This document provides an overview of the application layer of the OSI model. It discusses application layer functions like providing an interface for applications to access the network. It describes application layer protocols like HTTP, FTP, SMTP, and DNS. It also covers topics like client-server and peer-to-peer networking, application layer software, and application layer protocols and port numbers.

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Network Fundamentals
Last Update: 12/6/2010
Abdekhalik Elsaid Mosa
abdu.elsaid@yahoo.com
http://abdelkhalik.staff.scuegypt.edu.eg/

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Application Layer
• OSI: is a layered, abstract representation created as a guideline
for network protocol design.
• Application Layer, provides human interface to the network.

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Application Layer
• Application Layer, functions:
Provides the interface between the applications and network.
• Presentation Layer functions:
Handles the conversion of data between different formats.
 Encoding and decoding.
 Encryption and decryption.
 Compression and decompression.
• Session Layer functions:
Maintains dialogs between source and destination applications.
 Create session
 Manage and maintain session
 Terminate session
Most applications, like web browsers
Include functionality of the OSI
layers 5, 6 and 7.

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Application Layer Software
• The 2 forms of S/W programs that provide access to the network.
1.Network-Aware applications:
are able to communicate directly
with the protocol stack.
Ex: E-mail clients, and web browser
2.Application layer services:
are the programs that interface
with the network and prepare the
data for transfer.
Ex: network print spooling

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User Applications, Services, and Protocols
• Applications: Provide the human interface.
• Services: Establish an interface to the network.
• Protocols: Are rules and formats that govern how data is treated.
• The (applications,
services, and protocols)
may be used by a single
exe. Program.
Ex: Telnet, FTP

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Servers
• Servers usually are repositories of data.
• The server runs a service, sometimes called a server daemon.
• Daemons run in the background and are not under an end user's
direct control.
• Daemons are described as "listening"
for a request from a client.
• When a daemon "hears"
a request from a client:
It exchanges appropriate
messages with the client,
and then sends the
requested data to the client.

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Client-Server Model
• Client: the device requesting Information.
• Server: the device which responds to the request.
• Centralized Administration.
• Security is easier to enforce.

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Peer-to-Peer (P2P) Networking and Applications
• Peer-to-peer networking involves two distinct forms:
1. Peer-to-peer network design
2. Peer-to-peer applications.

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Peer-to-Peer Network Design
• Two or more computers are connected via a network and can
share resources without having a dedicated server.
• End device (peer) can function as either a server or a client.
• Decentralized Administration.
• Security is difficult to enforce.
• Used in small home networks
for file sharing and games.
• One computer might
assume the role of
server for one transaction
while simultaneously serving as a client for another.

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Peer-to-Peer Applications
• P2P applications allows a device to act as both a client and a
server within the same communication.
• Every client is a server and every server a client.
• Peer-to-peer applications can be used on peer-to-peer
networks, client/server networks, and across the Internet.
• Some P2P applications use a hybrid system.

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Application layer Protocols and Port numbers
• The Transport layer uses port number addressing .
• Port numbers identify applications and Application layer services.
• Server programs generally use predefined port numbers that are
commonly known by clients.
• Examples:
Telnet - TCP Port 23DNS - TCP/UDP Port 53
DHCP - UDP Port 67HTTP - TCP Port 80
FTP - TCP Ports 20 and 21SMTP - TCP Port 25
POP - UDP Port 110

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Domain Name System (DNS)
• Devices are labeled with numeric IP addresses.
• Domain Names were created to convert the numeric address into
a simple, recognizable name. Ex: IP: 198.133.219.25 DN: www.cisco.com
• DNS client is sometimes called DNS Resolver.
• A DNS Server provides name resolution using the name daemon.
• The DNS server stores different types of resource records (RRs)
used to resolve names.
• These records contain the
name, address, and others.
nslookup
• Displays default DNS server for
your host.

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DNS Servers Hierarchy

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DNS Name Resolution
Step 1:
• The DNS resolver sends a
recursive query to its Local
DNS server.
• Requests IP address for "www.example.com".
• The Local DNS server is responsible for resolving the name.
– Cannot refer the DNS client to another DNS server.
1

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Step 2:
• Local DNS Server forwards
the query to a
Root DNS server.
Step 3:
• Root DNS server
Makes note of .com suffix
Returns a list of IP addresses for TLD Servers responsible for
.com.
DNS Name Resolution
1
2
2
3

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• Root DNS Servers: There are
13 Root DNS servers (labeled
A through M)
• TLD Servers
– Responsible for domains
such as .com, edu, org,
.net, .uk.
– There are redundant
servers throughout the
world.
DNS Name Resolution

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Step 4:
• The local DNS server sends
query for www.example.com
to one of the TLD servers.
Step 5:
• TLD Server
– Makes note of example.com
– Returns IP address for authoritative server example.com
(such as dns.example.com server)
4
4
5
DNS Name Resolution

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Step 6:
• Local DNS server sends
query directly to DNS server
for example.com
Step 7:
• example.com DNS server
responds with its IP address
for www.example.com
6
6
7
DNS Name Resolution

