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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Definition of the "stun" or "stuns" URI . . . . . . . . . . . 3
3.1. URI Scheme Syntax . . . . . . . . . . . . . . . . . . . . 3
3.2. URI Scheme Semantics . . . . . . . . . . . . . . . . . . 3 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 5
5.1. "stun" URI Registration . . . . . . . . . . . . . . . . . 5
5.2. "stuns" URI Registration . . . . . . . . . . . . . . . . 5 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 6 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 7 8
Appendix B. Design Notes . . . . . . . . . . . . . . . . . . . . 7 8
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 also implies that those endpoints and/or applications are to must
be provisioned with the appropriate configurations
required configuration to identify the
STUN server. Having an inconsistent syntax
has its drawbacks adds ambiguity and can
result in non-interoperable solutions. It
can result in solutions that are ambiguous and have implementation
limitations on the different aspects of the syntax and alike. 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. This would also prevent the
shortcomings inherent with encoding similar information in non-
uniform syntaxes such as the ones proposed in the WebRTC standards
[WEBRTC], for example.
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
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.
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
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 "stun" "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
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>.
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.
A more detailed account of the reasoning behind this is available
at <http://blog.marc.petit-huguenin.org/2012/09/on-design-of-stun-
and-turn-uri-formats.html>.
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.
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