Application-Layer Protocol Negotiation (ALPN)

wikipedia:ALPN

curl --help
      --no-alpn       Disable the ALPN TLS extension

openssl s client -connect

See also

• TLS: OpenSSL, LibreSSL, WolfSSL, X.509, .pem, SNI, CT, OCSP, Mbed TLS, mTLS, ALPN, your connection is not private, SSL Certificate Checker, Wildcard certificate, JA3 fingerprint, sslcan, TLS inspection