MVS address spaces relationship between TCP/IP and CICS

Figure 1 shows the relationship between TCP/IP and CICS® in terms of MVS™ address spaces.

Figure 1. MVS address spaces

In this diagram, TCP/IP, CICS region A, and CICS region B all reside in separate address spaces.

Within each CICS region, server and client processes are allocated subtask numbers. TCP/IP treats each CICS region together with its application programs as a client application. Because of this, the address space and subtask of each CICS TCP/IP application is called its CLIENTID. This applies to CICS TCP/IP servers as well as to clients.

A single task can support up to 65535 sockets. However, the maximum number of sockets that the TCP/IP address space can support is determined by the value of MAXSOCKETS. Therefore, using multiple tasks, a single CICS region can support a number of sockets up to the setting of MAXSOCKETS, which has a maximum possible value of 16 777 215.

MAXFILEPROC limits the number of sockets per process. Because CICS is considered a process, MAXFILEPROC can limit the number of files allocated for the CICS region. Ensure that MAXFILEPROC is set to accommodate the total number of sockets used by all tasks running in the region.

The structure of CLIENTID is shown in Table 1. With CICS TCP/IP, the domain is always AF_INET, so the name (that is, address space) and subtask are the items of interest.

C structure

COBOL structure

struct clientid {
    int       domain;
    char      name[8];
    char      subtaskname[8];
    char      reserved[20];
};

CLIENTID STRUCTURE:
     01 CLIENTID.
       02 DOMAIN PIC 9(8) BINARY.
       02 NAME PIC X(8).
       02 TASK PIC X(8).
       02 RESERVED PIC X(20).