Sec-WebSocket-Protocol
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
The HTTP Sec-WebSocket-Protocol
request and response header is used in the WebSocket opening handshake to negotiate a sub-protocol to use in the communication.
This can be a well understood protocol, such as SOAP or WAMP, or a custom protocol understood by the client and server.
In a request the header specifies one or more WebSocket sub-protocols that the web application would like to use, in order of preference. These can be added as protocol values in multiple headers, or as comma separate values added to a single header.
In a response it specifies the sub-protocol selected by the server. This must be the first sub-protocol that the server supports from the list provided in the request header.
The request header is automatically added and populated by the browser using values specified by the application in the protocols
argument to the WebSocket()
.
The sub-protocol selected by the server is made available to the web application in WebSocket.protocol
.
Header type | Request header, Response header |
---|---|
Forbidden header name | Yes (Sec- prefix) |
Syntax
Request:
Sec-WebSocket-Protocol: <sub-protocols>
Response:
Sec-WebSocket-Protocol: <selected-sub-protocol>
Directives
<sub-protocols>
-
A comma-separated list of sub-protocol names, in the order of preference. The sub-protocols may be selected from the IANA WebSocket Subprotocol Name Registry, or may be a custom name jointly understood by the client and the server.
Examples
WebSocket opening handshake
The sub-protocol is specified in the original WebSocket handshake request.
The request below shows that the client prefers soap
, but also supports wamp
.
GET /chat HTTP/1.1
Host: example.com:8000
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13
Sec-WebSocket-Protocol: soap, wamp
Specifying the protocols like this has the same effect:
Sec-WebSocket-Protocol: soap
Sec-WebSocket-Protocol: wamp
The response from the server will include the Sec-WebSocket-Protocol
header, selecting the first sub-protocol that it supports from the client's preferences.
Below that is shown as soap
:
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
Sec-WebSocket-Protocol: soap
Specifications
Specification |
---|
The WebSocket Protocol # section-11.3.4 |
Browser compatibility
BCD tables only load in the browser
See also
Sec-WebSocket-Accept
Sec-WebSocket-Key
Sec-WebSocket-Version
Sec-WebSocket-Extensions
- The WebSocket handshake and Subprotocols in Writing WebSocket servers