IETF RFC: Ask question to the author Is it common for a graduate student to ask a question directly the author of an IETF RFC?

I have a subtle doubt about an aspect of a protocol addressed by a certain RFC and I have already searched online and posted question on stack exchange but I haven't found an answer yet.

Should I contact the author directly to the address shown in the RFC?

Thank you.
An IETF RFC may often be thought of as a certain peculiar form of scientific publication: they are frequently cited as such, feed research programs, and can indeed significantly count toward the evaluation of one's research career.  Certainly that has been the way of it for people who I have known who have authored RFCs or otherwise participated in the IETF.

As such, contacting the author of an IETF RFC is not particularly different than contacting the author of a scientific paper: make sure you do your homework first (as it sounds like you've done), so that you don't embarrass yourself or waste the authors' time, and then feel free to contact them.  Their contact information is there for a reason.