Extended Tokens and Stateless Clients in the Constrained Application Protocol (CoAP)
RFC 8974
Internet Engineering Task Force (IETF) K. Hartke
Request for Comments: 8974 Ericsson
Updates: 7252, 8323 M. Richardson
Category: Standards Track Sandelman
ISSN: 2070-1721 January 2021
Extended Tokens and Stateless Clients
in the Constrained Application Protocol (CoAP)
Abstract
This document provides considerations for alleviating Constrained
Application Protocol (CoAP) clients and intermediaries of keeping
per-request state. To facilitate this, this document additionally
introduces a new, optional CoAP protocol extension for extended token
lengths.
This document updates RFCs 7252 and 8323 with an extended definition
of the "TKL" field in the CoAP message header.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8974.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
1.1. Terminology
2. Extended Tokens
2.1. Extended Token Length (TKL) Field
2.2. Discovering Support
2.2.1. Extended-Token-Length Capability Option
2.2.2. Trial and Error
2.3. Intermediaries
3. Stateless Clients
3.1. Serializing Client State
3.2. Using Extended Tokens
3.3. Transmitting Messages
4. Stateless Intermediaries
4.1. Observing Resources
4.2. Block-Wise Transfers
4.3. Gateway Timeouts
4.4. Extended Tokens
5. Security Considerations
5.1. Extended Tokens
5.2. Stateless Clients and Intermediaries
6. IANA Considerations
6.1. CoAP Signaling Option Number
7. References
7.1. Normative References
7.2. Informative References
Appendix A. Updated Message Formats
A.1. CoAP over UDP
A.2. CoAP over TCP/TLS
A.3. CoAP over WebSockets
Acknowledgements
Authors' Addresses
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] is a RESTful
application-layer protocol for constrained environments [RFC7228].
In CoAP, clients (or intermediaries in the client role) make requests
to servers (or intermediaries in the server role), which satisfy the
requests by returning responses.
While a request is ongoing, a client typically needs to keep some
state that it requires for processing the response when that arrives.
Identification of this state is done in CoAP by means of a token: an
opaque sequence of bytes that is chosen by the client and included in
the CoAP request and that is returned by the server verbatim in any
resulting CoAP response (Figure 1).
+-----------------+ request with +------------+
| | | state identifier | |
| | | as token | |
| .-<-+->------|--------------------->|------. |
| _|_ | | | |
| / \ stored | | | |
| \___/ state | | | |
| | | | | |
| '->-+-<------|<---------------------|------' |
| | | response with | |
| v | token echoed back | |
+-----------------+ +------------+
Client Server
Figure 1: Token as an Identifier for Request State
In some scenarios, it can be beneficial to reduce the amount of state
that is stored at the client at the cost of increased message sizes.
A client can opt into this by serializing (parts of) its state into
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