HTTP Developer’s Handbook

Upgrade

The Upgrade header provides a method for the Web server and Web client to negotiate a change in protocol. The typical negotiation takes place with the client requesting the upgrade in protocol in the HTTP request, such as the following example request:

GET / HTTP/1.1 
Host: 127.0.0.1 
Upgrade: TLS/1.0 
Connection: Upgrade 

The Web server, as discussed in the section on status codes, will use a 101 Switching Protocols status code in the HTTP response to indicate a willingness to upgrade. Along with this, it also includes an Upgrade header and the same value for the Connection general header as used in the HTTP request:

HTTP/1.1 101 Switching Protocols 
Upgrade: TLS/1.0, HTTP/1.1 
Connection: Upgrade 

After the Web client receives a successful ...