TCP/IP Model

During the period of 1970, the Defense Advanced Research Projects Agency (DARPA) was underway to create an open standard network model. This network model came to be known as the TCP/IP Model. By 1985, the TCP/IP model started gaining more importance and support from vendors and ultimately replaced the OSI model. Currently, Internet Engineering Task Force (IETF) maintaining the TCP/IP model and its related protocols.

The TCP/IP model was on the path of development when the OSI standard published. The TCP/IP model is not the same as the OSI model. OSI is a seven-layered standard, but TCP/IP is a four-layered standard. The OSI model has been very important in the growth and development of the TCP/IP standard, and that is why much OSI terminology applied to TCP/IP.

Both models are open to standard networking models.  However, the TCP/IP model has found more acceptances today and the TCP/IP protocol suite is more commonly uses. Just like the OSI reference model, the TCP/IP model takes a layered approach. In this section, we will look at all the layers of the TCP/IP model and various protocols used in those layers.

The TCP/IP model is a reduced version of the OSI reference model consisting of the following 4 layers:

The functions of these four layers are comparable to the functions of the seven layers of the OSI model. Figure below illustrates the comparison between the layers of the two models.

TCP/IP

The following sections discuss the four layers and protocols in those layers in detail.

As we can see from the above figure, the presentation and session layers are not there in the TCP/IP model. Also, note that the Network Access Layer in the TCP/IP model combines the functions of Datalink Layer and Physical Layer.

Application Layer of TCP/IP model

The application layer is the topmost layer of the TCP/IP model. It is present on the top of the Transport layer. Application layer defines TCP/IP application protocols and how host programs interface with Transport layer services to use the network.

The Application Layer of the TCP/IP Model consists of various protocols that do all the functions of the OSI model’s Application, Presentation, and Session layers. This includes interaction with the application, data translation and encoding, dialogue control and communication coordination between systems.

Application layer includes all the higher-level protocols like DNS (Domain Naming System), HTTP (Hypertext Transfer Protocol), TelnetSSH, FTP (File Transfer Protocol), TFTP (Trivial File Transfer Protocol), SNMP (Simple Network Management Protocol),SMTP (Simple Mail Transfer Protocol) , DHCP (Dynamic Host Configuration Protocol), X Windows, RDP (Remote Desktop Protocol) etc.

Transport Layer of TCP/IP model

Transport Layer is the third layer of the TCP/IP model. The position of the Transport layer is between the Application layer and the Internet layer. The purpose of the Transport layer is to permit devices on the source and destination hosts to carry on a conversation. Transport layer defines the level of service and status of the connection used when transporting data. The main protocols included at the Transport layer are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

TCP/IP transport layer’s function is the same as the OSI layer’s transport layer. It concerned with end-to-end transportation of data and setups up a logical connection between the hosts.

TCP is a connection-oriented and reliable protocol that uses windowing to control the flow and provides ordered delivery of the data in segments. On the other hand, UDP simply transfers the data without the bells and whistles. Though these two protocols are different in many ways, they do the same function of transferring data and they use a concept called port numbers to do this.

Internet Layer of TCP/IP model

Once TCP and UDP have segmented the data and have added their headers, they send the segment down to the Network layer. The destination host may reside in a different network far from the host divided by multiple routers. It is the task of the Internet Layer to make sure that the segment moved across the networks to the destination network

Internet layer pack data into data packets known as IP datagram, which contains source and destination address (logical address or IP address) information that forwards the datagram between hosts and across networks. The Internet layer is also responsible for the routing of  IP datagram.

Packet switching network depends on a connectionless internetwork layer. This layer is known as the Internet layer. Its job is to allow hosts to insert packets into any network and have them to deliver independently to the destination. At the destination side data; packets may seem in a different order than they were sent. It is the job of the higher layers to rearrange them to deliver them to proper network applications operating at the Application layer.

The main protocols included at Internet layer are IP (Internet Protocol), ICMP (Internet Control Message Protocol), ARP (Address Resolution Protocol), RARP (Reverse Address Resolution Protocol) and IGMP (Internet Group Management Protocol).

The Internet layer of the TCP/IP model corresponds to the Network layer of the OSI reference model in function. It provides logical addressing, path determination, and forwarding.

Network Access Layer of TCP/IP model

The Network Access Layer is the first layer of the TCP/IP model. Network Access Layer defines details of how data is physically sent through the network. It is also including how bits are electrically or optically signalled by hardware devices that interface directly with a network medium, such as coaxial cable, optical fiber, or twisted pair copper wire. The important protocols of  Network Access Layer are Ethernet, Token Ring, FDDI, X.25 also Frame Relay.

LAN architecture is the most popular among those listed above is Ethernet. Ethernet uses an Access Method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection) to access the media when Ethernet operates in a shared media.

IN CSMA/CD Access Method, every host has equal access to the medium and can place data on the wire when the wire is free from traffic or in the idle position. When a host wants to place data on the wire. It will check the wire to find out whether another host is already using the medium.

If there is traffic already in the medium, the host will wait and if there is no traffic, it will place the data in the medium. But, if two systems place data on the medium at the same instance they will collide with each other, destroying the data. If the data destroyed during transmission. This data will need to re-transmit. After the collision, each host will wait for a small interval of time and again the data will re-transmit.