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RFC 8126

Guidelines for Writing an IANA Considerations Section in RFCs

Pages: 47
Best Current Practice: 26
Obsoletes:  5226
Part 2 of 3 – Pages 15 to 34
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4. Choosing a Registration Policy and Well-Known Policies

A registration policy is the policy that controls how new assignments in a registry are accepted. There are several issues to consider when defining the registration policy. If the registry's namespace is limited, assignments will need to be made carefully to prevent exhaustion.
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   Even when the space is essentially unlimited, it is still often
   desirable to have at least a minimal review prior to assignment in
   order to:

   o  prevent the hoarding of or unnecessary wasting of values.  For
      example, if the space consists of text strings, it may be
      desirable to prevent entities from obtaining large sets of strings
      that correspond to desirable names (existing company names, for

   o  provide a sanity check that the request actually makes sense and
      is necessary.  Experience has shown that some level of minimal
      review from a subject matter expert is useful to prevent
      assignments in cases where the request is malformed or not
      actually needed (for example, an existing assignment for an
      essentially equivalent service already exists).

   Perhaps most importantly, unreviewed extensions can impact
   interoperability and security.  See [RFC6709].

   When the namespace is essentially unlimited and there are no
   potential interoperability or security issues, assigned numbers can
   usually be given out to anyone without any subjective review.  In
   such cases, IANA can make assignments directly, provided that IANA is
   given detailed instructions on what types of requests it should
   grant, and it is able to do so without exercising subjective

   When this is not the case, some level of review is required.
   However, it's important to balance adequate review and ease of
   registration.  In many cases, those making registrations will not be
   IETF participants; requests often come from other standards
   organizations, from organizations not directly involved in standards,
   from ad-hoc community work (from an open-source project, for
   example), and so on.  Registration must not be unnecessarily
   difficult, unnecessarily costly (in terms of time and other
   resources), nor unnecessarily subject to denial.

   While it is sometimes necessary to restrict what gets registered
   (e.g., for limited resources such as bits in a byte, or for items for
   which unsupported values can be damaging to protocol operation), in
   many cases having what's in use represented in the registry is more
   important.  Overly strict review criteria and excessive cost (in time
   and effort) discourage people from even attempting to make a
   registration.  If a registry fails to reflect the protocol elements
   actually in use, it can adversely affect deployment of protocols on
   the Internet, and the registry itself is devalued.
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   Therefore, it is important to think specifically about the
   registration policy, and not just pick one arbitrarily nor copy text
   from another document.  Working groups and other document developers
   should use care in selecting appropriate registration policies when
   their documents create registries.  They should select the least
   strict policy that suits a registry's needs, and look for specific
   justification for policies that require significant community
   involvement (those stricter than Expert Review or Specification
   Required, in terms of the well-known policies).  The needs here will
   vary from registry to registry, and, indeed, over time, and this BCP
   will not be the last word on the subject.

   The following policies are defined for common usage.  These cover a
   range of typical policies that have been used to describe the
   procedures for assigning new values in a namespace.  It is not
   strictly required that documents use these terms; the actual
   requirement is that the instructions to IANA be clear and
   unambiguous.  However, use of these terms is strongly recommended
   because their meanings are widely understood.  Newly minted policies,
   including ones that combine the elements of procedures associated
   with these terms in novel ways, may be used if none of these policies
   are suitable; it will help the review process if an explanation is
   included as to why that is the case.  The terms are fully explained
   in the following subsections.

      1.   Private Use
      2.   Experimental Use
      3.   Hierarchical Allocation
      4.   First Come First Served
      5.   Expert Review
      6.   Specification Required
      7.   RFC Required
      8.   IETF Review
      9.   Standards Action
      10.  IESG Approval

   It should be noted that it often makes sense to partition a namespace
   into multiple categories, with assignments within each category
   handled differently.  Many protocols now partition namespaces into
   two or more parts, with one range reserved for Private or
   Experimental Use while other ranges are reserved for globally unique
   assignments assigned following some review process.  Dividing a
   namespace into ranges makes it possible to have different policies in
   place for different ranges and different use cases.

