strategy for identifying DTD/schema artifacts

I'm in the process 
of cleaning up the SIDSC specializations and catalog.
 
My question is, what 
is our going-forward strategy for identifying DITA DTD/schema 
resources?
 
Should I use a 
public ID such as "-//OASIS//DTD SIDSC Component//EN"? Or should I use oasis 
namespace identifier in a URN, such as 
"urn:oasis:names:sc:sidsc:dtd:component.dtd:1.2"?
 
What are the 
guidelines for constructing public IDs?
 
The catalog shipped 
with the OT has schema URNs as a system ID; can a URN be used as a public 
ID?
 
Should every MOD and 
ENT file also have a publicID/URN? Or is it sufficient to provide a publicID/URN 
for the top-level shells and use relative URIs as system IDs to reference 
MOD/ENT files?
 
I suppose the URN 
would only be available for organizations with a registered namespace 
identifier, so most user local shells and specializations would not be able to 
use URNs. Aesthetically, a mixed approach for public identifiers would be 
unappealing, but harmless, I presume.
 
 
Thanks,
 
-seth