IntrotoNetworkingChapter2.pdf

Chapter 2

Network Architecture

To engineer and build a system as complex as the Internet, en-

gineers try to break a single challenging problem into a set of

smaller problems that can be solved independently and then put

back together to solve the original large problem. The engineers

who built the first internets broke the overall problem into four

basic subproblems that could be worked on independently by dif-

ferent groups.

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Figure 2.1: The Four-Layer TCP/IP Model

They gave these four areas of engineering the following names:

(1) Link, (2) Internetwork, (3) Transport, and (4) Application. We

visualize these different areas as layers stacked on top of each

other, with the Link layer on the bottom and the Application layer

on the top. The Link layer deals with the wired or wireless con-

nection from your computer to the local area network and the

Application layer is what we as end users interact with. A web

13

14 CHAPTER 2. NETWORK ARCHITECTURE

browser is one example of an application in this Internet architec-

ture.

We informally refer to this model as the “TCP/IP model” in refer-

ence to the Transport Control Protocol (TCP) used to implement

the Transport layer and Internet Protocol (IP) used to implement

the Internetwork layer.

We will take a quick look at each of the layers, starting from the

“bottom” of the stack.

2.1 The Link Layer

The Link layer is responsible for connecting your computer to its

local network and moving the data across a single hop. The

most common Link layer technology today is wireless network-

ing. When you are using a wireless device, the device is only

sending data a limited distance. A smartphone communicates

with a tower that is a few kilometers away. If you are using your

smartphone on a train, it needs to switch to a new tower every

few minutes when the train is moving. A laptop that is connected

to a WiFi network is usually communicating with a base station

within 200 meters. A desktop computer that is connected using

a wired connection is usually using a cable that is 100 meters

long or shorter. Link layer technologies are often shared amongst

multiple computers at the same location.

The Link layer needs to solve two basic problems when dealing

with these shared local area networks. The first problem is how

to encode and send data across the link. If the link is wireless,

engineers must agree on which radio frequencies are to be used

to transmit data and how the digital data is to be encoded in the

radio signal. For wired connections, they must agree on what

voltage to use on the wire and how fast to send the bits across

the wire. For Link layer technologies that use fiber optics, they

must agree on the frequencies of light to be used and how fast to

send the data.

In addition to agreeing on how to send data using a shared

medium such as a wireless network, they also need to agree on

how to cooperate with other computers that might want to send

data at the same time. If all the computers on the network tried

to transmit whenever they had data to send, their messages

would collide. The result would be chaos, and receiving stations

would only receive noise. So we need to find a fair way to allow

each station to wait its turn to use the shared network.

2.1. THE LINK LAYER 15

The idea of breaking a large message into packets and then send-

ing each packet separately makes this sharing easier. If only one

computer wants to send data, it will send its packets one right

after another and move its data across the network as quickly as

it can. But if three computers want to send data at the same time,

each computer will send one packet and then wait while the other

two computers send packets. After each of the other computers

sends a packet, the first computer will send its next packet. This

way the computers are sharing access to the network in a fair

way.

But how does a computer know if other computers want to send

data at the same time? Engineers designed an ingenious method

to solve this problem called “Carrier Sense Multiple Access with

Collision Detection”, or CSMA/CD. It is a long name for a simple

and elegant concept. When your computer wants to send data,

it first listens to see if another computer is already sending data

on the network (Carrier Sense). If no other computer is sending

data, your computer starts sending its data. As your computer is

sending data it also listens to see if it can receive its own data. If

your computer receives its own data, it knows that the channel

is still clear and continues transmitting. But if two computers

started sending at about the same time, the data collides, and

your computer does not receive its own data. When a collision is

detected, both computers stop transmitting, wait a bit, and retry

the transmission. The two computers that collided wait different

lengths of time to retry their transmissions to reduce the chances

of a second collision.

When your computer finishes sending a packet of data, it pauses

to give other computers that have been waiting a chance to send

data. If another computer senses that your computer has stopped

sending data (Carrier Sense) and starts sending its own packet,

your computer will detect the other computer’s use of the net-

work and wait until that computer’s packet is complete before

attempting to send its next packet.

