URI Scheme for the Session Traversal Utilities for NAT (STUN) Protocol
RFC 7064

Internet Engineering Task Force (IETF)                     S. Nandakumar
Request for Comments: 7064                                  G. Salgueiro
Category: Standards Track                                       P. Jones
ISSN: 2070-1721                                            Cisco Systems
                                                       M. Petit-Huguenin
                                                      Impedance Mismatch
                                                           November 2013

 URI Scheme for the Session Traversal Utilities for NAT (STUN) Protocol

Abstract

   This document specifies the syntax and semantics of the Uniform
   Resource Identifier (URI) scheme for the Session Traversal Utilities
   for NAT (STUN) protocol.

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/rfc7064.

Copyright Notice

   Copyright (c) 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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Definition of the "stun" or "stuns" URI . . . . . . . . . . .   3
     3.1.  URI Scheme Syntax . . . . . . . . . . . . . . . . . . . .   3
     3.2.  URI Scheme Semantics  . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  "stun" URI Registration . . . . . . . . . . . . . . . . .   5
     5.2.  "stuns" 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

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1.  Introduction

   This document specifies the syntax and semantics of the Uniform
   Resource Identifier (URI) scheme for the Session Traversal Utilities
   for NAT (STUN) protocol.

   STUN is a protocol that serves as a tool for other protocols in
   dealing with Network Address Translator (NAT) traversal.  It can be
   used by an endpoint to determine the IP address and port allocated to
   it by a NAT, to perform connectivity checks between two endpoints,
   and as a keepalive protocol to maintain NAT bindings.  RFC 5389
   [RFC5389] defines the specifics of the STUN protocol.

   The "stun" and "stuns" URI schemes are used to designate a stand-
   alone STUN server or any Internet host performing the operations of a
   STUN server in the context of STUN usages (Section 14 of RFC 5389
   [RFC5389]).  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 STUN server to carry out the
   STUN protocol.  This implies that endpoints and/or applications must
   be provisioned with the appropriate configuration to identify the
   STUN server. Having an inconsistent syntax adds ambiguity and can
   result in non-interoperable solutions and implementation limitations.
   The "stun" and "stuns" URI schemes help alleviate most of these
   issues by providing a consistent way to describe, configure, and
   exchange the information identifying a STUN server.

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.  Definition of the "stun" or "stuns" URI

3.1.  URI Scheme Syntax

   "stun" and "stuns" URIs have the following formal ABNF syntax
   [RFC5234]:

   stunURI       = scheme ":" host [ ":" port ]
   scheme        = "stun" / "stuns"

   <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 "stun" and "stuns" URI

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   schemes are hierarchical URIs.  Developers MUST NOT use a generic
   hierarchical URI parser to parse a "stun" or "stuns" URI.

3.2.  URI Scheme Semantics

   The "stun" and "stuns" URI schemes are used to designate a stand-
   alone STUN server or any Internet host performing the operations of a
   STUN server in the context of STUN usages (Section 14 of RFC 5389
   [RFC5389]).  The STUN protocol supports sending messages over UDP,
   TCP, or TLS-over-TCP.  The "stuns" URI scheme MUST be used when STUN
   is run over TLS-over-TCP (or in the future DTLS-over-UDP), and the
   "stun" scheme MUST be used otherwise.

   The required <host> part of the "stun" URI denotes the STUN server
   host.

   For the optional DNS discovery procedure mentioned in Section 9 of
   RFC 5389, the "stun" URI scheme implies UDP as the transport protocol
   for SRV lookup, and the "stuns" URI scheme indicates TCP as the
   transport protocol.

   As specified in [RFC5389], the <port> part, if present, denotes the
   port on which the STUN server is awaiting connection requests.  If it
   is absent, the default port is 3478 for both UDP and TCP.  The
   default port for STUN over TLS is 5349 as per Section 9 of [RFC5389].

