Network Working Group                                         S. Bradner
Request for Comments: 4775                                       Harvard
BCP: 125                                               B. Carpenter, Ed.
Category: Best Current Practice                                T. Narten
                                                                     IBM
                                                           December 2006

Procedures for Protocol Extensions and Variations

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 Notice

Copyright (C) The IETF Trust (2006).

Abstract

   This document discusses procedural issues related to the
   extensibility of IETF protocols, including when it is reasonable to
   extend IETF protocols with little or no review, and when extensions
   or variations need to be reviewed by the IETF community.  Experience
   has shown that extension of protocols without early IETF review can
   carry risk.  The document also recommends that major extensions to or
   variations of IETF protocols only take place through normal IETF
   processes or in coordination with the IETF.

   This document is directed principally at other Standards Development
   Organizations (SDOs) and vendors considering requirements for
   extensions to IETF protocols.  It does not modify formal IETF
   processes.

   1. Introduction ....................................................2
   2. Technical Risks in Extensions ...................................3
   3. General Considerations ..........................................4
      3.1. The Importance of Interoperability .........................4
      3.2. Registered Values and the Importance of IANA Assignments ...5
      3.3. Significant Extensions Require Technical Review ............5
      3.4. Quality and Consistency ....................................6
      3.5. The Role of Formal Liaisons ................................6
   4. Procedure for Review of Extensions ..............................7
   5. Some Specific Issues ...........................................10
   6. Intellectual Property ..........................................10
   7. Security Considerations ........................................10
   8. IANA Considerations ............................................11
   9. Acknowledgements ...............................................11
   10. References ....................................................11
      10.1. Normative References .....................................11
      10.2. Informative References ...................................12

1. Introduction

   BCP 9 [RFC2026] is the current definition of the IETF standards
   track.  This process applies not only to the initial definition of a
   protocol, but also to any subsequent updates, such that continued
   interoperability can be guaranteed.  However, it is not always clear
   whether extensions to a protocol should be made within the IETF
   process, especially when they originate outside the IETF community.
   This document lays down guidelines and procedures for such
   extensions.

   When developing protocols, IETF Working Groups (WGs) typically
   include mechanisms whereby these protocols can be extended in the
   future.  It is, of course, a good principle to design extensibility
   into protocols; a common definition of a successful protocol is one
   that becomes widely used in ways not originally anticipated.  Well-
   designed extensibility mechanisms facilitate the evolution of
   protocols and help make it easier to roll out incremental changes in
   an interoperable fashion.  At the same time, experience has shown
   that extensibility features should be limited to what is clearly
   necessary when the protocol is developed, and any later extensions
   should be done carefully and with a full understanding of the base
   protocol, existing implementations, and current operational practice.
   However, it is not the purpose of this document to describe the
   architectural principles of sound extensibility design.

   When extensions to IETF protocols are made within the IETF, the
   normal IETF process is followed, including the normal processes for
   IETF-wide review and IESG approval.  Extensions developed in this way
   should respect the same architectural principles and technical
   criteria as any other IETF work.

   In addition to the IETF itself, other Standards Development
   Organizations (SDOs), vendors, and technology fora may identify a
   requirement for an extension to an IETF protocol.  The question
   addressed by this document is how such bodies should proceed.  There
   are several possible scenarios:

   1.  The requirement is straightforward and within the scope of
       whatever extension mechanism the base protocol includes.

   2.  The requirement is, or may be, outside that scope, and:
       1.  The IETF still has an active WG in the area;
       2.  The IETF has no active WG, but has relevant expertise;
       3.  The IETF no longer has a nucleus of relevant expertise.

   Especially in the latter three cases, there are technical risks in
   extension design, described in the next section.  These risks are
   higher when extensions to IETF protocols are made outside the IETF
   and without consulting the IETF.

   This document is focused on appropriate procedures and practices to
   minimize the chance that extensions developed outside the IETF will
   encounter these risks and, therefore, become useless or, worse,
   damaging to interoperability.  Architectural considerations are
   documented elsewhere.  This document is directed principally at other
   SDOs and vendors considering requirements for extensions to IETF
   protocols.  It does not modify formal IETF processes.

   The IETF claims no special position.  Everything said here about IETF
   protocols would apply with equal force to protocols specified by any
   SDO.  The IETF should follow whatever procedures another SDO lays
   down for extensions to its own protocols, if the IETF identifies a
   need for such an extension.

