I have been tasked to review this document on behalf of the OPS directorate.  This document describes a means by which link relationships may be used to advertise SIP trunking capabilities for an ITSP.  The document is short, sweet, to the point, and clear.  Overall, I had one question and a small nit.

In section 3, it is stated that an ITSP may use an authentication framework...

Should this language be stronger and normative?  That is, should this be a SHOULD?  Or perhaps this is best left to the document on the capability set document.  As it stands now, it felt like an aside as I read it.

My nit is that in the example href, should the URL be https://capserver.ssp1.example.com?