Network Working Group                                       B. Carpenter
Internet-Draft                                             B. Aboba (ed)
Intended Status: Informational                               S. Cheshire
Expires: April 30, 2012                      Internet Architecture Board
                                                         30 October 2011

             Design Considerations for Protocol Extensions


   This document discusses issues related to the extensibility of
   Internet protocols, with a focus on architectural design
   considerations.  It is intended to assist designers of both base
   protocols and extensions.  Case studies are included.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on April 30, 2012.

IAB                           Informational                     [Page 1]

Internet-Draft    Design Considerations for Extensions   30 October 2011

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

IAB                           Informational                     [Page 2]

Internet-Draft    Design Considerations for Extensions   30 October 2011

Table of Contents

1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
  1.1   Requirements Language  . . . . . . . . . . . . . . . . . .  5
2.  Routine and Major Extensions . . . . . . . . . . . . . . . . .  5
  2.1   When is an Extension Routine?  . . . . . . . . . . . . . .  5
  2.2   What Constitutes a Major Extension?  . . . . . . . . . . .  6
3.  Architectural Principles . . . . . . . . . . . . . . . . . . .  7
  3.1   Limited Extensibility  . . . . . . . . . . . . . . . . . .  8
  3.2   Design for Global Interoperability . . . . . . . . . . . .  8
  3.3   Architectural Compatibility  . . . . . . . . . . . . . . .  9
  3.4   Protocol Variations  . . . . . . . . . . . . . . . . . . . 11
  3.5   Testability  . . . . . . . . . . . . . . . . . . . . . . . 12
  3.6   Parameter Parameter Registration . . . . . . . . . . . . . 13
  3.7   Extensions to Critical Protocols . . . . . . . . . . . . . 15
4.  Considerations for the Base Protocol . . . . . . . . . . . . . 15
  4.1   Version Numbers  . . . . . . . . . . . . . . . . . . . . . 17
  4.2   Reserved Fields  . . . . . . . . . . . . . . . . . . . . . 20
  4.3   Encoding Formats . . . . . . . . . . . . . . . . . . . . . 20
  4.4   Parameter Space Design . . . . . . . . . . . . . . . . . . 21
  4.5   Cryptographic Agility  . . . . . . . . . . . . . . . . . . 23
  4.6   Transport  . . . . . . . . . . . . . . . . . . . . . . . . 24
  4.7   Handling of Unknown Extensions . . . . . . . . . . . . . . 25
5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 26
6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26
7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
  7.1   Normative References . . . . . . . . . . . . . . . . . . . 27
  7.2   Informative References . . . . . . . . . . . . . . . . . . 27
Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . . . 31
IAB Members . .  . . . . . . . . . . . . . . . . . . . . . . . . . 31
Appendix A - Examples  . . . . . . . . . . . . . . . . . . . . . . 32
  A.1   Already documented cases . . . . . . . . . . . . . . . . . 32
  A.2   RADIUS Extensions  . . . . . . . . . . . . . . . . . . . . 32
  A.3   TLS Extensions . . . . . . . . . . . . . . . . . . . . . . 33
  A.4   L2TP Extensions  . . . . . . . . . . . . . . . . . . . . . 35
Change log . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37

IAB                           Informational                     [Page 3]

Internet-Draft    Design Considerations for Extensions   30 October 2011

1.  Introduction

   When developing protocols, IETF Working Groups (WGs) often include
   mechanisms whereby these protocols can be extended in the future.  It
   is a good principle to design extensibility into protocols; as
   described in "What Makes for a Successful Protocol" [RFC5218], 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.

   When an initial protocol design is extended, there is always a risk
   of unintended consequences, such as interoperability problems or
   security vulnerabilities.  This risk is especially high if the
   extension is performed by a different team than the original
   designers, who may stray outside implicit design constraints or
   assumptions.  As a result, extensions should be done carefully and
   with a full understanding of the base protocol, existing
   implementations, and current operational practice.

   The proliferation of extensions, even well designed ones, can be
   costly.  As noted in "Simple Mail Transfer Protocol" [RFC5321]
   Section 2.2.1:

      Experience with many protocols has shown that protocols with few
      options tend towards ubiquity, whereas protocols with many options
      tend towards obscurity.

      Each and every extension, regardless of its benefits, must be
      carefully scrutinized with respect to its implementation,
      deployment, and interoperability costs.

   This is hardly a recent concern.  "TCP Extensions Considered Harmful"
   [RFC1263] was published in 1991.  "Extend" or "extension" occurs in
   the title of more than 400 existing Request For Comment (RFC)
   documents.  Yet generic extension considerations have not been
   documented previously.

   The purpose of this document is to describe the architectural
   principles of sound extensibility design, in order to minimize such
   risks.   Formal procedures for extending IETF protocols are discussed
   in "Procedures for Protocol Extensions and Variations" BCP 125

   The rest of this document is organized as follows: Section 2
   discusses routine and major extensions.  Section 3 describes
   architectural principles for protocol extensibility.  Section 4
   explains how designers of base protocols can take steps to anticipate

IAB                           Informational                     [Page 4]

Internet-Draft    Design Considerations for Extensions   30 October 2011

   and facilitate the creation of such subsequent extensions in a safe
   and reliable manner.

   Readers are advised to study the whole document, since the
   considerations are closely linked.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119

2.  Routine and Major Extensions

   To assist extension designers and reviewers, protocol documents
   should provide guidelines explaining how extensions should be
   performed, and guidance on the appropriate use of protocol extension
   mechanisms should be developed.

   Protocol components that are designed with the specific intention of
   allowing extensibility should be clearly identified, with specific
   and complete instructions on how to extend them.  This includes the
   process for adequate review of extension proposals: do they need
   community review and if so how much and by whom?

   The level of review required for protocol extensions will typically
   vary based on the nature of the extension.  Routine extensions may
   require minimal review, while major extensions may require wide
   review.  Guidance on which extensions may be considered 'routine' and
   which ones are 'major' are provided in the sections that follow.

2.1.  When is an Extension Routine?

   An extension may be considered 'routine' if its handling is opaque to
   the protocol itself (e.g. does not substantially change the pattern
   of messages and responses).  For this to apply, no changes to the
   base protocol can be required, nor can changes be required to
   existing and currently deployed implementations, unless they make use
   of the extension.  Furthermore, existing implementations should not
   be impacted.  This typically requires that implementations be able to
   ignore 'routine' extensions without ill-effects.

   Examples of routine extensions include the Dynamic Host Configuration
   Protocol (DHCP) vendor-specific option [RFC2132], Remote
   Authentication Dial In User Service (RADIUS) Vendor-Specific
   Attributes [RFC2865], the enterprise Object IDentifier (OID) tree for
   Management Information Base (MIB) modules, vendor Multipurpose

IAB                           Informational                     [Page 5]

Internet-Draft    Design Considerations for Extensions   30 October 2011

   Internet Mail Extension (MIME) types, and some classes of (non-
   critical) certification extensions.  Such extensions can safely be
   made with minimal discussion.

   Processes that allow routine extensions with minimal or no review
   should be used sparingly (such as the "First Come First Served"
   (FCFS) allocation policy described in "Guidelines for Writing an IANA
   Considerations Section in RFCs" [RFC5226]).  In particular, they
   should be limited to cases that are unlikely to result in
   interoperability problems, or security or operational exposures.

   Experience has shown that even routine extensions may benefit from
   review by experts.  For example, even though DHCP carries opaque
   data, defining a new option using completely unstructured data may
   lead to an option that is unnecessarily hard for clients and servers
   to process.

2.2.  What Constitutes a Major Extension?

   Major extensions may have characteristics leading to a risk of
   interoperability failure.  Where these characteristics are present,
   it is necessary to pay close attention to backward compatibility with
   implementations and deployments of the unextended protocol, and to
   the risk of inadvertent introduction of security or operational

   Extension designers should examine their design for the following

      1.  Modifications or extensions to the underlying protocol.  This
      can include specification of additional transports (see Section
      4.6), changing protocol semantics or defining new message types
      that may require implementation changes in existing and deployed
      implementations of the protocol, even if they do not want to make
      use of the new functions.  A base protocol that does not uniformly
      permit "silent discard" of unknown extensions may automatically
      enter this category, even for apparently minor extensions.
      Handling of "unknown" extensions is discussed in more detail in
      Section 4.7.

      2.  Changes to the basic architectural assumptions.  This may
      include architectural assumptions that are explicitly stated or
      those that have been assumed by implementers.  For example, this
      would include adding a requirement for session state to a
      previously stateless protocol.

