Now let's take a little closer look at TCP, and let's talk about the Transmission Control Protocol. Now TCP is one of the core Protocols in the TCP/IP Suite, and as a matter of fact, is one of the two original, if you want to call them, charter members of TCP, and TCP is actually a compliment to the Internet Protocol, that's in TCP/IP as well. Now TCP as it turns out, really is the foundation of most Internet applications, and we'll talk about Internet applications in a little more detail in some other places in the course, but when you're thinking Internet application, think world wide web. We think of the world wide web as a separate network whatever, and it's not. It is a functionality that happens on all of these interconnected computers all over the world, and we just know the application, or the functionality in these browsers, as the world wide web. Email is an Internet application. We just type email addresses, and send them off, and magically they arrive, and people respond to us. And the underlying Protocols in the TCP/IP Suite are making that magic happen for email. And then of course file transfers. Now what exactly does TCP do for us? Well it provides reliable, pay attention to that word, reliable ordered delivery of data packets. Now it works with IP, or the Internet Protocol to break the data down into smaller chunks which we call packets, and then it establishes a session, or it establishes a connection. It dials the number if you will, with the destination computer. And then it sends the packets. Now probably the most unique thing about TCP that you need to remember, and we're back to that word reliable, is it verifies the delivery on the destination. And if it sees that it's putting the packets together, and it's numbering them and putting them back into order, it sees that one of the packets has not been received, it will kind of wait for a few minutes, and there's a functionality called a TCP Window, that once we go past a certain point, and it hasn't received that particular packet, it assumes it got lost, or damaged out on the network, or whatever, and it will re-request that particular packet. So it makes sure that our entire message or all of our data gets there, and it can be a reassembled properly. And I've already mentioned that it re-orders packets on the destination, if they're out of order, and most often, they will be. Now did I mention reliable? And I know I'm beating a dead horse here, but when you think TCP on the Exam, and in real life, think reliable delivery OK? And you could also call that Stateful. Now here's a little diagram, and you want to watch this as it pertains to TCP, and then as we move through other videos and talk about other Protocols, think back to this basic animation. We have two computers and I want to send the message, this is a test, from this computer to this computer, and I'm going to use TCP to do that. Now, the first thing that happens, TCP is going to work with the IP Protocol and break that sentence down into shorter packets. Now this is an over simplification, but notice, our original statement, this is a test, has now been broken down into four discreet packets. It then establishes that session across the network, across the wire, to the destination PC, and then it moves that stuff across the wire. And notice, it has arrived on the other side, but if you'll also notice, that the packets arrived out of order. These might have taken different routes, they might have got caught in network congestion in the routers, but anyway one arrived before the other, and TCP will re-order these, to get it back in the right order. And then that is has been delivered. So that's what TCP does, it takes care of all that functionality, but this little part about, and I didn't put it in the animation, but if we never receive the A here, if we never receive this datagram, TCP would have send a message back to the source PC, and said hey, I didn't get this particular datagram, can you re-send it? And it would say sure, and it would re-send it, and it would come, and TCP would re-order, and make sure everybody's in the right order, and then the message would be here. So that's a good rough view, simplistic view of TCP, and if you can understand that TCP is reliable, and that it's breaking things into packets, moving them across the network, making sure everybody gets there, putting them in order, then you know what you need to know at this point about TCP.
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