"Delayed Acknowledgments
As specified in RFC 1122: Requirements for Internet Hosts -- Communication Layers, Transmission Control Protocol (TCP) uses delayed acknowledgments (ACKs) to reduce the number of packets sent on the transmission media.
In Windows Transmission Control Protocol/Internet Protocol (TCP/IP), the stack takes a common approach to implementing delayed ACKs. As data is received by TCP on a connection, the stack only sends an acknowledgment back if one of the following conditions is met:
No ACK was sent for the previous segment received.
A segment is received, but no other segment arrives within 200 milliseconds for that connection.
In other words, an ACK is sent for every other TCP segment received on a connection, unless the delayed ACK timer expires after 200 milliseconds pass.
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