I’ve finally got confirmation that the SIP ALPN identifier is registered. https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype… For Kamailio, this could assist when we multiplex multiple protocols on the same port, like we do for HTTPs and SIP/TLS many times. ALPN Is part of the TLS handshake, indicating what protocol comes after TLS is done. For it to help, both phones and servers needs to support it. I do think it would be helpful. Cheers, /O