Internet Engineering Task Force (IETF)
Request for Comments: 7443
Category: Informational
ISSN: 2070-1721
P. Patil
T. Reddy
G. Salgueiro
M. Petit-Huguenin
Impedance Mismatch
January 2015

Application-Layer Protocol Negotiation (ALPN) Labels

for Session Traversal Utilities for NAT (STUN) Usages


Application-Layer Protocol Negotiation (ALPN) labels for Session Traversal Utilities for NAT (STUN) usages, such as Traversal Using Relays around NAT (TURN) and NAT discovery, are defined in this document to allow an application layer to negotiate STUN usages within the Transport Layer Security (TLS) connection. ALPN protocol identifiers defined in this document apply to both TLS and Datagram Transport Layer Security (DTLS).

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

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). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see 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

Copyright Notice

Copyright © 2015 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 ( 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   4.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     4.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   4
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1. Introduction

   STUN can be securely transported using TLS-over-TCP (referred to as
   TLS [RFC5246]), as specified in [RFC5389], or TLS-over-UDP (referred
   to as DTLS [RFC6347]), as specified in [RFC7350].

ALPN [RFC7301] enables an endpoint to positively identify an application protocol in TLS/DTLS and distinguish it from other TLS/ DTLS protocols. With ALPN, the client sends the list of supported application protocols as part of the TLS/DTLS ClientHello message. The server chooses a protocol and sends the selected protocol as part of the TLS/DTLS ServerHello message. Application protocol negotiation can thus be accomplished within the TLS/DTLS handshake, without adding network round-trips.

STUN protocol usages, such as TURN [RFC5766], can be used to identify the purpose of a flow without initiating a session.

This document proposes the following ALPN labels to identify STUN protocol [RFC5389] usages.

   'stun.turn':  Label to identify the specific use of STUN over (D)TLS
      for TURN (Section 4.6 of [RFC7350]).
   'stun.nat-discovery':  Label to identify the specific use of STUN
      over (D)TLS for NAT discovery (Section 4.1 of [RFC7350]).

2. IANA Considerations

The following entries have been added to the "Application-Layer Protocol Negotiation (ALPN) Protocol IDs" registry established by [RFC7301].

The "stun.turn" label identifies the use of TURN usage (D)TLS:


Traversal Using Relays around NAT (TURN)

Identification Sequence: 0x73 0x74 0x75 0x6E 0x2E 0x74 0x75 0x72 0x6E ("stun.turn")

      Specification: This document (RFC 7443)

The "stun.nat-discovery" label identifies the use of STUN for the purposes of NAT discovery over (D)TLS:

Protocol: NAT discovery using Session Traversal Utilities for NAT (STUN)

Identification Sequence: 0x73 0x74 0x75 0x6E 0x2E 0x6e 0x61 0x74 0x2d 0x64 0x69 0x73 0x63 0x6f 0x76 0x65 0x72 0x79 ("stun.nat-discovery")

      Specification: This document (RFC 7443)

3. Security Considerations

The ALPN STUN protocol identifier does not introduce any specific security considerations beyond those detailed in the TLS ALPN Extension specification [RFC7301]. It also does not impact the security of TLS/DTLS session establishment or application data exchange.

4. References

4.1. Normative References

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008,
   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008, <>.
   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012,
   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, July 2014,
   [RFC7350]  Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport
              Layer Security (DTLS) as Transport for Session Traversal
              Utilities for NAT (STUN)", RFC 7350, August 2014,

4.2. Informative References

   [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,


This work benefited from the discussions and invaluable input by the various members of the TRAM working group. These include Simon Perrault, Paul Kyzivat, Brandon Williams, and Andrew Hutton. Special thanks to Martin Thomson and Oleg Moskalenko for their constructive comments, suggestions, and early reviews that were critical to the formulation and refinement of this document.

Barry Leiba, Stephen Farrell, Adrian Farrel, and Richard Barnes provided useful feedback during IESG review. Thanks to Russ Housley for his Gen-ART review and Adam Langley for his IETF LC review comments as TLS expert.

The authors would also like to express their gratitude to the TRAM WG chairs Gonzalo Camarillo and especially Simon Perrault, who also acted as document shepherd. Lastly, we also want to thank the Transport Area Director Spencer Dawkins for his support and careful reviews.

Authors' Addresses

Prashanth Patil
Cisco Systems, Inc.


   Tirumaleswar Reddy
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103


   Gonzalo Salgueiro
   Cisco Systems, Inc.
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709
   United States


Marc Petit-Huguenin
Impedance Mismatch
United States