Network Working Group T. Narten
Request for Comments: 2434 IBM
BCP: 26 H. Alvestrand
Category: Best Current Practice Maxware
Guidelines for Writing an IANA Considerations Section in RFCs
Status of this Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (1998). All Rights Reserved.
Many protocols make use of identifiers consisting of constants and
other well-known values. Even after a protocol has been defined and
deployment has begun, new values may need to be assigned (e.g., for a
new option type in DHCP, or a new encryption or authentication
algorithm for IPSec). To insure that such quantities have consistent
values and interpretations in different implementations, their
assignment must be administered by a central authority. For IETF
protocols, that role is provided by the Internet Assigned Numbers
In order for the IANA to manage a given name space prudently, it
needs guidelines describing the conditions under which new values can
be assigned. If the IANA is expected to play a role in the management
of a name space, the IANA must be given clear and concise
instructions describing that role. This document discusses issues
that should be considered in formulating a policy for assigning
values to a name space and provides guidelines to document authors on
the specific text that must be included in documents that place
demands on the IANA.
Table of Contents
Status of this Memo.......................................... 1
1. Introduction............................................. 2
2. Issues To Consider....................................... 3
3. Registration maintenance................................. 6
4. What To Put In Documents................................. 7
5. Applicability to Past and Future RFCs.................... 8
6. Security Considerations.................................. 8
7. Acknowledgments.......................................... 9
8. References............................................... 9
9. Authors' Addresses....................................... 10
10. Full Copyright Statement................................. 11
Many protocols make use of fields that contain constants and other
well-known values (e.g., the Protocol field in the IP header [IP] or
MIME types in mail messages [MIME-REG]). Even after a protocol has
been defined and deployment has begun, new values may need to be
assigned (e.g., a new option type in DHCP [DHCP] or a new encryption
or authentication algorithm for IPSec [IPSEC]). To insure that such
fields have consistent values and interpretations in different
implementations, their assignment must be administered by a central
authority. For IETF protocols, that role is provided by the Internet
Assigned Numbers Authority (IANA).
In this document, we call the set of possible values for such a field
a "name space"; its actual content may be a name, a number or another
kind of value. The assignment of a specific value to a name space is
called an assigned number (or assigned value). Each assignment of a
number in a name space is called a registration.
In order for the IANA to manage a given name space prudently, it
needs guidelines describing the conditions under which new values
should be assigned. This document provides guidelines to authors on
what sort of text should be added to their documents, and reviews
issues that should be considered in formulating an appropriate policy
for assigning numbers to name spaces.
Not all name spaces require centralized administration. In some
cases, it is possible to delegate a name space in such a way that
further assignments can be made independently and with no further
(central) coordination. In the Domain Name System, for example, the
IANA only deals with assignments at the higher-levels, while
subdomains are administered by the organization to which the space
has been delegated. As another example, Object Identifiers (OIDs) as
defined by the ITU are also delegated [ASSIGNED]. When a name space
can be delegated, the IANA only deals with assignments at the top
This document uses the terms 'MUST', 'SHOULD' and 'MAY', and their
negatives, in the way described in RFC 2119 [KEYWORDS]. In this case,
"the specification" as used by RFC 2119 refers to the processing of
protocols being submitted to the IETF standards process.
2. Issues To Consider
The primary issue to consider in managing a name space is its size.
If the space is small and limited in size, assignments must be made
carefully to insure that the space doesn't become exhausted. If the
space is essentially unlimited, on the other hand, it may be
perfectly reasonable to hand out new values to anyone that wants one.
Even when the space is essentially unlimited, however, it is usually
desirable to have a minimal review to prevent the hoarding of or
unnecessary wasting of a space. For example, if the space consists of
text strings, it may be desirable to prevent organizations from
obtaining large sets of strings that correspond to the "best" names
(e.g., existing company names).
A second consideration is whether it makes sense to delegate the name
space in some manner. This route should be pursued when appropriate,
as it lessens the burden on the IANA for dealing with assignments.
In some cases, the name space is essentially unlimited, and assigned
numbers can safely be given out to anyone. When no subjective review
is needed, the IANA can make assignments directly, provided that the
IANA is given specific instructions on what types of requests it
should grant, and what information must be provided before a request
for an assigned number will be considered. Note that the IANA will
not define an assignment policy; it should be given a set of
guidelines that allow it to make allocation decisions with little
In most cases, some review of prospective allocations is appropriate,
and the question becomes who should perform the review and how
rigorous the review needs to be. In many cases, one might think that
an IETF Working Group (WG) familiar with the name space at hand
should be consulted. In practice, however, WGs eventually disband, so
they cannot be considered a permanent evaluator. It is also possible
for name spaces to be created through individual submission
documents, for which no WG is ever formed.
One way to insure community review of prospective assignments is to
have the requester submit a document for publication as an RFC. Such
an action insures that the IESG and relevant WGs review the
assignment. This is the preferred way of insuring review, and is
particularly important if any potential interoperability issues can
arise. For example, many assignments are not just assignments, but
also involve an element of protocol specification. A new option may
define fields that need to be parsed and acted on, which (if
specified poorly) may not fit cleanly with the architecture of other
options or the base protocols on which they are built.
