Original subtitles

If you need to move a box from one place to the other.

A common way to accomplish that is to get a moving truck.

We can put boxes in the moving truck.

Move that box to another location and unload the moving truck.

In the case of networking, the road that we are driving on is the network.

So if you've got a wireless network, a DSL network, a cable modem network,

or you have an Ethernet switch, that is the road that we're using

to transport this particular package in this metaphor,

the truck itself is what we will consider Internet Protocol or IP.

We've designed all of these networks so that this type of truck can move.

Most efficiently between one point and another.

In each moving truck is a box, and inside of that box is your data.

The moving truck has no idea what's inside of that box.

All it knows is that it has one job,

which is to put the box in the back of the truck, transport

it across the network, and unload that box on the other side.

If you were to open up the box, there would be a certain type

of information inside. It may be specific

to a certain application or particular function on your network.

You can think of the

information in this box as belonging to a particular room in your house.

So when the box is delivered, that box needs to be moved

to the bathroom, the kitchen, or the living room.

It's this encapsulation process that allows us to move all kinds of data

across the network.

So we would have application data we would put inside of a box.

We would place that box inside of a truck.

That truck would then move across the network.

And on the other side we remove the box from the truck.

We open up the box and we take out the application data.

If you were to look

at this visually, this is the way that our networks operate.

On one side may be your device, a workstation, a client,

a laptop computer, and on the other side is a server.

This could be a web server or a mail server,

or any other type of server that you'd be connecting to.

This client is going to send

information to this server, and if this is across an Ethernet network,

then everything inside of that Ethernet packet is what we call

our Ethernet payload.

In Ethernet, we also have a header at the beginning of this particular frame

and an Ethernet trailer that's at the end of the frame.

This Ethernet payload could have anything inside of it,

but as you're probably aware, the most popular protocol we use on

our networks is the Internet Protocol or IP.

So our Ethernet payload will have an IP header.

And then there will be an IP payload within that particular part of the frame.

Obviously the IP payload has information inside of it.

This could be TCP data with a TCP header and a TCP payload.

And as you probably can expect,

that TCP payload can be separated out into different types of data.

So for this entire Ethernet frame inside we have IP inside of IP.

We have TCP and inside of TCP we have Http data.

Let's drill down into this IP

packet at the TCP protocol and the UDP protocol

TCP and UDP are transported inside of that IP packet.

We commonly say that they are encapsulated within IP, and they are

two very common ways to move data from one part of the network to the other.

You may be using TCP for some applications and UDP for other applications.

You might also hear someone refer to TCP or UDP

as operating at the transport layer of the OSI model.

Sometimes we refer to this as ozone layer four.

You may think that IP is all you would need to be able to move data

from one part of the network to the other, and in many ways you would be correct.

But TCP and UDP add additional capabilities that IP can't provide.

For example, these provide multiplexing so that you can have many different

applications on your system communicating to a separate server all simultaneously.

So your workstation is sending a lot of information for a lot of

different applications to the server, and the server is able

to determine what applications are in use through the use of this multiplexing.

Let's really

break down the difference between TCP and UDP.

TCP stands for the Transmission Control Protocol.

We often refer to this as a connection oriented protocol.

That's because there is a formal process to set up the flow from one device

to the other, and a formal process to tear down that flow.

When the conversation is over.

We sometimes refer to TCP as reliable delivery.

This doesn't mean that TCP somehow works better or faster

than other protocols on the network.

It means the TCP has a built in system to ensure that data that has been sent

has been verified as being received on the other side.

This allows us with a number of different features behind the scenes,

and one of the most important is that TCP can reorder messages

that may have been received out of order, which sometimes can happen on networks

that have multiple links to a single location,

and TCP can manage a retransmission process

so that if any data is not received by the destination,

that information can be sent from the source.

TCP also has a flow control mechanism so that if a device feels

that it's receiving information too quickly,

it can tell the other side to slow down the process

so that information can be received at a more reasonable rate.

UDP is the User Datagram Protocol.

With TCP we had a connection oriented flow, but UDP is a connection less flow.

There's no formal process to set up a traffic flow,

and there's no formal process to tear that down at the end.

UDP simply sends data from one place to the other, and

it's a very simple transaction to be able to send information across the network

because there's no acknowledgment being sent by the destination device.

We refer to UDP as unreliable.

Again, this doesn't mean that UDP does not work as well as

any other protocol on the network.

It only means that we have no receipt or any knowledge that the information

that we've sent was really received by the device on the other side.

With TCP, we had a way to regulate the flow of communication across the network.

