IFSM 310 – Cloud Computing vs. Client-Server Computing - DUE FRIDAY

Chapter 7

Application Layer

We have been working from the bottom to the top of our four-

layer TCP/IP network model and we are finally at the top. The Ap-

plication layer is where the networked software like web browsers,

mail programs, video players, or networked video players operate.

We as users interact with these applications and the applications

interact with the network on our behalf.

!"#$%&'"(

)&&*+,#(+'$

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

1+$0

Figure 7.1: The Application Layer

7.1 Client and Server Applications

It is important to remember that two parts are required for a net-

worked application to function. The architecture for a networked

73

74 CHAPTER 7. APPLICATION LAYER

application is called “client/server”. The server portion of the ap-

plication runs somewhere on the Internet and has the information

that users want to view or interact with. The client portion of the

application makes connections to the server application, retrieves

information, and shows it to the user. These applications use the

Transport layer on each of their computers to exchange data.

Figure 7.2: Client/Server Applications

To browse a web address like www.khanacademy.org, you must

have a web application running on your computer. When you type

an address into your web browser, it connects to the appropriate

web server, retrieves pages for you to view, and then shows you

the pages.

The web browser on your computer sends a request to connect

to www.khanacademy.org. Your computer looks up the domain

name to find the corresponding IP address for the server and

makes a transport connection to that IP address, then begins to

request data from the server over that network connection. When

the data is received, the web browser shows it to you. Sometimes

web browsers display a small animated icon to let you know that

the data is being retrieved across the network.

On the other end of the connection is another application called a

“web server”. The web server program is always up and waiting

for incoming connections. So when you want to see a web page,

you are connecting to a server application that is already running

and waiting for your connection.

In a sense, the Transport, Internetwork, and Link layers, along

7.2. APPLICATION LAYER PROTOCOLS 75

with the Domain Name System, are like a telephone network for

networked applications. They “dial up” different server applica-

tions on the network and have “conversations” with those appli-

cations to exchange data.

7.2 Application Layer Protocols

Just like people talking on telephones, each pair of network appli-

cations needs a set of rules that govern the conversation. In most

cultures, when your phone rings and you pick up the phone you

say “Hello”. Normally the person who made the call (the client

person) is silent until the person who picked up the phone (the

server person) says “Hello”. If you have ever called someone

who does not follow this simple rule, it can be quite confusing.

You probably would assume that the connection was not working,

hang up, and retry the call.

A set of rules that govern how we communicate is called a “proto-

col”. The definition of the word protocol is “a rule which describes

how an activity should be performed, especially in the field of

diplomacy.” The idea is that in formal situations, we should be-

have according to a precise set of rules. We use this word to

describe networked applications, because without precise rules,

applications have no way to establish and manage a conversa-

tion. Computers like precision.

There are many different networked applications and it is impor-

tant that each networked application have a well-documented

protocol so that all servers and clients can interoperate. Some

of these protocols are intricate and complex.

The protocol that describes how a web browser communicates

with a web server is described in a number of large documents

starting with this one:

https://tools.ietf.org/html/rfc7230

The formal name of the protocol between web clients and web

servers is the “HyperText Transport Protocol”, or HTTP for short.

When you put “http:” or “https:” on the beginning of a URL that

you type into the browser, you are indicating that you would like

to retrieve a document using the HTTP protocol.

If you were to read the above document, and go to section 5.3.2

on page 41, you find the exact text of what a web client is sup-

posed to send to a web server:

76 CHAPTER 7. APPLICATION LAYER

Figure 7.3: Application Protocols

GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1

One of the reasons that HTTP is so successful is that it is relatively

simple compared to most client/server protocols. Even though

the basic use of HTTP is relatively simple, there is a lot of detail

that allows web clients and servers communicate efficiently and

transfer a wide range of information and data. Six different doc-

uments describe the HTTP protocol, in a total of 305 pages. That

might seem like a lot of detail, but the key in designing protocols

is to think through all possible uses of the protocol and describe

each scenario carefully.

7.3 Exploring the HTTP Protocol

In this section we will manually exercise the HTTP protocol by

pretending to be a web browser and sending HTTP commands to

a web server to retrieve data. To play with the HTTP protocol, we

will use one of the earliest Internet applications ever built.

