TCP Flags

by cisnix on November 9, 2010

  • URG – Urgent pointer flag tells the peer that the urgent data needs to be sent immediately before any data processed from the queu. When a peer receives a tcp segment with urgent pointer flag set, It stops processing other segments and gives the priority to the segment with the URG flag set.
  • ACK – Acknowledgement bit indicates that tcp segment is received successfully. After initial SYN packet this flag should be set all the time.
  • PSH – Send data to the upper layer and give priority as well.
  • RST – Reset or abort the connection.
  • SYN – Initial TCP connection bit. Part of the three-way handshake.
  • FIN – Graceful termination of a tcp connection.
  • CWR – Congestion Window Reduced (CWR) flag indicates that it received a tcp segment with ECE bit set and notified congestion control mechanism.
  • ECE – ECN-Echo flag has two different meanings,

  • SYN flag is set, the peer is ECN capable
  • SYN flag is not set, a packet with Congestion Experienced CE flag in IP header set was received during normal transmission