Internet-Draft CoAP Early Data July 2022
Tschofenig & Fossati Expires 7 January 2023 [Page]
CoRE Working Group
Intended Status:
Standards Track
H. Tschofenig
Arm Limited
T. Fossati
Arm Limited

Early Data Option for CoAP


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.

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Source for this draft and an issue tracker can be found at

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Table of Contents

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:

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.

3. Open Issues

A list of open issues can be found at

4. Security Considerations


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

Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, , <>.
Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", RFC 9147, DOI 10.17487/RFC9147, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Thomson, M., Nottingham, M., and W. Tarreau, "Using Early Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, , <>.
Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, , <>.

Authors' Addresses

Hannes Tschofenig
Arm Limited
Thomas Fossati
Arm Limited