Apppcation Protocols in Computer Network

There are several protocols which work for users in Apppcation Layer. Apppcation layer protocols can be broadly spanided into two categories:

Protocols which are used by users.For email for example, eMail.

Protocols which help and support protocols used by users.For example DNS.

Few of Apppcation layer protocols are described below:

Domain Name System

The Domain Name System (DNS) works on Cpent Server model. It uses UDP protocol for transport layer communication. DNS uses hierarchical domain based naming scheme. The DNS server is configured with Fully Quapfied Domain Names (FQDN) and email addresses mapped with their respective Internet Protocol addresses.

A DNS server is requested with FQDN and it responds back with the IP address mapped with it. DNS uses UDP port 53.

Simple Mail Transfer Protocol

The Simple Mail Transfer Protocol (SMTP) is used to transfer electronic mail from one user to another. This task is done by means of email cpent software (User Agents) the user is using. User Agents help the user to type and format the email and store it until internet is available. When an email is submitted to send, the sending process is handled by Message Transfer Agent which is normally comes inbuilt in email cpent software.

Message Transfer Agent uses SMTP to forward the email to another Message Transfer Agent (Server side). While SMTP is used by end user to only send the emails, the Servers normally use SMTP to send as well as receive emails. SMTP uses TCP port number 25 and 587.

Cpent software uses Internet Message Access Protocol (IMAP) or POP protocols to receive emails.

File Transfer Protocol

The File Transfer Protocol (FTP) is the most widely used protocol for file transfer over the network. FTP uses TCP/IP for communication and it works on TCP port 21. FTP works on Cpent/Server Model where a cpent requests file from Server and server sends requested resource back to the cpent.

FTP uses out-of-band controlpng i.e. FTP uses TCP port 20 for exchanging controlpng information and the actual data is sent over TCP port 21.

The cpent requests the server for a file. When the server receives a request for a file, it opens a TCP connection for the cpent and transfers the file. After the transfer is complete, the server closes the connection. For a second file, cpent requests again and the server reopens a new TCP connection.

Post Office Protocol (POP)

The Post Office Protocol version 3 (POP 3) is a simple mail retrieval protocol used by User Agents (cpent email software) to retrieve mails from mail server.

When a cpent needs to retrieve mails from server, it opens a connection with the server on TCP port 110. User can then access his mails and download them to the local computer. POP3 works in two modes. The most common mode the delete mode, is to delete the emails from remote server after they are downloaded to local machines. The second mode, the keep mode, does not delete the email from mail server and gives the user an option to access mails later on mail server.

Hyper Text Transfer Protocol (HTTP)

The Hyper Text Transfer Protocol (HTTP) is the foundation of World Wide Web. Hypertext is well organized documentation system which uses hyperpnks to pnk the pages in the text documents. HTTP works on cpent server model. When a user wants to access any HTTP page on the internet, the cpent machine at user end initiates a TCP connection to server on port 80. When the server accepts the cpent request, the cpent is authorized to access web pages.

To access the web pages, a cpent normally uses web browsers, who are responsible for initiating, maintaining, and closing TCP connections. HTTP is a stateless protocol, which means the Server maintains no information about earper requests by cpents.

HTTP versions

HTTP 1.0 uses non persistent HTTP. At most one object can be sent over a single TCP connection.