ORA-3136 Inbound Connection Time out

We are getting frequently the following error in alert log files of production server.It was running fine for last 3 months.There is no changes in listener configuration.

WARNING: inbound connection timed out (ORA-3136)

In Sqlnet.log file the following error is getting displayed.

TNS-12535: TNS:operation timed out

   nt main err code: 0
   nt secondary err code: 0
   nt secondary err code: 0
   nt OS err code: 0
   nt OS err code: 0
   ns secondary err code: 12606
   nt main err code: 0
   nt secondary err code: 0
   nt OS err code: 0
   TNS-12535: TNS:operation timed out
   ns secondary err code: 12606
   nt main err code: 0
   nt secondary err code: 0
   nt OS err code: 0
   TNS-12535: TNS:operation timed out
   ns secondary err code: 12606
   nt main err code: 0
   nt secondary err code: 0
   nt OS err code: 0

When i searched on the net,they are telling that it might be an network issue.But when i asked the network team , they said there is no firewall /port block issues.Please help me to resolve the issues.

Regards
Krishna
9585354917

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