Fair enough, I meant: "In my opinion that's false."
What's the purpose of the "MUST" in the (draft) RFC if it's optional? Right, it wouldn't say "MUST". I'm quite sure "optional" is not what the RFC editors meant.
Their suggested example algorithm is all within the window, not outside. So staying within the window is mandatory IMO. That's based on the usage of the "MUST" terminology before the example.