2. Technical Risks in Extensions

   Extensions may be developed without full understanding of why the
   existing protocol was designed the way that it is -- e.g., what ideas
   were brought up during the original development and rejected because
   of some problem with them.  Also, extensions could unintentionally
   negate some key function of the existing protocol (such as security
   or congestion control).  Design choices can be made without analyzing
   their impact on the protocol as a whole, and basic underlying

   architectural principles of the protocol can be violated.  Also,
   there is a risk that mutually incompatible extensions may be
   developed independently.

   Of course, the IETF itself is not immune to such mistakes, suggesting
   a need for WGs to document their design decisions (including paths
   rejected) and some rationale for those decisions, for the benefit of
   both those within the IETF and those outside the IETF, perhaps years
   later.

   Documentation of non-IETF extensions can sometimes be hard to obtain,
   so assessing the quality of the specification, verifying
   interoperability, and verifying compatibility with other extensions
   (including past and future extensions) can be hard or impossible.

   A set of interrelated extensions to multiple protocols typically
   carries a greater danger of interoperability issues or
   incompatibilities than a simple extension.  Consequently, it is
   important that such proposals receive earlier and more in-depth
   review than unitary extensions.

   All that can be said about extensions applies with equal or greater
   force to variations -- in fact, by definition, protocol variations
   damage interoperability.  They must, therefore, be intensely
   scrutinized.  An extension adds features and, if well designed,
   allows interoperability between old and new implementations.  A
   variation modifies features in such a way that old and new
   implementations may not interoperate.  Throughout this document, what
   is said about extensions also applies to variations.

3. General Considerations

3.1. The Importance of Interoperability

   According to its Mission Statement [RFC3935], the IETF produces high
   quality, relevant technical and engineering documents, including
   protocol standards.  The mission statement goes on to say that the
   benefit of these standards to the Internet "is in interoperability -
   that multiple products implementing a standard are able to work
   together in order to deliver valuable functions to the Internet's
   users".

   One consequence of this mission is that the IETF designs protocols
   for the single Internet.  The IETF expects its protocols to work the
   same everywhere.  Protocol extensions designed for limited
   environments may be reasonable provided that products with these
   extensions interoperate with products without the extensions.
   Extensions that break interoperability are unacceptable when products

   with and without the extension are mixed.  It is the IETF's
   experience that this tends to happen on the Internet even when the
   original designers of the extension did not expect this to happen.

   Another consequence of this definition of interoperability is that
   the IETF values the ability to exchange one product implementing a
   protocol with another.  The IETF often specifies mandatory-to-
   implement functionality as part of its protocols so that there is a
   core set of functionality sufficient for interoperability that all
   products implement.  The IETF tries to avoid situations where
   protocols need to be profiled to specify which optional features are
   required for a given environment, because doing so harms
   interoperability on the Internet as a whole.

   The IETF, and in particular the IESG, will apply these considerations
   when evaluating protocol extensions proposed inside or outside the
   IETF.

3.2. Registered Values and the Importance of IANA Assignments

   An extension is often likely to make use of additional values added
   to an existing IANA registry (in many cases, simply by adding a new
   "TLV" (type-length-value) field).  It is essential that such new
   values are properly registered by the applicable procedures,
   including expert review where applicable (see BCP 26, [RFC2434]).
   Extensions may even need to create new IANA registries in some cases.

   Experience shows that the importance of this is often underestimated
   during extension design; designers sometimes assume that a new
   codepoint is theirs for the asking, or even simply for the taking.
   This is hazardous; it is far too likely that someone just taking a
   protocol value will find that the same value will later be formally
   assigned to another function, thus guaranteeing an interoperability
   problem.

   In many cases, IANA assignment requests trigger a thorough technical
   review of the proposal by a designated IETF expert reviewer.
   Requests are sometimes refused after such a review.  Thus, extension
   designers must pay particular attention to any needed IANA
   assignments and to the applicable criteria.

3.3. Significant Extensions Require Technical Review

   Some extensions may be considered minor (e.g., adding a
   straightforward new TLV to an application protocol, which will only
   impact a subset of hosts) and some may be considered major (e.g.,
   adding a new IP option type, which will potentially impact every node
   on the Internet).  This is essentially a matter of judgment.  It

   could be argued that anything requiring at most Expert Review in
   [RFC2434] is probably minor, and anything beyond that is major.
   However, even an apparently minor extension may have unforeseen
   consequences on interoperability.  Thus, the distinction between
   major and minor is less important than ensuring that the right amount
   of technical review takes place in either case.  In general, the
   expertise for such review lies within the same SDO that developed the
   original protocol.  Therefore, the expertise for such review for IETF
   protocols lies within the IETF.

