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

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Utilisation of the SYN and ACK flags during the three-way-handshake used by TCP

The TCP header, shown in the TCP chapter of Computer Networking: Principles, Protocols and Practice contains several flags, including the SYN and the ACK flags. These two flags play an important role during the establishment of a connection. The three segments below correspond to the three-way handshake that is used to establish a TCP connection.


Acknowledgement numbers

Can you infer the value of the SYN and ACK flags in the three segments below ?

# Length Summary Status
0 24 bytes Transmission Control Protocol, Src Port: 53708, Dst Port: 1234, Seq: 2250678587, Len: 0  
1 24 bytes Transmission Control Protocol, Src Port: 1234, Dst Port: 53708, Seq: 3164923748, Ack: 2250678588, Len: 0  
2 20 bytes Transmission Control Protocol, Src Port: 53708, Dst Port: 1234, Seq: 2250678588, Ack: 3164923749, Len: 0  

0000  d1cc04d28626a13b  0000000060??7080  
0010  00000000020405a0                    

0000  ÑÌ.Ò.&¡;....`?p.
0010  ........        
  • Transmission Control Protocol, Src Port: 53708, Dst Port: 1234, Seq: 2250678587, Len: 0
    • Source Port: 53708
      • Destination Port: 1234
        • Sequence number: 2250678587
          • Acknowledgment number: 0
            • 0110 .... = Header Length: 24 bytes (6)
              • Flags
                • 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  04d2d1ccbca4e764  8626a13c60??7080  
                                      0010  00000000020405a0                    

                                      0000  .ÒÑ̼¤çd.&¡<`?p.
                                      0010  ........        
                                      • Transmission Control Protocol, Src Port: 1234, Dst Port: 53708, Seq: 3164923748, Ack: 2250678588, Len: 0
                                        • Source Port: 1234
                                          • Destination Port: 53708
                                            • Sequence number: 3164923748
                                              • Acknowledgment number: 2250678588
                                                • 0110 .... = Header Length: 24 bytes (6)
                                                  • Flags
                                                    • 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  d1cc04d28626a13c  bca4e76550??7080  
                                                                          0010  00000000                            

                                                                          0000  ÑÌ.Ò.&¡<¼¤çeP?p.
                                                                          0010  ....            
                                                                          • Transmission Control Protocol, Src Port: 53708, Dst Port: 1234, Seq: 2250678588, Ack: 3164923749, Len: 0
                                                                            • Source Port: 53708
                                                                              • Destination Port: 1234
                                                                                • Sequence number: 2250678588
                                                                                  • Acknowledgment number: 3164923749
                                                                                    • 0101 .... = Header Length: 20 bytes (5)
                                                                                      • Flags
                                                                                        • 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