426 Upgrade Required

Takeaways

1. The server refuses to perform the request using the current protocol but might be willing to do so after the client upgrades to a different protocol.
2. The server must send an Upgrade header field in a 426 response to indicate the required protocol(s).

Example

HTTP/1.1 426 Upgrade Required
Upgrade: HTTP/3.0
Connection: Upgrade
Content-Length: 53
Content-Type: text/plain

This service requires use of the HTTP/3.0 protocol.