      3.  New usage scenarios not originally intended or investigated.
      This can potentially lead to operational difficulties when

IAB                           Informational                     [Page 6]

Internet-Draft    Design Considerations for Extensions   30 October 2011

      deployed, even in cases where the "on-the-wire" format has not
      changed.  For example, the level of traffic carried by the
      protocol may increase substantially, packet sizes may increase,
      and implementation algorithms that are widely deployed may not
      scale sufficiently or otherwise be up to the new task at hand.
      For example, a new DNS Resource Record (RR) type that is too big
      to fit into a single UDP packet could cause interoperability
      problems with existing DNS clients and servers.

      4. Changes to the extension model.  Adverse impacts are very
      likely if the base protocol contains an extension mechanism and
      the proposed extension does not fit into the model used to create
      and define that mechanism.  Extensions that have the same
      properties as those that were anticipated when an extension
      mechanism was devised are much less likely to be disruptive than
      extensions that don't fit the model.

      5. Changes to protocol syntax.  changes to protocol syntax bring
      with them the potential for backward compatibility issues.  If at
      all possible, extensions should be designed for compatibility with
      existing syntax, so as to avoid interoperability failures.

3.  Architectural Principles

   This section describes basic principles of protocol extensibility:

      1. Extensibility features should be limited to what is reasonably
      anticipated when the protocol is developed.

      2. Protocol extensions should be designed for global

      3. Protocol extensions should be architecturally compatible with
      the base protocol.

      4. Protocol extension mechanisms should not be used to create
      incompatible protocol variations.

      5. Extension mechanisms need to be testable.

      6. Protocol parameter assignments need to be coordinated to avoid
      potential conflicts.

      7. Extensions to critical protocols require special care.

IAB                           Informational                     [Page 7]

Internet-Draft    Design Considerations for Extensions   30 October 2011

3.1.  Limited Extensibility

   Designing a protocol for extensibility may have the perverse side
   effect of making it easy to construct incompatible extensions.
   Consequently, protocols should not be made more extensible than
   clearly necessary at inception, and the process for defining new
   extensibility mechanisms should ensure that adequate review of
   proposed extensions will take place before widespread adoption.

3.2.  Design for Global Interoperability

   The IETF mission [RFC3935] is to create interoperable protocols for
   the global Internet, not a collection of different incompatible
   protocols (or "profiles") for use in separate private networks.
   Experience shows that separate private networks often end up using
   equipment from the same vendors, or end up having portable equipment
   like laptop computers move between them, and networks that were
   originally envisaged as being separate can end up being connected

   As a result, extensions cannot be designed for an isolated
   environment; instead, extension designers must assume that systems
   using the extension will need to interoperate with systems on the
   global Internet.

   A key requirement for interoperable extension design is that the base
   protocol must be well designed for interoperability, and that
   extensions must have unambiguous semantics.  Ideally, the protocol
   mechanisms for extension and versioning should be sufficiently well
   described that compatibility can be assessed on paper.  Otherwise,
   when two "private" extensions encounter each other on a public
   network, unexpected interoperability problems may occur.

   Consider a "private" extension installed on a work computer which,
   being portable, is sometimes connected to a home network or a hotel
   network.  If the "private" extension is incompatible with an
   unextended version of the same protocol, problems will occur.

   Similarly, problems can occur if "private" extensions conflict with
   each other.  For example, imagine the situation where one site chose
   to use DHCP [RFC2132] option code 62 for one meaning, and a different
   site chose to use DHCP option code 62 for a completely different,
   incompatible, meaning. It may be impossible for a vendor of portable
   computing devices to make a device that works correctly in both

   One approach to solving this problem has been to reserve parts of an
   identifier namespace for "limited applicability" or "site-specific"

IAB                           Informational                     [Page 8]

Internet-Draft    Design Considerations for Extensions   30 October 2011

   use, such as "X-" headers in email messages [RFC0822], or "P-"
   headers in SIP [RFC3427].  This problem with this approach is that
   when a "limited applicability" or "site-specific" use turns out to
   have applicability elsewhere, other vendors will then implement that
   "X-"/"P-" header for interoperability, and the "X-"/"P-" header
   becomes a de-facto standard, meaning that it is no longer true that
   any header beginning "X-"/"P-" is "limited applicability" or "site-

   As a result, the notion of "X-" headers was removed from the Internet
   Message Format standard when it was updated in 2001 [RFC2822].
   Similarly, within SIP, [RFC5727] Section 4 deprecated the guidance
   provided in [RFC3427] on the creation of "P-" headers:

      In keeping with the IETF tradition of "running code and rough
      consensus", it is valid to allow for the development of SIP
      extensions that are either not ready for Standards Track, but
      might be understood for that role after some running code or are
      private or proprietary in nature because a characteristic
      motivating them is usage that is known not to fit the Internet
      architecture for SIP.  In the past, header fields associated with
      those extensions were called "P-" header fields for "preliminary",
      "private", or "proprietary".

      However, the "P-" header field process has not served the purpose
      for which it was designed -- namely, to restrict to closed
      environments the usage of mechanisms the IETF would not (yet)
      endorse for general usage.  In fact, some "P-" header fields have
      enjoyed widespread implementation; because of the "P-" prefix,
      however, there seems to be no plausible migration path to
      designate these as general-usage header fields without trying to
      force implausible changes on large installed bases.

      Accordingly, this specification deprecates the previous [RFC3427]
      guidance on the creation of "P-" header fields....  The future use
      of any header field name prefix ("P-" or "X-" or what have you) to
      designate SIP header fields of limited applicability is

3.3.  Architectural Compatibility

   Since protocol extension mechanisms may impact interoperability, it
   is important that they be architecturally compatible with the base

   As part of the definition of new extension mechanisms, it is
   important to address whether the mechanisms make use of features as
   envisaged by the original protocol designers, or whether a new

IAB                           Informational                     [Page 9]

Internet-Draft    Design Considerations for Extensions   30 October 2011

   extension mechanism is being invented.  If a new extension mechanism
   is being invented, then architectural compatibility issues need to be

   To assist in the assessment of architectural compatibility, protocol
   documents should provide guidelines explaining how extensions should
   be performed,  and guidance on the appropriate use of protocol
   extension mechanisms should be developed.  Protocol components that
   are designed with the specific intention of allowing extensibility
   should be clearly identified, with specific and complete instructions
   on how to extend them.  This includes the process for adequate review
   of extension proposals: do they need community review and if so how
   much and by whom?

   Documents relying on extension mechanisms need to explicitly identify
   the mechanisms being relied upon.  For example, a document defining
   new data elements should not implicitly define new data types or
   protocol operations without explicitly describing those dependencies
   and discussing their impact.  Where extension guidelines are
   available, mechanisms need to indicate whether they are compliant
   with those guidelines and if not, why not.

   Examples of extension guidelines documents include:

      1. "Guidelines for Extending the Extensible Provisioning Protocol
      (EPP)" [RFC3735], which provides guidelines for use of EPP's
      extension mechanisms to define new features and object management

      2. "Guidelines for Authors and Reviewers of MIB Documents" BCP 111
      [RFC4181], which provides guidance to protocol designers creating
      new MIB modules.

      3. "Guidelines for Authors of Extensions to the Session Initiation
      Protocol (SIP)" [RFC4485], which outlines guidelines for authors
      of SIP extensions.

      4. "Considerations for Lightweight Directory Access Protocol
      (LDAP) Extensions" BCP 118 [RFC4521], which discusses
      considerations for designers of LDAP extensions.

      5. "RADIUS Design Guidelines" BCP 158 [RFC6158], which provides
      guidelines for the design of attributes used  by the Remote
      Authentication Dial In User Service (RADIUS) protocol.

IAB                           Informational                    [Page 10]

Internet-Draft    Design Considerations for Extensions   30 October 2011

3.4.  Protocol Variations

   Protocol variations - specifications that look very similar to the
   original but don't interoperate with each other or with the original
   - are even more harmful to interoperability than extensions. In
   general, such variations should be avoided.  Causes of protocol
   variations include incompatible protocol extensions, uncoordinated
   protocol development, and poorly designed "profiles".

   Protocol extension mechanisms should not be used to create
   incompatible forks in development.  An extension may lead to
   interoperability failures unless the extended protocol correctly
   supports all mandatory and optional features of the unextended base
   protocol, and implementations of the base protocol operate correctly
   in the presence of the extensions.  In addition, it is necessary for
   an extension to interoperate with other extensions.

