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Versions: 00 01 02 03                                                   
Network Working Group                                           E. Wilde
Internet-Draft                                          February 1, 2016
Intended status: Informational
Expires: August 4, 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 [1].

   Online access to all versions and files is available on GitHub [2].

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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   This Internet-Draft will expire on August 4, 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  TCP/UDP Port Numbers  . . . . . . . . . . . . . . . . . .   4
     2.2.  Language Tags . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Web Linking . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Why to use Registries . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Openness and Extensibility  . . . . . . . . . . . . . . .   6
     3.2.  Limited Namespaces  . . . . . . . . . . . . . . . . . . .   6
     3.3.  Design/Usage Review . . . . . . . . . . . . . . . . . . .   7
     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  . . . . . . . . . . . . . . . . . . .  10
     5.4.  Registry History  . . . . . . . . . . . . . . . . . . . .  10
     5.5.  Runtime vs. Design-Time . . . . . . . . . . . . . . . . .  10
     5.6.  Registry Access . . . . . . . . . . . . . . . . . . . . .  10
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   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 technology or standard that has stable parts

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   (the technology/standard itself), as well as some parts that are
   supposed to evolve while the technology/standard itself remains
   stable and unchanged.

   A registry essentially is a pattern of how to separate those two
   aspects of a technology/standard, allowing the technology/standard 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.

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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
   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 4646 [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 standard 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.

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   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 4646 [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
   Internet stay in sync.  Thus, the management of the namespaces
   created for language tags by RFC 4646 [1] is mostly delegated to ISO,
   instead of being managed by IANA itself.

   Nevertheless, RFC 4646 [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.

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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

   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.

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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 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 technology/standard
   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.

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   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

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

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   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 [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 5.5.  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 technology/standard.  All the issues describes in
   Section 3 are reasonable motivations, and in many cases it is more
   than just one of them.

   For users of a technology/standard, 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 users 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.

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      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
      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


5.1.  Implementation Support

   ... implement a snapshot, fail over in a controlled way ...

5.2.  Registry Application

   ... template ...

5.3.  Registry Stability

   ... changes beyond editorial ones ...

   ... removal of a registry entry ...

5.4.  Registry History

   ... history of all registry updates ...

5.5.  Runtime vs. Design-Time

   ... snapshot ...

5.6.  Registry Access

   ... API ...

6.  References

   [1]        Phillips, A. and M. Davis, "Tags for Identifying
              Languages", RFC 4646, DOI 10.17487/RFC4646, September
              2006, <http://www.rfc-editor.org/info/rfc4646>.

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

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   [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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