The number of seconds of inactivity from client side
which the TN3270 Server allows to elapse before
sending DO-TIMING-MARK to the TN3270 client. If the
client does not reply within thirty minutes of such
a TIMING-MARK sending, the server disconnect the tn3270
session. Zero seconds indicates that no keepalives
will be sent.
65535 (not valid in server context) indicates that the
default keepalive value for an LU is taken from whatever
value had been defined in the server context for this
CIP card.