Network Fundamentals: Ch3 - Application Layer Functionality and Protocols

Suez Canal University – Faculty of Computers & Informatics - Cisco Local Academy

Network Fundamentals

CHP 3: APPLICATION LAYER FUNCTIONALITY AND PROTOCOLS

Last Update: 12/6/2010

Abdekhalik Elsaid [email protected]

http://abdelkhalik.staff.scuegypt.edu.eg/

mailto:[email protected]



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 directlywith 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 design2. 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 23 DNS - TCP/UDP Port 53DHCP - UDP Port 67 HTTP - TCP Port 80FTP - TCP Ports 20 and 21 SMTP - 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.

http://www.cisco.com/


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 suffixReturns a list of IP addresses for TLD Servers responsible

for .com.

DNS Name Resolution

1

2 23


• 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.comto 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

http://www.example.com/


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 OKDate: Fri, 22 Feb 2008 16:34:18 GMTServer: Apache/2.0.52 (Red Hat)Last-Modified: Thu, 15 Nov 2007 19:33:12 GMTContent-Length: 15137Connection: closeContent-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 MTAor to a MDA.

MTA uses SMTP toroute 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 ping3. Query: for file location4. query hit: as a reply to a query 5. Push: as a download request


Thank You..

Network Fundamentals: Ch3 - Application Layer Functionality and Protocols

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Transcript of Network Fundamentals: Ch3 - Application Layer Functionality and Protocols