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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Universal DID Native Addressing (UDNA) — Community Group Draft</title>
<meta name="description" content="UDNA provides a universal, decentralized addressing layer built on W3C DIDs, Verifiable Credentials, and DIDComm. Community Group Draft / Incubation specification.">
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<body>
<!-- ABSTRACT -->
<section id="abstract">
<p>
<strong>Universal DID Native Addressing (UDNA)</strong> defines a lightweight, extensible addressing
scheme that makes <a href="https://www.w3.org/TR/did-core/">Decentralized Identifiers (DIDs)</a> native
to resource location and service discovery on the Web. UDNA extends DID from identity resolution to
resource addressing, providing a uniform <code>udna://</code> URI scheme that encodes a DID and a resource
path, enabling secure, cryptographically verifiable interactions across decentralized systems.
This specification builds on W3C Recommendations ([[DID-CORE]], [[VC-DATA-MODEL]]) and the DIDComm v2
messaging protocol ([[DIDCOMM]]). It is an incubation deliverable of the
<strong>UDNA W3C Community Group</strong> and does not represent consensus of the broader
W3C Membership.
</p>
<p>
UDNA completes the addressing stack for decentralized identity: where DNS maps names to IP addresses,
and DIDs map identifiers to DID Documents, UDNA maps identity to resources.
</p>
</section>
<!-- STATUS OF THIS DOCUMENT -->
<section id="sotd">
<p>
This document is a <strong>Community Group Draft</strong> produced by the
<a href="https://www.w3.org/groups/cg/did-native-addr/">W3C Community Group for DID Native Addressing</a>.
It is not a W3C Standard nor on the W3C Standards Track. Publication by the W3C does not imply
endorsement by the W3C Membership. This is a draft document and may be updated, replaced, or
obsoleted by other documents at any time. It is inappropriate to cite this document as other than
work in progress.
</p>
<p>
This specification is being developed to explore the intersection of decentralized identifiers,
native addressing, and universal service discovery. The goal is to mature the proposal toward a
future W3C Working Group deliverable. The CG welcomes
<a href="https://github.com/w3c-cg/udna/issues">issues and pull requests</a> on GitHub. Before
contributing, please review the
<a href="https://www.w3.org/community/about/agreements/cla/">W3C Community Contributor License Agreement</a>.
</p>
<p>
The CG has received early reviews from implementers of Solid, DIDComm, and Hyperledger Aries, and
expects to conduct multiple interoperability events during 2026.
</p>
</section>
<!-- 1. ARCHITECTURE OVERVIEW -->
<section id="architecture-overview" class="informative">
<h2>Architecture Overview</h2>
<p><em>This section is informative.</em></p>
<p>
UDNA operates as a bridge layer between URI-based resource addressing and decentralized identity
infrastructure. It completes the addressing stack for decentralized identity:
</p>
<ul>
<li><strong>DNS</strong> → maps names to IP addresses</li>
<li><strong>DIDs</strong> → map identifiers to DID Documents</li>
<li><strong>UDNA</strong> → maps identity to resources</li>
</ul>
<p>
The following diagrams illustrate the core resolution flow from a UDNA address
to a concrete resource endpoint, and the component relationships within the UDNA ecosystem.
