIME attribute MAY be included, with semantics identical to the
   UDP case.

   The procedures for authentication of the Allocate request and
   processing of success and failure responses are identical to those
   for UDP.

   Once a success response is received, the TCP connection to the TURN
   server is called the control connection for that allocation.

4.2.  Refreshing an Allocation

   The procedures for refreshing an allocation are identical to those
   for UDP.  Note that the Refresh MUST be sent on the control
   connection.

4.3.  Initiating a Connection

   To initiate a TCP connection to a peer, a client MUST send a Connect
   request over the control connection for the desired allocation.  The
   Connect request MUST include an XOR-PEER-ADDRESS attribute containing
   the transport address of the peer to which a connection is desired.

   If the connection is successfully established, the client will
   receive a success response.  That response will contain a
   CONNECTION-ID attribute.  The client MUST initiate a new TCP
   connection to the server, utilizing the same destination transport
   address to which the control connection was established.  This
   connection MUST be made using a different local transport address.
   Authentication of the client by the server MUST use the same method
   and credentials as for the control connection.  Once established, the
   client MUST send a ConnectionBind request over the new connection.
   That request MUST include the CONNECTION-ID attribute, echoed from
   the Connect Success response.  When a response to the ConnectionBind
   request is received, if it is a success, the TCP connection on which
   it was sent is called the client data connection corresponding to the
   peer.

   If the result of the Connect request was an Error Response, and the
   response code was 447 (Connection Timeout or Failure), it means that
   the TURN server was unable to connect to the peer.  The client MAY
   retry with the same XOR-PEER-ADDRESS attribute, but MUST wait at
   least 10 seconds.

   As with any other request, multiple Connect requests MAY be sent
   simultaneously.  However, Connect requests with the same XOR-PEER-
   ADDRESS parameter MUST NOT be sent simultaneously.

4.4.  Receiving a Connection

   After an Allocate request is successfully processed by the server,
   the client will start receiving a ConnectionAttempt indication each
   time a peer for which a permission has been installed attempts a new
   connection to the relayed transport address.  This indication will
   contain CONNECTION-ID and XOR-PEER-ADDRESS attributes.  If the client
   wishes to accept this connection, it MUST initiate a new TCP
   connection to the server, utilizing the same destination transport
   address to which the control connection was established.  This
   connection MUST be made using a different local transport address.
   Authentication of the client by the server MUST use the same method
   and credentials as for the control connection.  Once established, the
   client MUST send a ConnectionBind request over the new connection.
   That request MUST include the CONNECTION-ID attribute, echoed from
   the ConnectionAttempt indication.  When a response to the
   ConnectionBind request is received, if it is a success, the TCP
   connection on which it was sent is called the client data connection
   corresponding to the peer.

4.5.  Sending and Receiving Data

   Once a client data connection is established, data sent on it by the
   client will be relayed as-is to the peer by the server.  Similarly,
   data sent by the peer to the server will be relayed as-is to the
   client over the data connection.

4.6.  Data Connection Maintenance

   The client MUST refresh the allocation (corresponding to a data
   connection) using the Refresh request as defined in [RFC5766] for as
   long as it wants to keep the data connection alive.

   When the client wishes to terminate its relayed connection to the
   peer, it closes the data connection to the server.

      Note: No mechanism for keeping alive the NAT bindings (potentially
      on the client data connection as well as on the peer data
      connection) is included.  This service is not provided by TURN-
      TCP.  If such a feature is deemed necessary, it can be implemented
      higher up the stack, in the application protocol being tunneled
      inside TURN-TCP.  Also, TCP keep-alives MAY be used to keep the
      NAT bindings on the client data connection alive.

5.  TURN Server Behavior

5.1.  Receiving a TCP Allocate Request

   The process is similar to that defined in [RFC5766], Section 6.2,
   with the following exceptions:

   1.  If the REQUESTED-TRANSPORT attribute is included and specifies a
       protocol other than UDP or TCP, the server MUST reject the
       request with a 442 (Unsupported Transport Protocol) error.  If
       the value is UDP, and if UDP transport is allowed by local
       policy, the server MUST continue with the procedures of [RFC5766]
       instead of this document.  If the value is UDP, and if UDP
       transport is forbidden by local policy, the server MUST reject
       the request with a 403 (Forbidden) error.

   2.  If the client connection transport is not TCP or TLS, the server
       MUST reject the request with a 400 (Bad Request) error.

   3.  If the request contains the DONT-FRAGMENT, EVEN-PORT, or
       RESERVATION-TOKEN attribute, the server MUST reject the request
       with a 400 (Bad Request) error.

   4.  A TCP relayed transport address MUST be allocated instead of a
       UDP one.

   5.  The RESERVATION-TOKEN attribute MUST NOT be present in the
       success response.

   If all checks pass, the server MUST start accepting incoming TCP
   connections on the relayed transport address.  Refer to Section 5.3
   for details.

5.2.  Receiving a Connect Request

   When the server receives a Connect request, it processes the request
   as follows.

   If the request is received on a TCP connection for which no
   allocation exists, the server MUST return a 437 (Allocation Mismatch)
   error.

   If the server is currently processing a Connect request for this
   allocation with the same XOR-PEER-ADDRESS, it MUST return a 446
   (Connection Already Exists) error.

   If the server has already successfully processed a Connect request
   for this allocation with the same XOR-PEER-ADDRESS, and the resulting
   client and peer data connections are either pending or active, it
   MUST return a 446 (Connection Already Exists) error.

