TCP Header for an FTP Client → Server Message

Ports chosen

• Well-known port (destination) = 21 — FTP control connection is always terminated at port 21 on the server.
• Ephemeral port (source) = 52345 — any unused port in the dynamic/private range 49152–65535 chosen by the client (e.g. 15151 or 52345 are both acceptable).

Lengths

• TCP header with no options = 5 words × 4 = 20 bytes → Data Offset = 5.
• Data = 40 bytes → total segment length = 20 + 40 = 60 bytes; total TCP length field in IP = 60.
• Checksum field is filled with 0000 as instructed (in real transmission it would be computed over the pseudo-header + TCP header + data).

Header entries

#	Field	Value	Notes
1	Source Port	52345	ephemeral port (client)
2	Destination Port	21	FTP well-known port (server)
3	Sequence Number	1000	(example) sequence number of first data byte
4	Acknowledgment Number	5000	(example) next byte expected from server
5	Data Offset	5	20-byte header, no options
6	Reserved	0	3 bits (4 bits + 4-bit NS in some texts)
7	Control Bits	URG=0, ACK=1, PSH=1, RST=0, SYN=0, FIN=0	acknowledging earlier data + pushing this data
8	Window	8192	advertised receive window (example)
9	Checksum	0x0000	all zeros, as specified
10	Urgent Pointer	0	URG bit is 0, field not used
11	Options	none	Data Offset stays 5 (20 bytes)
12	Data (payload)	40 bytes	e.g. an FTP command or file block

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Source Port = 52345       |    Dest Port = 21            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Sequence Number = 1000                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Acknowledgment Number = 5000                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Data |r|A|P|R|S|F|            Window = 8192                   |
| Offset| |C|S|S|Y|I|                                    0 0 0 0  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Checksum = 0x0000       |     Urgent Pointer = 0        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Data (40 bytes) ...                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Result: Source 52345 → Destination 21, 20-byte header + 40 bytes of data = 60-byte TCP segment, checksum 0000.