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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RFC 7064 STUN URI November 2013
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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RFC 7064 STUN URI November 2013
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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RFC 7064 STUN URI November 2013
 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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RFC 7064 STUN URI November 2013
 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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