The HTTP Upgrade request and response header can be used to upgrade an already-established client/server connection to a different protocol (over the same transport protocol). For example, it can be used by a client to upgrade a connection from HTTP/1.1 to HTTP/2, or an HTTP(S) connection to a WebSocket connection.
Warning: HTTP/2 explicitly disallows the use of this mechanism and header; it is specific to HTTP/1.1.
SyntaxA comma-separated list of one or more protocols:
Upgrade: <protocol>[/<protocol_version>]
Upgrade: <protocol>[/<protocol_version>], â¦, <protocolN>[/<protocol_versionN>]
<protocol>
Protocols are listed, comma-separated, in order of descending preference.
<protocol_version> Optional
An optional protocol version may be provided prefixed with a / forward slash.
The Upgrade header field may be used by clients to invite a server to switch to one (or more) of the listed protocols, in descending preference order. For example, the client might send a GET request as shown, listing the preferred protocols to switch to (in this case example/1 and foo/2):
GET /index.html HTTP/1.1
Host: www.example.com
Connection: upgrade
Upgrade: example/1, foo/2
Note: The Connection header with type upgrade must always be sent with the Upgrade header.
The server can ignore the request, for any reason, in which case it should respond as though the Upgrade header had not been sent (for example, with a 200 OK). If the server will upgrade the connection, it must:
Send back a 101 Switching Protocols response status with an Upgrade header that specifies the protocol(s) being switched to. For example:
HTTP/1.1 101 Switching Protocols
Upgrade: foo/2
Connection: Upgrade
Send a response to the original request using the new protocol (the server may only switch to a protocol with which it can complete the original request).
A server may also send the header as part of a 426 Upgrade Required response, to indicate that the server won't perform the request using the current protocol, but might do so if the protocol is changed. The client can then request a protocol change using the process above.
More detail and examples are provided in the topic Protocol upgrade mechanism.
ExamplesThe following request lists multiple protocols in descending preference:
Connection: upgrade
Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
This is a common combination of headers to use to begin upgrading a HTTP connection to WebSockets. See Upgrading to a WebSocket connection for more information.
Connection: Upgrade
Upgrade: websocket
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