Mani Raj Paul - TCP - Flags

TCP Flags


  • Flags (1-bit each)

    • NS - Nonce Sum bit is used by Explicit Congestion Notification signaling process.
    • CWR - When a host receives packet with ECE bit set, it sets Congestion Windows Reduced to acknowledge that ECE received.
    • ECE -It has two meanings:
      • If SYN bit is clear to 0, then ECE means that the IP packet has its CE (congestion experience) bit set.
      • If SYN bit is set to 1, ECE means that the device is ECT capable.
    • URG - It indicates that Urgent Pointer field has significant data and should be processed.
    • ACK - It indicates that Acknowledgement field has significance. If ACK is cleared to 0, it indicates that packet does not contain any acknowledgement.
    • PSH - When set, it is a request to the receiving station to PUSH data (as soon as it comes) to the receiving application without buffering it.
    • RST - Reset flag has the following features:
      • It is used to refuse an incoming connection.
      • It is used to reject a segment.
      • It is used to restart a connection.
    • SYN - This flag is used to set up a connection between hosts.
    • FIN - This flag is used to release a connection and no more data is exchanged thereafter. Because packets with SYN and FIN flags have sequence numbers, they are processed in correct order.
  • Windows Size  - This field is used for flow control between two stations and indicates the amount of buffer (in bytes) the receiver has allocated for a segment, i.e. how much data is the receiver expecting.
  • Checksum - This field contains the checksum of Header, Data and Pseudo Headers.
  • Urgent Pointer  - It points to the urgent data byte if URG flag is set to 1.
  • Options  - It facilitates additional options which are not covered by the regular header. Option field is always described in 32-bit words. If this field contains data less than 32-bit, padding is used to cover the remaining bits to reach 32-bit boundary.

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