This TransportProtocol sends OPC UA Connection Protocol messages over WebSockets.
WebSockets is a bi-directional protocol for communication via a web server which is commonly used by browser based applications to allow the web server to asynchronously send information to the client. WebSockets uses the same default port as HTTP or HTTPS and initiates communication with an HTTP request. This makes it very useful in environments where firewalls limit traffic to the ports used by HTTP or HTTPS.
WebSockets use HTTP, however, in practice a WebSocket connection is only initiated with a HTTP GET request and the web server provides an HTTP response. After that exchange, all traffic uses the binary framing protocol defined by RFC 6455.
A Server that supports the WebSockets transport shall publish one or more Endpoints with the scheme ‘opc.wss’. The TransportProfileUri shall be one of the URIs for WebSockets transports defined in OPC 10000-7. The TransportProfileUri specifies the encoding and security protocol used to construct the OPC UA messages sent via the WebSocket.
The SecurityMode and SecurityPolicyUri of the Endpoint control the security applied to the messages sent via the WebSocket. This allows the messages to be secure even if the WebSocket connection is established via untrusted HTTPS proxies.
Figure 17 summarizes the complete process for establishing communication over a WebSocket.
Figure 17 – Setting up Communication over a WebSocket
The figure assumes the opcua+uacp protocol mapping (see 7.5.2).