DCD: Dead Connection Detection ，可以用于检测、标记僵死而没有断开会session，再由PMON进行清理，释放资源。
开启DCD，只需要在服务端的sqlnet.ora文件中添加SQLNET.EXPIRE_TIME参数，单位为分钟：
SQLNET.EXPIRE_TIME = 10 |
如果时间达到这个值，server端就是发出一个”probe” packet 给客户端，如要客户断是正常的，这个packet就被忽略，timer重新计时；如果客户端异常中断，则server端就会收到一个消息，用以释放连接。
DCD还可以用于防止防火墙的timeout，例如：
某个系统RMAN备份，在结束时，报：
released channel: dev_0 released channel: dev_1 released channel: dev_2 released channel: dev_3 released channel: dev_4 released channel: dev_5 released channel: dev_6 released channel: dev_7 released channel: dev_8 released channel: dev_9 released channel: dev_10 released channel: dev_11 RMAN-00571: =========================================================== RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS =============== RMAN-00571: =========================================================== RMAN-06004: ORACLE error from recovery catalog database: ORA-03135: connection lost contact ORACLE error from recovery catalog database: ORA-03114: not connected to ORACLE |
参考文章(ID 729811.1)：
+ Is the rman catalog Database running on a different system than the Target Database? If so, verify with the System Administrators what the TCP/IP timeout is set to. If there is any firewall between two systems, set the value of SQLNET.EXPIRE_TIME to less than the TCP timeout value of the firewall. + Increase the value of SQLNET.EXPIRE_TIME in the sqlnet.ora file on both target servers if no firewall exists between two systems. + Increase the value of keepalive interval. |
以上这个RMAN报错例子就是由于防火墙设置timeout原因，当client和server在timeout时间内没有数据传输的时候，会话就会被防火墙断开。而设置SQLNET.EXPIRE_TIME参数，使其小于防火墙的timeout时间，就可以避免这一情况的发生。
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