TCP/IP Reference Model

The TCP/IP Reference Model

The ARPANET was a research network sponsored by the DoD (U.S. Department of Defence). It eventually connected hundreds of universities and government installations,using leased telephone lines. When satellite and radio networks were added later, the existing protocols had trouble inter-working with them, so a new reference architecture was needed. Thus, from nearly the beginning, the ability to connect multiple networks in a seamless way was one of the major design goals.This architecture later became known as the TCP/IP Reference Model, after itstwo primary protocols. 

The Link Layer

All the requirements of DoD led to the choice of a packet-switching network based on a connection less layer that runs across different networks.

The lowest layer in the model, the link layer describes what links such as serial lines and classic Ethernet must do to meet the needs of this connection less internet layer. It is not really a layer at all, in the normal sense of the term, but rather an interface between hosts and transmission links. The Internet Layer

The internet layer is the linchpin (hub) that holds the whole architecture together.It is shown in Fig. as corresponding roughly to the OSI network layer. Its job is to permit hosts to inject packets into any network and have them travel independently to the destination (potentially on a different network). They may even arrive in a completely 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.

The internet layer defines an official packet format and protocol called IP(Internet Protocol), plus additionally coordinating protocol called ICMP (Internet Control Message Protocol) that helps it function. The job of the internet layer is to deliver IP packets where they are supposed to go. Packet routing is clearly a major issue here, as is congestion (though IP has not proven effective at avoiding congestion).

The Transport Layer

The layer above the internet layer in the TCP/IP model is now usually called the transport layer. It is designed to allow peer entities on the source and destination hosts to 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 segments 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 as low receiver with more messages than it can handle.

The second protocol in this layer, UDP (User Datagram Protocol), is an unreliable, connection less 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 Application Layer

The TCP/IP model does not have session or presentation layers. No need for them was perceived. Instead, applications simply include any session and presentation functions that they require.

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).