I rarely read IETF documents. But as a practicing lawyer for about 15 years, I've read, interpreted, drafted, advised on, and litigated countless uses of the word "shall" in agreements (as well as in statutes and regulations). In fact my first pro bono case in BigLaw, on behalf of a nonprofit in a corporate election case, hinged on it. Shall is not this mysterious ambiguous beast the link makes it out to be.
Legal text doesn't come with a list of definitions that attempt to lock e.g. "shall" down in stone.