</p>
<figure class="uml-diagram" id="fig-resolution-flow">
<figcaption>Figure 1: UDNA Resolution Flow — Sequence Diagram</figcaption>
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<!-- Participants -->
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<text x="400" y="45" text-anchor="middle" font-weight="bold" fill="#005a9c">UDNA Resolver</text>
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<text x="710" y="45" text-anchor="middle" font-weight="bold" fill="#005a9c">DID Registry</text>
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<!-- Messages -->
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<text x="235" y="112" text-anchor="middle" font-size="11" fill="#333">udna://did:web:example.com/api/data</text>
<line x1="400" y1="160" x2="708" y2="160" stroke="#333" stroke-width="1.5" marker-end="url(#arrow)"/>
<text x="555" y="152" text-anchor="middle" font-size="11" fill="#333">GET did:web:example.com</text>
<line x1="710" y1="200" x2="402" y2="200" stroke="#333" stroke-width="1.5" marker-end="url(#arrow)"/>
<text x="555" y="192" text-anchor="middle" font-size="11" fill="#333">DID Document + service endpoints</text>
<!-- Internal processing -->
<line x1="400" y1="240" x2="400" y2="280" stroke="#333" stroke-width="1.5"/>
<text x="420" y="265" text-anchor="start" font-size="11" fill="#666">Parse service endpoints</text>
<line x1="400" y1="280" x2="400" y2="320" stroke="#333" stroke-width="1.5"/>
<text x="420" y="305" text-anchor="start" font-size="11" fill="#666">Select matching service</text>
<line x1="400" y1="320" x2="400" y2="360" stroke="#333" stroke-width="1.5"/>
<text x="420" y="345" text-anchor="start" font-size="11" fill="#666">Construct target URL</text>
<line x1="400" y1="360" x2="400" y2="400" stroke="#333" stroke-width="1.5"/>
<text x="420" y="385" text-anchor="start" font-size="11" fill="#666">Validate result</text>
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<text x="290" y="440" text-anchor="middle" font-size="11" fill="#856404">ResolutionResult</text>
<text x="290" y="455" text-anchor="middle" font-size="10" fill="#856404">{ targetUrl, didDoc, ... }</text>
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<!-- Error path -->
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<text x="510" y="510" text-anchor="middle" font-size="11" fill="#721c24">ResolutionError</text>
<text x="510" y="525" text-anchor="middle" font-size="10" fill="#721c24">{ error, description }</text>
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<text x="480" y="560" text-anchor="middle" font-size="10" fill="#999">Success path →</text>
<text x="480" y="575" text-anchor="middle" font-size="10" fill="#999">Error path - - →</text>
</svg>
</figure>
<figure class="uml-diagram" id="fig-component-model">
<figcaption>Figure 2: UDNA Component Model</figcaption>
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<!-- DID Registry -->
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<text x="400" y="58" text-anchor="middle" font-weight="bold" fill="#155724">DID Registry</text>
<text x="400" y="78" text-anchor="middle" font-size="11" fill="#155724">(Ledger, Web, etc.)</text>
<text x="400" y="95" text-anchor="middle" font-size="10" fill="#6c757d">Stores DID Documents</text>
<!-- UDNA Resolver -->
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<text x="400" y="208" text-anchor="middle" font-weight="bold" fill="#004085">UDNA Resolver</text>
<text x="400" y="230" text-anchor="middle" font-size="11" fill="#004085">Parsing → DID Resolution</text>
<text x="400" y="248" text-anchor="middle" font-size="11" fill="#004085">→ Service Selection</text>
<text x="400" y="266" text-anchor="middle" font-size="11" fill="#004085">→ URL Construction</text>
<text x="400" y="288" text-anchor="middle" font-size="10" fill="#6c757d">Exposes resolve(udnaURI) API</text>
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<text x="400" y="418" text-anchor="middle" font-size="11" fill="#856404">(HTTP, DIDComm, etc.)</text>
<text x="400" y="438" text-anchor="middle" font-size="10" fill="#6c757d">Receives final URL</text>
<!-- Client -->
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<text x="80" y="238" text-anchor="middle" font-weight="bold" fill="#383d41">Client</text>
<text x="80" y="255" text-anchor="middle" font-size="10" fill="#6c757d">App / Browser</text>
<!-- Gateway -->
<rect x="30" y="330" width="100" height="60" rx="8" fill="#f8f9fa" stroke="#adb5bd" stroke-width="1.5" stroke-dasharray="5,3"/>
<text x="80" y="355" text-anchor="middle" font-weight="bold" fill="#495057">Gateway</text>
<text x="80" y="375" text-anchor="middle" font-size="10" fill="#6c757d">(Optional)</text>