4.  Security Considerations

   The "stun" and "stuns" URI schemes do not introduce any specific
   security issues beyond the security considerations discussed in
   [RFC3986].  These URI schemes are intended for use in specific
   environments that involve NAT traversal.  Users of the scheme need to
   carefully consider the security properties of the context in which
   they are using them.

   Although a "stun" or "stuns" URI does not itself include the username
   or password that will be used to authenticate the STUN 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 "stuns" 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 "stuns" URI.  For this
   reason, a STUN client MUST ensure that the username, password,
   "stuns" URI, and any other security-relevant parameters are received
   with equivalent security before using the "stuns" URI.  Receiving
   those parameters over another TLS session can provide the appropriate
   level of security if both TLS sessions are similarly parameterized,
   e.g., with commensurate strength ciphersuites.

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5.  IANA Considerations

   This section contains the registration information for the "stun" and
   "stuns" URI schemes (in accordance with [RFC4395]).  Note that these
   URI schemes are intended for use in very specific NAT traversal
   environments and should not be used otherwise on the open Web or
   Internet.

5.1.  "stun" URI Registration

   URI scheme name: stun

   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 "stun" URI scheme is intended to be used by applications with
      a need to identify a STUN server to be used for NAT traversal.

   Interoperability considerations: N/A

   Security considerations: See Section 4

   Contact: Suhas Nandakumar <snandaku@cisco.com>

   Author/Change controller: The IESG

   References: RFC 7064

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5.2.  "stuns" URI Registration

   URI scheme name: stuns

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

   Interoperability considerations: N/A

   Security considerations: See Section 4

   Contact: Suhas Nandakumar <snandaku@cisco.com>

   Author/Change controller: The IESG

   References: RFC 7064

6.  Acknowledgements

   The authors would like to extend a very special thanks to Cullen
   Jennings for bringing to our attention to WebRTC's need for this
   document, as well as his detailed review and thoughtful comments on
   this document.

   This document has benefited from extensive discussion and review of
   many of the members of the RTCWEB and BEHAVE working groups.  The
   authors would also like to acknowledge Ted Hardie, Bjoern Hoehrmann,
   Russ Housley, Subramanian Moonesamy, Hadriel Kaplan, Graham Klyne,
   Peter Saint-Andre, Ted Lemon, Barry Leiba, Pete Resnick, Spencer
   Dawkins, Stephen Farrell, and Harald Alvestrand for their invaluable
   input, reviews, feedback comments, and suggestions that helped to
   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

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   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.

7.2.  Informative References

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999.

   [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", BCP 35, RFC
              4395, February 2006.

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

   [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>.

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Appendix A.  Examples

   Table 1 shows examples for the "stun" and "stuns" URI schemes.  For
   all these examples, the <host> component is populated with
   "example.org".

                          +-----------------------+
                          | URI                   |
                          +-----------------------+
                          | stun:example.org      |
                          | stuns:example.org     |
                          | stun:example.org:8000 |
                          +-----------------------+

                                  Table 1

Appendix B.  Design Notes

   o  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 need for
      ";proto=" for the STUN URI cannot be sufficiently explained, and
      supporting it would render an incomplete specification.  This
      would also result in lost symmetry between the TURN and STUN URIs.

   o  Following the advice of Section 2.2 of [RFC4395], and because the
      STUN URI does not describe a hierarchical structure, the STUN URIs
      are opaque.

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Authors' Addresses

   Suhas Nandakumar
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   EMail: snandaku@cisco.com

   Gonzalo Salgueiro
   Cisco Systems
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709
   USA

   EMail: gsalguei@cisco.com

   Paul E. Jones
   Cisco Systems
   7025 Kit Creek Road
   Research Triangle Park, NC  27709
   USA

   EMail: paulej@packetizer.com

   Marc Petit-Huguenin
   Impedance Mismatch

   EMail: petithug@acm.org

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