在工作中巡检数据库，发现数据库告警日志出现以下错误信息
Fatal NI connect error 12170.
  VERSION INFORMATION:
        TNS for Linux: Version 11.1.0.7.0 - Production
        Oracle Bequeath NT Protocol Adapter for Linux: Version 11.1.0.7.0 - Production
        TCP/IP NT Protocol Adapter for Linux: Version 11.1.0.7.0 - Production
  Time: 31-3月 -2017 11:47:56
  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
  Client address: (ADDRESS=(PROTOCOL=tcp)(HOST=10.234.235.138)(PORT=59344))
Fri Mar 31 13:47:55 2017
 
 
***********************************************************************
 
Fatal NI connect error 12170.
 
  VERSION INFORMATION:
        TNS for Linux: Version 11.1.0.7.0 - Production
        Oracle Bequeath NT Protocol Adapter for Linux: Version 11.1.0.7.0 - Production
        TCP/IP NT Protocol Adapter for Linux: Version 11.1.0.7.0 - Production
  Time: 31-3月 -2017 13:47:55
  Tracing not turned on.
  Tns error struct:
    ns main err code: 12535

于是在metalink上查询了相关报错信息
触发原因：
是由于程序异常断开导致超时，11g R1如果出现如上的错误信息会写入到sqlnet.log,11g R2会写入到alert.log，
其实出现此错误是正常的现象。
解决方法

The following parameter, set at the **RDBMS_HOME/network/admin/sqlnet.ora, can resolve this kind of problem.  DCD or SQLNET.EXPIRE_TIME can mimic data transmission between the server and the client during long periods of idle time. 

SQLNET.EXPIRE_TIME=n  Where <n> is a non-zero value set in minutes.  

Once this change is in place, there is NO need to restart the listener or the database.  The change will be in place for all newly spawned server processes following the change.