APPLICATION LAYER
The application layer is responsible for providing services to the users.
It provides user interfaces and support for services such as E-mail, file access and transfer,
access to system resources, surfing the world wide web(WWW), and network management.
DNS client/server program can support an e-mail program to find the IP address of an e-mail
recipient.
In a flat name space, a name is assigned to an address.
Node in the tree has a domain name. A full domain name is a sequence of labels separated by
dots (.)
A domain is a subtree of the domain name space.
The country domains section uses two-character country abbreviations (e.g., us for United
States).
DNS can use the services of UDP or TCP using the well-known port number 53.
The Domain Name System(DNS) is aclient/server application that identifies each host on the
Internet with a unique user-friendly name.
There are two types of DNS messages:
(1) queries
(2) responses
DNS uses the services of UDP for messages of less than 512 bytes; otherwise i.e more than
512 bytes TCP is used.
TELNET is a general-purpose client/server application program.
TELNET is an abbreviation for TErminaL NETwork.
TELNET uses only one TCP connection. The server uses the well-known port number 23.
TELNET is a client/server application that allows a user to log on to a remote machine, giving
the user access to the remote system.
SMTP is used two times, between the sender and the sender's mail server and between the two
mail servers.
SMTP simply defines how commands and responses must be sent back and forth.
SMTP uses commands and responses to transfer messages between an MTA client and an
MTA server.
Transferring files from one computer to another is one of the most common tasks expected
from a networking or internetworking environment. As a matter of fact, the greatest volume of
data exchange in the Internet today is due to
file transfer.
File Transfer Protocol (FTP) is the standard mechanism provided by TCP/IP for copying a file
from one host to another.
FTP uses the services of TCP. It needs two TCP connections. The well-known port number 21
is used for the control connection and the well-known port number 20 for the data connection.
One of the programs used for file transfer in the Internet is File Transfer Protocol (FTP).
FTP requires two connections for data transfer:
(1) control connection
(2) data connection.
FTP uses two well- known TCP ports
Port 21 ----. Control Connection.
Port 20. ---- Data connection
The WWW has aunique combination of flexibility, portability, and user-friendly features that
distinguish it from other services provided by the Internet.
The WWW project was initiated by
CERN
(European Laboratory for Particle Physics) to create a system to handle distributed.
Many different protocols can retrieve a document; among them are FTP or HTTP. The most
common today is HTTP.
Hyper text Markup Language (HTML) is a language for creating Web pages.
Hyper text Transfer Protocol (HTTP) is a protocol used mainly to access data on the World Wide
Web.
HTTP functions as a combination of FTP and SMTP.
It is similar to FTP because it transfers files and uses the services of TCP.
Unlike SMTP, the HTTP messages are not destined to be read by humans; they are read and
interpreted by the HTTP server and HTTP client (browser).
SMTP messages are stored and forwarded,
HTTP messages are delivered immediately.
HTTP uses the services of TCP port number 80.
The World Wide Web (WWW) is a repository of information linked together from points all over
the world.
HTTP uses a TCP connection to transfer files.
The uniform resource locator (URL) consists of a method, host computer, optional port number,
and path name to locate information on the WWW.
Simple Network Management Protocol (SNMP) is a frame work for managing devices in an
internet using the TCP/IP protocol suite.
It provides a set of fundamental operations for monitoring and maintaining an internet.
SNMP uses the concept of manager and agent. That is, a manager, usually a host, controls and
monitors a set of agents, usually routers.
SNMP is an application-level protocol in which a few manager stations control a set of agents.
SNMP defines the format of packets exchanged between a manager and an agent. It reads and
changes the status (values) of objects (variables) in SNMP packets.
SNMP uses the services of UDP on two well-known ports, 161 and 162.
Streaming stored audio/video refers to on-demand requests for compressed audio/video files.
Streaming live audio/video refers to the broadcasting of radio and TV programs through the
Internet.
Real-time traffic needs the support of multicasting.
TCP, with all its sophistication, is not suitable for interactive multimedia traffic because we
cannot allow retransmission of packets.
UDP is more suitable for interactive multimedia traffic. UDP supports multicasting and has no
retransmission strategy.
However, UDP has no provision for time-stamping, sequencing, or mixing.
A new transport protocol, Real-time Transport Protocol (RTP), provides these missing features.
Joint Photographic Experts Group (JPEG) is a method to compress pictures and graphics.
Moving Pictures Experts Group (MPEG) is a method to compress video.
Real-time multimedia traffic requires both UDP and Real-time Transport Protocol (RTP).
