Internet DRAFT - draft-wilde-registries
draft-wilde-registries
Network Working Group E. Wilde
Internet-Draft July 30, 2019
Intended status: Informational
Expires: January 31, 2020
The Use of Registries
draft-wilde-registries-03
Abstract
Registries for Internet and Web protocols fulfill a wide range of
tasks, ranging from low-level networking aspects such as packet type
identifiers, all the way up to application-level protocols and
standards. This document summarizes some of the reasons of why,
when, and how to use registries. It serves as an informative
reference for specification writers considering whether to create and
manage a registry, allowing them to better understand some of the
issues associated with certain design and operational decisions.
Note to Readers
Please discuss this draft on the ART mailing list
(<https://www.ietf.org/mailman/listinfo/art>).
Online access to all versions and files is available on GitHub
(<https://github.com/dret/I-D/tree/master/registries>).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 31, 2020.
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Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. TCP/UDP Port Numbers . . . . . . . . . . . . . . . . . . 4
2.2. Language Tags . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Web Linking . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Domain Name System (DNS) . . . . . . . . . . . . . . . . 6
2.5. non-IANA/DNS example . . . . . . . . . . . . . . . . . . 6
3. Why use Registries . . . . . . . . . . . . . . . . . . . . . 6
3.1. Openness and Extensibility . . . . . . . . . . . . . . . 7
3.2. Limited Namespaces . . . . . . . . . . . . . . . . . . . 7
3.3. Design/Usage Review . . . . . . . . . . . . . . . . . . . 8
3.4. Identifier Design . . . . . . . . . . . . . . . . . . . . 8
3.5. Identifier Lifecycle . . . . . . . . . . . . . . . . . . 9
3.6. Documentation Requirements . . . . . . . . . . . . . . . 9
3.7. Centralized Lookup . . . . . . . . . . . . . . . . . . . 10
4. When to use Registries . . . . . . . . . . . . . . . . . . . 10
5. Barrier to Entry Issues . . . . . . . . . . . . . . . . . . . 11
5.1. Non-Semantic/Reserved Entries . . . . . . . . . . . . . . 11
5.2. Entry Levels . . . . . . . . . . . . . . . . . . . . . . 12
5.3. Separation by Value Syntax . . . . . . . . . . . . . . . 12
6. How to use Registries . . . . . . . . . . . . . . . . . . . . 13
6.1. Registry Operations . . . . . . . . . . . . . . . . . . . 14
6.2. Registry Creation . . . . . . . . . . . . . . . . . . . . 15
6.3. Registry Interaction . . . . . . . . . . . . . . . . . . 15
6.4. Implementation Support . . . . . . . . . . . . . . . . . 16
6.5. Registry Stability . . . . . . . . . . . . . . . . . . . 16
6.6. Registry History . . . . . . . . . . . . . . . . . . . . 16
6.7. Registry Access . . . . . . . . . . . . . . . . . . . . . 17
6.8. Runtime vs. Design-Time . . . . . . . . . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
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Appendix A. W3C Examples . . . . . . . . . . . . . . . . . . . . 20
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 22
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
Specifications for technologies and standards in computer networking
have long used the concept of a "Registry" as a place where well-
known information is available and managed. In most case, the main
reason to use a registry is to create a specification that has stable
parts (the specification itself), as well as some parts of it that
are supposed to evolve while the specification itself remains stable
and unchanged.
In essence, a registry is a pattern of how to separate those two
aspects of a specification, allowing the specification to remain
stable, while the parts of it managed in the registry can evolve over
time by updating the registry contents. For specification writers,
this has proven to be a useful and successful pattern. The "Protocol
Registries" maintained by the "Internet Assigned Numbers Authority
(IANA)" have steadily increased in number. At the time of writing
(early 2016), the IANA Protocol Registry [IANA-Protocol-Registry]
contains 1903 individual registries. This number indicates that
registries as a "protocol specification pattern" are quite popular
and successful.
Deciding whether a specification should use a registry is not an easy
task. It involves identifying those parts that should be kept stable
(in the specification itself), and those that should be managed in
one or more registries for ongoing management and evolution. Even
after identifying this split, it is necessary to define how exactly
the registry part(s) should be managed, involving questions such as
submission procedures, review processes, revocation/change
management, and access to the registry contents for the worldwide
developer community.