This simple mechanism works well when only one computer

wants to send data. It also works well when many computers

want to send data at the same time. When only one computer is

sending data, that computer can make good use of the shared

network by sending packets one after another, and when many

computers want to use the shared network at the same time,

each computer gets a fair share of the link.

Some link layers, like a cellular connection for a smartphone, a

WiFi connection, or a satellite or cable modem, are shared con-

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Figure 2.2: Carrier Sense/Collision Detection

nections and need techniques like CSMA/CD to insure fair access

to the many different computers connected to the network. Other

link layers like fiber optic cables and leased lines are generally

not shared and are used for connections between routers. These

non-shared connections are still part of the Link layer.

The engineers working on Link layer technologies focus solving

the issues so computers can transmit data across a single link

that ranges in distance from a few meters to as long as hundreds

of kilometers. But to move data greater distances, we need to

send our packets through multiple routers connected by multiple

link layers. Each time our packet passes through another link

layer from one router to another we call it a “hop”. To send data

halfway around the world, it will pass through about 20 routers,

or make 20 “hops”.

2.2 The Internetwork Layer (IP)

Once your packet destined for the Internet makes it across the

first link, it will be in a router. Your packet has a source address

2.2. THE INTERNETWORK LAYER (IP) 17

and destination address and the router needs to look at the des-

tination address to figure out how to best move your packet to-

wards its destination. With each router handling packets destined

for any of many billions of destination computers, it’s not possible

for every router to know the exact location and best route to ev-

ery possible destination computer. So the router makes its best

guess as to how to get your packet closer to its destination.

Each of the other routers along the way also does its best to get

your packet closer to the destination computer. As your packet

gets closer to its final destination, the routers have a better idea

of exactly where your packet needs to go. When the packet

reaches the last link in its journey, the link layer knows exactly

where to send your packet.

We use a similar approach to route ourselves when going on hol-

iday. A holiday trip also has many hops. Perhaps the first hop is

driving your car or taking a cab or bus to a train station. Then

you take a local train from your small town to a larger city. In the

larger city you take a long-distance train to a large city in another

country. Then you take another local train to the small village

where you will stay for your holiday. When you get off the train,

you take a bus, and when you get off the bus, you walk to your

hotel.

If you were on the train between the two large cities and you

asked the conductor the exact location of your hotel in the small

village, the conductor would not know. The conductor only knows

how to get you closer to your destination, and while you are on

the long-distance train that is all that matters. When you get on

the bus at your destination village, you can ask the bus driver

which stop is closest to your hotel. And when you get off the bus

at the right bus stop, you can probably ask a person on the street

where to find the hotel and get an exact answer.

The further you are from your destination, the less you need to

know the exact details of how to get there. When you are far

away, all you need to know is how to get “closer” to your destina-

tion. Routers on the Internet work the same way. Only the routers

that are closest to the destination computer know the exact path

to that computer. All of the routers in the middle of the journey

work to get your message closer to its destination.

But just like when you are traveling, unexpected problems or de-

lays can come up that require a change in plans as your packets

are sent across the network.

Routers exchange special messages to inform each other about

18 CHAPTER 2. NETWORK ARCHITECTURE

Figure 2.3: A Multi-Step Trip

any kind of traffic delay or network outage so that packets can

be switched from a route that is no longer working to a different

route. The routers that make up the core of the Internet are smart

and adapt quickly to both small and large outages or failures of

network connections. Sometimes a connection slows down be-

cause it is overloaded. Other times a connection is physically bro-

ken when a construction crew mistakenly digs up a buried wire

and cuts it. Sometimes there is a natural disaster like a hurricane

or typhoon that shuts down the routers and links in a large ge-

ographical area. The routers quickly detect these outages and

reroute around them if possible.

But sometimes things go wrong and packets are lost. Dealing with

lost packets is the reason for the next layer in our architecture.

