This explains how the TCP congestion control mechanism works:
What is this TCP Window Thing?
A TCP window the amount of outstanding (unacknowledged by the recipient) data a sender can send on a particular connection before it gets an acknowledgment back from the receiver that it has gotten some of it.
For example if a pair of hosts are talking over a TCP connection that has a TCP window size of 64 KB (kilobytes), the sender can only send 64 KB of data and then it must stop and wait for an acknowledgment from the receiver that some or all of the data has been received. If the receiver acknowledges that all the data has been received then the sender is free to send another 64 KB. If the sender gets back an acknowledgment from the receiver that it received the first 32 KB (which could happen if the second 32 KB was still in transit or it could happen if the second 32 KB got lost), then the sender could only send another 32 KB since it can't have more than 64 KB of unacknowledged data outstanding (the second 32 KB of data plus the third).
Why have a TCP window?
The primary reason for the window is congestion control. The whole network connection, which consists of the hosts at both ends, the routers in between and the actual connections themselves (be they fiber, copper, satellite or whatever) will have a bottleneck somewhere that can only handle data so fast. Unless the bottleneck is the sending speed of the transmitting host, then if the transmitting occurs too fast the bottleneck will be surpassed resulting in lost data. The TCP window throttles the transmission speed down to a level where congestion and data loss do not occur.
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