Ambiguity in the interpretation of [Optional] in XKMS

After some thought and analysis, my feeling is that XKMS has
an ambiguous use of the term [Optional] when referring to
elements and attributes.

The interpretations that this term can have here are:

- An element or attribute that a client or a server may
  choose to include in a message.
- Implementation of a given element or attribute is 
  required/recommended/optional

The point I want to make here is that we may have an optional 
element, but whose implementation is required by a server.  If the
implementation is optional, the server may then decide to ignore
it. However, it should not ignore it if the implementation is required.

In few cases, the spec actually says that the implementation is optional. 
Most of the time it just says optional and it lets the reader the guesswork
as to whether the inclusion of the element/attribute or its support by a
client/server is optional.

In my opinion, this is is a source of confusion and of potential
interoperability problems. The spec should be more precise here, while
still leaving freedom of choice to the user.

You'll find here below a list of all the elements and attributes 
that are optional.  I think it would really be good to have 
some extra text that says if their implementation is optional,
recommended or required. We could add this as an appendix too.

Thoughts?

-jose

[88] MessageAbstractType

<ds:Signature> [Optional]
<OpaqueClientData>[Optional]
Nonce [Optional]

[97] RequestAbstractType

<PendingNotification> [Optional]
OriginalRequestId [Optional]
ResponseLimit [Optional]

[114] <Result>
<RequestSignatureValue> [Optional]
ResultMinor [Optional]
RequestId [Optional]

[134] <StatusResult>
Success [Optional]
Failed [Optional]
Pending [Optional]

[172]  KeyBindingAbstractType

Id [Optional]
<ds:KeyInfo>  [Optional]

[189]  <UnverifiedKeyBinding>

ValidityInterval> [Optional]

[191]  <ValidityInterval>

NotBefore     [Optional]
NotOnOrAfter     [Optional]

[213]  <QueryKeyBinding>

<TimeInstant> [Optional]

[282]  <PrototypeKeyBinding>

<ValidityInterval> [Optional]
<RevocationCodeIdentifier> [Optional]

[291] <Authentication>

KeyBindingAuthentication>     [Optional]
<NotBoundAuthentication>    [Optional]

[311]  <RegisterRequest>

<ProofOfPossesion>     [Optional]

[313]  <RegisterResult>

<PrivateKey>     [Optional]

[315]  <ReissueRequest>

<ProofOfPossesion>     [Optional]

[325]  <RecoverResult>

<PrivateKey>    [Optional]