use of Scoping element in AuthnRequest to assist with front-channel attribute aggregation

[Scott Koranda]

Hello,

If this is not an appropriate question for this list I would be
grateful if you could point me to the correct list.

I have come across some work on so-called "front-channel attribute
aggregation" that is using a <Scoping> element as part of an
<AuthnRequest> element to include the entityID of an IdP already used
to authenticate the subject. The entityID for the IdP is included in
an <IDPList> element.

The <AuthnRequest> is sent from the SP to an "attribute authority"
that is then using the entityID of the IdP to cause the browser to go
through another authentication with the IdP but this time sending the
assertion to the attribute authority, which then uses the targeted ID
sent by the IdP to the attribute authority to look up attributes for
the subject.

The <AuthnRequest> looks something like this:

<samlp:AuthnRequest
  xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
  AssertionConsumerServiceURL="https://myservice.edu/some/acs"
  Destination="https://attribute.authority.edu/some/url"
  ID="_b8f5f2a1361f90a4611caa60b908dc28"
  IssueInstant="2013-11-21T11:59:30Z"
  ProtocolBinding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST"
  Version="2.0">
  <saml:Issuer
    xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion">https://myservice.edu</saml:Issuer>
  <samlp:NameIDPolicy AllowCreate="1"/>
  <samlp:Scoping>
    <samlp:IDPList>
      <samlp:IDPEntry ProviderID="https://some.idp.edu"/>
    </samlp:IDPList>
  </samlp:Scoping>
</samlp:AuthnRequest>

Questions:

1) Is there any defined profile that addresses this type of
front-channel attribute query or aggregation?

2) Any comments on the use of an <AuthnRequest> to the attribute
authority to solicit attributes about the subject?

3) Any comments on the use of the <Scoping> element to signal to the
attribute authority to which IdP to direct the browser to complete the
assertion of a name identifier from the IdP to the attribute
authority?

Thank you for your consideration.

Cheers,

Scott K