Upgrade
| Upgrade |
The Upgrade header provides the sender of a message with a means of broadcasting the desire to use another, perhaps completely different, protocol. For instance, an HTTP/1.1 client could send an HTTP/1.0 request to a server and include an Upgrade header with the value "HTTP/1.1", allowing the client to test the waters and see whether the server speaks HTTP/1.1.
If the server is capable, it can send an appropriate response letting the client know that it is okay to use the new protocol. This provides an efficient way to move to other protocols. Most servers currently are only HTTP/1.0-compliant, and this strategy allows a client to avoid confusing a server with too many HTTP/1.1 headers until it determines whether the server is indeed capable of speaking HTTP/1.1.
When a server sends a 101 Switching Protocols response, it must include this header.
Type
General header
Basic Syntax
Upgrade: 1# protocol
Example
Upgrade: HTTP/2.0