TCP/IP on z/OS
The standard internet networking used by z/OS, provided by its Communications Server and run as a started task.
Also written TCPIP, z/OS Communications Server, OSA, NETSTAT, Sysplex Distributor
z/OS uses TCP/IP, the same networking as the internet, for almost all its connections today. It is provided by z/OS Communications Server, together with VTAM, and it runs as a started task, often called TCPIP.
Its settings, such as the system's IP addresses, network routes and which ports belong to which applications, are kept in a profile dataset read when TCP/IP starts. The network adapters on the mainframe are called OSA cards.
Many familiar services run on it: the TN3270 server that 3270 emulators connect to, FTP and SFTP for file transfer, SSH into Unix System Services, web servers, and the connections that MQ, DB2 and CICS use. In a sysplex, a feature called Sysplex Distributor can spread incoming connections across several systems, so a service keeps working if one system is down.
The NETSTAT command, available in TSO and in the Unix shell, shows the connections and ports in use. Network specialists look after the configuration. For others, it matters mainly when a connection fails and you need to know whether the problem is the network or the application.
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- VTAMThe part of z/OS that connects 3270 terminal sessions and SNA networks to applications such as TSO and CICS.
- File transferMoving data between the mainframe and other systems, where character encoding and record structure both need attention.
- 3270 terminalThe classic mainframe display device, and the protocol still used today to talk to z/OS over a network.
- Unix System ServicesA full Unix environment built into z/OS, with a shell, directories and files alongside the traditional mainframe world.
- Started taskA long-running program that the system starts and leaves running, rather than a job that runs once and finishes.
- z/OS ConnectIBM software that makes mainframe programs and data available as REST APIs, so web and mobile applications can use them.