[security-services] Status code proposal

[Scott Cantor <cantor.2@osu.edu>]

SAML needs an extensible, more fully described status code mechanism.
General support for an approach based on, but not directly importing,
the SOAP fault approach has been expressed. I suggest a modified design
copied from a proposal for hierachical fault codes in SOAP, from Martin
Gudgin.

I don't know the current likelihood of it (or something like it) being
adopted for SOAP 1.3, but it seemed to be favorably received in various
forms. I selected the variant I liked the best, but others are possible.

This is a hierarchical Status structure to be placed inside Response as
a required element. The Status element contains a nested Code tree in
which the top level Value attribute is from a small defined set that
SAML implementations must be able to create/interpret, while allowing
arbitrary detail to be nested inside, for applications prepared to
interpret further.

I mirrored some of SOAP's top level fault codes, while keeping SAML's
Success code, which doesn't exist in SOAP, since faults mean errors, not
status. We could rename this to Fault and dump Success if desired. I
also eliminated the Error vs Failure distinction, which seems to be
intended to "kind of" mean Receiver/Sender, which is better made
explicit. Unknown didn't make sense to me either. Please provide
clarifications if these original codes should be kept.

The proposed schema (unchecked, so somebody should proof it) is as
follows, replacing the current string enumeration of StatusCodeType:

<simpleType name="StatusCodeEnumType">
    <restriction base="QName">
        <enumeration value="samlp:Success"/>
        <enumeration value="samlp:VersionMismatch"/>
        <enumeration value="samlp:Receiver"/>
        <enumeration value="samlp:Sender"/>
    </restriction>
</simpleType>
<complexType name="StatusDetailType">
    <sequence>
        <any namespace="##any" minOccurs="0" maxOccurs="unbounded"
processContents="lax"/>
        <anyAttribute namespace="##any" processContents="lax"/>
    </sequence>
</complexType>
<complexType name="StatusCodeType">
    <sequence>
        <element name="Code" type="samlp:SubStatusCodeType"
minOccurs="0"/>
    </sequence>
    <attribute name="Value" type="sampl:StatusCodeEnumType"
use="required"/>
</complexType>
<complexType name="SubStatusCodeType">
    <sequence>
        <element name="Code" type="samlp:SubStatusCodeType"
minOccurs="0"/>
    </sequence>
    <attribute name="Value" type="Qname" use="required"/>
</complexType>
<complexType name="StatusType">
    <sequence>
        <element name="Code" type="samlp:StatusCodeType"/>
        <element name="Message" type="string" minOccurs="0"/
maxOccurs="unbounded"/>
        <element name="Detail" type="samlp:StatusDetailType"
minOccurs="0"/>
    </sequence>
</complexType>

In Response, replacing the StatusCode attribute, add:

<element name="Status" type="samlp:StatusType"/>

Some draft text for a new section, perhaps under 2.6.2 Response,
describing the structure might be:

SAML responses MUST include a Status element describing the outcome of
the requested operation in as much detail as the receiver desires to
express.

2.6.2.1 Element Status

The Status element contains a top-level status code, and optional
message and detail information.

<Code> [Required]
The top-level Code element MUST contain a Value attribute equal to one
of the StatusCodeEnumType values. It MAY contain additional Code
elements containing Value attributes equal to arbitrary Qnames. A
receiver SHOULD provide nested Code elements to fully describe error
conditions when possible, but a sender MUST be able to correctly process
the Response in a reasonable fashion by examining only the top-level
Code element. Four top-level values are defined:

Success - The request succeeded.

VersionMismatch - The receiver could not process the request because the
version was incorrect.

Receiver - The request could not be performed due to an error at the
receiving end.

Sender - The request could not be performed due to an error in the
sender or in the request.

<Message> [Optional]
Any number of Message elements may be included to describe the success
or failure condition in human-readable fashion. Multiple languages may
be included for I18N purposes.

<Detail> [Optional]
Arbitrary well-formed XML content may be included to pass application or
implementation-specific information to the sender. A receiver MUST NOT
require the sender understand the contents of Detail in order to process
the response in a reasonable fashion.

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Comments welcome.

-- Scott