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In 5-layer model.

Why we need additional transport layer? I read these two points

Users who interact with software have no control over the sub-net; Transport Service can provide a more reliable service than any the underlying Network Service might, because IP is not reliable protocol.

Now my doubt is why can't we directly interact with the IP layer from application layer.If we interact what would be the problem that we face?

Why can't we implement process-process communications & reliability everything in application layer?

Is the high level languages doesn't support to implement protocols?

  • You can implement your own protocol for reliability and process communication, but why would you want to? How does your application know that an IP packet it receives is for your application and not for another application process? – Ron Trunk May 15 '15 at 13:32
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Of course you can directly interact with the IP layer. The question is not about "why you can't", but about "why you wouldn't want/need to".

Since you mention higher level languages, the reason why they exist is the same as the reason the transport layer exists. A higher level language makes things easier by providing abstraction. You can code your application in an assembly language, but you will be spending a lot of effort on problems that have been hidden by higher level languages.

Most applications will benefit from a network which provides features like process identification, reliable communication, in order delivery or flow control. So it makes sense implementing these features once in a standardized protocol, instead of having to include custom solutions in every application.

There are situations in which designing your own solution can be beneficial or even necessary, but in most cases using TCP or UDP will just work and save you a lot of hassle. Doing it yourself can also be very educational and make you appreciate the work that has been done by others.

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