TCP Communication Flags

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, also known as synchronize, notifies the transmission.


ACK, also known as acknowledgment, confirms the receipt.


FIN, also known as finish, announces that it will not send more transmissions to remote system.


RST, also known as reset, aborts the connection in response to the error


PSH, also known as push, indicates that the sender is starting of finishing sending buffer data.


URG, also known as urgent, instructs to process data as soon as possible.


NS is used for ECN-nonce – concealment protection.


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, 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

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