



Internet Engineering Task Force (IETF)                     M. S. Lenders
Request for Comments: 9952                                    TU Dresden
Category: Informational                                        C. Amsüss
ISSN: 2070-1721                                                         
                                                           T. C. Schmidt
                                                             HAW Hamburg
                                                             M. Wählisch
                                        TU Dresden & Barkhausen Institut
                                                              March 2026


  Application-Layer Protocol Negotiation (ALPN) ID for CoAP over DTLS

Abstract

   This document specifies an Application-Layer Protocol Negotiation
   (ALPN) ID for Constrained Application Protocol (CoAP) services that
   are secured by 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 candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9952.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction
   2.  Application-Layer Protocol Negotiation (ALPN) IDs
   3.  Security Considerations
   4.  IANA Considerations
   5.  References
     5.1.  Normative References
     5.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   Application-Layer Protocol Negotiation (ALPN) enables communicating
   parties to agree on an application-layer protocol during a Transport
   Layer Security (TLS) handshake using an ALPN ID [RFC7301].  This ALPN
   ID can be discovered for services as part of Service Bindings (SVCBs)
   via the DNS, using SVCB resource records with the "alpn" Service
   Parameter Keys [RFC9460].  As an example, applications that use the
   Constrained Application Protocol (CoAP) [RFC7252] can obtain this
   information as part of the discovery of DNS over CoAP (DoC) servers
   (see Section 3.2 of [RFC9953]) that deploy TLS 1.3 [RFC8446] as well
   as Datagram Transport Layer Security (DTLS) 1.2 or 1.3 [RFC6347]
   [RFC9147] to secure their messages.  This document specifies an ALPN
   ID for CoAP services that are secured by DTLS.  An ALPN ID for CoAP
   services secured by TLS has already been specified in [RFC8323].

2.  Application-Layer Protocol Negotiation (ALPN) IDs

   For CoAP over TLS, an ALPN ID is defined as "coap" in [RFC8323].  As
   it is not advisable to reuse the same ALPN ID for a different
   transport layer, an ALPN for CoAP over DTLS is registered in
   Section 4.

   ALPN ID values have variable length.  For CoAP over DTLS, a short
   value ("co") is allocated, as this can avoid fragmentation of Client
   Hello and Server Hello messages in constrained networks with link-
   layer fragmentation, such as 6LoWPAN [RFC4944].

   To discover CoAP services that secure their messages with TLS or
   DTLS, the ALPN IDs "coap" and "co" can be used, respectively, in the
   same manner as for any other service secured with TLS, as described
   in [RFC9460].  The discovery of CoAP services that rely on other
   security mechanisms is out of the scope of this document.

3.  Security Considerations

   Any security considerations for ALPN (see [RFC7301]) and SVCB
   resource records (see [RFC9460]) also apply to this document.

4.  IANA Considerations

   IANA has added the following entry to the "TLS Application-Layer
   Protocol Negotiation (ALPN) Protocol IDs" registry in the "Transport
   Layer Security (TLS) Extensions" registry group.

   +==================+=========================+=====================+
   | Protocol         | Identification Sequence | Reference           |
   +==================+=========================+=====================+
   | CoAP (over DTLS) | 0x63 0x6f ("co")        | [RFC7252], RFC 9952 |
   +------------------+-------------------------+---------------------+

        Table 1: TLS Application-Layer Protocol Negotiation (ALPN)
                          Protocol IDs Registry

   Note that [RFC7252] does not define the use of the ALPN TLS extension
   during the DTLS connection handshake.  This document does not change
   this behavior and thus does not establish any rules like those in
   Section 8.2 of [RFC8323].

5.  References

5.1.  Normative References

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <https://www.rfc-editor.org/info/rfc7301>.

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/info/rfc9147>.

   [RFC9460]  Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
              and Parameter Specification via the DNS (SVCB and HTTPS
              Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
              November 2023, <https://www.rfc-editor.org/info/rfc9460>.

5.2.  Informative References

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
              <https://www.rfc-editor.org/info/rfc4944>.

   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,
              <https://www.rfc-editor.org/info/rfc8323>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC9953]  Lenders, M. S., Amsüss, C., Gündoğan, C., Schmidt, T. C.,
              and M. Wählisch, "DNS over CoAP (DoC)", RFC 9953,
              DOI 10.17487/RFC9953, March 2026,
              <https://www.rfc-editor.org/info/rfc9953>.

Acknowledgments

   We would like to thank Rich Salz for the expert review on the "co"
   ALPN ID allocation.  We would also like to thank Mohamed Boucadair
   and Ben Schwartz for their early reviews before WG adoption of this
   specification and Esko Dijk, Thomas Fossati, and Marco Tiloca for
   their feedback and comments.

   This work was supported in parts by the German Federal Ministry of
   Research, Technology, and Space (BMFTR) under the grant numbers
   16KIS1386K (TU Dresden) and 16KIS1387 (HAW Hamburg) within the
   research project PIVOT and under the grant numbers 16KIS1694K (TU
   Dresden) and 16KIS1695 (HAW Hamburg) within the research project
   C-ray4edge.

Authors' Addresses

   Martine Sophie Lenders
   TUD Dresden University of Technology
   Helmholtzstr. 10
   D-01069 Dresden
   Germany
   Email: martine.lenders@tu-dresden.de


   Christian Amsüss
   Email: christian@amsuess.com


   Thomas C. Schmidt
   HAW Hamburg
   Berliner Tor 7
   D-20099 Hamburg
   Germany
   Email: t.schmidt@haw-hamburg.de


   Matthias Wählisch
   TUD Dresden University of Technology & Barkhausen Institut
   Helmholtzstr. 10
   D-01069 Dresden
   Germany
   Email: m.waehlisch@tu-dresden.de
