FMH - Function Management Header

Enhanced Definition

A Function Management Header (FMH) is a control block used within IBM's Systems Network Architecture (SNA) to carry specific control information between logical units (LUs) during a session. It provides instructions for data formatting, presentation services, and transaction management, enabling structured program-to-program communication.

Key Characteristics

- SNA Component: Integral to SNA, operating primarily at the presentation services layer to manage the structure and meaning of data exchanged between applications.
- Control Information: Contains metadata about the user data that follows, such as data stream type, compression indicators, security parameters, and transaction identifiers.
- Multiple Types: Various FMH types exist (e.g., FMH-5 for program-to-program data, FMH-7 for data stream structure) each serving a specific purpose in defining how data is presented and processed.
- Session Management: Used during the establishment and ongoing management of LU-to-LU sessions, particularly for LU 6.2 (APPC) conversations.
- Application Transparency: While fundamental to SNA, FMHs are often handled transparently by access methods like VTAM and communication subsystems like CICS, abstracting their complexity from most application developers.
- Structured Data Exchange: Facilitates the exchange of structured data, ensuring that the receiving application correctly interprets the incoming information and performs necessary actions.

Use Cases

- CICS Intersystem Communication (ISC): CICS uses FMHs extensively for transaction routing, function shipping, and distributed program link (DPL) between different CICS regions or to other subsystems (like IMS).
- APPC/MVS Applications: Programs utilizing Advanced Program-to-Program Communication (APPC) rely on FMHs to exchange conversation control information and structured application data between peer applications.
- IMS-to-CICS Communication: FMHs are crucial for enabling synchronized and asynchronous communication between IMS transactions and CICS transactions, ensuring proper data mapping and transaction integrity.
- Distributed Data Management (DDM): In DDM environments, FMHs are used to manage remote file access and database operations, providing control information for data transfer and processing across networked systems.
- SNA Gateway Functions: Used by gateway products to translate and route data between SNA networks and other network architectures, often involving the manipulation of FMH information.

Related Concepts

FMHs are a cornerstone of SNA (Systems Network Architecture), providing the intelligence for VTAM (Virtual Telecommunications Access Method) to properly route and deliver data. They are fundamental to LU 6.2 (APPC), enabling peer-to-peer program communication by defining the structure and control of conversations. Subsystems like CICS and IMS heavily leverage FMHs for their intersystem communication (ISC) capabilities, allowing transactions and programs to interact seamlessly across different regions or systems.

Best Practices:

Leverage High-Level APIs: For most application development, utilize high-level communication APIs and protocols (e.g., CICS ISC, APPC verbs) that abstract the complexities of FMH handling, rather than attempting explicit FMH manipulation.
Monitor Communication Flows: When diagnosing communication issues, use tools like VTAM traces, CICS auxiliary traces, or network analyzers to inspect FMH contents and ensure they are correctly formed and exchanged.
Understand FMH Types: For advanced network programming or troubleshooting, familiarize yourself with the different FMH types and their specific functions to better understand data flow and control.
Performance Considerations: While FMHs are small, efficient processing by communication subsystems and access methods is crucial for overall network performance, especially in high-volume transaction environments.
Security Integration: Ensure that security mechanisms (e.g., SNA session-level security, CICS transaction security) are properly configured to protect the integrity and confidentiality of data exchanged via FMHs.