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TCP/IP Guide by Charles M. Kozierok

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TCP Noncontiguous Acknowledgment Handling and Selective Acknowledgment (SACK)

Computer science people sometimes use the term elegant to describe a simple but effective solution to a problem or need. I think the term applies fairly well to the cumulative acknowledgment method that is part of the TCP sliding window system. With a single number, returned in the Acknowledgment Number field of a TCP segment, the device sending the segment can acknowledge not just a single segment it has received from its connection peer, but possibly several of them. We saw how this works in the discussion of the fundamentals of sliding windows in Chapter 46, and again in the previous discussion of retransmissions.

Even the most elegant technique has certain weaknesses, ...

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