nt secondary err code: 110

 I am trying to diagnose intermittent TNS-12535 errors on the system.  The entry in the sqlnet.ora log says
Fatal NI connect error 12170.

  VERSION INFORMATION:
        TNS for Linux: Version 10.2.0.4.0 - Production
        Oracle Bequeath NT Protocol Adapter for Linux: Version 10.2.0.4.0 - Production
        TCP/IP NT Protocol Adapter for Linux: Version 10.2.0.4.0 - Production
  Time: 01-JUN-2010 10:25:28
  Tracing not turned on.
  Tns error struct:
    ns main err code: 12535
    TNS-12535: TNS:operation timed out
    ns secondary err code: 12560
    nt main err code: 505
    TNS-00505: Operation timed out
    nt secondary err code: 110
    nt OS err code: 0


I was curious to know what " nt secondary err code: 110" was.