This document is intended to provide an overview to specification
developers in terms of why, when, and how to use registries. It is
not meant to provide definitive guidance, but mostly intended as a
reference to consider the different ways in which the general
"registry pattern" can be used, and what the possible side-effects of
some of these solutions may be.
This document also has to tackle a difficult question: Registries as
a pattern can be managed and hosted in a large variety of ways. On
the other hand, the IANA Protocol Registry mentioned above provides a
simple and hosted place for specifications to run their registries.
As all existing platforms, the IANA Protocol Registry comes with
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certain constraints in terms of how a registry can be defined,
managed, and run. This document acknowledges the existence and
popularity of the IANA Protocol Registry, but also tries to be open
enough so that if specification writer choose to do so for whatever
reason, they should also be able to use registries other than one in
the IANA Protocol Registry
2. Examples
The following list of examples is intended to be illustrative of some
of the existing registries, what kind of identifiers they are using,
and how they are managed. This list is not intended to highlight
these registries in any special way other than explaining some of the
specifics of the managed namespaces. As mentioned above, the list of
IANA-managed registries is long (around 2000 individual registries),
and the examples listed here have been selected somewhat randomly
just to illustrate certain points.
In order to highlight the fact that IANA-managed registries are only
one way of how to establish and run a registry, Section 2.4 briefly
talks about DNS as an example for the same basic function (providing
a managed namespace of identifiers), but one that is based on
different constraints and thus results in a different operational
model and implementation.
2.1. TCP/UDP Port Numbers
The registry for TCP/UDP port numbers [RFC6335] is one of the oldest
well-known registries on the Internet. Because of its core
importance for how the Internet functions, it has been around for a
long time, there is a long history about managing and running it, and
thus the most up-to-date document about it is relatively new (RFC
6335 [RFC6335] from August 2011).
The namespace of ports is limited. Port numbers in TCP and UDP are
16-bit numbers, yielding a namespace of 65'536 port numbers. The
port numbers are subdivided into three ranges of system ports
(0-1023), user ports (1024-49151), and dynamic ports (49152-65535).
RFC 6335 only treats system and user port numbers as assignable,
whereas dynamic port numbers cannot be assigned at all.
The TCP/UDP port number registry is a good example for a limited and
popular namespace, and thus managing this registry follows a
disciplined review process. RFC 6335 [RFC6335] defines specific
guidance about assignment, de-assignment, reuse, revocation, and
transfer of numbers. This level of detail may not be required for
all registries, but it is a good demonstration of what may be
necessary in case of constrained and popular namespaces.
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2.2. Language Tags
The ability to identify human languages is important for many
scenarios and applications on the Internet, and thus RFC 5646
[RFC5646] defines how to do this. The specification uses a registry
to manage the actual language identifiers, because this list is
constantly evolving and thus better separated from the specification
defining the language tag format itself.
Apart from the primary language subtag (identifying for example the
English language with the identifier "en"), the language tag format
supports additional subtags for extended languages, scripts, regions,
variants, extensions, and private use. The primary language subtag
uses 2- and 3-letter identifiers that are taken from ISO 639
[ISO.639.1988]. There is a provision for longer identifiers to exist
(and be directly registered with IANA), but the goal is to manage
actual registration through ISO 639, and use the namespace and
identifiers established by this standard.
While the namespace of the primary language subtags is rather
restricted (2- and 3-letter identifiers), IANA's registry itself does
not need to be directly concerned with its use and management, as
this is handled by ISO through their ISO 639 process.
Without going into the details of how the other subtag namespaces are
defined and registered, it should suffice to mention that one of the
main goals of RFC 5646 is to ensure that the language registry of ISO
639 (as well as some others, such as the script registry of ISO
15924) and language tags as used on the Internet stay in sync. Thus,
the management of the namespaces created for language tags by RFC
5646 [RFC5646] is mostly delegated to ISO, instead of being managed
by IANA itself.
Nevertheless, RFC 5646 [RFC5646] does make provisions about the
stability of entries in the various namespaces, so that the meaning
of language tags remains stable over time. This includes provisions
that existing values are not going to be changed, and that even
values withdrawn by ISO remain valid and will simply be marked as
"deprecated" in the respective IANA registry.
2.3. Web Linking
Web links can be typed by link relation types, and RFC 8288 [RFC8288]
defines a model for how this can be done, and a registry for well-
known values. One interesting aspect of the model is that the value
space is divided: On the one had there are well-known and registered
values identified by strings, and on the other hand it is possible to
use URIs, in which case no registration is required. This means that
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the namespace for the link relation type registry is that of strings,
meaning that it is not highly constrained.