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Step 8:
• Local DNS server sends the IP
of www.example.com to the
DNS client.
• DNS Caching: When a DNS
server receives a DNS reply ,
it can cache the information
in its local memory.
• ipconfig /displaydns: Displays cashed DNS entries.
• ipconfig /flushdns: Manually deletes entries.
8
7
DNS Name Resolution

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DHCP – Dynamic Host Configuration Protocol

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DHCP
• DHCP automates the assignment
of IP address, Subnet mask, Default
gateway, DNS Server
• DHCP servers can be:
–Server on LAN
–Router
–Server at ISP
• DHCP addresses are not
permanently assigned to hosts but
are leased for a period of time

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Telnet
• Allows a user to remotely
access another device.
• A connection using Telnet is
called a Virtual Terminal (VTY)
session.
• Telnet clients (Teraterm
,Hyperterm)

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• The Telnet server runs a service called Telnet daemon.
• Telnet supports user authentication, but does not encrypt data.
• Telnet transfers data as plain text.
• Secure Shell (SSH) protocol offers an alternate and secure
method for server access.
• SSH benefits over Telnet
– Stronger authentication
– Encryption
• As a best practice, network professionals should always use SSH
in place of Telnet, whenever possible.
Telnet and SSH

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HTTP (HyperText Transfer Protocol)
HTTP: developed to publish and retrieve HTML pages, “data
transfer”.

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HTTP Request Message
Request Line
• GET: Browser/client is requesting an object.
• /~index/: Browser is requesting this object in this directory
(default is index.html).
• HTTP/1.1: Browser implements the HTTP/1.1.
GET /~index/ HTTP/1.1
Accept-Language: en-us
User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0;
SLCC1; .NET CLR 2.0.50727; Media Center PC 5.0; .NET CLR
3.0.04506; InfoPath.1)
Host: www.cisco.com
Connection: Keep-Alive
Request line

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Header Lines
• Accept-Language: User prefers this language of the object
• User-Agent: The browser type making the request
• Host: Host on which the object resides
• Connection: Client/browser is telling the server to keep this TCP
connection Open, known as a persistent connection.
GET /~ index / HTTP/1.1
Accept-Language: en-us
User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0;
SLCC1; .NET CLR 2.0.50727; Media Center PC 5.0; .NET CLR
3.0.04506; InfoPath.1)
Host: www.cisco.com
Connection: Keep-Alive

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HTTP Response Message
HTTP/1.1 200 OK
Date: Fri, 22 Feb 2008 16:34:18 GMT
Server: Apache/2.0.52 (Red Hat)
Last-Modified: Thu, 15 Nov 2007 19:33:12 GMT
Content-Length: 15137
Connection: close
Content-Type: text/html
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
 Response message:
Status line
Header lines
Entity body
 HTTP is not a secure protocol.
 For secure communication, the HTTP Secure
(HTTPS) is used for accessing or posting web server
information.

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FTP (File Transfer Protocol)
• FTP was developed to allow for file transfers between a client
and a server.
• Used to push and pull files from a server running the FTP
daemon (FTPd).
• FTP requires two connections:
1. Control Connection:
- For commands and replies.
- Port 21.
2. Data Connection:
- For the actual file transfer.
- Port 20.

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– Mail User agent (MUA) “Mail Client”
• Allows messages to be sent and
places received messages into the
client's mailbox.
• GUI user agents: Outlook, Eudora,
Messenger
– Mail servers: Stores user mail boxes,
communicates with local user agents and
other mail servers.
– SMTP: allows you to send e-mail from
either a client or a server.
– POP: allows you to receive e-mail
messages from an e-mail server ,
(Mail access protocol)
SMTP – POP3

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The e-mail server operates two separate processes:
 Mail Transfer Agent (MTA)
 Mail Delivery Agent (MDA)
MTA: is used to forward
e-mail either to another MTA
or to a MDA.
MTA uses SMTP to
route email between
servers.
SMTP – POP3

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MDA: governs transfer of email
from mail servers to clients.
SMTP: an outbound e-mail
delivery protocol.
POP/POP3: an inbound e-mail
delivery protocol.
SMTP – POP3

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SMB: is a client/server file sharing protocol.
Developed by IBM to describe the structure of shared network
resources, such as directories, files, printers, and serial ports.
Microsoft windows and Apple Macintosh operating systems
support resource sharing using the SMB protocol.
SMB Protocol

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• Peers (hosts) act as both clients and servers.
• The actual file transfer usually rely on HTTP services.
• No centralized file server.
• Many client applications are available for accessing the Gnutella network, including:
BearShare, Gnucleus, LimeWire, Morpheus, WinMX and XoloX
P2P File Sharing and Gnutella Protocol
The Gnutella protocol defines five
different packet types:
1. Ping: for device discovery
2. Pong: as a reply to a ping
3. Query: for file location
4. query hit: as a reply to a query
5. Push: as a download request

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