Problem Statement for Discovery of Network-designated OSCORE-based Resolvers

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Table of Contents

1. Introduction

[RFC9460] specifies the "SVCB" ("Service Binding") DNS resource records to lookup information on how to communicate with a service. Service Parameters (SvcParams) are used to carry that information. On top of that, options to discover DNS resolvers that allow for encrypted DNS resolution are specified in other document. These use either DNS ([RFC9461], [RFC9462]) or, in a local network, Router Advertisements or DHCP ([RFC9463]). These specifications use SvcParams to carry information required for configuration of such resolvers. So far, however, only DNS transfer protocols based on Transport Layer Security (TLS) are supported, namely DNS over TLS (DoT) [RFC7858], DNS over HTTPS (DoH) [RFC8484], and DNS over Dedicated QUIC (DoQ) [RFC9250].¶

DNS over CoAP [I-D.ietf-core-dns-over-coap] provides a solution for encrypted DNS in constrained environments. In such scenarios, the usage of DoT, DoH, DoQ, or similar TLS-based solutions is often not possible. The Constrained Application Protocol (CoAP) [RFC7252], the transfer protocol for DoC, is mostly agnostic to the transport layer, i.e., CoAP can be transported over UDP, TCP, or WebSockets [RFC8323], and even less common transports such as Bluetooth GATT [I-D.amsuess-core-coap-over-gatt] or SMS [lwm2m] are discussed. A future iteration of [I-D.ietf-core-transport-indication] will cover the selection of this transport via SVCB records.¶

Furthermore, CoAP offers three security modes:¶

• No Security: This plain CoAP mode does not support any encryption. It is not recommended when using [I-D.ietf-core-dns-over-coap] but inherits core CoAP features such as block-wise transfer [RFC7959] for datagram-based segmentation. Such features are beneficial in constrained settings even without encryption.¶

• Transport Security: CoAP may use DTLS when transferred over UDP [RFC7252] and TLS when transferred over TCP [RFC8323].¶

• Object Security: Securing content objects can be achieved using OSCORE [RFC8613]. OSCORE can be used either as an alternative or in addition to transport security.¶

  OSCORE keys have a limited lifetime and need to be set up, for example through an EDHOC key exchange [RFC9528], which may use credentials from trusted ACE Authorization Server (AS) as described in the ACE EDHOC profile [I-D.ietf-ace-edhoc-oscore-profile]. As an alternative to EDHOC, keys can be set up by such an AS as described in the ACE OSCORE profile [RFC9203].¶

The case of no security will be sufficiently covered by [I-D.ietf-core-transport-indication]. [RFC8323] and [I-D.lenders-core-coap-dtls-svcb] cover the case for transport security. However, there is still a gap for object security. This document provides a problem statement for what is needed to fill this gap.¶

For simplicity, we will talk about the discovery CoAP servers in the following, even though the discovery and configuration of DoC servers over DDR and DNR is currently the main use case for this, as [RFC9176] already provides resource discovery, and consequently CoAP service discovery, for constrained environments.¶

2. Terminology

The terms “DoC server” and “DoC client” are used as defined in [I-D.ietf-core-dns-over-coap].¶

The terms “constrained node” and "constrained network" are used as defined in [RFC7228].¶

SvcParams denotes the field in either DNS SVCB/HTTPS records as defined in [RFC9460], or DHCP and RA messages as defined in [RFC9463]. SvcParamKeys are used as defined in [RFC9460].¶

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.¶

3. Problem Space

The first and most important point of discussion for the discoverability of CoAP is if and what new SvcParamKeys need to be defined and what is already there.¶

[RFC9460] defines the “alpn” key, which is used to identify the protocol suite of a service binding using its Application-Layer Protocol Negotiation (ALPN) ID [RFC7301]. While this is useful to identify classic transport layer security, the question is raised if this is needed or even helpful for when there is only object security. There is an ALPN ID for CoAP over TLS that is defined in [RFC8323]. As using the same ALPN ID for different transport layers is not recommended, another ALPN ID for CoAP over DTLS is introduced in [I-D.lenders-core-coap-dtls-svcb]. Object security may be selected in addition to transport layer security or without it. Additionally, different CoAP transports can be selected, which may be orthogonal to the transport security. For instance, DTLS can be used over transports other than UDP. The selection of CoAP transport protocols will be covered in future versions of [I-D.ietf-core-transport-indication]. Defining an ALPN ID for each combination of object security, mode of transport layer security, and transport protocol might not be viable or scalable. For some ways of setting up object security, additional information is needed, such as an establishment options for an encryption context with EDHOC or an authentication server (AS) with ACE.¶

Beyond the SvcParamKeys, there is the question of what the field values of the Encrypted DNS Options defined in [RFC9463] might be with EDHOC or ACE EDHOC. While most fields map, “authentication-domain-name” (ADN) and its corresponding ADN length field may not matter when authentication is driven by Authorization for Constrained Environments (ACE) [RFC9203] [I-D.ietf-ace-edhoc-oscore-profile].¶

4. Objectives

SVCB records are not meant and should not be used to exchange security contexts, so this eliminates scenarios that use pre-shared keys with OSCORE. This leaves 2 base scenarios for OSCORE, which may occur in combination, with scenarios using transport security, or alternative transport protocols:¶

• DoC over OSCORE using EDHOC, and¶

• DoC over OSCORE using ACE.¶

We mostly need to answer the question for additional SvcParamKeys. [RFC9460] defines the keys “mandatory”, “alpn”, “no-default-alpn”, “port”, “ipv4hint”, and “ipv6hint”. Additionally, [I-D.ietf-core-dns-over-coap] defines “docpath” which carries the path for the DNS resource at the DoC server as a CBOR sequence.¶

Since “alpn” is needed for transport layer security, the type of object security (OSCORE using EDHOC, OSCORE using ACE, OSCORE using EDHOC using ACE), needs to be conveyed in a different SvcParamKey. The semantics and necessacity of the authenticator-domain-name field in [RFC9463] needs to be evaluated in each case.¶

When using ACE, more SvcParamKeys might be needed, such as the OAuth audience, the scope or the authentication server URI.¶

Defining these SvcParamKeys, including their value formats and spaces, as well as the behavior definition for authenticator-domain-name field, shall be part of future work.¶

5. Security Considerations

TODO Security¶

6. IANA Considerations

This document has no IANA considerations.¶

Appendix A. Change Log

Acknowledgments

TODO acknowledge.¶

Authors' Addresses