The “telnet” application was first developed in 1968, and was

7.3. EXPLORING THE HTTP PROTOCOL 77

developed according to one of the earliest standards for the Inter-

net:

https://tools.ietf.org/html/rfc15

This standard is only five pages long and at this point, you prob-

ably can easily read and understand most of what is in the docu-

ment. The telnet client application is so old that it is effectively a

dinosaur, as it comes from “prehistoric” times in terms of the age

of the Internet. The Internet was created in 1985 by the NSFNet

project and the precursor to the NSFNet called the ARPANET was

brought up in 1969. Telnet was designed and built even before

the first TCP/IP network was in production.

Interestingly, the telnet application is still present in most mod-

ern operating systems. You can access telnet from the terminal

(command line) in Macintosh and Linux. The telnet application

was also present in Windows 95 through Windows XP, but is not

included in later versions of Windows. If you have a later version

of Windows, you can download and install a telnet client to do the

examples in this section.

Telnet is a simple application. Run telnet from the command line

(or terminal) and type the following command:

telnet www.dr-chuck.com 80

The first parameter is a domain name (an IP address would work

here as well) and a port to connect to on that host. We use the

port to indicate which application server we would like to connect

to. Port 80 is where we typically expect to find an HTTP (web)

server application on a host. If there is no web server listening

on port 80, our connection will time out and fail. But if there is

a web server, we will be connected to that web server and what-

ever we type on our keyboard will be sent directly to the server.

At this point, we need to know the HTTP protocol and type the

commands precisely as expected. If we don’t know the protocol,

the web server will not be too friendly. Here is an example of

things not going well:

telnet www.dr-chuck.com 80

Trying 198.251.66.43...

Connected to www.dr-chuck.com.

Escape character is '^]'.

HELP

<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">

78 CHAPTER 7. APPLICATION LAYER

<html><head>

<title>501 Method Not Implemented</title>

...

</body></html>

Connection closed by foreign host.

We type “telnet” in the terminal requesting a connection to port

80 (the web server) on the host www.dr-chuck.com. You can see

as our transport layer is looking up the domain name, finding

that the actual address is “198.251.66.43”, and then making a

successful connection to that server. Once we are connected, the

server waits for us to type a command followed by the enter or

return key. Since we don’t know the protocol, we type “HELP” and

enter. The server is not pleased, gives us an error message, and

then closes the connection. We do not get a second chance. If we

do not know the protocol, the web server does not want to talk to

us.

But let’s go back and read section 5.3.2 of the RFC-7230 docu-

ment and try again to request a document using the correct syn-

tax:

telnet www.dr-chuck.com 80

Trying 198.251.66.43...

Connected to www.dr-chuck.com.

Escape character is '^]'.

GET http://www.dr-chuck.com/page1.htm HTTP/1.0

HTTP/1.1 200 OK

Last-Modified: Sun, 19 Jan 2014 14:25:43 GMT

Content-Length: 131

Content-Type: text/html

<h1>The First Page</h1>

<p>

If you like, you can switch to the

<a href="http://www.dr-chuck.com/page2.htm">

Second Page</a>.

</p>

Connection closed by foreign host.

We make the connection to the web browser again using telnet,

then we send a GET command that indicates which document we

want to retrieve. We use version 1.0 of the HTTP protocol because

7.3. EXPLORING THE HTTP PROTOCOL 79

Figure 7.4: Hacking HTTP By Hand

it is simpler than HTTP 1.1. Then we send a blank line by pressing

“return” or “enter” to indicate that we are done with our request.

Since we have sent the proper request, the host responds with a

series of headers describing the document, followed by a blank

line, then it sends the actual document.

The headers communicate metadata (data about data) about the

document that we have asked to retrieve. For example, the first

line contains a status code.

In this example, the status code of “200” means that things went

well. A status code of “404” in the first line of the headers in-

dicates that the requested document was not found. A status

code of “301” indicates that the document has moved to a new

location.

The status codes for HTTP are grouped into ranges: 2XX codes in-

dicate success, 3XX codes are for redirecting, 4XX codes indicate

that the client application did something wrong, and 5xx codes

indicate that the server did something wrong.