With a rather large namespace, it is possible to accommodate a larger
set of entries. However, it is still required that additions to the
registry are done by following a process that requires describing the
requested entries, and referring to a document that contains their
definition and some context.
In addition, RFC 8288 [RFC8288] does define some constraints for how
registered link relation types have to be defined. A submission
process and reviews by designated experts are used to make sure that
these constraints are met when new entries are submitted for
inclusion in the registry.
2.4. Domain Name System (DNS)
The Domain Name System (DNS) specified in RFCs 1034 [RFC1034] and
1035 [RFC1035] is a distributed infrastructure for hosting a variety
of entry types. In the context of the DNS, the entry types are
defined by various Resource Record (RR) types. However, since the
DNS is used for specific purposes, it is not a generic key/value
store. Instead, the assumption is that the key always is a DNS name,
whereas the value then is determined by the RR(s) available for that
domain.
It could be argued that the DNS is not a typical registry because its
design reflects the fact that entries are updated rather frequently,
that entries therefore are not stable, and that registry access is a
runtime issue Section 6.8. Because of these differences, it may be a
bit of a stretch to call the DNS a "registry", but it is a well-know
and well-established key/value lookup mechanism for IETF
specifications, and therefore deserves mention in this document.
2.5. non-IANA/DNS example
(( Still looking for an example of a registry that's established/used
by an IETF spec and that is neither managed by IANA, nor part of DNS
Section 2.4. If you have a suggestion, please let me know. ))
3. Why use Registries
Establishing and using a registry can be done for a number of
reasons. The following sections list some of these reasons, and in
many cases, registries are used for at least some of the reasons
described here.
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The list of reasons why to establish a registry may seem long, and in
some cases it may feel as if these reasons are valid reasons for
managing established long-term values, but that having a registry
seems like a potential barrier to entry for short-term
experimentation and other not-quite-permanent scenarios. Section 5
discusses this issue and how specifications can deal with it.
3.1. Openness and Extensibility
Registries separate a specification into a stable part that is
represented by the specification itself, and a dynamic part that is
represented by one or more registries that are established by the
specification. This pattern allows a specification to remain stable,
while still having well-defined parts that are allowed to evolve over
time.
In order for this pattern to work well, the specification should
clearly state what implementations should do when encountering
unknown values in those locations where allowable values are managed
in a registry. The two most popular processing models are to either
silently ignore such a value and continue as if the value was not
present at all, or to raise an error and notify higher layers of the
fact that something unknown was encountered.
Depending on the way values are managed, it is also possible to
distinguish between values that are supposed to be registered, and
those that are not supposed to be registered and have to be
considered unregistered extensions. The link relation types
described in Section 2.3 use such an approach, defining that a link
relation is either a string and supposed to be a registered value, or
a URI in which case it is not supposed to be a registered value.
This strategy works when it is possible to clearly separate the
namespace of the place where values are expected into ones that are
considered to be registered, and those that are not. This can be
done lexically, or by having some kind of flag that indicates whether
a value is supposed to be well-known, or an unregistered extension.
3.2. Limited Namespaces
Historically, registries started managing the limited namespace of
identifier fields in protocol packets or other low-level mechanisms
such as the port numbers described in Section 2.1, often limited to a
small number of bits or bytes. Carefully managing this limited set
of available identifiers was important, as was a way to allow new
values to be added without having to update the protocol
specification itself.
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The higher the level is on which registries are used, the more likely
it is that namespaces at least on the technical level are not overly
constrained. For example, the link relation types described in
Section 2.3 are using strings as identifiers without imposing a
length limitation, meaning that the set of possible identifiers is
virtually inexhaustible. However, even in this case, the set of
helpful and meaningful identifiers (i.e., names that are human-
readable and partly self-describing) is limited, and thus even in
this case, the realistically useful namespace is much more limited
that the theoretical one.
3.3. Design/Usage Review
Registries are established in the context of a given specification,
and provide a mechanism to make this specification extensible by
allowing the registry to evolve over time. However, the context of
the specification often has a clear design rationale for why a
registry is established for a certain set of values. Any value added
or changed in the registry should fit into this context, and having a
registry provides an opportunity to have design and usage reviews
before the registry gets updated.
For design and usage reviews to work well, the most crucial aspect is
that the context of the registry is well-defined, and states clearly
what kind of expectations the design and usage review will be
checking. Often this review process is implemented using a mailing
list and designated experts, so that registration requests as well as
results of the deign and usage review are done openly and
transparently.
