I understand and agree with you that IPv4 and IPv6 are OSI layer 3 protocols and UDP and TCP are layer 4. At the same time, the OSI model above the fourth layer

I understand and agree with you that IPv4 and IPv6 are OSI layer 3 protocols and UDP and TCP are layer 4. At the same time, the OSI model above the fourth layer has been widely considered mostly useless, and some have called into question the whole thing. Yes, RFCs exist that point to what you are saying. But again, I can 100% implement TCP on top of UDP without ever thinking about IP packets. And no, I don't care which layer 3 protocol TCP is using underneath, so long as I can initialize the socket properly and get the local/remote addresses. Take a look at a complete example of a TCP server: https://www.cs.cmu.edu/afs/cs/academic/class/15213-f99/www/class26/tcpserver.c https://www.cs.cmu.edu/afs/cs/academic/class/15213-f99/www/c... Where in there do you see anything about ARP or ND? Or hops vs TTL? Or IP packet header sizes? Or header checksums? Or priority flags? You can take this code, add a couple of macros for getting the correct AF_ value and structs and the code would be 100% abstracted away from any mention of the underlying layer 3 protocol. Google has been developing transport layer protocols on top of UDP for years now. Why? Because getting a new layer 4 protocol to work is nearly impossible. Yet, if we all agreed that it was something we wanted, they could easily do so, which makes these new protocols functionally transport layer. If it helps you sleep at night, you can say that they are temporarily exiled to be encapsulated in UDP, but really they'll return some day to their proper place in the OSI model.