<!-- Arrows -->
<line x1="400" y1="110" x2="400" y2="175" stroke="#28a745" stroke-width="1.5" marker-end="url(#arrow)"/>
<text x="420" y="145" text-anchor="start" font-size="10" fill="#28a745">DID Resolution</text>
<line x1="130" y1="240" x2="275" y2="240" stroke="#333" stroke-width="1.5" marker-end="url(#arrow)"/>
<text x="200" y="232" text-anchor="middle" font-size="10" fill="#333">resolve()</text>
<line x1="400" y1="300" x2="400" y2="365" stroke="#ffc107" stroke-width="1.5" marker-end="url(#arrow)"/>
<text x="420" y="335" text-anchor="start" font-size="10" fill="#856404">targetUrl →</text>
<!-- Gateway path -->
<line x1="130" y1="360" x2="275" y2="355" stroke="#adb5bd" stroke-width="1" stroke-dasharray="5,3" marker-end="url(#arrow)"/>
<text x="200" y="350" text-anchor="middle" font-size="9" fill="#6c757d">via gateway</text>
<!-- Extension registry -->
<rect x="580" y="180" width="190" height="120" rx="8" fill="#f8f9fa" stroke="#adb5bd" stroke-width="1.5" stroke-dasharray="5,3"/>
<text x="675" y="208" text-anchor="middle" font-weight="bold" fill="#495057">Extension Registry</text>
<text x="675" y="232" text-anchor="middle" font-size="10" fill="#6c757d">Batch Operations</text>
<text x="675" y="250" text-anchor="middle" font-size="10" fill="#6c757d">Gateway Forwarding</text>
<text x="675" y="268" text-anchor="middle" font-size="10" fill="#6c757d">Resource Integrity</text>
<line x1="520" y1="230" x2="575" y2="230" stroke="#adb5bd" stroke-width="1" stroke-dasharray="5,3"/>
</svg>
</figure>
</section>
<!-- 2. INTRODUCTION -->
<section id="introduction">
<h2>Introduction</h2>
<p>
The Web's addressing model is built on URLs that bind authority to domain names, which in turn rely
on centralized DNS and certificate authorities. While this has enabled massive growth, it introduces
identity silos, privacy leaks, and intermediary dependency. Decentralized Identifiers (DIDs) offer
self-sovereign, cryptographically verifiable identities, but they lack a native addressing scheme
to directly locate resources, APIs, or services tied to a DID.
</p>
<p>
<strong>UDNA fills this gap.</strong> UDNA extends DID from identity resolution to resource addressing.
By introducing the <code>udna://</code> URI scheme and a deterministic resolution mechanism, UDNA allows
any entity with a DID to publish service endpoints, resources, and sub-paths that can be resolved in a
secure, privacy-preserving way. Conceptually:
</p>
<ul>
<li><strong>DNS</strong> → maps names to IP addresses</li>
<li><strong>DIDs</strong> → map identifiers to DID Documents</li>
<li><strong>UDNA</strong> → maps identity to resources</li>
</ul>
<p>
UDNA leverages [[DID-CORE]] for identity, [[DIDCOMM]] for secure messaging, and [[VC-DATA-MODEL]]
for authorization policies.
</p>
<aside class="note" title="Incubation status">
<p>
This draft is a proposal incubated by the UDNA Community Group. It has received early reviews
from implementers of Solid, DIDComm, and Hyperledger Aries. The CG expects to conduct multiple
interoperability events during 2026.
</p>
</aside>
</section>
<!-- CONFORMANCE -->
<section id="conformance">
<p>
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and
notes in this specification are non-normative. Everything else in this specification is normative.
</p>
<p>
The key words <em class="rfc2119">MAY</em>, <em class="rfc2119">MUST</em>,
<em class="rfc2119">MUST NOT</em>, <em class="rfc2119">OPTIONAL</em>,
<em class="rfc2119">RECOMMENDED</em>, <em class="rfc2119">REQUIRED</em>,
<em class="rfc2119">SHALL</em>, <em class="rfc2119">SHALL NOT</em>,
<em class="rfc2119">SHOULD</em>, and <em class="rfc2119">SHOULD NOT</em>
in this document are to be interpreted as described in
<a href="https://www.rfc-editor.org/rfc/rfc2119">BCP 14</a> [[RFC2119]] [[RFC8174]]
when, and only when, they appear in all capitals, as shown here.
</p>
<section id="conformance-classes">
<h3>Conformance Classes</h3>
<p>
This specification defines the following conformance classes:
</p>
<dl class="conformance-class">
<dt><dfn>UDNA Resolver</dfn></dt>
<dd>
An implementation that accepts a [=UDNA Address=] and returns a
[=ResolutionResult=] or [=ResolutionError=]. A conformant resolver MUST
implement the <a href="#processing-model">Processing Model</a> and produce
outputs conforming to the <a href="#resolution-result-data-model">Resolution
Result Data Model</a> and <a href="#error-handling">Error Handling</a> sections.
</dd>
<dt><dfn>UDNA Client</dfn></dt>
<dd>
An application that constructs and dereferences [=UDNA Address=]es, either by
using a [=UDNA Resolver=] directly or by interacting with a [=UDNA Gateway=].
