*** IMPORTANT NOTICE ***

THIS SPECIFICATION IS BEING SUPERSEDED by the DID-Linked Resources Specification hosted by the Utility Foundry Working Group — please see that specification for all future work.

Document Status

This document is a Draft Deliverable of the ToIP Foundation Technical Stack Working Group. 

The current version is Working Draft 0103.

Introduction

This is a specification for an extension to the W3C Decentralized Identifiers (DIDs) 1.0 specification to support the resource parameter as listed in the W3C DID Specification Registries 1.0. It specifies the syntax and normative behavior for usage of the parameter and the requirements for DID methods that support the parameter.

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The purpose of this specification is to specify how a DID URL may include a parameter that instructs a DID resolver to request the associated verifiable data registry (VDR) to directly return a digital resource identified by a decentralized identifier (DID). This parameter is only available for DID URLs whose DID method supports the parameter. The parameter also supports requesting the resource in a particular media type. 

Motivations

The process of resolving a DID and dereferencing a DID URL (meaning a DID that includes an additional path, query, and/or fragment component as defined by the ABNF in section 3.2 of the DID 1.0 specification) is shown in figure 1 from section 7.2 of the spec:

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However an exception to this normal 2-step resolution/dereferencing process is when the DID itself identifies a digital resource that can be returned directly from by the VDR of the associated DID method. This behavior may be desirable:

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1. The DID document is not retrieved by the resolver as part of the dereferencing process. Rather the resolver makes a call to the VDR with the DID URL including a resource parameter conformant with this specification. The VDR then follows the specification of the associated DID method to retrieve the identified digital resource and return that resource to the resolver directly.
2. The resource parameter serves a different purpose than the alsoKnownAs parameter. The purpose of the alsoKnownAs parameter is to map a DID to an alternative identifier for the same DID subject. This alternative identifier can be any other URI, e.g., a URL, or a URN, or another DID. When the DID is resolved, the value of the alsoKnownAs parameter is returned as a property of the DID document. It is then up to the client to decide what action to take with this alternative identifier. For example, if the value is a URL, the client may choose to dereferencing dereference this URL to retrieve the identified Web resource. In this case, the the alsoKnownAs parameter essentially serves as a redirection mechanism managed by the DID controller. By contrast, the resource parameter:
   1. Does not involve an alternative identifier for the DID subject. The DID is the only identifier needed. (Although the DID document may still include an alsoKnownAs parameter if desired.)
   2. Does not involve any redirection. The DID directly identifies the target digital resource.
   3. Does not require a separate dereferencing step. The DID URL is effectively resolved and dereferenced in one step.
   4. Does not involve a service endpoint in the associated DID document. In fact the use of any content in the DID document (other than the DID itself) is an anti-pattern because the purpose of the resource parameter is to enable a DID URL to resolve directly to the digital resource—including the ability to specify the requested media type of the resource—so that developers are completely abstracted from the mechanics of how the VDR actually returns the identified resource to the requesting DID resolver.

The Resource Parameter

To enable this combined resolution/dereferencing behavior, this specification defines a DID URL parameter named resource. If a DID method specification supports this parameter, and if a DID URL using that method includes the parameter value resource=truewith a valid value (either true or a valid media type name), then when a resolver calls the associated VDR using that DID URL, the VDR returns the identified digital resource, not the DID document.

IMPORTANT: A DID URL that includes the resource=true parameter does still has an associated have an associated DID document like any other DID. However this if the DID URL includes the resource parameter, the associated DID document is not involved in the combined resolution/dereferencing step. If the DID alone is resolved (without the resource=true parameter parameter), it will still return the associated DID document. There is nothing special about this DID document. It still describes and controls interactions with the identified digital resource just like any other DID document. For example:

• If the DID document contains only an id property (whose value must be the DID) and no verification methods, then the DID document cannot be updated and the identified digital resource is a static resource that cannot be versioned.
• If the DID document contains one or more verification methods, then the DID controller has a method for versioning the identified digital resource and/or the DID document itself.

If the DID document contains one or more alsoKnownAs property values, those values still identify other instances of the identified digital resource available via other URIs, URLs, or DIDs. (See also the recommendation below to include an alsoKnownAs property whose value is a hashlink to the identified digital resource.)

Syntax

The parameter name MUST be resource. The parameter type value MUST be a Boolean for which the valid values are true and falseone of the following:

1. A Boolean value of true. The parameter value of true MUST be case-sensitive and MUST be spelled out exactly. No other renderings of true, i.e.  T, TRUE, 1, -1, and so forth — are equivalent.