   Similarly, it will often be useful to specify multiple policies in
   parallel, with each policy being used under different circumstances.
   For more discussion of that topic, see Section 4.12.
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   Examples of RFCs that specify multiple policies in parallel:

      LDAP [RFC4520]
      TLS ClientCertificateType Identifiers [RFC5246] (as detailed in
      the subsections below)
      MPLS Pseudowire Types Registry [RFC4446]

4.1. Private Use

Private Use is for private or local use only, with the type and purpose defined by the local site. No attempt is made to prevent multiple sites from using the same value in different (and incompatible) ways. IANA does not record assignments from registries or ranges with this policy (and therefore there is no need for IANA to review them) and assignments are not generally useful for broad interoperability. It is the responsibility of the sites making use of the Private Use range to ensure that no conflicts occur (within the intended scope of use). Examples: Site-specific options in DHCP [RFC2939] Fibre Channel Port Type Registry [RFC4044] TLS ClientCertificateType Identifiers 224-255 [RFC5246]

4.2. Experimental Use

Experimental Use is similar to Private Use, but with the purpose being to facilitate experimentation. See [RFC3692] for details. IANA does not record assignments from registries or ranges with this policy (and therefore there is no need for IANA to review them) and assignments are not generally useful for broad interoperability. Unless the registry explicitly allows it, it is not appropriate for documents to select explicit values from registries or ranges with this policy. Specific experiments will select a value to use during the experiment. When code points are set aside for Experimental Use, it's important to make clear any expected restrictions on experimental scope. For example, say whether it's acceptable to run experiments using those code points over the open Internet or whether such experiments should be confined to more closed environments. See [RFC6994] for an example of such considerations. Example: Experimental Values in IPv4, IPv6, ICMPv4, ICMPv6, UDP, and TCP Headers [RFC4727]
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4.3. Hierarchical Allocation

With Hierarchical Allocation, delegated administrators are given control over part of the namespace and can assign values in that part of the namespace. IANA makes allocations in the higher levels of the namespace according to one of the other policies. Examples: o DNS names - IANA manages the top-level domains (TLDs), and, as [RFC1591] says: Under each TLD may be created a hierarchy of names. Generally, under the generic TLDs the structure is very flat. That is, many organizations are registered directly under the TLD, and any further structure is up to the individual organizations. o Object Identifiers - defined by ITU-T recommendation X.208. According to <>, some registries include * IANA, which hands out OIDs under the "Private Enterprises" branch, * ANSI, which hands out OIDs under the "US Organizations" branch, and * BSI, which hands out OIDs under the "UK Organizations" branch. o URN namespaces - IANA registers URN Namespace IDs (NIDs [RFC8141]), and the organization registering an NID is responsible for allocations of URNs within that namespace.

4.4. First Come First Served

For the First Come First Served policy, assignments are made to anyone on a first come, first served basis. There is no substantive review of the request, other than to ensure that it is well-formed and doesn't duplicate an existing assignment. However, requests must include a minimal amount of clerical information, such as a point of contact (including an email address, and sometimes a postal address) and a brief description of how the value will be used. Additional information specific to the type of value requested may also need to be provided, as defined by the namespace. For numbers, IANA generally assigns the next in-sequence unallocated value, but other values may be requested and assigned if an extenuating circumstance exists. With names, specific text strings can usually be requested.
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   When creating a new registry with First Come First Served as the
   registration policy, in addition to the contact person field or
   reference, the registry should contain a field for change controller.
   Having a change controller for each entry for these types of
   registrations makes authorization of future modifications more clear.
   See Section 2.3.

   It is important that changes to the registration of a First Come
   First Served code point retain compatibility with the current usage
   of that code point, so changes need to be made with care.  The change
   controller should not, in most cases, be requesting incompatible
   changes nor repurposing a registered code point.  See also Sections
   9.4 and 9.5.

