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I'm confused in differentiating the OSI Model and TCP/IP suite. As per my knowledge. OSI is a structure on top of which Networks are built and TCP/IP is the suite of protocols that operate on these 7 layers of OSI. Correct me if I'm wrong. Also, is TCP/IP a replacement to OSI and is used IN PLACE of OSI? or both these work COLLECTIVELY to form the Network? any good source to clarify my confusion will be appreciated.

Thanks.

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  • This sounds like it'd be better asked on serverfault or something as it doesn't strictly relate to programmers.
    – Doug T.
    Feb 4, 2012 at 12:58

3 Answers 3

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The OSI stack is a rather theoretical model (and standard) for networking layers, but it has little practical relevance and mainly survives as a subject of CS classes.

TCP/IP is a suite of concrete networking protocols that have seen overwhelming adoption on the internet. It roughly maps to the layers 3 and 4 in the OSI model.

Concrete protocols rarely fit the OSI model perfectly, e.g. layers 1 and 2 are usually combined, as are layers 6 and 7 (often also 5).

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  • So shall I consider both OSI Model and TCP/IP two different stacks built for same purpose, but TCP/IP adopted widely while OSI remained something to be studied in CS and not in use in real world anymore?
    – Kushal
    Feb 5, 2012 at 19:33
  • @Kush: basically yes, though the other big difference is that TCP/IP is a working implementation and OSI only an abstract model you cannot actually use. Feb 6, 2012 at 8:23
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The OSI model is a networking model. DECNet has evolved from a 4 layer model to the OSI model.

TCP/IP is a different model and implementation of a network stack.

As they are different models and implementations, there is normally no question of interoperation between the two. If this needs to be done, a network bridge might be used.

In terms of modelling some of the OSI layers can be loosely mapped to the TCP/IP layers, though it is more granular so several OSI layers sometimes map to a single TCP/IP layer.

See this short article comparing the two architectures.

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  • I'm not sure what you mean by "never been implemented". There are technologies that implement each of the 7 layers as defined in the OSI model. The TCP/IP model is just another way of looking at networking. Both define networking, from the physical connections between two networked devices to applications that transmit data over that link.
    – Thomas Owens
    Feb 4, 2012 at 10:59
  • @ThomasOwens - I mean a whole network stack that is directly modelled on the OSI model. I don't know of one. Yes, there are individual parts, but now a whole, as far as I know.
    – Oded
    Feb 4, 2012 at 11:04
  • @Oded, I think, but may be mistaken, that DECnet had evolved in an OSI stack. Decnet is no more really relevant nowadays, but you can still think trace, X Windows has still syntax supporting DECnet for instance. Feb 4, 2012 at 11:22
  • @AProgrammer - Looks like you are right. It has evolved into one: en.wikipedia.org/wiki/DECnet
    – Oded
    Feb 4, 2012 at 11:23
  • There certainly used to be "directly inspired by OSI" network stacks. The IS-IS routing protocol is a multi-protocol IGP carried over OSI.
    – Vatine
    Feb 4, 2012 at 11:45
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Please forget the fact that TCP/IP came before OSI Model for a second and everything will be clear.

OSI Model

Take any specification you can think of that your teacher/professor/business analyst ever gave you and that would be sort of equivalent to OSI Model. It is a model, or a guide for network communication. And take any implementation of the specs that your teacher/professor/business analyst gave you/anyone else, whether you/anyone else did it in Java, C# and so on and that would be sort of equivalent to TCP/IP. In other words, one is a specification, if you may call it that, and the other is an implementation. As stated here and quoted below (emphasis mine):

OSI model is a conceptual model that characterizes and standardizes how different software and hardware components involved in a network communication should divide labor and interact with one another. It has seven layers.

The most important words from the above definition is conceptual model meaning it is relating to or based on mental concepts. In other words, it is sort of like a blue print.

TCP/IP

TCI/IP is not conceptual but it is a concrete implementation of the OSI. Here is a visual of how TCP/IP implements each layer.

enter image description here

Please note how the OSI Model has Application Layer, Presentation Layer, and Session Layer but TCIP/IP has decided to include the implementation for all the aforementioned layers and called it Application Layer. This layer even within TCP/IP has multiple implementations such as HTTP, SMTP and so on.

In fact you or anyone can take the OSI Model and implement it any way you want and call it MyOSI or whatever you want. You can use a horse, car anything you want. Seriously, no Joke!

Seriously, No Joke!

Someone humorously submitted an RFC 1149 to use pigeons to implement the Data Link Layer and Physical Layer and called it IP over Avian Carriers (IPoAC). Then a company in South Africa, to prove how slow the country's internet connection is, actually went ahead and used pigeons instead of the Telkom ADSL line. The pigeon won by far!!. You can search it online for example here and I quote below:

Cape Town - Winston, a homing pigeon, has made history by beating a Telkom ADSL line in delivering 4GB of data from Howick to Hillcrest, outside Durban in just 2 hours 6 minutes and 57 seconds, whereas the ADSL download was "still just under four percent complete" at 11:45.

Conclusion

Back to why OSI Model came after TCP/IP? Perhaps, TCP/IP was so successful that some of the ideas were borrowed for the OSI Model. But if you know why, for the pedantics, please edit my answer.

In conclusion, if the OSI Model is like a blue print, then TCP/IP is like a house/building etc.

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