Basically they’re violating RFC-3390.

You’re supposed to check the speed the receiver can handle before sending another packet, and then up your speed if it’s good, or stabilise if it isn’t. What they do is instead of waiting for the return packet confirming or denying the uprate in speed is good (initial window / IW), they pump multiple packets into the reciever straight up.

Google pumps around 8 or 9, Microsoft pumps in around 43(!). They’re not the only ones doing this: Amazon, Facebook, Cisco, they all do this!

Ben Strong’s Blog: Google and Microsoft Cheat on Slow-Start. Should You?.