Question

Illustrate with a diagram how basic TCP Windowing works.

Answer 1

`Hey,

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TCP uses “windowing” which means that a sender will send one or more data segments and the receiver will acknowledge one or all segments. When we start a TCP connection, the hosts will use a receive buffer where we temporarily store data before the application can process it.

When the receiver sends an acknowledgment, it will tell the sender how much data it can transmit before the receiver will send an acknowledgment. We call this the window size. Basically, the window size indicates the size of the receive buffer.

Typically the TCP connection will start with a small window size and every time when there is a successful acknowledgement, the window size will increase. Here’s an example:

Above we have two hosts, the host on the left side will send one segment and the host on the right side will send an acknowledgment in return. Since the acknowledgement was successful, the windows size will increase:

The host on the left side is now sending two segments and the host on the right side will return a single acknowledgment. Everything is working fine so the window size will increase even further:

The host is now sending four segments and the host on the right side responds with a single acknowledgment.

In the example above the window size keeps increasing as long as the receiver sends acknowledgments for all our segments or when the window size hits a certain maximum limit. When the receiver doesn’t send an acknowledgment within a certain time period (called the round-trip time) then the window size will be reduced.

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