Subtitles of the Movie

Yet another component of TCP you need to be aware of is Windowing. Now, before I tell you what windowing is, let me explain a little bit. Not all hosts or computers can send or receive data as fast as any other host on the network and this might be due to different reasons. A limitation may be a slow, old network card that's in use in one of the hosts or one of the OSs in an older version of OS or even different implementations of the TCP/IP Stack on operating systems may affect how much data can be sent or received. Another factor could be network connection speeds. For example, a computer using a gigabit Ethernet sending data to a computer that's using a 10100 Ethernet Card may have some problems sending and receiving data at the same rate. In order to get around this, TCP uses a concept called Windowing. Now, we call these Windows or Sliding Windows and, of course, don't get this confused with the operating system Windows. This has nothing to do with Windows. TCP's Windows, however, basically enable two hosts in a TCP conversation to determine how much data can be sent and received at a time. This is what we call Flow Control between these two computers; determining the amount of data that the receiving computer can accept and that determines how much data the sending computer will send. Now, this Window size is a number that specifies how much data can be received by the receiving computer at any given time. It's a 16-bit number and what happens is every time there's a TCP segment acknowledgement sent from the receiver back to the sender, it includes the TCP Widow size. That tells how much data can be sent by the sending computer before waiting on an acknowledgement from the receiving computer. So the sending computer will send data until it hits the Window size. Then it will stop sending data and wait on an acknowledgement so that it knows that the receiving computer actually received all the data it sent. Then it'll start sending data again. Now, as network conditions improve or as the computer has the ability to accept more data into its buffer, it may send a larger Window size. So this is why we call it Sliding Windows, because it can vary. During a demonstration that we'll have at the end of this section, we'll cover Window sizes and I'll show you what it looks like.