Traversal Using Relays around NAT (TURN) Uniform Resource Identifiers

Abstract

This document specifies the syntax of Uniform Resource Identifier (URI) schemes for the Traversal Using Relays around NAT (TURN) protocol. It defines two URI schemes to provision the TURN Resolution Mechanism (RFC 5928).

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/rfc7065.

Copyright Notice

Copyright © 2013 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.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Definitions of the "turn" and "turns" URI . . . . . . . . . .   4
     3.1.  URI Scheme Syntax . . . . . . . . . . . . . . . . . . . .   4
     3.2.  URI Scheme Semantics  . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  "turn" URI Registration . . . . . . . . . . . . . . . . .   5
     5.2.  "turns" URI Registration  . . . . . . . . . . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .   8
   Appendix B.  Design Notes . . . . . . . . . . . . . . . . . . . .   8

1. Introduction

This document specifies the syntax and semantics of the Uniform Resource Identifier (URI) scheme for the Traversal Using Relays around NAT (TURN) protocol.

The TURN protocol is a specification allowing hosts behind NAT to control the operation of a relay server. The relay server allows hosts to exchange packets with its peers. The peers themselves may also be behind NATs. RFC 5766 [RFC5766] defines the specifics of the TURN protocol.

The "turn" and "turns" URI schemes are used to designate a TURN server (also known as a relay) on Internet hosts accessible using the TURN protocol. With the advent of standards such as WebRTC [WEBRTC], we anticipate a plethora of endpoints and web applications to be able to identify and communicate with such a TURN server to carry out the TURN protocol. This implies that endpoints and/or applications must be provisioned with the appropriate configuration to identify the TURN server. Having an inconsistent syntax adds ambiguity and can result in non-interoperable solutions and implementation limitations. The "turn" and "turns" URI schemes help alleviate most of these issues by providing a consistent way to describe, configure, and exchange the information identifying a TURN server.

[RFC5928] defines a resolution mechanism to convert a secure flag, a host name or IP address, a potentially empty port, and a potentially empty transport to a list of IP address, port, and TURN transport tuples.

To simplify the provisioning of TURN clients, this document defines the "turn" and "turns" URI schemes that can carry the four components needed for the resolution mechanism.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] when they appear in ALL CAPS. When these words are not in ALL CAPS (such as "should" or "Should"), they have their usual English meanings, and are not to be interpreted as RFC 2119 key words.

3. Definitions of the "turn" and "turns" URI

3.1. URI Scheme Syntax

The "turn" and "turns" URIs have the following formal ABNF syntax [RFC5234]:

   turnURI       = scheme ":" host [ ":" port ]
                   [ "?transport=" transport ]
   scheme        = "turn" / "turns"
   transport     = "udp" / "tcp" / transport-ext
   transport-ext = 1*unreserved

<host> and <port> are specified in [RFC3986]. While these two ABNF productions are defined in [RFC3986] as components of the generic hierarchical URI, this does not imply that the "turn" and "turns" schemes are hierarchical URIs. Developers MUST NOT use a generic hierarchical URI parser to parse a "turn" or "turns" URI.

The <host>, <port>, and <transport> components are passed without modification to the [RFC5928] algorithm. <secure> is set to false if <scheme> is equal to "turn", and set to true if <scheme> is equal to "turns" and passed to the [RFC5928] algorithm with the other components.

3.2. URI Scheme Semantics

The "turn" and "turns" URI schemes are used to designate a TURN server (also known as a relay) on Internet hosts accessible using the TURN protocol. The TURN protocol supports sending messages over UDP, TCP, or TLS-over-TCP. The "turns" URI scheme MUST be used when TURN is run over TLS-over-TCP (or, in the future, DTLS-over-UDP), and the "turn" scheme MUST be used otherwise.

The required <host> part of the "turn" URI denotes the TURN server host.

As specified in [RFC5766] and [RFC5928], the <port> part, if present, denotes the port on which the TURN server is awaiting connection requests. If it is absent, the default port is 3478 for both UDP and TCP. The default port for TURN over TLS is 5349.

4. Security Considerations

Security considerations for the resolution mechanism are discussed in Section 5 of [RFC5928]. Note that this section contains normative text defining authentication procedures to be followed by turn clients when TLS is used.

The "turn" and "turns" URI schemes do not introduce any specific security issues beyond the security considerations discussed in [RFC3986].

Although a "turn" or "turns" URI does not itself include the username or password that will be used to authenticate the TURN client, in certain environments, such as WebRTC, the username and password will almost certainly be provisioned remotely by an external agent at the same time as a "turns" URI is sent to that client. Thus, in such situations, if the username and password were received in the clear, there would be little or no benefit to using a "turns" URI. For this reason, a TURN client MUST ensure that the username, password, "turns" URI, and any other security-relevant parameters are received with equivalent security before using the "turns" URI. Receiving those parameters over another TLS session can provide the appropriate level of security, if both TLS sessions are similarly parameterised, e.g., with commensurate strength ciphersuites.

5. IANA Considerations

This section contains the registration information for the "turn" and "turns" URI Schemes (in accordance with [RFC4395]).