3.4. Identifier Design
Depending on the namespace, managing the registry namespace may
follow certain guidelines. For numeric values, there may be certain
number ranges that are supposed to be used in certain ways. For
string values, there may be some convention or best practices on how
to mint identifiers so that the namespace contains values that are
following these principles.
Note that this is different from the design and usage review
Section 3.3. Whereas the design and usage review is about testing
whether the meaning associated with a new value follow the
constraints defined in the context that established the registry, the
identifier design simply checks for how the registered values are
chosen. It thus is a lower bar than a design and usage review, but
still requires a review process that allows to propose new values,
and provides some feedback about whether these values follow the
guidelines or not.
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3.5. Identifier Lifecycle
The main reason for registries to exist in contrast to just including
a set of predefined values in the underlying specification itself is
the ability for these values to change over time. However,
registries only make sense if there is some sense of stability to
their contents, so that looking at existing registry values at some
point in time can be assumed to be a reasonable snapshot for some
amount of time.
Usually, registry entries are added at a modest pace, and an
implementation not supporting the latest additions shouldn't fail,
but simply implement some default behavior when encountering
unsupported values. This pattern ensures that the namespace can
evolve separately from the landscape of implementations.
However, adding identifiers is the easiest aspects of registry
updates. Things get more complicated when it comes to updating and
removing entries. The reason why these things are more complicated
is that implementations depending on an identifier having certain
semantics will behave incorrectly when the registry has been updated
for this identifier with either a change in semantics, or a
withdrawal of the entry.
For this reason, it often makes sense to include rules in the
management of the registry about if/how entries can be updated, or
removed. One popular approach is disallow updates with breaking
changes, and to allow withdrawal but keep the identifier and mark it
as "deprecated". This way it can be ensured that no incompatible
entry will be created by somebody using an identifier that was
previously used and removed.
The exact way how this process is defined depends on the context and
purpose of the registry, and also on the namespace size. Tightly
constrained namespaces mean that values probably should be managed
more carefully, so that the registry does not run out of values.
Also, while impossible to predict reliably, it is also important to
look at the possible lifetime of implementations (that will use
snapshots of the registry at some point in time), and on the long-
term effects of having outdated registry snapshots in
implementations.
3.6. Documentation Requirements
Registering a value means that people encountering this value should
be able to learn about what it represents. This means that there
should be documentation associated with it that can be used to learn
more about the value's meaning. Many registries at least require a
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short explanation to be submitted with a registration request, so
that the registry itself can list those texts as helpful
explanations.
Going further, many registries also require links to more detailed
specifications, so that people looking for complete explanations of
the meaning of registered values can follow those links and will find
specifications or at least explanations. The exact requirement on
what such a link must refer to is something that the specification
creating the registry has to define. One popular requirement is that
it must be publicly available information, so that anybody looking
for it can openly access it.
3.7. Centralized Lookup
With a registry containing all current values (and possibly listing
changed/deprecated ones as well) along with some registration
metadata, they provide valuable information for anybody looking for
information about registered values in the registry namespace. All
IANA protocol registries [IANA-Protocol-Registry] are openly
accessible on the Web, allowing everybody to lookup the current state
of all these registries.
Even though centralized lookup is an important aspect of openness and
extensibility Section 3.1, the usual usage model of these lookup
facilities is to use them at design-time rather than at runtime
Section 6.8. This means that the central lookup facilities are meant
to be used by developers, and not by the implementations created by
those developers. For the latter model a much more scalable
infrastructure would be required, and thus it is important to
consider the fact if the namespace managed by a registry fits this
model of being useful for developer lookup at design-time, and for
value lookup at runtime.
4. When to use Registries
Based on the examples given in Section 2 and the possible reasons
described in Section 3, the next question is how for designers to
decide when they should establish one or more registries to
complement a specification. All the issues describes in Section 3
are reasonable motivations, and in many cases it is more than just
one of them.
For developers using a specification, it is helpful if the
specification clearly describes which reasons were most important
when deciding to establish one or more registries. This is even more
true for developers who are looking to update the registry, because
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they should be aware of the reasons that were considered when the
registry was created.
For every registry that is established, it is helpful if a
specification explains the following general aspects:
What were the main design rationales behind establishing the
registry? The reasons described in Section 3 may be a good
starting point to pick from.