A conformant client MUST construct UDNA addresses according to
<a href="#scheme-syntax"></a> and SHOULD validate [=ResolutionResult=] objects
against the validity rules in <a href="#result-validity"></a>.
</dd>
<dt><dfn>UDNA Gateway</dfn> (OPTIONAL)</dt>
<dd>
An intermediary service that exposes UDNA resolution through HTTP endpoints,
enabling clients without native UDNA support to dereference UDNA addresses.
A conformant gateway MUST NOT modify the semantics of the resolved
<code>targetUrl</code> and MUST preserve all path, query, and fragment components
from the original UDNA address. Gateways are defined further in
<a href="#gateway-model"></a>.
</dd>
</dl>
</section>
</section>
<!-- 3. TERMINOLOGY -->
<section id="terminology">
<h2>Terminology</h2>
<p>
This section defines the terms used throughout this specification.
</p>
<dl>
<dt><dfn>UDNA Address</dfn></dt>
<dd>
A URI conforming to the <code>udna</code> scheme, composed of a DID component, an optional
service segment, and a resource path. Formally specified in <a href="#udna-scheme"></a>.
</dd>
<dt><dfn>DID Resolution</dfn></dt>
<dd>
The process defined in [[DID-CORE]] by which a DID string is dereferenced to obtain a
DID Document containing public keys and service endpoints.
</dd>
<dt><dfn>Native Addressing</dfn></dt>
<dd>
An addressing model where the identifier of an entity (a DID) is an integral part of the
resource locator, enabling direct, verifiable interactions without external naming authorities.
</dd>
<dt><dfn>Service Endpoint</dfn></dt>
<dd>
A network address defined within a DID Document under the <code>service</code> array property,
expressed as a URI string, a set of URI strings, or a DIDComm endpoint object.
</dd>
<dt><dfn>UDNARoot Service</dfn></dt>
<dd>
The default service entry used by UDNA resolution when no explicit service segment is present.
Identified by <code>type: "UDNARoot"</code> or the fragment identifier <code>#udna-root</code>
in the DID Document. A DID Document MUST contain at most one UDNARoot service entry.
</dd>
<dt><dfn>ResolutionResult</dfn></dt>
<dd>
A structured object returned on successful resolution, containing the constructed target URL,
the resolved DID Document, metadata, and cryptographic verification materials. Defined in
<a href="#resolution-result-data-model"></a>.
</dd>
<dt><dfn>ResolutionError</dfn></dt>
<dd>
A structured error object returned when resolution fails. Defined in
<a href="#error-handling"></a>.
</dd>
<dt><dfn>Processing Model</dfn></dt>
<dd>
The normative sequence of steps a resolver MUST execute to transform a UDNA address into
a ResolutionResult or ResolutionError. Defined in <a href="#processing-model"></a>.
</dd>
</dl>
</section>
<!-- 4. UDNA URI SCHEME -->
<section id="udna-scheme">
<h2>UDNA URI Scheme</h2>
<section id="scheme-syntax">
<h3>Scheme Syntax</h3>
<p>The <code>udna</code> URI scheme follows the ABNF syntax below (extending [[RFC3986]]):</p>
<pre class="nohighlight">
udna-URI = "udna://" did-string [ "/" service-id ] path [ "?" query ] [ "#" fragment ]
did-string = scheme ":" method-specific-id
; a valid DID per [[DID-CORE]]
service-id = *( unreserved / pct-encoded )
path = *( "/" segment )
query = *( pchar / "/" / "?" )
fragment = *( pchar / "/" / "?" )
</pre>
<p>A complete example illustrating all components:</p>
<pre class="nohighlight">
udna://did:example:123456/service/api/resource?version=1#section
</pre>
</section>
<section id="scheme-components">
<h3>Components</h3>
<dl>
<dt>DID (<em>mandatory</em>)</dt>
<dd>
A valid DID string per [[DID-CORE]], e.g., <code>did:key:z6Mkp…</code> or
<code>did:web:example.com</code>. The DID forms the authority component of the
[=UDNA Address=].
</dd>
<dt>Service segment (<em>optional</em>)</dt>
<dd>
Identifies a specific service endpoint by matching the <code>id</code> fragment of
a service entry in the resolved DID Document. If omitted, the resolver selects the
[=UDNARoot Service=].
</dd>
<dt>Path</dt>
<dd>
A hierarchical path that is opaque to the UDNA resolution layer but meaningful to
the target service after resolution.
