TCP And Sliding Windows

In order to allow fast traffic over connections with high bandwidth and high latency (round-trip packet delay), several packets are sent at the same time by the sending host. After they are all acknowledged, the host will send more packets. The total size of un-acknowledged packets pending is called "the window size".

The window size may be modified while the connection is active. If the receiving host may sustain more bandwidth, it may enlarge the window size. If it receives too much traffic,  it may request that the window size becomes smaller. TCP supports such window size negotiation.