My current favorite value for the id selector advertising HTTP header "X-ID-Selector":
- version has a constant value until we change the spec for this HTTP header
- name is one from a OSIS defined list
- capabilities is a string of capabilities.
- "+" indicated support for a capability.
- "-" indicates missing support for a capability
These three capabilities are the first (and currently only) ones I think that matter to a relying party.
What needs to be defined is a list of capabilities that define the relationship between the RP and the id selector, and are relevant to the RP in regard of what response is generated to the HTTP request.
Someone suggested a "morehere" part of the HTTP header... The value of this would be the URL of the capabilties metadata e.g.: morehere='https://xmldap.org/capabilities?name=%n&idv=%idv' or morehere='http://axel.nennker.de/c?name=%n&idv=%idv'. I must say: I don't like this but if somebody conjures an example of the returned metadata and this is much longer or complicate than the capabilities part of the header value proposed here, than I might see sense in this capabilities by reference approach.
This image shows (a local install of ) the XMLDAP relying party displaying the value of the X-ID-Selector HTTP header: