DoD Network Model or TCP / IP Model

DoD Network Model or TCP / IP Model

Previously, we discussed about OSI Model, the Open Systems Interconnection Model, which divided networks into 7 layers namely - Application Layer, Presentation Layer, Session Layer, Transport Layer, Network Layer, Data-Link Layer and Physical Layer. We also stated that the network communication is defined only by the lower 4 layers. Now, we will see the model designed and adopted by Department of Defense (DoD) to connect their networks, which eventually is popularly referred to or known as TCP / IP Model. Hence forth, will refer it as TCP / IP Model.

The major difference between both the models is the number of stacked layers into which the network communication is divided logically. In OSI Model the layers are 7, but in TCP / IP Model it is only 4 layers. These four layers are Application Layer, Transport Layer, Internet Layer and Network Access Layer.

In TCP / IP Model the application, presentation and session layers of OSI are clubbed together into a single layer and named it as Application Layer. The Layer 4 of OSI Model i.e. Transport Layer is same in TCP / IP Model as well, but Layer 3 of OSI Model i.e. Network Layer is named as Internet Layer. The last layer in TCP / IP Model has clubbed together the Layer 2 and Layer 1 of OSI Model into one single Network Access Layer.

Can be graphically represented as, 

It acts as an interface to the user.

means of end-to-end host communication.

Network discovery, Network to Network communication.

Physical Network communication and Host discovery.

Today, all the networks are run based on the TCP / IP Network Model, but the technical people still refer to OSI Network Model as for designing, implementing and troubleshooting the networks irrespective of its base being TCP / IP Model.

TCP / IP Model is easy to implement, hence, the industry used it and went on developing it. Even then, it lacks the clarity of OSI Model in defining the functionality at each layer. Both the models divide the communication into layers logically.

Why people call it as TCP / IP Model ? TCP and IP are nothing but the two main protocols on which the networks of today are working. TCP at Transport Layer and IP at Internet Layer. TCP means Transmission Control Protocol and IP means Internet Protocol.

Until now we discussed the two network models and the constituent layers of each model. Now, let us shift our focus to each individual layer as per the OSI Model definition for better understanding of the functions that each layer carries out.

OSI Network Model

OSI NETWORK MODEL (7 layer model) :

 The communication between two hosts has been sub-divided into 7 layers by OSI Model.

Layer 7 - Application Layer    --  How the application decides whether it needs to communicate another host..

Layer 6 - Presentation Layer  --  How the data should be presented to the user for their understanding.

Layer 5 - Session Layer           --  How the application session is established or maintained.

Layer 4 - Transport Layer       --  How the communication must be established and maintain error free communication.

Layer 3 - Network Layer         --  How to reach the destination network, if both the hosts belong to different networks.

Layer 2 - Data-Link Layer      --  How to reach the destination host within the network.

Layer 1 - Physical Layer         --  How to send the data over the physical medium.

For ease of understanding we can divide these 7 layers into 2 sub-categories. such as 

Layer 7, Layer 6 and Layer 5 as Non-Communication Layers run on the host system. These layers are not concerned with the network communication

Layer 4, Layer 3, Layer2 and Layer 1 as Communication Layers. These are the basis on which the communication is established between two hosts. 

At Cisco, we are concerned only with the communication part of the OSI Model. We are not concerned how the applications run, present the data or the application's session is maintained (or how long it is maintained). We are concerned with the establishment of communication when an application wants to reach another application on a different host for data sharing or requirement, finding and resolving of the destination network, destination host etc. How is this achieved ? All the different aspects related to this end is our concern. So, now we know what is our duty.

All networks refer to OSI Model for learning or understanding the aspects of Network. Do they implement OSI Model in reality. To learn the answer to this we need to see the other model DoD Network Model.

Network Models

NETWORK MODELS

What are network models and why should we know them ?

There are basically 2 types of network models 

1. OSI Network Model proposed by International Organization for Standardization.

2. DoD Network Model defined by Department of Defense, more commonly known as TCP/IP model.

These models will give us the insight into the different aspects of network, such as establishing, maintenance of sessions for communication between 2 different hosts, etc. This is achieved by sub-dividing the communication into different layers based on the services that is carried at each layer. There are subtle differences between these two models eventhough they follow the layered architecture for defining the communication.

Earlier, the network equipments were vendor specific, i.e. Vendor 'A' products were compatible with his own products. An organization has to purchase every equipment from one manufacturer, it cannot change the vendor. If it wanted to purchase from a new manufacturer then it needs to change its entire network equipments and purchase all the products from one manufacturer. 

By defining these models the industry could define some standards for each network equipment so that a compatibility between different manufactured equipments. Let us see how these models helped in achieving this major breakthrough.