client changed its mind and closed the connection immediately after initiating it
client crashed
firewall kills the connection
some oracle timeout set on client
解决方案：
Because the entry from listener.log contains only CONNECT_DATA and CID related information we need to check the client configuration for any sqlnet timeouts:
possible timeouts in sqlnet.ora in client oracle home:
sqlnet.outbound_connect_time
sqlnet.recv_timeout
sqlnet.send_timeout
tcp_connect_timeout
检查客户端目录中sqlnet.ora的超时设置，通常是这个引起的
possible timeout in client connect descriptor (hardcoded in client application or in client tnsnames.ora):
connect_timeout
检查客户端应用或客户单tnsnames.ora中的超时参数
在CentOS 6.4下安装Oracle 11gR2(x64)