I studied networking in the university, 20 years ago. We did study OSI as well as TCP/IP and the rest of the actual Internet stack, and it was blatantly obvious that the latter doesn't really conform to the former - protocols straddling boundaries etc. For example, TCP is not a strictly transport-layer protocol, since it also handles sessions.

When asked, our prof readily admitted that the OSI model was one of those design-by-committee experiments in purity that quickly broke down IRL.