What indicates a problem with connection establishment in the debug output?

The inability to establish a TCP connection indicates a problem with connection establishment because it signifies that the client and server are unable to complete the handshake required for communication. In a typical TCP connection process, a three-way handshake is initiated where the client sends a SYN packet, the server responds with a SYN-ACK packet, and finally, the client sends an ACK packet back to the server. If this process fails at any point, it results in an inability to establish the connection, which is a clear indicator of an issue.

In the context of debug output, observing messages that reflect failure to complete this handshake directly points to problems such as network issues, incorrect configurations, or unresponsive servers preventing the establishment of the connection. This could manifest as timeouts or errors that may result in the client being unable to send data to the server.

While receiving an ACK response indicates that a connection is properly established, a lack of response from the server could either signify no connectivity or a problem during the handshake, but it is not as definitive as an outright inability to establish the connection. Errors in phase 1 negotiation relate to specific protocols like IPsec, which could cause issues but don’t inherently reflect a general TCP connectivity issue in all scenarios.