   As noted in "Uncoordinated Protocol Development Considered Harmful"
   [RFC5704] Section 1, incompatible forks in development can result
   from the uncoordinated adaptation of a protocol, parameter or code-

      In particular, the IAB considers it an essential principle of the
      protocol development process that only one SDO maintains design
      authority for a given protocol, with that SDO having ultimate
      authority over the allocation of protocol parameter code-points
      and over defining the intended semantics, interpretation, and
      actions associated with those code-points.

3.4.1.  Profiles

   Profiling is a common technique for improving interoperability within
   a target environment or set of scenarios.  Generally speaking, there
   are two approaches to profiling:

   a) Removal or downgrading of normative requirements (thereby creating
   potential interoperability problems);

   b) Elevation of normative requirement levels (such as from a
   MAY/SHOULD to a MUST) in order to improve interoperability by
   narrowing potential implementation choices.

   While approach a) is potentially harmful, approach b) may be
   beneficial, but is typically only necessary when the underlying
   protocol is ill-defined enough to permit non-interoperable yet
   compliant implementations.

   In order to avoid creating interoperability problems when profiled

IAB                           Informational                    [Page 11]

Internet-Draft    Design Considerations for Extensions   30 October 2011

   implementations interact with others over the Global Internet,
   profilers need to remain cognizant of the implications of normative

   As noted in "Key words for use in RFCs to Indicate Requirement
   Levels" [RFC2119] Section 6, imperatives are to be used with care,
   and as a result, their removal within a profile is likely to result
   in serious consequences:

      Imperatives of the type defined in this memo must be used with
      care and sparingly.  In particular, they MUST only be used where
      it is actually required for interoperation or to limit behavior
      which has potential for causing harm (e.g., limiting
      retransmissions)  For example, they must not be used to try to
      impose a particular method on implementors where the method is not
      required for interoperability.

   As noted in [RFC2119] Sections 3 and 4, recommendations cannot be
   removed from profiles without serious consideration:

      there may exist valid reasons in particular circumstances to
      ignore a particular item, but the full implications must be
      understood and carefully weighed before choosing a different

   Even the removal of optional features and requirements can have
   consequences.  As noted in [RFC2119] Section 5, implementations which
   do not support optional features still retain the obligation to
   ensure interoperation with implementations that do:

      An implementation which does not include a particular option MUST
      be prepared to interoperate with another implementation which does
      include the option, though perhaps with reduced functionality. In
      the same vein an implementation which does include a particular
      option MUST be prepared to interoperate with another
      implementation which does not include the option (except, of
      course, for the feature the option provides.)

3.5.  Testability

   Experience has shown that it is insufficient merely to correctly
   specify extensibility and backwards compatibility in an RFC.  It is
   also important that implementations respect the compatibility
   mechanisms; if not, non-interoperable pairs of implementations may
   arise.  The TLS case study (Appendix A.3) shows how important this
   can be.

   In order to determine whether protocol extension mechanisms have been

IAB                           Informational                    [Page 12]

Internet-Draft    Design Considerations for Extensions   30 October 2011

   properly implemented, testing is required.  However, for this to be
   possible, test cases need to be developed.  If a base protocol
   document specifies extension mechanisms but does not utilize them or
   provide examples, it may not be possible to develop effective test
   cases based on the base protocol specification alone.  As a result,
   base protocol implementations may not be properly tested and non-
   compliant extension behavior may not be detected until these
   implementations are widely deployed.

   To encourage correct implementation of extension mechanisms, base
   protocol specifications should clearly articulate the expected
   behavior of extension mechanisms and should include examples of
   correct and incorrect extension behavior.

3.6.  Protocol Parameter Registration

   An extension is often likely to make use of additional values added
   to an existing IANA registry.  To avoid conflicting usage of the same
   value, as well as to prevent potential difficulties in determining
   and transferring parameter ownership, it is essential that all new
   values are properly registered by the applicable procedures.  If this
   is not done, there is nothing to prevent two different extensions
   picking the same value.  When these two extensions "meet" each other
   on the Internet, failure is inevitable.

   A surprisingly common case of this is misappropriation of assigned
   Transmission Control Protocol (TCP) (or User Datagram Protocol (UDP))
   registered port numbers.  This can lead to a client for one service
   attempting to communicate with a server for another service.
   Numerous cases could be cited, but not without embarrassing specific

   For general rules see [RFC5226], and for specific rules and
   registries see the individual protocol specification RFCs and the
   IANA web site.  While in theory a "standards track" or "IETF
   consensus" parameter allocation policy may be instituted to encourage
   protocol parameter registration or to improve interoperability, in
   practice problems can arise if the procedures result in so much delay
   that requesters give up and "self-allocate" by picking presumably-
   unused code points.

3.6.1.  Experimental and Local Use

   In some cases, it may be appropriate to use values designated as
   "experimental" or "local use" in early implementations of an
   extension.  For example, "Experimental Values in IPv4, IPv6, ICMPv4,
   ICMPv6, UDP and TCP Headers" [RFC4727] discusses experimental values
   for IP and transport headers, and "Definition of the Differentiated

IAB                           Informational                    [Page 13]

Internet-Draft    Design Considerations for Extensions   30 October 2011

   Services Field (DS Field) in the IPv4 and IPv6 Headers" [RFC2474]
   defines experimental/local use ranges for differentiated services
   code points.

   Such values should be used with care and only for their stated
   purpose: experiments and local use.  They are unsuitable for
   Internet-wide use, since they may be used for conflicting purposes
   and thereby cause interoperability failures.  Packets containing
   experimental or local use values must not be allowed out of the
   domain in which they are meaningful.

   As noted in [RFC5226] Section 4.1:

      For private or local use... No attempt is made to prevent multiple
      sites from using the same value in different (and incompatible)
      ways...  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).

   "Assigning Experimental and Testing Numbers Considered Useful" BCP 82
   [RFC3692] Section 1 provides additional guidance on the use of
   experimental code points:

      Numbers in the experimentation range.... are not intended to be
      used in general deployments or be enabled by default in products
      or other general releases.  In those cases where a product or
      release makes use of an experimental number, the end user must be
      required to explicitly enable the experimental feature and
      likewise have the ability to chose and assign which number from
      the experimental range will be used for a specific purpose (i.e.,
      so the end user can ensure that use of a particular number doesn't
      conflict with other on-going uses).  Shipping a product with a
      specific value pre-enabled would be inappropriate and can lead to
      interoperability problems when the chosen value collides with a
      different usage, as it someday surely will.

      From the above, it follows that it would be inappropriate for a
      group of vendors, a consortia, or another Standards Development
      Organization to agree among themselves to use a particular value
      for a specific purpose and then agree to deploy devices using
      those values.  By definition, experimental numbers are not
      guaranteed to be unique in any environment other than one where
      the local system administrator has chosen to use a particular
      number for a particular purpose and can ensure that a particular
      value is not already in use for some other purpose.

IAB                           Informational                    [Page 14]

Internet-Draft    Design Considerations for Extensions   30 October 2011

3.7.  Extensions to Critical Protocols

   Some protocols (such as Domain Name Service (DNS) and Border Gateway
   Protocol (BGP)) have become critical components of the Internet
   infrastructure.  When such protocols are extended, the potential
   exists for negatively impacting the reliability and security of the
   global Internet.

   As a result, special care needs to be taken with these extensions,
   such as taking explicit steps to isolate existing uses from new ones.
   For example, this can be accomplished by requiring the extension to
   utilize a different port or multicast address, or by implementing the
   extension within a separate process, without access to the data and
   control structures of the base protocol.

   Experience has shown that even when a mechanism has proven benign in
   other uses,  unforseen issues may result when adding it to a critical
   protocol.  For example, both ISIS and OSPF support opaque Link State
   Attributes (LSAs) which are propagated by intermediate nodes that
   don't understand the LSA.  Within Interior Gateway Protocols (IGPs),
   support for opaque LSAs has proven useful without introducing

   However, within BGP, 'attribute tunneling' has resulted in large
   scale routing instabilities, since remote nodes may reset the LOCAL
   session if the tunneled attributes are malformed or aren't
   understood.  This has required modification to BGP error handling, as
   noted in "Error Handling for Optional Transitive Attribute BGP
   Attributes" [Transitive].

   In general, when extending protocols with local failure conditions,
   tunneling of attributes that may trigger failures in non-adjacent
   nodes should be avoided.  This is particularly problematic when the
   originating node receives no indicators of remote failures it may
   have triggered.

