Application Layer - OSI Model

It is the top most layer of OSI Model. Manipulation of data(information) in various ways
is done in this layer which enables user or software to get access to the network. Some
services provided by this layer includes: E-Mail, transferring files, distributing the results
to user, directory services, network resources, etc.
The Application Layer contains a variety of protocols that are commonly needed by
users. One widely-used application protocol is HTTP (HyperText Transfer Protocol),
which is the basis for the World Wide Web. When a browser wants a web page, it sends
the name of the page it wants to the server using HTTP. The server then sends the
page back.
Other Application protocols that are used are: File Transfer Protocol (FTP), Trivial
File Transfer Protocol (TFTP), Simple Mail Transfer protocol (SMTP)
, TELNET, Domain Name System (DNS) etc.

Functions of Application Layer

1. Mail Services: This layer provides the basis for E-mail forwarding and storage.
2. Network Virtual Terminal: It allows a user to log on to a remote host. The
application creates software emulation of a terminal at the remote host. User's
computer talks to the software terminal which in turn talks to the host and vice
versa. Then the remote host believes it is communicating with one of its own
terminals and allows user to log on.
3. Directory Services: This layer provides access for global information about
various services.
4. File Transfer, Access and Management (FTAM): It is a standard mechanism to
access files and manages it. Users can access files in a remote computer and
manage it. They can also retrieve files from a remote computer.

Design Issues with Application Layer

There are commonly reoccurring problems that occur in the design and implementation
of Application Layer protocols and can be addressed by patterns from several different
pattern languages:

• Pattern Language for Application-level Communication Protocols

 • Service Design Patterns

• Patterns of Enterprise Application Architecture

• Pattern-Oriented Software Architecture

File Transfer Protocol (FTP)

File Transfer Protocol(FTP) is an application layer protocol which moves files between
local and remote file systems. It runs on the top of TCP, like HTTP. To transfer a file, 2
TCP connections are used by FTP in parallel: control connection and data connection.

control connection
For sending control information like user identification, password, commands to change
the remote directory, commands to retrieve and store files, etc., FTP makes use of
control connection. The control connection is initiated on port number 21.

data connection
For sending the actual file, FTP makes use of data connection. A data connection is
initiated on port number 20. FTP sends the control information out-of-band as it uses a
separate control connection. Some protocols send their request and response header
lines and the data in the same TCP connection. For this reason, they are said to send
their control information in-band. HTTP and SMTP are such examples.

FTP Session
When a FTP session is started between a client and a server, the client initiates a
control TCP connection with the server side. The client sends control information over
this. When the server receives this, it initiates a data connection to the client side. Only
one file can be sent over one data connection. But the control connection remains
active throughout the user session. As we know HTTP is stateless i.e. it does not have
to keep track of any user state. But FTP needs to maintain a state about its user
throughout the session.
Data Structures : FTP allows three types of data structures :

1. File Structure – In file-structure there is no internal structure and the file is

considered to be a continuous sequence of data bytes.
2. Record Structure – In record-structure the file is made up of sequential records.
3. Page Structure – In page-structure the file is made up of independent indexed
pages.

Virtual terminal
A virtual terminal is a secure web page in your internet browser where you can process
card payments online. It is called “virtual” because it functions like a credit card terminal,
but through a non-physical software application rather than physical terminal.

Usually, the virtual terminal is embedded on a page you can access when you’re logged
in to your payment provider’s account online. You process the payment by manually
entering the customer’s card details – which you may be given over the phone, a mail
order form or through an online chat – and submitting it for processing. The customer’s
card will then be charged, and the money transferred to your payment provider account
or bank account, minus applicable processing fees.

Advantages of virtual terminals

• You can take payments quickly and easily without your customer being
present. But if the customer is present, you will need a card reader that
can perform a chip and PIN or contactless transaction instead, for security
and verification reasons.
• You don’t need to invest in expensive point-of-sale software or traditionally
expensive credit card machines. If you don’t sell anything in person, you
don’t even need to invest in a cash drawer.
• There is more freedom and flexibility – transactions can be carried out
anywhere, from any device and at any time, as long as you have an
internet connection.

