Introduction to:

7 Layers of Open System Interconnection (OSI)

Model

Open Systems Interconnection ( OSI) Model• International standard organization (ISO) established a

committee in 1977 to develop an architecture for computer communication.

• Open Systems Interconnection (OSI) reference model is the result of this effort.

• In 1984, the Open Systems Interconnection (OSI) reference model was approved as an international standard for communications architecture.

• Term “open” denotes the ability to connect any two systems which conform to the reference model and associated standards.

OSI Reference Model• The OSI model is now considered the primary Architectural

model for inter-computer communications.

• The OSI model describes how information or data makes its way from application programmes (such as spreadsheets) through a network medium (such as wire) to another application programme located on another network.

• The OSI reference model divides the problem of moving information between computers over a network medium into SEVEN smaller and more manageable problems .

• This separation into smaller more manageable functions is known as layering.

OSI Reference Model: 7 Layers

OSI: A Layered Network Model• The process of breaking up the functions or tasks of

networking into layers reduces complexity.

• Each layer provides a service to the layer above it in the protocol specification.

• Each layer communicates with the same layer’s software or hardware on other computers.

• The lower 4 layers (transport, network, data link and physical —Layers 4, 3, 2, and 1) are concerned with the flow of data from end to end through the network.

• The upper four layers of the OSI model (application, presentation and session—Layers 7, 6 and 5) are orientated more toward services to the applications.

• Data is Encapsulated with the necessary protocol information as it moves down the layers before network transit.

OSI: A Layered Network Model

OSI Encapsulation Process

OSI Model Layer 7:Application Layer

What is Application Layer?• is the highest level in the Open System Interconnection (OSI)

model and is the level that is closest to you — or furthest away from you if you are at the other end of the connection. The application layer effectively moves data between your computer and the server.

• Provides the interface between the applications on either end of the network.

• Interface between human and data network

• It is responsible for displaying data and images to the user in a human-recognizable format and to interface with the presentation layer below it.

• It focuses on the end to end communication of data source and destinations.

Components of Application Layer • Applications

- provide people with a way to create messages

• Application Layer Services

- establish an interface to the network

• Application Layer Protocols

- provide the rules and formats that govern how data is treated

2 Types of Service Elements of Application

Layer

CASEs

- Common Application Service Elements

- useful to a variety of application processes

Example:

• association control that establishes, maintains and terminates connections with a peer application entity and commitment concurrence and recovery that ensure integrity of distributed transactions.

SASEs- Specific Application Service Elements

- generally satisfy particular needs of application processes.

Examples:

• TCP/IP protocol stack and include FTP (file transfer protocol)

• SMNP ( simple management network protocol)

• telnet ( Virtual terminal protocol)

• SMTP ( simple mail transfer protocol)

Application Layer Architecture

Client/Server Model

Peer-to-Peer (P2P) Model

Application Layer Services and Protocols

Client/Server ModelAdvantages:

- Centralized administration

- Security is easier to enforce

• Client

- device requesting information (initiates the data exchange)

- Can also UPLOAD data to the servers

• Server

- device responding to the request

- Any device that responds to requests from client applications.

- Relies on support from the lower layer functions to distinguish between services and conversations.

- Server relies on a service called a server daemon – runs in the background and ‘listens’ for requests for that service. It can then exchange messages as appropriate & send requested data.

Client/Server Model

Communication Process Between the Application Layer On A Client Computer and Server

1. The process starts at the application layer with someone retrieving and opening an e-mail in an e-mail program, such as Microsoft Outlook.

2. The server’s Post Office Protocol version 3 (POP3) service responds to the request for the user’s e-mail.

3. The data passes through the presentation, session, transport, network, data link, and physical layers, across the physical media, and backs up through all those layers on the client side of the connection.

4. The data arrives at the application layer on the destination computer, and is then displayed using a program such as Outlook.

Peer-to-Peer (P2P) Model

Two Distinct Forms of Peer-to-Peer (P2P)

•P2P Network

•P2P Application

P2P Network

• Two or more computers are connected and are able to share resources without having a dedicated server

• Every end device can function as a client or server on a ‘per request’ basis

• Resources are decentralized (information can be located anywhere)

• Difficult to enforce security and policies

• User accounts and access rights have to be set individually on each peer device

P2P Network Example

P2P Application

• Running applications in hybrid mode allows for a centralized directory of files even though the files themselves may be on multiple machines

• Unlike P2P networks, a device can act as both the client and server within the same communication

• Each device must provide a user interface and run a background service.

• Can be used on P2P networks, client/server networks and across the internet.

P2P Application Example

Application Layer Services and Protocols

Uses of Application Layer Protocols

• Defining processes at either end of the communication

• Defining the types of messages

• Defining the syntax of messages

• Defining the meaning of any informational fields

• Defining how message are sent and the expected response

• Defining the interaction with the next lower layer

Domain Name System (DNS)– Converts/Resolves domain name into an IP Address.

– Matches domain names with IP addresses

– is a client/server service

– sometimes called the DNS resolver, supports name resolution for the other network applications and other services that need it.