This is a HyperText Markup Language (HTML) document, so it is

marked up with tags like <h1> and <p>. When your browser

receives the document in HTML format, it looks at the markup in

the document, interprets it, and presents you a formatted version

of the document.

80 CHAPTER 7. APPLICATION LAYER

7.4 The IMAP Protocol for Retrieving Mail

The HTTP protocol is only one of many client/server application

protocols used on the Internet. Another common protocol is used

so that a mail application running on your computer can retrieve

mail from a central server. Since your personal computer might

not be turned on at all times, when mail is sent to you it is sent to

a server and stored on that server until you turn on your computer

and retrieve any new email.

Like many application standards, the Internet Message Access

Protocol (IMAP) is described in a series of Request For Comment

(RFC) documents starting with this RFC:

https://tools.ietf.org/html/rfc3501

IMAP is a more complicated protocol than the web protocol, so

we won’t be able to use the telnet command to fake the protocol.

But if you were going to develop a mail reading application, you

could carefully read this document and develop code to have a

successful conversation with a standards-compliant IMAP server.

Here is a simple example from section 6.3.1 of the above docu-

ment showing what the client (C:) sends and how the server (S:)

responds:

C: A142 SELECT INBOX

S: * 172 EXISTS

S: * 1 RECENT

S: * OK [UNSEEN 12] Message 12 is first unseen

S: * OK [UIDVALIDITY 3857529045] UIDs valid

S: * OK [UIDNEXT 4392] Predicted next UID

S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)

S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited

S: A142 OK [READ-WRITE] SELECT completed

The messages that are sent by the client and server are not de-

signed to be viewed by an end user so they are not particularly

descriptive. These messages are precisely formatted and are sent

in a precise order so that they can be generated and read by net-

worked computer applications on each end of the connection.

7.5. FLOW CONTROL 81

7.5 Flow Control

When we looked at the Transport layer, we talked about the “win-

dow size”, which was the amount of data that the Transport layer

on the sending computer will send before pausing to wait for an

acknowledgement.

!"#$%& '&(()*&

+$),(-"$./ 0)1&$

2&(.3,).3",/ +$),(-"$./

0)1&$

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

""""""""######

""""""""$$%$

#%#%#

Figure 7.5: Buffering in the Transport Layer

In this figure, we see a message broken into packets, with some

of the packets sent and acknowledged. Six packets have been

sent but not yet acknowledged and the sending Transport layer

has reached the limit of the transmit window, so it is pausing un-

til it receives an acknowledgement from the receiving computer’s

Transport layer. The receiving computer has received three pack-

ets, one of which is out of order.

When we were looking at this example before from the point of

view of the Transport layer, we ignored where the packets to be

sent were coming from and where the packets were going to in

the receiving computer. Now that we are looking at the Applica-

tion layer, we can add the two applications that are the source

and the destination of the stream of data.

Let’s assume the web browser has made a transport connection

to the web server and has started downloading an image file. The

web server has opened the image file and is sending the data

from the file to its Transport layer as quickly as possible. But the

Transport layer must follow the rules of window size, so it can only

send a certain amount of data at a time. When the window fills up,

the web server is paused until the Transport layer on the destina-

tion computer has started to receive and acknowledge packets.

As the Transport layer on the destination computer starts to re-

82 CHAPTER 7. APPLICATION LAYER

ceive packets, reconstruct the stream of data, and acknowledge

packets, it delivers the reconstructed stream of packets to the

web browser application display on the user’s screen. Sometimes

on a slow connection you can see your browser “paint” pictures

as the data is downloaded. On a fast connection the data comes

so quickly that the pictures appear instantaneously.

If we redraw our picture of packets in the Transport layer, adding

both of the application layers where the packets are in the middle

of an image, now it looks like this:

!"#$%"&'"& ())*+,-.+/0

1&-02)/&. 1&-02)/&. !"#$3&/42"& ())*+,-.+/0

50."&0".4/&6$ -07$8+06 8-9"&2

!!"###### !!"$$%$ &&&&

&&&&

&&!! !!!'''''

