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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 FIN flag

TCP segments contain flags. These are defined in Section 3.1 of RFC793.

The packet trace below corresponds to the establishment of a TCP connection by a client that sent one byte of data and then decided to close the connection immediately.


Utilisation of the FIN flag

In this trace, can you infer which segments had their FIN flag set ?

# Length Summary Status
0 24 bytes Transmission Control Protocol, Src Port: 53716, Dst Port: 1234, Seq: 0, Len: 0
1 24 bytes Transmission Control Protocol, Src Port: 1234, Dst Port: 53716, Seq: 0, Ack: 1, Len: 0
2 20 bytes Transmission Control Protocol, Src Port: 53716, Dst Port: 1234, Seq: 1, Ack: 1, Len: 0
3 21 bytes Transmission Control Protocol, Src Port: 53716, Dst Port: 1234, Seq: 1, Ack: 1, Len: 1  
4 20 bytes Transmission Control Protocol, Src Port: 1234, Dst Port: 53716, Seq: 1, Ack: 2, Len: 0  
5 20 bytes Transmission Control Protocol, Src Port: 53716, Dst Port: 1234, Seq: 2, Ack: 1, Len: 0  
6 20 bytes Transmission Control Protocol, Src Port: 1234, Dst Port: 53716, Seq: 1, Ack: 3, Len: 0  
7 20 bytes Transmission Control Protocol, Src Port: 53716, Dst Port: 1234, Seq: 3, Ack: 2, Len: 0  

0000  d1d404d2cb01e4f8  0000000060027080  
0010  4f900000020405a0                    

0000  ÑÔ.ÒË.äø....`.p.
0010  O.......        
  • Transmission Control Protocol, Src Port: 53716, Dst Port: 1234, Seq: 0, Len: 0
    • Source Port: 53716
      • Destination Port: 1234
        • TCP Segment Len: 0
          • Sequence number: 0 (relative sequence number)
            • 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  04d2d1d48253f648  cb01e4f960127080  
                                              0010  4f900000020405a0                    

                                              0000  .ÒÑÔ.SöHË.äù`.p.
                                              0010  O.......        
                                              • Transmission Control Protocol, Src Port: 1234, Dst Port: 53716, Seq: 0, Ack: 1, Len: 0
                                                • Source Port: 1234
                                                  • Destination Port: 53716
                                                    • TCP Segment Len: 0
                                                      • Sequence number: 0 (relative sequence number)
                                                        • Acknowledgment number: 1 (relative ack 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  d1d404d2cb01e4f9  8253f64950107080  
                                                                                          0010  4f8c0000                            

                                                                                          0000  ÑÔ.ÒË.äù.SöIP.p.
                                                                                          0010  O...            
                                                                                          • Transmission Control Protocol, Src Port: 53716, Dst Port: 1234, Seq: 1, Ack: 1, Len: 0
                                                                                            • Source Port: 53716
                                                                                              • Destination Port: 1234
                                                                                                • TCP Segment Len: 0
                                                                                                  • Sequence number: 1 (relative sequence number)
                                                                                                    • Acknowledgment number: 1 (relative ack 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
                                                                                                                                0000  d1d404d2cb01e4f9  8253f64950??7080  
                                                                                                                                0010  4f8d00000a                          

                                                                                                                                0000  ÑÔ.ÒË.äù.SöIP?p.
                                                                                                                                0010  O....           
                                                                                                                                • Transmission Control Protocol, Src Port: 53716, Dst Port: 1234, Seq: 1, Ack: 1, Len: 1
                                                                                                                                  • Source Port: 53716
                                                                                                                                    • Destination Port: 1234
                                                                                                                                      • TCP Segment Len: 1
                                                                                                                                        • Sequence number: 1 (relative sequence number)
                                                                                                                                          • Acknowledgment number: 1 (relative ack number)
                                                                                                                                            • 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
                                                                                                                                                          • .... ...1 .... = Acknowledgment: Set
                                                                                                                                                            • .... .... 1... = Push: Set
                                                                                                                                                              • .... .... .0.. = Reset: Not set
                                                                                                                                                                • .... .... ..0. = Syn: Not set
                                                                                                                                                                  • .... .... ...? = Fin: ?
                                                                                                                                                                • Window size value: 28800
                                                                                                                                                                  • Checksum: 0x4f8d [unverified]
                                                                                                                                                                    • Urgent pointer: 0
                                                                                                                                                                      • TCP payload (1 byte)
                                                                                                                                                                      0000  04d2d1d48253f649  cb01e4fa50??7080  
                                                                                                                                                                      0010  4f8c0000                            

                                                                                                                                                                      0000  .ÒÑÔ.SöIË.äúP?p.
                                                                                                                                                                      0010  O...            
                                                                                                                                                                      • Transmission Control Protocol, Src Port: 1234, Dst Port: 53716, Seq: 1, Ack: 2, Len: 0
                                                                                                                                                                        • Source Port: 1234
                                                                                                                                                                          • Destination Port: 53716
                                                                                                                                                                            • TCP Segment Len: 0
                                                                                                                                                                              • Sequence number: 1 (relative sequence number)
                                                                                                                                                                                • Acknowledgment number: 2 (relative ack number)
                                                                                                                                                                                  • 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
                                                                                                                                                                                                • .... ...1 .... = Acknowledgment: Set
                                                                                                                                                                                                  • .... .... 0... = Push: Not set
                                                                                                                                                                                                    • .... .... .0.. = Reset: Not set
                                                                                                                                                                                                      • .... .... ..0. = Syn: Not set
                                                                                                                                                                                                        • .... .... ...? = Fin: ?
                                                                                                                                                                                                      • Window size value: 28800
                                                                                                                                                                                                        • Checksum: 0x4f8c [unverified]
                                                                                                                                                                                                          • Urgent pointer: 0
                                                                                                                                                                                                          0000  d1d404d2cb01e4fa  8253f64950??7080  
                                                                                                                                                                                                          0010  4f8c0000                            

