TCP Sliding Window Acknowledgment System For Data Transport, Reliability and Flow Control
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Dealing With Missing Acknowledgments
But wait what about bytes 42 through 45 in our example? Well, until segment #3 (containing bytes 37 to 41) shows up, the receiving device will not send an acknowledgment for those bytes, nor any others that show up after it. The sender will be able to send the new bytes added to Category #3: bytes 52 to 56. It will then stop, with the window stuck on bytes 37 to 41.
Of course, like any PAR system, TCP includes a system for timing transmissions and retransmitting. Eventually, the TCP device will re-send the lost segment, and hopefully this time it will arrive again. Unfortunately, one drawback of TCP is that since it does not separately acknowledge segments, this may mean retransmitting other segments that actually were received by the recipient (such as the segment with bytes 42 to 45). This starts to get very complex, as discussed in the topic on TCP retransmissions.
I should explicitly point out that despite the length of this topic, the preceding is just a summary description of the overall operation of sliding windows, and one that does not include all of the modifications used in modern TCP! As you can see, the sliding window mechanism is at the heart of the operation of TCP as a whole.
In the section that describes segments and discusses data transfer we will see in more detail how TCP transmitters decide how and when to create segments for transmission. The section describing TCP's reliability and data flow control features will provide much more information on how sliding windows enable a device to manage the flow of data to it on a TCP connection. It also discusses special problems that can arise if windows size is not carefully managed, and how problems such as congestion are avoided in TCP implementations through key changes to the basic sliding window mechanism described in this topic.
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