4.  Considerations for the Base Protocol

   Good extension design depends on a well designed base protocol.
   Interoperability stems from a number of factors, including:

      1.  A well-written base protocol specification.  Does the base
      protocol specification make clear what an implementor needs to
      support and does it define the impact that individual operations
      (e.g. a message sent to a peer) will have when invoked?

      2.  Design for deployability.  This includes understanding what
      current implementations do and how a proposed extension will

IAB                           Informational                    [Page 15]

Internet-Draft    Design Considerations for Extensions   30 October 2011

      interact with deployed systems.  Is it clear when a proposed
      extension (or its proposed usage) will operationally stress
      existing implementations or the underlying protocol itself if
      widely deployed?  If this is not explained in the base protocol
      specification, is this covered in an extension design guidelines

      3.  Design for backward compatibility.  Does the base protocol
      specification describe how to determine the capabilities of a
      peer, and negotiate the use of extensions?  Does it indicate how
      implementations handle extensions that they do not understand?  Is
      it possible for an extended implementation to negotiate with an
      unextended peer to find a common subset of useful functions?

      4.  Respecting underlying architectural or security assumptions.
      Is there a document describing the underlying architectural
      assumptions, as well as considerations that have arisen in
      operational experience?  Or are there undocumented considerations
      that have arisen as the result of operational experience, after
      the original protocol was published?

      For example, will backward compatibility issues arise if
      extensions reverse the flow of data, allow formerly static
      parameters to be changed on the fly, or change assumptions
      relating to the frequency of reads/writes?

      5. Minimizing impact on critical infrastructure.  For a protocol
      that represents a critical element of Internet infrastructure, it
      is important to explain when it is appropriate to isolate new uses
      of the protocol from existing ones.

      For example, is it explained when a proposed extension (or usage)
      has the potential for negatively impacting critical infrastructure
      to the point where explicit steps would be appropriate to isolate
      existing uses from new ones?

      6. Data model extensions.  Is there a document that explains when
      a protocol extension is routine and when it represents a major

      For example, is it clear when a data model extension represents a
      major versus a routine change?  Are there guidelines describing
      when an extension (such as a new data type) is likely to require a
      code change within existing implementations?  in -0.3i

IAB                           Informational                    [Page 16]

Internet-Draft    Design Considerations for Extensions   30 October 2011

4.1.  Version Numbers

   Any mechanism for extension by versioning must include provisions to
   ensure interoperability, or at least clean failure modes.  Imagine
   someone creating a protocol and using a "version" field and
   populating it with a value (1, let's say), but giving no information
   about what would happen when a new version number appears in it.
   That's bad protocol design and description; it should be clear what
   the expectation is and how you test it.  For example, stating that
   1.X must be compatible with any version 1 code, but version 2 or
   greater is not expected to be compatible, has different implications
   than stating that version 1 must be a proper subset of version 2.

   An example of an under-specified versioning mechanism is provided by
   the MIME-Version header, originally defined in "MIME (Multipurpose
   Internet Mail Extensions)" [RFC1341].  As noted in [RFC1341] Section

      A MIME-Version header field....uses a version number to  declare
      a  message  to  be  conformant  with  this specification and
      allows  mail  processing  agents  to distinguish  between  such
      messages and those generated by older or non-conformant software,
      which is  presumed to lack such a field.

   Beyond this, [RFC1341] provided little guidance on versioning
   behavior, or even the format of the MIME-Version header, which was
   specified to contain "text".  [RFC1521] which obsoleted [RFC1341],
   better defined the format of the version field, but still did not
   clarify the versioning behavior:

      Thus, future format specifiers, which might replace or extend
      "1.0", are constrained to be two integer fields, separated by a
      period.  If a message is received with a MIME-version value other
      than "1.0", it cannot be assumed to conform with this

      It is not possible to fully specify how a mail reader that
      conforms with MIME as defined in this document should treat a
      message that might arrive in the future with some value of MIME-
      Version other than "1.0".  However, conformant software is
      encouraged to check the version number and at least warn the user
      if an unrecognized MIME- version is encountered.

   Thus, even though [RFC1521] defined a MIME-Version header with a
   syntax suggestive of a "Major/Minor" versioning scheme, in practice
   the MIME-Version header was little more than a decoration.

   A better example is ROHC (Robust Header Compression).  ROHCv1

IAB                           Informational                    [Page 17]

Internet-Draft    Design Considerations for Extensions   30 October 2011

   [RFC3095] supports a certain set of profiles for compression
   algorithms.  But experience had shown that these profiles had
   limitations, so the ROHC WG developed ROHCv2 [RFC5225].  A ROHCv1
   implementation does not contain code for the ROHCv2 profiles.  As the
   ROHC WG charter said during the development of ROHCv2:

      It should be noted that the v2 profiles will thus not be
      compatible with the original (ROHCv1) profiles, which means less
      complex ROHC implementations can be realized by not providing
      support for ROHCv1 (over links not yet supporting ROHC, or by
      shifting out support for ROHCv1 in the long run). Profile support
      is agreed through the ROHC channel negotiation, which is part of
      the ROHC framework and thus not changed by ROHCv2.

   Thus in this case both backwards-compatible and backwards-
   incompatible deployments are possible.  The important point is a
   clearly thought out approach to the question of operational
   compatibility.  In the past, protocols have utilized a variety of
   strategies for versioning, many of which have proven problematic.
   These include:

      1. No versioning support.  This approach is exemplified by
      Extensible Authentication Protocol (EAP) [RFC3748] as well as
      Remote Authentication Dial In User Service (RADIUS) [RFC2865],
      both of which provide no support for versioning.  While lack of
      versioning support protects against the proliferation of
      incompatible dialects, the need for extensibility is likely to
      assert itself in other ways, so that ignoring versioning entirely
      may not be the most forward thinking approach.

      2. Highest mutually supported version (HMSV).  In this approach,
      implementations exchange the version numbers of the highest
      version each supports, with the negotiation agreeing on the
      highest mutually supported protocol version.  This approach
      implicitly assumes that later versions provide improved
      functionality, and that advertisement of a particular version
      number implies support for all lower version numbers.  Where these
      assumptions are invalid, this approach breaks down, potentially
      resulting in interoperability problems.  An example of this issue
      occurs in Protected Extensible Authentication Protocol [PEAP]
      where implementations of higher versions may not necessarily
      provide support for lower versions.

      3. Assumed backward compatibility.  In this approach,
      implementations may send packets with higher version numbers to
      legacy implementations supporting lower versions, but with the
      assumption that the legacy implementations will interpret packets
      with higher version numbers using the semantics and syntax defined

IAB                           Informational                    [Page 18]

Internet-Draft    Design Considerations for Extensions   30 October 2011

      for lower versions.  This is the approach taken by Port-Based
      Access Control [IEEE-802.1X].  For this approach to work, legacy
      implementations need to be able to accept packets of known types
      with higher protocol versions without discarding them;  protocol
      enhancements need to permit silent discard of unsupported
      extensions; implementations supporting higher versions need to
      refrain from mandating new features when encountering legacy

      4. Major/minor versioning.  In this approach, implementations with
      the same major version but a different minor version are assumed
      to be backward compatible, but implementations are required to
      negotiate a mutually supported major version number.  This
      approach assumes that implementations with a lower minor version
      number but the same major version can safely ignore unsupported
      protocol messages.

      5. Min/max versioning.  This approach is similar to HMSV, but
      without the implied obligation for clients and servers to support
      all versions back to version 1, in perpetuity.  It allows clients
      and servers to cleanly drop support for early versions when those
      versions become so old that they are no longer relevant and no
      longer required.  In this approach, the client initiating the
      connection reports the highest and lowest protocol versions it
      understands.  The server reports back the chosen protocol version:

       a. If the server understands one or more versions in the client's
       range, it reports back the highest mutually understood version.

       b. If there is no mutual version, then the server reports back
       some version that it does understand (selected as described
       below).  The connection is then typically dropped by client or
       server, but reporting this version number first helps facilitate
       useful error messages at the client end:

        * If there is no mutual version, and the server speaks any
        version higher than client max, it reports the lowest version it
        speaks which is greater than the client max.  The client can
        then report to the user, "You need to upgrade to at least
        version <xx>."

        * Else, the server reports the highest version it speaks.  The
        client can then report to the user, "You need to request the
        server operator to upgrade to at least version <min>."

