Behave                                                 S. Perreault, Ed.
Internet-Draft                                                  Viagenie
Intended status: Standards Track                            J. Rosenberg
Expires: November 5, 2009                                  Cisco Systems
                                                             May 4, 2009


Traversal Using Relays around NAT (TURN) Extensions for TCP Allocations
                   draft-ietf-behave-turn-tcp-03.txt

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   This Internet-Draft will expire on November 5, 2009.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Abstract

   This specification defines an extension of Traversal Using Relays
   around NAT (TURN), a relay protocol for NAT traversal, to allows a



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   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 STUN server.


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 . . . . . . . . . . . . . . . . .  6
     4.3.  Initiating a Connection  . . . . . . . . . . . . . . . . .  7
     4.4.  Receiving a Connection . . . . . . . . . . . . . . . . . .  7
     4.5.  Sending and Receiving Data . . . . . . . . . . . . . . . .  7
     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 an Allocated Port  . . . . .  9
     5.4.  Receiving a ConnectionBind Request . . . . . . . . . . . . 10
     5.5.  Data Connection Maintenance  . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  New STUN Methods . . . . . . . . . . . . . . . . . . . . . 11
     6.2.  New STUN Attributes  . . . . . . . . . . . . . . . . . . . 11
       6.2.1.  CONNECTION-ID  . . . . . . . . . . . . . . . . . . . . 11
       6.2.2.  New STUN response codes  . . . . . . . . . . . . . . . 11
     6.3.  Security Considerations  . . . . . . . . . . . . . . . . . 11
     6.4.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . 11
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12














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1.  Introduction

   Traversal Using Relays around NAT (TURN) [I-D.ietf-behave-turn] 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) [I-D.ietf-mmusic-ice] form 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 which allows a client to
   obtain a TCP allocation.  It also allows the client to initiate
   connections from that allocation to peers, and accept connection
   requests from peers made towards that allocation.


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].






















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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 port, 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 which 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 must have a TCP or TLS
   connection to its TURN server.  Using that connection, it sends an
   Allocate request.  That request contains a REQUESTED-TRANSPORT



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   attribute, which indicates a TCP-based allocation is desired.  A
   server which supports this extension will allocate a TCP port and
   begin listening for connection requests on that port.  It then
   returns the allocated port to the client in the resposne 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
   allocated address, it issues a Connect request to the TURN server
   over the control connection.  That request contains a XOR-PEER-
   ADDRESS attribute identifying the peer IP address and port 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.

   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.  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 1-to-1.  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 are
   forwarded to the peer, again without encapsulation or framing of any
   kind.  Once a connection has been bound using the ConnectionBind
   request, TURN processing is no longer permitted on the connection.

   In a similar way, when a peer opens a TCP connection towards the
   allocated port, 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



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   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, utilizing an internal buffer.  However, back pressure is
   used in order to achieve end-to-end flow control.  If the buffer from
   client to peer fills up, the TURN server ceases to read off the
   client data connection, which causes TCP backpressure through the OS
   towards the client.


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 [I-D.ietf-behave-turn].  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 TCP.

   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.







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4.3.  Initiating a Connection

   To initiate a TCP connection to a peer, a client MUST send a Connect
   request over the control channel for the desired allocation.  The
   Connect request MUST include a XOR-PEER-ADDRESS attribute containing
   the IP address and port 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 IP address
   and port to which the control connection was established.  This
   connection MUST be made using a different local IP address and/or
   port.  Once established, the client MUST send a ConnectionBind
   request.  That request MUST include the CONNECTION-ID attribute,
   mirrored from the Connect Success response.  When a response to the
   ConnectionBind request is recevied, 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 a Error Response, and the
   response code was XXX, it means that the TURN server was unable to
   connect to the peer.  The client MAY retry, but MUST wait at least 10
   seconds.

