The TCP Windows Scale option

The TCP Window Scale option, defined in RFC1323 and described in the TCP chapter of Computer Networking: Principles, Protocols and Practice allows endhosts to use a larger window than TCP's default 64 KBytes window.

Paste this command into your terminal:

The password to connect is

Impact of the window scale option

The window scale option has been correctly negotiated on this connection. Assuming that that the client will use on the entire connection the same window size that it announced in the SYN, can you infer the value of the receive window that it will advertise inside its first segments ?

#	Length	Summary
0	28 bytes	`Transmission Control Protocol, Src Port: 53736, Dst Port: 1234, Seq: 1395955212, Len: 0`
1	28 bytes	`Transmission Control Protocol, Src Port: 1234, Dst Port: 53736, Seq: 2170178573, Ack: 1395955213, Len: 0`
2	20 bytes	`Transmission Control Protocol, Src Port: 53736, Dst Port: 1234, Seq: 1395955213, Ack: 2170178574, Len: 0`
3	22 bytes	`Transmission Control Protocol, Src Port: 53736, Dst Port: 1234, Seq: 1395955213, Ack: 2170178574, Len: 2`

0000 d1e804d25334960c 0000000070027080
0010 00000000020405a0 01030309

0000 Ñè.ÒS4......p.p.
0010 ............

Transmission Control Protocol, Src Port: 53736, Dst Port: 1234, Seq: 1395955212, Len: 0
- Source Port: 53736
- Destination Port: 1234
- Sequence number: 1395955212
- Acknowledgment number: 0
- 0111 .... = Header Length: 28 bytes (7)
- 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
- Calculated window size: 28800
- Urgent pointer: 0
- Options: (8 bytes), Maximum segment size, No-Operation (NOP), Window scale
  - TCP Option - Maximum segment size: 1440 bytes
    - Kind: Maximum Segment Size (2)
    - Length: 4
    - MSS Value: 1440
  - TCP Option - No-Operation (NOP)
    - Kind: No-Operation (1)
  - TCP Option - Window scale: 9 (multiply by 512)
    - Kind: Window Scale (3)
    - Length: 3
    - Shift count: 9
    - Multiplier: 512

0000 04d2d1e8815a4c0d 5334960d70127080
0010 00000000020405a0 01030309

0000 .ÒÑè.ZL.S4..p.p.
0010 ............

Transmission Control Protocol, Src Port: 1234, Dst Port: 53736, Seq: 2170178573, Ack: 1395955213, Len: 0
- Source Port: 1234
- Destination Port: 53736
- Sequence number: 2170178573
- Acknowledgment number: 1395955213
- 0111 .... = Header Length: 28 bytes (7)
- 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
- Calculated window size: 28800
- Urgent pointer: 0
- Options: (8 bytes), Maximum segment size, No-Operation (NOP), Window scale
  - TCP Option - Maximum segment size: 1440 bytes
    - Kind: Maximum Segment Size (2)
    - Length: 4
    - MSS Value: 1440
  - TCP Option - No-Operation (NOP)
    - Kind: No-Operation (1)
  - TCP Option - Window scale: 9 (multiply by 512)
    - Kind: Window Scale (3)
    - Length: 3
    - Shift count: 9
    - Multiplier: 512

0000 d1e804d25334960d 815a4c0e5010????
0010 00000000

0000 Ñè.ÒS4...ZL.P.??
0010 ....

Transmission Control Protocol, Src Port: 53736, Dst Port: 1234, Seq: 1395955213, Ack: 2170178574, Len: 0
- Source Port: 53736
- Destination Port: 1234
- Sequence number: 1395955213
- Acknowledgment number: 2170178574
- 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: 57
- Calculated window size: ?
- Urgent pointer: 0

0000 d1e804d25334960d 815a4c0e5018????
0010 00000000610a

0000 Ñè.ÒS4...ZL.P.??
0010 ....a.

Transmission Control Protocol, Src Port: 53736, Dst Port: 1234, Seq: 1395955213, Ack: 2170178574, Len: 2
- Source Port: 53736
- Destination Port: 1234
- Sequence number: 1395955213
- Acknowledgment number: 2170178574
- 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: 57
- Calculated window size: ?
- Urgent pointer: 0
- TCP payload (2 bytes)

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

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