   A working group or any other entity that is developing a protocol
   based on a First Come First Served code point has to be extremely
   careful that the protocol retains wire compatibility with current use
   of the code point.  Once that is no longer true, the new work needs
   to change to a different code point (and register that use at the
   appropriate time).

   It is also important to understand that First Come First Served
   really has no filtering.  Essentially, any well-formed request is


      SASL mechanism names [RFC4422]
      LDAP Protocol Mechanisms and LDAP Syntax [RFC4520]

4.5. Expert Review

For the Expert Review policy, review and approval by a designated expert (see Section 5) is required. While this does not necessarily require formal documentation, information needs to be provided with the request for the designated expert to evaluate. The registry's definition needs to make clear to registrants what information is necessary. The actual process for requesting registrations is administered by IANA (see Section 1.2 for details). (This policy was also called "Designated Expert" in earlier editions of this document. The current term is "Expert Review".) The required documentation and review criteria, giving clear guidance to the designated expert, should be provided when defining the registry. It is particularly important to lay out what should be considered when performing an evaluation and reasons for rejecting a request. It is also a good idea to include, when possible, a sense
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   of whether many registrations are expected over time, or if the
   registry is expected to be updated infrequently or in exceptional
   circumstances only.

   Thorough understanding of Section 5 is important when deciding on an
   Expert Review policy and designing the guidance to the designated

   Good examples of guidance to designated experts:

      Extensible Authentication Protocol (EAP) [RFC3748], Sections 6 and
      North-Bound Distribution of Link-State and TE Information Using
      BGP [RFC7752], Section 5.1

   When creating a new registry with Expert Review as the registration
   policy, in addition to the contact person field or reference, the
   registry should contain a field for change controller.  Having a
   change controller for each entry for these types of registrations
   makes authorization of future modifications more clear.  See
   Section 2.3.


      EAP Method Types [RFC3748]
      HTTP Digest AKA algorithm versions [RFC4169]
      URI schemes [RFC7595]
      GEOPRIV Location Types [RFC4589]

4.6. Specification Required

For the Specification Required policy, review and approval by a designated expert (see Section 5) is required, and the values and their meanings must be documented in a permanent and readily available public specification, in sufficient detail so that interoperability between independent implementations is possible. This policy is the same as Expert Review, with the additional requirement of a formal public specification. In addition to the normal review of such a request, the designated expert will review the public specification and evaluate whether it is sufficiently stable and permanent, and sufficiently clear and technically sound to allow interoperable implementations. The intention behind "permanent and readily available" is that a document can reasonably be expected to be findable and retrievable long after IANA assignment of the requested value. Publication of an RFC is an ideal means of achieving this requirement, but Specification Required is intended to also cover the case of a
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   document published outside of the RFC path, including informal

   For RFC publication, formal review by the designated expert is still
   requested, but the normal RFC review process is expected to provide
   the necessary review for interoperability.  The designated expert's
   review is still important, but it's equally important to note that
   when there is IETF consensus, the expert can sometimes be "in the
   rough" (see also the last paragraph of Section 5.4).

   As with Expert Review (Section 4.5), clear guidance to the designated
   expert should be provided when defining the registry, and thorough
   understanding of Section 5 is important.

   When specifying this policy, just use the term "Specification
   Required".  Some specifications have chosen to refer to it as "Expert
   Review with Specification Required", and that only causes confusion.


