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Author(s) Olivier Bonaventure
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Category tags tcp

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The acknowledgement numbers in the three-way-handshake used by TCP

The TCP header, shown in the TCP chapter of Computer Networking: Principles, Protocols and Practice contains a sequence number and an acknowledgement number. The three segments below correspond to the three-way handshake that is used to establish a TCP connection.


Acknowledgement numbers

Can you infer the acknowledgement numbers of the second and third TCP segments exchanged in the three-way-handshake below ?


0000  d1d004d2fac2af4b  0000000060027080  
0010  4f900000020405a0                    

0000  ÑÐ.Òú¯K....`.p.
0010  O.......        
  • Transmission Control Protocol, Src Port: 53712, Dst Port: 1234, Seq: 4207062859, Len: 0
    • Source Port: 53712
      • Destination Port: 1234
        • TCP Segment Len: 0
          • Sequence number: 4207062859
            • Acknowledgment number: 0
              • 0110 .... = Header Length: 24 bytes (6)
                • Flags: 0x002 (SYN)
                  • 000. .... .... = Reserved: Not set
                    • ...0 .... .... = Nonce: Not set
                      • .... 0... .... = Congestion Window Reduced (CWR): Not set
                        • .... .0.. .... = ECN-Echo: Not set
                          • .... ..0. .... = Urgent: Not set
                            • .... ...0 .... = Acknowledgment: Not set
                              • .... .... 0... = Push: Not set
                                • .... .... .0.. = Reset: Not set
                                  • .... .... ..1. = Syn: Set
                                    • .... .... ...0 = Fin: Not set
                                    • Window size value: 28800
                                      • Checksum: 0x4f90 [unverified]
                                        • Urgent pointer: 0
                                          • Options: (4 bytes), Maximum segment size
                                            • TCP Option - Maximum segment size: 1440 bytes
                                              • Kind: Maximum Segment Size (2)
                                                • Length: 4
                                                  • MSS Value: 1440