What are the comparisons of the OSI and TCP/IP Reference Models

 What are the comparisons of the OSI and TCP/IP Reference Models?

Comparison of the OSI and TCP/IP Reference Models:

             The OSI and TCP/IP reference models have much in common. Both are based on the

concept of a stack of independent protocols. Also, the functionality of the layers is roughly

similar. For example, in both models the layers up through and including the transport layer

are there to provide an end-to-end, network-independent transport service to processes

wishing to communicate. These layers form the transport provider. Again in both models, the

layers above transport are application-oriented users of the transport service. 

             Despite these fundamental similarities, the two models also have many differences

Three concepts are central to the OSI model: 

1.      services. 

2.      Interfaces. 

3.      Protocols. 

Probably the biggest contribution of the OSI model is to make the distinction between

these three concepts explicit. Each layer performs some services for the layer above it. The

service definition tells what the layer does, not how entities above it access it or how the layer

works. It defines the layer's semantics. 

  A layer's interface tells the processes above it how to access it. It specifies what the

parameters are and what results to expect. It, too, says nothing about how the layer works

inside. 

  Finally, the peer protocols used in a layer are the layer's own business. It can use any

protocols it wants to, as long as it gets the job done (i.e., provides the offered services). It can

also change them at will without affecting software in higher layers. 

  The TCP/IP model did not originally clearly distinguish between service, interface, and

protocol, although people have tried to retrofit it after the fact to make it more OSI-like. For

example, the only real services offered by the internet layer are SEND IP PACKET and

RECEIVE IP PACKET.

As a consequence, the protocols in the OSI model are better hidden than in the

TCP/IP model and can be replaced relatively easily as the technology changes. Being able to

make such changes is one of the main purposes of having layered protocols in the first place.

The OSI reference model was devised before the corresponding protocols were invented. 

This ordering means that the model was not biased toward one particular set of

protocols, a fact that made it quite general. The downside of this ordering is that the designers

did not have much experience with the subject and did not have a good idea of which

functionality to put in which layer.

 Another difference is in the area of connectionless versus connection-oriented

communication. The OSI model supports both connectionless and connection-oriented

communication in the network layer, but only connection-oriented communication in the

transport layer, where it counts (because the transport service is visible to the users). The

TCP/IP model has only one mode in the network layer (connectionless) but supports both

modes in the transport layer, giving the users a choice. This choice is especially important for

simple request-response protocols.