TCP Example

TCP ACK

Analyze a TCP ACK packet, the final step of the three-way handshake. Understand the ACK flag, sequence and acknowledgment numbers, and how the connection is established.

Hex Data

00 14 01 BB 00 00 00 02 00 00 00 03 50 10 20 00 20 00 00 00 00 00 00 00 00
Note: Only ACK flag set to 1, connection establishment completed
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handshakeack

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
2
Acknowledgment Number
Next sequence number the sender expects to receive
3
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)
0b101000000010000
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 Protocol Visualizer - Parse & Analyze TCP PacketsTCP Header Format and Fields Explained - ByteLensTCP Three-Way Handshake Packet Breakdown | ByteLensTCP Flags Explained - SYN, ACK, FIN, RST, PSH, URGTCP SYN Packet Example - ByteLensTCP SYN-ACK Packet Example - ByteLens

About TCP

Transmission Control Protocol (RFC 793)

Fields: 10
Endianness: big
Specification: RFC 793