Читать книгу CCNP Enterprise Certification Study Guide: Implementing and Operating Cisco Enterprise Network Core Technologies - Ben Piper, David Higby Clinton - Страница 45

The TCP Reset (RST) flag is useful for cleaning up old and duplicate connections. Generally, when a TCP receives a segment for a port that it's not listening on, it will respond with a RST, letting the sender know that the connection was refused. This can happen if the sender tries to send data over an old connection that it believes is still open.

In another case, if a TCP doesn't receive any ACKs from a host, it will abort the connection by sending a RST. This may happen if there's a unidirectional link or someone abruptly shuts down the host, precluding it from gracefully terminating the connection. On the off chance that the host receives the RST, it will abandon the connection and may attempt to open a new one.