Network Working Group R. Bush
Request for Comments: 3967 IIJ
BCP: 97 T. Narten
Category: Best Current Practice IBM Corporation
December 2004 Clarifying when Standards Track Documents may Refer
Normatively to Documents at a Lower Level
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 (2004).
IETF procedures generally require that a standards track RFC may not
have a normative reference to another standards track document at a
lower maturity level or to a non standards track specification (other
than specifications from other standards bodies). For example, a
standards track document may not have a normative reference to an
informational RFC. Exceptions to this rule are sometimes needed as
the IETF uses informational RFCs to describe non-IETF standards or
IETF-specific modes of use of such standards. This document
clarifies and updates the procedure used in these circumstances.
The Internet Standards Process [RFC2026] Section 4.2.4 specifies the
Standards track specifications normally must not depend on other
standards track specifications which are at a lower maturity level
or on non standards track specifications other than referenced
specifications from other standards bodies.
One intent is to avoid creating a perception that a standard is more
mature than it actually is.
It should also be noted that Best Current Practice documents
[RFC1818] have generally been considered similar to Standards Track
documents in terms of what they can reference. For example, a
normative reference to an Experimental RFC has been considered an
improper reference per [RFC2026].
1.1. Normative References
Within an RFC, references to other documents fall into two general
categories: "normative" and "informative". Broadly speaking, a
normative reference specifies a document that must be read to fully
understand or implement the subject matter in the new RFC, or whose
contents are effectively part of the new RFC, as its omission would
leave the new RFC incompletely specified. An informative reference
is not normative; rather, it provides only additional background
An exact and precise definition of what is (and is not) a normative
reference has proven challenging in practice, as the details and
implications can be subtle. Moreover, whether a reference needs to
be normative can depend on the context in which a particular RFC is
being published in the first place. For example, in the context of
an IETF Standard, it is important that all dependent pieces be
clearly specified and available in an archival form so that there is
no disagreement over what constitutes a standard. This is not always
the case for other documents.
The rest of this section provides guidance on what might (and might
not) be considered normative in the context of the IETF standards
In the IETF, it is a basic assumption that implementors must have a
clear understanding of what they need to implement in order to be
fully compliant with a standard and to be able to interoperate with
other implementations of that standard. For documents that are
referenced, any document that includes key information an implementer
needs would be normative. For example, if one needs to understand a
packet format defined in another document in order to fully implement
a specification, the reference to that format would be normative.
Likewise, if a reference to a required algorithm is made, the
reference would be normative.
Some specific examples:
- If a protocol relies on IPsec to provide security, one cannot
fully implement the protocol unless the specification for IPsec is
available; hence, the reference would be normative.
The referenced specification would likely include details about
specific key management requirements, which transforms are
required and which are optional, etc.
- In MIB documents, an IMPORTS clause by definition is a normative
- When a reference to an example is made, such a reference need not
be normative. For example, text such as "an algorithm such as the
one specified in [RFCxxxx] would be acceptable" indicates an
informative reference, since that cited algorithm is just one of
several possible algorithms that could be used.
2. The Need for Downward References
There are a number of circumstances in which an IETF document may
need to make a normative reference to a document at a lower maturity
level, but such a reference conflicts with Section 4.2.4 of
[RFC2026]. For example:
o A standards track document may need to refer to a protocol or
algorithm developed by an external body but modified, adapted, or
profiled by an IETF informational RFC, for example, MD5 [RFC1321]
and HMAC [RFC2104]. Note that this does not override the IETF's
duty to see that the specification is indeed sufficiently clear to
enable creation of interoperable implementations.
o A standards document may need to refer to a proprietary protocol,
and the IETF normally documents proprietary protocols using
o A migration or co-existence document may need to define a
standards track mechanism for migration from, and/or co-existence
with, an historic protocol, a proprietary protocol, or possibly a
non-standards track protocol.
o There are exceptional procedural or legal reasons that force the
target of the normative reference to be an informational or
historical RFC or to be at a lower standards level than the
o A BCP document may want to describe best current practices for
experimental or informational specifications.
3. The Procedure to Be Used
For Standards Track or BCP documents requiring normative reference to
documents of lower maturity, the normal IETF Last Call procedure will
be issued, with the need for the downward reference explicitly
documented in the Last Call itself. Any community comments on the
appropriateness of downward references will be considered by the IESG
as part of its deliberations.
Once a specific down reference to a particular document has been
accepted by the community (e.g., has been mentioned in several Last
Calls), an Area Director may waive subsequent notices in the Last
Call of down references to it. This should only occur when the same
document (and version) are being referenced and when the AD believes
that the document's use is an accepted part of the community's
understanding of the relevant technical area. For example, the use
of MD5 [RFC1321] and HMAC [RFC2104] is well known among
This procedure should not be used if the proper step is to move the
document to which the reference is being made into the appropriate
category. It is not intended as an easy way out of normal process.
Rather, the procedure is intended for dealing with specific cases
where putting particular documents into the required category is
problematic and unlikely ever to happen.
4. Security Considerations
This document is not known to create any new vulnerabilities for the
Internet. On the other hand, inappropriate or excessive use of the
process might be considered a downgrade attack on the quality of IETF
standards or, worse, on the rigorous review of security aspects of
This document is the result of discussion within the IESG, with
particular contribution by Harald Alvestrand, Steve Bellovin, Scott
Bradner, Ned Freed, Allison Mankin, Jeff Schiller, and Bert Wijnen.
8. Full Copyright Statement
Copyright (C) The Internet Society (2004).
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