What are TCP Flags?

You may see references to TCP flags in various places. These are control bits that indicate different connection states or information about how a packet should be handled.

They may be abbreviated even more to the first letter of the flag.

A list from Wikipedia:

• CWR – Congestion Window Reduced (CWR) flag is set by the sending host to indicate that it received a TCP segment with the ECE flag set (added to header by RFC 3168).
• ECE (ECN-Echo) – indicate that the TCP peer is ECN capable during 3-way handshake (added to header by RFC 3168).
• URG – indicates that the URGent pointer field is significant
• ACK – indicates that the ACKnowledgment field is significant (Sometimes abbreviated by tcpdump as ".")
• PSH – Push function
• RST – Reset the connection (Seen on rejected connections)
• SYN – Synchronize sequence numbers (Seen on new connections)
• FIN – No more data from sender (Seen after a connection is closed)