Internet-Draft dereferenceable identifiers December 2023
Bormann & Amsüss Expires 21 June 2024 [Page]
Thing-to-Thing Research Group
Intended Status:
C. Bormann
Universität Bremen TZI
C. Amsüss

The "dereferenceable identifier" pattern


In a protocol or an application environment, it is often important to be able to create unambiguous identifiers for some meaning (concept or some entity).

Due to the simplicity of creating URIs, these have become popular for this purpose. Beyond the purpose of identifiers to be uniquely associated with a meaning, some of these URIs are in principle dereferenceable, so something can be placed that can be retrieved when encountering such a URI.

The present -01 version is a stub to draw some attention to the opportunity that this pattern would benefit from a common description, documenting its benefits and pitfalls, and some mitigations for the latter.

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1. Introduction

(Please see abstract.)

1.1. Terminology





The information that is retrieved by dereferencing a dereferenceable identifier.


Dereferencing requires some server infrastructure to actually provide the information. Simplifying the potential complexity of this infrastructure, the entity (entities) controlling the operation of this server infrastructure, including the name spaces in use (e.g., DNS names, URI paths on a server) are called the operator(s) of the dereferenceable identifier.


A directed identifier is an identifier that has been specifically minted to not just identify the intended entity, but also context information such as the intended use, or intended consumer of the identifier.

Directed information is information that is tailored to the implicit context of a specific dereferencing access, such as the accessing IP address or other ancillary parameters. (Content negotiation alone is not "directed information", as it is explicitly triggered by the dereferencing entity.)


A unique identifier is an identifier that is unique for the entity; i.e., no other identifiers are in use (or intended to be in use).

2. Examples for "dereferenceable identifiers"

This section is intended to present a number of examples where dereferenceable identifiers are in use in a protocol, including existing discussion about constraints on their usage, the benefits claimed for this constrained usage, and remaining issues.

2.1. Protocol and Protocol Version identifiers

Many protocols based on XML or JSON include a protocol or protocol version identifier in the heading to a data item.

E.g., [JSO] defines a language for data models that contain an identifier to the language version in use, here The model that can be retrieved from this URI in turn contains further dereferenceable identifiers that point to further details.

Section 8.1.1 of [JSO] has this:

If this URI identifies a retrievable resource, that resource SHOULD be of media type "application/schema+json".

So it acknowledges that the dereferenceability is optional, but does place further restrictions on what can be the result of a successful dereference: another one of these data models, which in turn contain further dereferenceable identifiers.

2.2. Concept identifiers

The Problem Details for HTTP APIs format [PROBLEM] uses a dereferenceable identifier for its "type" field. The value is a URI that "identifies the specific "problem type" (e.g., "out of credit")" (Section 1 of [PROBLEM]).

Section 3.1.1 of [PROBLEM] has this:

If the type URI is a locator (e.g., those with an "http" or "https" scheme), dereferencing it SHOULD provide human-readable documentation for the problem type (e.g., using HTML [HTML5]).

but then warns:

However, consumers SHOULD NOT automatically dereference the type URI, unless they do so when providing information to developers (e.g., when a debugging tool is in use).

Section 4 of [PROBLEM] further details:

A problem type URI SHOULD resolve to HTML [HTML5] documentation that explains how to resolve the problem.

This becomes even more interesting as Section 4.2 of [PROBLEM] then gives this advice:

Registrations MAY use the prefix "" for the type URI.

A reference to the place where registrations for these items are managed is certainly desirable, however, the implications on the management of fragment identifiers in the HTML documents that IANA generates from registration information are an example for the increased complexity dereferenceable identifiers may place on the owners of the URI space employed.


There are a lot more examples in published RFCs; add them to this document.

3. Pitfalls

3.1. Server overload

If a data item containing dereferenceable identifier(s) becomes widely distributed, naive implementations that handle such a data item might dereference these identifiers as part of a routine operation. Many definitions of dereferenceable identifiers contain admonitions that such a behavior can cause an implosion of requests on the server(s) for the URI.

3.2. Longevity of identifiers

Dereferenceable URIs usually contain domain names, whose ownership can change. As a result, and for other reasons as well, parts of the name space of an origin may come under new administration, which can change the policies that apply to resources made available there.

These are problems of such URIs in general (and can be mitigated by going to a non-dereferenceable kind of URIs such as one based on the 'tag' uri scheme [TAG]). However, the problems are exacerbated by their use as a dereferenceable identifier. The new owner/administrator might more easily accept that a certain chunk of their URI space should not be used (which suffices for a non-dereferenceable identifier based on this kind of URI namespace) than that certain content needs to be offered there (potentially presenting non-trivial loads, some mechanisms needed to update that information, and legal liabilities that are hard to assess).

3.3. Redirect ambiguities

Dereferencing an identifier may involve following some redirections; whether that following is actually implied, or desired (or even desirable) is rarely being discussed.

4. IANA Considerations

This document makes no concrete requests on IANA, but does point out that IANA resources might be a good target for a certain class of dereferenceable identifiers.

5. Security considerations

The ability to create a denial of service attack by pointing a dereferenceable identifier into a popular data item that is widely distributed is implied by the discussion in Section 2, alongside with some recommendations for implementers that would mitigate such attacks. A problem with such recommendations is that they need to be followed by implementations that are using dereferenceable identifiers, which might not care much.

6. Privacy considerations

Dereferencing an identifier leaves a wide-spread data trail, ranging from host name lookups visible on the network to the absolute URI (i.e., the URI without its fragment identifier) visible to the operator of the identifier. Moreover, the operator might gain additional data about the requester, e.g. from a User-Agent header.

By minting directed (e.g., single-use) dereferencable identifiers and assigning short cache lifetimes to the dereferenced resource, the originator of a document can track dereferencing clients whenever they process the document the identifier has been created for.

7. Informative References

WHATWG, "HTML — Living Standard", n.d., <>.
Wright, A., Andrews, H., Hutton, B., and G. Dennis, "JSON Schema: A Media Type for Describing JSON Documents", Work in Progress, Internet-Draft, draft-bhutton-json-schema-01, , <>.
Nottingham, M., Wilde, E., and S. Dalal, "Problem Details for HTTP APIs", RFC 9457, DOI 10.17487/RFC9457, , <>.
Kindberg, T. and S. Hawke, "The 'tag' URI Scheme", RFC 4151, DOI 10.17487/RFC4151, , <>.


Christian Amsüss pointed out that this document would be good to have.

Authors' Addresses

Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Christian Amsüss