Transcript Layer 7- Application Layer

Application Layer Protocols
Learning Objectives
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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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• 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.
OSI
TCP/IP
Application: Layer 7
Interface to operating
System
Presentation: Layer 6
Coding, encryption,
compression
Session: Layer 5
Create and maintain
dialogues between
applications
Application
Well Known Application Protoc
File Transfer:
•File Transfer Protocol (FTP)
•Trivial File Transfer Protocol (TFTP)
APPLICATION
Transfer to Application
Virtual Terminal Session,
Compression & Encryption
Email:
•Simple Mail Transfer Protocol (SMTP)
•Post Office Protocol 3 (POP3)
Establish, send, close
session,
Authenticators, Master
/ Slave
TRANSPORT
Web Browsing:
•Hyper Text Transfer Protocol (HTTP)
INTERNET
LOCAL NETWORK
N
I
C
Network Management:
•Simple Network Management
Protocol (SNMP)
Name Resolution:
•Domain Name Service (DNS)
Host A
•Application layer protocols describe
the format of the requests and
responses between clients and
servers
Host B
Host C
Download
FTP Server
Data flowing from a server to a client is known as download.
Client / Server Processes
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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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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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Client
1.
Web browser uses HTTP to
request a particular webpage 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.
HTML
WWW Page
Web Browser
Web Server
HTTP Request
HTML
WWW Page
•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.
•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.
Client – Web Browser
WWW Server
HTML
Web Page
Web Page Transfer Phases:
2.
HTTP Transfer – Server
transfers HTML web page
using HTTP
3.
Connection Termination –
After web page is transferred
successfully, the client
terminates the connection.
TRANSPORT
INTERNET
LOCAL NETWORK
HTTP Server
Data(80)
Request
Connection Establishment –
Client makes connection to
TCP port 80 on the web
server. Server commences the
connection phase.
Respons
e
1.
Request
Respons
e
HTTP Client
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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.
Uniform Resource Locater (URL)
URLs are presented in a standard format:
Method
://
Host
:
Port
/
Path
•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.
Simple Name Resolution
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
Phill
192.168.1.100
Lisa
192.168.1.101
Bazil
192.168.1.102
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
Switch
Name Resolution Using DNS
What is
IP Address
DNS
Server:
of Bazil?
192.168.1.1
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Phill
192.168.1.100
DNS Client
DNS Server:
192.168.1.1
03
Lisa
192.168.1.101
DNS Client
DNS Server:
192.168.1.1
03
Bazil
192.168.1.102
DNS Client
•A single DNS server
holds the host table.
DNS Server
192.168.1.103
•Client DNS services
request host/IP address
mappings from the
server.
Switch
Host Table:
•Phill
-=192.168.1.100
Bazil
192.168.1.103
•Lisa - 192.168.1.101
•Bazil - 192.168.1.102
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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