What does the connection-state=established matcher signify in connection tracking?

The connection-state=established matcher in connection tracking is used to identify packets that belong to an active, ongoing connection. Specifically, it signifies that the packet in question is part of a previously established connection, which typically means that it is a part of a data exchange—like a reply packet sent from a server back to a client after the client has initiated a request.

This state is crucial for maintaining efficient communication across a network by allowing for the tracking of established sessions. When a packet is classified as established, it indicates that the connection has gone through the initial setup phase (such as the TCP three-way handshake), and communication can occur in both directions without needing to repeatedly go through the connection establishment process. By distinguishing established connections from new, untracked, or time-out connections, the network device can employ optimized handling of the data flows, enhancing performance and reducing overhead.

The other options address different conditions within connection tracking that don't apply in the same way. For example, the initiation of a new TCP connection is represented by a different state, as it reflects packets that are beginning the process of establishing a connection. Similarly, packets that do not correspond to any known connection fall under another state, which identifies them as out-of-scope for current connection tracking