What is delayed ACK in TCP and what trade-off does it introduce?

• A. A receiver may delay an ACK up to about 0.5 RTT to piggyback on data or group ACKs; can increase time to detect losses
• B. A receiver sends ACKs immediately with no delay
• C. ACKs are never delayed and are used only for flow control
• D. Delay ACK prevents loss detection but improves throughput

Answer: (A)

Delayed ACK in TCP means the receiver postpones sending an acknowledgment for a short period, typically up to about half a round-trip time, to see if it can piggyback the ACK on outgoing data or acknowledge multiple segments with a single ACK. This reduces the number of ACK packets, cutting overhead and potentially improving throughput.

The trade-off is slower feedback to the sender about received data, which can delay loss detection and the start of retransmission if something was lost. In other words, delaying ACKs saves bandwidth but can increase the time it takes to notice and respond to a missing segment.

Other statements don’t fit because ignoring the delay and sending ACKs immediately reduces the efficiency benefits of delayed ACK, and claiming ACKs are never delayed or that delay prevents loss detection while boosting throughput oversimplifies and misstates the behavior.