Extension of the Datagram Transport Layer Security (DTLS) Profile for Authentication and Authorization for Constrained Environments (ACE) to Transport Layer Security (TLS)

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Table of Contents

1. Introduction

The Authentication and Authorization for Constrained Environments (ACE) framework [RFC9200] defines an architecture for lightweight authentication between Client, Resource Server (RS), and Authorization Server (AS) where the Client and RS may be constrained. The Datagram Transport Layer Security (DTLS) Profile for Authentication and Authorization for Constrained Environments (ACE) [RFC9202] only specifies the use of Datagram Transport Layer Security (DTLS) [RFC9147] for transport-layer security between the nodes in the ACE architecture but works equally well for Transport Layer Security (TLS) [RFC8446]. For many constrained implementations, Constrained Application Protocol (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 Resource Server, 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 Object Security for Constrained RESTful Environments (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 Authorization Server to Client (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 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.¶

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.¶

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. Non-constrained Clients and Resource Servers SHOULD support both TLS and DTLS.¶

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 the Resource Server (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 Resource Server 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.¶

Acknowledgments

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

Authors' Addresses