Telnet Keep-Alive Time-Out Parameter

Resolving The Problem

The TCP protocol utilizes a mechanism to probe if the client is still active. The TCP keep-alive parameter (TCPKEEPALV) specifies at which intervals the protocol begins to probe the client. When the value in this parameter has been reached, the TCP protocol will send a TCP keep-alive request to the client. If the client does not respond to the first request, the protocol will re-send the request until the client responds or until the tenth request has been sent. If the client does not respond to any of these requests, the protocol will assume the client is no longer active (for example, PC hanging or re-booting, network drop, and so on).

The Telnet server takes advantage of the TCP keep-alive functionality with its Session keep-alive parameter (TIMMRKTIMO). For Telnet, the recovery action depends on the type of virtual device associated with the user's interactive job. If it is an auto-selected virtual device (for example, QPADEV0002), then the interactive job is always ended. If it is a named virtual device, (for example, PC52S1), then the action specified in the system value QDEVRCYACN is taken. Once action has been taken on the interactive job, the virtual device is recovered to a VARY ON PENDING status to allow a new connection.

Whereas the Telnet Session keep-alive parameter is used to detect and act upon clients no longer responding, the system value QINACTITV is used to act upon inactivity by the user (for example, user went to lunch still logged onto the session). The QINACTITV value defines the length of time an interactive job can be in DSPW status before action is taken. For auto-selected virtual devices (for example, QPADEV0003), the interactive job is always ended. For named virtual devices (for example, PC52S1), the system value QINACTMSGQ defines what action is to be taken with the interactive job. The virtual device then goes into a SIGNON DISPLAY status and the user is presented with the sign-on screen. For further information on system value QINACTITV, refer to document New, QINACTITV System Value Explained. To link to New immediately, click here .

How to Access the Parameters and Values

The TCP keep-alive parameter (TCPKEEPALV) can be set using the Change TCP Attributes (CHGTCPA) command. The value defaults to 120 minutes. Changes to the TCP Attributes take affect immediately.

The Telnet Session keep-alive parameter (TIMMRKTIMO) can be set using the Change Telnet Attributes (CHGTELNA) command. The value defaults to *CALC (on most systems this equates to 600 seconds).

System values can be displayed and changed using the appropriate options from the Work with System Values screen. This screen can be accessed with the WRKSYSVAL command:

WRKSYSVAL QDEVRCYACN