Imagine: a hundred temperature sensors in a warehouse, each with 10 KB RAM, sending data to a technician's mobile tablet. HTTP is too heavy, MQTT requires a broker. The solution is CoAP (Constrained Application Protocol, described in RFC 7252). As mobile developers, we often face the integration of such a protocol. CoAP's semantics are the same: GET, POST, PUT, DELETE, response codes (2.05 Content = 200, 4.04 Not Found = 404). But it runs over UDP, takes tens of bytes instead of hundreds, and is designed for devices with 10 KB RAM and battery power. A mobile app communicating directly with such devices or through a CoAP proxy is a niche but growing scenario: Thread networks (Matter), industrial sensors, medical wearables. CoAP mobile app integration is a growing niche. CoAP IoT communication is ideal for constrained devices.
How CoAP solves reliability over UDP
CoAP defines two message types:
- CON (Confirmable) – sender waits for ACK. On timeout, exponential backoff and retry (default: 2 attempts, 2–32 second interval). Analogous to TCP guarantees.
- NON (Non-confirmable) – fire and forget. For high-frequency telemetry.
Each CON message carries a Message ID (2 bytes) for deduplication — the receiver caches the last processed IDs and ignores duplicates. Cache lifetime is EXCHANGE_LIFETIME (247 seconds per RFC). When working through NAT from mobile to IoT network, note that NAT bindings also expire (~30 seconds for UDP through many carrier NATs). CoAP's congestion control mechanism implements exponential backoff with a random factor, adhering to RFC 7252's RECOMMENDED values.
What is Observe and how to implement it on mobile?
RFC 7641 adds the Observe option — an analog of WebSocket subscription for CoAP. The client sends GET /sensor/temperature with Observe: 0 (subscribe). The server sends the current value and then notifications on each change. Observe: 1 unsubscribes. The Observe CoAP feature allows server push, reducing network traffic by up to 90% compared to polling every 5 seconds.
This is key for mobile IoT clients: instead of polling every 5 seconds, a single request triggers server pushes. The caveat: per RFC, the server maintains a list of observers. When the client's IP changes (mobile data, Wi-Fi → 4G), the server is unaware — you must re-register the Observe from the new address. In practice, on each network change (detected via ConnectivityManager.NetworkCallback on Android, NWPathMonitor on iOS), we resend all active Observe requests.
DTLS: security over UDP
CoAP without encryption is unsafe for production. DTLS (Datagram TLS, RFC 6347) is the TLS analog for UDP. The handshake is heavier than TCP TLS (4–6 RTT vs 1–2 for TLS 1.3), which is critical for devices with slow CPUs. DTLS CoAP security is essential for production.
CoAP security profiles:
- NoSec – no encryption. Only for isolated networks.
- PreSharedKey (PSK) – symmetric key flashed into the device at the factory. Most common in industrial scenarios.
- RawPublicKey – no PKI infrastructure, but with public keys.
- Certificate – full PKI. Rare on constrained devices.
For a mobile client connecting to PSK devices: store PSK keys securely (Keychain / EncryptedSharedPreferences) and implement DTLS handshake via a library with PSK support.
CoAP vs MQTT: which to choose for IoT?
CoAP wins over MQTT in message size (2-3 times smaller) and requires no broker, but loses in guaranteed delivery and complex queuing support. Specifically, CoAP header is only 4 bytes, which is 8 times smaller than an HTTP header and 3 times smaller than MQTT's minimum header. For "sensor-app" scenarios with infrequent requests, CoAP is more efficient; for constant data streams with many endpoints, MQTT is better.
Client libraries for mobile platforms
CoAP libraries for mobile are significantly fewer than MQTT. Real options:
| Platform | Library | DTLS | Observe |
|---|---|---|---|
| Android | Californium (Eclipse) | + (via Scandium) | + |
| iOS | libcoap (C, via FFI) | + | + |
| Flutter | coap (pub.dev) | partial | + |
| React Native | none ready | — | — |
For React Native, the only workable path is a native module (Californium on Android, libcoap on iOS through Objective-C bridge) or a CoAP-to-HTTP proxy on the backend.
Californium (org.eclipse.californium:californium-core) is the most mature implementation. The most mature CoAP Android library is Californium. Observe via CoapClient.observe() with CoapHandler. DTLS via the separate Scandium artifact. Example of PSK connection initialization:
DtlsConnectorConfig config = new DtlsConnectorConfig.Builder() .setPskStore(new StaticPskStore("device-id", pskBytes)) .build(); DTLSConnector connector = new DTLSConnector(config); CoapEndpoint endpoint = new CoapEndpoint.Builder() .setConnector(connector).build(); CoapClient client = new CoapClient("coaps://192.168.1.100/sensor/temperature"); client.setEndpoint(endpoint); For iOS, libcoap is compiled via CocoaPods with a custom podspec or via SPM as a binary target (needs rebuild for arm64/x86_64). For CoAP iOS integration, we use libcoap via FFI. That's not a 15-minute task.
CoAP through a proxy: when direct is not needed
If devices are in an isolated IoT network and the mobile client must communicate over the internet, a CoAP-to-HTTP proxy (or CoAP-to-MQTT proxy) removes the complexity from the mobile side. Eclipse Hono, AWS IoT Core with CoAP endpoint, or a self-hosted Californium proxy. The mobile client works over plain HTTPS/WebSocket, the proxy translates to CoAP. We lose direct Observe (need emulation via SSE or WebSocket at the proxy level) but gain simplicity and TCP reliability on the client side.
DTLS handshake implementation details
When using PSK, the handshake consists of 4 steps: ClientHello, ServerHello (with PSK identity), ChangeCipherSpec, Finished. On constrained devices, this can take up to 2 seconds. We recommend caching the DTLS session (session ID or session ticket) for reconnections.What's included in our work
- Development of the exchange protocol (resource model, payload formats).
- Implementation of Observe on the mobile side with network change handling.
- DTLS configuration (PSK or certificates) and integration with Keychain/EncryptedSharedPreferences.
- Testing on real devices and network emulators.
- Integration documentation (diagrams, request examples).
- Consultation on infrastructure choices (proxy or no proxy).
- Typical project cost ranges from $15,000 to $30,000 depending on complexity and platform coverage.
We are a team of mobile developers with 10+ years of experience and 40+ projects in IoT. If you have a similar task, contact us — we'll assess the complexity and propose an architecture for your scenario.
Assessment and timelines
CoAP integration is a non-standard task. Timelines depend on the scenario: if through a proxy — 2–3 weeks. Direct CoAP with DTLS on native platforms — 4–8 weeks, including Californium/libcoap setup, DTLS handshake, and Observe. We always verify the device security profile and available network infrastructure before estimating.