   Protocols generally do not need any version-negotiation mechanism
   more complicated than the mechanisms described here.  The nature of
   protocol version-negotiation mechanisms is that, by definition, they

IAB                           Informational                    [Page 19]

Internet-Draft    Design Considerations for Extensions   30 October 2011

   don't get widespread real-world testing until *after* the base
   protocol has been deployed for a while, and its deficiencies have
   become evident. This means that, to be useful, a protocol version
   negotiation mechanism should be simple enough that it can reasonably
   be assumed that all the implementers of the first protocol version at
   least managed to implement the version-negotiation mechanism

4.2.  Reserved Fields

   Protocols commonly include one or more "reserved" fields, clearly
   intended for future extensions.  It is good practice to specify the
   value to be inserted in such a field by the sender (typically zero)
   and the action to be taken by the receiver when seeing some other
   value (typically no action).  In packet format diagrams, such fields
   are typically labeled "MBZ", to be read as, "Must Be Zero on
   transmission, Must Be Ignored on reception."

   A common mistake of inexperienced protocol implementers is to think
   that "MBZ" means that it's their software's job to verify that the
   value of the field is zero on reception, and reject the packet if
   not.  This is a mistake, and such software will fail when it
   encounters future versions of the protocol where these previously
   reserved fields are given new defined meanings.  Similarly, protocols
   should carefully specify how receivers should react to unknown
   extensions (headers, TLVs etc.), such that failures occur only when
   that is truly the intended outcome.

4.3.  Encoding Formats

   Using widely-supported encoding formats leads to better
   interoperability and easier extensibility.

   As described in "IAB Thoughts on Encodings for International Domain
   Names" [RFC6055], the number of encodings should be minimized and
   complex encodings are generally a bad idea.  As soon as one moves
   outside the ASCII repertoire, issues relating to collation and/or
   comparison arise that extensions must handle with care.

   An example is the Simple Network Management Protocol (SNMP) Structure
   of Managed Information (SMI).  Guidelines exist for defining the
   Management Information Base (MIB) objects that SNMP carries
   [RFC4181].  Also, multiple textual conventions have been published,
   so that MIB designers do not have to reinvent the wheel when they
   need a commonly encountered construct.  For example, the "Textual
   Conventions for Internet Network Addresses" [RFC4001] can be used by
   any MIB designer needing to define objects containing IP addresses,
   thus ensuring consistency as the body of MIBs is extended.

IAB                           Informational                    [Page 20]

Internet-Draft    Design Considerations for Extensions   30 October 2011

4.4.  Parameter Space Design

   In some protocols the parameter space is either infinite (e.g. Header
   field names) or sufficiently large that it is unlikely to be
   exhausted.  In other protocols, the parameter space is finite, and in
   some cases, has proven inadequate to accommodate demand.  Common
   mistakes include:

   a. A version field that is too small (e.g. two bits or less).  When
   designing a version field, existing as well as potential versions of
   a protocol need to be taken into account.  For example, if a protocol
   is being standardized for which there are existing implementations
   with known interoperability issues, more than one version for "pre-
   standard" implementations may be required.  If two "pre-standard"
   versions are required in addition to a version for an IETF standard,
   then a two-bit version field would only leave one additional version
   code-point for a future update, which could be insufficient.  This
   problem was encountered during the development of the PEAPv2 protocol

   b. A small parameter space (e.g. 8-bits or less) along with a First
   Come, First Served (FCFS) allocation policy.  In general, an FCFS
   allocation policy is only appropriate in situations where parameter
   exhaustion is highly unlikely.  In situations where substantial
   demand is anticipated within a parameter space, the space should
   either be designed to be sufficient to handle that demand, or vendor
   extensibility should be provided to enable vendors to self-allocate.
   The combination of a small parameter space, an FCFS allocation
   policy, and no support for vendor extensibility is particularly
   likely to prove ill-advised.  An example of such a combination was
   the design of the original 8-bit EAP Method Type space [RFC2284].

   Once the potential for parameter exhaustion becomes apparent, it is
   important that it be addressed as quickly as possible.  Protocol
   changes can take years to appear in implementations and by then the
   exhaustion problem could become acute.

   Options for addressing a protocol parameter exhaustion problem

Rethinking the allocation regime
     Where it becomes apparent that the size of a parameter space is
     insufficient to meet demand, it may be necessary to rethink the
     allocation mechanism, in order to prevent or delay parameter space
     exhaustion.  In revising parameter allocation mechanisms, it is
     important to consider both supply and demand aspects so as to avoid
     unintended consequences such as self-allocation or the development
     of black markets for the re-sale of protocol parameters.

IAB                           Informational                    [Page 21]

Internet-Draft    Design Considerations for Extensions   30 October 2011

     For example, a few years after approval of RFC 2284 [RFC2284], it
     became clear that the combination of a FCFS allocation policy and
     lack of support for vendor-extensions had created the potential for
     exhaustion of the EAP Method Type space within a few years.  To
     address the issue, [RFC3748] Section 6.2 changed the allocation
     policy for EAP Method Types from FCFS to Expert Review, with
     Specification Required.  Since this allocation policy revision did
     not change the demand for EAP Method Types, it would have been
     likely to result in self-allocation within the standards space, had
     mechanisms not been provided to expand the method type space
     (including support for vendor-specific method types).

Support for vendor-specific parameters
     If the demand that cannot be accommodated is being generated by
     vendors, merely making allocation harder could make things worse if
     this encourages vendors to self-allocate, creating interoperability
     problems.  In such a situation, support for vendor-specific
     parameters should be considered, allowing each vendor to self-
     allocate within their own vendor-specific space based on a vendor's
     Private Enterprise Code (PEC).  For example, in the case of the EAP
     Method Type space, [RFC3748] Section 6.2 also provided for an
     Expanded Type space for "functions specific only to one vendor's

Extensions to the parameter space
     If the goal is to stave off exhaustion in the face of high demand,
     a larger parameter space may be helpful.  Where vendor-specific
     parameter support is available, this may be achieved by allocating
     an PEC for IETF use. Otherwise it may be necessary to try to extend
     the size of the parameter fields, which could require a new
     protocol version or other substantial protocol changes.

Parameter reclamation
     In order to gain time, it may be necessary to reclaim unused
     parameters.  However, it may not be easy to determine whether a
     parameter that has been allocated is in use or not, particularly if
     the entity that obtained the allocation no longer exists or has
     been acquired (possibly multiple times).

Parameter Transfer
     When all the above mechanisms have proved infeasible and parameter
     exhaustion looms in the near future, enabling the transfer of
     ownership of protocol parameters can be considered as a means for
     improving allocation efficiency.  However, enabling transfer of
     parameter ownership can be far from simple if the parameter
     allocation process was not originally designed to enable title
     searches and ownership transfers.

IAB                           Informational                    [Page 22]

Internet-Draft    Design Considerations for Extensions   30 October 2011

     A parameter allocation process designed to uniquely allocate code-
     points is fundamentally different from one designed to enable title
     search and transfer.  If the only goal is to ensure that a
     parameter is not allocated more than once, the parameter registry
     will only need to record the initial allocation.  On the other
     hand, if the goal is to enable transfer of ownership of a protocol
     parameter, then it is important not only to record the initial
     allocation, but also to track subsequent ownership changes, so as
     to make it possible to determine and transfer title.  Given the
     difficulty of converting from a unique allocation regime to one
     requiring support for title search and ownership transfer, it is
     best for the desired capabilities to be carefully thought through
     at the time of registry establishment.

4.5.  Cryptographic Agility

   Extensibility with respect to cryptographic algorithms is desirable
   in order to provide resilience against the compromise of any
   particular algorithm.  "Guidance for Authentication, Authorization,
   and Accounting (AAA) Key Management" BCP 132 [RFC4962] Section 3
   provides some basic advice:

      The ability to negotiate the use of a particular cryptographic
      algorithm provides resilience against compromise of a particular
      cryptographic algorithm...  This is usually accomplished by
      including an algorithm identifier and parameters in the protocol,
      and by specifying the algorithm requirements in the protocol
      specification.  While highly desirable, the ability to negotiate
      key derivation functions (KDFs) is not required.  For
      interoperability, at least one suite of mandatory-to-implement
      algorithms MUST be selected...

