TCP/IP model define 4 layers that are as follows:
1) Internet layer :
Packet switching network depends upon a connectionless internetwork
layer. This layer is known as internet layer, is the linchpin that holds
the whole design together. Its job is to allow hosts to insert packets
into any network and have them to deliver independently to the
destination. They may appear in a different order than
they were sent in
each case it is a job of higher layers to rearrange them in order to
deliver them to proper destination.
The internet layer specifies an official packet format and protocol
known as internet protocol. The job of internet layer is to transport IP
packets to appropriate destination. Packet routing is very essential
task in order to avoid congestion. For these reason it is said that
TCP/IP internet layer perform same function as that of OSI network
2) Transport layer :
In the TCP/IP model, the layer above the internet layer is known as
transport layer. It is developed to permit entities on the source and
destination hosts to carry on a conversation. It specifies 2 end-to-end
1)TCP (Transmission Control Protocol)
2)UDP (User Datagram Protocol)
It is a reliable connection-oriented protocol that permits a byte stream
originating on one machine to be transported without error on any
machine in the internet. It divides the incoming byte stream into
discrete message and passes each one onto the internet layer. At the
destination, the receiving TCP process collects the received message
into the output stream. TCP deals with flow control to make sure a fast
sender cannot swamp a slow receiver with more message than it can
It is an unreliable, connectionless protocol for applications that do
not want TCP's sequencing on flow control and wish to offer their own.
It is also used for client-server type request-reply queries and
applications in which prompt delivery is more important than accurate
delivery such as transmitting speech or video.
Application Layer :
In TCP/IP model, session or presentation layer are not present.
Application layer is present on the top of the Transport layer. It
includes all the higher-level protocols which are virtual terminal
(TELNET), file transfer (FTP) and electronic mail (SMTP).
The virtual terminal protocol permits a user on one machine to log into a
distant machine and work there. The file transfer protocol offers a way
to move data efficiently from one machine to another. Electronic mail
was used for file transfer purpose but later a specialized protocol was
developed for it.
The Application Layer defines following protocols
File Transfer Protocol (FTP)
It was designed to permit reliable transfer of files over different
platforms. At the transport layer to ensure reliability, FTP uses TCP.
FTP offers simple commands and makes the differences in storage methods
across networks transparent to the user. The FTP client is able to
interact with any FTP server; therefore the FTP server must also be able
to interact with any FTP client. FTP does not offer a user interface,
but it does offer an application program interface for file transfer.
The client part of the protocol is called FTP and the server part of the
protocol is known as FTPd. The suffix "d" means Daemon this is a legacy
from Unix computing where a daemon is a piece of software running on a
server that offers a service.
Hyper Text Transfer Protocol
HTTP permits applications such as browsers to upload and download web
pages. It makes use of TCP at the transport layer again to check
reliability. HTTP is a connectionless protocol that sends a request,
receives a response and then disconnects the connection. HTTP delivers
HTML documents plus all the other components supported within HTML such
Simple Mail Transfer Protocol
By using TCP, SMTP sends email to other computers that support the
TCP/IP protocol suite. SMTP provides an extension to the local mail
services that existed in the early years of LANs. It supervises the
email sending from the local mail host to a remote mail host. It is not
reliable for accepting mail from local users or distributing received
mail to recipients this is the responsibility of the local mail system.
SMTP makes use of TCP to establish a connection to the remote mail host,
the mail is sent, any waiting mail is requested and then the connection
is disconnected. It can also return a forwarding address if the
intended recipient no longer receives email at that destination. To
enable mail to be delivered across differing systems, a mail gateway is
Simple Network Management Protocol
For the transport of network management information, SNMP is used as
standardized protocol. Managed network devices can be cross-examined by a
computer running to return details about their status and level of
activity. Observing software can also trigger alarms if certain
performance criteria drop below acceptable restrictions. At the
transport layer SNMP protocol uses UDP. The use of UDP results in
decreasing network traffic overheads.
4) The Host to Network Layer:
Below the internet layer is great void. The TCP/IP reference model does
not really say such about what happen here, except to point out that the
host has connect to the network using some protocol so it can transmit
IP packets over it. This protocol is not specified and varies from host
to host and network to network.