[scim] ExternalId Case Sensitivity

[Shelley <[e-mail]>]

Section 3.1 indicates that "externalId" has "caseExact" as "true". However,
I would like our service provider's "externalId" attribute to be
*case-insensitive*, but since "externalId" is a "Common Attribute", service
providers cannot change its attribute characteristics [1]:

For backward compatibility, some existing schema definitions MAY list
> common attributes as part of the schema.  The attribute
> characteristics (see Section 2.2) listed here SHALL take precedence
> over older definitions that may be included in existing schemas.
>

I have a few related questions:

   - *What was the reasoning behind making externalId caseExact*?
      - From the perspective of *case preservation*, I can understand the
      use of "caseExact" for externalIds; however, for *filtering*, it
      still seems appropriate to search *case-insensitively*.
      - I've seen some past discussions where "externalId" had "caseExact"
      as "false" (e.g. [2,3]), but it seems to be "true" in the final
RFC, and I
      was unable to trace that decision back to a discussion.
      - If I'm not mistaken, externalId was case-insensitive implicitly in
      SCIM 1.1 as well.


   - More generally, perhaps future-thinking (unless this has already been
   discussed), perhaps "caseExact" may be over-simplified? There are
   potentially 3 different uses for caseExact, and they may not all share a
   value:
      - *Case Preservation*: Must the server preserve the case of the
      attribute?
      - *Uniqueness*: If the attribute is unique, must it be
      case-sensitively unique?
      - *Filtering*: Should the attribute be filtered case-sensitively?
      (Perhaps allowing clients to specify case-sensitive filter
operators may be
      more appropriate than making this part of the attribute's
characteristics?)


   - Should service providers be allowed to *change the "common attribute"
   characteristics* to account for cases such as these? The spec currently
   seems to forbid this.

[1] https://tools.ietf.org/html/rfc7643#section-3.1
[2] https://mailarchive.ietf.org/arch/msg/scim/ev7DC2ZUTtaIam4RmcbhSyCF9xE/
[3] https://mailarchive.ietf.org/arch/msg/scim/NgMipeLaeLlOwpPkrg2Jak7W2-M/