Osi vs Tcpip

Comparison and Critique of Reference Models

Comparison

Comparison OSI Model

TCP / IP Model

General Model

Specific to TCP/IP network and cannot be used to describe any other network

7 Layers

4 Layers

OSI makes distinction between service, TCP/IP does not distinguish between interface and protocol service, interface and protocol It is devised before the corresponding protocols were invented

In TCP/IP, the protocols came first and the model was really just a description of the existing protocols

Connection-oriented and connectionless communication in network layer

Only connectionless communication in network layer

Only connection-oriented communication in transport layer

Both connection-oriented and connectionless communication in transport layer

Critique of OSI Model and Protocols 1. Bad timing: 

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The competing TCP/IP protocols were already in widespread use by research universities by the time the OSI protocols appeared. The academic market was large enough that many vendors had begun cautiously offering TCP/IP products and when OSI came around, they did not want to support a second protocol stack.

Critique of OSI Model and Protocols 2. Bad technology:  Both OSI model and the protocols are flawed. 

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Two of the layers (session and presentation) are nearly empty, whereas two other layers (data link and network) are overfull. The OSI model, along with the associated service definitions and protocols, is extraordinarily complex. In addition to being incomprehensible, another problem with OSI is that some functions, such as addressing, flow control and error control, reappear again and again in each layer.

Critique of OSI Model and Protocols 3. Bad implementation:  Initial implementation was huge, unwieldy and slow.  In contrast, one of the first implementation of TCP/IP was part of Berkeley UNIX and was quite good. 4. Bad politics : 

TCP/IP was advertised as a better technology than OSI model, though it was inferior.

Critique of the TCP/IP Reference Model 

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It is not much of a guide for designing new networks using new technologies. Not a general reference model, hence cannot describe any protocol stack other than TCP/IP. Host-to-network layer is rather an interface Does not distinguish (or even mention) data link and physical layers. Although the IP and TCP protocols were carefully thought out and well implemented, many of the other protocols were designed and implemented ad-hoc. Some of them are a bit of an embarrassment now. 

For example, TELNET was designed for a ten-character per second mechanical Teletype terminal. It knows nothing of graphical user interfaces and mice.

The Hybrid Reference Model 

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In summary, despite of its problems, the OSI model (minus the session and presentation layers) has proven to be exceptionally useful for discussing computer networks. In contrast, the OSI protocols have not become popular. The reverse is true of TCP/IP: the model is practically nonexistent, but the protocols are widely used. Hence, a new hybrid model is designed for study purpose as below.

Virtual-Circuit vs. Datagram Networks