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

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The source and destination ports in the TCP headers

The TCP header, shown in the TCP chapter of Computer Networking: Principles, Protocols and Practice contains a source port and a destination port. The two segments below correspond to the beginning of a TCP connection.


TCP source and destination ports

The first TCP segment was sent by a client to a server to establish a connection. From the response returned by the server, can you infer the source and destination ports used in the first segment ?

# Length Summary Status
0 24 bytes Transmission Control Protocol, Src Port: ????, Dst Port: ????, Seq: 0, Len: 0  
1 24 bytes Transmission Control Protocol, Src Port: 1234, Dst Port: 53710, Seq: 0, Ack: 1, Len: 0

0000  ????????c9613aec  0000000060027080  
0010  00000000020405a0                    

0000  ????Éa:ì....`.p.
0010  ........        
  • Transmission Control Protocol, Src Port: ????, Dst Port: ????, Seq: 0, Len: 0
    • Source Port: ?
    • Destination Port: ?
    • Sequence number: 0 (relative sequence number)
      • 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
                      • .... ...0 .... = Acknowledgment: Not set
                        • .... .... 0... = Push: Not set
                          • .... .... .0.. = Reset: Not set
                            • .... .... ..1. = Syn: Set
                              • .... .... ...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  04d2d1ce93aaa513  c9613aed60127080  
                                      0010  00000000020405a0                    

                                      0000  .ÒÑÎ.ª¥.Éa:í`.p.
                                      0010  ........        
                                      • Transmission Control Protocol, Src Port: 1234, Dst Port: 53710, Seq: 0, Ack: 1, Len: 0
                                        • Source Port: 1234
                                          • Destination Port: 53710
                                            • Sequence number: 0 (relative sequence number)
                                              • Acknowledgment number: 1 (relative ack number)
                                                • 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
                                                              • .... ...1 .... = Acknowledgment: Set
                                                                • .... .... 0... = Push: Not set
                                                                  • .... .... .0.. = Reset: Not set
                                                                    • .... .... ..1. = Syn: Set
                                                                      • .... .... ...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