Network Fundamentals: Ch3 - Application Layer Functionality and Protocols

Suez Canal University – Faculty of Computers & Informatics - Cisco Local Academy
Network Fundamentals
Last Update: 12/6/2010
Abdekhalik Elsaid Mosa
abdu.elsaid@yahoo.com
http://abdelkhalik.staff.scuegypt.edu.eg/

Suez Canal University – Faculty of Computers & Informatics - Local Cisco Academy
Application Layer
• OSI: is a layered, abstract representation created as a guideline
for network protocol design.
• Application Layer, provides human interface to the network.

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.

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

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

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.

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

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.

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.

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.

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

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.

DNS Servers Hierarchy

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

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

• 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

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

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

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

DHCP – Dynamic Host Configuration Protocol

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

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

• 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

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

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

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

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.

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.

– 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

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

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

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

• 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

Network Fundamentals: Ch3 - Application Layer Functionality and Protocols

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