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 attemps a new connection to the allocated address.  This
   indication will contain a CONNECTION-ID and a 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 IP address and port to which the control connection was
   established.  This connection MUST be made using a different local IP
   address and/or port.  Once established, the client MUST send a
   ConnectionBind request.  That request MUST include the CONNECTION-ID
   attribute, mirrorred 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.  Data on a data connection MUST NOT



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   be interpreted as STUN messages.

4.6.  Data Connection Maintenance

   The client MUST refresh the allocation corresponding to a data
   connection, using the Refresh request as defined in
   [I-D.ietf-behave-turn], 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.


5.  TURN Server Behavior

5.1.  Receiving a TCP Allocate Request

   The process is similar to that defined in [I-D.ietf-behave-turn],
   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, the server MUST continue with the procedures of
       [I-D.ietf-behave-turn] instead of this document.)

   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



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   for details.

5.2.  Receiving a Connect Request

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

   If the request is received on a control connection for which no
   allocation exists, the server MUST return a 437 (Allocation Mismatch)
   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 a XOR-PEER-ADDRESS attribute, or if
   such attribute is invalid, the server MUST return a 400 (Bad Request)
   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 XXX (Connection Timeout or Failure)
   error.

   If the connection is successful, it is now called a peer data
   connection.  The server MUST buffer any data received from the peer.
   Data MUST NOT be lost.  It is up to the server to adjust its
   advertised TCP receive window should the buffer size become a
   limiting factor.

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

5.3.  Receiving a TCP Connection on an Allocated Port

   When a server receives an incoming TCP connection on a relayed
   transport, it processes 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.  Data MUST NOT be lost.  It is up to the server to adjust its
   advertised TCP receive window should the buffer size become a



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   limiting factor.

   The server then sends a ConnectionAttempt indication to the client
   over the control connection.  The indication MUST include a XOR-PEER-
   ADDRESS attribute containing the peer's 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 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 Listen request was
   successfully processed and before this ConnectionBind request was
   received.

   Data received on a client or peer data connection MUST NOT be
   interpreted as a STUN message.

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 or times out, the server MUST
   close the corresponding peer data connection.

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




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6.  IANA Considerations

   This specification defines several new STUN methods, STUN attributes,
   and STUN response codes.  This section directs IANA to add these new
   protocol elements to the IANA registry of STUN protocol elements.

6.1.  New STUN Methods

   This section lists the codepoints for the new STUN methods defined in
   this specification.  See elsewhere in this document for the semantics
   of these new methods.

   0x0007 :  Connect           (only request/response semantics defined)
   0x0008 :  ConnectionBind    (only request/response semantics defined)
   0x0009 :  ConnectionAttempt (only indication semantics defined)

6.2.  New STUN Attributes

   This STUN extension defines the following new attributes:

   0x002A :  CONNECTION-ID

6.2.1.  CONNECTION-ID

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

6.2.2.  New STUN response codes

   446    Connection Already Exists
   447    Connection Timeout or Failure

6.3.  Security Considerations

   The methods, attribute, and error response codes defined in this
   document do not have any special security considerations beyond those
   for other attributes and Error response codes.  All the security
   considerations applicable to STUN [RFC5389] and TURN
   [I-D.ietf-behave-turn] are applicable to this document as well.

6.4.  Acknowledgements

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

   The authors would also like to thank Marc Petit-Huguenin for his
   comments and suggestions.




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7.  References

7.1.  Normative References

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

   [I-D.ietf-behave-turn]
              Rosenberg, J., Mahy, R., and P. Matthews, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)",
              draft-ietf-behave-turn-14 (work in progress), April 2009.

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

7.2.  Informative References

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-19 (work in progress), October 2007.


Authors' Addresses

   Simon Perreault (editor)
   Viagenie
   2600 boul. Laurier, suite 625
   Quebec, QC  G1V 4W1
   Canada

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














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   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

   Phone: +1 973 952-5000
   Email: jdrosen@cisco.com
   URI:   http://www.jdrosen.net










































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