2.3 The Transport Layer (TCP)

The Internetwork layer is both simple and complex. It looks at

a packet’s destination address and finds a path across multiple

network hops to deliver the packet to the destination computer.

2.3. THE TRANSPORT LAYER (TCP) 19

But sometimes these packets get lost or badly delayed. Other

times the packets arrive at their destination out of because

a later packet found a quicker path through the network than an

earlier packet. Each packet contains the source computer’s ad-

dress, the destination computer’s address, and an offset of where

this packet “fits” relative to the beginning of the message. Know-

ing the offset of each packet from the beginning of the message

and the length of the packet, the destination computer can recon-

struct the original message even if the packets were received out

of .

As the destination computer reconstructs the message and deliv-

ers it to the receiving application, it periodically sends an acknowl-

edgement back to the source computer indicating how much of

the message it has received and reconstructed. But if the des-

tination computer finds that parts of the reconstructed message

are missing, this probably means that these packets were lost

or badly delayed. After waiting a bit, the destination computer

sends a request to the source computer to resend the data that

seems to be missing.

The sending computer must store a copy of the parts of the orig-

inal message that have been sent until the destination computer

acknowledges successful receipt of the packets. Once the source

computer receives the acknowledgment of successful receipt of

a portion of the message, it can discard the data that has been

acknowledged and send some more data.

The amount of data that the source computer sends before wait-

ing for an acknowledgement is called the “window size”. If the

window size is too small, the data transmission is slowed because

the source computer is always waiting for acknowledgments. If

the source computer sends too much data before waiting for an

acknowledgment, it can unintentionally cause traffic problems by

overloading routers or long-distance communication lines. This

problem is solved by keeping the window size small at the begin-

ning and timing how long it takes to receive the first acknowledge-

ments. If the acknowledgments come back quickly, the source

computer slowly increases the window size and if the acknowl-

edgements come back slowly, the source computer keeps the

window size small so as not to overload the network. Just like at

the Link layer, a little courtesy on the Internet goes a long way

toward ensuring good use of the shared network infrastructure.

This strategy means that when the network has high-speed con-

nections and is lightly loaded the data will be sent quickly, and

if the network is heavily loaded or has slow connections the data

20 CHAPTER 2. NETWORK ARCHITECTURE

will be slowed down to match the limitations of the network con-

nections between the source and destination computers.

2.4 The Application Layer

The Link, Internetwork, and Transport layers work together to

quickly and reliably move data between two computers across

a shared network of networks. With this capability to move data

reliably, the next question is what networked applications will be

built to make use of these network connections.

When the first widely used Internet came into being in the mid-

1980s, the first networked applications allowed users to log in to

remote computers, transfer files between computers, send mail

between computers, and even do real-time text chats between

computers.

In the early 1990s, as the Internet came to more people and com-

puters’ abilities to handle images improved, the World Wide Web

application was developed by scientists at the CERN high-energy

physics facility. The web was focused on reading and editing net-

worked hypertext documents with images. Today the web is the

most common network application in use around the world. But

all the other older Internet applications are still in wide use.

Each application is generally broken into two halves. One half of

the application is called the “server”. It runs on the destination

computer and waits for incoming networking connections. The

other half of the application is called the “client” and runs on the

source computer. When you are browsing the web using software

like Firefox, Chrome, or Internet Explorer, you are running a “web

client” application which is making connections to web servers

and displaying the pages and documents stored on those web

servers. The Uniform Resource Locators (URLs) that your web

browser shows in its address bar are the web servers that your

client is contacting to retrieve documents for you to view.

When we develop the server half and the client half of a net-

worked application, we must also define an “application protocol”

that describes how the two halves of the application will exchange

messages over the network. The protocols used for each applica-

tion are quite different and specialized to meet the needs of the

particular application. Later we will explore some of these Appli-

cation layer protocols.

2.5. STACKING THE LAYERS 21

2.5 Stacking the Layers

We usually show the four different layers (Link, Internetwork,

Transport, and Application) stacked on top of each other with

the Application layer at the top and the Link layer at the bottom.

