Some host VTAM® applications are set up to inquire whether a secondary LU is active. If the LU is active, the application initiates a session. The LUMAP option, KEEPOPEN, causes the LU to be activated when the LU is assigned to the connection, and to remain active for as long as the LU is assigned to the client by this mapping statement.
LU assignment is different for TN3270E and TN3270 connections. TN3270E connections have an LU assigned during connection negotiation. TN3270 connections do not have an LU assigned until the VTAM application name is known.
When the host VTAM application initiates a session with the secondary LU, the host must issue an INQUIRE to see if the LU is active with an OPEN ACB. This INQUIRE will fail for Telnet LUs because Telnet does not open the ACB until a session request is sent from Telnet to VTAM. When the user gets the solicitor (or USSMSG10) screen, Telnet has not opened the ACB of the LU assigned to a TN3270E connection. TN3270 and LineMode connections do not have LUs assigned yet. If the KEEPOPEN parameter is coded on the LUMAP statement used by Telnet to assign an LU during Early Lookup for a terminal TN3270E connection, Telnet opens the ACB before sending the solicitor (or USSMSG10) screen. At that time a user can either log on to an application as usual or wait for a host application to INQUIRE about the LU and initiate a session. TN3270 connections will not have an LU assigned until an application name is known. When the name is known, an LU is assigned to the connection, the ACB of the LU is opened, and a session request is issued. If the KEEPOPEN parameter is coded on the LUMAP statement used by Telnet to assign the LU, the LU stays assigned to the connection with the ACB open until the connection is dropped. If profile statements define LUs uniquely to different applications, a second logon to a different application might fail. When KEEPOPEN is mapped to a connection, the MSG07 and LUSESSIONPEND functions are in effect whether or not they were explicitly coded. When a session is ended, the connection remains and the ACB remains open. Only a client disconnect, a Telnet error, or the KEEPINACTIVE/INACTIVE timers will cause a KEEPOPEN connection to be dropped. The KEEPINACTIVE timer is used whenever the Telnet LU is not in session with a VTAM application. Otherwise, the INACTIVE timer is used. See Connection persistence for information about ending idle KEEPOPEN connections.