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RFC 6709 - Design Considerations for Protocol Extensions

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Internet Architecture Board (IAB)                           B. Carpenter
Request for Comments: 6709                                 B. Aboba, Ed.
Category: Informational                                      S. Cheshire
ISSN: 2070-1721                                           September 2012

             Design Considerations for Protocol Extensions


   This document discusses architectural issues related to the
   extensibility of Internet protocols, with a focus on design
   considerations.  It is intended to assist designers of both base
   protocols and extensions.  Case studies are included.  A companion
   document, RFC 4775 (BCP 125), discusses procedures relating to the
   extensibility of IETF protocols.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Architecture Board (IAB)
   and represents information that the IAB has deemed valuable to
   provide for permanent record.  It represents the consensus of the
   Internet Architecture Board (IAB).  Documents approved for
   publication by the IAB are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2012 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.

Table of Contents

   1. Introduction ....................................................3
      1.1. Requirements Language ......................................4
   2. Routine and Major Extensions ....................................4
      2.1. What Constitutes a Major Extension? ........................4
      2.2. When is an Extension Routine? ..............................6
   3. Architectural Principles ........................................7
      3.1. Limited Extensibility ......................................7
      3.2. Design for Global Interoperability .........................8
      3.3. Architectural Compatibility ...............................12
      3.4. Protocol Variations .......................................13
      3.5. Testability ...............................................16
      3.6. Protocol Parameter Registration ...........................16
      3.7. Extensions to Critical Protocols ..........................17
   4. Considerations for the Base Protocol ...........................18
      4.1. Version Numbers ...........................................19
      4.2. Reserved Fields ...........................................22
      4.3. Encoding Formats ..........................................23
      4.4. Parameter Space Design ....................................23
      4.5. Cryptographic Agility .....................................26
      4.6. Transport .................................................27
      4.7. Handling of Unknown Extensions ............................28
   5. Security Considerations ........................................29
   6. References .....................................................30
      6.1. Normative References ......................................30
      6.2. Informative References ....................................30
   7. Acknowledgments ................................................35
   8. IAB Members at the Time of Approval ............................35
   Appendix A.  Examples .............................................36
      A.1. Already-Documented Cases ..................................36
      A.2. RADIUS Extensions .........................................36
      A.3. TLS Extensions ............................................39
      A.4. L2TP Extensions ...........................................41

1.  Introduction

   When developing protocols, IETF Working Groups (WGs) often include
   mechanisms whereby these protocols can be extended in the future.  It
   is often a good principle to design extensibility into protocols; as
   described in "What Makes for a Successful Protocol" [RFC5218], a
   "wildly 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.  However, at the
   same time, experience has shown that extensions carry the risk of
   unintended consequences, such as interoperability issues, operational
   problems, or security vulnerabilities.

   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 Comments (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
   and facilitate the creation of such subsequent extensions in a safe
   and reliable manner.  Section 5 discusses security considerations.
   Appendix A provides case studies.

   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 "Key words for use in
   RFCs to Indicate Requirement Levels" BCP 14 [RFC2119].

2.  Routine and Major Extensions

   The risk of unintended consequences from an extension 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, it is highly desirable for the original
   designers to articulate the design constraints and assumptions, so as
   to enable extensions to be done carefully and with a full
   understanding of the base protocol, existing implementations, and
   current operational practice.

   To assist extension designers and reviewers, protocol documents
   should provide guidelines explaining how extensions should be
   performed, and guidance on how protocol extension mechanisms should
   be used.

   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' is provided in the sections that follow.

2.1.  What Constitutes a Major Extension?

   Major extensions may have characteristics leading to a risk of
   interoperability failures, security vulnerabilities, or operational
   problems.  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 potential for
   inadvertent introduction of security or operational exposures.

   Extension designers should examine their design for the following

   1.  Modifications or extensions to the underlying protocol.  An
       extension document should be considered to update the underlying
       protocol specification if an implementation of the underlying
       protocol would need to be updated to accommodate the extension.
       This should not be necessary if the underlying protocol was
       designed with a modular interface.  Examples of extensions
       modifying the underlying protocol 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
       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.  Similarly, the
       additional traffic that results from an extension to a routing
       protocol could have a detrimental impact on the performance or
       stability of implementations that do not implement the extension.

