WEBVTT

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 Hello and welcome to this video
 on the TCP three-way handshake.

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 In this video, the topics I'm going
 to cover is the purpose of the three

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-way handshake. I'm going to define
 things called TCBs and sockets.

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 We're going to look at the difference
 between TCP active opens and TCP

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 passive opens. We're going to look
 at what specifically within the TCP

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 header is used to work with
 the three-way handshake.

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 I'm going to do an animation of
 how TCP connections are created.

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 So the TCP three-way handshake,
 what is its purpose?

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 So if you've been watching any of my
 videos on TCP or you've learned about

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 TCP elsewhere, you've probably learned
 that one of the big differentiators

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 of TCP versus UDP is that TCP is what's
 called connection oriented.

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 And one of the things that I stress when
 I stress what that means is that

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 TCP before it sends any data to a destination,
 first validates, is that

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 destination alive?

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 Is he actually there?

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 If the destination is there, then he
 says, okay, let me negotiate some

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 parameters with you up front that's going
 to control the flow of the sequence

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 of data we're going to exchange
 back and forth.

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 The TCP three-way handshake is the
 critical component in doing all of

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 that. So it starts the process
 of opening a connection.

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 It proves the existence
 of the destination.

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 It establishes what the initial sequence
 numbers are going to be for TCP

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 segments. And there's an exchange
 of parameters that takes place.

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 Now, in order to understand the TCP three
-way handshake, we have to understand

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 some terminology first.

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 This terminology of
 TCBs and sockets.

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 So what is a TCB?

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 It stands for a transmission
 control block.

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 Transmission control block.

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 And like this says here, this is a
 block of memory space allocated by

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 a CPU to maintain state information
 for a single TCP session.

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 Now, in that TCP, in that TCB, that
 transmission control block, a big

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 part of it is TCP socket
 information.

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 So what is in a transmission
 control block?

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 Well, if I was to draw it here, just
 to give you sort of a conceptual

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 idea, let's say that
 you are your source.

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 So you, the source, are 1.1.1.

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 And you just opened
 up a web browser.

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 So that's going to be HTTP.

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 And you're going to be going
 to a destination of 2222.

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 Well, a transmission control block
 is going to be created for that.

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 So for example, part of that transmission
 control block is going to be

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 your IP information.

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 1.1.1.1 is going to be you.

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 You're going to be going to 2222.

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 Another part of that transmission control
 block is going to be the destination

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 port number you're going to use.

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 Which in this case, if you're doing
 HTTP, is going to be port number 80,

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 you're going to come up with a random
 source port number to keep track

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 of this individual session of information,
 which might be 51,002.

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 So that's going to be in there.

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 You're also going to have, as data is
 coming in and going out, that data

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 is going to be stored in memory.

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 Okay, maybe for this particular session,
 memory cell location number A,

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 all the way through memory cell location
 number Z is where all that memory

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 is going to be stored.

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 So that's also going to be here in
 this transmission control block.

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 Well, to say memory location A through
 Z, that's going to be where all

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 the information is
 actually stored.

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 And a lot of other stuff, your sliding
 window mechanism, you know, bytes

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 received, bytes transmitted, everything
 TCP needs to keep track of the

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 session and the flow of the session
 and what's currently outstanding,

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 what currently needs
 to be acknowledged.

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 All of that will be kept here inside
 this transmission control block.

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 So, how is this created?

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 How's the transmission
 control block created?

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 Well, I just showed you one way.

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 One way is where you decide you're
 going to start at HTTP.

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 You pulled open a web browser.

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 You pulled open a web page.

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 There's two ways this
 could happen.

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 So one way is what's called
 an active open.

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 And that's what I just drew.

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 And we'll look at that again
 on the next slide.

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 And there's also something
 called passive opens.

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 So just to recap what I just talked about,
 TCP active opens, this happens

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 on you. Your TCP client, your smart
 phone, your tablet, your laptop.

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 The moment you open up an application,
 the moment you open up a new browser

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 and your web browser, anything that
 you're going to do, you're going to

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 do it. And the moment you open up a
 new browser, you're going to open

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 up a transmission control block.

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 And this is that transmission control
 block is not going to be created

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 until you start the
 application request.

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 So that's why we call
 it an active open.

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 You had to actively do something to
 get TCP to recognize, oh, I should

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 probably create a TCP.

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 Then once your transmission control
 block is created, now you can send

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 out your TCP send.

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 Keith, what's a TCP send?

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 Oh, we'll get there.

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 That's coming up.

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 But just be aware that you can't send
 out anything related to TCP until

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 your transmission control block is
 sitting there ready and waiting to

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 go. By the way, on this slide it says
 ISN, that stands for initial sequence

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 number. The initial sequence number
 that you're going to be putting in

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 your very first TCP segment.

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 Now what about the other side?

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 The TCP server. So you've got your TCP
 server sitting out there and they're

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 recognizing in advance
 certain things.

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 So for example, if I have a web server,
 okay, if I have a web server,

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 a Linux or a Ubuntu server or something
 like that, and it's running Apache

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 or some web server application, I'm
 going to put my files in there, my

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 graphics, whatever my web page content
 is, and then I'm going to basically

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 turn on the web service
 in that server.

