Chapter 10: Application Layer

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Chapter 10:

Application Layer

Introduction to Networks

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Chapter 10: Objectives

By the end of this chapter, you will be able to:

 Explain how the functions of the application layer, session layer,

and presentation layer work together to provide network services to

end user applications.

 Describe how common application layer protocols interact with end

user applications.

 Describe, at a high level, common application layer protocols that

provide Internet services to end-users, including WWW services

and email.

 Describe application layer protocols that provide IP addressing

services, including DNS and DHCP.

 Describe the features and operation of well-known application layer

protocols that allow for file sharing services, including: FTP, File

Sharing Services, SMB protocol.

 Explain how data is moved across the network, from opening an

application to receiving data.

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Chapter 10

10.0 Introduction

10.1 Application Layer Protocols

10.2 Well-Known Application Layer Protocols and Service

10.3 The Message Heard Around the World

10.4 Summary

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10.1 Application Layer

Protocols

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Application, Session and Presentation

OSI and TCP/IP Models Revisited

The key parallels are in the transport and network layer.

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Application Session and Presentation

Application Layer

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Application, Session and Presentation

Presentation and Session Layers

 Presentation layer

 Coding and conversion of application layer data

 Data compression

 Data encryption for the transmission and decryption of

data upon receipt by the destination

 Session layer

 Functions, creates, and maintains dialogs between source

and destination applications

 Handles the exchange of information to initiate dialogs,

keep them active, and to restart sessions

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Application, Session and Presentation

Presentation and Session Layers (cont.)

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Application, Session and Presentation

TCP/IP Application Layer Protocols

 Domain Name Service Protocol (DNS) – used to resolve Internet

names to IP addresses

 Telnet – a terminal emulation protocol used to provide remote access

to servers and networking devices

 Bootstrap Protocol (BOOTP) – a precursor to the DHCP protocol, a

network protocol used to obtain IP address information during bootup

 Dynamic Host Control Protocol (DHCP) – used to assign an IP

address, subnet mask, default gateway and DNS server to a host

 Hypertext Transfer Protocol (HTTP) – used to transfer files that

make up the Web pages of the World Wide Web

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Application, Session and Presentation

TCP/IP Application Layer Protocols (cont.)

 File Transfer Protocol (FTP) - used for interactive file transfer

between systems

 Trivial File Transfer Protocol (TFTP) - used for connectionless

active file transfer

 Simple Mail Transfer Protocol (SMTP) - used for the transfer of

mail messages and attachments

 Post Office Protocol (POP) - used by email clients to retrieve email

from a remote server

 Internet Message Access Protocol (IMAP) – another protocol for

email retrieval

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How Application Protocols Interact with End-User Applications

Peer-to-Peer Networks

Both devices are considered equal in the communication.

The roles of client and server are set on a per request basis.

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How Application Protocols Interact with End-User Applications

Peer-to-Peer Applications

Client and server in the same communication.

Both can initiate a communication and are considered

equal in the communication process.

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How Application Protocols Interact with End-User Applications

Common P2P Applications

 With P2P applications, each computer in the network running the

application can act as a client or a server for the other computers in

the network running the application.

 Common P2P applications include:

 eDonkey

 eMule

 Shareaza

 BitTorrent

 Bitcoin

 LionShare

 Some P2P applications are based on the Gnutella protocol which

enables people to share files on their hard disks with others

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How Application Protocols Interact with End-User Applications

Client-Server Model

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How Application Protocols Interact with End-User Applications

Client-Server Model

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10.2 Well-Known

Application Layer

Protocols and Services

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Common Application Layer Protocols

Application Layer Protocols Revisited

Three application layer protocols involved in everyday work or play

include:

 HTTP to browse the web.

 Simple Mail Transfer Protocol (SMTP) to enable users to send email.

 Post Office Protocol (POP) to enable users to receive email.

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Common Application Layer Protocols

Hypertext Transfer Protocol and Hypertext

Markup Language

Example URL: http://www.cisco.com/index.html

1. First, the browser interprets the three parts of the URL:

 http (the protocol or scheme)

 www.cisco.com (the server name)

 index.html (the specific file name requested)

2. Browser checks with a name server to convert www.cisco.com into

a numeric address

3. Using the HTTP protocol requirements sends a GET request to the

server and asks for the file index.html

4. Server sends the HTML code for this web page

5. Browser deciphers the HTML code and formats the page

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Common Application Layer Protocols

HTTP and HTTPS

 Developed to publish and retrieve HTML pages

 Used for data transfer

 Specifies a request/response protocol

 Three common message types are GET, POST, and PUT

 GET is a client request for data

 POST and PUT are used to send messages that upload data to the

web server

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Common Application Layer Protocols

SMTP, POP, and IMAP

 Typically use an application

called a Mail User Agent

(email client)

 Allows messages to be sent

 Places received messages

into the client's mailbox

 SMTP - Send email from

either a client or a server

 POP - Receive email

messages from an email

server

 IMAP - Internet Message

Access Protocol

 Email client provides the

functionality of both

protocols within one

application

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Common Application Layer Protocols

SMTP, POP, and IMAP (cont.)