   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.  Also, changes to the extension model
       itself (including changes limiting further extensibility) can
       create interoperability problems.

   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.

   6.  Interrelated extensions to multiple protocols.  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

   7.  Changes to the security model.  Changes to the protocol security
       model (or even addition of new security mechanisms within an
       existing framework) can introduce security vulnerabilities or
       adversely impact operations.  Consequently, it is important that
       such proposals undergo security as well as operational review.
       Security considerations are discussed in Section 5.

   8.  Performance impact.  An extension that impacts performance can
       have adverse consequences, particularly if the performance of
       existing deployments is affected.

2.2.  When is an Extension Routine?

   An extension may be considered 'routine' if it does not meet the
   criteria for being considered a 'major' extension and 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

   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, and vendor Multipurpose
   Internet Mail Extension (MIME) types.  Such extensions can safely be
   made with minimal discussion.

   Processes that allow routine extensions with minimal or no review
   (such as "First Come First Served" (FCFS) allocation [RFC5226])
   should be used sparingly.  In particular, they should be limited to
   cases that are unlikely to result in interoperability problems or in
   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.

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 components require special care.  A
       critical component is one whose failure can lead to Internet-wide
       reliability and security issues or performance degradation.

3.1.  Limited Extensibility

   Protocols should not be made more extensible than clearly necessary
   at inception, in order to enable optimization along dimensions (e.g.,
   bandwidth, state, memory requirements, deployment time, latency,
   etc.) important to the most common use cases.

   The process for defining new extensibility mechanisms should ensure
   that adequate review of proposed extensions will take place before
   widespread adoption.

   As noted in "What Makes for a Successful Protocol" [RFC5218], "wildly
   successful" protocols far exceed their original goals, in terms of
   scale, purpose (being used in scenarios far beyond the initial
   design), or both.  This implies that all potential uses may not be
   known at inception.  As a result, extensibility mechanisms may need
   to be revisited as additional use cases reveal themselves.  However,
   this does not imply that an initial design needs to take all
   potential needs into account at inception.

3.2.  Design for Global Interoperability

   Section 3.1 of "Procedures for Protocol Extensions and Variations"
   BCP 125 [RFC4775] notes:

      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.

   Since the global Internet is more than a collection of incompatible
   protocols (or "profiles") for use in separate private networks,
   implementers supporting extensions in shipping products or multi-site
   experimental usage must assume that systems will need to interoperate
   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" or "experimental" extensions encounter each other
   on a public network, unexpected interoperability problems may occur.
   However, as noted in the Transport Layer Security (TLS) case study
   (Appendix A.3), it is not sufficient to design extensibility
   carefully; it also must be implemented carefully.

3.2.1.  Private Extensions

   Experience shows that separate private networks often 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 later.

   Consider a "private" extension installed on a work computer that,
   being portable, is sometimes connected to networks other than the
   work network, like 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"
   use, such as "X-" headers in email messages [RFC822] or "P-" headers
   in SIP [RFC3427].  However, as noted in "Deprecating the "X-" Prefix
   and Similar Constructs in Application Protocols" [RFC6648], Appendix

      The primary problem with the "X-" convention is that
      unstandardized parameters have a tendency to leak into the
      protected space of standardized parameters, thus introducing the
      need for migration from the "X-" name to a standardized name.
      Migration, in turn, introduces interoperability issues (and
      sometimes security issues) because older implementations will
      support only the "X-" name and newer implementations might support
      only the standardized name.  To preserve interoperability, newer
      implementations simply support the "X-" name forever, which means

      that the unstandardized name has become a de facto standard (thus
      obviating the need for segregation of the name space into
      standardized and unstandardized areas in the first place).

   As a result, the notion of "X-" headers from the 1982 Internet
   Message Format standard [RFC822] was removed when the specification
   was updated in 2001 [RFC2822].  Within SIP, the guidance published in
   2002 regarding "P-" headers [RFC3427] was deprecated eight years
   later in Section 4 of the 2010 update [RFC5727].  More generally, as
   noted in Section 1 of the "X-" prefix deprecation document [RFC6648]:

      This document generalizes from the experience of the email and SIP
      communities by doing the following:

      1.  Deprecates the "X-" convention for newly defined parameters in
          application protocols, including new parameters for
          established protocols.  This change applies even where the
          "X-" convention was only implicit, and not explicitly
          provided, such as was done for email in [RFC822].