– TCP/UDP connection is port 53

– Uses different types of resource records to actually resolve the name/IP address issues

Resolving DNS Addresses

DNS Servers resolve names to IP addresses. It would be difficult to remember the IP address of every website we like to visit, but we can remember names.

Different Types of Resource Records of DNS Server

• A: An end device address

• NS: An authoritative name server

• CNAME: The canonical name (or fully qualified domain name [FQDN]) for an alias; used when multiple services have the single network address but each service has its own entry in DNS

• MX: Mail exchange record; maps a domain name to a list of mail exchange servers for that domain

DNS Hierarchy

Example of Top-Level Domains:

-Root Servers-Top-Level Domain Servers (TLD)-Secondary Level Domain Servers

Telnet- allows users to login to a host from a remote location and take control as if they were sitting at the machine (virtual connection)

- Telecommunication Networ

- Developed in the early 1970’s – among the oldest of the application layer protocols and services in the TCP/IP protocol suite.

- Allows users to emulate text-based terminal devices over the network using software.

- A connection is known as a ‘virtual terminal (vty)’ session.

- Can be run from the command prompt on a PC.

- You can use the device as if you were sitting there with all the rights and priorities that you username will offer you.

- Disadvantages: Doesn’t support encryption like SSH. All data is transferred as plain text. It can be easily intercepted and understood.

- If security is a concern, you should use Secure Shell (SSH) protocol. Provides for remote logins with stronger authentication than telnet.

- Network Professionals should always use SSH whenever possible.

Telnet Example

E-mail services and SMTP/POP protocols• E-mail is the most popular network service.

• Requires several applications and services

• POP or POP3

- Post Office Protocol

- deliver email from server to client (incoming messages)

- Protocol for e-mail client application, like Microsoft Outlook, to retrieve e-mail from an e-mail server.

• SMTP

-Simple Message Transfer Protocol

-handles outbound messages from clients

-Protocol for sending mail from one computer to another. This could be from an e-mail client, like Microsoft Outlook, to an e-mail server, or from one e-mail server to another.

E-mail services and SMTP/POP protocols

• MUA

- Mail User Agent

- E-mail client

- allows messages to be sent/retrieved to and from your mailbox

• MTA

- Mail Transfer Agent

- Server receiving or forwarding emails

• MDA

- Mail Delivery Agent

- Server delivering E-mail to client

E-mail services and SMTP/POP protocols

WWW Service and HTTPWeb Browser

- establishes a connection to the web service running on the server using HTTP. URLs and URIs (uniform resource identifiers) are the names most people associate with web addresses.

- the client applications computers use to connect to the World Wide Web and access resources stored on a web server. As with most server processes, the web server runs as a background service and makes different types of files available.

http://www.cisco.com/web-server.htm.

Three Parts of the URL

• http: The protocol or scheme

• www.cisco.com: The server name

• web-server.htm: The specific filename requested

HTTP

- one of the protocols in the TCP/IP suite, was originally developed to publish and retrieve HTML pages and is now used for distributed, collaborative information systems.

- Protocol for transferring data from web servers to web browser applications like Microsoft Internet Explorer.

- is used across the world wide web for data transfer and is one of the most used application protocols.

- specifies a request/response protocol.

- Not a secure protocol

Three common message types:

•GET

•POST

•PUT

HTTPS

- Secure HTTP Protocol

- Used for accessing and posting web server information

- can use authentication and encryption to secure data as it travels between the client and server

- Specifies additional rules for passing data between the application layer and the transport layer.

Dynamic Host Configuration Protocol (DHCP)- enables devices to obtain IP addresses and other information

from a DHCP server.

- allows a host to obtain an IP address dynamically when it connects to the network.

- DHCP-distributed addresses are not permanently assigned to hosts but are only leased for a period of time.

- makes it possible for you to access the Internet using wireless hotspots

- can pose a security risk because any device connected to the network can receive an address.

DHCP Protocol

FTP Server• was developed to allow file transfers between a client and a server.

• An FTP client is an application that runs on a computer that is used to push and pull files from a server running the FTP daemon (FTPd).

• A 2 connection step for file transfer between client and server

• 1st Connection

-Control Connection/Traffic

-Consists of client commands and server replies.

-Client establishes this connection to the server on TCP port 21.

• 2nd Connection

-Data Connection

-For the actual file transfer and is created every time a file is transferred.

-Client establishes this connection to the server on TCP port 20.

FTP Process

SMB Protocol

• is a client/server file-sharing protocol.

• IBM developed SMB in the late 1980s to describe the structure of shared network resources, such as directories, files, printers, and serial ports.

• clients establish a long-term connection to servers

• describes file system access and indicates how clients can make requests for files.

• It also describes the interprocess communication.

• All SMB messages share a common format.

SMB messages can perform the following tasks:

• Start, authenticate, and terminate sessions

• Control file and printer access

• Allow an application to send or receive messages to or from another device

SUMMARY

THANK YOU! PRESENTED BY:

BATRONEL, Mydene

CUETO, Jocel

EREÑO, Aileen

FRANE, Kristine Marie

GARCIA, Remiel

RIVERA, Madelyn

VILLAPANDO, Aldreen