      This requirement does not mean that a protocol must support both
      public-key and symmetric-key cryptographic algorithms.  It means
      that the protocol needs to be structured in such a way that
      multiple public-key algorithms can be used whenever a public-key
      algorithm is employed.  Likewise, it means that the protocol needs
      to be structured in such a way that multiple symmetric-key
      algorithms can be used whenever a symmetric-key algorithm is

   In practice, the most difficult challenge in providing cryptographic
   agility is providing for a smooth transition in the event that a
   mandatory-to-implement algorithm is compromised.  Since it may take
   significant time to provide for widespread implementation of a
   previously undeployed alternative, it is often advisable to recommend
   implementation of alternative algorithms of distinct lineage in
   addition to those made mandatory-to-implement, so that an alternative

IAB                           Informational                    [Page 23]

Internet-Draft    Design Considerations for Extensions   30 October 2011

   algorithm is readily available.  If such a recommended alternative is
   not in place, then it would be wise to issue such a recommendation as
   soon as indications of a potential weakness surface.  This is
   particularly important in the case of potential weakness in
   algorithms used to authenticate and integrity-protect the
   cryptographic negotiation itself, such as KDFs or message integrity
   checks (MICs).  Without secure alternatives to compromised KDF or MIC
   algorithms, it may not be possible to secure the cryptographic
   negotiation while retaining backward compatibility.

4.6.  Transport

   In the past, IETF protocols have been specified to operate over
   multiple transports.  Often the protocol was originally specified to
   utilize a single transport,  but limitations were discovered in
   subsequent deployment, so that additional transports were
   subsequently specified.

   In a number of cases, the protocol was originally specified to
   operate over UDP, but subsequent operation disclosed one or more of
   the following issues, leading to the specification of alternative

      a. Payload fragmentation (often due to the introduction of
      extensions or additional usage scenarios);

      b. Problems with congestion control, transport reliability or

      c. Lack of deployment in multicast scenarios, which had been a
      motivator for UDP transport.

   On the other hand, there are also protocols that were originally
   specified to operate over reliable transport that have subsequently
   defined transport over UDP, due to one or more of the following

      d. NAT traversal concerns that were more easily addressed with UDP

      e. Scalability problems, which could be improved by UDP transport.

   Since specification of a single transport offers the highest
   potential for interoperability, protocol designers should carefully
   consider not only initial but potential future requirements in the
   selection of a transport protocol.  Where UDP transport is selected,
   the guidance provided in "Unicast UDP Usage Guidelines for
   Application Designers" [RFC5405].  should be taken into account.

IAB                           Informational                    [Page 24]

Internet-Draft    Design Considerations for Extensions   30 October 2011

   After significant deployment has occurred, there are few satisfactory
   options for addressing problems with the originally selected
   transport.  While specification of additional transports is possible,
   removal of a widely implemented transport protocol is likely to
   result in interoperability problems and should be avoided.

   Mandating support for the initially selected transport, while
   designating additional transports as optional may have limitations.
   Since optional transport protocols are typically introduced due to
   the advantages they afford in certain scenarios, in those situations
   implementations not supporting optional transport protocols may
   exhibit degraded performance or may even fail.

   While mandating support for multiple transport protocols may appear
   attractive, designers need to realistically evaluate the likelihood
   that implementers will conform to the requirements.  For example,
   where resources are limited (such as in embedded systems),
   implementers may choose to only support a subset of the mandated
   transport protocols, resulting in non-interoperable protocol

4.7.  Handling of Unknown Extensions

   IETF protocols have utilized several techniques for handling of
   unknown extensions.  One technique (often used for vendor-specific
   extensions) is to specify that unknown extensions be "silently

   While this approach can deliver a high level of interoperability,
   there are situations in which it is problematic.  For example, where
   security functionality is involved, "silent discard" may not be
   satisfactory, particularly if the recipient does not provide feedback
   as to whether it supports the extension or not.  This can lead to
   operational security issues that are difficult to detect and correct,
   as noted in Appendix A.2 and "common RADIUS Implementation Issues and
   Suggested Fixes" [RFC5080] Section 2.5.

   In order to ensure that a recipient supports an extension, a
   recipient encountering an unknown extension may be required to
   explicitly reject it and to return an error, rather than proceeding.
   This can be accomplished via a "Mandatory" bit in a TLV-based
   protocol such as L2TP [RFC2661], or a "Require" or "Proxy-Require"
   header in a text-based protocol such as SIP [RFC3261] or HTTP

   Since a mandatory extension can result in an interoperability failure
   when communicating with a party that does not support the extension,
   this designation may not be permitted for vendor-specific extensions,

IAB                           Informational                    [Page 25]

Internet-Draft    Design Considerations for Extensions   30 October 2011

   and may only be allowed for standards-track extensions.  To enable
   fallback operation with degraded functionality, it is good practice
   for the recipient to indicate the reason for the failure, including a
   list of unsupported extensions.  The initiator can then retry without
   the offending extensions.

   Typically only the recipient will find itself in the position of
   rejecting a mandatory extension, since the initiator can explicitly
   indicate which extensions are supported, with the recipient choosing
   from among the supported extensions.  This can be accomplished via an
   exchange of TLVs, such as in IKEv2 [RFC5996] or Diameter [RFC3588],
   or via use of "Accept", "Accept-Encoding", "Accept-Language", "Allow"
   and "Supported" headers in a text-based protocol such as SIP
   [RFC3261] or HTTP [RFC2616].

5.  Security Considerations

   An extension must not introduce new security risks without also
   providing adequate counter-measures, and in particular it must not
   inadvertently defeat security measures in the unextended protocol.
   Thus, the security analysis for an extension needs to be as thorough
   as for the original protocol - effectively it needs to be a
   regression analysis to check that the extension doesn't inadvertently
   invalidate the original security model.

   This analysis may be simple (e.g. adding an extra opaque data element
   is unlikely to create a new risk) or quite complex (e.g. adding a
   handshake to a previously stateless protocol may create a completely
   new opportunity for an attacker).

   When the extensibility of a design includes allowing for new and
   presumably more powerful cryptographic algorithms to be added,
   particular care is needed to ensure that the result is in fact
   increased security.  For example, it may be undesirable from a
   security viewpoint to allow negotiation down to an older, less secure

6.  IANA Considerations

   [RFC Editor: please remove this section prior to publication.]

   This document has no IANA Actions.

IAB                           Informational                    [Page 26]

Internet-Draft    Design Considerations for Extensions   30 October 2011

7.  References

7.1.  Normative References

[RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC4775]  Bradner, S., Carpenter, B., and T. Narten, "Procedures for
           Protocol Extensions and Variations", BCP 125, RFC 4775,
           December 2006.

[RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
           Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.

7.2.  Informative References

           Institute of Electrical and Electronics Engineers, "Local and
           Metropolitan Area Networks: Port-Based Network Access
           Control", IEEE Standard 802.1X-2004, December 2004.

[PEAP]     Palekar, A., Simon, D., Salowey, J., Zhou, H., Zorn, G. and
           S. Josefsson, "Protected EAP Protocol (PEAP) Version 2",
           draft-josefsson-pppext-eap-tls-eap-10.txt, Expired Internet
           draft (work in progress), October 2004.

[RFC0822]  Crocker, D., "Standard for the format of ARPA Internet text
           messages", STD 11, RFC 822, August 1982.

[RFC1263]  O'Malley, S. and L. Peterson, "TCP Extensions Considered
           Harmful", RFC 1263, October 1991.

[RFC1341]  Freed, N. and N. Borenstein, "MIME (Multipurpose Internet
           Mail Extensions): Mechanisms for Specifying and Describing
           the Format of Internet Message Bodies", RFC 1341, June 1992.

[RFC1521]  Borenstein, N. and N. Freed, "MIME (Multipurpose Internet
           Mail Extensions) Part One: Mechanisms for Specifying and
           Describing the Format of Internet Message Bodies", RFC 1521,
           September 1993.

[RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
           Extensions", RFC 2132, March 1997.

[RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
           2246, January 1999.

IAB                           Informational                    [Page 27]

Internet-Draft    Design Considerations for Extensions   30 October 2011

[RFC2284]  Blunk, L. and J. Vollbrecht, "PPP Extensible Authentication
           Protocol (EAP)", RFC 2284, March 1998.

[RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black, "Definition
           of the Differentiated Services Field (DS Field) in the IPv4
           and IPv6 Headers", RFC 2474, December 1998.

[RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,
           L., Leach, P., and T. Berners-Lee, "Hypertext Transfer
           Protocol -- HTTP/1.1", RFC 2616, June 1999.

[RFC2661]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G.,
           and B. Palter, "Layer Two Tunneling Protocol "L2TP"", RFC
           2661, August 1999.

[RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)",RFC 2671,
           August 1999.

[RFC2822]  Resnick, P., "Internet Message Format", RFC 2822, April 2001.

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

[RFC3095]  Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
           Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K.,
           Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke,
           T., Yoshimura, T., and H. Zheng, "RObust Header Compression
           (ROHC): Framework and four profiles: RTP, UDP, ESP, and
           uncompressed", RFC 3095, July 2001.

[RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
           Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
           Session Initiation Protocol", RFC 3261, June 2002.

[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.

[RFC3575]  Aboba, B., "IANA Considerations for RADIUS (Remote
           Authentication Dial In User Service)", RFC 3575, July 2003.

[RFC3588]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J.
           Arkko, "Diameter Base Protocol", RFC 3588, September 2003.

[RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record (RR)
           Types", RFC 3597, September 2003.

IAB                           Informational                    [Page 28]

Internet-Draft    Design Considerations for Extensions   30 October 2011

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

[RFC3735]  Hollenbeck, S., "Guidelines for Extending the Extensible
           Provisioning Protocol (EPP)", RFC 3735, March 2004.

[RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H.
           Lefkowetz, "Extensible Authentication Protocol (EAP)", RFC
           3748, June 2004.

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

[RFC4001]  Daniele, M., Haberman, B., Routhier, S., and J.
           Schoenwaelder, "Textual Conventions for Internet Network
           Addresses", RFC 4001, February 2005.

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

[RFC4366]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
           and T. Wright, "Transport Layer Security (TLS) Extensions",
           RFC 4366, April 2006.

[RFC4485]  Rosenberg, J. and H. Schulzrinne, "Guidelines for Authors of
           Extensions to the Session Initiation Protocol (SIP)", RFC
           4485, May 2006.

[RFC4521]  Zeilenga, K., "Considerations for Lightweight Directory
           Access Protocol (LDAP) Extensions", BCP 118, RFC 4521, June

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

[RFC4929]  Andersson, L. and A. Farrel, "Change Process for
           Multiprotocol Label Switching (MPLS) and Generalized MPLS
           (GMPLS) Protocols and Procedures", BCP 129, RFC 4929, June

[RFC4962]  Housley, R. and B. Aboba, "Guidance for Authentication,
           Authorization, and Accounting (AAA) Key Management", BCP 132,
           RFC 4962, July 2007.

[RFC5080]  Nelson, D. and A. DeKok, "Common Remote Authentication Dial
           In User Service (RADIUS) Implementation Issues and Suggested
           Fixes", RFC 5080, December 2007.

IAB                           Informational                    [Page 29]

Internet-Draft    Design Considerations for Extensions   30 October 2011

[RFC5218]  Thaler, D., and B. Aboba, "What Makes for a Successful
           Protocol?", RFC 5218, July 2008.

[RFC5225]  Pelletier, G. and K. Sandlund, "RObust Header Compression
           Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and UDP-
           Lite", RFC 5225, April 2008.

[RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
           (TLS) Protocol Version 1.2", RFC 5246, August 2008.

[RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
           October 2008.

[RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
           for Application Designers", RFC 5405 (BCP 145), November

[RFC5704]  Bryant, S. and M. Morrow, "Uncoordinated Protocol Development
           Considered Harmful", RFC 5704, November 2009.

[RFC5727]  Peterson, J., Jennings, C. and R. Sparks, "Change Process for
           the Session Initiation Protocol (SIP) and the Real-time
           Applications and Infrastructure Area", BCP 67, RFC 5727,
           March 2010.

[RFC5996]  Kaufman, C., Hoffman, P., Nir, Y. and P. Eronen, "Internet
           Key Exchange Protocol Version 2 (IKEv2)", RFC 5996, September

[RFC6055]  Thaler, D., Klensin, J. and S. Cheshire, "IAB Thoughts on
           Encodings for Internationalized Domain Names", RFC 6055,
           February 2011.

[RFC6158]  DeKok, A. and G. Weber, "RADIUS Design Guidelines", BCP 158,
           RFC 6158,  March 2011.

           Scudder, J., Chen, E., Mohapatra, P. and K. Patel, "Revised
           Error Handling for BGP UPDATE Messages", Internet draft (work
           in progress), draft-ietf-idr-optional-transitive-04, October,

IAB                           Informational                    [Page 30]

Internet-Draft    Design Considerations for Extensions   30 October 2011


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

   That 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 and others.

   Valuable comments and suggestions on the current form of the document
   were made by Loa Andersson, Jari Arkko, Leslie Daigle, Phillip
   Hallam-Baker, Ted Hardie, Alfred Hoenes, John Klensin, Danny
   McPherson, Eric Rescorla, Adam Roach and Pekka Savola.

   The text on TLS experience was contributed by Yngve Pettersen.

IAB Members at the Time of Approval

   Bernard Aboba
   Ross Callon
   Alissa Cooper
   Spencer Dawkins
   Joel Halpern
   Russ Housley
   David Kessens
   Olaf Kolkman
   Danny McPherson
   Jon Peterson
   Andrei Robachevsky
   Dave Thaler
   Hannes Tschofenig

IAB                           Informational                    [Page 31]

Internet-Draft    Design Considerations for Extensions   30 October 2011

Appendix A.  Examples

   This section discusses some specific examples, as case studies.

A.1.  Already documented cases

   There are certain documents that specify a change process or describe
   extension considerations for specific IETF protocols:

      The SIP change process [RFC3427], [RFC4485], [RFC5727]
      The (G)MPLS change process (mainly procedural) [RFC4929]
      LDAP extensions [RFC4521]
      EPP extensions [RFC3735]
      DNS extensions [RFC2671][RFC3597]
      SMTP extensions [RFC5321]

   It is relatively common for MIBs, 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.2.  RADIUS Extensions

   The RADIUS [RFC2865] protocol was designed to be extensible via
   addition of Attributes to a Data Dictionary on the server, without
   requiring code changes.  However, this extensibility model assumed
   that Attributes would conform to a limited set of data types and that
   vendor extensions would be limited to use by vendors, in situations
   in which interoperability was not required.  Subsequent developments
   have stretched those assumptions.

   Section 6.2 of the RADIUS specification [RFC2865] defines a mechanism
   for Vendor-Specific extensions (Attribute 26), and states that use of
   Vendor-Specific extensions:

      should be encouraged instead of allocation of global attribute
      types, for functions specific only to one vendor's implementation
      of RADIUS, where no interoperability is deemed useful.

   However, in practice usage of Vendor-Specific Attributes (VSAs) has
   been considerably broader than this.  In particular, VSAs have been
   used by Standards Development Organizations (SDOs) to define their
   own extensions to the RADIUS protocol.

   This has caused a number of problems.  Since the VSA mechanism was
   not designed for interoperability, VSAs do not contain a "mandatory"
   bit.  As a result, RADIUS clients and servers may not know whether it
   is safe to ignore unknown attributes.  For example, Section 5 of the
   RADIUS specification [RFC2865] states:

IAB                           Informational                    [Page 32]

Internet-Draft    Design Considerations for Extensions   30 October 2011

      A RADIUS server MAY ignore Attributes with an unknown Type.  A
      RADIUS client MAY ignore Attributes with an unknown Type.

   However, in the case where the VSAs pertain to security (e.g.
   Filters) it may not be safe to ignore them, since the RADIUS
   specification [RFC2865] also states:

      A NAS that does not implement a given service MUST NOT implement
      the RADIUS attributes for that service.  For example, a NAS that
      is unable to offer ARAP service MUST NOT implement the RADIUS
      attributes for ARAP.  A NAS MUST treat a RADIUS access-accept
      authorizing an unavailable service as an access-reject instead."

   Detailed discussion of the issues arising from this can be found in
   "Common Remote Authentication Dial In User Service (RADIUS)
   Implementation Issues and Suggested Fixes" [RFC5080] Section 2.5.

   Since it was not envisaged that multi-vendor VSA implementations
   would need to interoperate, the RADIUS specification [RFC2865] does
   not define the data model for VSAs, and allows multiple sub-
   attributes to be included within a single Attribute of type 26.
   However, this enables VSAs to be defined which would not be
   supportable by current implementations if placed within the standard
   RADIUS attribute space.  This has caused problems in standardizing
   widely deployed VSAs, as discussed in "RADIUS Design Guidelines"
   BCP 158 [RFC6158].