</dd>
<dt>Query and Fragment</dt>
<dd>
Standard URI semantics per [[URL]], interpreted by the target service after resolution
is complete.
</dd>
</dl>
<aside class="note" title="DID method agnosticism">
<p>
UDNA does not introduce new DID methods. Any DID method is compatible provided its
DID Document conforms to [[DID-CORE]] and expresses valid service endpoints.
</p>
</aside>
</section>
</section>
<!-- 5. PROCESSING MODEL -->
<section id="processing-model">
<h2>Processing Model</h2>
<p>
This section defines the normative processing steps that a [=UDNA Resolver=] MUST
execute. It is separated from the data model to enable independent evolution of
processing logic and output structures.
</p>
<section id="resolution-algorithm">
<h3>Resolution Algorithm</h3>
<p>
To resolve a [=UDNA Address=], a conformant [=UDNA Resolver=] MUST perform the
following steps in order:
</p>
<ol>
<li>
<strong>Parse</strong> the UDNA address to extract the DID string, service segment (if present),
path, query, and fragment components per the syntax in
<a href="#scheme-syntax"></a>. If parsing fails, return a [=ResolutionError=] of type
<code>invalidUri</code>.
</li>
<li>
<strong>Validate</strong> that the extracted DID string is a valid DID per [[DID-CORE]].
If invalid, return a [=ResolutionError=] of type <code>invalidDid</code>.
</li>
<li>
<strong>Resolve</strong> the DID to a DID Document using the appropriate DID method resolver, as
specified in [[DID-CORE]]. If the DID method is not supported, return a [=ResolutionError=]
of type <code>methodNotSupported</code>. If resolution fails, return a [=ResolutionError=]
of type <code>notFound</code>.
</li>
<li>
<strong>Perform service selection:</strong>
<ul>
<li>
If a service segment is present, locate the entry in the DID Document's
<code>service</code> array whose <code>id</code> fragment matches the given
segment. If no match is found, return a [=ResolutionError=] of type
<code>serviceNotFound</code>.
</li>
<li>
If no service segment is provided, select the entry whose <code>type</code>
contains <code>"UDNARoot"</code> or whose <code>id</code> fragment equals
<code>udna-root</code>. If neither is found, return a [=ResolutionError=] of type
<code>defaultServiceNotFound</code>.
</li>
</ul>
</li>
<li>
<strong>Obtain</strong> the <code>serviceEndpoint</code> value from the selected service entry.
This value MAY be a URI string, a set of URI strings, or a DIDComm endpoint object.
If the value is unsupported or malformed, return a [=ResolutionError=] of type
<code>invalidEndpoint</code>.
</li>
<li>
<strong>Construct</strong> the final target URL by appending the path, query, and fragment
to the resolved endpoint base URI. If the endpoint is a URI template ([[RFC6570]]),
template expansion MUST be applied using available context variables. If template
expansion fails, return a [=ResolutionError=] of type
<code>templateExpansionFailed</code>.
</li>
<li>
<strong>Build</strong> a [=ResolutionResult=] containing: <code>targetUrl</code>,
<code>didDocument</code>, <code>didDocumentMetadata</code>,
<code>service</code>, and <code>verificationMethods</code>.
</li>
<li>
<strong>Validate</strong> the built ResolutionResult against the
<a href="#result-validity">validity rules</a>. If validation fails, return a
[=ResolutionError=] with an appropriate error description.
</li>
<li>
<strong>Return</strong> the validated [=ResolutionResult=].
</li>
</ol>
<p>
Resolvers SHOULD implement caching with appropriate TTL derived from DID Document
metadata (<code>created</code>, <code>updated</code>). Resolvers MUST respect HTTP
cache headers when resolving <code>did:web</code> identifiers.
</p>
</section>
<section id="resolution-example">
<h3>Example Resolution</h3>
<aside class="example" title="Resolving a did:web UDNA Address">
<pre>
Input: udna://did:web:example.com/app/profile#me
Step 1 — Parse:
DID = did:web:example.com
path = /app/profile
fragment = me
Step 2 — Resolve DID → DID Document (simplified):
{
"@context": "https://www.w3.org/ns/did/v1",
"id": "did:web:example.com",
"service": [{
"id": "#udna-root",
"type": "UDNARoot",
"serviceEndpoint": "https://api.example.com/udna"
}]
}
Step 3 — No service segment → select UDNARoot:
serviceEndpoint = "https://api.example.com/udna"
Step 4 — Construct target URL:
https://api.example.com/udna/app/profile#me
Step 5 — Build ResolutionResult (validated):
{ targetUrl: "https://api.example.com/udna/app/profile#me", ... }
</pre>
</aside>
</section>
<section id="result-validity">
<h3>ResolutionResult Validity</h3>
<p>
A [=ResolutionResult=] is considered valid if and only if ALL of the following
conditions are satisfied:
</p>
<div class="validity-rule">
<strong>VR-1:</strong> <code>targetUrl</code> MUST be a valid absolute URI as defined
by [[RFC3986]].
