Other parts of this series of standards are written to be independent of the technology used for implementation. This approach means OPC UA is a flexible specification that will continue to be applicable as technology evolves. On the other hand, this approach means that it is not possible to build an OPC UA application with the information contained in OPC 10000-1 through to OPC 10000-5 because important implementation details have been left out.
This standard defines Mappings between the abstract specifications and technologies that can be used to implement them. The Mappings are organized into three groups: DataEncodings, SecurityProtocols and TransportProtocols. Different Mappings are combined together to create StackProfiles. All OPC UA applications shall implement at least one StackProfile and can only communicate with other OPC UA applications that implement the same StackProfile.
This standard defines the DataEncodings in Clause 5, the SecurityProtocols in Clause 5.4 and the TransportProtocols in 6.7.6. The StackProfiles are defined in OPC 10000-7.
All communication between OPC UA applications is based on the exchange of Messages. The parameters contained in the Messages are defined in OPC 10000-4; however, their format is specified by the DataEncoding and TransportProtocol. For this reason, each Message defined in OPC 10000-4 shall have a normative description which specifies exactly what shall be put on the wire. The normative descriptions are defined in the appendices.
A Stack is a collection of software libraries that implement one or more StackProfiles. The interface between an OPC UA application and the Stack is a non-normative API which hides the details of the Stack implementation. An API depends on a specific DevelopmentPlatform. Note that the datatypes exposed in the API for a DevelopmentPlatform may not match the datatypes defined by the specification because of limitations of the DevelopmentPlatform. For example, Java does not support an unsigned integer which means that any Java API will need to map unsigned integers onto a signed integer type.
Figure 1 illustrates the relationships between the different concepts defined in this standard.
Figure 1 – The OPC UA Stack Overview
The layers described in this specification do not correspond to layers in the OSI 7-layer model [X200]. Each OPC UA StackProfile should be treated as a single Layer 7 (application) protocol that is built on an existing Layer 5, 6 or 7 protocol such as TCP/IP, TLS or HTTP. The SecureChannel layer is always present even if the SecurityMode is None. In this situation, no security is applied but the SecurityProtocol implementation shall maintain a logical channel with a unique identifier. Users and administrators are expected to understand that a SecureChannel with SecurityMode set to None cannot be trusted unless the application is operating on a physically secure network or a low-level protocol such as IPSec is being used.