   If the request does not contain an XOR-PEER-ADDRESS attribute, or if
   such attribute is invalid, the server MUST return a 400 (Bad Request)
   error.

   If the new connection is forbidden by local policy, the server MUST
   reject the request with a 403 (Forbidden) error.

   Otherwise, the server MUST initiate an outgoing TCP connection.  The
   local endpoint is the relayed transport address associated with the
   allocation.  The remote endpoint is the one indicated by the XOR-
   PEER-ADDRESS attribute.  If the connection attempt fails or times
   out, the server MUST return a 447 (Connection Timeout or Failure)
   error.  The timeout value MUST be at least 30 seconds.

   If the connection is successful, it is now called a peer data
   connection.  The server MUST buffer any data received from the
   client.  The server adjusts its advertised TCP receive window to
   reflect the amount of empty buffer space.

   The server MUST include the CONNECTION-ID attribute in the Connect
   success response.  The attribute's value MUST uniquely identify the
   peer data connection.

   If no ConnectionBind request associated with this peer data
   connection is received after 30 seconds, the peer data connection
   MUST be closed.

5.3.  Receiving a TCP Connection on a Relayed Transport Address

   When a server receives an incoming TCP connection on a relayed
   transport address, it processes the request as follows.

   The server MUST accept the connection.  If it is not successful,
   nothing is sent to the client over the control connection.

   If the connection is successfully accepted, it is now called a peer
   data connection.  The server MUST buffer any data received from the
   peer.  The server adjusts its advertised TCP receive window to
   reflect the amount of empty buffer space.

   If no permission for this peer has been installed for this
   allocation, the server MUST close the connection with the peer
   immediately after it has been accepted.

   Otherwise, the server sends a ConnectionAttempt indication to the
   client over the control connection.  The indication MUST include an
   XOR-PEER-ADDRESS attribute containing the peer's transport address,
   as well as a CONNECTION-ID attribute uniquely identifying the peer
   data connection.

   If no ConnectionBind request associated with this peer data
   connection is received after 30 seconds, the peer data connection
   MUST be closed.

5.4.  Receiving a ConnectionBind Request

   When a server receives a ConnectionBind request, it processes the
   request as follows.

   If the client connection transport is not TCP or TLS, the server MUST
   return a 400 (Bad Request) error.

   If the request does not contain the CONNECTION-ID attribute, or if
   this attribute does not refer to an existing pending connection, the
   server MUST return a 400 (Bad Request) error.

   Otherwise, the client connection is now called a client data
   connection.  Data received on it MUST be sent as-is to the associated
   peer data connection.

   Data received on the associated peer data connection MUST be sent
   as-is on this client data connection.  This includes data that was
   received after the associated Connect or request was successfully
   processed and before this ConnectionBind request was received.

5.5.  Data Connection Maintenance

   If the allocation associated with a data connection expires, the data
   connection MUST be closed.

   When a client data connection is closed, the server MUST close the
   corresponding peer data connection.

   When a peer data connection is closed, the server MUST close the
   corresponding client data connection.

6.  IANA Considerations

   This specification defines several new STUN methods, STUN attributes,
   and STUN error codes.  IANA added these new protocol elements to the
   Session Traversal Utilities for NAT (STUN) Parameters registry.

6.1.  New STUN Methods

   This section lists the codepoints for the new STUN methods defined in
   this specification.  See Sections 4 and 5 for the semantics of these
   new methods.

   0x000a :  Connect
   0x000b :  ConnectionBind
   0x000c :  ConnectionAttempt

6.2.  New STUN Attributes

   This STUN extension defines the following new attributes:

   0x002a :  CONNECTION-ID

6.2.1.  CONNECTION-ID

   The CONNECTION-ID attribute uniquely identifies a peer data
   connection.  It is a 32-bit unsigned integral value.

6.3.  New STUN Error Codes

   446    Connection Already Exists
   447    Connection Timeout or Failure

7.  Security Considerations

   After a TCP connection is established between the server and a peer,
   and before a ConnectionBind request is received from the client, the
   server buffers all data received from the peer.  This protocol
   specification lets the server drop the connection if the buffer size
   is about to exceed a limit defined by local policy.  This policy
   should ensure that memory resources are not exceeded.  See also
   [RFC4732], Section 2.1.3.

   All the security considerations applicable to STUN [RFC5389] and TURN
   [RFC5766] are applicable to this document as well.

8.  Acknowledgements

   Thanks to Rohan Mahy and Philip Matthews for their initial work on
   getting this document started.

   The authors would also like to thank Alfred E. Heggestad, Ari
   Keranen, Marc Petit-Huguenin, Dave Thaler, and Dan Wing for their
   comments and suggestions.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.

9.2.  Informative References

   [RFC4732]  Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
              Service Considerations", RFC 4732, December 2006.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.

Authors' Addresses

   Simon Perreault (editor)
   Viagenie
   2875 boul. Laurier, suite D2-630
   Quebec, QC  G1V 2M2
   Canada

   Phone: +1 418 656 9254
   EMail: simon.perreault@viagenie.ca
   URI:   http://www.viagenie.ca


   Jonathan Rosenberg
   jdrosen.net
   Monmouth, NJ
   US

   EMail: jdrosen@jdrosen.net
   URI:   http://www.jdrosen.net