So each end host keeps what's called a sliding window to manage the number of packets that have been sent out to regulate it. And that sliding window will respond to congestion conditions as we will see next. But under TCP each sender keeps a congestion window, or sliding window, if you will. which is really the limit on the number of outstanding or unacknowledged, meaning by outstanding, packets, that a device can have at one time. Right? So it's the number of packets you can send out without knowing for sure if they're going to ever get to the receiver. And so, when an acknowledgement receives then you slide the window and you sent, going out sort of like this. [UNKNOWN] and the current window size, or CWND, we call the congestion window size CWND for an abbreviation. The congestion window size is three, then that means that the sender can send three packets, one, two, three out into the network. And then once [INAUDIBLE] receives an acknowledgement for one of the packets, it can slide the window by one and send another one, because now there's only two outstanding, so one more would make three. And then once the acknowledgement comes back for the second packet, you can slide them and send the fifth packet, because then three, four and five make three. So notice that at any given time, there's only three outstanding packets and the window size is staying fixed at three. So, it would've been really way too conservative if they had just allowed you to only send one packet out at a time. because then we'd, we'd have very slow transmission rates and it would be just way too cautious. So there's a, there's a, there's, there's a trade-off between the the window size though and how much congestion we have, but this is a, it seemed to be acceptable as very good trade-off in terms of TCP. so it's really similar to Netflix's DVD policy where you're paying the monthly rental rate. so, it allows you a fixed number of movies at one time. Right? So if you're window in Netflix was three, it would mean that any given time you can have three, movies outstanding that you haven't returned yet. And, you can pay for a larger window. Right? So if you, if you pay more each month, then you can have four or five outstanding movies at any given time. Then once you return one you can get another one. But at any time, you're only allowed four or five, it's really also for security reasons too. Right? because they don't want you to just take 100 movies and never return any of them. So it's for both of those instances. But just keep the idea of a sliding window in mind. it's a very very important concept for TCP to understand that once an acknowledgement is received, then we slide the window and we can send another packet. We're fixed in this window is really confining us to how many packets we're allowed to have out one time.