Extension of the CoAP-DTLS Profile for ACE to TLS

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Table of Contents

1. Introduction

[RFC9202] only specifies the use of DTLS [RFC6347] [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 feature is supported by the OSCORE profile [RFC9203] but is lacking in the DTLS profile.¶

This document updates [RFC9202] by specifying that the profile applies to TLS as well as DTLS. The same access rights are valid in case transport layer security is provided by either DTLS or TLS, and the same access token can be used. 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. 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).¶

In case the ace_profile parameter indicates the use of the DTLS profile for ACE as defined in [RFC9202], the Client MAY try to connect to the Resource Server via TLS, or try TLS and DTLS in parallel to accelerate the session setup.¶

As resource-constrained devices are not expected to support both transport layer security mechanisms, a Client that implements either TLS or DTLS but not both might fail in establishing a secure communication channel with the Resource Server altogether. This error SHOULD be handled by the Client in the same way as unsupported ACE profiles. If the Client is modified accordingly or it learns that the Resource Server has been, the Client may try to connect to the Resource Server using the transport layer security mechanism that was previously not mutually supported.¶

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 for the establishment of the security association as well (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.¶

4. IANA Considerations

The following updates have been done for the "ACE Profiles" registry for the profile with Profile ID 1 and Profile name 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: [RFC-XXXX]¶

5. Security Considerations

The security consideration and requirements in TLS 1.3 [RFC8446] and BCP 195 [RFC7525] [RFC8996] also apply to this document.¶

Acknowledgments

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

Authors' Addresses