In a TLS connection, the client and server uses the Application Layer Protocol Negotiation (ALPN) extension to find out the application protocol. In the starting of the TLS handshake, the client takes the help of the ALPN extension for sending the number of application protocols reinforced to the server during the ClientHello phase. When the server receives the ClientHello message, it selects the preferred protocols from the number of protocols received from the client and informs the client through the ServerHello message. Even if no protocol is chosen, the server confirms back to the client in the ServerHello message:

Using the preceding set of methods, the communication is established in two stages:

• Client-side protocol negotiation

• Server-side protocol negotiation

Within Server-Side Protocol Negotiation, there is a default and custom negotiation mechanisms. If no negotiated ALPN is decided, the server can throw an exception or ignore the ALPN values sent by the client.