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It is a nice comfort to be
able to rely on something

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and have it happen.

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And so many applications, such
as HTTP, and SSH and telnet--

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those application layer services
use, at the transport layer,

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TCP.

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And one of the
benefits of using TCP

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is that it's connection-oriented
and reliable.

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Meaning, there are going
to be sequence numbers

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and acknowledgment
numbers to validate

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that the communications
are working between the two

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devices.

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And one of the terms that
I've heard thrown around

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is a thing called the Protocol
Data Unit, which I just

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don't ever imagine using
that phrase in a conversation

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with a friend.

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Instead of calling it
a protocol data unit,

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we'd simply call it a
segment, for example, if we're

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talking about data at layer 4.

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Or we would call a protocol data
unit at layer 3, the network

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layer, we call that a
packet, or layer 2, we

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refer to that protocol
data unit as a frame.

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And that layer 1,
the physical layer,

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we're just dealing
with individual bits.

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So those would be the quote
unquote "protocol data units"

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at those respective layers.

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And as traffic is moved
across the network,

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various network devices focus
on and pay close attention

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to various layers.

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And so for the purpose of
moving a packet, for example,

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between one network
and another, that

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would be a router's function.

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And it's focusing on the
information at the network

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layer inside of each packet.

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We also have some devices on
our networks called switches.

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And one of the advantages
of a layer 2 switch

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is it has the ability to look
at the information at layer 2

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in the frames and
make forwarding

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decisions on a local network
based on that information

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and frame.

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And then we have devices
that the physical layer like

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a hub, or a physical
network adapter card,

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or cables that
are all considered

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to be layer 1
devices because they

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don't focus or process on
anything higher in the protocol

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stack.

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I wanted to focus,
in this Nugget,

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also on the TCP three-way
handshake, which

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is the magic which
allows TCP to do

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connection-oriented and reliable
communications between itself

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and another party.

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So when it comes
to TCP, setting up

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a reliable connection-oriented
session between two devices,

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it involves three transmissions.

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And they refer to it as
TCP's three-way handshake.

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And to set the stage
for this, let's

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imagine that Bob is going
to be acting as a client,

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and that Lois's computer
is running an HTTP server

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service on her computer.

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So we can just imagine
that Lois's computer

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is acting as a web server.

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And the TCP three-way
handshake goes like this.

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The individual or entity
that's initiating it

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is going to send
over a SYN request.

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Now, that's not a
SYN request like,

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hey, let's go rob somebody.

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This is a
synchronization request.

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And I put a B here to
represent that this

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is Bob's synchronization
request that he is sending over

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to Lois's computer.

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And in the TCP
header, there's going

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to be a little flag that
marked a little bit that's

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on that says, hey, this
is a synchronization.

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And there's also going to be
a little field for a sequence

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number.

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And let's just imagine
that Bob pulled

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from the air, the
number 951,057,939.

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And that's the sequence number
Bob's computer chose to use.

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And it sends it over to Lois.

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Also, as part of
that TCP segment,

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if Bob was requesting
web services,

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it would be going to
the well-known port

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of TCP port number 80.

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And if Lois is
listening on port 80,

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because she's running
the web server software,

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she can go ahead and
respond back to Bob.

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So if Bob sent a
SYN request, Lois

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is going to send an
acknowledgment of Bob's SYN

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request.

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She's going to send back an
acknowledgment number that

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is going to be the sequence
number of Bob plus one.

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So the acknowledgment here
should be 951, 057,940.

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So Lois is simply taking the
sequence number she received

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an incrementing by one.

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And when Bob receives
that, he goes, great.

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Not only do I know that she's
talking to me or the server

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talking to me, but for
my next sequence number,

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I'm going to use that 940.

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Because behind the scenes,
that's what that really means.

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It's the next
sequence number that's

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expected to come from Bob.

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So Bob's a happy client.

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He's thinking, great,
she wants to talk to me.

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However, Lois also, the
server in this case,

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needs to synchronize her
sequence numbers with Bob

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the other direction because
life is a two-way street.

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So as part of that
reply, not only

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will include the acknowledgment
and the acknowledgment number,

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but Lois is also going to
send her own synchronization

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request to Bob.

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And let's imagine, for
fun, that Lois's computer

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running the web server software
chose a sequence number as part

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of the request of 290,218,379.

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So all that information,
including the sequence

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numbers in the acknowledgment
numbers and the ACK and the SYN

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flags are all sent
back in one message

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back to Bob, at which
point, Bob needs

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to go ahead and acknowledge
the synchronization request

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that he got from Lois.

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So he's going to send
a final message back.

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So I've got a question for you.

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If Lois sent over
this sequence number

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as part of her SYN
request, what would

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be the appropriate
acknowledgment number that Bob

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should send back to Lois?

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[MUSIC PLAYING]

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And if we consider the
logic on the previous one,

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the acknowledgment is going
to be the initial sequence

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number plus 1.

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So if Louis's initial sequence
number is 290,218,379,

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Bob's acknowledgment
number to acknowledge

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that he received that
should be one more,

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which would be 290,218,380.

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These devices can now
send data back and forth.

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They can manage their
sequence numbers

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in acknowledgment
numbers to make sure

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that all the data that's
sent from one party

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is acknowledged
by the other side.

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And that, my friend,
is how TCP establishes

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reliable communications by
using its three-way handshake.

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I hope this has been
informative for you.

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And I'd like to thank
you for viewing.