Another major use of ALPN is in automated certificate issuance. It was discovered in early 2018 that the SNI based method used by Automatic Certificate Management Environment (ACME) protocol, for domain validation over https port, was insecure. This prompted a new challenge method for domain validation over ALPN.
ALPN protocol id “acme-tls/1” is assigned for this purpose. A supporting client and server will use “acme-tls/1” in ALPN extension to perform the domain validation. An interesting fact to note is, there is no application data.
The protocol consists of a TLS handshake in which the required validation information is transmitted. The “acme-tls/1” protocol does not carry application data. Once the handshake is completed, the client MUST NOT exchange any further data with the server and MUST immediately close the connection.