IntrotoNetworkingChapter1.pdf

Chapter 1

Introduction

Using the Internet seems pretty easy. We go to a web address

and up comes a page. Or we go to our favorite social site and

see pictures of our friends, families, and pets. But it takes a lot

of complex software and hardware to make the Internet seem so

simple. The design of the technologies that make today’s Inter-

net work started in the 1960s, and there were over 20 years of

research into how to build internetworking technologies before

the first “Internet” was built in the late 1980s by academics in a

project called NSFNet. Since then, the research and development

into improving network technologies has continued as networks

have become far larger and faster and globally distributed with

billions of computers.

In to better understand how today’s Internet works, we will

take a look at how humans and computers have communicated

using technology over the years.

1.1 Communicating at a Distance

Imagine a group of five people in a room sitting in a circle. As long

as they are courteous and don’t have more than one conversation

at the same time, it’s quite natural for any person to talk to any

other person in the room. They just need to be able to hear each

other and coordinate how to use the shared space in the room.

But what if we put these people in different rooms so they can

no longer see or hear each other? How could pairs of people

communicate with each other then? One way might be to run a

wire between each pair of people with a microphone on one end

and a speaker on the other end. Now everyone could still hear all

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the conversations. They would still need to be courteous to make

sure that there was only one conversation going on at the same

time.

Each person would need four speakers (one for each of the other

people) and enough pieces of wire to connect all the microphones

and speakers. This is a problem with five people and it gets far

worse when there are hundreds or thousands of people.

Using wires, microphones, and speakers is how early telephone

systems from the 1900s allowed people to make phone calls. Be-

cause they could not have separate wires between every pair of

telephones, these systems did not allow all pairs of people to be

connected at the same time. Each person had a single connection

to a human “operator”. The operator would connect two wires to-

gether to allow a pair of people to talk, and then disconnect them

when the conversation was finished.

Figure 1.1: Connecting Using Telephone Operators

The first local telephone systems worked well when a customer’s

home or business was close to the operator’s building and a wire

could be strung directly from the operator’s building to the per-

son’s home.

1.1. COMMUNICATING AT A DISTANCE 3

But what if thousands people who are hundreds of kilometers

apart need to be able to communicate? We can’t run 100-

kilometer wires from each home to a single central office. What

the telephone companies did instead was to have many central

offices and run a few wires between the central offices, then

share connections between central offices. For long distances, a

connection might run through a number of central offices. Before

the advent of fiber optic, long-distance telephone calls were

carried between cities on poles with lots of separate wires. The

number of wires on the poles represented the number of possible

simultaneous long-distance phone calls that could use those

wires.

Figure 1.2: Long-Distance Telephone Poles

Since the cost of the wires went up as the length of the wire

increased, these longer connections between offices were quite

expensive to install and maintain, and they were scarce. So in the

early days of telephones, local calls were generally quite inexpen-

sive. But long-distance calls were more expensive and they were

charged by the minute. This made sense because each minute

you talked on a long-distance call, your use of the long-distance

wires meant no one else could use them. The telephone compa-

nies wanted you to keep your calls short so their long-distance

lines would be available for other customers.

When telephone companies started using fiber optic, more ad-

vanced techniques were used to carry many simultaneous long-

distance conversations on a single fiber. When you look at an old

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photo and see lots of wires on a single pole, it generally means

they were telephone wires and not used to carry electricity.

1.2 Computers Communicate Differently

When humans talk on the phone, they make a call, talk for a

while, and then hang up. Statistically, most of the time, humans

are not talking on the phone. At least they weren’t before ev-

eryone had smartphones. But computers, including the applica-

tions on your smartphone, communicate differently than humans

do. Sometimes computers send short messages to check if an-

other computer is available. Computers sometimes send medium-

sized information like a single picture or a long email message.

And sometimes computers send a lot of information like a whole

movie or a piece of software to install that might take minutes or

even hours to download. So messages between computers can

be short, medium, or long.

In the earliest days of connecting computers to one another, pairs

of computers were connected with wires. The simplest way to

send data from one computer to another was to line up the out-

going messages in a queue and send the messages one after

another as fast as the computers and the wires could carry the

data. Each message would wait for its turn until the messages

ahead of it were sent, and then it would get its chance to be sent

across the connection.

