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

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Inferring acknowledgment numbers

TCP, as explained in the TCP chapter of Computer Networking: Principles, Protocols and Practice includes a sequence number in each segment that carries data and uses acknowledgements.


Inferring acknowledgement numbers

During this connection, the client sent some data in successive segments and then the server replied with a an acknowledgement that confirms the reception of all the previous segments. Can you infer the acknowledgement number of the segment returned by the server ?

# Length Summary Status
0 28 bytes Transmission Control Protocol, Src Port: 53710, Dst Port: 1234, Seq: 3378592493, Ack: 2477434132, Len: 8
1 27 bytes Transmission Control Protocol, Src Port: 53710, Dst Port: 1234, Seq: 3378592501, Ack: 2477434132, Len: 7
2 29 bytes Transmission Control Protocol, Src Port: 53710, Dst Port: 1234, Seq: 3378592508, Ack: 2477434132, Len: 9
3 29 bytes Transmission Control Protocol, Src Port: 53710, Dst Port: 1234, Seq: 3378592517, Ack: 2477434132, Len: 9
4 20 bytes Transmission Control Protocol, Src Port: 1234, Dst Port: 53710, Seq: 2477434132, Ack: ????, Len: 0  

0000  d1ce04d2c9613aed  93aaa51450187080  
0010  0000000073647173  6473710a          

0000  ÑÎ.ÒÉa:í.ª¥.P.p.
0010  ....sdqsdsq.    
  • Transmission Control Protocol, Src Port: 53710, Dst Port: 1234, Seq: 3378592493, Ack: 2477434132, Len: 8
    • Source Port: 53710
      • Destination Port: 1234
        • Sequence number: 3378592493
          • Acknowledgment number: 2477434132
            • 0101 .... = Header Length: 20 bytes (5)
              • Flags: 0x018 (PSH, 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
                          • .... ...1 .... = Acknowledgment: Set
                            • .... .... 1... = Push: Set
                              • .... .... .0.. = Reset: Not set
                                • .... .... ..0. = Syn: Not set
                                  • .... .... ...0 = Fin: Not set
                                  • Window size value: 28800
                                    • Urgent pointer: 0
                                      • TCP payload (8 bytes)
                                      0000  d1ce04d2c9613af5  93aaa51450187080  
                                      0010  0000000073716471  73640a            

                                      0000  ÑÎ.ÒÉa:õ.ª¥.P.p.
                                      0010  ....sqdqsd.     
                                      • Transmission Control Protocol, Src Port: 53710, Dst Port: 1234, Seq: 3378592501, Ack: 2477434132, Len: 7
                                        • Source Port: 53710
                                          • Destination Port: 1234
                                            • Sequence number: 3378592501
                                              • Acknowledgment number: 2477434132
                                                • 0101 .... = Header Length: 20 bytes (5)
                                                  • Flags: 0x018 (PSH, 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
                                                              • .... ...1 .... = Acknowledgment: Set
                                                                • .... .... 1... = Push: Set
                                                                  • .... .... .0.. = Reset: Not set
                                                                    • .... .... ..0. = Syn: Not set
                                                                      • .... .... ...0 = Fin: Not set
                                                                      • Window size value: 28800
                                                                        • Urgent pointer: 0
                                                                          • TCP payload (7 bytes)
                                                                          0000  d1ce04d2c9613afc  93aaa51450187080  
                                                                          0010  0000000073716471  736473710a        

                                                                          0000  ÑÎ.ÒÉa:ü.ª¥.P.p.
                                                                          0010  ....sqdqsdsq.   
                                                                          • Transmission Control Protocol, Src Port: 53710, Dst Port: 1234, Seq: 3378592508, Ack: 2477434132, Len: 9
                                                                            • Source Port: 53710
                                                                              • Destination Port: 1234
                                                                                • Sequence number: 3378592508
                                                                                  • Acknowledgment number: 2477434132
                                                                                    • 0101 .... = Header Length: 20 bytes (5)
                                                                                      • Flags: 0x018 (PSH, 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
                                                                                                  • .... ...1 .... = Acknowledgment: Set
                                                                                                    • .... .... 1... = Push: Set
                                                                                                      • .... .... .0.. = Reset: Not set
                                                                                                        • .... .... ..0. = Syn: Not set
                                                                                                          • .... .... ...0 = Fin: Not set
                                                                                                          • Window size value: 28800
                                                                                                            • Urgent pointer: 0
                                                                                                              • TCP payload (9 bytes)
                                                                                                              0000  d1ce04d2c9613b05  93aaa51450187080  
                                                                                                              0010  0000000073716473  716473710a        

                                                                                                              0000  ÑÎ.ÒÉa;..ª¥.P.p.
                                                                                                              0010  ....sqdsqdsq.   
                                                                                                              • Transmission Control Protocol, Src Port: 53710, Dst Port: 1234, Seq: 3378592517, Ack: 2477434132, Len: 9
                                                                                                                • Source Port: 53710
                                                                                                                  • Destination Port: 1234
                                                                                                                    • Sequence number: 3378592517
                                                                                                                      • Acknowledgment number: 2477434132
                                                                                                                        • 0101 .... = Header Length: 20 bytes (5)
                                                                                                                          • Flags: 0x018 (PSH, 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
                                                                                                                                      • .... ...1 .... = Acknowledgment: Set
                                                                                                                                        • .... .... 1... = Push: Set
                                                                                                                                          • .... .... .0.. = Reset: Not set
                                                                                                                                            • .... .... ..0. = Syn: Not set
                                                                                                                                              • .... .... ...0 = Fin: Not set
                                                                                                                                              • Window size value: 28800
                                                                                                                                                • Urgent pointer: 0
                                                                                                                                                  • TCP payload (9 bytes)
                                                                                                                                                  0000  04d2d1ce93aaa514  ????????50107080  
                                                                                                                                                  0010  00000000                            

                                                                                                                                                  0000  .ÒÑÎ.ª¥.????P.p.
                                                                                                                                                  0010  ....            
                                                                                                                                                  • Transmission Control Protocol, Src Port: 1234, Dst Port: 53710, Seq: 2477434132, Ack: ????, Len: 0
                                                                                                                                                    • Source Port: 1234
                                                                                                                                                      • Destination Port: 53710
                                                                                                                                                        • Sequence number: 2477434132
                                                                                                                                                          • Acknowledgment number: ?
                                                                                                                                                          • 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
                                                                                                                                                                        • .... ...1 .... = Acknowledgment: Set
                                                                                                                                                                          • .... .... 0... = Push: Not set
                                                                                                                                                                            • .... .... .0.. = Reset: Not set
                                                                                                                                                                              • .... .... ..0. = Syn: Not set
                                                                                                                                                                                • .... .... ...0 = Fin: Not set
                                                                                                                                                                                • Window size value: 28800
                                                                                                                                                                                  • Urgent pointer: 0