Directory cache operation: Lack of download channel specification

As far as it seems, the directory specifcation makes no clear statement on which channel relays download from authorities? Ordinary TCP + HTTP or a full-blown Tor circuit using RELAY_BEGIN_DIR.

With pure reason alone, the first one makes more sense (not even sure if the latter one is technically possible) and is slightly implied by URLs such as http://<hostname>/tor/micro/d/<D1>-<D2>-<D3>. However, I would still prefer a paragraph or sentence stating the relays download from authorities outside of the Tor network or something.