OSI RING · L2 watches L5 across the closure

As a ladder, L2 carries L5 and can't watch it — encapsulation forbids it. Bend the stack into a ring and the floor (below L1) meets the ceiling (above L7): the wire-level observer wraps around to face the session. L2 sits across the diameter from L5. The witness rides a tangent ring at the L2/floor point — it attests that frames flow and who sent them (L2's job: identity, bare metal) but never what they mean (L5's payload). Presence at the fork, blind to content. The airgap (no-ack) is the gap the sightline crosses but the payload never does.