I did a normal download of The Birth & Death of JavaScript with Wireshark turned on. Scrolling through the capture, I see packet after packet being received successfully.

For example, a packet with sequence number 563,321 arrived. Like all TCP packets, it had a "next sequence number", which is the number used for the following packet. This packet's "next sequence number" was 564,753. The next packet did, in fact, have sequence number 564,753, so everything was good. This happens thousands of times per second once the connection gets up to speed.

Occasionally, my computer sends a message to the server saying, for example, "I've received all packets up to and including packet number 564,753." That's an ACK, for acknowledgement: my computer acknowledges receipt of the server's packets. On a new connection, the Linux kernel sends an ACK after every ten packets. This is controlled by the TCP_INIT_CWND constant, which we can see defined in the Linux kernel's source code.

(The CWND in TCP_INIT_CWND stands for congestion window: the amount of data allowed in flight at once. If the network becomes congested – overloaded – then the window size will be reduced, slowing packet transmission.)

Ten packets is about 14 KB, so we're limited to 14 KB of data in flight at a time. This is part of TCP slow start: connections begin with small congestion windows. If no packets are lost, the receiver will continually increase the congestion window, allowing more packets in flight at once.

Eventually, a packet will be lost, so the receive window will be decreased, slowing transmission. By automatically adjusting the congestion window, as well as some other parameters, the sender and receiver keep data moving as quickly as the network will allow, but no quicker.

This happens on both sides of the connection: each side ACKs the other side's messages, and each side maintains its own congestion window. Asymmetric windows allow the protocol to take full advantage of network connections with asymmetric upstream and downstream bandwidth, like most residential and mobile Internet connections.