Everything over CoAP
draft-amsuess-core-coap-kitchensink-00
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draft-amsuess-core-coap-kitchensink-00
CoRE C. Amsüss
Internet-Draft 2 March 2022
Intended status: Informational
Expires: 3 September 2022
Everything over CoAP
draft-amsuess-core-coap-kitchensink-00
Abstract
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 (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://gitlab.com/chrysn/coap-kitchensink.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 3 September 2022.
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Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Applications . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. Publish-Subscribe services . . . . . . . . . . . . . . . 2
2.2. Remote configuration . . . . . . . . . . . . . . . . . . 3
2.2.1. Network status monitoring . . . . . . . . . . . . . . 3
2.3. Software updates . . . . . . . . . . . . . . . . . . . . 4
2.4. Network file system . . . . . . . . . . . . . . . . . . . 4
2.5. Network address resolution . . . . . . . . . . . . . . . 4
2.6. Time service . . . . . . . . . . . . . . . . . . . . . . 5
2.7. Terminal access . . . . . . . . . . . . . . . . . . . . . 5
2.8. Chat services . . . . . . . . . . . . . . . . . . . . . . 6
2.9. E-Mail . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.10. Video streaming . . . . . . . . . . . . . . . . . . . . . 6
3. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Normative References . . . . . . . . . . . . . . . . . . 6
3.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. CoAP . . . . . . . . . . . . . . . . . . . . . . . . 8
Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
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.
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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 MQTT
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.
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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.
It has never been specified and described; that gap is closed in
Appendix A.
Competing with WebDAV, NFS, FTP
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.
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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 SNTP, NTP
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
https://forum.riot-os.org/t/coap-remote-shell/3340/5
(https://forum.riot-os.org/t/coap-remote-shell/3340/5). 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 SSH
Strong points The head-of-line blocking that occasionally plagues
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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 https://comatrix.eu/ (https://comatrix.eu/) has
demonstrated that the Matrix chat protocol can be simplified to the
point where it becomes usable transparently with constrained devices.
2.9. 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,
mailto:office@example.com?subject=Sensor%20failure), no attempts are
known to send or receive E-Mail over CoAP.
2.10. 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
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[RFC7252] 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>.
3.2. Informative References
[I-D.ietf-core-coap-pubsub]
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-09, 30 September 2019,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
coap-pubsub-09>.
[I-D.ietf-core-comi]
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, 17
January 2021, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-comi-11>.
[RFC132] White, J., "Typographical Error in RFC 107", RFC 132,
DOI 10.17487/RFC0132, April 1971,
<https://www.rfc-editor.org/rfc/rfc132>.
[I-D.ietf-suit-manifest]
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-16, 25 October 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-suit-
manifest-16>.
[I-D.draft-lenders-dns-over-coap]
Lenders, M. S., Amsüss, C., Gündoğan, C., Schmidt, T. C.,
and M. Wählisch, "DNS Queries over CoAP (DoC)", Work in
Progress, Internet-Draft, draft-lenders-dns-over-coap-02,
25 October 2021, <https://datatracker.ietf.org/doc/html/
draft-lenders-dns-over-coap-02>.
[RFC15] Carr, C., "Network subsystem for time sharing hosts",
RFC 15, DOI 10.17487/RFC0015, September 1969,
<https://www.rfc-editor.org/rfc/rfc15>.
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[I-D.ietf-core-interfaces]
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, 11 March
2019, <https://datatracker.ietf.org/doc/html/draft-ietf-
core-interfaces-14>.
[I-D.ietf-core-fasor]
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, 19 October 2020,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
fasor-01>.
[I-D.bhattacharyya-core-a-realist]
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, 5 February 2019,
<https://datatracker.ietf.org/doc/html/draft-
bhattacharyya-core-a-realist-02>.
Appendix A. CoAP
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):
* Directories are unconditionally represented by URIs with a
trailing slash; files by those without one.
The GET operation is used to list them (for there is no PROPFIND
operation in CoAP). Different media types migth be used depending
on the capabilities of the parties, with application/link-format
as a base line.
(Note that application/link-format is not particularly efficient
for this purpose, as the directory listing needs to repeat the
requested resource's full path for each entry).
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* Paths are constructed by placing directory names and either an
empty string (for the trailing slash) or the unescaped file name
in Uri-Path options.
* Clients may attempt to treat any URI composed of the file server
entry URI and additional path segments as files on the file
server. Consequently, any additional services the file server may
provide (e.g., as resources specified in extensions) are
necessarily assigned URIs with a query, for these can not be
inadvertedly constructed in an attempt to access a file.
* For lack of a HEAD option, file metadata can only be obtained by
performing a GET on the directory containing the file, or a FETCH
for efficiency if suitable media types are defined.
All metadata is provided on a best-effort basis, and the supported
directory formats limit what can be expressed. Typical supported
metadata are the media type (expressed as ct in link format) and
the size (sz).
* If write support is available and permissions allow, a client can
create files by performing a PUT operation on a previously
nonexistent resource.
Files can be overwritten by PUTting a new representation. Files
sent with block-wise transfer should be processed atomically by
the server unless explicitly negotiated otherwise. (On POSIX file
systems, this can be implemented without additional state by
storing the blocks in a temporary file whose name contains the
original file name and the block key of the request, and renaming
it to the target name when receiving the last block).
* Files and directories can be removed using the DELETE operation,
subject ot the same conditions as writing to resources.
Deleting directories is recursive; client that desire POSIX
semantics need to check whether the directory is empty and use the
If-Match option with the empty directory's ETag to avoid race
conditions.
* Regular CoAP extensions apply if the parties support them, for
example:
- Observation can be used to watch files or directories for
changes.
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- ETags (e.g. derived from the file's stats) can be used to
ensure that large files are assembled correctly by the client,
and to perform file updates with optimistic locking by using
the If-Match option.
* Additional features can be specified and advertised separately,
either per resoource or by a named specification that provides
templates for further operations.
For example, a specification might say that when a file system is
advertised with a given interface (if parameter of link format),
for each file and directory there is an associated resource at
?acl that describes access control applicable to that file, and
can be used with GET and PUT as per the ACL's policies.
Additional operations can use their custom media types and
methods, and possibly create more resources. For example, a
server-to-server copy (again, advertised by a suitable interface
parameter) could provide a ?copy resource under any directory, to
which a CBOR list containing two CRIs (source and destination)
would be posted. That specification might describe that if copies
are not completed instantly, the response might indicate a new
location using Uri-Path and Uri-Query options (the latter might be
necessary to not conflict with existing files) which tracks the
status of the operation.
Appendix B. Change log
(Initial version)
Author's Address
Christian Amsüss
Austria
Email: christian@amsuess.com
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