What are the management policies for the registry? Depending on a
variety of factors such as namespace size, expected frequency of
updates, level of review before acceptance, required level of
documentation, and possibly others, management can be rather
lightweight or a carefully managed process.
What is the size of the namespace and the expected rate of how it
will be used and possibly exhausted?
Even if it makes sense to establish a registry based on the reasons
given in Section 3, and if a specification makes use of this pattern,
it is possible that registries are a potential barrier to entry.
Section 5 discusses how these issues can be addressed.
5. Barrier to Entry Issues
For some of the reasons described in Section 3, specification authors
may decide to establish one or more registries, as described in
Section 4. However, it may be the case that having such a registry
can make it harder for developers to experiment with implementations
(without violating the specification), and/or that there should be a
more "lightweight" way for establishing some identifiers than for
others.
This section presents some design options for this issue, and
discusses how these design options are influenced by the registry's
identifier namespace, and the registration policy.
5.1. Non-Semantic/Reserved Entries
One possible approach is to explicitly reserve parts of the available
namespace for "non-semantic" purposes. This means that whenever such
a value is encountered, it is impossible to conclude based on this
value alone which concept it represents. Instead, the interpretation
of the value depends on additional information and/or on context, and
usually the exact mechanism is outside of the scope of the
specification.
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One example for this method is the TCP/UDP port example given in
Section 2.1. The namespace for port numbers is two bytes and thus
numbers from 0 to 65535. However, numbers in the range 49152-65535
are considered to be "dynamic ports", and are reserved for uses that
are outside of the scope of registered values.
5.2. Entry Levels
A different pattern to differentiate values is to have different
"levels" that values can be registered for. This effectively
partitions the registry into different classes, which sometime follow
certain naming conventions, sometimes not. In the latter case, when
encountering a value, it is necessary to consult the registry to
understand which level a value is associated with.
The levels often carry differentiations in a variety of factors, such
as how established represented concepts are, how well and/or how open
they are documented, how the expected stability of the entry is, and
what level of scrutiny is applied during the review process when
registering a new value. Specifications have a wide latitude
defining those levels and specifying what they represent and how they
affect value semantics and registration procedures.
As one example, media types [RFC6838] establish a "standards tree"
and as well as other trees ("vendor" and "personal"). Entries in
these trees are distinguished by prefix or lack thereof. The general
idea is that the non-standard trees contain entries that are not
quite at the same level, significance-wise, as the standards tree.
As another example, URI schemes [RFC7595] differentiate registrations
into "permanent" and "provisional" schemes. In addition, URI schemes
can also be registered as being "historical". A "provisional"
registration is intended to be used in cases where URI deployment is
likely to happen outside of a private environment, but still
controlled by a private party. It is also possible to update a
registration from "provisional" to "permanent" once the associated
protocol has become stable or more widely deployed.
The classification of registry entries into "permanent" and
"provisional" is a pattern being followed by some IETF specifications
and registries as well, for example by the registration procedures
for message header fields [RFC3864].
5.3. Separation by Value Syntax
While the examples given in Section 5.2 sometimes use ways to
differentiate entry levels by name, this is just a convenience so
that it is possible to determine a value's level by value syntax. In
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this case, the value syntax is enforced by registration policy,
meaning that during the registration process it is ensured that when
a registration request for a certain level is made, the registered
value conforms to the required syntax. However, in this case, all
values still need to be registered.
A different approach is to separate the value range itself by syntax,
and only require registration for partitions of that range. For
example, link relation types as described in Section 2.3 allow
strings and URIs as values. For strings, it is expected that any
used values are registered. For URIs, however, no such registration
is required or even possible. This means that "private link relation
types" can be safely identified by URI values, and since it is
possible to make these dereferencable, it is also possible to make
these values self-documenting and/or self-describing by making human-
and/or machine-readable information available at that URI.
While this separation by value syntax can be useful, it is important
to note that by design, any approach that reflects value
classification through value syntax automatically means that values
cannot moved between categories. Renaming established and deployed
values is an expensive thing to do, which means that the approach of
separation by value syntax should be carefully considered, in
particular in light of the fact that it makes it impossible for
values to be moved between categories. This may be acceptable for
some scenarios, but may be undesirable in other. RFC 6648 [RFC6648]
discusses this problem starting from prefix-based value syntax
approaches (the famous "X-" for extension values), but is not limited
to discussing this prefix-based approach alone.
