The servers of Microsoft and Google are trying so hard to offer you a speedy download that they slow down all other uses of the internet. How does this happen? The creators of the basic internet protocols had to work with significantly slower internet speeds than we have today. That’s why, when laying down the foundations of TCP (Transmission Control Protocol), one of the core protocols of the internet, they implemented a congestion control strategy, called slow-start.
Slow-start is designed to avoid sending more data than the network is capable of transmitting. It requires the server to start sending small amounts of data first (initial window), and increase the number of segments until an acknowledgement signal is not received. Slow-start assumes that unacknowledged segments are due to network congestion (which is one of its weaknesses – we will talk about this later).
From Google’s point of view, slow-start is an obstacle that would slow down the load time of their website, that’s why they just cheat it. They can save unnecessary round trips by using a larger initial window than they are supposed to. Some other major websites cheat to some extent too, but there is only one other company that pushes the boat as far as Google, or even further, and that’s Microsoft.
Microsoft is skipping the slow-start altogether and set the initial window to the full buffer size the client can receive. As you might know, Microsoft has a monthly Patch Tuesday, when they release major updates for most of their products at the same time on the whole planet (at 18:00 or 17:00 GMT). As a result of this and the fact their servers skip slow-start, the internet speed of other uses can be significantly slowed from computers downloading updates. The impact can be noticed especially on home and smaller business networks.