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Versions: 00 01 02 03                                                   
Network Working Group                                           E. Wilde
Internet-Draft                                          February 3, 2016
Intended status: Informational
Expires: August 6, 2016

                         The Use of Registries


   Registries on the Internet and the Web fulfill a wide range of tasks,
   ranging from low-level networking aspects such as packet type
   identifiers, all the way to application-level protocols and
   standards.  This document summarizes some of the reasons of why and
   how to use registries, and how some of them are operated.  It serves
   as a 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

Note to Readers

   Please discuss this draft on the apps-discuss mailing list

   Online access to all versions and files is available on GitHub

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   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on August 6, 2016.

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Copyright Notice

   Copyright (c) 2016 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
   Provisions Relating to IETF Documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  TCP/UDP Port Numbers  . . . . . . . . . . . . . . . . . .   3
     2.2.  Language Tags . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Web Linking . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Why to use Registries . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Openness and Extensibility  . . . . . . . . . . . . . . .   5
     3.2.  Limited Namespaces  . . . . . . . . . . . . . . . . . . .   6
     3.3.  Design/Usage Review . . . . . . . . . . . . . . . . . . .   6
     3.4.  Identifier Design . . . . . . . . . . . . . . . . . . . .   7
     3.5.  Identifier Lifecycle  . . . . . . . . . . . . . . . . . .   7
     3.6.  Documentation Requirements  . . . . . . . . . . . . . . .   8
     3.7.  Centralized Lookup  . . . . . . . . . . . . . . . . . . .   9
   4.  When to use Registries  . . . . . . . . . . . . . . . . . . .   9
   5.  How to use Registries . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Implementation Support  . . . . . . . . . . . . . . . . .  10
     5.2.  Registry Application  . . . . . . . . . . . . . . . . . .  10
     5.3.  Registry Stability  . . . . . . . . . . . . . . . . . . .  11
     5.4.  Registry History  . . . . . . . . . . . . . . . . . . . .  11
     5.5.  Registry Access . . . . . . . . . . . . . . . . . . . . .  12
     5.6.  Runtime vs. Design-Time . . . . . . . . . . . . . . . . .  12
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

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

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   parts (the specification itself), as well as some parts that are
   supposed to evolve while the specification itself remains stable and

   A registry essentially is a pattern of how to separate those two
   aspects of a specification, allowing the specification to remain
   stable, while the parts of it referencing the registry can evolve
   over time by changing 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 [5] contains 1903
   individual registries.  This number indicates that registries as a
   "protocol specification pattern" are quite popular and successful.

   Deciding if 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 on
   some of these solutions may be.

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 to
   illustrate certain points.

2.1.  TCP/UDP Port Numbers

   The registry for TCP/UDP port numbers [3] 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

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   is a long history about managing and running it, and thus the most
   up-to-date document about it is relatively new (RFC 6335 [3] from
   August 2011).

   The namespace of ports is a very limited one.  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).  IANA 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 very limited
   and very popular namespace, and thus managing this registry follows a
   very disciplined review process.  RFC 6335 [3] specifies very
   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.

2.2.  Language Tags

   The ability to identify human languages is important for many
   scenarios and applications on the Internet, and thus RFC 5646 [1]
   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 [4].
   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 all the other subtags
   namespaces are defined and registered, it should suffice to mention
   that one of the main goals of RFC 5646 [1] 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

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   Internet stay in sync.  Thus, the management of the namespaces
   created for language tags by RFC 5646 [1] is mostly delegated to ISO,
   instead of being managed by IANA itself.

   Nevertheless, RFC 5646 [1] 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 5988 [2]
   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
   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 5988 [2] 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.

3.  Why to 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.

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,

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   while still having well-defined parts that are allowed to evolve over

   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.

3.2.  Limited Namespaces

   Historically, registries started managing the very 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
   very 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.

   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 very
   constrained.  For example, the link relation types described in
   Section 2.3 are using strings as identifiers without imposing a
   length limitations, 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

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   the specification very 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

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.

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.

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   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

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 the very least
   require a very short explanation to be submitted with a registration
   request, so that the registry itself can list those texts as helpful

   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.

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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 [5] are openly accessible on the Web,
   allowing everybody to lookup the current state of all these

   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 5.6.  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
   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
      updated, level of review before acceptance, required level of

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      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?

5.  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.

5.1.  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.

5.2.  Registry Application

   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.

   Following some structure in the application process help with keeping
   a better record of requested and performed updates of a registry,
   which can be helpful when it comes to maintaining and providing a
   registry history Section 5.4.

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5.3.  Registry Stability

   Since implementations often use registries based on snapshots
   Section 5.1, 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 very 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.

5.4.  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.

   Some registries have mailing lists associated with them which can be
   regarded as some sort of history.  However, this is a very 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).

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5.5.  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 designed to be used as 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.

   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 5.2 or history access Section 5.4.  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.

5.6.  Runtime vs. Design-Time

   Most implementations use registry snapshots (complete or partial) for
   using a registry's contents Section 5.1.  If a registry provides API
   access, then it would be possible to author implementations that use
   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

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   mechanisms in place to protect itself against suffering under too
   much load.

6.  References

   [1]        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>.

   [2]        Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,

   [3]        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,

   [4]        International Organization for Standardization, "Code for
              the representation of names of languages, 1st edition",
              ISO Standard 639, 1988.

   [5]        "IANA Protocol Registry", <http://www.iana.org/protocols>.

Appendix A.  Acknowledgements

   Thanks for comments and suggestions provided by ...

Author's Address

   Erik Wilde

   Email: erik.wilde@dret.net
   URI:   http://dret.net/netdret/

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