CoAP
HTTP's tiny cousin for constrained devices — request/response over UDP when even MQTT is too heavy.
In one line
CoAP (Constrained Application Protocol) brings the familiar web model — GET/PUT/POST on URLs — to devices so small that running TCP and HTTP would drain the battery.
How it works
CoAP runs over UDP with a 4-byte header. Requests and responses mirror REST
semantics, so a sensor exposes resources like coap://device/vitals/hr that a gateway
simply GETs. Reliability is opt-in per message ("confirmable" messages get
acknowledgements), and the observe extension turns a GET into a subscription — the
device pushes a notification whenever the value changes. DTLS adds the security layer
TLS provides for TCP.
Where it shows up in digital health
Battery-powered wearables and implant-adjacent sensors where every milliwatt counts; smart-home health devices behind a hub; constrained nodes that translate to MQTT or HTTP at a gateway. The practical rule of thumb you'll use in the IoT lab: CoAP at the edge, MQTT between gateway and cloud, FHIR after that.