3 The end-to-end (transport) layer of the internet model maps to the transport layer of the OSI model.
The internet model implements reliable end-to-end communication over many intervening networks using
the transaction control protocol (TCP). TCP insures that, even if parts of a message travel by different
routes to the destination, the message will be reassembled in the correct sequence, and presented without
duplication, omission, or corruption of data.
4 Finally, the application layer of the internet model subsumes the session, presentation, and application
layers of the OSI model. As the name implies, application programs have responsibility for the application
software layer. For instance, web browser applications have responsibility for formatting and displaying
the data sent to them from distant web servers.
Why has the internet model persisted in practice instead of being replaced by the more general and more
layered OSI model? A big part of the answer is that the internet model was implemented first, and users found
that its model, though simpler, was successful. By the time the OSI model was widely promulgated, the internet
model was already in wide use. The advantages of a more elaborate layering of services were not persuasive to
those already successful with the existing protocols.
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