</div>
<div class="validity-rule">
<strong>VR-2:</strong> <code>didDocument</code> MUST conform to the DID Document data
model defined in [[DID-CORE]], Section 4.
</div>
<div class="validity-rule">
<strong>VR-3:</strong> <code>service</code> MUST correspond to an entry present in the
<code>didDocument.service</code> array, matched by <code>id</code> fragment.
</div>
<div class="validity-rule">
<strong>VR-4:</strong> <code>didDocumentMetadata</code> MUST include the
<code>created</code> or <code>updated</code> timestamp fields as specified in
[[DID-CORE]].
</div>
<div class="validity-rule">
<strong>VR-5:</strong> <code>verificationMethods</code> MUST be a subset of the
verification methods present in the <code>didDocument</code>, filtered to those
relevant to the selected service or DID subject.
</div>
<p>
A resolver that produces an invalid ResolutionResult SHALL report the violation
as an internal error and MUST NOT return the invalid result to the caller.
</p>
</section>
</section>
<!-- 5a. RESOLUTION RESULT DATA MODEL -->
<section id="resolution-result-data-model">
<h2>Resolution Result Data Model</h2>
<p>
Conformant [=UDNA Resolvers=] MUST return resolution results conforming to the
following structure. The data model extends DID Resolution metadata ([[DID-CORE]])
with UDNA-specific fields.
</p>
<section id="resolution-result-structure">
<h3>Structure</h3>
<pre class="json">
{
"@context": "https://w3c-cg.github.io/udna/contexts/udna-v1.jsonld",
"type": "UDNAResolutionResult",
"targetUrl": "https://api.example.com/udna/app/profile#me",
"didDocument": {
"@context": "https://www.w3.org/ns/did/v1",
"id": "did:web:example.com",
"service": [ ... ]
},
"didDocumentMetadata": {
"created": "2025-01-01T00:00:00Z",
"updated": "2025-06-01T00:00:00Z",
"deactivated": false
},
"service": {
"id": "#udna-root",
"type": "UDNARoot",
"serviceEndpoint": "https://api.example.com/udna"
},
"verificationMethods": [
{
"id": "did:web:example.com#keys-1",
"type": "Ed25519VerificationKey2020",
"controller": "did:web:example.com",
"publicKeyMultibase": "zH3C2AVvLMv6gmMNam3uVAjZpfkcJCwDwnZn6z3wXmqPV"
}
]
}
</pre>
<dl>
<dt><code>@context</code></dt>
<dd>
A JSON-LD context URI that defines the vocabulary for the UDNA resolution result.
The context is defined at
<a href="https://w3c-cg.github.io/udna/contexts/udna-v1.jsonld">
w3c-cg.github.io/udna/contexts/udna-v1.jsonld
</a>.
</dd>
<dt><code>type</code></dt>
<dd>
MUST be <code>"UDNAResolutionResult"</code>.
</dd>
<dt><code>targetUrl</code></dt>
<dd>
The fully constructed URL derived from the selected service endpoint, path,
query, and fragment. This is the concrete resource location.
</dd>
<dt><code>didDocument</code></dt>
<dd>
The resolved DID Document per [[DID-CORE]]. MUST be present when resolution
succeeds.
</dd>
<dt><code>didDocumentMetadata</code></dt>
<dd>
Metadata about the DID Document resolution, including timestamps and
deactivation status, as defined in [[DID-CORE]].
</dd>
<dt><code>service</code></dt>
<dd>
The specific service entry selected by the resolution algorithm. This enables
clients to inspect service-level metadata and endpoint configuration.
</dd>
<dt><code>verificationMethods</code></dt>
<dd>
An array of verification methods from the DID Document that are associated with
the selected service or the DID subject. Clients MAY use these to authenticate
the resolved endpoint or verify responses.
