9.4.5.1 TCP/IP Applications using the Sockets API for Assembler

VSE/ESA applications based on the SOCKET Assembler macro cannot be used on an OS/390 system. They have to be recoded for the OS/390 TCP/IP.

9.4.5.2 TCP/IP Applications using the Preprocessor API

The HLL preprocessor API which is available on VSE/ESA for PL/I, Assembler and COBOL is not compatible with the OS/390 TCP/IP interfaces. Therefore, these programs have to be recoded for the OS/390 system as well.

9.4.5.3 TCP/IP Applications using the BSD/C Sockets

The BSD (²Berkeley²) C sockets interface on VSE/ESA is almost compatible to the C socket API on OS/390. Only some additional (proprietary) functions or parameters of the BSD/C interface are not supported by TCP/IP on OS/390. This is why VSE/ESA TCP/IP applications based on the BSD/C sockets usually can be migrated to OS/390 with only minor code changes.

9.4.5.4 TCP/IP Applications using the LE/VSE C Socket API

It is highly recommended to use the IBM C for VSE/ESA compiler, the IBM Language Environment for VSE/ESA (LE/VSE) C run-time environment and the LE/VSE C socket interface to write TCP/IP applications on VSE/ESA. These are compatible with the OS/390 X/Open (XPG4.2) compliant socket interfaces. This assures the maximum in compatibility and portability for cross platform development. In this case, migrating the applications is just a matter of relinking them on the OS/390 system. More information about sockets programming can be found in the TCP/IP for VSE/ESA User¢s Guide, SC33-6601.

The VSE applications don¢t have to be necessarily C program since you can use the LE/VSE C socket API also from within other languages using the ILC (InterLanguage Communication). This is described in the book Writing Interlanguage Communication Applications, SC33-6686.

9.4.5.5 CGI Programs

If you are running VSE/ESA as a Web server and therefore have implemented CGI (Common Gateway Interface) programs on the VSE system, all these programs have to be rewritten on OS/390 since the CGI interface on OS/390 is totally different to the one on VSE/ESA.

9.4.6 Security

Security is an important consideration for an OS/390 system, especially if it¢ s connected to large TCP/IP networks or even the Internet. TCP/IP on OS/390 has some built-in internal security mechanism and relies on the services of an external security manager such as IBM Resource Access Control Facility (RACF).

Basic TCP/IP security definitions on VSE (such as user ID/password) can be easily defined in RACF for the OS/390 system. If you have implemented your own security exit on VSE, similar exits can be written for the FTP server function on OS/390. Furthermore, RACF can be used to protect whole libraries or single resources from unauthorized TCP/IP access.

Additionally, the OS/390 system can be run as a firewall to secure the system against users coming through the TCP/IP network.

Generally, you can achieve a higher level of security on the OS/390 system which, of course requires a little more effort to set it up.

196VSE to OS/390 Migration Workbook

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IBM OS/390 5.1 TCP/IP Applications using the Sockets API for Assembler, 5.2 TCP/IP Applications using the Preprocessor API
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OS/390 specifications

IBM OS/390, a versatile operating system, was a cornerstone in enterprise environments and played a pivotal role in mainframe computing. Released in the mid-1990s, OS/390 combined the strengths of IBM's MVS (Multiple Virtual Storage) with new features and enhancements, targeting scalability, reliability, and performance in demanding business applications.

One of the key features of OS/390 was its robust support for multiple users and processes. The system allowed thousands of concurrent users to access applications and data, ensuring high availability and minimizing downtime—a critical requirement for many large organizations. This scalability was supported through various enhancements in memory management and processor scheduling, enabling optimal resource allocation across diverse workloads.

OS/390 was known for its superior workload management capabilities. The Workload Manager (WLM) component allowed administrators to define service policies, specifying how system resources would be allocated according to the priority of tasks. This ensured that critical business processes received the necessary resources while less critical tasks were managed more flexibly.

Another significant characteristic of OS/390 was its commitment to security. The operating system provided comprehensive security features, including user authentication, data encryption, and auditing capabilities. This focus on security was vital for organizations handling sensitive data, ensuring compliance with regulations and safeguarding against unauthorized access.

OS/390 also supported advanced technologies that facilitated integration and development. The system included features like the IBM CICS (Customer Information Control System) for transaction processing and IMS (Information Management System) for database management. These technologies allowed organizations to build robust, high-performance applications tailored to specific business needs.

The ease of network integration was another strength of OS/390. With the advent of the Internet and global connectivity, OS/390 systems could easily interface with various network protocols, enabling businesses to operate in a connected world. This inclusion paved the way for many organizations to expand their capabilities and offer new services, driving digital transformation.

In conclusion, IBM OS/390 represented a significant advancement in mainframe technology, combining scalability, security, and robust workload management. Its rich feature set and support for critical enterprise applications solidified its role as a vital component of many organizations' IT infrastructures, ensuring they could meet their operational challenges head-on while supporting future growth. As technology continues to evolve, the legacy of OS/390 remains influential in the realm of computing.
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