Early Data Option for CoAP

Internet-Draft	CoAP Early Data	July 2022
Tschofenig & Fossati	Expires 7 January 2023	[Page]

Abstract

This document defines mechanisms that allow clients to communicate with servers about CoAP requests that are sent in early data. Techniques are described that use these mechanisms to mitigate the risk of replay.¶

Discussion Venues

This note is to be removed before publishing as an RFC.¶

Discussion of this document takes place on the Constrained RESTful Environments Working Group mailing list (core@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/core/.¶

Source for this draft and an issue tracker can be found at https://github.com/thomas-fossati/draft-coap-early-data.¶

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 7 January 2023.¶

Copyright Notice

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

1. Introduction

TLS and DTLS 1.3 provide a "zero round-trip time" (0-RTT) feature, the mechanics of which are described in Section 2.3 of [TLS13].¶

This feature provides a significant performance enhancement by enabling a client to send data to a server whom it has already spoken to in the first TLS handshake flight. However, TLS does not provide inherent replay protections for 0-RTT data, therefore the application running atop the TLS session has to take care of that. Specifically, Appendix E.5 of [TLS13] establishes that:¶

Application protocols MUST NOT use 0-RTT data without a profile that defines its use. That profile needs to identify which messages or interactions are safe to use with 0-RTT and how to handle the situation when the server rejects 0-RTT and falls back to 1-RTT.¶

This document defines the application profile for CoAP [CoAP] to allow clients and servers to exchange CoAP requests that are sent in early data. It also describes how to mitigate the risk of replay. The design is inspired by [RFC8470].¶

1.1. Requirements Language

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.¶

2. 0-RTT Data

For a given request, the level of tolerance to replay risk is specific to the resource it operates upon (and therefore only known to the origin server). In general, if processing a request does not have state-changing side effects, the consequences of replay are not significant. The server can choose whether it will process early data before the TLS handshake completes.¶

It is RECOMMENDED that origin servers allow resources to explicitly configure whether early data is appropriate in requests.¶

This document specifies the Early-Data option, which indicates that the request has been conveyed in early data and that a client understands the 4.25 (Too Early) status code. The semantic follows [RFC8470].¶

Table 1: Early-Data Option
No.	C	U	N	R	Name	Format	Length	Default	E
TBD	x				Early-Data	empty	0	(none)	x

Note that 4.25 is just the suggested CoAP response code, which has not been registered yet.¶

5. IANA Considerations

5.1. New Option

IANA is asked to add the Option defined in Table 2 to the CoAP Option Numbers registry.¶

Table 2: Early-Data Option
Number	Name	Reference
TBD	Early-Data	RFCthis

5.2. New Response Code

IANA is asked to add the Response Code defined in Table 3 to the CoAP Response Code registry.¶

Table 3: Too Early Response Code
Code	Description	Reference
4.25	Too Early	RFCthis

6. Normative References

[CoAP]: Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014, <https://www.rfc-editor.org/rfc/rfc7252>.
[DTLS13]: Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022, <https://www.rfc-editor.org/rfc/rfc9147>.
[RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174]: Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8470]: Thomson, M., Nottingham, M., and W. Tarreau, "Using Early Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September 2018, <https://www.rfc-editor.org/rfc/rfc8470>.
[TLS13]: Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, <https://www.rfc-editor.org/rfc/rfc8446>.