Sec-WebSocket-Protocol header
BaselineWidely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
The HTTPSec-WebSocket-Protocolrequest andresponse header is used in theWebSocket openinghandshake to negotiate asub-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 theprotocols argument to theWebSocket().The sub-protocol selected by the server is made available to the web application inWebSocket.protocol.
| Header type | Request header,Response header |
|---|---|
| Forbidden request header | Yes (Sec- prefix) |
Syntax
Sec-WebSocket-Protocol: <sub-protocols>Directives
<sub-protocols>A comma-separated list of sub-protocol names, in the order of preference.The sub-protocols may be selected from theIANA WebSocket Subprotocol Name Registry, or may be a custom name jointly understood by the client and the server.
As a response header, this is a single sub-protocol that the server selected.
Examples
WebSocket opening handshake
The sub-protocol is specified in the original WebSockethandshake request.The request below shows that the client preferssoap, but also supportswamp.
GET /chat HTTP/1.1Host: example.com:8000Upgrade: websocketConnection: UpgradeSec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==Sec-WebSocket-Version: 13Sec-WebSocket-Protocol: soap, wampSpecifying the protocols like this has the same effect:
Sec-WebSocket-Protocol: soapSec-WebSocket-Protocol: wampThe response from the server will include theSec-WebSocket-Protocol header, selecting the first sub-protocol that it supports from the client's preferences.Below that is shown assoap:
HTTP/1.1 101 Switching ProtocolsUpgrade: websocketConnection: UpgradeSec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=Sec-WebSocket-Protocol: soapSpecifications
| Specification |
|---|
| The WebSocket Protocol # section-11.3.4 |
Browser compatibility
See also
Sec-WebSocket-AcceptSec-WebSocket-KeySec-WebSocket-VersionSec-WebSocket-Extensions- The WebSocket handshake andSubprotocols inWriting WebSocket servers