#%#%#

&()

Figure 7.6: Buffering in the Application and Transport Layers

The web server is reading the image file (‘F’) and sending it as

a stream to the web browser as quickly as it can send the data.

The source Transport layer has broken the stream into packets

and used IP to send the packets to the destination computer.

The Transport layer has sent six packets (‘S’) and has stopped

sending because it has reached its window size and paused the

web server. Three of those six packets have made it to the Trans-

port layer on the destination computer (‘R’) and three of the pack-

ets are still making their way across the Internet (‘S’).

As the destination Transport layer pieces the stream back to-

gether, it both sends an acknowledgement (ACK) and delivers

the data to the receiving application (the web browser). The web

browser reconstructs the image (‘A’) and displays it to the user

as the data is received.

A key thing to notice in this picture is that the transport layers do

not keep the packets for the entire file. They only retain packets

that are “in transit” and unacknowledged. Once packets are ac-

knowledged and delivered to the destination application, there is

no reason for either the source or destination Transport layer to

hold on to the packets.

When the acknowledgment flows back from the destination

computer to the source computer, the Transport layer on the

7.6. WRITING NETWORKED APPLICATIONS 83

source computer unpauses the web server application and the

web server continues to read data from the file and send it to the

source Transport layer for transmission.

This ability to start and stop the sending application to make sure

we send data as quickly as possible without sending data so fast

that they clog up the Internet is called “flow control”. The applica-

tions are not responsible for flow control, they just try to send or

receive data as quickly as possible and the two transport layers

start and stop the applications as needed based on the speed and

reliability of the network.

7.6 Writing Networked Applications

The applications which send and receive data over the network

are written in one or more programming languages. Many pro-

gramming languages have libraries of code that make it quite

simple to write application code to send and receive data across

the network. With a good programming library, making a connec-

tion to an application running on a server, sending data to the

server, and receiving data from the server is generally as easy as

reading and writing a file.

As an example, the code below is all it takes in the Python pro-

gramming language to make a connection to a web server and

retrieve a document:

!"#$%&'($)*+& ''' ",($)*'-'($)*+&.($)*+&/($)*+&.01234567'($)*+&.89:;286<50=> ",($)*.)$??+)&//@AAA.#,B!?C.)$"@7'DE>> ",($)*.(+?F/@G56'H&&#IJJAAA.#,B!?C.)$"J)$F+J%$"+$.&K&'L66MJN.EO?O?@>

AH!P+'6%Q+I ''''FR&R'-'",($)*.%+)S/TNU> ''''!C'/'P+?/FR&R>'V'N'>'I ''''''''W%+R* ''''#%!?&'FR&R

",($)*.)P$(+/>

Figure 7.7: Programming with Sockets in Python

While you may or not know the Python programming language,

the key point is that it only takes ten lines of application code to

make and use a network connection. This code is simple because

the Transport, Internetwork, and Link layers so effectively solve

84 CHAPTER 7. APPLICATION LAYER

the problems at each of their layers that applications using the

network can ignore nearly all of the details of how the network is

implemented.

In the Python application, in this line of code

mysock.connect(('www.py4inf.com', 80))

we have specified that we are connecting to the application that

is listening for incoming connections on port 80 on the remote

computer www.py4inf.com.

By choosing port 80 we are indicating that we want to connect to

a World Wide Web server on that host and are expecting to com-

municate with that server using the HyperText Transport Protocol.

7.7 Summary

The entire purpose of the lower three layers (Transport, Internet-

work, and Link) is to make it so that applications running in the

Application layer can focus the application problem that needs to

be solved and leave virtually all of the complexity of moving data

across a network to be handled by the lower layers of the network

model.

Because this approach makes it so simple to build networked

applications, we have seen a wide range of networked applica-

tions including web browsers, mail applications, networked video

games, network-based telephony applications, and many others.

And what is even more exciting is that it is easy to experiment

and build whole new types of networked applications to solve

problems that have not yet been imagined.

7.8 Glossary

HTML: HyperText Markup Language. A textual format that

marks up text using tags surrounded by less-than and

greater-than characters. Example HTML looks like: <p>This

is <strong>nice</strong></p>.