In some cases, however, the burden of publishing an RFC in order to
get an assignment is excessive. However, it is generally still useful
(and sometimes necessary) to discuss proposed additions on a mailing
list dedicated to the purpose (e.g., the firstname.lastname@example.org for
media types) or on a more general mailing list (e.g., that of a
current or former IETF WG). Such a mailing list provides a way for
new registrations to be publicly reviewed prior to getting assigned,
or to give advice for persons who want help in understanding what a
proper registration should contain.
While discussion on a mailing list can provide valuable technical
expertise, opinions may vary and discussions may continue for some
time without resolution. In addition, the IANA cannot participate in
all of these mailing lists and cannot determine if or when such
discussions reach consensus. Therefore, the IANA cannot allow
general mailing lists to fill the role of providing definitive
recommendations regarding a registration question. Instead, the IANA
will use a designated subject matter expert. The IANA will rely on a
"designated expert" to advise it in assignment matters. That is, the
IANA forwards the requests it receives to a specific point-of-contact
(one or a small number of individuals) and acts upon the returned
recommendation from the designated expert. The designated expert can
initiate and coordinate as wide a review of an assignment request as
may be necessary to evaluate it properly.
Designated experts are appointed by the relevant Area Director of the
IESG. They are typically named at the time a document that creates a
new numbering space is published as an RFC, but as experts originally
appointed may later become unavailable, the relevant Area Director
will appoint replacements if necessary.
Any decisions made by the designated expert can be appealed using the
normal IETF appeals process as outlined in Section 6.5 of [IETF-
PROCESS]. Since the designated experts are appointed by the IESG,
they may be removed by the IESG.
The following are example policies, some of which are in use today:
Private Use - 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. There is no need for IANA to review
such assignments and assignments are not generally useful for
Examples: Site-specific options in DHCP [DHCP] have
significance only within a single site. "X-foo:" header
lines in email messages.
Hierarchical allocation - Delegated managers can assign values
provided they have been given control over that part of the
name space. IANA controls the higher levels of the namespace
according to one of the other policies.
Examples: DNS names, Object Identifiers
First Come First Served - Anyone can obtain an assigned number, so
long as they provide a point of contact and a brief
description of what the value would be used for. For
numbers, the exact value is generally assigned by the IANA;
with names, specific names are usually requested.
Examples: vnd. (vendor assigned) MIME types [MIME-REG], TCP
and UDP port numbers.
Expert Review - approval by a Designated Expert is required.
Specification Required - Values and their meaning must be
documented in an RFC or other permanent and readily available
reference, in sufficient detail so that interoperability
between independent implementations is possible.
Examples: SCSP [SCSP]
IESG Approval - New assignments must be approved by the IESG, but
there is no requirement that the request be documented in an
RFC (though the IESG has discretion to request documents or
other supporting materials on a case-by-case basis).
IETF Consensus - New values are assigned through the IETF
consensus process. Specifically, new assignments are made via
RFCs approved by the IESG. Typically, the IESG will seek
input on prospective assignments from appropriate persons
(e.g., a relevant Working Group if one exists).
Examples: SMTP extensions [SMTP-EXT], BGP Subsequent Address
Family Identifiers [BGP4-EXT].
Standards Action - Values are assigned only for Standards Track
RFCs approved by the IESG.
Examples: MIME top level types [MIME-REG]
It should be noted that it often makes sense to partition a name
space into several categories, with assignments out of each category
handled differently. For example, the DHCP option space [DHCP] is
split into two parts. Option numbers in the range of 1-127 are
globally unique and assigned according to the Specification Required
policy described above, while options number 128-254 are "site
specific", i.e., Local Use. Dividing the name space up makes it
possible to allow some assignments to be made with minimal review,
while simultaneously reserving some part of the space for future use.
3. Registration maintenance
Registrations are a request for an assigned number, including the
related information needed to evaluate and document the request. Even
after a number has been assigned, some types of registrations contain
additional information that may need to be updated over time. For
example, mime types, character sets, language tags, etc. typically
include more information than just the registered value itself.
Example information can include point of contact information,
security issues, pointers to updates, literature references, etc. In
such cases, the document must clearly state who is responsible for
maintaining and updating a registration. It is appropriate to:
- Let the author update the registration, subject to the same
constraints and review as with new registrations.
- Allow some mechanism to attach comments to the registration, for
cases where others have significant objections to claims in a
registration, but the author does not agree to change the
- Designate the IESG or another authority as having the right to
reassign ownership of a registration. This is mainly to get
around the problem when some registration owner cannot be
reached in order to make necessary updates.
