The TCP/IP profile

The TCP/IP profile is read by TCP/IP when it is started. If a change needs to be made to the TCP/IP configuration after it has been started, TCP/IP can be made to re-read the profile dynamically (or read a new profile altogether).

Having extolled the versatility of JCL, it would be unfair not to provide a sample for starting the TCP/IP stack, so refer to Figure 1.

Figure 1. Sample JCL for TCP/IP task

//TCPIP   PROC
//TCPIP   EXEC PGM=EZBTCPIP,
//PROFILE DD DISP=SHR,DSN=SYS1.PARMLIB(PROFILE)
//SYSTCPD DD DISP=SHR,DSN=SYS1.PARMLIB(TCPDATA)

This JCL sample is simplified and would not execute if submitted to the job entry subsystem (JES), but it contains the basic elements pertinent to TCP/IP. Most of the missing statements would be parameters unique to an organization's specific requirements.

The first statement identifies our JCL as a started task procedure. The second statement identifies the program to be executed, which in this case is EZBTCPIP.

Tip: The z/OS environment includes a convention of program (load module) and message prefixes that are generally standardized across each application environment. For the TCP/IP environment, the message and module prefix is always EZn. Hence many messages begin with EZA or EZB, and as we can see, the module that starts TCP/IP itself begins with EZB.

Because a z/OS console can be a very busy place, being able to recognize such a prefix can immediately provide a system operator with a context for a given message.

The third statement is a DD statement that assigns a pointer to the data set member PROFILE found within data set SYS1.PARMLIB. The TCP/IP started task automatically searches for a file (data set) allocated to (pointed to) by a DD statement (pointer handle) called PROFILE at startup.

There are other methods of allowing the TCP/IP program to search other locations for profile information. However, the first place that TCP/IP searches is the PROFILE DD statement. Other methods can be used if an organization has such a need.