IFSM 310 – Stage 4 Assignment – Due Sunday

Chapter 10

Wrap Up

It has been said that building the Internet solved the world’s most

complex engineering problem to date. The design and engineer-

ing of the Internet started well over 50 years ago. It has been

continuously improving and evolving over the past 50 years and

will continue to evolve in the future.

The Internet now connects billions of computers using many thou-

sands of routers and link-level connections. The Internet is so

complex that it is never fully operational, The Internet is less

about being “perfect” and more about adapting to problems, out-

ages, errors, lost data, and many other unforeseen problems. The

Internet is designed to be flexible and adapt to whatever prob-

lems are encountered.

!"#$%&'"(

)&&*+,#(+'$

-$(."$.(/'"0

1+$0

Figure 10.1: The Four-Layer Model

In order to build an overall solution that works at scale, it was

109

110 CHAPTER 10. WRAP UP

important to break the Internet engineering problems down into

four distinct layers:

• The Link/Physical layer includes all of the complex engineer-

ing required to move data across a single “hop”, whether the

hop is a wireless WiFi, wired Ethernet, fiber optic, or satellite

connection.

• The Internetwork Protocol (IP) layer is how data is routed

across a series of hops to get quickly and efficiently from one

of a billion source computers to any of a billion destination

computers. The IP layer dynamically adjusts and reroutes

data based on network load, link performance, or network

outages. While the IP layer is highly reliable and fast, at

times it loses or even throws away data. The IP layer is not

responsible for insuring the overall reliability of the network.

It just moves the data the best that it can.

• The Transport layer compensates for any imperfections in

the IP or Link layers. The Transport layer makes sure that

any lost packets are retransmitted and packets that arrive

out of order are put back into order before being passed on

to the receiving application. The Transport layer also acts as

flow control between the sending and receiving applications

to make sure that data is moved quickly when the network is

fast and the links are not overloaded, and to slow the trans-

fer of data when using slower or heavily loaded links. The

data flow and rate limitation in the Transport layer allow the

Internet to continue to function smoothly even when it is

heavily loaded.

• The other three layers make the use of the network very

simple for the Application Layer. An application can make a

network connection and send/receive data on that connec-

tion with just a few lines of code. By making the use of the

network simple, applications can focus on solving the end-

user problems they need to solve. Because it is so easy for

applications to use the network in new and different ways,

we have seen the emergence of a wide range of highly inno-

vative applications that work without any changes required

to the Internet protocols.

Without breaking the problem of engineering and building the In-

ternet into these four distinct layers, it would be far more difficult

to build and deploy ever-improving versions of the network. And

111

if every single application needed to be fully aware of all of the

complex details required to use the Internet, it would greatly limit

the richness and diversity of the networked applications that we

have today.

It is amazing to realize what has been accomplished in building

the Internet over the past 50 years. But in a way, we have only

just begun the engineering journey of building networked appli-

cations. It does not take much to imagine an Internet where ev-

ery light switch, lightbulb, refrigerator, table, automobile, road-

way, flying drone, and chair has an Internet address and they all

want to communicate with one another. New engineering issues

will have to be solved, and perhaps even the four-layer network

model will need to evolve to meet these new engineering chal-

lenges.

But just like brilliant engineers designed and evolved network pro-

tocols to move from hundreds of network-connected computers

to billions of network-connected computers, our present and fu-

ture engineers will certainly solve the problems and challenges

we will face as the network evolves to connect trillions of comput-

ers.