A boundary is not a request made of someone else. It is a description of what you will do. “Stop shouting at me” is a request — it depends on the other person's cooperation. “If the volume goes up, I'll leave the room and we'll continue tomorrow” is a boundary — it depends only on you.
That distinction is why most “boundaries” fail. Phrased as requests, they hand power to precisely the person violating them.
Working boundaries have three parts: they are stated plainly (not hinted at), they are applied consistently (a boundary enforced one time in three trains the other person to push), and they carry a cost you are willing to pay. A boundary you are not prepared to hold is an empty threat, and a relationship learns that fast.
They are not an act of aggression. Paradoxically, clear boundaries make relationships safer: the other person knows where they stand without having to guess.