According to this article by Alf Hiltebeitel, the Mahabharata has "three interwoven frame stories" (page 4). What he calls the "outermost" frame is Vyasa's recounting of the story to five of his disciples. An inner "generational frame" has one of those disciples (Vaisampayana) reciting the epic at King Janamejaya's snake sacrifice. In the third frame, which Hiltebeitel calls the ""outer" cosmological frame", a bard (Ugrasravas) who was present at the snake sacrifice relates the story he heard there to the rishis of the Naimisa forest.
My question is, how exactly does this interlocking frame structure work? I see how Ugrasravas's narration frames Vaisampayana's (which is why the former is called the "outer" frame and the latter the "inner") because it recounts Vaisampayana's narration and naturally takes place at some point after the snake sacrifice. But how is Vyasa's narration - the first narration - the "outermost"? I would've thought that Hiltebeitel would have called it the innermost.