      Diffserv-aware TE Bandwidth Constraints Model Identifiers
      TLS ClientCertificateType Identifiers 64-223 [RFC5246]
      ROHC Profile Identifiers [RFC5795]

4.7. RFC Required

With the RFC Required policy, the registration request, along with associated documentation, must be published in an RFC. The RFC need not be in the IETF stream, but may be in any RFC stream (currently an RFC may be in the IETF, IRTF, IAB, or Independent Submission streams [RFC5742]). Unless otherwise specified, any type of RFC is sufficient (currently Standards Track, BCP, Informational, Experimental, or Historic). Examples: DNSSEC DNS Security Algorithm Numbers [RFC6014] Media Control Channel Framework registries [RFC6230] DANE TLSA Certificate Usages [RFC6698]

4.8. IETF Review

(Formerly called "IETF Consensus" in the first edition of this document.) With the IETF Review policy, new values are assigned only through RFCs in the IETF Stream -- those that have been shepherded through the IESG as AD-Sponsored or IETF working group documents
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   [RFC2026] [RFC5378], have gone through IETF Last Call, and have been
   approved by the IESG as having IETF consensus.

   The intent is that the document and proposed assignment will be
   reviewed by the IETF community (including appropriate IETF working
   groups, directorates, and other experts) and by the IESG, to ensure
   that the proposed assignment will not negatively affect
   interoperability or otherwise extend IETF protocols in an
   inappropriate or damaging manner.

   Unless otherwise specified, any type of RFC is sufficient (currently
   Standards Track, BCP, Informational, Experimental, or Historic).


      IPSECKEY Algorithm Types [RFC4025]
      TLS Extension Types [RFC5246]

4.9. Standards Action

For the Standards Action policy, values are assigned only through Standards Track or Best Current Practice RFCs in the IETF Stream. Examples: BGP message types [RFC4271] Mobile Node Identifier option types [RFC4283] TLS ClientCertificateType Identifiers 0-63 [RFC5246] DCCP Packet Types [RFC4340]

4.10. IESG Approval

New assignments may be approved by the IESG. Although there is no requirement that the request be documented in an RFC, the IESG has the discretion to request documents or other supporting materials on a case-by-case basis. IESG Approval is not intended to be used often or as a "common case"; indeed, it has seldom been used in practice. Rather, it is intended to be available in conjunction with other policies as a fall-back mechanism in the case where one of the other allowable approval mechanisms cannot be employed in a timely fashion or for some other compelling reason. IESG Approval is not intended to circumvent the public review processes implied by other policies that could have been employed for a particular assignment. IESG Approval would be appropriate, however, in cases where expediency is desired and there is strong consensus (such as from a working group) for making the assignment.
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   Before approving a request, the IESG might consider consulting the
   community, via a "call for comments" that provides as much
   information as is reasonably possible about the request.


      IPv4 Multicast address assignments [RFC5771]
      IPv4 IGMP Type and Code values [RFC3228]
      Mobile IPv6 Mobility Header Type and Option values [RFC6275]

4.11. Using the Well-Known Registration Policies

Because the well-known policies benefit from both community experience and wide understanding, their use is encouraged, and the creation of new policies needs to be accompanied by reasonable justification. It is also acceptable to cite one or more well-known policies and include additional guidelines for what kind of considerations should be taken into account by the review process. For example, for media-type registrations [RFC6838], a number of different situations are covered that involve the use of IETF Review and Specification Required, while also including specific additional criteria the designated expert should follow. This is not meant to represent a registration procedure, but to show an example of what can be done when special circumstances need to be covered. The well-known policies from "First Come First Served" to "Standards Action" specify a range of policies in increasing order of strictness (using the numbering from the full list in Section 4): 4. First Come First Served No review, minimal documentation. 5 and 6 (of equal strictness). 5. Expert Review Expert review with sufficient documentation for review. 6. Specification Required Significant stable public documentation sufficient for interoperability. 7. RFC Required Any RFC publication, IETF or a non-IETF Stream. 8. IETF Review
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      RFC publication, IETF Stream only, but need not be Standards

   9. Standards Action
      RFC publication, IETF Stream, Standards Track or BCP only.

   Examples of situations that might merit IETF Review or Standards
   Action include the following:

   o  When a resource is limited, such as bits in a byte (or in two
      bytes, or four), or numbers in a limited range.  In these cases,
      allowing registrations that haven't been carefully reviewed and
      agreed to by community consensus could too quickly deplete the
      allowable values.

