When an unexpected TCP packet arrives at a host, that host usually responds by sending a reset packet back on the same connection. A reset packet is simply one with no payload and with the
RST bit set in the TCP header flags.
There are a few circumstances in which a TCP packet might not be expected; the two most common are:
The packet is an initial
SYN packet trying to establish a connection to a server port on which no process is listening.
Other circumstances are possible, but are unlikely outside of malicious behavior such as attempts to hijack a TCP connection.