XRD for host-meta

[Will Norris <will@willnorris.com>]

One of the things talked about at IIW was how there is movement toward  
establishing the "/.well-known/" directory to serve as a container for  
well-known files of various types.  This makes the /host-meta file  
somewhat obsolete for many use cases, since anyone can simply register  
a filename within the .well-known directory directly.  The main group  
left with a real use for host-meta then was the XRD community, as host- 
meta is still the place where you define the Link Pattern used to get  
the XRD document for a given URI.  Since we were the only ones who  
still cared, we generally agreed that it made sense to drop the  
existing plain-text format for host-meta, and instead use XRD.  Two  
major reasons for this:
  - consumers were going to have to parse XRD anyway, so why use two  
different formats?
  - host-meta needs to be signed.  XRDs are going to be signable also  
so again, why use two different formats?

So with that in mind....

I've recently been going through a number of the XRD use-cases, and I  
can't actually figure out how to use XRD for a host-meta document.   
One particular piece of the puzzle doesn't seem to fit -- what is the  
<Subject> ?  The current host-meta draft states:

> Note that the metadata provided by a host-meta resource is  
> explicitly scoped to apply to the entire authority (in the URI  
> [RFC3986] sense) associated with it

host-meta is about an authority, but <Subject> is a URI.  This makes  
sense, because XRD is intended to describe a resource.  Authorities  
are not resources.  You could fudge it by converting the authority  
"example.com" into "http://example.com/";, but now the XRD is just  
wrong.  It's saying that it is describing the specific resource "http://example.com/ 
", when it's really intending to describe the entire authority.

How big of a problem is this?

I've actually come across a number of potential wrinkles, but this one  
was fairly discrete and easy to explain first.

-will