I think it´s a very simple doubt but also very important, and i can´t find the answer. If you are flying on a Federal Airway that at some point cross a class B airspace (for example)...do you need authorization or can you keep flying without one?
If you are flying your IFR flight plan which you got clearance for, then you have permission already.
If you are flying VFR you need to ask ATC for clearance.
Class B airspace requires clearance for all traffic and separation is also provided by ATC for all traffic.