But because this is a single conversation between one device and another,

there's no flow control on UDP and no way to determine

whether a device should slow down or send information faster.

If you compare these

two protocols, it does sound like UDP is not as functional

and therefore may not be the best choice for sending information over our network.

But in reality, UDP plays a very important role

in being able to send information very quickly over the network.

UDP is most associated with real time communication

communication where you can't stop retransmit information

and then catch up with yourself.

If you're on a phone call, there's no way to rewind the conversation

and send a packet that may have been missed a second or two ago.

With UDP, we simply send the data.

If it makes it to the other side than we were successful.

If the information was dropped along the way,

we simply keep the conversation going.

An example of protocols that use this connection.

This form of communication is Dhcp, which is the Dynamic

Host Configuration Protocol.

This is the protocol

that we use to automatically assigned IP addresses to our devices.

And another protocol that uses UDP is Tftp or the trivial file

transfer Protocol, which uses UDP as its transport mechanism.

Since both of these protocols are using UDP,

information is simply sent across the network with no type of acknowledgment

that the data was received on the other side.

It's up to the application, therefore, to keep track

of who has received information and who has not received information.

So in the example we gave before, Dhcp is responsible

for making sure that information may have been received by the other side.

So if it sends information and doesn't receive a response,

Dhcp is responsible for rescinding that data over the network.

With TCP,

we receive an acknowledgment for any packets that are sent over the network.

An example of protocols that take advantage

of this return receipt functionality is Https.

That's the Hypertext Transfer Protocol.

Secure, commonly used to send information in our web browsers

and protocols like Secure Shell or SSH, which provide us with an encrypted form

of terminal communication between our systems.

If our Https data between a web server and a client

somehow loses a packet between point A and point

B, TCP will recognize that that packet was missing.

It will ask to retransmit that information and the retransmitted data

is sent over the network.

All of this happens automatically with TCP and Https.

And SSH don't have to worry about managing the process of getting data

from one side to the other.

So far in our

moving truck metaphor, we know that we have our IP delivery truck.

This truck is moving information from one physical address

in the world of networking, this is one IP address

and it's delivering it to another IP address.

Just as every house that's on your block has a unique mailing address.

Every computer that's inside your network has a unique IP address.

At this point, our moving truck IP has taken information

from one IP address and moved it to another IP address.

Once that box is received at the destination IP address,

there is more information that needs to be examined on the label

to determine where that box is to go inside of the house.

Inside of your house, for example, you have many different rooms.

There's a bathroom, a kitchen, a living room, and a bedroom.

And when this box is delivered,

we have to determine what room is going to receive that box.

In the case of TCP and UDP, there's an additional piece

of information that's added to all of those conversations.

And that piece of information is a port number.

That port number determines what room in the house is going to receive this data.

Or in the case of an actual server,

what application on the server is going to receive this data?

The port numbers written on the outside of the box.

So when the boxes received at the front door, we can look at the box

and see, oh, that box needs to go to the bedroom.

In the case of port numbers,

each room has a number, and we know that the bedroom is port 80.

We have a living room, a port for 43, our bathroom is port

25 and the kitchen will be port 123.

When we receive the box at the front door, we look at the port number.

It says port 80, so we can move this box

inside of the house and deliver it into the bedroom.

In the case of our server, we have four different services

running on the same IP address.

We have a web server sending unencrypted data on port 80.

We have a web server sending encrypted data on port 443.

We've got a mail server on port 25 and a time server on port 123.

When this packet is received by our IP address at the front door,

we examine the port number.

And if the port number says this is for the service running at port 443,

that packet is delivered into that service.

That's running on that device.

This is where the multiplexing feature comes from that I mentioned earlier.

This front door is going to be receiving a lot of boxes destined for this

IP address.

And the port number allows us to know exactly what

service running on this device will be receiving that data.

So to complete this traffic flow

between these two devices, we need some information.

The first would be the server's IP address a protocol

that would be in use, such as TCP or UDP.

And for that protocol

what server application port number should be assigned for that?

In the example we had before, we had the house

that had four different ports running inside of that house for port

80, port for 43, port 25, and port 123.

Those are all associated with this server IP address.

The protocol and the application port numbers.

The client communicating with that server also has an IP address.

It's communicating using TCP or UDP.

And there are port numbers that it is using to send that data

so that when a response is received, we know exactly what that response

is associated with.

It's important on our server that the port numbers we're communicating

with are well known.

For example, if the browser on a client wants to communicate with a web server,

we know that that web server commonly uses TCP port 80 and TCP port 443.

