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Long dispatch times for the SO TCB in CICS TS

Technote (troubleshooting)


You are observing transactions waiting to be dispatched on the SO TCB in CICS Transaction Server for z/OS (CICS TS). You see tasks using the SO TCB for as long as 30 seconds. You might see many transactions waiting for the SO TCB and this can result in the region hitting the maximum number of transactions (MXT) limit.


A Domain Name Server (DNS) request is being done on the SO TCB and the DNS server is not responding. Your TCP/IP resolver timeout (RESOLVERTIMEOUT) parameter is set to 30 seconds which is high. A DNS server is usually very fast. If you do not get a response in 2 or 3 seconds it is unlikely you will get a response at all.

As part of Secure Sockets Layer (SSL) optimization in CICS TS V5.3:

  • If you are using CICS native SSL support the DNS requests will be done on an S8 TCB instead of the SO TCB
  • If you are using AT-TLS for SSL support the SO TCB will be used

Resolving the problem


Related information

MXT system initialization parameter

Product Alias/Synonym

CICS/TS CICS TS CICS Transaction Server

Document information

More support for: CICS Transaction Server

Software version: 4.1, 4.2, 5.1, 5.2, 5.3

Operating system(s): z/OS

Reference #: 1996418

Modified date: 17 January 2017

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