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RFC 6062

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

Pages: 13
Proposed Standard
Errata

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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.
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Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   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
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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].
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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 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
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   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.
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   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.
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   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
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   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
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       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.
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   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.
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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
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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.
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   [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