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 Now that server is not
 sending anything out.

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 By turning on the web service, I'm
 basically turning on his listening

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 ability to listen to incoming
 TCP requests.

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 So he is creating what's called a passive
 connection or what's called

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 a passive open. So he's going to create
 a transmission control block just

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 like I sort of drew, but some of the
 elements in there are going to be

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 undefined. For example, in that block,
 he's going to put down his own

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 IP address, but the IP address of the
 other end of the session, well,

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 that hasn't happened yet.

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 So that's going to be empty.

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 That's waiting to be filled in by some
 client saying, hey, I'd like to

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 talk to you. The port numbers.

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 Well, for talking about I just started
 up the HTTP service in that server.

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 He will create the
 port number of 80.

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 He's listening on that.

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 But what's the port
 number coming in?

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 He doesn't know yet.

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 He's passively waiting
 to find that out.

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 So that's what a passive
 open is all about.

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 All right. So let's see.

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 How does TCP initiate
 a connection?

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 You may have heard of the
 TCP three way handshake.

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 What exactly is happening there?

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 And in order to do that, we're going to
 be focusing primarily on the control

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 flags field inside the TCP header.


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 Okay. So number one, a transmission
 control block is created as a result

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 of a passive open on the receiver.


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 Like I said, the receiver is,
 for example, your web server.

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 You just went in there and you
 activated the web application.

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 He's now listening.

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 He has just created
 a passive open.

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 Now he sits there for minutes or hours
 or days until somebody needs to

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 use him. Now, few miles or thousands
 of miles away, somebody opens up

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 their client and they
 create an active open.

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 They open up their web browser and they
 type in Google.com or something

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 like that. That creates a transmission
 control block on them, which is

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 concerned an active open.

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 So what are they going to do?

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 Well, they're going to send a TCP
 message from sender to receiver.

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 Now here's the thing.

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 When you open up your web browser, most
 likely, like 99% of us, it defaults

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 to some home page, right?

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 Whatever your home page is.

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 Hopefully your home
 page is I need.com.

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 If it's not, change
 it there right now.

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 But let's say your home
 page is not I need.com.

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 Let's say it's Google as an
 example, typical home page.

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 All right. So here you go.

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 You just open up your browser.

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 You open up a tab in your browser.


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 Go into Google. All right.

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 With a little box where you can
 type in, please give me I and E.

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 Okay. So, but in order to display that,
 okay, now to you, it looks like,

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 oh, there it is.

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 It displayed it.

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 But things had to happen in the background
 to see that Google tab in the

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 first place. So what had to happen is
 the HTTP process had to start up.

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 It had to create an HTTP get request
 or get message, which basically was

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 a message saying, hey, Google.com,
 I'm trying to get your home page.

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 I'm trying to get that.

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 So that's what the application
 creates.

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 Then HTTP says, hey,
 TCP, I need you.

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 I need to use your services.

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 So that's when this happened.

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 The active open, and we create a transmission
 control block with all those

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 fields I talked about in there.

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 Source IP address, destination IP address,
 port numbers, pointers to the

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 various memory locations and
 where those are going to be.

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 Transmission control
 block was open.

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 Now, we can't send this HTTP get message
 yet because we don't even know

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 if the Google web server
 is alive at this point.

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 So now we have to do the three-way handshake
 to find out if he's alive.

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 So this is the process
 that happens.

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 So the process that happens is TCP
 sends its very first segment.

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 Now, this segment does not have
 your HTTP inside of it.

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 It's basically just a TCP
 header all by itself.

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 There's no data behind it.

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 It's an IP header followed by
 a TCP header, and that's it.

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 And in the TCP header, in the control
 field, let's go back here.

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 Let's just go back to this
 for just a second.

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 Remember how the TCP header had the control
 flags that we see right here?

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 Well, one of those control flags
 is called the SIN flag.

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 It stands for Synchronize, meaning I
 want to synchronize a session with

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 you. This is my very
 first TCP segment.

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 I'd like to synchronize with you.

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 Synchronize like sequence numbers.


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 There's another flag in here
 called the ACK flag.

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 So now that we know that, let's
 go back to where we just were.

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 Okay, so here we see this.

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 So here we see this is our TCP segment
 going out and it has the SIN bit

0:12:21.080000 --> 0:12:23.640000
 set in the control flags.

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 What does X stand for?

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 That's his initial
 sequence number.

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 Our TCP header has to have
 a sequence number in it.

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 So this is going to be
 some random number.

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 You can't predict what
 that's going to be.

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 It's going to be some
 random number.

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 In this case, we're
 just saying it's X.

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 So right now the sender is
 saying, hey, are you there?

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 And by the way, this is going to be
 going to, if we were talking about

0:12:44.820000 --> 0:12:53.320000
 HTTP, the destination port
 number in here will be 80.

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 And then we'll have an IP
 header in front of this.

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 So this is the sender's way of saying,
 you know, knock, knock, knock,

0:13:00.480000 --> 0:13:02.560000
 receiver. Are you alive?

