Oracle 官方的解說 :
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| A SQLNET.EXPIRE_TIME=10 solution is to make this backend "not so" passive, using the DCD (dead connection detection) to figure out if the communication is still possible. |
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| Simply, set in the $ORACLE_HOME/network/admin/sqlnet.ora, in the server side SQLNET.EXPIRE_TIME=10 (10 minutes, for example). With this parameter in place, after 10 minutes of inactivity, the server send a small 10 bytes probe packet to the client. If this packet is not acknowledge, the connection will be closed and the associated resources will be released. |
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| There are two benefits with this DCD |
| 1. If the SQLNET.EXPIRE_TIME is less than the FW connection idle time-out, then the firewall *may* consider this packet as activity, and the idle time-out (firewall blackout) will never happen until both the client and the server processes are alive. |
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| 2. If the SQLNET.EXPIRE_TIME (let's say a little bit higher) than the FW idle limit, then , as soon as the blackout happens , the RDBMS will know and will close the connection. |
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| The first case is recommended when the connection comes from another application server , and the second makes sense for client applications. |
| DCD works at the application level and also works on top of TCP/IP protocol. If you have set the SQLNET.EXPIRE_TIME=10 then do not expect that the connections will be flagged as dead exactly after 10 minutes of the blackout or network outage. Please see Note:151972.1 "Dead Connection Detection (DCD) Explained" for details on DCD. The TCP timeout and TCP retransmission values also adds to this time. |
| PLEASE NOTE: |
| DCD was never designed to be used as a "virtual traffic generator" as we are wanting to use it for here. This is merely a useful side-effect of the feature. |
| In fact, some later firewalls and updated firewall firmware may not see DCD packets as a valid traffic possibly because the packets that DCD sends are actually empty packets. Therefore, DCD may not work as expected and the firewall / switch may still terminate TCP sockets that are idle for the period monitored, even when DCD is enabled and working. |
In such cases, the firewall timeout should be increased or users should not leave the application idle for longer than the idle time out configured on the firewall.
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