   o  When thorough community review is necessary to avoid extending or
      modifying the protocol in ways that could be damaging.  One
      example is in defining new command codes, as opposed to options
      that use existing command codes: the former might require a strict
      policy, where a more relaxed policy could be adequate for the
      latter.  Another example is in defining protocol elements that
      change the semantics of existing operations.

   o  When there are security implications with respect to the resource,
      and thorough review is needed to ensure that the new usage is
      sound.  Examples of this include lists of acceptable hashing and
      cryptographic algorithms, and assignment of transport ports in the
      system range.

   When reviewing a document that asks IANA to create a new registry or
   change a registration policy to any policy more stringent than Expert
   Review or Specification Required, the IESG should ask for
   justification to ensure that more relaxed policies have been
   considered and that the more strict policy is the right one.

   Accordingly, document developers need to anticipate this and document
   their considerations for selecting the specified policy (ideally, in
   the document itself; failing that, in the shepherd writeup).
   Likewise, the document shepherd should ensure that the selected
   policies have been justified before sending the document to the IESG.

   When specifications are revised, registration policies should be
   reviewed in light of experience since the policies were set.

4.12. Using Multiple Policies in Combination

In some situations, it is necessary to define multiple registration policies. For example, registrations through the normal IETF process
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   might use one policy, while registrations from outside the process
   would have a different policy applied.

   Thus, a particular registry might want to use a policy such as "RFC
   Required" or "IETF Review" sometimes, with a designated expert
   checking a "Specification Required" policy at other times.

   The alternative to using a combination requires either that all
   requests come through RFCs or that requests in RFCs go through review
   by the designated expert, even though they already have IETF review
   and consensus.

   This can be documented in the IANA Considerations section when the
   registry is created, for example:

      IANA is asked to create the registry "Fruit Access Flags" under
      the "Fruit Parameters" group.  New registrations will be permitted
      through either the IETF Review policy or the Specification
      Required policy [BCP26].  The latter should be used only for
      registrations requested by SDOs outside the IETF.  Registrations
      requested in IETF documents will be subject to IETF review.

   Such combinations will commonly use one of {Standards Action, IETF
   Review, RFC Required} in combination with one of {Specification
   Required, Expert Review}.  Guidance should be provided about when
   each policy is appropriate, as in the example above.

4.13. Provisional Registrations

Some existing registries have policies that allow provisional registration: see URI Schemes [RFC7595] and Email Header Fields [RFC3864]. Registrations that are designated as provisional are usually defined as being more readily created, changed, reassigned, moved to another status, or removed entirely. URI Schemes, for example, allow provisional registrations to be made with incomplete information. Allowing provisional registration ensures that the primary goal of maintaining a registry -- avoiding collisions between incompatible semantics -- is achieved without the side effect of "endorsing" the protocol mechanism the provisional value is used for. Provisional registrations for codepoints that are ultimately standardized can be promoted to permanent status. The criteria that are defined for converting a provisional registration to permanent will likely be more strict than those that allowed the provisional registration. If your registry does not have a practical limit on codepoints, perhaps adding the option for provisional registrations might be
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   right for that registry as well.

5. Designated Experts

5.1. The Motivation for Designated Experts

Discussion on a mailing list can provide valuable technical feedback, but opinions often vary and discussions may continue for some time without clear resolution. In addition, IANA cannot participate in all of these mailing lists and cannot determine if or when such discussions reach consensus. Therefore, IANA relies on a "designated expert" for advice regarding the specific question of whether an assignment should be made. The designated expert is an individual who is responsible for carrying out an appropriate evaluation and returning a recommendation to IANA. It should be noted that a key motivation for having designated experts is for the IETF to provide IANA with a subject matter expert to whom the evaluation process can be delegated. IANA forwards requests for an assignment to the expert for evaluation, and the expert (after performing the evaluation) informs IANA as to whether or not to make the assignment or registration. In most cases, the registrants do not work directly with the designated experts. The list of designated experts for a registry is listed in the registry. It will often be useful to use a designated expert only some of the time, as a supplement to other processes. For more discussion of that topic, see Section 4.12.