6. How to use Registries
If a specification does introduce registries as part of how the
specification divides static and dynamic parts, then it is
interesting to look at how those registries will actually be used.
As in the previous sections, the list of topics included here is not
necessarily mutually exclusive, and it is likely that for any
established registry, there is more than one way how the registry is
being used.
For authors of specifications establishing registries, the following
list of possible ways how a registry might be used may be a good
starting point to consider the design options for the registry, such
as how to design the submission and update process, and how to
provide access to registry contents.
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6.1. Registry Operations
A registry is an abstract idea, mostly consisting of a separation of
a stable and fixed specifications, and associated registry-based
values that can be looked up and which may change over time. How a
registry is operated is a different matter. This is true for both
the workflows associated with registry operations, and the technical
implementation of those workflows.
For both the workflows, and their technical implementation, there are
a large number of issues to consider, and a large number of possible
solutions that satisfy those issues. For an organization like the
IETF, with a substantial stream of technical specifications, it makes
a lot of sense to establish both a common workflow, and a common
implementation, so that specifications can rely on this
infrastructure, instead of having to re-invent registry operations
every time they want to establish a registry.
In the context of IETF specifications, registries are typically
maintained by the "Internet Assigned Numbers Authority (IANA)".
Since there is a large number of registries, and they should be
maintained in a coherent, systematic, and efficient way, there is a
set of "Principles for Operation of Internet Assigned Numbers
Authority (IANA) Registries" which are described in RFC 7500
[RFC7500]. The key principles are defined as follows:
Ensure Uniqueness: The same protocol identifier must not be used
for more than one purpose.
Stable: Protocol identifier assignment must be lasting.
Predictable: The process for making assignments must not include
unexpected steps.
Public: The protocol identifiers must be made available in well-
known locations in a manner that makes them freely available to
everyone.
Open: The process that sets the policy for protocol identifier
assignment and registration must be open to all interested
parties.
Transparent: The protocol registries and their associated policies
should be developed in a transparent manner.
Accountable: Registry policy development and registry operations
need to be accountable to the affected community.
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The way RFC 7500 currently defines operational principles leaves some
possible issues open. For example, the principle of the identifiers
being "public" is described as the identifiers being "freely
available to everyone without restrictions." This means that it is
sufficient to make the identifiers available in human-readable form,
as opposed to the more specific ways in which machine-readable access
(as discussed in Section 6.7) would have to be enabled and possibly
managed.
While the exact requirements for a registry can be spelled out in the
"IANA Considerations" section of a specification establishing a
registry (see Section 6.2), there are hard limitations based on the
current implementation which is hosted by IANA. It is possible that
a different model may be implemented at a later time, but the current
model is biased towards email-based workflows and human-readable
registry access. If specification authors feel that IANA's
implementation will not fit their needs, then it is (at least
theoretically) possible for a specification to define its own
registry operations and infrastructure, but that would require a lot
of effort for a
6.2. Registry Creation
If a specification creates one or more registries, then RFC 5226
[RFC5226] sets some guidelines for the setup process and the
substance of each registry. Without repeating those guidelines here,
it should suffice to mention that those revolve around possible
status labels for assigned values (private use, experimental,
unassigned, reserved), and a number of predefined policies that
define how a registry is managed. RFC 5226 also mentions the fact
that a registry's management policy can change, in which case a new
specification is required that updates the definition of the
registry.
6.3. Registry Interaction
Since registry contents establish a controlled vocabulary, and each
registry has some policies around how that vocabulary can be updated,
it usually makes sense to have a template or a form that allows
applicants to prepare and submit update requests. It is up to the
registry to define how detailed this template is, whether applicants
are required to use it, and whether submission is implemented based
on that template. RFC 5226 [RFC5226] provides guidelines for
specification writers how to include any registry interactions in
their documents.
Following some structure in the interaction process help with keeping
a better record of requested and performed updates of a registry,
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which can be helpful when it comes to maintaining and providing a
registry history Section 6.6.
6.4. Implementation Support
The usual pattern for using registry-based identifiers in
implementations is to support some snapshot of the registry, which
either can be complete, or just a subset of the registry contents.
Implementation support then is based on the semantics associated with
registry values at the time of this snapshot. This means that any
registry updates changing semantics will affect and possibly break
those implementations, unless there is a strong policy to only allow
backwards-compatible changes to identifier semantics.
6.5. Registry Stability
Since implementations often use registries based on snapshots
Section 6.4, a key issue for registries is the stability of entries.
