Unit 6
Short Notes on following
There are several protocols which work for users in Application Layer. Application layer
protocols can be broadly divided 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.
Domain Name System
The Domain Name System (DNS) works on Client Server model. It uses UDP protocol
for transport layer communication. DNS uses hierarchical domain based naming
scheme. The DNS server is configured with Fully Qualified 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 client 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 client 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.
Client 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
Client/Server Model where a client requests file from Server and server sends requested
resource back to the client.
FTP uses out-of-band controlling i.e. FTP uses TCP port 20 for exchanging controlling
information and the actual data is sent over TCP port 21.
The client requests the server for a file. When the server receives a request for a file, it
opens a TCP connection for the client and transfers the file. After the transfer is complete,
the server closes the connection. For a second file, client 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 (client email software) to retrieve mails from mail server.
When a client 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 hyperlinks to link the pages in the text
documents. HTTP works on client server model. When a user wants to access any HTTP page
on the internet, the client machine at user end initiates a TCP connection to server on port
80. When the server accepts the client request, the client is authorized to access web pages.
To access the web pages, a client 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 earlier requests by clients.
HTTP versions
HTTP 1.0 uses non persistent HTTP. At most one object can be sent over a single TCP
connection.
HTTP 1.1 uses persistent HTTP. In this version, multiple objects can be sent over a
single TCP connection.
