upgradeRequired constant
int
const upgradeRequired
upgradeRequired = 426.
This response code is used when 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. The server sends an Upgrade header with this response to indicate the required protocol(s).
https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/426
Implementation
static const int upgradeRequired = 426;