[Geojson] clarification on Feature id

[Keith Jenkins]

In the example for FeatureCollection...
  http://wiki.geojson.org/GeoJSON_draft_version_5#FeatureCollection

...each Feature object has an "id" property, which isn't mentioned in
the specification section, 3.3.

Is this "id" property supposed to be required or optional?  (I guess
another possibility is that it should really be within "properties".)

Keith