> You're comparing apples with oranges, IP is one layer and TCP and UDP are on another.
That is what the books say, but I don't think it is right. When you consider that raw IPv4 doesn't work in practice because of NAT, UDP is a defacto minimum internet layer in practice.
NAT is a kludge. The existence of NAT does not remove the fact that both TCP and UDP are layered on top of it and there is plenty of stuff happening that is layered directly on IP besides UDP, for instance ICMP.
That is what the books say, but I don't think it is right. When you consider that raw IPv4 doesn't work in practice because of NAT, UDP is a defacto minimum internet layer in practice.