TCP Acknowledgments
Recall that a TCP connection is a bidirectional, full-duplex logical circuit that consists of outbound and inbound logical pipes for the inbound and outbound byte streams. To account for data sent and received, each byte in the outbound and inbound byte streams is numbered. These numbers are used by TCP for reliable data transfer and are independent of the actual data in the byte streams.
A TCP acknowledgment (ACK) is a TCP segment with the ACK flag set. In an ACK, the Acknowledgment Number field indicates the number for the next byte in the contiguous byte stream that the ACK’s sender expects to receive. Additionally, if the TCP Selective Acknowledgment (SACK) option is present, the ACK can indicate up to four blocks of noncontiguous ...
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