DTLS cookies are disabled - Vulnerable to amplification attacks

[NZSmartie]

mbedTLS is a stateless TLS library, which requires TLS sessions to be restarted if a client's cookie is invalid (or missing) before Hello Verification can succeed. The default cookie callback functions are not intended to work right out of the box for security reasons.
But I'm lazy and have concluded that since these devices will initially be running on a network with no access to the internet, the risk is low.

Please read mbed's DTLS Tutorial for more information, especially the heading MBEDTLS_ERR_SSL_HELLO_VERIFY_REQUIRED