What is the difference between "ORA-12571: TNS packet writer failure" and "ORA-03135: connection lost contact"?

I am working in an environment where we get production issues from tim开发者_StackOverflow社区e to time related to Oracle connections. We use ODP.NET from ASP.NET applications, and we suspect the firewall closes connections that have been in the connection pool too long.

Sometimes we get an "ORA-12571: TNS packet writer failure" error, and sometimes we get "ORA-03135: connection lost contact."

I was wondering if someone has run into this and/or has an understanding of the difference between the 2 errors.

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Using a mobile phone analogy:

• ORA-12571 (Failure) Means call is dropped.
• ORA-03135 (Connection Lost) Other party hung up.

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My understanding is that 3135 occurs when a connection is lost. This doesn't tell you why the connection was lost, though. It may have been terminated by the server because the server failed to recieve a response to a probe for a certain amount of time, and assumed that the connection was dead. Or (I'm not sure about this) the exact reverse of that: the client failed to recieve a probe response from the server for a certain amount of time, so it assumed the connection was lost. The "certain amount of time" is cotrolled by SQLNET.EXPIRE_TIME=[minutes] in sqlnet.ora.

As for 12571, my (again vague) understanding is that there was a sudden failure to send a packet during communication with the server, and that this is typically caused by some software or hardware interfering with the connection (either by design, or by error). For instance, if you pull out your ethernet cable and then try to execute a query, you'll probably get this. Or if a firewall or anti-malware application decides to block the traffic.