HTTP/1.1 Upgrade header
HTTP/1.1 introduced support for the Upgrade header field. In the exchange, the client begins by making a clear-text request, which is later upgraded to a newer http protocol version or switched to a different protocol. Connection upgrade must be requested by the client, if the server wants to enforce an upgrade it may send a "426 upgrade required" response. The client can then send a new request with the appropriate upgrade headers.
Use with TLS
One use is to begin a request on the normal http port but switch to Transport Layer Security (TLS). In practice such use is rare with the https URL scheme being a far more common way to initiate encrypted http.
The server returns a 426 status-code to alert legacy clients that the failure was client-related (400 level codes indicate a client failure: List of HTTP status codes).
This method for establishing a secure connection is advantageous because it:
- Does not require messy and problematic redirection and URL rewriting on the server side.
- Enables virtual hosting of secured websites (although HTTPS also allows this using Server Name Indication).
- Reduces the potential for user confusion by providing a single way to access a particular resource.
A disadvantage of this method is that the client cannot specify the requirement for a secure HTTP in the URI. Therefore a man-in-the-middle may maintain an unencrypted and unauthenticated connection with the client while maintaining an encrypted connection with the server.
Use with WebSockets
WebSocket also uses this mechanism to set up a connection with a HTTP server in a compatible way. The WebSocket Protocol has two parts: a handshake to establish the upgraded connection, then the actual data transfer. First, a client requests a websocket connection by using the "Upgrade: websocket" and "Connection: Upgrade" headers, along with a few protocol-specific headers to establish the version being used and set-up a handshake. The server, if it supports the protocol, replies with the same "Upgrade: websocket" and "Connection: Upgrade" headers and completes the handshake. Once the handshake is completed successfully, data transfer begins.
Use with HTTP/2
HTTP Upgrade mechanism is used to establish HTTP/2 starting from plain http. The client starts a HTTP/1.1 connection and sends "Upgrade: h2c" header. If the server supports HTTP/2, it replies with HTTP 101 Switching Protocol status code.