Internet-Draft Everything over CoAP July 2022
Amsüss Expires 13 January 2023 [Page]
Intended Status:
C. Amsüss

Everything over CoAP


The Constrained Application Protocol (CoAP) has become the base of applications both inside of the constrained devices space it originally aimed for and outside. This document gives an overview of applications that are, can, may, and would better not be implemented on top of CoAP.

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 (, which is archived at

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

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This Internet-Draft will expire on 13 January 2023.

Table of Contents

1. Introduction

[ See abstract for now ]

2. Applications

2.1. Publish-Subscribe services

Publish-subscribe services (pubsub) are a widespread tool for the some of the fundamental use cases of Internet of Things (IoT) protocols: acquiring sensor data and controlling actuators.

A pubsub implementation has been in development since shorlty after the original CoAP publication and is as of now still in draft status, as [I-D.ietf-core-coap-pubsub].

Competing with


Strong points

Once a topic is set up, data can be sent and received by CoAP clients that are not even aware of pubsub, as long as they can PUT or GET (possibly with observation) data to and from configured URIs.

Weak points

To implement a pubsub broker that supports arbitrarily many topics, some (potentially difficult-to-implement) compromises have to be made.

2.2. Remote configuration

The OMA LwM2M protocol (which caters for several applications at the granularity of this document) includes provisions for configuring and monitoring devices over the network, setting properties such as a time server and reading properties such as a network interface's packet count.

In parallel, the NETCONF protocol and its YANG modelling language have been ported to the constrained ecosystem as CORECONF [I-D.ietf-core-comi]. By using numeric identifiers with good compression properties, it can efficiently express data both from shared and from bespoke models in single requests.

Competing with

SNMP [ ? ], Puppet [ ? ]

2.2.1. Network status monitoring

Related to remote configuration, CoAP is used as the signalling channel of DOTS ([RFC132]).

Strong points

CoAP over UDP/DTLS provides operational signalling on links under attack, on which a TCP/TLS based connection would fail.

CoAP's consistency across transports makes it easy to adjust to situations in which UDP is uanvailable, sacrificing some properties but leaving the high-level protocol unmodified.

Weak points

CoAP's default parameters for flow control (such as PROBING_RATE) are unsuitable for this application and need to be customized.

2.3. Software updates

The SUIT manifest format [I-D.ietf-suit-manifest] can be used to describe firmware updates that can be performed over CoAP or any other protocol that is expressible in terms of URIs.

The OMA LwM2M protocol also contains provisions for firmware updates over CoAP.

2.4. Network file system

Using CoAP as a backend for a no-frills file service is a simple composition and is provided as a demo by the aiocoap library and a module in the RIOT operating system.

It has never been specified and described; that gap is closed in Appendix A.

Competing with


Strong points

CoAP protocol already provides random read access (through the Block2 option), optimistic locking and cache (through the ETag and If-Match options) and change notification (through the Observe option).

Files can be used in other CoAP protocols without the client's awareness (e.g. for SUIT)

Weak points

Transfer of large files is inefficient due to the repetition of file names in block-wise requests (mitigated when using CoAP-over-TCP and BERT).

Advanced file system functionality (file metadata, server-to-server copies, renaming, locking) would need additional specifications.

2.5. Network address resolution

The Domain Name System (DNS) can be utilized from CoAP using the mechanisms described in [I-D.draft-lenders-dns-over-coap].

Strong points

Savings in firmware complexity by using infrastructure shared with other applications.

Potential for traffic (and thus energy) reduction by using request-response binding.

Weak points

Not deployed in existing networks.

2.6. Time service

Whereas no attempt has been made yet to specify a time service over CoAP, a primitive time service can be assembled by creating a CoAP resource that returns the server's current time, e.g., in a UNIX time stamp represented in decimal ASCII, or in CBOR using any of timestamp tags (0, 1 or 1001).

Such services have been in use since at least 2013, and are easy to operate and scale.

Competing with


Strong points

Savings in firmware complexity by using infrastructure shared with other applications.

Compact messages.

Reuse of existing security associations.

Weak points

None of the advanced features of (S)NTP, such as distinction between receive and transmit timestamps. Not even leap seconds are advertised (but that can be mitigated by using a time scale that is not affected by them, such as TAI).

Generally only suitable for the last hop in time synchronization.

2.7. Terminal access

Virtual terminal access was one of the first network applications described in an RFC ([RFC15]), and popular to date.

There is no full specification yet as to how to express the data streams of character based user input and character based text responses in CoAP. Necessary components, as well as optional future extensions, have been sketched for the RIOT operating system at Unlike SSH, that sketch assumes the presence of a single virtual terminal (as opposed to one created per connection). On platforms with dynamic resources and per-process output capture, an SSH-like muliplexing can be created based on the resource collection pattern described in [I-D.ietf-core-interfaces].

Competing with


Strong points

The head-of-line blocking that occasionally plagues TCP based connections is eliminiated in favor of on-demand recovery (i.e., observing the last output will produce the latest chunk of output, and the terminal may recover skipped data later if it is still in the device's back-scroll buffer).

