https://devzone.nordicsemi.com/f/nordic-q-a/86527/thread-coaps-dts-handshake-failsHi, With the NRF Connect SDK version 1.9.1 it is not possible to setup a Thread Coaps connection. The DTLS handshake fails after Client Hello with an Alert message (Illegal Parameter). Error is triggered in the mbedTLS library =&gt; ssl_srv.c =&gt;en-USTelligent Community 13Mon, 04 Apr 2022 09:25:32 GMThttps://devzone.nordicsemi.com/thread/361439?ContentTypeID=1Mon, 04 Apr 2022 09:25:32 GMT137ad170-7792-4731-bb38-c0d22fbe4515:0e54dd62-dced-4047-8174-0887c16b3699Amanda Hsieh<p>Hi,&nbsp;</p> <p>Bug identified in mbedtls SSL Client:&nbsp;<a href="https://github.com/nrfconnect/sdk-mbedtls/blob/development/library/ssl_cli.c#L1248-L1256" rel="nofollow noopener noreferrer" target="_blank">https://github.com/nrfconnect/sdk-mbedtls/blob/development/library/ssl_cli.c#L1248-L1256</a></p> <p>ECJPAKE extension is added unconditionally to the Client Hello message.</p> <p>Bugfix can be fixed&nbsp;by adding the extension only when `uses_ec` is True (<a href="https://github.com/nrfconnect/sdk-mbedtls/blob/development/library/ssl_cli.c#L1228" rel="nofollow noopener noreferrer" target="_blank">https://github.com/nrfconnect/sdk-mbedtls/blob/development/library/ssl_cli.c#L1228</a>).</p> <p>Regards,<br />Amanda</p><div style="clear:both;"></div>