1. TCP/IP Model
TCP/IP is an abbreviation used for transmission control
protocol/Internet Protocol.
It was developed by Defence Advance Research Projects
Agency (ARPA, later DARPA) in late 1970's.
It is a set of communications protocol. It contains sets and
procedures for dealing with packet transport, media access,
session communication, file transfer, email and terminal
emulation.

2. The current internet is based on TCP/IP reference model.
The model was defined by keeping following goals in mind.
(i) The network should connect multiple networks
together.
(ii) The connection should withstand till the source and
destination machines are functioning.
(iii) The architecture should be so flexible that it should
be able to transfer data among different hardware and/or
software platforms.

4. Host to Network Layer
This is the lowest layer in TCP/IP reference model. The
functioning of this layer is generally different in different
networks. The main job of this layer is to inform the upper
layers about when the host is connected to the network; so
that they could start sending the data packets.
There are various networks like ARPANET, SATNET, X-25,
ATM, ETHERNET, Token ring etc. into existence. The host
has to connect to the network using some protocol, so that
it can send the IP packet over it. Protocols used in this
layer also vary from host to host and network to network.

5. Internet Layer
The functioning of internet layer of TCP/IP model is same
as the functioning of network layer of oSI model. At this
layer the Internet Protocol (IP) is used. The task of this
layer is to allow the host to insert packets into any
network and then make them travel independently to the
destination. The order in which the packets are received
can be different from the sequence in which they were
sent. Then the higher layers are supposed to arrange
them in the proper order. So routing of packets and
congestion control are important issues related to this
layer.

6. Main functions of the internet layer are
1. Internetworking
2. Addressing
3. Routing
4. Packetizing
5. Data Switching
6. Congestion Control etc.

7. Transport layer
The transport layer is situated above the internet layer of
TCP model. The functioning of this layer is same as the
functioning of transport layer in OSI model. In this layer,
the byte stream is divided into message and these
messages are passed on to the internet layer. The
transport layer provides the services like segmentation and
fragmentation of messages. It also handles the flow control
so that a fast sender should not overflow a slow receiver.

8. The protocols used in this layer are
1) TCP :It is a reliable connection oriented protocol. It allows a
byte stream transmitted from one machine to be delivered to
the other machine without introducing any errors. TCP also
handles the flow control.
2) UDP: (User Datagram Protocol) It is an unreliable,
connectionless protocol and used for the applications which do
not want the TCP's sequencing or flow control. It is used in
transmitting speech or video.

9. Main functions of transport layer are:
1. Segmenting and reassembly
2. Sequence Numbering
3. Connection control
4. Flow control
5. Error control
6. Multiplexing

10. Application Layer
This is the topmost layer of TCP/IP layer. It is the layer
seen by the application program and hence is user oriented
layer which provides services to support end user of
network directly. A message or data to be sent across
network enters the TCP/IP model at this point and then
travels down ward into then through transmission line unto
host-to-network layer of destination & then upwards up to
application layer of receiver end system. It provides
services that directly support user applications such as
database access, email and file transfer.

11. This layer uses the various protocol to transfer the data
between applications. Some common protocol used in this
layer are:
(a) FTP (File Transfer Protocol)
FTP is used for file transfer between inter network nodes.
b) SMTP (Simple Mail transfer Protocol)
It is used for exchanging e-mail.
c)Telnet
It is used for remote terminal emulation. It enables user to
access host­based
applications by emulating one of the
host's terminals. Telnet provides connectivity between
dissimilar operating systems. Telnet is a general purpose
client-server application program.

12. d) DNS (Domain Name Systems)
The DNS is used to convert the domain name into the IP
addresses. To identify an entity, TCP/IP protocol use the
IP address which uniquely identify the connection of a
host to the internet. However, people prefer to use name
instead of addresses. Therefore DNS maps a name to an
address and conversely.
(e) HTTP (Hypertext Transfer Protocol)
HTTP is an internet protocol designed for a specific
application, the World Wide Web (WWW).