Run DTLS on Embedded Devices

DTLS secures UDP transports — the usual choice for sensors, industrial gateways, and real-time control where TCP is too heavy or unsuitable. NoxTLS ships DTLS 1.2 (RFC 6347) and DTLS 1.3 (RFC 9147) alongside TLS.

When to use DTLS

Use DTLS	Use TLS instead
CoAP, MQTT-SN, custom UDP protocols	HTTPS, MQTT over TCP
Lossy links with application-level retry	Reliable streaming APIs
Multicast/broadcast-adjacent designs (unicast DTLS)	Browser-grade HTTPS

Start with the in-process demo

The fastest way to validate your build is dtls_psk_demo — no sockets required:

cmake -S . -B build -D BUILD_TESTS=OFF
cmake --build build --config Release
./binary/dtls_psk_demo
./binary/dtls_psk_demo 1.3
./binary/dtls_psk_demo --chacha 1.2

This exercises PSK handshakes for DTLS 1.2 and 1.3. See DTLS PSK demo for OpenSSL interop commands when you add real UDP.

DTLS on real UDP (embedded pattern)

NoxTLS does not own your socket API. You:

1. Open a UDP socket (or RTOS datagram endpoint).
2. Initialize dtls_context_t via noxtls_dtls12_context_init or noxtls_dtls13_context_init.
3. Feed incoming datagrams to the DTLS record layer; send outbound records on the same socket.
4. Tune MTU, retransmit, and CID options on the shared DTLS base — see DTLS API.

MTU and fragmentation

Embedded links often have small MTUs (1500 down to 576 or lower). Set MTU explicitly:

• noxtls_dtls_set_mtu — accounts for IP/UDP overhead and DTLS header variants
• Handshake fragmentation and reassembly are handled inside the stack (DTLS 1.2 flights; DTLS 1.3 unified header + ACK/retransmit)

Loss and replay

Mechanism	DTLS 1.2	DTLS 1.3
Retransmission	Flight timers, flight buffers	ACK ranges, RTT-aware resend
Replay protection	Epoch + sequence window	Per-epoch replay window
DoS mitigation	Cookie exchange (HelloVerifyRequest)	Cookie + anti-amplification limits

Deep dive: DTLS 1.3 guide and TLS component — DTLS sections.

PSK vs certificate handshakes

Mode	Best for	Sample
PSK	Closed device fleets with pre-shared keys	dtls_psk_demo, DTLS PSK test
Certificates	Interop with standard PKI or cloud brokers	Same cert tooling as TLS — Configure Certificates

PSK is the quickest path on constrained MCUs. Certificate-based DTLS uses the same X.509 stack as TLS once UDP I/O is wired.

Connection ID (DTLS 1.3)

For NAT-heavy cellular networks, enable Connection ID negotiation so datagrams can be routed after address/port changes. APIs live under DTLS API (RequestConnectionId, NewConnectionId, rotation hooks). See DTLS 1.3.

RFC 9147 alignment

DTLS 1.3 wire format and key derivation follow RFC 9147. Peers must use compatible NoxTLS builds; older experimental DTLS 1.3 peers are not interoperable.

Embedded build tips

cmake -S . -B build \
  -D NOXTLS_PROFILE=minimal_tls_client \
  -D BUILD_APPLICATIONS=OFF \
  -D BUILD_TESTS=OFF

Then enable only the DTLS/TLS versions you need in Configuration Guide. Trim SHA algorithms, PKC, and PQC features you will not use to save flash.

On Zephyr, add NoxTLS with add_subdirectory, set NOXTLS_OMIT_UT_SOURCES ON, link zephyr_interface, and implement entropy + UDP send/recv — see Port NoxTLS to Your Platform.

OpenSSL interop sanity check

When your UDP path works, verify against OpenSSL 3.x using the commands in DTLS PSK demo (DTLS 1.2/1.3 PSK examples). Swap PSK identity and cipher suite names to match your configuration.

Next steps

• TCP/HTTPS path: Build Your First TLS Client
• Certificates: Configure Certificates
• Platform port: Port NoxTLS to Your Platform