ALPN ID Specification for CoAP over DTLS
draft-ietf-core-coap-dtls-alpn-05

Constrained RESTful Environments                           M. S. Lenders
Internet-Draft                                                TU Dresden
Intended status: Informational                                 C. Amsüss
Expires: 12 February 2026                                               
                                                           T. C. Schmidt
                                                             HAW Hamburg
                                                             M. Wählisch
                                        TU Dresden & Barkhausen Institut
                                                          11 August 2025

                ALPN ID Specification for CoAP over DTLS
                   draft-ietf-core-coap-dtls-alpn-05

Abstract

   This document specifies an Application-Layer Protocol Negotiation
   (ALPN) ID for transport-layer-secured Constrained Application
   Protocol (CoAP) services.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at https://core-
   wg.github.io/coap-dtls-alpn/draft-ietf-core-coap-dtls-alpn.html.
   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-core-coap-dtls-alpn/.

   Discussion of this document takes place on the Constrained RESTful
   Environments Working Group mailing list (mailto:core@ietf.org), which
   is archived at https://mailarchive.ietf.org/arch/browse/core/.
   Subscribe at https://www.ietf.org/mailman/listinfo/core/.

   Source for this draft and an issue tracker can be found at
   https://github.com/core-wg/coap-dtls-alpn.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 12 February 2026.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Application-Layer Protocol Negotiation (ALPN) IDs . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  TLS ALPN for CoAP . . . . . . . . . . . . . . . . . . . .   3
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .   5
     A.1.  Since draft-ietf-core-coap-dtls-alpn-04 . . . . . . . . .   5
     A.2.  Since draft-ietf-core-coap-dtls-alpn-03 . . . . . . . . .   5
     A.3.  Since draft-ietf-core-coap-dtls-alpn-02 . . . . . . . . .   5
     A.4.  Since draft-ietf-core-coap-dtls-alpn-01 . . . . . . . . .   5
     A.5.  Since draft-ietf-core-coap-dtls-alpn-00 . . . . . . . . .   6
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

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 (SVCB)
   via the DNS, using SVCB resource records with the "alpn" Service
   Parameter Keys [RFC9460].  As an example, applications that use the

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   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 [I-D.ietf-core-dns-over-coap]) 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 transport
   layer security using 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 was defined as "coap" in [RFC8323].  As
   it is not advisable to re-use the same ALPN ID for a different
   transport layer, an ALPN for CoAP over DTLS is registered in
   Section 4.1.

   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 transport layer
   security, 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 on ALPN (see [RFC7301]) and SVCB resource
   records (see [RFC9460]) also apply to this document.

4.  IANA Considerations

   // RFC Ed.: throughout this section, please replace RFC-XXXX with the
   // RFC number of this specification and remove this note.

   This document has the following actions for IANA.

4.1.  TLS ALPN for CoAP

   The following entry has been added to the "TLS Application-Layer
   Protocol Negotiation (ALPN) Protocol IDs" registry, which is part of
   the "Transport Layer Security (TLS) Extensions" registry group.

   *  Protocol: CoAP (over DTLS)

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   *  Identification sequence: 0x63 0x6f ("co")

   *  Reference: [RFC7252] and [RFC-XXXX]

   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/rfc/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/rfc/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/rfc/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/rfc/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/rfc/rfc9460>.

5.2.  Informative References

   [I-D.ietf-core-dns-over-coap]
              Lenders, M. S., Amsüss, C., Gündoğan, C., Schmidt, T. C.,
              and M. Wählisch, "DNS over CoAP (DoC)", Work in Progress,
              Internet-Draft, draft-ietf-core-dns-over-coap-17, 24 July
              2025, <https://datatracker.ietf.org/doc/html/draft-ietf-
              core-dns-over-coap-17>.

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   [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/rfc/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/rfc/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/rfc/rfc8446>.

Appendix A.  Change Log

A.1.  Since draft-ietf-core-coap-dtls-alpn-04
      (https://datatracker.ietf.org/doc/draft-ietf-core-coap-dtls-
      alpn/04/)

   *  Address Deb Cooley's IESG ballot COMMENT

A.2.  Since draft-ietf-core-coap-dtls-alpn-03
      (https://datatracker.ietf.org/doc/draft-ietf-core-coap-dtls-
      alpn/03/)

   *  Make DTLS references normative

A.3.  Since draft-ietf-core-coap-dtls-alpn-02
      (https://datatracker.ietf.org/doc/draft-ietf-core-coap-dtls-
      alpn/02/)

   *  Address shepherd review

A.4.  Since draft-ietf-core-coap-dtls-alpn-01
      (https://datatracker.ietf.org/doc/draft-ietf-core-coap-dtls-
      alpn/01/)

   *  Address review by Esko Dijk

   *  Address review by Marco Tiloca

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A.5.  Since draft-ietf-core-coap-dtls-alpn-00
      (https://datatracker.ietf.org/doc/draft-ietf-core-coap-dtls-
      alpn/00/)

   *  Fix ALPN ID for CoAP over TLS

   *  Change intended status to Informational

Acknowledgments

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

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

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