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Author(s) Olivier Bonaventure
Deadline No deadline
Submission limit No limitation
Category tags TCP

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Reordering the three-way-handshake and infering the TCP flags

A TCP connection, as explained in the TCP chapter of Computer Networking: Principles, Protocols and Practice always starts with a three-way-handshake.


Reorder the three-way-handshake and infer the TCP flags

Can you correctly reorder the three segments that are used to establish a TCP connection and set the SYN and ACK flags correctly ?

# Length Summary Status
0 20 bytes Transmission Control Protocol, Src Port: 53720, Dst Port: 1234, Seq: 1541628760, Ack: 1554975570, Len: 0  
1 24 bytes Transmission Control Protocol, Src Port: 1234, Dst Port: 53720, Seq: 1554975569, Ack: 1541628760, Len: 0  
2 24 bytes Transmission Control Protocol, Src Port: 53720, Dst Port: 1234, Seq: 1541628759, Len: 0  

0000  d1d804d25be36358  5caf0b5250??7080  
0010  00000000                            

0000  ÑØ.Ò[ãcX\¯.RP?p.
0010  ....            
  • Transmission Control Protocol, Src Port: 53720, Dst Port: 1234, Seq: 1541628760, Ack: 1554975570, Len: 0
    • Source Port: 53720
      • Destination Port: 1234
        • Sequence number: 1541628760
          • Acknowledgment number: 1554975570
            • 0101 .... = Header Length: 20 bytes (5)
              • Flags: 0x010 (ACK)
                • 000. .... .... = Reserved: Not set
                  • ...0 .... .... = Nonce: Not set
                    • .... 0... .... = Congestion Window Reduced (CWR): Not set
                      • .... .0.. .... = ECN-Echo: Not set
                        • .... ..0. .... = Urgent: Not set
                          • .... ...? .... = Acknowledgment: ?
                          • .... .... 0... = Push: Not set
                            • .... .... .0.. = Reset: Not set
                              • .... .... ..?. = Syn: ?
                              • .... .... ...0 = Fin: Not set
                              • Window size value: 28800
                                • Urgent pointer: 0
                                0000  04d2d1d85caf0b51  5be3635860??7080  
                                0010  00000000020405a0                    

                                0000  .ÒÑØ\¯.Q[ãcX`?p.
                                0010  ........        
                                • Transmission Control Protocol, Src Port: 1234, Dst Port: 53720, Seq: 1554975569, Ack: 1541628760, Len: 0
                                  • Source Port: 1234
                                    • Destination Port: 53720
                                      • Sequence number: 1554975569
                                        • Acknowledgment number: 1541628760
                                          • 0110 .... = Header Length: 24 bytes (6)
                                            • Flags: 0x012 (SYN, ACK)
                                              • 000. .... .... = Reserved: Not set
                                                • ...0 .... .... = Nonce: Not set
                                                  • .... 0... .... = Congestion Window Reduced (CWR): Not set
                                                    • .... .0.. .... = ECN-Echo: Not set
                                                      • .... ..0. .... = Urgent: Not set
                                                        • .... ...? .... = Acknowledgment: ?
                                                        • .... .... 0... = Push: Not set
                                                          • .... .... .0.. = Reset: Not set
                                                            • .... .... ..?. = Syn: ?
                                                            • .... .... ...0 = Fin: Not set
                                                            • Window size value: 28800
                                                              • 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
                                                                    0000  d1d804d25be36357  0000000060??7080  
                                                                    0010  00000000020405a0                    

                                                                    0000  ÑØ.Ò[ãcW....`?p.
                                                                    0010  ........        
                                                                    • Transmission Control Protocol, Src Port: 53720, Dst Port: 1234, Seq: 1541628759, Len: 0
                                                                      • Source Port: 53720
                                                                        • Destination Port: 1234
                                                                          • Sequence number: 1541628759
                                                                            • 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
                                                                                            • .... ...? .... = Acknowledgment: ?
                                                                                            • .... .... 0... = Push: Not set
                                                                                              • .... .... .0.. = Reset: Not set
                                                                                                • .... .... ..?. = Syn: ?
                                                                                                • .... .... ...0 = Fin: Not set
                                                                                                • Window size value: 28800
                                                                                                  • 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