Weak points

The default retransmission characteristics of CoAP make operations painfully slow when encountering packet loss. Tuning of parameters or the implementation of advanced flow control as described in [I-D.ietf-core-fasor] are necessary for smooth operation.

On-demand recovery is incompatible with regular terminals, and requires either fully managed terminals (where the full output is reprinted when lost fragments are recovered) or accepting the loss of data where printed exceeding the network speed. (Data is still lost gracefully, as the loss is detected and can be indicated visually).

2.8. Chat services

The CoMatrix project has demonstrated that the Matrix chat protocol can be simplified to the point where it becomes usable transparently with constrained devices.

2.9. Web browsing

Competing with

HTTP [RFC9110] over its various transports; Gemini ([gemini]).

By virtue of cross proxying to HTTP, CoAP is generally capable of transporting web pages the same way as HTTP, albeit at a reduced feature set (in particular, most HTTP headers).

CoAP offers only niche benefits over HTTP when combined with HTML, the predominant markup language on the web: Any benefits of a more compact transport or implementation are dwarved by the typical size of pages and the complexity of the HTML ecosystem.

CoAP might be a suitable transport for Small Web environments such as Gemini [gemini], which can be rendered even by constrained devices.

2.10. E-Mail

While E-Mail was part of the considerations that led to the definition of the Proxy-Uri option (which would technically allow a cross-proxy to accept POST requests to, say,, no attempts are known to send or receive E-Mail over CoAP.

2.11. Video streaming

The use of CoAP for real time video streaming and telemetry from Unmanned Aerial Vehicles (UAVs) has been explored in [I-D.bhattacharyya-core-a-realist].

It is unclear whether CoAP could actually outperform unconstrained streaming protocols such as WebRTC, or whether devices that produce and consume video benefit from the constraints of CoAP.

3. References

3.1. Normative References

Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, , <>.

3.2. Informative References

solderpunk, "Project Gemini, speculative specification, v0.16.1", , <>.
Koster, M., Keranen, A., and J. Jimenez, "Publish-Subscribe Broker for the Constrained Application Protocol (CoAP)", Work in Progress, Internet-Draft, draft-ietf-core-coap-pubsub-10, , <>.
Veillette, M., Stok, P. V. D., Pelov, A., Bierman, A., and I. Petrov, "CoAP Management Interface (CORECONF)", Work in Progress, Internet-Draft, draft-ietf-core-comi-11, , <>.
White, J., "Typographical Error in RFC 107", RFC 132, DOI 10.17487/RFC0132, , <>.
Moran, B., Tschofenig, H., Birkholz, H., and K. Zandberg, "A Concise Binary Object Representation (CBOR)-based Serialization Format for the Software Updates for Internet of Things (SUIT) Manifest", Work in Progress, Internet-Draft, draft-ietf-suit-manifest-18, , <>.
Lenders, M. S., Amsüss, C., Gündoğan, C., Schmidt, T. C., and M. Wählisch, "DNS over CoAP (DoC)", Work in Progress, Internet-Draft, draft-lenders-dns-over-coap-04, , <>.
Carr, C., "Network subsystem for time sharing hosts", RFC 15, DOI 10.17487/RFC0015, , <>.
Shelby, Z., Koster, M., Groves, C., Zhu, J., and B. Silverajan, "Reusable Interface Definitions for Constrained RESTful Environments", Work in Progress, Internet-Draft, draft-ietf-core-interfaces-14, , <>.
Jarvinen, I., Kojo, M., Raitahila, I., and Z. Cao, "Fast-Slow Retransmission Timeout and Congestion Control Algorithm for CoAP", Work in Progress, Internet-Draft, draft-ietf-core-fasor-01, , <>.
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, , <>.
Bhattacharyya, A., Agrawal, S., Rath, H., Pal, A., and B. Purushothaman, "Adaptive RESTful Real-time Live Streaming for Things (A-REaLiST)", Work in Progress, Internet-Draft, draft-bhattacharyya-core-a-realist-02, , <>.

Appendix A. CoAP File Service

This sketches [ TBD: describes ] a file transfer protocol / remote file system built on top of CoAP.

A file server works similar to a WebDAV server, and follows these rules (which are sometimes expressed from the point of view of the server, but apply when a client maps them back into a file system in such a way that operations can round-trip):

Some implementation guidance should be provided around the interaction between idempotent requests that have no actual effect and preconditions: If a DELETE with If-Match is transmitted again on a new token (by a proxy relying on its idempotence), should the server respond Deleted rather than Not Found? (In this case, the client could at least know what to do with it, but) If a PUT with If-Match is transmitted again after it has been acted on, should the server respond Changed rather than Precondition Failed? (Probably "No" to both, as the former is easily recognizable by the client, and the latter would delay faulting by long, but still needs further thought.)

Appendix B. Change log

From -01 to -02:

From -00 to -01:

Author's Address

Christian Amsüss