The TCP/IP Reference Model

• TCP/IP means Transmission Control Protocol and Internet Protocol. It is the network model used in the current Internet architecture as well. 
• Protocols are set of rules which govern every possible communication over a network. 
• These protocols describe the movement of data between the source and destination or the internet. These protocols offer simple naming and addressing schemes.

Overview of TCP/IP reference model

• TCP/IP that is Transmission Control Protocol and Internet Protocol was developed by Department of Defence's Project Research Agency (ARPA, later DARPA) as a part of a research project of network interconnection to connect remote machines.
• The features that stood out during the research, which led to making the TCP/IP reference model were:     
• Support for a flexible architecture. Adding more machines to a network was easy.
• The network was robust, and connections remained intact untill the source and destination machines were functioning.
• The overall idea was to allow one application on one computer to talk to(send data packets) another application running on different computer.

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Description of different TCP/IP protocols

Layer 1: Host-to-network Layer

• Lowest layer of the all.
• Protocol is used to connect to the host, so that the packets can be sent over it.
• Varies from host to host and network to network.

Layer 2: Internet layer

• Selection of a packet switching network which is based on a connection less internetwork layer is called a internet layer.
• It is the layer which holds the whole architecture together.
• It helps the packet to travel independently to the destination.
• Order in which packets are received is different from the way they are sent.
• IP (Internet Protocol) is used in this layer.
• The various functions performed by the Internet Layer are:

1. Delivering IP packets
2. Performing routing
3. Avoiding congestion

Layer 3: Transport Layer

• It decides if data transmission should be on parallel path or single path.
• Functions such as multiplexing, segmenting or splitting on the data is done by transport layer.
• The applications can read and write to the transport layer.
• Transport layer adds header information to the data.
• Transport layer breaks the message (data) into small units so that they are handled more efficiently by the network layer.
• Transport layer also arrange the packets to be sent, in sequence.

Layer 4: Application Layer

• The TCP/IP specifications described a lot of applications that were at the top of the protocol stack. Some of them were TELNET, FTP, SMTP, DNS etc.
• TELNET is a two-way communication protocol which allows connecting to a remote machine and run applications on it.
• FTP(File Transfer Protocol) is a protocol, that allows File transfer amongst computer users connected over a network. It is reliable, simple and efficient.
• SMTP(Simple Mail Transport Protocol) is a protocol, which is used to transport electronic mail between a source and destination, directed via a route.
• DNS(Domain Name Server) resolves an IP address into a textual address for Hosts connected over a network.
• It allows peer entities to carry conversation.
• It defines two end-to-end protocols: TCP and UDP

TCP(Transmission Control Protocol) 

• It is a reliable connection-oriented protocol which handles byte-stream from source to destination without error and flow control.

UDP(User-Datagram Protocol)

• It is an unreliable connection-less protocol that do not want TCPs, sequencing and flow control. Eg: One-shot request-reply kind of service.

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Merits of TCP/IP model

•     It operated independently.
•     It is scalable.
•     Client/server architecture.
•     Supports a number of routing protocols.
•     Can be used to establish a connection between two computers.

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Demerits of TCP/IP

•     In this, the transport layer does not guarantee delivery of packets.
•     The model cannot be used in any other application.
•     Replacing protocol is not easy.
•     It has not clearly separated its services, interfaces and protocols.