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
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
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
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
OSI: A Layered Network Model
The process of breaking up the functions or tasks of networking into layers
Each layer provides a service to the layer above it in the protocol
Each layer communicates with the same layer’s software or hardware on
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
The upper four layers of the OSI model (application, presentation and
session—Layers 7, 6 and 5) are orientated more toward services to the
Data is Encapsulated with the necessary protocol information as it moves
down the layers before network transit.
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.
the interface between the applications on either end of the
• Interface between human and data network
• It is responsible for displaying data and images to the user in a humanrecognizable format and to interface with the presentation layer below it.
focuses on the end to end communication of data source and
Components of Application Layer
- 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
2 Types of Service Elements
of Application Layer
- Common Application Service Elements
- useful to a variety of application processes
control that establishes, maintains and
terminates connections with a peer application entity and
commitment concurrence and recovery that ensure
integrity of distributed transactions.
- Specific Application Service Elements
- generally satisfy particular needs of application processes.
TCP/IP protocol stack and include FTP (file transfer protocol)
SMNP ( simple management network protocol)
telnet ( Virtual terminal protocol)
SMTP ( simple mail transfer protocol)
Peer-to-Peer (P2P) Model
Application Layer Services and Protocols
- Centralized administration
- Security is easier to enforce
- device requesting information (initiates the data exchange)
- Can also UPLOAD data to the servers
- device responding to the request
Any device that responds to requests from client applications.
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.
Relies on support from the lower layer functions to distinguish between
services and conversations.
Communication Process Between the Application
Layer On A Client Computer and Server
The process starts at the application layer
with someone retrieving and opening an email in an e-mail program, such as
The server’s Post Office Protocol version 3
(POP3) service responds to the request for
the user’s e-mail.
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.
The data arrives at the application layer on
the destination computer, and is then
displayed using a program such as
Peer-to-Peer (P2P) Model
Two Distinct Forms of Peer-to-Peer (P2P)
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
• 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
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
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
• 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
-Top-Level Domain Servers (TLD)
-Secondary Level Domain Servers
Example of Top-Level
allows users to login to a host from a remote location and take control as if they were
sitting at the machine (virtual connection)
Allows users to emulate text-based terminal devices over the network using software.
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.
Developed in the early 1970’s – among the oldest of the application layer protocols and
services in the TCP/IP protocol suite.
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.
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.
- 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
Mail User Agent
allows messages to be sent/retrieved to and from your mailbox
Mail Transfer Agent
Server receiving or forwarding emails
Mail Delivery Agent
Server delivering E-mail to client
WWW Service and HTTP
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
Three Parts of the URL
• http: The protocol or scheme
• www.cisco.com: The server name
• web-server.htm: The specific filename requested
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:
Secure HTTP Protocol
Specifies additional rules for passing data between the application
layer and the transport layer.
Used for accessing and posting web server information
can use authentication and encryption to secure data as it travels
between the client and server
Dynamic Host Configuration Protocol
enables devices to obtain IP addresses and other information from a DHCP
allows a host to obtain an IP address dynamically when it connects to the
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.
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
- Control Connection/Traffic
- Consists of client commands and server replies.
- Client establishes this connection to the server on TCP port 21.
- 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.
• 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
• 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