Example

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1. A valid media type name. If the VDR supports the storage of the resource in multiple media types, this option enables the client to request return of a specific media type.

Examples

Without a Path Component

Following are examples of DID URLs that use the resource parameter but do not include a path component. These examples are based on the fictional did:example: DID method that includes supports the resource parameter:

did:example:21tDAKCERh95uGgKbJNHYp?resource=true

did:example:21tDAKCERh95uGgKbJNHYp?resource=json

did:example:21tDAKCERh95uGgKbJNHYp?resource=ld+json

did:example:21tDAKCERh95uGgKbJNHYp?resource=cbor

With a Path Component

The next set of examples are DID URLs that use the resource parameter and include a path component. In this case the target resource identified by the path component is relative to the resource identified by the DID itself. This pattern supports the ability of a VDR to organize digital resources in structures such as directory trees, collections, databases, and so on.

did:example:21tDAKCERh95uGgKbJNHYp/some/path?resource=true

did:example:21tDAKCERh95uGgKbJNHYp/some/longer/path?resource=json

did:example:21tDAKCERh95uGgKbJNHYp/uuid:33ad7beb-1abc-4a26-b892-466df4379a51/?resource=ld+json

did:example:21tDAKCERh95uGgKbJNHYp/resources/uuid:33ad7beb-1abc-4a26-b892-466df4379a51/?resource=cbor

DID Resolver and Verifiable Data Registry (VDR) Requirements

1. If a DID URL includes the resource parameter with a value of true, a conforming DID resolver MUST return the default media type of the digital resource identified by the DID from the VDR specified by the associated DID method provided such resource is available. 
   1. If the DID resolver is unable to return the identified resource, the resolver MUST return the error "Resource not found".
2. If a DID URL includes the resource parameter with a value of a valid media type name, a conforming DID resolver MUST return the requested media type of the digital resource identified by the DID from the VDR specified by the associated DID method provided such resource is available. 
   1. If the DID resolver is unable to return the identified resource in the requested media type, the resolver MUST return the error "Resource not found".
3. If a DID URL includes the resource parameter with any value other than true (for example, false)a value of false, a conforming DID resolver SHOULD ignore the parameter.
4. If the DID alone  alone is resolved resolved without the resource parameter, it MUST return the authoritative DID authoritative DID document as defined in in W3C Decentralized Identifiers (DIDs) 1.0. This specification adds no additional requirements to a conforming DID document.

DID Method Requirements

A DID method specification conforming to this specification to include support for the resource parameter:

1. MUST define:
   1. How the associated VDR shall map the DID to the identified digital resource ;and the identified media type.
   2. If the DID URL includes a path component, how the VDR shall process this path component in mapping to the identified resource.
   3. If the resource parameter has a value of true, how the VDR shall determine the default media type.
   4. How the VDR shall return the resource in response to a request from a conforming DID resolver.
   MAY define additional DID document properties to enable dereferencing to the target digital resource, however this is discouraged vs. the alternative of storing such dereferencing metadata in the VDR itself
   1. .
2. MUST define how this parameter will interact with the following additional DID parameters defined in section 3.2.1 of the DID Core Specification.
   
   1. versionId
      
      1. If the versionId parameter is included in the DID URL, the resolver MUST return the identified version of the identified digital resource.
      2. If the identified version does not exist, the resolver MUST return the error: "Version not found".
   2. versionTime
      
      1. If the versionTime parameter is included in the DID URL, the resolver MUST return the identified version of the identified digital resource.
      2. If the identified version does not exist, the resolver MUST return the error: "Version not found".
3. SHOULD include in the associated DID document an alsoKnownAs property containing at least one value that is a hashlink to the identified digital resource. This provides additional cryptographic verification of the binding between the DID and the identified digital resource.
4. SHOULD NOT define any additional DID document properties—such as a special service endpoint—to enable dereferencing to the target digital resource. This is an anti-pattern because it requires DID controllers to add custom properties to their DID documents in order to use the resource parameter. A much better solution is to store the mapping between the DID and the identified digital resource within the VDR itself (and specify how this works in the DID method specification). This way the VDR can evolve its internal storage mechanisms without requiring DID controllers to make any changes to their DID documents.

Registration with the W3C DID Specification Registries

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The authors wish to thank the editors and contributors to the W3C Decentralized Identifiers (DIDs) 1.0 specification and and in particular co-editor Amy Guy for her help. Also thanks to the cheqd team of Ankur Banerjee and Alex Tweeddale for their suggestions about including support for the DID path component.

Licensing

This is a publicly available specification published by the ToIP Foundation under the following licenses:

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