z/OS Communications Server Version 2.Release 4 IP IMS Sockets Guide IBM SC27-3653-40 Note: Before using this information and the product it supports, be sure to read the general information under “Notices” on page 303. This edition applies to Version 2 Release 4 of z/OS® (5650-ZOS), and to subsequent releases and modifications until otherwise indicated in new editions. Last updated: 2021-06-21 © Copyright International Business Machines Corporation 2000, 2020. US Government Users Restricted Rights – Use, duplication or disclosure restricted by GSA ADP Schedule Contract with IBM Corp. Contents Figures................................................................................................................. ix Tables................................................................................................................ xiii About this document.......................................................................................... xvii Who should read this document.............................................................................................................. xvii How this document is organized............................................................................................................. xviii How to use this document.......................................................................................................................xviii How to contact IBM service............................................................................................................... xviii Conventions and terminology that are used in this information............................................................ xviii How to read a syntax diagram...................................................................................................................xix Prerequisite and related information....................................................................................................... xxii Summary of changes for IP IMS Sockets Guide..................................................xxvii Changes made in z/OS Communications Server Version 2 Release 4.................................................. xxvii Changes made in z/OS Communications Server Version 2 Release 3.................................................. xxvii Changes made in z/OS Communications Server Version 2 Release 2.................................................. xxvii Chapter 1. Using TCP/IP in the IMS environment.................................................... 1 The role of IMS TCP/IP.................................................................................................................................1 IMS TCP/IP feature components.................................................................................................................2 The IMS Listener.....................................................................................................................................2 The IMS Assist module...........................................................................................................................2 The MVS TCP/IP socket application programming interface (Sockets Extended)............................... 2 Chapter 2. IMS TCP/IP...........................................................................................3 Using IMS with SNA or TCP/IP.....................................................................................................................3 TCP/IP internets...........................................................................................................................................3 Mainframe interactive processing..........................................................................................................4 Client/server processing........................................................................................................................ 4 TCP, UDP, and IP.....................................................................................................................................4 The socket API........................................................................................................................................5 Programming with sockets.......................................................................................................................... 5 Socket types........................................................................................................................................... 5 Addressing TCP/IP hosts........................................................................................................................6 A typical client/server program flow chart..................................................................................................7 Concurrent and iterative servers............................................................................................................8 The basic socket calls.................................................................................................................................. 9 Server TCP/IP calls.................................................................................................................................... 10 Server SOCKET call...............................................................................................................................10 Server BIND call................................................................................................................................... 11 Server LISTEN call................................................................................................................................11 Server ACCEPT call...............................................................................................................................11 Server GIVESOCKET and TAKESOCKET calls...................................................................................... 11 Server READ and WRITE calls..............................................................................................................12 Client TCP/IP calls..................................................................................................................................... 12 Client SOCKET call................................................................................................................................12 Client CONNECT call.............................................................................................................................12 Client Read/Write calls — the conversation.........................................................................................12 iii Client CLOSE call.................................................................................................................................. 13 Other socket calls...................................................................................................................................... 13 The SELECT call....................................................................................................................................13 IOCTL and FCNTL calls.........................................................................................................................15 GIVESOCKET and TAKESOCKET calls..................................................................................................15 What you need to run IMS TCP/IP.............................................................................................................16 A summary of what IMS TCP/IP provides................................................................................................. 17 Chapter 3. Principles of operation of the Listener and the Assist module............... 19 Overview of the Listener and the Assist module.......................................................................................19 The role of the IMS Listener.................................................................................................................19 The role of the IMS Assist module.......................................................................................................19 Client/server logic flow..............................................................................................................................20 How the connection is established......................................................................................................20 How the server exchanges data with the client...................................................................................22 How the IMS Listener manages multiple connection requests.......................................................... 25 Use of the IMS message queue........................................................................................................... 26 Call sequence for the IMS Listener......................................................................................................26 Application design considerations.......................................................................................................27 Restrictions for operation of the Listener and the Assist module............................................................ 28 Chapter 4. How to write an IMS TCP/IP client program......................................... 29 General client program logic flow............................................................................................................. 29 Explicit-mode client