5.1. "turn" URI Registration

URI scheme name: turn

Status:

           permanent

URI scheme syntax: See Section 3.1.

URI scheme semantics: See Section 3.2.

Encoding considerations: There are no encoding considerations beyond those in [RFC3986].

Applications/protocols that use this URI scheme name:

The "turn" URI scheme is intended to be used by applications with a need to identify a TURN server to be used for NAT traversal.

   Interoperability considerations: N/A

Security considerations: See Section 4.

   Contact: Marc Petit-Huguenin <petithug@acm.org>

Author/Change controller: The IESG

References:

               RFC 7065

5.2. "turns" URI Registration

URI scheme name: turns

Status:

           permanent

URI scheme syntax: See Section 3.1.

URI scheme semantics: See Section 3.2.

Encoding considerations: There are no encoding considerations beyond those in [RFC3986].

Applications/protocols that use this URI scheme name:

The "turns" URI scheme is intended to be used by applications with a need to identify a TURN server to be used for NAT traversal over a secure connection.

   Interoperability considerations: N/A

Security considerations: See Section 4.

   Contact: Marc Petit-Huguenin <petithug@acm.org>

Author/Change controller: The IESG

References:

               RFC 7065

6. Acknowledgements

Thanks to Margaret Wasserman, Magnus Westerlund, Juergen Schoenwaelder, Sean Turner, Ted Hardie, Dave Thaler, Alfred E. Heggestad, Eilon Yardeni, Dan Wing, Alfred Hoenes, and Jim Kleck for the comments, suggestions, and questions that helped improve "Traversal Using Relays around NAT (TURN) Uniform Resource Identifiers" by M. Petit-Huguenin (October 2011).

Many thanks to Cullen Jennings for his detailed review and thoughtful comments on "URI Scheme for Traversal Using Relays around NAT (TURN) Protocol" by S. Nandakumar, et al. (October 2011).

Thanks to Bjoern Hoehrmann, Dan Wing, Russ Housley, S. Moonesamy, Graham Klyne, Harald Alvestrand, Hadriel Kaplan, Tina Tsou, Spencer Dawkins, Ted Lemon, Barry Leiba, Pete Resnick, and Stephen Farrell for the comments, suggestions, and questions that helped improve this document.

The authors would also like to express their gratitude to Dan Wing for his assistance in shepherding this document. We also want to thank Gonzalo Camarillo, the Real-time Applications and Infrastructure Area Director, for sponsoring this document as well as his careful reviews.

7. References

7.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
   
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.
   
   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 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.
   
   [RFC5928]  Petit-Huguenin, M., "Traversal Using Relays around NAT
              (TURN) Resolution Mechanism", RFC 5928, August 2010.

7.2. Informative References

   [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", BCP 35, RFC
              4395, February 2006.
   
   [WEBRTC]   Bergkvist, A., Burnett, D., Jennings, C., and A.
              Narayanan, "WebRTC 1.0: Real-time Communication Between
              Browsers", World Wide Web Consortium WD
              WD-webrtc-20120821, August 2012,
              <http://www.w3.org/TR/2012/WD-webrtc-20120821>.

Appendix A. Examples

Table 1 shows how the <secure>, <port>, and <transport> components are populated from various URIs. For all these examples, the <host> component is populated with "example.org".

   +---------------------------------+----------+--------+-------------+
   | URI                             | <secure> | <port> | <transport> |
   +---------------------------------+----------+--------+-------------+
   | turn:example.org                | false    |        |             |
   | turns:example.org               | true     |        |             |
   | turn:example.org:8000           | false    | 8000   |             |
   | turn:example.org?transport=udp  | false    |        | UDP         |
   | turn:example.org?transport=tcp  | false    |        | TCP         |
   | turns:example.org?transport=tcp | true     |        | TLS         |
   +---------------------------------+----------+--------+-------------+

Table 1

Appendix B. Design Notes

• One recurring comment was to stop using the suffix "s" on the URI scheme, and to move the secure option to a parameter (e.g. ";proto=tls"). We decided against this idea because the STUN URI does not have a ";proto=" parameter and we would have lost the symmetry between the TURN and STUN URIs.

• Following the advice of Section 2.2 of RFC 4395, and because the TURN URI does not describe a hierarchical structure, the TURN URIs are opaque URIs.

• <password> is not used in the URIs because it is deprecated [RFC3986]. <username> and <auth> are not used in the URIs because they do not guide the resolution mechanism.

• As discussed at IETF 72 in Dublin, there are no generic parameters in the URI to prevent compatibility issues.

Authors' Addresses

Marc Petit-Huguenin
Impedance Mismatch

EMail:

          petithug@acm.org
   
   Suhas Nandakumar
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   US

EMail:

          snandaku@cisco.com
   
   Gonzalo Salgueiro
   Cisco Systems
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709
   US

EMail:

          gsalguei@cisco.com
   
   Paul E. Jones
   Cisco Systems
   7025 Kit Creek Road
   Research Triangle Park, NC  27709
   US

EMail:

          paulej@packetizer.com