TCP Segment Retransmission Timers and the Retransmission Queue
(Page 2 of 3)
Recognizing When a Segment is Fully Acknowledged
One issue we have yet to discuss is how we know when a segment has been fully acknowledged. Retransmissions are handled on a segment basis, but TCP acknowledgments, as we have seen, are done on a cumulative basis using sequence numbers. Each time a segment is sent by Device A to Device B, B looks at the value of the Acknowledgment Number field in the segment. All bytes with sequence numbers lower than the value of this field have been received by A.
Thus, a segment sent by B to A is considered acknowledged when all of the bytes that were sent in the segment have a lower sequence number than the latest Acknowledgment Number sent by B to A. This is determined by calculating the last sequence number of the segment using its first byte number (in the Sequence Number field) and length of the segments Data field.
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