FROM: 这个动画太棒了

动画中的文本：

1 - TCP send one window at a time and waiting for acks. Then send the next window. The 1st window contains 1 packet: Packet 0 is used to set up connection..

2 - Receiving ack0 triggers window sliding and sender transmitting the 2nd window of 2 packets: 1, 2. The left acked zone contains 1 acked packet, 0.

3 - When receiving an ack, extend the acked zone, slide the window, transmits 2 packets. Receiving acks of the 2nd window triggers sending 4 packets in the 3rd window.

4 - When receiving acks from the 3rd window, TCP sends the 4th window of 8 packets. Initially, TCP is in slow start state. The window size starts at 1, but keep doubling.

5 - When the 4th window is acked, TCP sends the 5th window of 8 packets. Now, window size growth is slowing down. It's in congestion avoidance state.

6 - Sliding window make TCP adaptive to network congestion. Transmission rate is clocked to ack receiving rate. Ack receiving is slowing down when congestion increases. Slower ack rate slows down transmission.