   There may sometimes be doubt whether a particular proposal is or is
   not truly a protocol extension.  When in doubt, it is preferable to
   err on the side of additional review.  However, it should be noted
   that if an 'extension' only consists of registering a new value with
   IANA in a First Come First Served registry [RFC2434], this document
   is not intended to require formal IETF review.  Informal review by
   experts may, nevertheless, be valuable.  In other cases (Section 5),
   there is a well-specified procedure for extensions that should be
   followed.

   The only safe rule is that, even if an extension appears minor to the
   person proposing it, early review by subject matter experts is
   advisable.  For protocols that have been developed in the IETF, the
   appropriate forum for such review is the IETF, either in the relevant
   WG or Area, or by individual IETF experts if no such WG exists.

3.4. Quality and Consistency

   In order to be adequately reviewed by relevant experts, a proposed
   extension must be documented in a clear and well-written
   specification that IETF subject matter experts have access to and can
   review.  Ideally, such a document would be published as an Internet
   Draft, using terminology and content that is sufficiently consistent
   with the unextended specification that these experts can readily
   identify the technical changes proposed at an early stage.

3.5. The Role of Formal Liaisons

   The IETF has formal liaisons in place with a number of SDOs;
   documentation of the liaison process is in [RFC4052], [RFC4053], and
   [RFC4691].  These liaison channels should be used as relevant for
   discussing and reviewing extensions, as should informal communication
   at the engineering level (for example, experts from other SDOs are
   welcome to participate in IETF meetings and mailing lists).  Where
   formal liaison does not exist, the point of contact in the IETF
   should be the Chairs of relevant WGs, the most appropriate Area
   Director, or, in case of doubt, the IESG as a whole.

4. Procedure for Review of Extensions

   In some cases, explicit provision is made in the relevant RFCs for
   extending individual IETF protocols.  Nothing in this document
   overrides such procedures.  Some such cases are mentioned in
   Section 5.

   There are several ways in which an extension to an IETF protocol can
   be considered for publication as an RFC:

   1.  Extensions to IETF protocols developed within the IETF will be
       subject to the normal IETF process, exactly like new designs.  It
       is not suggested that this is a panacea; appropriate cross-
       working-group and cross-area review is needed within the IETF to
       avoid oversights and mistakes.

   2.  Extensions to IETF protocols discussed in an IRTF Research Group
       may well be the prelude to regular IETF discussion.  However, a
       Research Group may desire to specify an experimental extension
       before the work is mature enough for IETF processing.  In this
       case, the Research Group is required to involve appropriate IETF
       or IANA experts in their process to avoid oversights.

   3.  Extensions to IETF protocols described in Independent Submissions
       to the RFC Editor are subject to IESG review, currently described
       in BCP 92 [RFC3932].  If appropriate, the IESG advises the RFC
       Editor that full IETF processing is needed, or that relevant IANA
       procedures need to be followed before publication can proceed.
       Note that Independent Submissions cannot be placed on the IETF
       Standards Track; they would need to enter full IETF processing.

   Where vendors or other SDOs identify a requirement for extending an
   IETF protocol, their first step should be to consider the scenarios
   listed in Section 1.  If the requirement is straightforward and
   within the scope of a documented extension mechanism, the way is
   clear, and the documented mechanism must be followed.  If these two
   conditions are not met, the next step should be to contact the
   relevant IETF Area Director to check whether there is an active WG in
   the area or, at least, relevant expertise available in the IETF.  At
   this point, it will be possible to select the most appropriate of the
   above three routes.  Regular IETF process is most likely to be
   suitable, assuming sufficient interest can be found in the IETF
   community.  IRTF process is unlikely to be suitable unless there is a
   genuine research context for the proposed extension.

   In the event that the IETF no longer has relevant expertise, there
   are still two choices to discuss with the Area Director: bring the
   work to the IETF (i.e., the IETF imports expertise) or reach mutual
   agreement to do the work elsewhere (i.e., the IETF explicitly exports
   change control).

   In the case of an SDO that identifies a requirement for a
   standardized extension, a standards development process within the
   IETF (while maintaining appropriate liaison) is strongly recommended
   in preference to publishing a non-IETF standard.  Otherwise, the
   implementor community will be faced with a standard split into two or
   more parts in different styles, obtained from different sources, with
   no unitary control over quality, compatibility, interoperability, and
   intellectual property conditions.  Note that, since participation in
   the IETF is open, there is no formality or restriction for
   participants in other SDOs choosing to work in the IETF as well.  In
   some cases (see Section 5), the IETF has well-defined procedures for
   this in place.

