Once both of the devices in a TCP connection have completed connection setup and have entered the ESTABLISHED state, the TCP software is in its normal operating mode. Bytes of data will be packaged into segments for transmission using the mechanisms described in the section on message formatting and data transfer. The sliding windows scheme will be used to control segment size and to provide flow control, congestion handling and retransmissions as needed.

Once in this mode, both devices can remain there indefinitely. Some TCP connections can be very long-lived indeed—in fact, some users maintain certain connections like Telnet sessions for hours or even days at a time. There are two circumstances that can cause a connection to move out of the ESTABLISHED state:

• Connection Termination: Either of the devices decides to terminate the connection. This involves a specific procedure that is covered in the next topic in this section.
  
  
• Connection Disruption: A problem of some sort occurs that causes the connection to be interrupted.

To allow TCP to live up to its job of being a reliable and robust protocol, it includes intelligence that allows it to detect and respond to various problems that can occur during an established connection. One of the most common is the half-open connection. This situation occurs when due to some sort of problem, one device closes or aborts the connection without the other one knowing about it. This means one device is in the ESTABLISHED state while the other may be in the CLOSED state (no connection) or some other transient state. This could happen if, for example, one device had a software crash and was restarted in the middle of a connection, or if some sort of glitch caused the states of the two devices to become unsynchronized.

To handle half-open connections and other problem situations, TCP includes a special reset function. A reset is a TCP segment that is sent with the RST flag set to one in its header. Generally speaking, a reset is generated whenever something happens that is “unexpected” by the TCP software. Some of the most common specific cases in which a reset is generated include:

• Receipt of any TCP segment from any device with which the device receiving the segment does not currently have a connection (other than a SYN requesting a new connection.)
  
  
• Receipt of a message with an invalid or incorrect Sequence Number or Acknowledgment Number field, indicating the message may belong to a prior connection or is spurious in some other way.
  
  
• Receipt of a SYN message on a port where there is no process listening for connections.