There are six TCP communication flags:
- Govern the establishment, maintenance and termination of a connection:
- SYN
- ACK
- FIN
- RST
- Provide instructions to the system:
- PSH
- URG
- Explicit Congestion Notification (ECN) related
- NS
- CWN
- ECE
List of TCP Communication Flags
SYN
SYN, also known as synchronize, notifies the transmission.
ACK
ACK, also known as acknowledgment, confirms the receipt.
FIN
FIN, also known as finish, announces that it will not send more transmissions to remote system.
RST
RST, also known as reset, aborts the connection in response to the error
PSH
PSH, also known as push, indicates that the sender is starting of finishing sending buffer data.
URG
URG, also known as urgent, instructs to process data as soon as possible.
NS
NS is used for ECN-nonce – concealment protection.
CWR
CWR, also known as congestion window reduced, indicate that it received a TCP segment with the ECE flag set and had responded in congestion control mechanism.
ECE
ECE, also known as ECN-Echo, has a dual role, depending on the value of the SYN flag. It indicates:
- If the SYN flag is set (1), that the TCP peer is Explicit Congestion Notification (ECN) capable.
- If the SYN flag is clear (0), that a packet with Congestion Experienced flag set (ECN=11) in the IP header was received during normal transmission. This serves as an indication of network congestion (or impending congestion) to the TCP sender.
External references
- Tannenbaum; “Computer Networks”
- CEHv10, module 3 “Scanning Networks”
- Wikipedia; “Transmission Control Protocol“