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CUE - Communication User Exit

Enhanced Definition

A Communication User Exit (CUE) is a user-written program or routine that allows system programmers to intercept and customize the behavior of communication subsystems on z/OS. It provides specific points within the communication stack (e.g., VTAM, TCP/IP) where custom logic can be injected to modify, enhance, or control data flow and connection management.

Key Characteristics

- Customization Point: Provides a predefined entry point within a z/OS communication component or product for user-supplied code.
- Low-Level Control: Operates at a fundamental level, allowing manipulation of network buffers, connection parameters, and protocol elements.
- Implementation Language: Typically written in Assembler for performance and direct system interaction, but can sometimes be implemented in C.
- Event-Driven: Invoked by the communication subsystem when specific events occur, such as connection establishment, data transmission, or session termination.
- Performance Critical: Due to its placement in the communication path, the code within a CUE must be highly optimized to avoid introducing latency or performance bottlenecks.
- Product-Specific: The exact nature, parameters, and invocation points of CUEs are specific to the communication product or component (e.g., VTAM exits, TCP/IP stack exits).

Use Cases

- Custom Security Enforcement: Implementing unique authentication, authorization, or access control mechanisms for network connections beyond standard z/OS security features.
- Data Transformation and Encryption: Encrypting or decrypting sensitive data, compressing/decompressing messages, or reformatting data streams as they traverse the network.
- Enhanced Logging and Auditing: Capturing detailed communication metadata or payload information for compliance, troubleshooting, or real-time monitoring purposes.
- Network Protocol Customization: Modifying standard protocol headers, adding proprietary information to data packets, or implementing non-standard routing logic.
- Connection Management: Dynamically altering connection parameters, influencing session setup/teardown, or performing custom resource allocation for network sessions.

Related Concepts

CUEs are a specific type of User Exit within the broader z/OS ecosystem, focusing exclusively on network communication. They often interact closely with VTAM (Virtual Telecommunications Access Method) for SNA networks or the z/OS TCP/IP stack for IP -based communication. Application programs running in CICS or IMS might indirectly benefit from or require CUEs for specific communication requirements, though CICS/IMS also have their own application-level user exits. CUEs are a core component of System Programming on z/OS, requiring deep knowledge of communication protocols and operating system internals.

Best Practices:

Minimize Logic: Keep the code within a CUE as concise and efficient as possible to prevent performance degradation of the communication subsystem.
Thorough Testing: Rigorously test CUEs in all possible scenarios, including error conditions and high-volume traffic, as errors can severely impact system stability.
Robust Error Handling: Implement comprehensive error checking and recovery mechanisms within the exit to ensure graceful failure and prevent system crashes.
Clear Documentation: Document the purpose, logic, entry/exit conditions, and any dependencies of the CUE meticulously for future maintenance and troubleshooting.
Version Control: Manage CUE source code and load modules under strict version control to track changes and facilitate rollbacks.