5.2. The Role of the Designated Expert

The designated expert is responsible for coordinating the appropriate review of an assignment request. The review may be wide or narrow, depending on the situation and the judgment of the designated expert. This may involve consultation with a set of technology experts, discussion on a public mailing list, consultation with a working group (or its mailing list if the working group has disbanded), etc. Ideally, the designated expert follows specific review criteria as documented with the protocol that creates or uses the namespace. See the IANA Considerations sections of [RFC3748] and [RFC3575] for specific examples. Designated experts are expected to be able to defend their decisions to the IETF community, and the evaluation process is not intended to be secretive or bestow unquestioned power on the expert. Experts are expected to apply applicable documented review or vetting procedures, or in the absence of documented criteria, follow generally accepted norms such as those in Section 5.3. Designated experts are generally
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   not expected to be "gatekeepers", setting out to make registrations
   difficult to obtain, unless the guidance in the defining document
   specifies that they should act as such.  Absent stronger guidance,
   the experts should be evaluating registration requests for
   completeness, interoperability, and conflicts with existing protocols
   and options.

   It has proven useful to have multiple designated experts for some
   registries.  Sometimes those experts work together in evaluating a
   request, while in other cases additional experts serve as backups,
   acting only when the primary expert is unavailable.  In registries
   with a pool of experts, the pool often has a single chair responsible
   for defining how requests are to be assigned to and reviewed by
   experts.  In other cases, IANA might assign requests to individual
   members in sequential or approximate random order.  The document
   defining the registry can, if it's appropriate for the situation,
   specify how the group should work -- for example, it might be
   appropriate to specify rough consensus on a mailing list, within a
   related working group or among a pool of designated experts.

   In cases of disagreement among multiple experts, it is the
   responsibility of those experts to make a single clear recommendation
   to IANA.  It is not appropriate for IANA to resolve disputes among
   experts.  In extreme situations, such as deadlock, the designating
   body may need to step in to resolve the problem.

   If a designated expert has a conflict of interest for a particular
   review (is, for example, an author or significant proponent of a
   specification related to the registration under review), that expert
   should recuse himself.  In the event that all the designated experts
   are conflicted, they should ask that a temporary expert be designated
   for the conflicted review.  The responsible AD may then appoint
   someone or the AD may handle the review.

   This document defines the designated expert mechanism with respect to
   documents in the IETF stream only.  If other streams want to use
   registration policies that require designated experts, it is up to
   those streams (or those documents) to specify how those designated
   experts are appointed and managed.  What is described below, with
   management by the IESG, is only appropriate for the IETF stream.

5.2.1. Managing Designated Experts in the IETF

Designated experts for registries created by the IETF are appointed by the IESG, normally upon recommendation by the relevant Area Director. They may be appointed at the time a document creating or updating a namespace is approved by the IESG, or subsequently, when the first registration request is received. Because experts
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   originally appointed may later become unavailable, the IESG will
   appoint replacements as necessary.  The IESG may remove any
   designated expert that it appointed, at its discretion.

   The normal appeals process, as described in [RFC2026], Section 6.5.1,
   applies to issues that arise with the designated expert team.  For
   this purpose, the designated expert team takes the place of the
   working group in that description.

