Constrained Application Protocol (COAP)#
The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained networks in the Internet of Things. The protocol is designed for machine-to-machine (M2M) applications such as smart energy and building automation.
Based on Representational State Transfer (REST).
Header Format:#
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) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- Ver: Version Number (currently 0b01)
- T: Type (0: Confirmable, 1: Non-confirmable, 2: Acknowledgement, 3: Reset)
- Code: Upper 3 MSBs: Class, 5 LSBs: Class details
- Message ID: used to prevent duplicates, same for the ACK
- Token: used to correlate request and response
- Payload Marker:
Options#
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| delta |length | 1 byte
+---------------+
\ delta ext. \ 0-2 byte
+---------------+
\ length ext. \ 0-2 byte
+---------------+
\ value \ 0 or more bytes
+---------------+
Features#
- Embedded web transfer protocol (
coap://) - UDP binding with reliability and multicast support
- GET, POST, PUT, DELETE methods
- URI support
- 4 byte header
- Subset of MIME types and HTTP response codes
REST request: CON GET /light
- Problem: REST paradigm is often "PULL" type Solution: COAP observation Fragmentation at application layer to avoid fragmentation at IPv6 layer.