Application Layer
The application layer is a layer in the Open Systems Interconnection (OSI) seven-layer
model in the TCP/IP protocol suite. It consists of protocols that focus on process-to-process
communication across an IP network and provides a firm communication interface and end-
user services.
The application layer is the seventh layer of the OSI model and the only one that
directly interacts with the end user. The application layer provides many services, including:
● File transfer
● Web surfing
● Web chat
● Network data sharing
● Virtual terminals
● Various file and data operations
The application layer provides full end-user access to a variety of shared network
services for efficient OSI model data flow. This layer has many responsibilities, including error
handling and recovery, data flow over a network and full network flow. It is also used to
develop network-based applications. The functions of the application layer are:
● Facilitate the user to use the services of the network
● Develop network-based applications
● Provide user services like user login, naming network devices, formatting messages,
and e-mails, transfer of files etc.
● Error handling and recovery of the message as a whole.
Application Layer Protocols
This layer uses a number of protocols, the main among which are as follows:
● Hypertext Transfer Protocol (HTTP)
➔ The underlying protocol for world wide web; it defines how hypermedia
messages are formatted and transmitted
● File Transfer Protocol (FTP)
➔ A client-server based protocol for transfer of files between client and
server over the network
● Simple Mail Transfer Protocol (SMTP)
➔ Lays down the rules and semantics for sending and receiving electronic
mails (e-mails)
● Domain Name Service (DNS)
➔ A naming system for devices in networks; it provides services for
translating domain names to IP addresses
● Telecommunications Network (TELNET)
➔ Provides bi-directional text-oriented services for remote login to the
hosts over the network
● Simple Network Management Protocol (SNMP)
➔ Used for managing, monitoring the network and for organizing
information about the networked devices
Well-Known Application Layer Protocols and Services
The application layer is present at the top of the OSI model. It is the layer through
which users interact. It provides services to the user.
1. Telecommunications Network (TELNET)
❏ Used for managing files on the internet and initial set up of devices such as
switches; allows Telnet client to access the resources of the Telnet server
Command :
telnet [\\RemoteServer]
\\RemoteServer : Specifies the name of the server to which
you want to connect
2. File Transfer Protocol (FTP)
❏ A program that promotes sharing of files via remote computers with reliable
and efficient data transfer
Command :
ftp machinename
3. Trivial File Transfer Protocol (TFTP)
❏ An internet software utility for transferring files that is simpler to use than the
File Transfer Protocol (FTP) but less capable; it is used where user
authentication and directory visibility are not required
Command :
tftp [ options... ] [host [port]] [-c command]
4. Network File System (NFS)
❏ Allows remote hosts to mount file systems over a network and interact with
those file systems as though they are mounted locally; enables system
administrators to consolidate resources onto centralized servers on the network
Command :
service nfs start
5. Simple Mail Transfer Protocol (SMTP)
❏ Works closely with something called the Mail Transfer Agent (MTA) to send your
communication to the right computer and email inbox; using a process called
“store and forward,” SMTP moves your email on and across networks
Command :
MAIL FROM:<mail@abc.com?
6. Line Printer Daemon (LPD)
❏ A network printing protocol for submitting print jobs to a remote printer
Command :
lpd [ -d ] [ -l ] [ -D DebugOutputFile]
7. X Window
❏ It defines a protocol for the writing of graphical user interface–based
client/server applications; primarily used in networks of interconnected
mainframes
Command :
Run xdm in runlevel 5
8. Simple Network Management Protocol (SNMP)
❏ Gathers data by polling the devices on the network from a management station
at fixed or random intervals, requiring them to disclose certain information; a
way that servers can share information about their current state, and also a
channel through which an administrate can modify predefined values
Command :
snmpget -mALL -v1 -cpublic snmp_agent_Ip_address sysName.0
9. Domain Name Service (DNS)
❏ Translates domain names into IP Addresses, which computers can understand;
provides a list of mail servers which accept Emails for each domain name
Command :
ipconfig /flushdns
10. Dynamic Host Configuration Protocol (DHCP)
❏ Enables a server to automatically assign an IP address to a computer from a
defined range of numbers configured for a given network
Command :
clear ip dhcp binding {address | * }
Sources:
https://www.techopedia.com/definition/6006/application-layer
https://www.tutorialspoint.com/The-Application-Layer-in-TCP-IP-Model
https://www.geeksforgeeks.org/protocols-application-layer/
