TCP Example

TCP FIN

Analyze a TCP FIN packet used to close a TCP connection. Understand the FIN and ACK flags, sequence numbers, acknowledgment numbers, and graceful TCP teardown behavior.

Hex Data

00 14 01 BB 01 00 00 10 02 00 00 11 50 11 20 00 20 00 00 00 00 00 00 00 00
Note: FIN flag requests connection closure
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teardownfin

Field Breakdown

Source Port
Source port number
20
FTP Data
Destination Port
Destination port number
443
HTTPS
Sequence Number
Sequence number of the first data byte in this segment
16777232
Acknowledgment Number
Next sequence number the sender expects to receive
33554449
Data Offset
Size of TCP header in 32-bit words (minimum 5, maximum 15)
80
Reserved
Reserved for future use (must be zero)
0b1010000
Flags
Control flags (NS, CWR, ECE, URG, ACK, PSH, RST, SYN, FIN)
0b101000000010001
Window Size
Size of the receive window (flow control)
8192
Checksum
Checksum for error detection
0x2000
Urgent Pointer
Offset from sequence number indicating last urgent data byte
0

Related Resources

TCP Packet Parser & Header Decoder | ByteLensTCP Header Format and Fields Explained - ByteLensTCP Three-Way Handshake Packet Breakdown | ByteLensTCP Flags Explained - SYN, ACK, FIN, RST, PSH, URGTCP SYN Packet Example - ByteLensTCP ACK Packet Example - ByteLens

About TCP

Transmission Control Protocol (RFC 793)

Fields: 10
Endianness: big
Specification: RFC 793