Every web server we communicate with will use those same port numbers,

so that we know exactly where that service is located on that IP address.

Because these port numbers

are usually permanent, we refer to them as non ephemeral ports.

This means they are non temporary port numbers that tend to be

the same every time we access that device.

Port number 80 is commonly associated with Http port

number 443 is commonly associated with Https.

And if you go to any web server,

that's usually the port numbers that will be in use.

If you were to look at those port numbers,

they're commonly between port zero and port 1023.

But these port numbers can really be anything as long as they're port numbers

that are commonly known and well known across multiple devices.

When you're communicating to the server, you need a port number

on your device that you can associate with this particular traffic flow.

These are usually temporary port numbers.

And once that traffic flow is over, will no longer use that port number.

We refer to these as ephemeral ports or temporary port numbers, and commonly

an operating system will assign a port number between

1024 and 65,535.

But this is often configured

in the operating system itself, and it's assigned in real time.

As you're using these applications.

If you were to look at a protocol decode of these conversations, both TCP and UDP

can therefore use any port number between 0 and 65,535.

As we mentioned,

most services are going to use non ephemeral or non temporary port numbers.

But that's not always the case.

There are some applications that use dynamic port numbers

that can change dramatically from one device to the other.

Just keep in mind that this is simply a number associated with that service.

And if we know that, then we're able to communicate

with that service and have a conversation to send data.

You might also think that you could change the port number

on the server to something that is not well known,

and that would be more secure because it might hide the application

or keep other people from accessing that application.

But port numbers are not designed to be a security mechanism.

They're simply designed

to allow you to access those services on that particular device.

It's relatively easy to use a port scanner to find all of the open ports

on a particular server, and then begin to do more research

to determine what service is really running on that port.

This means when you access all of the different sites

that you visit on the internet, that all of those sites are going to be

using the same port numbers, which are well known.

This allows you to simply type in the name of the website,

and you're immediately connected to that site and able to transfer data.

If all of those different websites used completely different port numbers,

we would have to have another mechanism in place to somehow determine

what the appropriate port number might be for that individual site.

You can see why having well known port numbers makes the process so much simpler.

Although these port numbers can range between port zero and port 65,535,

TCP has its own set of port numbers that are different than UDP port numbers.

This means that there could be a service running on TCP port 80,

but a completely different service running on UDP port 80.

As you can imagine, having one service running on TCP port 80

and another service running on UDP port 80 could be a bit confusing,

which is why we don't tend to do that in normal operation.

So let's take a scenario from the picture we looked at earlier.

We have a client on the left side.

Its IP address is ten .0.0.1.

It is communicating to a server on the other side.

Its IP address is ten .0.0.2.

And you can see there is web server traffic communicating over TCP port 80

to this server.

The server is also a voice over IP

server using traffic communicating over UDP port 5004.

The server is also an email server communicating over TCP port 143.

We want to send information from this client to the server.

We'll send web server traffic with Http data inside of it will send VoIP traffic.

There's the VoIP data inside of that packet, and email

traffic with email data inside of the packet.

If we were to look at this a little bit closer,

we would see the comparison of port numbers on both sides of the conversation.

So we have these two devices the client the server.

And you can see the source IP address of ten .0.0.1.

That's our client.

And the destination IP address of ten .0.0.2 which is our server.

When the client wants to send data to the server it's the Http data.

So we know we're going to be sending data

to a destination port, a port 80 for our web traffic.

We know that that's going to use TCP data.

And we know the well known port for web traffic for this

server is TCP destination port 80.

But we need some port number to send this information from.

So this client will pick a random port number.

And in this example this client pick the random port number up 3000.

So the TCP source port is 3000.

Heading to a TCP destination port which is a well known port,

a port 80 on the server to be able to send the Http data.

At the same time,

this client wants to communicate to the server using voice over IP.

Our source of destination IP is the same.

You can see that UDP is being used in this scenario because we are using

VoIP traffic and VoIP traffic uses UDP.

The destination port is 5004, which is the well-known port number

for this VoIP server, and we picked a random port number

to send this traffic over UDP using port 7100.

The same thing applies for the third conversation

that's occurring simultaneously,

where the source and destination IP address in this example are identical.

The destination port number is TCP port 143.

Because this is email traffic, and this client picked a random source

port of TCP port 40 407 to send this email data.

You can see that

we're sending a lot of information simultaneously across the network.

But because we're using IP addresses and port numbers, the server knows exactly

where this traffic goes.

Once it's received by this destination device.