While it is clear that new entries can be added (this after all is
the minimal use case of registries: the ability to add identifiers
without the need for updating the specification itself), things get
more complicated when it comes to updates of existing entries, or
removal of exiting entries.
When it comes to updating entries, then often the goal is to avoid
breaking changes. This means that entries can only be updated in a
way that the updated semantics are backwards-compatible with the old
semantics. This way, implementations based on the old semantics can
safely use those and will not conflict when encountering data or
implementations assuming the updated semantics.
In terms of removal, it also is important to consider whether removed
entries should remain registered and blocked for future
registrations, so that they cannot be re-used (which essentially
would be equivalent to making a breaking change to an existing
entry). If such a policy is in-place, then technically speaking
there is no actual "removal" of a value rom the registry. Instead, a
value can be updated to be deprecated, but it remains in the registry
so that it is not re-assigned.
6.6. Registry History
While not strictly necessary for registry usage and management, in
terms of openness and transparency it can be helpful to provide a
registry history. This way it is possible to recreate all actions
that changed the registry, and to reconstruct the state of the
registry at any point in time.
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Some registries have mailing lists associated with them which can be
regarded as some sort of history. However, this is a weak form of
history since reconstructing the registry history and its state
requires to read all the emails, and infer the resulting registry
actions. Having access to the actual actions in machine-readable
form can make it much easier to access and recreate registry history.
Since registry usually are highly structured (often tabular models
with small number of columns), they would lend themselves well to
representing any updates to the contents in a similarly structured
way, along with some metadata about the entry update (such as date,
applicant, expert, links to email archives, and similar ways to
contextualize the entry update).
6.7. Registry Access
Depending on the intended use of a registry, an important question is
how developers and/or implementations can access the registry. While
the current IANA registries can be accessed via HTTP, they are
clearly intended and designed to be used as human-readable HTML pages
for developers. Alternative or complementary models could provide
API-based access, with documented and stable ways how to provide
machine-based access to registry contents.
However, if an API is considered and provided, then an important
question is whether it is intended for accessing registry contents
only, or providing full-fledged access to all services of the
registry, such as updates Section 6.3 or history access Section 6.6.
The former kind of access is easier to accomplish, because the sets
of provided is smaller, and the requirements for authentication and
authorization are probably simpler.
In addition to the question of how the registry API would be
designed, a more important question may be how it would be managed.
Any Internet-wide registry that provided API access would have to
carefully consider the implications of providing such a service,
mostly in terms of registry operations. Such as API could easily
become overloaded, and then would become a possible point of failure.
Also, it could become a point of attack, both in terms of denial or
service attacks, or in attempts to use the API to access the registry
in unauthorized ways.
6.8. Runtime vs. Design-Time
Most implementations use registry snapshots (complete or partial) for
using a registry's contents Section 6.4. If a registry provides API
access, then it would be possible to author implementations that use
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registry contents at runtime. However, there are two important
concerns about this possibility:
Having access to registry contents may be of little use other than
learning about the existence of new identifiers. In most cases, a
registry entry alone will not be sufficient to understand the
semantics of a new entry encountered at runtime. If that is the
case, then all an implementation can do is to verify that it
encountered a new entry that is a valid identifier according to
the current registry state, but it cannot implement the behavior
associated with that new entry.
If the model of the registry allows meaningful implementation
behavior by runtime updates, then this can result in this registry
becoming overwhelmed by the number of accesses. After all,
dynamic implementation behavior then may be preferable over the
more traditional snapshot implementation pattern, which then
results in the majority of implementations converging to the
runtime access model.
Because of the second issue in particular, any registry supporting
and intended for runtime access should make sure that provisions are
in place to control registry access. This is no different from any
other service on the Internet or Web that also needs to have
mechanisms in place to protect itself against suffering under too
much load.
7. IANA Considerations
This document has no IANA actions.
8. References
[IANA-Protocol-Registry]
"IANA Protocol Registry", <http://www.iana.org/protocols>.
[ISO.639.1988]
International Organization for Standardization, "Code for
the representation of names of languages, 1st edition",
ISO Standard 639, 1988.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
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[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<http://www.rfc-editor.org/info/rfc3864>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
September 2009, <http://www.rfc-editor.org/info/rfc5646>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<http://www.rfc-editor.org/info/rfc6335>.