                                                                                                                                                                                                          0000  ÑÔ.ÒË.äú.SöIP?p.
                                                                                                                                                                                                          0010  O...            
                                                                                                                                                                                                          • Transmission Control Protocol, Src Port: 53716, Dst Port: 1234, Seq: 2, Ack: 1, Len: 0
                                                                                                                                                                                                            • Source Port: 53716
                                                                                                                                                                                                              • Destination Port: 1234
                                                                                                                                                                                                                • TCP Segment Len: 0
                                                                                                                                                                                                                  • Sequence number: 2 (relative sequence number)
                                                                                                                                                                                                                    • Acknowledgment number: 1 (relative ack number)
                                                                                                                                                                                                                      • 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
                                                                                                                                                                                                                                    • .... ...1 .... = Acknowledgment: Set
                                                                                                                                                                                                                                      • .... .... 0... = Push: Not set
                                                                                                                                                                                                                                        • .... .... .0.. = Reset: Not set
                                                                                                                                                                                                                                          • .... .... ..0. = Syn: Not set
                                                                                                                                                                                                                                            • .... .... ...? = Fin: ?
                                                                                                                                                                                                                                          • Window size value: 28800
                                                                                                                                                                                                                                            • Checksum: 0x4f8c [unverified]
                                                                                                                                                                                                                                              • Urgent pointer: 0
                                                                                                                                                                                                                                              0000  04d2d1d48253f649  cb01e4fb50??7080  
                                                                                                                                                                                                                                              0010  4f8c0000                            

                                                                                                                                                                                                                                              0000  .ÒÑÔ.SöIË.äûP?p.
                                                                                                                                                                                                                                              0010  O...            
                                                                                                                                                                                                                                              • Transmission Control Protocol, Src Port: 1234, Dst Port: 53716, Seq: 1, Ack: 3, Len: 0
                                                                                                                                                                                                                                                • Source Port: 1234
                                                                                                                                                                                                                                                  • Destination Port: 53716
                                                                                                                                                                                                                                                    • TCP Segment Len: 0
                                                                                                                                                                                                                                                      • Sequence number: 1 (relative sequence number)
                                                                                                                                                                                                                                                        • Acknowledgment number: 3 (relative ack number)
                                                                                                                                                                                                                                                          • 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
                                                                                                                                                                                                                                                                        • .... ...1 .... = Acknowledgment: Set
                                                                                                                                                                                                                                                                          • .... .... 0... = Push: Not set
                                                                                                                                                                                                                                                                            • .... .... .0.. = Reset: Not set
                                                                                                                                                                                                                                                                              • .... .... ..0. = Syn: Not set
                                                                                                                                                                                                                                                                                • .... .... ...? = Fin: ?
                                                                                                                                                                                                                                                                              • Window size value: 28800
                                                                                                                                                                                                                                                                                • Checksum: 0x4f8c [unverified]
                                                                                                                                                                                                                                                                                  • Urgent pointer: 0
                                                                                                                                                                                                                                                                                  0000  d1d404d2cb01e4fb  8253f64a50??7080  
                                                                                                                                                                                                                                                                                  0010  4f8c0000                            

                                                                                                                                                                                                                                                                                  0000  ÑÔ.ÒË.äû.SöJP?p.
                                                                                                                                                                                                                                                                                  0010  O...            
                                                                                                                                                                                                                                                                                  • Transmission Control Protocol, Src Port: 53716, Dst Port: 1234, Seq: 3, Ack: 2, Len: 0
                                                                                                                                                                                                                                                                                    • Source Port: 53716
                                                                                                                                                                                                                                                                                      • Destination Port: 1234
                                                                                                                                                                                                                                                                                        • TCP Segment Len: 0
                                                                                                                                                                                                                                                                                          • Sequence number: 3 (relative sequence number)
                                                                                                                                                                                                                                                                                            • Acknowledgment number: 2 (relative ack number)
                                                                                                                                                                                                                                                                                              • 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
                                                                                                                                                                                                                                                                                                            • .... ...1 .... = Acknowledgment: Set
                                                                                                                                                                                                                                                                                                              • .... .... 0... = Push: Not set
                                                                                                                                                                                                                                                                                                                • .... .... .0.. = Reset: Not set
                                                                                                                                                                                                                                                                                                                  • .... .... ..0. = Syn: Not set
                                                                                                                                                                                                                                                                                                                    • .... .... ...? = Fin: ?
                                                                                                                                                                                                                                                                                                                  • Window size value: 28800
                                                                                                                                                                                                                                                                                                                    • Checksum: 0x4f8c [unverified]
                                                                                                                                                                                                                                                                                                                      • Urgent pointer: 0