Expand description

Low-level representation of CoAP messages.

The most notable item in kwap_msg is Message; a CoAP message very close to the actual byte layout.


CoAP messages have some attributes whose size is dynamic:

  • The message payload (in http terms: the request/response body)
  • the number of options (in http terms: headers)
  • the value of an option (in http terms: header value)

Message does not require an allocator and has no opinions about what kind of collection it uses internally to store these values.

It solves this problem by being generic over the collections it needs and uses an Array trait to capture its idea of what makes a collection useful.

This means that you may use a provided implementation (for Vec or tinyvec::ArrayVec) or provide your own collection (see the custom collections example)

//! Note: both of these type aliases are exported by `kwap_msg` for convenience.

use tinyvec::ArrayVec;
use kwap_msg::{Message, Opt};

//                        Message Payload byte buffer
//                        |
//                        |        Option Value byte buffer
//                        |        |
//                        |        |        Array of options in the message
//                        vvvvvvv  vvvvvvv  vvvvvvvvvvvvvvvvv
type VecMessage = Message<Vec<u8>, Vec<u8>, Vec<Opt<Vec<u8>>>>;

// Used like: `ArrayVecMessage<1024, 256, 16>`; a message that can store a payload up to 1024 bytes, and up to 16 options each with up to a 256 byte value.
type ArrayVecMessage<
       const PAYLOAD_SIZE: usize,
       const OPT_SIZE: usize,
       const NUM_OPTS: usize,
     > = Message<
           ArrayVec<[u8; PAYLOAD_SIZE]>,
           ArrayVec<[u8; OPT_SIZE]>,
           ArrayVec<[Opt<ArrayVec<[u8; OPT_SIZE]>>; NUM_OPTS]>,

It may look a little ugly, but a core goal of kwap is to be platform- and alloc-agnostic.


This crate uses criterion to measure performance of the heaped & heapless implementations in this crate as well as coap_lite::Packet.

In general, kwap_msg::VecMessage performs identically to coap_lite (+/- 5%), and both are much faster than kwap_msg::ArrayVecMessage.


Serializing to bytes

Click to expand chart


Deserializing from bytes

Click to expand chart



CoAP Code Registries

Iterator yielded by EnumerateOptNumbers, wrapping an Iterator over Opts.

Message ID

Message struct


The “Option Delta” is the difference between this Option’s Number and the previous Option’s number.

Option Numbers

Option Value Formats

Payloads and Representations


Version of the CoAP protocol that the message adheres to.


Whether a code is for a request, response, or empty message

Errors encounterable while parsing a message from bytes

Errors encounterable while parsing an option from bytes


Indicates if this message is of type Confirmable (0), Non-confirmable (1), Acknowledgement (2), or Reset (3).

Proxy Unsafe or Safe-to-Forward and NoCacheKey

Proxy Unsafe or Safe-to-Forward and NoCacheKey


Creates an iterator which gives the current opt’s number as well as the option.

Trait for converting a sequence of bytes into some data structure

Trait allowing fallible conversion into bytes

Type Definitions

Message that uses static fixed-capacity stack-allocating byte buffers

Message that uses Vec byte buffers