5.3. Designated Expert Reviews

In the years since [RFC2434] was published and put to use, experience has led to the following observations: o A designated expert must respond in a timely fashion, normally within a week for simple requests to a few weeks for more complex ones. Unreasonable delays can cause significant problems for those needing assignments, such as when products need code points to ship. This is not to say that all reviews can be completed under a firm deadline, but they must be started, and the requester and IANA should have some transparency into the process if an answer cannot be given quickly. o If a designated expert does not respond to IANA's requests within a reasonable period of time, either with a response or with a reasonable explanation for the delay (some requests may be particularly complex), and if this is a recurring event, IANA must raise the issue with the IESG. Because of the problems caused by delayed evaluations and assignments, the IESG should take appropriate actions to ensure that the expert understands and accepts his or her responsibilities, or appoint a new expert. o The designated expert is not required to personally bear the burden of evaluating and deciding all requests, but acts as a shepherd for the request, enlisting the help of others as appropriate. In the case that a request is denied, and rejecting the request is likely to be controversial, the expert should have the support of other subject matter experts. That is, the expert must be able to defend a decision to the community as a whole. When a designated expert is used, the documentation should give clear guidance to the designated expert, laying out criteria for performing an evaluation and reasons for rejecting a request. In the case where there are no specific documented criteria, the presumption should be that a code point should be granted unless there is a compelling reason to the contrary (and see also Section 5.4). Reasons that have been used to deny requests have included these:
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   o  Scarcity of code points, where the finite remaining code points
      should be prudently managed, or where a request for a large number
      of code points is made and a single code point is the norm.

   o  Documentation is not of sufficient clarity to evaluate or ensure

   o  The code point is needed for a protocol extension, but the
      extension is not consistent with the documented (or generally
      understood) architecture of the base protocol being extended and
      would be harmful to the protocol if widely deployed.  It is not
      the intent that "inconsistencies" refer to minor differences "of a
      personal preference nature".  Instead, they refer to significant
      differences such as inconsistencies with the underlying security
      model, implying a change to the semantics of an existing message
      type or operation, requiring unwarranted changes in deployed
      systems (compared with alternate ways of achieving a similar
      result), etc.

   o  The extension would cause problems with existing deployed systems.

   o  The extension would conflict with one under active development by
      the IETF, and having both would harm rather than foster

   Documents must not name the designated expert(s) in the document
   itself; instead, any suggested names should be relayed to the
   appropriate Area Director at the time the document is sent to the
   IESG for approval.  This is usually done in the document shepherd

   If the request should also be reviewed on a specific public mailing
   list, its address should be specified.
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5.4. Expert Reviews and the Document Lifecycle

Review by the designated expert is necessarily done at a particular point in time and represents review of a particular version of the document. While reviews are generally done around the time of IETF Last Call, deciding when the review should take place is a question of good judgment. And while rereviews might be done when it's acknowledged that the documentation of the registered item has changed substantially, making sure that rereview happens requires attention and care. It is possible, through carelessness, accident, inattentiveness, or even willful disregard, that changes might be made after the designated expert's review and approval that would, if the document were rereviewed, cause the expert not to approve the registration. It is up to the IESG, with the token held by the responsible Area Director, to be alert to such situations and to recognize that such changes need to be checked. For registrations made from documents on the Standards Track, there is often expert review required (by the registration policy) in addition to IETF consensus (for approval as a Standards Track RFC). In such cases, the review by the designated expert needs to be timely, submitted before the IESG evaluates the document. The IESG should generally not hold the document up waiting for a late review. It is also not intended for the expert review to override IETF consensus: the IESG should consider the review in its own evaluation, as it would do for other Last Call reviews.

6. Well-Known Registration Status Terminology

The following labels describe the status of an assignment or range of assignments: Private Use: Private use only (not assigned), as described in Section 4.1. Experimental: Available for general experimental use as described in [RFC3692]. IANA does not record specific assignments for any particular use. Unassigned: Not currently assigned, and available for assignment via documented procedures. While it's generally clear that any values that are not registered are unassigned and available for assignment, it is sometimes useful to explicitly specify that situation. Note that this is distinctly different from "Reserved".
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      Reserved:  Not assigned and not available for assignment.
            Reserved values are held for special uses, such as to extend
            the namespace when it becomes exhausted.  "Reserved" is also
            sometimes used to designate values that had been assigned
            but are no longer in use, keeping them set aside as long as
            other unassigned values are available.  Note that this is
            distinctly different from "Unassigned".