The reason we show them this way is because each layer makes

use of the layers above and below it to achieve networked

communications.

All four layers run in your computer where you run the client ap-

plication (like a browser), and all four layers also run in the des-

tination computer where the application server is running. You

as the end user interact with the applications that make up the

top layer of the stack, and the bottom layer represents the WiFi,

cellular, or wired connection between your computer and the rest

of the Internet.

The routers that forward your packets from one to another to

move your packets towards their destination have no understand-

ing of either the Transport or Application layers. Routers oper-

ate at the Internetwork and Link layers. The source and destina-

tion addresses at the Internetwork layer are all that is needed for

routers to move your packets across the series of links (hops) to

get them to the destination. The Transport and Application lay-

ers only come into play after the Internetwork layer delivers your

packets to the destination computer.

If you wanted to write your own networked application, you would

likely only talk to the Transport layer and be completely uncon-

cerned about the Internetwork and Link layers. They are essen-

tial to the function of the Transport layer, but as you write your

program, you do not need to be aware of any of the lower-layer

details. The layered network model makes it simpler to write net-

worked applications because so many of the complex details of

moving data from one computer to another can be ignored.

Up next, we will talk about these four layers in more detail.

2.6 Glossary

client: In a networked application, the client application is the

one that requests services or initiates connections.

fiber optic: A data transmission technology that encodes data

using light and sends the light down a very long strand of thin

22 CHAPTER 2. NETWORK ARCHITECTURE

glass or plastic. Fiber optic connections are fast and can cover

very long distances.

offset: The relative position of a packet within an overall mes-

sage or stream of data.

server: In a networked application, the server application is the

one that responds to requests for services or waits for incoming

connections.

window size: The amount of data that the sending computer is

allowed to send before waiting for an acknowledgement.

2.7 Questions

You can take this quiz online at http://www.net-intro.com/quiz/

1. Why do engineers use a “model” to organize their approach

to solving a large and complex problem?

a) Because it allows them to build something small and test it

in a wind tunnel

b) Because talking about a model delays the actual start of the

hard work

c) Because they can break a problem down into a set of smaller

problems that can be solved independently

d) Because it helps in developing marketing materials

2. Which is the top layer of the network model used by TCP/IP

networks?

a) Application

b) Transport

c) Internetwork

d) Link

3. Which of the layers concerns itself with getting a packet of

data across a single physical connection?

a) Application

b) Transport

c) Internetwork

2.7. QUESTIONS 23

d) Link

4. What does CSMA/CD stand for?

a) Carrier Sense Multiple Access with Collision Detection

b) Collision Sense Media Access with Continuous Direction

c) Correlated Space Media Allocation with Constant Division

d) Constant State Multiple Address Channel Divison

5. What is the goal of the Internetwork layer?

a) Insure that no data is lost while enroute

b) Get a packet of data moved across multiple networks from

its source to its destination

c) Make sure that only logged-in users can use the Internet

d) Insure than WiFi is fairly shared across multiple computers

6. In addition to the data, source, and destination addresses,

what else is needed to make sure that a message can be

reassembled when it reaches its destination?

a) An offset of where the packet belongs relative to the begin-

ning of the message

b) A location to send the data to if the destination computer is

down

c) A compressed and uncompressed version of the data in the

packet

d) The GPS coordinates of the destination computer

7. What is “window size”?

a) The sum of the length and width of a packet

b) The maximum size of a single packet

c) The maximum number of packets that can make up a mes-

sage

d) The maximum amount of data a computer can send before

receiving an acknowledgement

8. In a typical networked client/server application, where does

the client application run?

a) On your laptop, desktop, or mobile computer

24 CHAPTER 2. NETWORK ARCHITECTURE

b) On a wireless access point

c) On the closest router

d) In an undersea fiber optic cable

9. What does URL stand for?

a) Universal Routing Linkage

b) Uniform Retransmission Logic

c) Uniform Resource Locator

d) Unified Recovery List