Explain the TCP/IP Reference Model.

Explain the TCP/IP Reference Model.

The TCP/IP Reference Model:

The TCP/IP reference model was developed prior to OSI model. The major design goals

of this model were,

1. To connect multiple networks together so that they appear as a single network.

2. To survive after partial subnet hardware failures.

3. To provide a flexible architecture.

Unlike OSI reference model, TCP/IP reference model has only 4 layers. They are, 

Computer Networks

1. Host-to-Network Layer

2. Internet Layer

3. Transport Layer

4. Application Layer

Host-to-Network Layer:  The TCP/IP reference model does not really say much about what happens here, except to point out that the host has to connect to the network using some protocol so it can send IP packets to it. This protocol is not defined and varies from host to host and network to network. 

Internet Layer: 

This layer, called the internet layer, is the linchpin that holds the whole architecture

together. Its job is to permit hosts to inject packets into any network and have they travel

independently to the destination (potentially on a different network). They may even arrive in

a different order than they were sent, in which case it is the job of higher layers to rearrange

them, if in-order delivery is desired. Note that ''internet'' is used here in a generic sense, even

though this layer is present in the Internet. 

The internet layer defines an official packet format and protocol called IP (InternetProtocol). The job of the internet layer is to deliver IP packets where they are supposed to go. Packet routing is clearly the major issue here, as is avoiding congestion. For these reasons, it

is reasonable to say that the TCP/IP internet layer is similar in functionality to the OSI

network layer. Fig.6.1 shows this correspondence.

The Transport Layer:

The layer above the internet layer in the TCP/IP model is now usually called the Transport carry on a conversation, just as in the OSI transport layer. Two end-to-end transport protocols have been defined here. The first one, TCP (Transmission Control Protocol), is a reliable connection-oriented protocol that allows a byte stream originating on one machine to be delivered without error on any other machine in the internet. It fragments the incoming byte stream into discrete messages and passes each one on to the internet layer. At the destination, the receiving TCP process reassembles the received messages into the output stream. TCP also handles flow control to make sure a fast sender cannot swamp a slow receiver with more  messages than it can handle. 

The second protocol in this layer, UDP (User Datagram Protocol), is an unreliable, connectionless protocol for applications that do not want TCP's sequencing or flow control

and wish to provide their own. It is also widely used for one-shot, client-server-type request-

reply queries and applications in which prompt delivery is more important than accurate

delivery, such as transmitting speech or video. The relation of IP, TCP, and UDP

 The Application Layer: 

The TCP/IP model does not have session or presentation layers. 

On top of the transport layer is the application layer. It contains all the higher-level

protocols. The early ones included virtual terminal (TELNET), file transfer (FTP), and

electronic mail (SMTP), as shown in Fig.6.2. The virtual terminal protocol allows a user on

one machine to log onto a distant machine and work there. The file transfer protocol provides

a way to move data efficiently from one machine to another. Electronic mail was originally

just a kind of file transfer, but later a specialized protocol (SMTP) was developed for it.

 Many other protocols have been added to these over the years: the Domain Name System (DNS) for mapping host names onto their network addresses, NNTP, the protocol for moving USENET news articles around, and HTTP, the protocol for fetching pages on the World Wide Web, and many others.