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.

Paste this command into your terminal:

The password to connect is

Utilisation of the FIN flag

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

#	Length	Summary
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

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

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