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?