TCP Window Field Explained | Generated by AI
Question: In the TCP header structure, the 16-bit window field is used to implement which of the following TCP mechanisms?
A. Connection control
B. Flow control
C. Data transmission
D. Error control
Answer:
The correct answer is B. Flow control
Explanation:
The window field (also called receive window or advertised window, 16 bits long) in the TCP header is the primary mechanism TCP uses for flow control.
What flow control means here:
The receiver tells the sender:
“How much data (in bytes) I am currently willing and able to accept right now.”
This value is put in the 16-bit window field and sent back to the sender in every acknowledgment segment (ACK).
The sender is not allowed to send more data than the window size the receiver has advertised — this prevents the receiver’s buffer from overflowing. This is exactly what flow control means in TCP.
Quick comparison with other options:
-
A. Connection control
→ Mainly refers to connection establishment and termination (SYN, ACK, FIN flags and state machine), not the window field. -
C. Data transmission
→ Too vague. The window field controls how much data can be sent, but it is not the mechanism that “performs” data transmission itself. -
D. Error control
→ Handled mainly by sequence number, acknowledgment number, checksum, and retransmission mechanism — not by the window field.
Key point to remember:
The 16-bit window size field in the TCP header is used for sliding window flow control (receiver-based flow control).
Correct answer: B. Flow control
References:
- None (standard TCP protocol knowledge from RFC 793 and modern TCP specifications)