IEC 62541-4:2025

Overview

IEC 62541-4:2025 - "OPC Unified Architecture - Part 4: Services" defines the abstract OPC UA Services used for all interactions between OPC UA Clients and Servers. It specifies the collection of abstract Remote Procedure Calls (RPCs) - discovery, secure channel and session management, node and view operations, attributes, methods, monitoring and subscriptions, history, query, redundancy and auditing - without mandating a specific transport or encoding. Concrete mappings (for example UA-TCP / UA-SC / UA-Binary) are provided in IEC 62541-6. This fourth edition (2025) replaces the 2020 edition and includes technical revisions and new service behaviors, security and subscription clarifications.

Key Topics

• Service Set model: Discovery, SecureChannel, Session, NodeManagement, View, Query, Attribute, Method, MonitoredItem, Subscription and related service behaviors.
• Request/response semantics and headers for OPC UA RPC-style services.
• Security and certificates: procedures for creating SecureChannels, determining if an application certificate is trusted, authentication (user impersonation and identity tokens) and continuous security checks.
• Subscriptions & monitored items: models for creating, modifying and publishing monitored data and events, including re-establishment, retransmission and durable subscriptions.
• Redundancy & re-establishing connections: server/client redundancy and failover behaviors.
• Data types & filters: definitions for DataValue, ContentFilter, AggregateFilter and monitoring filters.
• Service-specific behaviors: method calls (including optional method arguments), history read/update, query semantics, and session lifecycle (Create/Activate/Close).
• Annex A: BNF for browse path names.
• Notable additions in Edition 4: ECC support, new status codes (e.g., Bad_ServerTooBusy, Good_PasswordChangeRequired), INVALID entries for enumerations, subscription retransmission handling, IssuedIdentityToken expiration behavior, and tighter certificate and client/server checks.

Applications

• Implementing OPC UA Servers and Clients across industrial automation, IIoT, energy, building management and process control.
• Engineering secure, interoperable communications using OPC UA Services for discovery, browsing, read/write, method invocation, eventing and historical access.
• Developing SDKs, embedded device stacks and gateway products that conform to OPC UA service semantics.
• Auditing, security architects, and integrators implementing certificate trust, secure channels and session authorization.

Related Standards

• IEC 62541-6 - OPC UA: Mappings (concrete encodings and transports such as UA-Binary over UA-TCP).
• IEC 62541-7 - OPC UA: Profiles (which services are required for specific conformance profiles).

Keywords: IEC 62541-4:2025, OPC UA Services, OPC Unified Architecture, SecureChannel, UA-TCP UA-SC UA-Binary, subscriptions, monitored items, industrial interoperability.