OPC UA Connection Protocol (UACP) is an abstract protocol that establishes a full duplex channel between a Client and Server. Concrete implementations of the UACP can be built with any middleware that supports full-duplex exchange of messages including TCP/IP and WebSockets. The term “TransportConnection” describes the middleware specific connection used to exchange messages. For example, a socket is the TransportConnection for TCP/IP. TransportConnections allow responses to be returned in any order and allow responses to be returned on a different physical TransportConnection if communication failures cause temporary interruptions.

The OPC UA Connection Protocol is designed to work with the SecureChannel implemented by a layer higher in the stack. For this reason, the OPC UA Connection Protocol defines its interactions with the SecureChannel in addition to the wire protocol.