OSI Model: serving a layer bellow [duplicate]

Question

Several references of the OSI model states the following:

• A layer serves the layer above it and is served by the layer below it.
• Each layer provides services to the next-higher layer and shields the upper layer from the details of how the services below it are actually implemented.

Therefore, the Network Layer (#3) should provide services for the Transport Layer (#4) and not the other way around. However, there are some routing protocols (Network Layer) that uses UDP and TCP services (Transport Layer), for example:

• RIP (Routing Information Protocol) uses UDP
• BGP (Border Gate Protocol) uses TCP

How do we reconcile that? Am I missing something?

Answer 1

This is the same answer (copied and pasted) for this question: In which OSI/TCP-IP model layers do BGP, RIP protocols belong?

You have to remember that models like OSI are just that, models. They are theoretical. The real world doesn't fall neatly into these models. For the most part, routing is a layer-3 function, but, as you pointed out, BGP uses a layer-4 protocol to communicate with other BGP speakers in order to do what is normally considered a layer-3 function.

Many network protocols fall into a gray area, or are considered in one layer while using another layer. Take ARP for instance. It resolves layer-3 addresses to layer-2 addresses. Which layer should it be considered to be in?

Understanding the models is useful, but the models are not mandated by any organization, and you are free to create protocols and functions that do not follow any model.