The Oracle Net listener is an application positioned on top of the Oracle Net foundation layer. The database receives an initial connection from a client application through the listener.

The listener brokers client requests, handing off the requests to the Oracle database server. Every time a client requests a network session with a database, the listener receives the initial request.

The following diagram illustrates the various layers on the client and database during an initial connection. As shown in the diagram, the listener is at the top layer of the server-side network stack.

• About Service Registration
  
• About the Listener and Connection Requests
  
• About Oracle Restart
  
• About Blocked Connection Requests
  
• Understanding Database Server Process Architecture
  
• Understanding Oracle Connection Manager Architecture
  
• Complete Architecture
  
• Reverse Connection Using CMAN Tunnels
  Starting with Oracle Database 21c, you can use secure tunnels to connect to an Oracle Database instance, which is inside a network that supports only outbound connections.