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 If you are alive, are you listening
 to HTTP on port number 80?

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 Can you let me know
 if you're there?

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 That's the purpose of this
 initial first TCP segment.

0:13:13.360000 --> 0:13:19.280000
 Now the receiver, if he is alive,
 has to respond to that.

0:13:19.280000 --> 0:13:23.020000
 So he receives the sin
 with sequence number X.

0:13:23.020000 --> 0:13:25.400000
 He's going to generate
 his own sin.

0:13:25.400000 --> 0:13:30.880000
 Because remember, the very first time
 I talk to you in this direction,

0:13:30.880000 --> 0:13:33.520000
 I raise the sin flag.

0:13:33.520000 --> 0:13:35.160000
 You only see that once.

0:13:35.160000 --> 0:13:39.200000
 Now you and I might end up exchanging
 thousands or tens of thousands of

0:13:39.200000 --> 0:13:43.580000
 TCP segments over the course of our
 talking, but the very first one I

0:13:43.580000 --> 0:13:46.440000
 send to you, I'm going
 to raise the sin flag.

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 And the very first one you send to me,
 you're going to raise the sin flag.

0:13:51.000000 --> 0:13:55.700000
 After that, we'll lower that flag and
 we will never exchange the sin flag

0:13:55.700000 --> 0:14:00.620000
 ever again. So right here, this is
 the very first time the receiver is

0:14:00.620000 --> 0:14:02.340000
 talking to the sender.

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 So he generates his own sin and he puts
 his own sequence number in there.

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 So this is kind of interesting.

0:14:08.940000 --> 0:14:13.140000
 Notice how there's a sequence number
 that started in this direction and

0:14:13.140000 --> 0:14:16.500000
 there's another sequence number
 that starts in this direction.

0:14:16.500000 --> 0:14:20.220000
 So this X is going to increment every
 single time something goes from

0:14:20.220000 --> 0:14:24.820000
 left to right and Y is going to increment
 every single time something

0:14:24.820000 --> 0:14:28.920000
 goes from right to left.

0:14:28.920000 --> 0:14:35.200000
 So notice that in this sin, here the
 server, the receiver saying, yep,

0:14:35.200000 --> 0:14:38.940000
 I'm alive, I'm here,
 I acknowledge you.

0:14:38.940000 --> 0:14:43.680000
 So in the acknowledgment packet, I put
 whatever your number was plus one.

0:14:43.680000 --> 0:14:48.560000
 So if the initial sequence number was,
 for example, 85, here he'll say

0:14:48.560000 --> 0:14:55.880000
 86, which means I'm acknowledging that
 I have received up to but not including

0:14:55.880000 --> 0:14:56.620000
 any other sequence number.

0:14:56.620000 --> 0:15:00.580000
 This is called forward explicit acknowledgments
 here, the way TCP does

0:15:00.580000 --> 0:15:08.800000
 that. So the sender now knows
 that the receiver is alive.

0:15:08.800000 --> 0:15:14.860000
 But just like the sender just got an acknowledgment
 of his sequence number,

0:15:14.860000 --> 0:15:18.260000
 he needs to acknowledge the
 receiver sequence number.

0:15:18.260000 --> 0:15:23.540000
 And this is the third part
 of the three way handshake.

0:15:23.540000 --> 0:15:27.580000
 He generates what's called an ACK.


0:15:27.580000 --> 0:15:30.460000
 Now this is just another
 TCP segment.

0:15:30.460000 --> 0:15:35.920000
 All three of these arrows here represent
 three distinct IP packets.

0:15:35.920000 --> 0:15:40.140000
 Each IP packet has a TCP header
 with no data behind it.

0:15:40.140000 --> 0:15:44.780000
 All we're doing is using the TCP header
 to raise and lower the sin or

0:15:44.780000 --> 0:15:49.380000
 ACK flags and to set
 sequence numbers.

0:15:49.380000 --> 0:15:58.560000
 So at this point, because the sender
 has sent out this third thing right

0:15:58.560000 --> 0:16:00.900000
 here, he goes into the
 established state.

0:16:00.900000 --> 0:16:03.640000
 He says, OK, this is established.

0:16:03.640000 --> 0:16:07.620000
 I'm ready to start sending
 data to the receiver.

0:16:07.620000 --> 0:16:10.840000
 He can see me. I know he's there.

0:16:10.840000 --> 0:16:12.920000
 He's acknowledged my
 sequence number.

0:16:12.920000 --> 0:16:14.300000
 We're good to go.

0:16:14.300000 --> 0:16:18.400000
 And as soon as this TCP segment gets
 all the way over to the receiver

0:16:18.400000 --> 0:16:23.740000
 and the receiver sees, OK, he acknowledged
 my sequence number.

0:16:23.740000 --> 0:16:28.860000
 Now the receiver can go into
 the established state.

0:16:28.860000 --> 0:16:34.180000
 And that is how the TCP three
 way handshake works.

0:16:34.180000 --> 0:16:37.400000
 Thank you for watching this video,
 and I hope you found it useful.