When the computers were in the same building, the building

owner could run wires to connect them. If the computers were

in the same town, the owners of the computers generally had

to lease wires from the telephone companies to connect their

computers. They often would have the phone company connect

the wires together in their central office so that it was not

necessary for one computer to “dial” the other computer to

send data. These leased lines were convenient for computer

communications because they were “always on”, but they were

also quite expensive because they were used 24 hours a day.

When the computers were even farther away, in different cities,

the leased lines were extended using the longer wires connect-

ing the central offices. Since there were so few wires between

central offices, these long-distance leased lines were quite ex-

pensive and their cost increased dramatically as the length of the

leased line increased. But if you had enough money, you could

lease direct connections between your computers so they could

1.3. EARLY WIDE AREA STORE-AND-FORWARD NETWORKS 5

exchange data. This worked pretty well as long as you were only

using one brand of computers, because each computer company

had their own way of using telephone wires to connect their com-

puters together and send data.

1.3 Early Wide Area Store-and-Forward

Networks

In the 1970s and 1980s, people working at universities around

the world wanted to send each other data and messages using

these computer-to-computer connections. Since the cost for each

connection was so high and increased with distance, computers

generally only had connections to other nearby computers. But

if the computer that you were connected to was connected to

another computer and that computer in turn was connected to

another computer, and so on, you could send a message a long

distance as long as each of the computers along the route of the

message agreed to store and forward your message.

Figure 1.3: Store-and-Forward Networks

Over time, with relatively few connections you could send data

long distances across a patchwork of network connections as long

as you were patient. Along the way, after your message reached

one computer, it would have to wait until its turn came to be

sent to the next computer along the route. A message would

arrive at an intermediate computer, be stored for a while (perhaps

hours, depending on traffic), and then be forwarded one more

connection (or “hop”).

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Sending entire messages one at a time this way, a message might

take minutes, hours, or even days to arrive at its ultimate desti-

nation, depending on the traffic at each of the hops. But even if

it took a few hours for an email message to find its way from one

part of the country to another, this was still much quicker and

easier than sending a letter or postcard.

1.4 Packets and Routers

The most important innovation that allowed messages to move

more quickly across a multi-hop network was to break each mes-

sage into small fragments and send each fragment individually.

In networking terms, these pieces of messages are called “pack-

ets”. The idea of breaking a message into packets was pioneered

in the 1960s, but it was not widely used until the 1980s because it

required more computing power and more sophisticated network-

ing software.

When messages are broken into packets and each packet is sent

separately, if a short message was sent after a large message

had begun, the short message did not have to wait until the entire

long message was finished. The first packet of the short message

only had to wait for the current packet of the large message to be

finished. The system alternated sending packets from the long

and short messages until after a while the short message was

completely sent and the long message resumed making full use

of the network connection.

Breaking the message into packets also greatly reduced the

amount of storage needed in the intermediate computers be-

cause instead of needing to store an entire message for as long

as a few hours, the intermediate computer only needed to store

a few packets for a few seconds while the packets waited for

their turns on the outbound link.

As networks moved away from the store-and-forward approach,

they started to include special-purpose computers that special-

ized in moving packets. These were initially called “Interface Mes-

sage Processors” or “IMPs” because they acted as the interface

between general-purpose computers and the rest of the network.

Later these computers dedicated to communications were called

“routers” because their purpose was to route the packets they

received towards their ultimate destination.

By building routers that specialized in moving packets across mul-

tiple hops, it became simpler to connect computers from multiple

1.5. ADDRESSING AND PACKETS 7

Figure 1.4: Sending Packets

vendors to the same network. To connect any computer to the

network, now all you needed to do was connect it to one router

and then the rest of the communication details were handled by

the other routers.

When multiple computers at one location were connected to-

gether in a “Local Area Network” (or LAN) using physical wiring,

you would connect a router to the local area network. By sending

data through the router, all the computers on the local area

network could send data across the “Wide Area Network” (or

WAN).

1.5 Addressing and Packets

In the early store-and-forward networks it was important to know

the source and destination computers for every message. Each

computer was given a unique name or number that was called

the “address” of the computer. To send a message to another

computer, you needed to add the source and destination address

to the message before sending the message along its way. By

having a source and destination address in each message, the

computers that stored and forwarded the message would be able

to pick the best path for the message if more than one path was

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available.

When a long message was split into much smaller packets and

each packet was sent individually, the source and destination ad-

dresses had to be added to each packet, so that routers could

choose the best path to forward each packet of the message. In

addition to the source and destination addresses, it was also nec-

essary to add data to each packet indicating the “offset” or po-

sition of the packet in the overall message so that the receiving

computer could put the packets back together in the right

to reconstruct the original message.