            Reserved values can be released for assignment by the change
            controller for the registry (this is often the IESG, for
            registries created by RFCs in the IETF stream).

      Known Unregistered Use:  It's known that the assignment or range
            is in use without having been defined in accordance with
            reasonable practice.  Documentation for use of the
            assignment or range may be unavailable, inadequate, or
            conflicting.  This is a warning against use, as well as an
            alert to network operators who might see these values in use
            on their networks.

7. Documentation References in IANA Registries

Usually, registries and registry entries include references to documentation (RFCs or other documents). The purpose of these references is to provide pointers for implementors to find details necessary for implementation, NOT to simply note what document created the registry or entry. Therefore: o If a document registers an item that is defined and explained elsewhere, the registered reference should be to the document containing the definition, not to the document that is merely performing the registration. o If the registered item is defined and explained in the current document, it is important to include sufficient information to enable implementors to understand the item and to create a proper implementation. o If the registered item is explained primarily in a specific section of the reference document, it is useful to include a section reference. For example, "[RFC4637], Section 3.2", rather than just "[RFC4637]". o For documentation of a new registry, the reference should provide information about the registry itself, not just a pointer to the creation of it. Useful information includes the purpose of the registry, a rationale for its creation, documentation of the process and policy for new registrations, guidelines for new
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      registrants or designated experts, and other such related
      information.  But note that, while it's important to include this
      information in the document, it needn't all be in the IANA
      Considerations section.  See Section 1.1.

8. What to Do in "bis" Documents

On occasion, an RFC is issued that obsoletes a previous edition of the same document. We sometimes call these "bis" documents, such as when RFC 4637 is to be obsoleted by draft-ietf-foo-rfc4637bis. When the original document created registries and/or registered entries, there is a question of how to handle the IANA Considerations section in the "bis" document. If the registrations specify the original document as a reference, those registrations should be updated to point to the current (not obsolete) documentation for those items. Usually, that will mean changing the reference to be the "bis" document. There will, though, be times when a document updates another, but does not make it obsolete, and the definitive reference is changed for some items but not for others. Be sure that the references always point to the correct, current documentation for each item. For example, suppose RFC 4637 registered the "BANANA" flag in the "Fruit Access Flags" registry, and the documentation for that flag is in Section 3.2. The current registry might look, in part, like this: Name Description Reference -------- ------------------- --------- BANANA Flag for bananas [RFC4637], Section 3.2 If draft-ietf-foo-rfc4637bis obsoletes RFC 4637 and, because of some rearrangement, now documents the flag in Section 4.1.2, the IANA Considerations of the bis document might contain text such as this: IANA is asked to change the registration information for the BANANA flag in the "Fruit Access Flags" registry to the following: Name Description Reference -------- ------------------- --------- BANANA Flag for bananas [[this RFC]], Section 4.2.1
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   In many cases, if there are a number of registered references to the
   original RFC and the document organization has not changed the
   registered section numbering much, it may simply be reasonable to do

      Because this document obsoletes RFC 4637, IANA is asked to change
      all registration information that references [RFC4637] to instead
      reference [[this RFC]].

   If information for registered items has been or is being moved to
   other documents, then the registration information should be changed
   to point to those other documents.  In most cases, documentation
   references should not be left pointing to the obsoleted document for
   registries or registered items that are still in current use.  For
   registries or registered items that are no longer in current use, it
   will usually make sense to leave the references pointing to the old
   document -- the last current reference for the obsolete items.  The
   main point is to make sure that the reference pointers are as useful
   and current as is reasonable, and authors should consider that as
   they write the IANA Considerations for the new document.  As always:
   do the right thing, and there is flexibility to allow for that.

   It is extremely important to be clear in your instructions regarding
   updating references, especially in cases where some references need
   to be updated and others do not.

(page 34 continued on part 3)

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