DCD: Dead Connection Detection ，可以用於檢測、標記僵死而沒有斷開會session，再由PMON進行清理，釋放資源。
開啟DCD，只需要在服務端的sqlnet.ora檔案中新增SQLNET.EXPIRE_TIME引數，單位為分鐘：

SQLNET.EXPIRE_TIME = 10

如果時間達到這個值，server端就是發出一個”probe” packet 給客戶端，如要客戶斷是正常的，這個packet就被忽略，timer重新計時；如果客戶端異常中斷，則server端就會收到一個訊息，用以釋放連線。

DCD還可以用於防止防火牆的timeout，例如：

某個系統RMAN備份，在結束時，報：

released channel: dev_0
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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

參考文章(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.

以上這個RMAN報錯例子就是由於防火牆設定timeout原因，當client和server在timeout時間內沒有資料傳輸的時候，會話就會被防火牆斷開。而設定SQLNET.EXPIRE_TIME引數，使其小於防火牆的timeout時間，就可以避免這一情況的發生。

SQLNET.EXPIRE_TIME引數—之防火牆短連線

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