--http2

Use HTTP/2.

For HTTPS, this means curl negotiates HTTP/2 in the TLS handshake. curl does this by default.

For HTTP, this means curl attempts to upgrade the request to HTTP/2 using the Upgrade: request header.

When curl uses HTTP/2 over HTTPS, it does not itself insist on TLS 1.2 or higher even though that is required by the specification. A user can add this version requirement with --tlsv1.2.