This attribute specifies the maximum amount of time that the IWF
allows to elapse without frames being exchanged. The link is
permitted to remain idle with no frames being exchanged on the
data link. Upon expiry of this timer, the IWF polls its peer for
status. Since information about the T403 timer is not carried in the
Exchange Identification (XID) negotiations at call setup time, both
sides operate with different timer values.
Changes to this attribute do not affect existing calls.