Constrained RESTful Environments
Internet Engineering Task Force (IETF) M. S. Lenders
Internet-Draft
Request for Comments: 9952 TU Dresden
Intended status:
Category: Informational C. Amsüss
Expires: 12 February 2026
ISSN: 2070-1721
T. C. Schmidt
HAW Hamburg
M. Wählisch
TU Dresden & Barkhausen Institut
11 August 2025
ALPN
March 2026
The Application-Layer Protocol Negotiation (ALPN) ID Specification for CoAP
the Constrained Application Protocol (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
Status of This Document Memo
This note document is to be removed before publishing as not 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.
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https://datatracker.ietf.org/doc/draft-ietf-core-coap-dtls-alpn/.
Discussion of this document takes place on the Constrained RESTful
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published for this draft and an issue tracker can be found at
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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) (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 [I-D.ietf-core-dns-over-coap]) [PRE-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 transport
layer security TLS 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 is defined as "coap" in [RFC8323]. As
it is not advisable to re-use reuse the same ALPN ID for a different
transport layer, an ALPN for CoAP over DTLS is registered in
Section 4.1. 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 transport layer
security, 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 on for 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
IANA has added 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 registry in the "Transport
Layer Security (TLS) Extensions" registry group.
* Protocol:
+==================+=========================+=====================+
| Protocol | Identification Sequence | Reference |
+==================+=========================+=====================+
| CoAP (over DTLS)
* Identification sequence: | 0x63 0x6f ("co")
* Reference: [RFC7252] and [RFC-XXXX] | [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, 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>. <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/rfc/rfc7252>.
<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/rfc/rfc7301>. <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/rfc/rfc9147>.
<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/rfc/rfc9460>. <https://www.rfc-editor.org/info/rfc9460>.
5.2. Informative References
[I-D.ietf-core-dns-over-coap]
[PRE-RFC9953]
Lenders, M. S., Amsüss, C., Gündoğan, C., Schmidt, T. C.,
and M. Wählisch, "DNS over CoAP the Constrained Application
Protocol (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>. RFC PRE-9953, DOI 10.17487/PRE-RFC9953,
March 2026, <https://www.rfc-editor.org/info/rfc9953>.
[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>.
<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/rfc/rfc8323>.
<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/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
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
<https://www.rfc-editor.org/info/rfc8446>.
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 review reviews before WG adoption of this draft
specification 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