Information

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

Tags

Sign in

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 ?

# Length Summary Status
0 24 bytes Transmission Control Protocol, Src Port: 53712, Dst Port: 1234, Seq: 4207062859, Len: 0
1 24 bytes Transmission Control Protocol, Src Port: 1234, Dst Port: 53712, Seq: 4037529898, Ack: ????, Len: 0  
2 20 bytes Transmission Control Protocol, Src Port: 53712, Dst Port: 1234, Seq: ????, Ack: ????, Len: 0  

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
                                              0000  04d2d1d0f0a7d12a  ????????60127080  
                                              0010  4f900000020405a0                    

                                              0000  .ÒÑÐð§Ñ*????`.p.
                                              0010  O.......        
                                              • Transmission Control Protocol, Src Port: 1234, Dst Port: 53712, Seq: 4037529898, Ack: ????, Len: 0
                                                • Source Port: 1234
                                                  • Destination Port: 53712
                                                    • TCP Segment Len: 0
                                                      • Sequence number: 4037529898
                                                        • Acknowledgment number: ?
                                                        • 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
                                                                      • .... ...1 .... = Acknowledgment: 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
                                                                                        0000  d1d004d2????????  ????????50107080  
                                                                                        0010  4f8c0000                            

                                                                                        0000  ÑÐ.Ò????????P.p.
                                                                                        0010  O...            
                                                                                        • Transmission Control Protocol, Src Port: 53712, Dst Port: 1234, Seq: ????, Ack: ????, Len: 0
                                                                                          • Source Port: 53712
                                                                                            • Destination Port: 1234
                                                                                              • TCP Segment Len: 0
                                                                                                • Sequence number: ?
                                                                                                • 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
                                                                                                                        • Checksum: 0x4f8c [unverified]
                                                                                                                          • Urgent pointer: 0