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# coap-message

CoAP message abstraction
========================

This crate defines interfaces for readable and writable [CoAP] messages, which are used in the
Internet of Things to transfer state in representations (REST). With these interfaces, it
allows CoAP servers and clients to be written independently of the actual CoAP library that is
used to serialize and deserialize the messages to the protocol.

[CoAP]: https://coap.space/

The defined traits only cover the properties of CoAP at the request/response exchange level,
not of the transport specific details. In particular, they do not expose tokens, message IDs or
message types: Those are not even present in some transports, and in general there is no need
for libraries to expose them. In the same vein, no methods are described to allocate messages:
For constrained devices, that may not even be possible, and with CoAP libraries that support
multiple configured transports, the type of message allocated may easily depend on the intended
use. They do not cover remote addresses either, as these too are transport specific.

The crate also provides some trivial implementations of the types: [Code] for [u8], and
[OptionNumber] for [u16].

Usage
-----

A larger chunk of examples and demos is available at [coap-message-demos], which
illustrates both how to build coap-message based handlers, and how to use them in practice with
different implementations of CoAP. Most of the applications in there use coap-message through
[the implementations of coap-handler], which provide short-cuts for common cases.

[coap-message-demos]: https://codeberg.org/chrysn/coap-tools/src/branch/main/coap-message-demos
[the implementations of coap-handler]: https://docs.rs/coap-handler-implementations/latest/coap_handler_implementations/

If you are on the implementing side of the message traits, you may want to have a look at
[coap-message-utils], which contains useful building blocks.

[coap-message-utils]: https://crates.io/crates/coap-message-utils

Error handling
--------------

The readable message types are designed with minimal fallability -- for example, iterating over
the options can not raise a "premature end of message" style error, and the payload is always
"present" (albeit possibly empty). This encodes the concept that having a message type implies
validity of the message.

Libraries that insist on minimizing the cycles spent parsing (as typical in embedded
situations) can opt to express the formatting error as a critical, not-safe-to-forward option
(eg. the private-use number 65535). As consumers of a message that has not been checked in
advance must check all options before acting on the message anyway, this ensures that the error
does not go unnoticed.

Operations on writable messages are fallible. The typical error cause there is overflowing the
size of message that the transport can carry (the path MTU on UDP, or the peer's
Max-Message-Size on TCP) or the size of the allocated buffer. Errors resulting from the stack
not implementing certain features (eg. not implementing all options or codes) can be checked
ahead of attempted use because they use the associated types [Code] and [OptionNumber] that are
created fallibly.

License: MIT OR Apache-2.0
