11gr2 RAC was newly installed in the front. One day, during the patrol, the following error was found in the alert log
fatal Ni connect error 12170

VERSION INFORMATION:
TNS for Linux: Version 11.2.0.4.0 – Production
Oracle Bequeath NT Protocol Adapter for Linux: Version 11.2.0.4.0 – Production
TCP/IP NT Protocol Adapter for Linux: Version 11.2.0.4.0 – Production
Time: 23-MAY-2019 10:49:06
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 = 172.16.100.116) (Port = 38136))
Thu May 23 10:49:19 2019
trigger cause:
it is caused by abnormal program disconnection Timeout, 11g R1 will write the above error message to sqlnet.log, 11g R2 will write to alert.log,
in fact, this error is normal

If we analyze in depth why these errors occur, the details are as follows:
the whole steps of a client connection:
1. The client initiates a connection listening
2. The listening starts an exclusive process (server process, which is commonly known as loca = no process) to receive the connection
3. After the exclusive process is started, the client starts a connection listening, Listening will pass the connection to the exclusive process
4. The exclusive process shakes hands with the client through the connection
5. The information exchange between the exclusive process and the client needs to establish a session
6. The session is opened

When the client closes in steps 3 to 4 above, when the exclusive process tries to contact the client and finds that the connection is closed, it will report the error we see

If you don’t want the information to be printed in the alert log, set it as follows:
set diag in sqlnet.ora_ ADR_ Enabled = off
set
diag in listener.ora_ ADR_ ENABLED_ = Off
Restart monitoring