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Simple Mail Transfer Protocol (SMTP)

 transfers mail

 message must be formatted properly

 SMTP processes must be running on both the client and server

 message header must have a properly formatted recipient email

address and a sender

 uses port 25

Post Office Protocol (POP)

 enables a workstation to retrieve mail from a mail server

 mail is downloaded from the server to the client and then deleted on

the server

 uses port 110

 POP does not store messages

 POP3 is desirable for an ISP, because it alleviates their responsibility

for managing large amounts of storage for their email servers

Common Application Layer Protocols

SMTP, POP, and IMAP (cont.)

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Common Application Layer Protocols

SMTP, POP, and IMAP (cont.)

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Common Application Layer Protocols

SMTP, POP, and IMAP (cont.)

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Everyday Application Layer Protocols

SMTP, POP, and IMAP (cont.)

 MDA accepts a piece of email from MTA and performs the actual

delivery.

 MDA receives all the inbound mail from the MTA and places it into

mailboxes.

 MDA can also resolve final delivery issues, such as virus scanning,

spam filtering, and return-receipt handling.

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Common Application Layer Protocols

SMTP, POP, and IMAP (cont.)

Simple Mail Transfer Protocol (SMTP)

 Transfers mail reliably and efficiently

Post Office Protocol (POP)

 Enables a workstation to retrieve mail from a mail server

 With POP, mail is downloaded from the server to the client and then

deleted on the server

Internet Message Access Protocol (IMAP)

 Another protocol that to retrieves email messages

 Unlike POP, when the user connects to an IMAP-capable server,

copies of the messages are downloaded to the client application

 Original messages are kept on the server until manually deleted

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Providing IP Addressing Services

Domain Name Service

A human legible

name is resolved

to its numeric

network device

address by the

DNS protocol.

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Providing IP Addressing Services

Domain Name Service (cont.)

A human legible

name is resolved

to its numeric

network device

address by the

DNS protocol.

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Providing IP Addressing Services

Domain Name Service (cont.)

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Providing IP Addressing Services

DNS Message Format

 DNS server stores different types of resource records used to resolve

names

 Contains the name, address, and type of record.

 Record types are:

 A – An end device address

 NS – An authoritative name server

 CNAME – The canonical name for an alias; used when multiple

services have the single network address, but each service has its

own entry in DNS

 MX – Mail exchange record; maps a domain name to a list of mail

exchange servers

 Unable to resolve the name using its stored records, contacts other

servers.

 Server temporarily stores the numbered address that matches the name

in cache memory.

 Windows ipconfig /displaydns displays all cached DNS.

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Providing IP Addressing Services

DNS Hierarchy

Examples top-

level domains:

.au - Australia

.co - Colombia

.com - business

or industry

.jp - Japan

.org - non-profit

organization

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Providing IP Addressing Services

nslookup

 Operating system utility called nslookup allows the user to manually

query the name servers to resolve a given host name

 Utility can be used to troubleshoot name resolution issues and to verify

the current status of the name servers

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Providing IP Addressing Services

Dynamic Host Configuration Protocol

 DHCP allows a host to obtain an IP address dynamically.

 DHCP server is contacted and address requested - chooses address

from a configured range of addresses called a pool and “leases” it to

the host for a set period.

 DHCP used for general purpose hosts such as end user devices,

and static addressing is used for network devices such as gateways,

switches, servers and printers.

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Providing IP Addressing Services

Dynamic Host Configuration Protocol (cont.)

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Providing IP Addressing Services

DHCP Operation

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Providing File Sharing Services

File Transfer Protocol

 FTP allow data transfers between a client and a server.

 FTP client is an application that runs on a computer that is used to

push and pull data from a server running an FTP daemon.

 To successfully transfer data, FTP requires two connections between

the client and the server, one for commands and replies, the other for

the actual file transfer.

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Providing File Sharing Services

File Transfer Protocol (cont.)

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Providing File Sharing Services

Server Message Block

 Clients establish

a long term

connection to

servers.

 After the

connection is

established, the

user can access

the resources on

the server as if

the resource is

local to the client

host.

SMB is a client-server, request-response protocol. Servers

can make their resources available to clients on the network.

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Providing File Sharing Services

Server Message Block (cont.)

A file may be copied from PC to PC with Windows

Explorer using the SMB protocol.

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10.3 The Message Heard

Around the World

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Move It!

The Internet of Things

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Move It!

Message Travels Through a Network

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Move It!

Message Travels Through a Network (cont.)

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Move It!

Message Travels Through a Network (cont.)

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Move It!

Getting the Data to the End Device

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Move It!

Getting the Data through the Internetwork

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Move It!

Getting the Data to the Right Application

At the end device, the service port number directs the data to

the correct conversation.

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Application Layer

Summary

 Applications are computer programs with which the user interacts and

which initiate the data transfer process at the user’s request.

 Services are background programs that provide the connection

between the application layer and the lower layers of the networking

model.

 Protocols provide a structure of agreed-upon rules and processes that

ensure services running on one particular device can send and receive

data from a range of different network devices.

 HTTP supports the delivery of web pages to end devices.

 SMTP, POP, and IMAP support sending and receiving email.

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Application Layer

Summary

 SMB and FTP enable users to share files.

 P2P applications make it easier for consumers to seamlessly share

media.

 DNS resolves the human legible names used to refer to network

resources into numeric addresses usable by the network

 All of these elements work together, at the application layer.

 The application layer enables users to work and play over the

Internet.

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