Transmission Control Protocol (TCP) (RFC 793) is one of the main protocols in the Internet Protocol (IP) suite, and is so prevalent that it is frequently referenced together with IP as TCP/IP. TCP is considered a reliable transport protocol because it provides error-checking while transmitting and receiving segments, acknowledges segments received, and reorders segments that arrive in the wrong order. TCP also requests and provides retransmission of segments that were dropped. TCP is stateful and connection-oriented, meaning a connection between the sender and receiver is established for the duration of the session. TCP provides flow control of packets, so it can handle congestion over networks.

TCP performs a handshake during session setup to initiate and acknowledge a session. After the data is transferred, the session is closed in an orderly manner, where each side transmits a FIN packet and acknowledges it with an ACK packet. The handshake that initiates the TCP session is often a three-way handshake (an exchange of three messages) between the initiator and the listener, or it could be a variation, such as a four-way or five-way split handshake or a simultaneous open. The TCP Split Handshake Drop explains how to Prevent TCP Split Handshake Session Establishment.

Applications that use TCP as their transport protocol include Hypertext Transfer Protocol (HTTP), HTTP Secure (HTTPS), File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP), Telnet, Post Office Protocol version 3 (POP3), Internet Message Access Protocol (IMAP), and Secure Shell (SSH).

The following topics describe details of the PAN-OS implementation of TCP.

You can configure packet-based attack protection and thereby drop IP, TCP, and IPv6 packets with undesirable characteristics or strip undesirable options from packets before allowing them into the zone. You can also configure flood protection, specifying the rate of SYN connections per second (not matching an existing session) that trigger an alarm, cause the firewall to randomly drop SYN packets or use SYN cookies, and cause the firewall to drop SYN packets that exceed the maximum rate.