Layer 7- Application Layer

Application Layer Protocols

Describe how the functions of the three upper OSI model layers provide network services to end user applications.

Describe how the TCP/IP Application Layer protocols provide the services specified by the upper layers of the OSI model.

Define how people use the Application Layer to communicate across the information network.

Describe the function of well-known TCP/IP applications, such as the World Wide Web and email, and their related services (HTTP, DNS, SMB, DHCP, SMTP/POP, and Telnet).

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Application Layer Protocols & Functionality

Learning Objectives

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Applications – The Interface Between Human and Data Networks

• The Application layer is the top layer of both the OSI and TCP/IP models•Application layer protocols are used to exchange data between programs running on the source and destination hosts. •There are many Application layer protocols and new protocols are always being developed.

Session: Layer 5Create and maintain dialogues betweenapplications

Presentation: Layer 6Coding, encryption,compression

Application: Layer 7Interface to operatingSystem

Application

OSI TCP/IP

NIC

LOCAL NETWORK

INTERNET

TRANSPORT

APPLICATION

Establish, send, close session, Authenticators, Master / Slave

Virtual Terminal Session, Compression & Encryption

Transfer to Application

Well Known Application Protocols

File Transfer:•File Transfer Protocol (FTP)•Trivial File Transfer Protocol (TFTP)

Email:•Simple Mail Transfer Protocol (SMTP)•Post Office Protocol 3 (POP3)

Web Browsing:•Hyper Text Transfer Protocol (HTTP)

Network Management:•Simple Network Management Protocol (SNMP)

Name Resolution:•Domain Name Service (DNS)

Client / Server Processes

FTP Server

Host A

Host B

Host C

Download

Data flowing from a server to a client is known as download.

•Application layer protocols describe the format of the requests and responses between clients and servers

In a client/server network, the server runs a service, or process, sometimes called a server daemon , typically running in the background.

Daemons are described as "listening" for a

request from a client, because they are programmed to respond whenever the server receives a request for the service provided by the daemon.

When a daemon "hears" a request from a client, it exchanges appropriate messages with the client, as required by its protocol, and proceeds to send the requested data to the client in the proper format.

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Client / Server Processes

Peer-to-Peer Networks In a peer-to-peer network, two or more computers are

connected via a network and can share resources (i.e. printers and files) without having a dedicated server.

Every connected end device (known as a peer) can function as either a server or a client. One computer might assume the role of server for one transaction while simultaneously serving as a client for another.

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

Application Layer Services

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The World Wide Web (WWW) is a repository of information stored on web pages, linked together from points all over the world.

Web pages are written in a language called Hypertext Mark-Up Language (HTML), and stored on web servers.

To retrieve an HTML web page, a client/server protocol called Hyper Text Transfer Protocol (HTTP) is used.

HTML web pages are displayed on a users PC by web browser software clients.

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WWW and HTTP

Web Browsing Processes

Web ServerClient

HTMLWWW Page

Web Browser HTTP Request

HTMLWWW Page

1. Web browser uses HTTP to request a particular web-page from a web server.

2. Web server responds, using HTTP to send the HTML web page page to the web browser.

3. The web browser formats the web page for display on the client PC.

Hyper Text Mark-up Language (HTML)

•Language used for creating web pages.

•Mark-up language formats a web page independently from the process that created it – provides a standard way for web browsers to interpret web pages.

•Uses only ASCII characters for both the main web page text and the formatting instructions.

Hyper Text Transfer Protocol (HTTP)

•HTTP is used mainly to access data on the WWW.

•Functions like a combination of FTP and SMTP.

•Used to transfer files using TCP and well-known port 80 – there is no control connection required.

•Web browser utilises an HTTP client, while a web server runs an HTTP server.

•The HTML data transferred by HTTP is not readable by the user – it has to be interpreted by a web-browser.

LOCAL NETWORK

INTERNET

TRANSPORT

(80)

Client – Web Browser WWW Server

Data

HTTP Client HTTP ServerR

equest

Resp

ons

e Request

Resp

ons

e

Web Page Transfer Phases:

1. Connection Establishment – Client makes connection to TCP port 80 on the web server. Server commences the connection phase.

2. HTTP Transfer – Server transfers HTML web page using HTTP

3. Connection Termination – After web page is transferred successfully, the client terminates the connection.

HTML Web Page

The navigation of web pages is achieved using locators called Uniform Resource Locators (URLs).

These allow a user to access sites without using IP addresses.

The URL is a standard for specifying any kind of information on the Internet, and defines four things: protocol, host computer, port and path.

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Uniform Resource Locater (URL)

Method Host://•Method – the protocol used to retrieve data (usually HTTP)

•Host – alias of the web server where the data is located. Normally prefixed with ‘WWW’ signifying a web server.

•Port – Transport layer port that the web server is using – not a mandatory field, as port 80 is used by default.

•Path – the location of the data on the web server – the ‘/’ indicates directories and subdirectories.

: Port / Path

Uniform Resource Locater (URL)

URLs are presented in a standard format:

Simple Name Resolution

Phill192.168.1.100

Lisa192.168.1.101

Bazil192.168.1.102

Host Table:•Phill - 192.168.1.100•Lisa - 192.168.1.101•Bazil - 192.168.1.102

Host Table:•Phill - 192.168.1.100•Lisa - 192.168.1.101•Bazil - 192.168.1.102

Host Table:•Phill - 192.168.1.100•Lisa - 192.168.1.101•Bazil - 192.168.1.102

Switch

Name Resolution can be achieved using a host table, mapping all the host names in a network to their respective IP addresses

All host tables need to be changed every time a new PC is added to the network- this can be time consuming on a large network

Bazil = 192.168.1.103

What is IP Address of Bazil?

Name Resolution Using DNS

Phill192.168.1.100

Lisa192.168.1.101

Bazil192.168.1.102

Switch

•A single DNS server holds the host table.

•Client DNS services request host/IP address mappings from the server. Host Table:

•Phill - 192.168.1.100•Lisa - 192.168.1.101•Bazil - 192.168.1.102

DNS Server192.168.1.103

DNS Server: 192.168.1.103

DNS Server: 192.168.1.103

DNS Server: 192.168.1.103

DNS Client DNS Client DNS Client

Conclusions Today we looked at how the functions of the three upper OSI

model layers provide network services to end user applications◦ Protocol used by WWW