[Tagging] Tag ref on motorway_link

[Martin Vonwald (imagic)]

Hi!

I'm wondering what ref should be used on slip roads/ramps of a motorway (not the junction node, but the way tagged with motorway_link). Up to now I've seen:
* the reference of the junction
* the reference of the motorway
* the reference of the junction not in the ref tag but in junction:ref
* nothing (neither ref nor junction:ref)

In the Wiki (English and German) I couldn't find an answer.

Any opinions/comments/hints are welcome.

Thanks,
Martin