4. What To Put In Documents
The previous sections presented some issues that should be considered
in formulating a policy for assigning well-known numbers and other
protocol constants. It is the Working Group and/or document author's
job to formulate an appropriate policy and specify it in the
appropriate document. In some cases, having an "IANA Considerations"
section may be appropriate. Specifically, documents that create an
name space (or modify the definition of an existing space) and that
expect the IANA to play a role in maintaining that space (e.g.,
serving as a repository for registered values) MUST document the
process through which future assignments are made. Such a section
MUST state clearly:
- whether or not an application for an assigned number needs to be
reviewed. If review is necessary, the review mechanism MUST be
specified. When a Designated Expert is used, documents MUST NOT
name the Designated Expert in the document itself; instead, the
name should be relayed to the appropriate IESG Area Director at
the time the document is sent to the IESG for approval.
- If the request should also be reviewed on a specific public
mailing list (such as the email@example.com for media types),
that mailing address should be specified. Note, however, that
use of a Designated Expert MUST also be specified.
- if the IANA is expected to make assignments without requiring an
outside review, sufficient guidance MUST be provided so that the
requests can be evaluated with minimal subjectivity.
Authors SHOULD attempt to provide guidelines that allow the IANA to
assign new values directly without requiring review by a Designated
Expert. This can be done easily in many cases by designating a range
of values for direct assignment by the IANA while simultaneously
reserving a sufficient portion of the name space for future use by
requiring that assignments from that space be made only after a more
Finally, it is quite acceptable to pick one of the example policies
cited above and refer to it by name. For example, a document could
say something like:
Following the policies outlined in [IANA-CONSIDERATIONS],
numbers in the range 0-63 are allocated as First Come First
Served, numbers between 64-240 are allocated through an IETF
Consensus action and values in the range 241-255 are reserved
for Private Use.
For examples of documents that provide good and detailed guidance to
the IANA on the issue of assigning numbers, consult [MIME-REG, MIME-
5. Applicability to Past and Future RFCs
For all existing RFCs that either explicitly or implicitly rely on
the IANA to evaluate assignments without specifying a precise
evaluation policy, the IANA will continue to decide what policy is
appropriate. The default policy has been first come, first served.
Changes to existing policies can always be initiated through the
normal IETF consensus process.
All future RFCs that either explicitly or implicitly rely on the IANA
to register or otherwise manage assignments MUST provide guidelines
for managing the name space.
6. Security Considerations
Information that creates or updates a registration needs to be
Information concerning possible security vulnerabilities of a
protocol may change over time. Likewise, security vulnerabilities
related to how an assigned number is used (e.g., if it identifies a
protocol) may change as well. As new vulnerabilities are discovered,
information about such vulnerabilities may need to be attached to
existing registrations, so that users are not mislead as to the true
security issues surrounding the use of a registered number.
An analysis of security issues is required for all parameters (data
types, operation codes, keywords, etc.) used in IETF protocols or
registered by the IANA. All descriptions of security issues must be
as accurate as possible regardless of level of registration. In
particular, a statement that there are "no security issues associated
with this type" must not given when it would be more accurate to
state that "the security issues associated with this type have not
Jon Postel and Joyce K. Reynolds provided a detailed explanation on
what the IANA needs in order to manage assignments efficiently, and
patiently provided comments on multiple versions of this document.
Brian Carpenter provided helpful comments on earlier versions of the
document. One paragraph in the Security Considerations section was
borrowed from [MIME-REG].
[ASSIGNED] Reynolds, J., and J. Postel, "Assigned
Numbers", STD 2, RFC 1700, October 1994. See
[BGP4-EXT] Bates. T., Chandra, R., Katz, D. and Y.
Rekhter, "Multiprotocol Extensions for BGP-4",
RFC 2283, February 1998.
[DHCP-OPTIONS] Alexander, S. and R. Droms, "DHCP Options and
BOOTP Vendor Extensions", RFC 2132, March 1997.
[IANA-CONSIDERATIONS] Alvestrand, H. and T. Narten, "Guidelines for
Writing an IANA Considerations Section in
RFCs", BCP 26, RFC 2434, October 1998.
[IETF-PROCESS] Bradner, S., "The Internet Standards Process --
Revision 3", BCP 9, RFC 2026, October 1996.
[IP] Postel, J., "Internet Protocol", STD 5, RFC
791, September 1981.
[IPSEC] Atkinson, R., "Security Architecture for the
Internet Protocol", RFC 1825, August 1995.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
[MIME-LANG] Freed, N. and K. Moore, "MIME Parameter Value
and Encoded Word Extensions: Character Sets,
Languages, and Continuations", RFC 2184, August
[MIME-REG] Freed, N., Klensin, J. and J. Postel,
"Multipurpose Internet Mail Extension (MIME)
Part Four: Registration Procedures", RFC 2048,
[SCSP] Luciani, J., Armitage, G. and J. Halpern,
"Server Cache Synchronization Protocol (SCSP)",
RFC 2334, April 1998.
[SMTP-EXT] Klensin, J., Freed, N., Rose, M., Stefferud, E.
and D. Crocker, "SMTP Service Extensions", RFC
1869, November 1995.
9. Authors' Addresses
3039 Cornwallis Ave.
PO Box 12195 - BRQA/502
Research Triangle Park, NC 27709-2195
Harald Tveit Alvestrand
Phone: +47 73 54 57 97
10. Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
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