</dd>
</dl>
</section>
</section>
<!-- 5b. ERROR HANDLING -->
<section id="error-handling">
<h2>Error Handling</h2>
<p>
When resolution fails, a conformant [=UDNA Resolver=] MUST return a [=ResolutionError=]
object using the following structure:
</p>
<section id="error-structure">
<h3>Error Structure</h3>
<pre class="json">
{
"error": "serviceNotFound",
"errorDescription": "No matching service entry found for 'api'",
"did": "did:web:example.com",
"serviceSegment": "api"
}
</pre>
<dl>
<dt><code>error</code></dt>
<dd>
A machine-readable error code drawn from the
<a href="#error-codes">error codes registry</a>.
</dd>
<dt><code>errorDescription</code></dt>
<dd>
A human-readable description of the error condition.
</dd>
<dt><code>did</code></dt>
<dd>
The DID string that was being resolved when the error occurred.
</dd>
<dt><code>serviceSegment</code></dt>
<dd>
The service segment that failed to match, if applicable. MAY be omitted
for errors not related to service selection.
</dd>
</dl>
</section>
<section id="error-codes">
<h3>Error Codes</h3>
<table class="simple">
<caption>Standard UDNA Resolution Error Codes</caption>
<thead>
<tr>
<th>Error Code</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>notFound</code></td>
<td>The DID could not be resolved to a DID Document.</td>
</tr>
<tr>
<td><code>serviceNotFound</code></td>
<td>A service segment was provided but no matching service entry exists.</td>
</tr>
<tr>
<td><code>defaultServiceNotFound</code></td>
<td>No service segment was provided and no UDNARoot service is declared.</td>
</tr>
<tr>
<td><code>invalidUri</code></td>
<td>The provided UDNA address could not be parsed.</td>
</tr>
<tr>
<td><code>invalidDid</code></td>
<td>The DID string extracted from the UDNA address is not a valid DID.</td>
</tr>
<tr>
<td><code>methodNotSupported</code></td>
<td>The DID method is not supported by this resolver.</td>
</tr>
<tr>
<td><code>invalidEndpoint</code></td>
<td>The selected service endpoint is malformed or unsupported.</td>
</tr>
<tr>
<td><code>templateExpansionFailed</code></td>
<td>URI template expansion failed for the selected endpoint.</td>
</tr>
</tbody>
</table>
</section>
</section>
<!-- 5c. CONTENT NEGOTIATION -->
<section id="content-negotiation">
<h2>Content Negotiation</h2>
<p>
When UDNA resolution produces an HTTP-based <code>targetUrl</code>, clients MAY
use standard HTTP content negotiation ([[RFC9110]], Section 12) when interacting
with the resolved endpoint. This section defines content types relevant to UDNA
interactions.
</p>
<p>
Services exposed via UDNA SHOULD support the following media types:
</p>
<ul>
<li>
<strong><code>application/json</code></strong> — Generic JSON responses for
RESTful service interactions.
</li>
<li>
<strong><code>application/ld+json</code></strong> — JSON-LD formatted responses
for linked data interactions, including credential verification.
</li>
<li>
<strong><code>application/didcomm-envelope+json</code></strong> — DIDComm v2
encrypted envelopes when the resolved endpoint is a DIDComm service
([[DIDCOMM]]).
</li>
</ul>
<p>
Clients MAY include an <code>Accept</code> header to indicate preferred content types.
Services SHOULD respond with an appropriate <code>Content-Type</code> header and
SHOULD return HTTP status <code>406 Not Acceptable</code> if unable to satisfy the
client's content type preferences.
</p>
<aside class="note" title="DIDComm vs HTTP endpoints">
<p>
UDNA does not require DIDComm. A service endpoint MAY be a plain HTTP URL.
However, when the endpoint type indicates DIDComm, the content negotiation
expectations defined in [[DIDCOMM]] take precedence.
</p>
</aside>
</section>
<!-- 5d. RESOLVER API INTERFACE -->
<section id="resolver-api">
<h2>Resolver API Interface</h2>
<p>
To promote interoperability across implementations, a conformant [=UDNA Resolver=]
MUST expose at minimum the following function signature:
</p>
<pre class="nohighlight">
function resolve(udnaURI: string): Promise<ResolutionResult | ResolutionError>;
</pre>
<p>Implementations MAY additionally provide:</p>
<ul>
<li>