   Naturally, SDOs can and do develop scenarios, requirements, and
   architectures based on IETF specifications.  It is only actual
   protocol extensions and changes that need to go through the IETF
   process.  However, there is large risk of wasted effort if
   significant investment is made in planning stages for use of IETF
   technology without early review and feedback from the IETF.  Other
   SDOs are encouraged to communicate informally or formally with the
   IETF as early as possible, to avoid false starts.  Early technical
   review in a collaborative spirit is of great value.  Each SDO can
   "own" its ideas and discuss them in its own fora, but should start
   talking to the IETF experts about those ideas the moment the idea is
   well formulated.  It is understood that close collaboration may be
   needed in order that the IETF experts correctly understand the
   systems architecture envisaged by the other SDO.  This is much
   preferable to a situation where another SDO presents the IANA and the
   IETF with a 'fait accompli.'

   Vendors that identify a requirement for an extension are strongly
   recommended to start informal discussion in the IETF and to publish a
   preliminary Internet Draft describing the requirements.  This will
   allow the vendor, and the community, to evaluate whether there is
   community interest and whether there are any major or fundamental
   issues.  However, in the case of a vendor that identifies a
   requirement for a proprietary extension that does not generate
   interest in the IETF (or IRTF) communities, an Independent Submission
   to the RFC Editor is strongly recommended in preference to publishing
   a proprietary document.  Not only does this bring the draft to the

   attention of the community, but it also ensures a minimum of review
   [RFC3932], and (if published as an RFC) makes the proprietary
   extension available to the whole community.

   If, despite these recommendations, a vendor or SDO does choose to
   publish its own specification for an extension to an IETF protocol,
   the following guidance applies:

   o  Extensions to IETF protocols should be well, and publicly,
      documented, and reviewed at an early stage by the IETF community
      to be sure that the extension does not undermine basic assumptions
      and safeguards designed into the protocol (such as security
      functions) or its architectural integrity.

   o  Vendors and other SDOs are formally requested to submit any such
      proposed publications for IETF review, and are invited to actively
      participate in the IETF process.  Submission may be by an
      established liaison channel if it exists, or by direct
      communication with the relevant WG or the IESG.  This should be
      done at an early stage, before a large investment of effort has
      taken place, in case basic problems are revealed.  When there is a
      formal liaison in place between the other SDO and the IETF, the
      liaison channel should be used to ensure that review takes place,
      both by relevant experts and by established review teams or
      Directorates within the IETF.  If there is no formal liaison, the
      other SDO or vendor should ask the IESG (or a relevant Area
      Director) to obtain such reviews.  Note that general aspects such
      as security, internationalization, and management may need review,
      as well as the protocol as such.

   o  In the case of extensions involving only routine IANA parameter
      assignments, for which there is an underlying IETF specification
      containing clear IANA Considerations, this request is satisfied as
      long as those considerations are satisfied (see [RFC2434]).
      Anything beyond this requires an explicit protocol review by
      experts within the IETF.

   o  Note that, like IETF specifications, such proposed publications
      must include an IANA Considerations section to ensure that
      protocol parameter assignments that are needed to deploy
      extensions are not made until after a proposed extension has
      received adequate review, and then to ensure that IANA has precise
      guidance on how to make those assignments.

5. Some Specific Issues

   It is relatively common for MIB modules, which are all, in effect,
   extensions of the SMI data model, to be defined or extended outside
   the IETF.  BCP 111 [RFC4181] offers detailed guidance for authors and
   reviewers.

   A number of protocols have foreseen experimental values for certain
   IANA parameters, so that experimental usages and extensions may be
   tested without need for a special parameter assignment.  It must be
   stressed that such values are not intended for production use or as a
   way to evade the type of technical review described in this document.
   See [RFC3692] and [RFC4727].

   RADIUS [RFC2865] is designed to carry attributes and allow definition
   of new attributes.  But it is important that discussion of new
   attributes involve the IETF community of experts knowledgeable about
   the protocol's architecture and existing usage in order to fully
   understand the implications of a proposed extension.  Adding new
   attributes without such discussion creates a high risk of
   interoperability or functionality failure.  For this reason among
   others, the IETF has an active RADIUS Extensions WG at the time of
   writing.

   There are certain documents that specify a change process for
   specific IETF protocols, such as:
      The SIP change process [RFC3427]
      The (G)MPLS change process [CHANGEPROC]

   This document does not override such specific change processes.