Disadvantages of virtual terminals

• You need extra security measures in place to take card-not-present
payments to avoid chargeback fees.
• Keyed-in payments require more effort (typing needs attention to detail)
than a card machine transaction that automatically processes a
contactless or chip and PIN card.
 • Fees are typically higher for transactions where the card/customer is not
present than for card machine payments.

Public Network
A public network is a type of network wherein anyone, namely the general public, has access
and through it can connect to other networks or the Internet. This is in contrast to a private
network, where restrictions and access rules are established in order to relegate access to a
select few. Since a public network has few or no restrictions, users need to be wary of possible
security risks when accessing it.

PRIVATE IP ADDRESS PUBLIC IP ADDRESS

Scope is local. Scope is global.

It is used to communicate outside the

It is used to communicate within the network. network.

Private IP addresses of the systems connected Public IP may differ in uniform or

in a network differ in a uniform manner. non-uniform manner.

It works only in LAN. It is used to get internet service.

It is used to load network operating system. It is controlled by ISP.

It is available in free of cost. It is not free of cost.

Private IP can be known by entering “ipconfig” Public IP can be known by searching

on command prompt. “what is my ip” on google.

Other Application layer Protocol

1. TELNET:

Telnet stands for the TELecomunications NETwork. It helps in terminal emulation. It

allows Telnet client to access the resources of the Telnet server. It is used for managing
the files on the internet. It is used for initial set up of devices like switches. The telnet
command is a command that uses the Telnet protocol to communicate with a remote
device or system. Port number of telnet is 23.

2. FTP:

FTP stands for file transfer protocol. It is the protocol that actually lets us transfer files.It
can facilitate this between any two machines using it. But FTP is not just a protocol but
it is also a program.FTP promotes sharing of files via remote computers with reliable
and efficient data transfer. Port number for FTP is 20 for data and 21 for control.

5. SMTP:

It stands for Simple Mail Transfer Protocol. It is a part of the TCP/IP protocol. Using a
process called “store and forward,” SMTP moves your email on and across networks. It
works closely with something called the Mail Transfer Agent (MTA) to send your
communication to the right computer and email inbox. Port number for SMTP is 25.

8. SNMP:

It stands for Simple Network Management Protocol. It gathers data by polling the
devices on the network from a management station at fixed or random intervals,
requiring them to disclose certain information. It is a way that servers can share
information about their current state, and also a channel through which an administrate
can modify pre-defined values. Port number of SNMP is 161(TCP) and 162(UDP).

DNS(Domain Name System)

An application layer protocol defines how the application processes running
on different systems, pass the messages to each other.

o DNS stands for Domain Name System.

o DNS is a directory service that provides a mapping between the name
of a host on the network and its numerical address.
o DNS is required for the functioning of the internet.
o Each node in a tree has a domain name, and a full domain name is a
sequence of symbols specified by dots.
 o DNS is a service that translates the domain name into IP addresses.
This allows the users of networks to utilize user-friendly names when
looking for other hosts instead of remembering the IP addresses.
o For example, suppose the FTP site at EduSoft had an IP address of
132.147.165.50, most people would reach this site by specifying
ftp.EduSoft.com. Therefore, the domain name is more reliable than IP
address.

DNS is a TCP/IP protocol used on different platforms. The domain name

space is divided into three different sections: generic domains, country
domains, and inverse domain.

Generic Domains
o It defines the registered hosts according to their generic behavior.
o Each node in a tree defines the domain name, which is an index to the DNS
database.
o It uses three-character labels, and these labels describe the organization type.

Label Description

Aero Airlines and aerospace companies

Biz Businesses or firms

Com Commercial Organizations

Coop Cooperative business Organizations

Edu Educational institutions

Gov Government institutions

Info Information service providers

Int International Organizations

Mil Military groups

museum Museum & other nonprofit organizations

Name Personal names

Net Network Support centers

Org Nonprofit Organizations

Pro Professional individual Organizations