What does modular protocol design mean?
Why does TLS not have modular protocol design?
What protocols have modular design? (IPSec, SSH)
By the definition given there, in a modular protocol design,
each method or scheme used in the protocol (such as the use of nonces or the application of a cryptographic schemes) has a clear goal which it is proven to achieve, and sub-protocols can be replaced without re-proving the security of the remaining protocol steps.
That's also the meaning used there.
The word modular is used with the meaning it has for an engineer: the ideal of building trustable complex systems from smaller trusted and well understood components, in a way such that a component can be replaced with any other having the required, stated properties. That is actually seldom done in real complex systems.
I pass as to if and why TLS, IPSec, SSH... follow, or try to follow, the modular protocol design principle, or not.