Internet DRAFT - draft-bormann-t2trg-deref-id
draft-bormann-t2trg-deref-id
Thing-to-Thing Research Group C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Informational C. Amsüss
Expires: 3 September 2024 2 March 2024
The "dereferenceable identifier" pattern
draft-bormann-t2trg-deref-id-03
Abstract
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 -03 revision discusses the continuum between entirely
// relying on the result of dereferencing an identifier and building
// in knowledge of all expected identifiers, and it mentions further
// pitfalls of using dereferenceable identifiers.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-bormann-t2trg-deref-id/.
Discussion of this document takes place on the t2trg Research Group
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Source for this draft and an issue tracker can be found at
https://github.com/cabo/deref-id.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Examples for "dereferenceable identifiers" . . . . . . . . . 4
2.1. Protocol and Protocol Version identifiers . . . . . . . . 4
2.2. Concept identifiers . . . . . . . . . . . . . . . . . . . 4
2.3. MORE EXAMPLES . . . . . . . . . . . . . . . . . . . . . . 5
3. Pitfalls . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Server overload . . . . . . . . . . . . . . . . . . . . . 5
3.2. Longevity of identifiers . . . . . . . . . . . . . . . . 5
3.3. Breakage due to incompatible changes . . . . . . . . . . 6
3.4. Redirect ambiguities . . . . . . . . . . . . . . . . . . 6
3.5. Other pitfalls . . . . . . . . . . . . . . . . . . . . . 6
4. Usage patterns between dereferencing and precise matching . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security considerations . . . . . . . . . . . . . . . . . . . 8
7. Privacy considerations . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
(Please see abstract.)
1.1. Terminology
Identifier
Dereferenceable
Dereference
_Information_: The information that is retrieved by dereferencing a
dereferenceable identifier.
Operator: 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.
Consumer: An entity that receives data containing a dereferenceable
identifier.
Directed: 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.)
Unique: 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).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [BCP14] when, and only when, they appear in all capitals, as
shown here.
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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 https://json-
schema.org/draft/2020-12/schema. 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:
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| 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 "https://iana.org/assignments/
| http-problem-types#" 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.
2.3. MORE EXAMPLES
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.
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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. Breakage due to incompatible changes
Dereferencing an identifier may produce different representations
over time. While these changes may be intentional and beneficial
(e.g. because terms are compatibly added to a resource describing
terms that are evolved together [COOL]), they can also cause breakage
in applications that previously dereferenced the identifier
successfully:
* There can be errors in the representation introduced by the
change.
* The operator and the consumer may disagree about what constitutes
a compatible change.
* An updated representation may exceed the consumer's capabilities,
e.g. not fitting in an allocated buffer.
* Even without intended changes to the representation, changes to
the channel may exclude certain consumers. For example, the
operator's web server may cease to accept the cipher suites
implemented in the consumer.
* When the operator's services are compromised, there may be
malicious changes in the representation.
3.4. 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.
3.5. Other pitfalls
Denial of service attacks are discussed in Section 6. Privacy
implications, in particular around single-use identifiers, are
discussed in Section 7.
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4. Usage patterns between dereferencing and precise matching
Consumers do not face a binary choice between dereferencing
dereferenceable identifiers and treating them as opaque. The space
between those extremes is continuous. Notable steps consumers can
take to mitigate pitfalls of dereferencing are:
1. Consumers that dereference may apply caching, which reduces
server load and bridges both outages and misconfigurations on the
server side.
These caches may adhere to the caching rules of the underlying
systems (DNS result life times, HTTP's freshness rules), but may
also stretch them if the alternative are failures or treating the
identifier as opaque.
2. Consumers may use caching proxy services provided by trusted
parties.
While this increases the susceptibility to service outages, it
immediately mitigates the privacy implications of having the
consumer's network address visible to the operator. Restrictive
policies at the proxy can further mitigate other issues. For
example, if the proxy's cache is eagerly populated by web spider
operations from public starting points and only ever serves
cached results to consumers, it defends against single-use URIs.
3. Consumer caches may be pre-populated as part of their firmware
update mechanisms.
In its extreme form, the consumer may not even be equipped to
dereference any identifiers outside of its cache, and the
dereferenced representation may already be part of the firmware
in ingested form to save runtime resources. Such a consumer
shares its properties with a consumer that treats dereferenceable
identifiers as opaque. However, the authors of the firmware can
make good use of the dereferenceable identifiers. For example,
they can dereference a known (or spidered) set of identifiers in
an automated fashion, with any suitable amount of caching or
manual verification.
5. 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.
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6. 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.
7. 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) dereferenceable 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.
Moreover, single-use identifiers can also be used to exfiltrate data
from originators whose network access is restricted through
dereferencing clients.
8. References
8.1. Normative References
[BCP14] Best Current Practice 14,
<https://www.rfc-editor.org/info/bcp14>.
At the time of writing, this BCP comprises the following:
Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References
[COOL] Sauermann, L. and R. Cyganiak, "Cool URIs for the Semantic
Web", 3 December 2008, <https://www.w3.org/TR/cooluris/>.
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[HTML5] WHATWG, "HTML — Living Standard", n.d.,
<https://html.spec.whatwg.org>.
[JSO] 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,
10 June 2022, <https://datatracker.ietf.org/doc/html/
draft-bhutton-json-schema-01>.
[PROBLEM] Nottingham, M., Wilde, E., and S. Dalal, "Problem Details
for HTTP APIs", RFC 9457, DOI 10.17487/RFC9457, July 2023,
<https://www.rfc-editor.org/rfc/rfc9457>.
[TAG] Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
RFC 4151, DOI 10.17487/RFC4151, October 2005,
<https://www.rfc-editor.org/rfc/rfc4151>.
Acknowledgements
Christian Amsüss pointed out that this document would be good to
have.
Authors' Addresses
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Christian Amsüss
Austria
Email: christian@amsuess.com
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