CoAP is the Constrained Application Protocol. It is a specialized web transfer protocol for constrained nodes and networks.

What is a constrained node?

A node is often an 8-bit microcontroller with small amounts of ROM and RAM. These are often the microcontrollers that are used in IoT devices.

What is an 8-bit microcontroller?

A microcontroller is a single computer chip with a CPU, memory and I/O peripherals. Microcontrollers are designed for embedded applications in compact devices as it does not need external components. They use low amounts of power and can be often powered using batteries. 8-bit microcontroller means that the microcontroller processes 8-bits of data at a time. For example, the size of the registers is 8 bits per register, the number of memory addresses is 2^8 = 256 addresses and the largest numbers that can be processed are 2^8 = 256 integers (0 - 255).

What is a constrained network?

A constrained network is a low-power or lossy network such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs). These networks often have high packet error rates and low throughput.

Back to CoAP

CoAP is similar to HTTP but it is optimized specifically for IoT machine to machine (M2M) data exchange.

CoAP uses a datagram-oriented transport such as UDP

                        +----------------------+
                        |      Application     |
                        +----------------------+
                        +----------------------+  \
                        |  Requests/Responses  |  |
                        |----------------------|  | CoAP
                        |       Messages       |  |
                        +----------------------+  /
                        +----------------------+
                        |          UDP         |
                        +----------------------+

                        Abstract Layering of CoAP

Message types:

Confirmable
Non-confirmable
Acknowledgement
Reset

Messaging Model

Each CoAP messages has a 4 byte header followed by binary options and a payload.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Ver| T |  TKL  |      Code     |          Message ID           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Token (if any, TKL bytes) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Options (if any) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1 1 1 1 1 1 1 1|    Payload (if any) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            Message Format

Request/Response Model

CoAP uses request and response semantics. Every message includes either a Method Code or a Response Code.

A request can be either

Confirmable (CON)
Non-confirmable (NON).

Confirmable requests guarantee message reliability. Confirmable messages will be resent until the client receives a successful Acknowledgement (ACK) message. If the response to a Confirmable request is immediately available, the response is added to the Acknowledgement message. This is known as a piggybacked response.

CoAP messages use a Token to match a request with a response.

        Client              Server       Client              Server
           |                  |             |                  |
           |   CON [0xbc90]   |             |   CON [0xbc91]   |
           | GET /temperature |             | GET /temperature |
           |   (Token 0x71)   |             |   (Token 0x72)   |
           +----------------->|             +----------------->|
           |                  |             |                  |
           |   ACK [0xbc90]   |             |   ACK [0xbc91]   |
           |   2.05 Content   |             |  4.04 Not Found  |
           |   (Token 0x71)   |             |   (Token 0x72)   |
           |     "22.5 C"     |             |   "Not found"    |
           |<-----------------+             |<-----------------+
           |                  |             |                  |

                Two GET Requests with Piggybacked Responses

On the other hand, if the server is not able to respond immediately with the resulting data for a Confirmable request, it can just reply with an empty Acknowledgement message. Then when the response is ready, it can reply to the client with a new Confirmable message. This is known as a separate response.

                        Client              Server
                           |                  |
                           |   CON [0x7a10]   |
                           | GET /temperature |
                           |   (Token 0x73)   |
                           +----------------->|
                           |                  |
                           |   ACK [0x7a10]   |
                           |<-----------------+
                           |                  |
                           ... Time Passes  ...
                           |                  |
                           |   CON [0x23bb]   |
                           |   2.05 Content   |
                           |   (Token 0x73)   |
                           |     "22.5 C"     |
                           |<-----------------+
                           |                  |
                           |   ACK [0x23bb]   |
                           +----------------->|
                           |                  |

                    A GET Request with a Separate Response

For the case of Non-confirmable requests, the server can reply with a Confirmable or Non-confirmable message.

                        Client              Server
                           |                  |
                           |   NON [0x7a11]   |
                           | GET /temperature |
                           |   (Token 0x74)   |
                           +----------------->|
                           |                  |
                           |   NON [0x23bc]   |
                           |   2.05 Content   |
                           |   (Token 0x74)   |
                           |     "22.5 C"     |
                           |<-----------------+
                           |                  |

              A Request and a Response Carried in Non-confirmable Messages

If the server has trouble with the incoming requests, they can send back a Reset (RST) message.

The same process can be repeated for PUT, POST and DELETE methods.

DTLS

CoAP uses Datagram TLS over UDP or DTLS.

CoAP for IoT