   In addition to extending RADIUS by use of VSAs, SDOs have also
   defined new values of the Service-Type attribute in order to create
   new RADIUS commands.  Since the RADIUS specification [RFC2865]
   defined Service-Type values as being allocated First Come, First
   Served (FCFS), this essentially enabled new RADIUS commands to be
   allocated without IETF review.  This oversight has since been fixed
   in "IANA Considerations for RADIUS" [RFC3575].

A.3.  TLS Extensions

   The Secure Sockets Layer (SSL) v2 protocol was developed by Netscape
   to be used to secure online transactions on the Internet.  It was
   later replaced by SSL v3, also developed by Netscape.  SSL v3 was
   then further developed by the IETF as the Transport Layer Security
   (TLS) 1.0 [RFC2246].

   The SSL v3 protocol was not explicitly specified to be extended.
   Even TLS 1.0 did not define an extension mechanism explicitly.
   However, extension "loopholes" were available.  Extension mechanisms
   were finally defined in "Transport Layer Security (TLS) Extensions"

IAB                           Informational                    [Page 33]

Internet-Draft    Design Considerations for Extensions   30 October 2011

      o  New versions
      o  New cipher suites
      o  Compression
      o  Expanded handshake messages
      o  New record types
      o  New handshake messages

   The protocol also defines how implementations should handle unknown

   Of the above extension methods, new versions and expanded handshake
   messages have caused the most interoperability problems.
   Implementations are supposed to ignore unknown record types but to
   reject unknown handshake messages.

   The new version support in SSL/TLS includes a capability to define
   new versions of the protocol, while allowing newer implementations to
   communicate with older implementations.  As part of this
   functionality some Key Exchange methods include functionality to
   prevent version rollback attacks.

   The experience with this upgrade functionality in SSL and TLS is
   decidedly mixed:

    o  SSL v2 and SSL v3/TLS are not compatible.  It is possible to use
       SSL v2 protocol messages to initiate a SSL v3/TLS connection, but
       it is not possible to communicate with a SSL v2 implementation
       using SSL v3/TLS protocol messages.
    o  There are implementations that refuse to accept handshakes using
       newer versions of the protocol than they support.
    o  There are other implementations that accept newer versions, but
       have implemented the version rollback protection clumsily.

   The SSL v2 problem has forced SSL v3 and TLS clients to continue to
   use SSL v2 Client Hellos for their initial handshake with almost all
   servers until 2006, much longer than would have been desirable, in
   order to interoperate with old servers.

   The problem with incorrect handling of newer versions has also forced
   many clients to actually disable the newer protocol versions, either
   by default, or by automatically disabling the functionality, to be
   able to connect to such servers.  Effectively, this means that the
   version rollback protection in SSL and TLS is non-existent if talking
   to a fatally compromised older version.

   SSL v3 and TLS also permitted expansion of the Client Hello and
   Server Hello handshake messages.  This functionality was fully
   defined by the introduction of TLS Extensions, which makes it

IAB                           Informational                    [Page 34]

Internet-Draft    Design Considerations for Extensions   30 October 2011

   possible to add new functionality to the handshake, such as the name
   of the server the client is connecting to, request certificate status
   information, indicate Certificate Authority support, maximum record
   length, etc.  Several of these extensions also introduce new
   handshake messages.

   It has turned out that many SSL v3 and TLS implementations that do
   not support TLS Extensions, did not, as required by the protocol
   specifications, ignore the unknown extensions, but instead failed to
   establish connections.  Several of the implementations behaving in
   this manner are used by high profile Internet sites, such as online
   banking sites, and this has caused a significant delay in the
   deployment of clients supporting TLS Extensions, and several of the
   clients that have enabled support are using heuristics that allow
   them to disable the functionality when they detect a problem.

   Looking forward, the protocol version problem, in particular, can
   cause future security problems for the TLS protocol.  The strength of
   the digest algorithms (MD5 and SHA-1) used by SSL and TLS is
   weakening.  If MD5 and SHA-1 weaken to the point where it is feasible
   to mount successful attacks against older SSL and TLS versions, the
   current error recovery used by clients would become a security
   vulnerability (among many other serious problems for the Internet).

   To address this issue, TLS 1.2 [RFC5246] makes use of a newer
   cryptographic hash algorithm (SHA-256) during the TLS handshake by
   default.  Legacy ciphersuites can still be used to protect
   application data, but new ciphersuites are specified for data
   protection as well as for authentication within the TLS handshake.
   The hashing method can also be negotiated via a Hello extension.
   Implementations are encouraged to implement new ciphersuites, and to
   enable the negotiation of the ciphersuite used during a TLS session
   to be governed by policy, thus enabling a more rapid transition away
   from weakened ciphersuites.

   The lesson to be drawn from this experience is that it isn't
   sufficient to design extensibility carefully; it must also be
   implemented carefully by every implementer, without exception.  Test
   suites and certification programs can help provide incentives for
   implementers to pay attention to implementing extensibility
   mechanisms correctly.

A.4.  L2TP Extensions

   Layer Two Tunneling Protocol (L2TP) [RFC2661] carries Attribute-Value
   Pairs (AVPs), with most AVPs having no semantics to the L2TP protocol
   itself.  However, it should be noted that L2TP message types are
   identified by a Message Type AVP (Attribute Type 0) with specific AVP

IAB                           Informational                    [Page 35]

Internet-Draft    Design Considerations for Extensions   30 October 2011

   values indicating the actual message type.  Thus, extensions relating
   to Message Type AVPs would likely be considered major extensions.

   L2TP also provides for Vendor-Specific AVPs.  Because everything in
   L2TP is encoded using AVPs, it would be easy to define vendor-
   specific AVPs that would be considered major extensions.

   L2TP also provides for a "mandatory" bit in AVPs.  Recipients of L2TP
   messages containing AVPs they do not understand but that have the
   mandatory bit set, are expected to reject the message and terminate
   the tunnel or session the message refers to.  This leads to
   interesting interoperability issues, because a sender can include a
   vendor-specific AVP with the M-bit set, which then causes the
   recipient to not interoperate with the sender.  This sort of behavior
   is counter to the IETF ideals, as implementations of the IETF
   standard should interoperate successfully with other implementations
   and not require the implementation of non-IETF extensions in order to
   interoperate successfully.  Section 4.2 of the L2TP specification
   [RFC2661] includes specific wording on this point, though there was
   significant debate at the time as to whether such language was by
   itself sufficient.

   Fortunately, it does not appear that the potential problems described
   above have yet become a problem in practice.  At the time of this
   writing, the authors are not aware of the existence of any vendor-
   specific AVPs that also set the M-bit.

Change log [RFC Editor: please remove this section]

   -09: 2011-10-30. Resolved remaining issues raised during Call for

   -08: 2011-10-22. Incorporated additional resolutions to issues raised
   during Call for Comment.

   -07: 2011-7-24.  Incorporated resolutions to issues raised during
   Call for Comment.

   -06: 2011-3-1.  Incorporated additional corrections and
   organizational improvements.

   -05: 2011-2-4.  Added to the security considerations section.

   -04: 2011-2-1.  Added a section on cryptographic agility.  Additional

   -03: 2011-1-25.  Updates and reorganization to reflect comments from
   the IETF community.

IAB                           Informational                    [Page 36]

Internet-Draft    Design Considerations for Extensions   30 October 2011

   -02: 2010-7-12.  Updates by Bernard Aboba

   -01: 2010-4-7.   Updates by Stuart Cheshire.

   draft-iab-extension-recs-00:   2009-4-24.   Updated boilerplate,
   author list.

   -04: 2008-10-24.  Updated author addresses, fixed editorial issues.

   -03: 2008-10-17.  Updated references, added material relating to

   -02: 2007-06-15.  Reorganized Sections 2 and 3.

   -01: 2007-03-04.  Updated according to comments, especially the
   wording about TLS, added various specific examples.

   draft-carpenter-extension-recs-00: original version, 2006-10-12.
   Derived from draft-iesg-vendor-extensions-02.txt dated 2004-06-04 by
   focusing on architectural issues; the procedural issues were moved to
   RFC 4775.

Authors' Addresses

   Brian Carpenter
   Department of Computer Science
   University of Auckland
   PB 92019
   Auckland,   1142
   New Zealand

   Email: brian.e.carpenter@gmail.com

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   EMail: bernard_aboba@hotmail.com

   Stuart Cheshire
   Apple Computer, Inc.
   1 Infinite Loop
   Cupertino, CA 95014

   EMail: cheshire@apple.com

IAB                           Informational                    [Page 37]