ALPN ID Specification for CoAP over DTLS

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Table of Contents

1. Introduction

Application-Layer Protocol Negotiation (ALPN) enable communicating parties to agree on an application-layer protocol during a Transport Layer Security (TLS) handshake using an ALPN ID. 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. As an example, this information can be obtained as part of the discovery of DNS over CoAP (DoC) servers (see [I-D.ietf-core-dns-over-coap]) that deploy TLS or DTLS to secure their messages. This document specifies an ALPN ID for CoAP services that are secured by transport security using DTLS. An ALPN ID for CoAP service 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 "coaps" 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.¶

ALPN ID values have variable length. Here, a short value ("co") is allocated for CoAP over DTLS, 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, ALPN IDs "coaps" and "co" can be used respectively in the same manner as for any other service secured with transport layer security, as described in [RFC9460]. Other authentication mechanisms are currently out of scope.¶

3. Security Considerations

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

4. IANA Considerations

5. References

Appendix A. Change Log

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.¶

Authors' Addresses