DXC - Data Transfer Control

Enhanced Definition

DXC, or Data Transfer Control, is a VTAM (Virtual Telecommunications Access Method) macro instruction and its associated control block used by application programs to define and manage the parameters for data transfer operations between logical units (LUs) within an SNA (Systems Network Architecture) network. It specifies crucial information for `SEND` and `RECEIVE` operations, enabling applications to exchange data over an established SNA session. `DXC` (Data Exchange Control or Data Transfer Control) is a parameter primarily used within IBM's `DFSMSdss` (Data Facility Storage Management Subsystem data set services) utility. It controls how `DFSMSdss` handles data sets during `DUMP`, `RESTORE`, `COPY`, or `MOVE` operations, particularly concerning data sets that might be concurrently accessed or open by other applications. Its main purpose is to ensure data integrity and availability during data transfer processes.

Key Characteristics

- VTAM Macro: DXC is invoked via a VTAM macro instruction in assembler programs, typically used in conjunction with ACB (Access Method Control Block) and RPL (Request Parameter List) macros to perform network I/O.
- Parameter Definition: It defines critical parameters such as the data buffer address, the length of the data buffer, the session identifier, and flags indicating the type of data transfer (e.g., normal data, expedited flow, definite response required).
- Control Block: When the DXC macro is assembled, it generates a DXC control block in memory, which VTAM uses internally to execute the requested data transfer operation, managing the actual transmission or reception of data.
- Session Context: A DXC always operates within the context of an established SNA session between two LUs, facilitating the structured exchange of application data units (Request/Response Units - RUs).
- Low-Level Control: It provides a low-level interface for direct control over data transfer, giving application programmers fine-grained command over how data is presented to and received from VTAM.

Use Cases

- Application-to-Application Communication: COBOL or Assembler programs acting as SNA LUs use DXC to send and receive data to/from other SNA applications, such as CICS regions communicating with other CICS regions or remote batch applications.
- Batch Data Transfer: Legacy batch applications requiring direct SNA communication for transferring files or messages to remote systems might explicitly use DXC for precise control over data segments and flow.
- Custom Network Services: Developing specialized network services or gateways on z/OS that require direct interaction with VTAM at the SNA protocol level to handle non-standard data flows or emulate specific LU types.
- Performance-Critical Applications: In scenarios where maximum control over data buffering and network interaction is paramount for performance, direct DXC usage can be employed, though often abstracted by higher-level APIs.

Related Concepts

DXC is fundamental to VTAM and SNA, providing the low-level interface for applications to interact with the network. It works in conjunction with an ACB (Access Method Control Block), which represents the application's access to VTAM, and an RPL (Request Parameter List), which describes a specific request (like SEND or RECEIVE). The DXC defines the data-specific parameters for the RPL, enabling the transfer of data between Logical Units (LUs) over an established SNA session. It's a building block for higher-level communication protocols often used in CICS and other transaction processing environments.

Best Practices:

Proper Buffer Management: Ensure that the data buffer specified in the DXC is correctly allocated, sized, and managed to prevent data truncation, storage overlay issues, or ABENDs.
Robust Error Handling: Implement comprehensive error handling by checking return codes from VTAM after SEND or RECEIVE operations to diagnose and respond to network, session, or buffer-related problems.
Session State Awareness: Always ensure that data transfer operations are attempted only when an SNA session is in an active and appropriate state (e.g., BIND complete, READY state) to avoid unexpected errors.
Performance Optimization: For high-volume data transfers, consider optimizing buffer sizes and understanding VTAM pacing mechanisms to maximize throughput and minimize network overhead, aligning with SNA best practices.
Security Considerations: While DXC itself doesn't directly provide security, ensure that the applications using it are part of a secure SNA environment, leveraging VTAM's security features like APPLID protection and session-level encryption where available.