1.6 Putting It All Together

So when we combine all this together we can understand the ba-

sic operation of today’s Internet. We have specialized computers

called “routers” that know how to route packets along a path from

a source to a destination. Each packet will pass through multiple

routers during its journey from the source computer to the desti-

nation computer.

Even though the packets may be part of a larger message, the

routers forward each packet separately based on its source and

destination addresses. Different packets from the same message

may take different routes from the source to the destination. And

sometimes packets even arrive out of ; a later packet might

arrive before an earlier packet, perhaps because of a data “traffic

jam”. Each packet contains an “offset” from the beginning of the

message so that the destination computer can reassemble the

packets in the correct to reconstruct the original message.

By creating a network using multiple short hops, the overall cost

of communicating across a large geographical area could be

spread across a large number of connecting groups and individ-

uals. Normally, packets would find the shortest path between

the source and destination, but if a link on that path was an

overloaded or broken, the routers could cooperate and reroute

traffic to take slightly longer paths that would get packets from a

source to a destination as quickly as possible.

The core of the Internet is a set of cooperating routers that move

packets from many sources to many destinations at the same

time. Each computer or local area network is connected to a

router that forwards the traffic from its location to the various des-

tinations on the Internet. A router might handle data from a single

1.7. GLOSSARY 9

Figure 1.5: Connecting Around the World

computer like a smartphone, from several computers in the same

building, or from thousands of computers connected to a univer-

sity campus network. The term “Internet” comes from the idea of

“internetworking”, which captures the idea of connecting many

networks together. Our computers connect to local networks and

the Internet connects the local networks together so all of our

computers can talk to each other.

1.7 Glossary

address: A number that is assigned to a computer so that mes-

sages can be routed to the computer.

hop: A single physical network connection. A packet on the In-

ternet will typically make several “hops” to get from its source

computer to its destination.

LAN: Local Area Network. A network covering an area that is

limited by the ability for an organization to run wires or the power

of a radio transmitter.

leased line: An “always up” connection that an organization

leased from a telephone company or other utility to send data

across longer distances.

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operator (telephone): A person who works for a telephone com-

pany and helps people make telephone calls.

packet: A limited-size fragment of a large message. Large mes-

sages or files are split into many packets and sent across the

Internet. The typical maximum packet size is between 1000 and

3000 characters.

router: A specialized computer that is designed to receive incom-

ing packets on many links and quickly forward the packets on the

best outbound link to speed the packet to its destination.

store-and-forward network: A network where data is sent

from one computer to another with the message being stored

for relatively long periods of time in an intermediate computer

waiting for an outbound network connection to become available.

WAN: Wide Area Network. A network that covers longer dis-

tances, up to sending data completely around the world. A WAN

is generally constructed using communication links owned and

managed by a number of different organizations.

1.8 Questions

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

1. What did early telephone operators do?

a) Maintained cell phone towers

b) Connected pairs of wires to allow people to talk

c) Installed copper wire between cities

d) Sorted packets as they went to the correct destination

2. What is a leased line?

a) A boundary between leased and owned telephone equip-

ment

b) A connection between a keyboard and monitor

c) A wire that ran from one phone company office to another

d) An “always on” telephone connection

3. How long might a message be stored in an intermediate com-

puter for a store-and-forward network?

1.8. QUESTIONS 11

a) less than a second

b) no more than four seconds

c) less than a minute

d) possibly as long as several hours

4. What is a packet?

a) A technique for wrapping items for shipping

b) A small box used for storage

c) A portion of a larger message that is sent across a network

d) The amount of data that could be stored on an early punched

card

5. Which of these is most like a router?

a) A mail sorting facility

b) A refrigerator

c) A high-speed train

d) An undersea telecommunications cable

6. What was the name given to early network routers?

a) Interfaith Message Processors

b) Internet Motion Perceptrons

c) Instant Message Programs

d) Interface Message Processors

7. In addition to breaking large messages into smaller seg-

ments to be sent, what else was needed to properly route

each message segment?

a) A source and destination address on each message segment

b) An ID and password for each message segment

c) A small battery to maintain the storage for each message

segment

d) A small tracking unit like a GPS to find lost messages

8. Why is it virtually free to send messages around the world

using the Internet?

a) Because governments pay for all the connections

b) Because advertising pays for all the connections

c) Because so many people share all the resources

d) Because it is illegal to charge for long-distance connections

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