If a network client is trying to connect to the wrong TCP port, the most likely response it would receive is a RST (reset) packet from the server. A RST packet is used to terminate a TCP connection abruptly, either because the server does not recognize the port number, or because the server has no application listening on that port, or because the server detects an error or an attack on the connection. A RST packet tells the client to stop sending any more data and close the connection immediately12 The other options are not as likely to be the responses for connecting to the wrong TCP port. A FIN (finish) packet is used to gracefully end a TCP connection, after both parties have exchanged all the data they need. A FIN packet indicates that the sender has no more data to send and is ready to close the connection. A FIN packet does not imply an error or a rejection, but rather a normal termination of the connection12 An ICMP (Internet Control Message Protocol) Time Exceeded message is used to inform the sender that a packet has been discarded because its time to live (TTL) value has reached zero. A TTL value is a counter that decrements by one every time a packet passes through a router. When the TTL value reaches zero, the packet is dropped and an ICMP Time Exceeded message is sent back to the sender. This message is used to prevent packets from looping endlessly in the network, and also to implement the traceroute tool, which shows the path and the hops that a packet takes to reach a destination. An ICMP Time Exceeded message is not related to the TCP port number, but rather to the IP routing and the TTL value3 A Redirect message is another type of ICMP message that is used to inform the sender that there is a better route to the destination. A Redirect message is sent by a router when it receives a packet from a host on the same network, and forwards the packet to another router on the same network. A Redirect message tells the sender to use the second router as the next hop, instead of the first router, to optimize the routing and avoid unnecessary hops. A Redirect message is also not related to the TCP port number, but rather to the IP routing and the network topology3 References:
* What causes a TCP/IP reset (RST) flag to be sent?
* RFC 792: Internet Control Message Protocol