Internet-Draft CoAP-DTLS Extension January 2023
Bergmann, et al. Expires 29 July 2023 [Page]
ACE Working Group
9202 (if approved)
Intended Status:
Standards Track
O. Bergmann
J. Preuß Mattsson
G. Selander

Extension of the CoAP-DTLS Profile for ACE to TLS


This document updates the CoAP-DTLS profile for ACE described in RFC 9202 by specifying that the profile applies to TLS as well as DTLS.

Status of This Memo

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

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This Internet-Draft will expire on 29 July 2023.

Table of Contents

1. Introduction

[RFC9202] only specifies the use of DTLS [RFC9147] but works equally well for TLS [RFC8446]. For many constrained implementations, CoAP over UDP [RFC7252] is the first choice, but when deploying ACE in networks controlled by other entities (such as the Internet), UDP might be blocked on the path between the client and the RS, and the client might have to fall back to CoAP over TCP [RFC8323] for NAT or firewall traversal. This dual support for security over TCP as well as UDP is already supported by the OSCORE profile [RFC9203].

This document updates [RFC9202] by specifying that the profile applies to TLS as well as DTLS. It only impacts the transport layer security channel between Client and Resource Server. The same access rights are valid in case transport layer security is provided by either DTLS or TLS. The same access token can be used by either DTLS or TLS between a given (Client, RS) pair. Therefore, the value coap_dtls in the ace_profile parameter of an AS-to-Client response or in the ace_profile claim of an access token indicates that either DTLS or TLS can be used for transport layer security.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

Readers are expected to be familiar with the terms and concepts described in [RFC9200] and [RFC9202].

3. Specific Changes to RFC 9202

The main changes to [RFC9202] specified in this document are limited to replacing "DTLS" with "DTLS/TLS" throughout the document. This essentially impacts the use of secure transport as described in the sections 3.2.2, 3.3.2, 4, and 5.

In addition to this, the Client and Resource Server behavior is updated to describe the case where either or both DTLS and TLS may be available, as described in the following section.

4. Connection Establishment

Following the procedures defined in [RFC9202], a Client can retrieve an Access Token from an Authorization Server (AS) in order to establish a security association with a specific Resource Server. The ace_profile parameter in the Client-to-AS request and AS-to-client response is used to determine the ACE profile that the Client uses towards the Resource Server (RS).

The ace_profile parameter indicates the use of the DTLS profile for ACE as defined in [RFC9202]. Therefore, the Client typically first tries using DTLS to connect to the Resource Server. If this fails the Client MAY try to connect to the Resource Server via TLS. The client can try TLS and DTLS in parallel to accelerate the connection setup. It is up to the implementation to handle the case where the RS reponds to both connection requests.

As resource-constrained devices are not expected to support both transport layer security mechanisms. Clients and Resource Servers SHOULD support DTLS and MAY support TLS. A Client that implements either TLS or DTLS but not both might fail in establishing a secure communication channel with the Resource Server altogether.

Note that a communication setup with an a priori unknown Resource Server typically employs an initial unauthorized resource request as illustrated in Section 2 of [RFC9202]. If this message exchange succeeds, the Client SHOULD first use the same underlying transport protocol also for the establishment of the security association to RS (i.e., DTLS for UDP, and TLS for TCP).

As a consequence, the selection of the transport protocol used for the initial unauthorized resource request also depends on the transport layer security mechanism supported by the Client. Clients that support either DTLS or TLS but not both SHOULD use the transport protocol underlying the supported transport layer security mechanism also for an initial unauthorized resource request to the RS as in Section 2 of [RFC9202].

5. IANA Considerations

The following updates have been done to the "ACE Profiles" registry for the profile with a "CBOR Value" field value of 1 and "Name" of "coap_dtls":

Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]" with the RFC number of this specification and delete this paragraph.

Description: Profile for delegating client Authentication and Authorization for Constrained Environments by establishing a Datagram Transport Layer Security (DTLS) or Transport Layer Security (TLS) channel between resource-constrained nodes.

Change Controller: IESG

Reference: [RFC9202] [RFC-XXXX]

6. Security Considerations

The security consideration and requirements in [RFC9202], TLS 1.3 [RFC8446], and BCP 195 [RFC8996] [RFC9325] also apply to this document.

7. References

7.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
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, , <>.
Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, , <>.
Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", RFC 9147, DOI 10.17487/RFC9147, , <>.
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and H. Tschofenig, "Authentication and Authorization for Constrained Environments Using the OAuth 2.0 Framework (ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, , <>.
Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and L. Seitz, "Datagram Transport Layer Security (DTLS) Profile for Authentication and Authorization for Constrained Environments (ACE)", RFC 9202, DOI 10.17487/RFC9202, , <>.

7.2. Informative References

Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS 1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, , <>.
Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson, "The Object Security for Constrained RESTful Environments (OSCORE) Profile of the Authentication and Authorization for Constrained Environments (ACE) Framework", RFC 9203, DOI 10.17487/RFC9203, , <>.
Sheffer, Y., Saint-Andre, P., and T. Fossati, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, , <>.


The authors would like to thank Marco Tiloca for reviewing this specification.

Authors' Addresses

Olaf Bergmann
Universität Bremen TZI
Bremen, D-28359
John Preuß Mattsson
Ericsson AB
SE-164 80 Stockholm
Göran Selander
Ericsson AB
SE-164 80 Stockholm