Upgrade
The HTTP 1.1 (only) Upgrade
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 2.0, or an HTTP or HTTPS connection into a WebSocket.
Warning: HTTP/2 explicitly disallows the use of this mechanism/header; it is specific to HTTP/1.1.
Header type | Request header, Response header |
---|---|
Forbidden header name | yes |
Overview
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"):
http
GET /index.html HTTP/1.1
Host: www.example.com
Connection: upgrade
Upgrade: example/1, foo/2
Note: Connection: upgrade
must be set whenever Upgrade
is sent.
The server can choose to ignore the request, for any reason, in which case it should just respond as though the Upgrade
header had not been sent (for example, with a 200 OK
).
If the server decides to upgrade the connection, it must:
- Send back a
101 Switching Protocols
response status with anUpgrade
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.
Syntax
http
Connection: upgrade
Upgrade: protocol_name[/protocol_version]
Notes:
- The
Connection
header with typeupgrade
must always be sent with theUpgrade
header (as shown above). - Protocols are listed, comma-separated, in order of descending preference. Protocol version is optional. For example:
http
Connection: upgrade
Upgrade: a_protocol/1, example, another_protocol/2.2
Directives
- any comma-separated list protocol names (each with optional protocol version)
-
One or more protocol names with optional version ("/" separated). The protocols are listed in order of descending preference.
Examples
http
Connection: upgrade
Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
http
Connection: Upgrade
Upgrade: websocket
Specifications
Specification |
---|
HTTP Semantics # field.upgrade |
HTTP Semantics # status.426 |
HTTP/2 # informational-responses |
Browser compatibility
BCD tables only load in the browser
See also
- Protocol upgrade mechanism
101
Switching Protocol
426
Upgrade Required
Connection