6. Intellectual Property

   All IETF documents fall under the IETF's intellectual property rules,
   BCP 78 [RFC3978] and BCP 79 [RFC3979], as amended.  In particular,
   there are restrictions on the production of derivative works, and
   there are rights that remain with the original authors.  Anybody
   outside the IETF considering an extension based on an IETF document
   must bear these legal restrictions and rights in mind.

7. Security Considerations

   An extension must not introduce new security risks without also
   providing an adequate counter-measure, and in particular it must not
   inadvertently defeat security measures in the unextended protocol.
   This aspect must always be considered during IETF review.

8. IANA Considerations

   The IETF requests IANA to pay attention to the requirements of this
   document when requested to make protocol parameter assignments for
   vendors or other SDOs, i.e., to respect the IANA Considerations of
   all RFCs that contain them, and the general considerations of BCP 26
   [RFC2434].

9. Acknowledgements

   This document is heavily based on an earlier draft under a different
   title by Scott Bradner and Thomas Narten.

   That earlier draft stated: The initial version of this document was
   put together by the IESG in 2002.  Since then, it has been reworked
   in response to feedback from John Loughney, Henrik Levkowetz, Mark
   Townsley, Randy Bush, Bernard Aboba, and others.

   Ted Hardie, Scott Brim, Dan Romascanu, Jari Arkko, Loa Andersson,
   Adrian Farrel, Roy Fielding, Keith Moore, Bernard Aboba, Elwyn
   Davies, Stephen Trowbridge, and Ted Ts'o also made valuable comments
   on this document.

   Sam Hartman contributed the section on interoperability.

   This document was produced using the xml2rfc tool [RFC2629].

10. References

10.1. Normative References

   [RFC2026]    Bradner, S., "The Internet Standards Process -- Revision
                3", BCP 9, RFC 2026, October 1996.

   [RFC2434]    Narten, T. and H. Alvestrand, "Guidelines for Writing an
                IANA Considerations Section in RFCs", BCP 26, RFC 2434,
                October 1998.

   [RFC3427]    Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J.,
                and B. Rosen, "Change Process for the Session Initiation
                Protocol (SIP)", BCP 67, RFC 3427, December 2002.

   [RFC3692]    Narten, T., "Assigning Experimental and Testing Numbers
                Considered Useful", BCP 82, RFC 3692, January 2004.

   [RFC3932]    Alvestrand, H., "The IESG and RFC Editor Documents:
                Procedures", BCP 92, RFC 3932, October 2004.

   [RFC3935]    Alvestrand, H., "A Mission Statement for the IETF",
                BCP 95, RFC 3935, October 2004.

   [RFC3978]    Bradner, S., "IETF Rights in Contributions", BCP 78,
                RFC 3978, March 2005.

   [RFC3979]    Bradner, S., "Intellectual Property Rights in IETF
                Technology", BCP 79, RFC 3979, March 2005.

   [RFC4052]    Daigle, L. and Internet Architecture Board, "IAB
                Processes for Management of IETF Liaison Relationships",
                BCP 102, RFC 4052, April 2005.

   [RFC4053]    Trowbridge, S., Bradner, S., and F.  Baker, "Procedures
                for Handling Liaison Statements to and from the IETF",
                BCP 103, RFC 4053, April 2005.

   [RFC4181]    Heard, C., "Guidelines for Authors and Reviewers of MIB
                Documents", BCP 111, RFC 4181, September 2005.

   [RFC4727]    Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
                ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.

10.2. Informative References

   [CHANGEPROC] Andersson, L. and A. Farrel, "Change Process for
                Multiprotocol Label Switching (MPLS) and Generalized
                MPLS  (GMPLS) Protocols and Procedures", Work in
                Progress, October 2006.

   [RFC2629]    Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
                June 1999.

   [RFC2865]    Rigney, C., Willens, S., Rubens, A., and W. Simpson,
                "Remote Authentication Dial In User Service (RADIUS)",
                RFC 2865, June 2000.

   [RFC4691]    Andersson, L., "Guidelines for Acting as an IETF Liaison
                to Another Organization", RFC 4691, October 2006.

Authors' Addresses

   Scott Bradner
   Harvard University
   29 Oxford St.
   Cambridge, MA  02138
   US

   EMail: sob@harvard.edu

   Brian Carpenter, Ed.
   IBM
   8 Chemin de Blandonnet
   1214 Vernier
   Switzerland

   EMail: brc@zurich.ibm.com

   Thomas Narten
   IBM
   3039 Cornwallis Ave.
   PO Box 12195 - BRQA/502
   Research Triangle Park, NC  27709-2195
   US

   EMail: narten@us.ibm.com

Full Copyright Statement

   Copyright (C) The IETF Trust (2006).

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Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.