HTTP: HyperText Transport Protocol. An Application layer proto-

col that allows web browsers to retrieve web documents from web

servers.

7.9. QUESTIONS 85

IMAP: Internet Message Access Protocol. A protocol that allows

mail clients to log into and retrieve mail from IMAP-enabled mail

servers.

flow control: When a sending computer slows down to make

sure that it does not overwhelm either the network or the desti-

nation computer. Flow control also causes the sending computer

to increase the speed at which data is sent when it is sure that

the network and destination computer can handle the faster data

rates.

socket: A software library available in many programming lan-

guages that makes creating a network connection and exchang-

ing data nearly as easy as opening and reading a file on your

computer.

status code: One aspect of the HTTP protocol that indicates

the overall success or failure of a request for a document. The

most well-known HTTP status code is “404”, which is how an HTTP

server tells an HTTP client (i.e., a browser) that it the requested

document could not be found.

telnet: A simple client application that makes TCP connections

to various address/port combinations and allows typed data to be

sent across the connection. In the early days of the Internet, tel-

net was used to remotely log in to a computer across the network.

web browser: A client application that you run on your computer

to retrieve and display web pages.

web server: An application that deliver (serves up) Web pages

7.9 Questions

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

1. Which layer is right below the Application layer?

a) Transport

b) Internetworking

c) Link Layer

d) Obtuse layer

2. What kind of document is used to describe widely used Ap-

plication layer protocols?

86 CHAPTER 7. APPLICATION LAYER

a) DHCP

b) RFC

c) APPDOC

d) ISO 9000

3. Which of these is an idea that was invented in the Applica-

tion layer?

a) 0f:2a:b3:1f:b3:1a

b) 192.168.3.14

c) www.khanacademy.com

d) http://www.dr-chuck.com/

4. Which of the following is not something that the Application

layer worries about?

a) Whether the client or server starts talking first

b) The format of the commands and responses exchanged

across a socket

c) How the window size changes as data is sent across a socket

d) How data is represented as it is sent across the network to

assure interoperability.

5. Which of these is an Application layer protocol?

a) HTTP

b) TCP

c) DHCP

d) Ethernet

6. What port would typically be used to talk to a web server?

a) 23

b) 80

c) 103

d) 143

7. What is the command that a web browser sends to a web

server to retrieve an web document?

a) RETR

7.9. QUESTIONS 87

b) DOCUMENT

c) 404

d) GET

8. What is the purpose of the “Content-type:” header when you

retrieve a document over the web protocol?

a) Tells the browser how to display the retrieved document

b) Tells the browser where to go if the document cannot be

found

c) Tells the browser whether or not the retrieved document is

empty

d) Tells the browser where the headers end and the content

starts

9. What common UNIX command can be used to send simple

commands to a web server?

a) ftp

b) ping

c) traceroute

d) telnet

10. What does an HTTP status code of “404” mean?

a) Document has moved

b) Successful document retrieval

c) Protocol error

d) Document not found

11. What characters are used to mark up HTML documents?

a) Less-than and greater-than signs < >

b) Exclamation points !

c) Square brackets [ ]

d) Curly brackets { }

12. What is a common application protocol for retrieving mail?

a) RFC

b) HTML

88 CHAPTER 7. APPLICATION LAYER

c) ICANN

d) IMAP

13. What application protocol does RFC15 describe?

a) telnet

b) ping

c) traceroute

d) www

14. What happens to a server application that is sending a large

file when the TCP layer has sent enough data to fill the win-

dow size and has not yet received an acknowledgement?

a) The application switches its transmission to a new socket

b) The application crashes and must be restarted

c) The application is paused until the remote computer

acknowledges that it has received some of the data

d) The closest gateway router starts to discard packets that

would exceed the window size

15. What is a “socket” on the Internet?

a) A way for devices to get wireless power

b) A way for devices to get an IP address

c) An entry in a routing table

d) A two-way data connection between a pair of client and

server applications

16. What must an application know to make a socket connection

in software?

a) The address of the server and the port number on the server

b) The route between the source and destination computers

c) Which part of the IP address is the network number

d) The initial size of the TCP window during transmission