The TCP/IP Reference Model
The TCP/IP reference model is a set of four communication protocols. It was developed by the Department of Defense in the 1960s and is named after the two main protocols used in the model, namely TCP and IP. The Ministry of Defense wanted the connections to remain intact as long as the source and target machines were running, even though some of the intermediate transmission lines or machines were suddenly down. Besides, since applications with different requirements were considered, from file transfer to real-time voice transmission, a flexible architecture was required.
The Link Layer
In The TCP/IP reference model, All these requirements have led to the choice of a packet-switching based network on a connectionless layer run in different networks. The lowest level of the model, the link layer, describes what links need to do, such as serial and traditional Ethernet interfaces, to meet the needs of this stand-alone Internet layer. It is not a level, in the usual sense of the term, but an interface between hosts and transmission lines. The sources on the TCP / IP model have little to say about it.
The Internet Layer
In The TCP/IP reference model, The level of the Internet is a key element that unites all the architecture. The following figure shows that it roughly corresponds to the OSI network layer. Its task is to allow hosts to enter packets on any network and send them independently to their destination (possibly to another network). They may even arrive in a completely different order than the one that was sent, in which case the work of the higher levels is to rearrange them if you wish to place an order. Keep in mind that the Internet is used herein general, although this layer is present on the Internet.
The Transport Layer
The level at the Internet layer in the TCP / IP model is now called the transport layer. It is designed to allow the peer objects of the source and destination hosts to conduct a dialog, such as on the OSI transport layer. Here, two transit transport protocols have been defined. The first is TCP (Transmission Control Protocol) and the second protocol at this level is UDP (User Datagram Protocol).
TCP is a robust, connection-oriented protocol that allows you to deliver a stream of bytes from one machine without error to another machine on the Internet. Segment the incoming byte stream into separate messages and send each to the Internet layer. At the destination, the destination TCP process reassembles the received messages in the output sequence. TCP also manages flow control to ensure that the fast sender cannot fill the slow recipient with more messages than he can handle.
UDP is an unreliable connectionless protocol for applications that do not want TCP sequencing or flow control and want to provide theirs. It is also widely used for occasional requests and applications of a client-server request, where fast delivery is more important than accurate delivery, such as voice or video.
The Application Layer
At the top of the Transport, the level is the application layer. It contains all the higher-level protocols. The first ones are the virtual terminal (TELNET), the file transfer (FTP) and the electronic mail (SMTP).
HTTP, Web page receiving protocol, and RTP, real-time multimedia streaming protocol, such as voice or movies.