[RFC6648] Saint-Andre, P., Crocker, D., and M. Nottingham,
"Deprecating the "X-" Prefix and Similar Constructs in
Application Protocols", BCP 178, RFC 6648,
DOI 10.17487/RFC6648, June 2012,
<http://www.rfc-editor.org/info/rfc6648>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<http://www.rfc-editor.org/info/rfc6838>.
[RFC7500] Housley, R., Ed. and O. Kolkman, Ed., "Principles for
Operation of Internet Assigned Numbers Authority (IANA)
Registries", RFC 7500, DOI 10.17487/RFC7500, April 2015,
<http://www.rfc-editor.org/info/rfc7500>.
[RFC7595] Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
and Registration Procedures for URI Schemes", BCP 35,
RFC 7595, DOI 10.17487/RFC7595, June 2015,
<http://www.rfc-editor.org/info/rfc7595>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
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Appendix A. W3C Examples
As an example of when registries can be useful, this appendix lists
"registries" defined by the World Wide Web Consortium (W3C). The W3C
does not currently have a registry model such as IETF's IANA, and
thus W3C specification authors have to come up with the own solutions
how to define and manage evolving sets of values.
The following list is not meant to be complete, but it does highlight
that a lack of registry capabilities in larger organizations can lead
to the "registry problem" being approached and solved in a variety of
ways, each of them requiring their own processes and infrastructure,
and each of them having their own side-effects.
Over time, various W3C specifications have used managed lists of
values. One of these specifications already is a stable
specification, it is the "XPointer" specification. XPointer has a
scheme registry which is managed as a manually updated Web page,
which has a documented registry policy.
Of the W3C drafts under development, one common model is to define
the values in a separate document, and then to evolve this document.
This has the advantage of decoupling the actual specification from
the set of managed values. The downside is that value evolution now
is based on the process defined for W3C documents, event though
that's not primarily the intention and the goal.
When following this approach, one set of specifications defines the
values in a Working Draft (WD) document. Here are the current W3C
specifications following this approach:
o Identifiers for WebRTC's Statistics API: https://www.w3.org/TR/
webrtc-stats/
o UI Events KeyboardEvent code Values: https://www.w3.org/TR/
uievents-code/
o UI Events KeyboardEvent key Values: https://www.w3.org/TR/
uievents-key/
o Basic Card Payment: https://www.w3.org/TR/payment-method-basic-
card/
o Payment Method Identifiers: https://www.w3.org/TR/payment-method-
id/
o Timing Entry Names Registry: https://www.w3.org/TR/timing-
entrytypes-registry/
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A second set of specifications also uses a separately evolved
document, but manages this as a Note (NOTE) document. Here are the
current W3C specifications following this approach:
o Encrypted Media Extensions Stream Format Registry:
https://www.w3.org/TR/eme-stream-registry/
o TTML Media Type Definition and Profile Registry:
https://www.w3.org/TR/ttml-profile-registry/
o Media Source Extensions Byte Stream Format Registry:
https://www.w3.org/TR/mse-byte-stream-format-registry/
o Trace Context Protocols Registry: https://www.w3.org/TR/trace-
context-protocols-registry/
Another approach is to keep the values inside the specification
itself. This has the side-effect that any change in the "registered"
values requires a change of the complete specification, which usually
is one is trying to avoid when identifying a potentially evolving set
of values. Here are the current W3C specifications following this
approach:
o Performance Timeline Level 2 "entryType": https://www.w3.org/TR/
performance-timeline-2/#dom-performanceentry-entrytype
o Permissions "Permission Registry": https://www.w3.org/TR/
permissions/#permission-registry
And finally, the last practice seen in W3C specifications is to use a
wiki as the managed list of values. Here are the current W3C
specifications following this approach:
o Web App Manifest Platform Values (wiki is located on GitHub):
https://github.com/w3c/manifest/wiki/Platforms
As can be seen from this list of practices, in larger organizations
it can become increasingly useful to support registries as a model of
how to define and manage evolving sets of values. Whether or not
this results in an organization creating and managing their own
registry infrastructure is one question. But apart from that, at
least identifying this as an issue that various teams are facing in
the organization, and giving them guidance on how to solve it, can
already help to better support teams, and avoid a fragmentation of
approaches that all create their own side-effects and technical debt
in the long run.
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Appendix B. Acknowledgements
Thanks for comments and suggestions provided by John Curran, Doug
Ewell, Graham Klyne, and Andrew Malis.
Author's Address
Erik Wilde
Email: erik.wilde@dret.net
URI: http://dret.net/netdret/
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