3.2.2.  Local Use

   Values designated as "experimental" or "local use" are only
   appropriate in limited circumstances such as in early implementations
   of an extension restricted to a single site.

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

   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.

   Section 1 of "Assigning Experimental and Testing Numbers Considered
   Useful" BCP 82 [RFC3692] provides 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.

      Once an extension has been tested and shown to be useful, a
      permanent number could be obtained through the normal assignment

   However, as noted in Appendix B of the "X-" prefix deprecation
   document [RFC6648], assigning a parameter block for experimental use
   is only necessary when the parameter pool is limited:

      "Assigning Experimental and Testing Numbers Considered Useful" ...
      implies that the "X-" prefix is also useful for experimental
      parameters.  However, BCP 82 addresses the need for protocol
      numbers when the pool of such numbers is strictly limited (e.g.,
      DHCP options) or when a number is absolutely required even for
      purely experimental purposes (e.g., the Protocol field of the IP
      header).  In almost all application protocols that make use of
      protocol parameters (including email headers, media types, HTTP
      headers, vCard parameters and properties, URNs, and LDAP field
      names), the name space is not limited or constrained in any way,
      so there is no need to assign a block of names for private use or
      experimental purposes....

      Therefore, it appears that segregating the parameter space into a
      standardized area and a unstandardized area has few, if any,
      benefits and has at least one significant cost in terms of

3.2.3.  Multi-Site Experiments

   Where an experiment is undertaken among a diverse set of experimental
   sites connected via the global Internet, the use of "experimental" or
   "local use" code points is inadvisable.  This might include, for
   example, sites that take a prototype implementation of some protocol
   and use that both within their site but, importantly, among the full
   set of other sites interested in that protocol.  In such a situation,
   it is impractical and probably impossible to coordinate the
   de-confliction of "experimental" code points.  Section 4.1 of the
   IANA Considerations guidelines document [RFC5226] notes:

      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).

   The Host Identity Protocol (HIP) [RFC5201] and the Locator/ID
   Separation Protocol [LISP] are examples where a set of experimental
   sites are collaborating among themselves, but not necessarily in a
   tightly coordinated way.  Both HIP and LISP have dealt with this by
   having unique non-experimental code points allocated to HIP and LISP,
   respectively, at the time of publication of their respective
   Experimental RFCs.

3.3.  Architectural Compatibility

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

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

   As part of the definition of a new extension, it is important to
   address whether the extension makes use of features as envisaged by
   the original protocol designers, or whether a new extension mechanism
   is being invented.  If a new extension mechanism is being invented,
   then architectural compatibility issues need to be addressed.

   To assist in the assessment of architectural compatibility, protocol
   documents should provide guidelines explaining how extensions should
   be performed, and guidance on how protocol extension mechanisms
   should be used.

   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 offer an explanation if they are not.

   Examples of documents describing extension guidelines 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.

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".

   Designing a protocol for extensibility may have the perverse side
   effect of making it easy to construct incompatible variations.
   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 Section 1 of "Uncoordinated Protocol Development
   Considered Harmful" [RFC5704], incompatible forks in development can
   result from the uncoordinated adaptation of a protocol, parameter, or
   code point:

      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.

   Note that problems can occur even when one Standards Development
   Organization (SDO) maintains design authority, if protocol parameter
   code points are reused.  As an example, EAP-FAST [RFC5421][RFC5422]
   reused previously assigned Extensible Authentication Protocol (EAP)
   type codes.  As described in the IESG note in the EAP-FAST document

      The reuse of previously assigned EAP Type Codes is incompatible
      with EAP method negotiation as defined in RFC 3748.

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).  This can be done in order to improve
       interoperability by narrowing potential implementation choices

       (such as when the underlying protocol is ill-defined enough to
       permit non-interoperable yet compliant implementations) or to
       meet specific operational requirements (such as enabling use of
       stronger cryptographic mechanisms than those mandated in the

   While approach a) is potentially harmful, approach b) may be

   In order to avoid interoperability problems when profiled
   implementations interact with others over the global Internet,
   profilers need to remain cognizant of the implications of removing
   normative requirements.  As noted in Section 6 of "Key words for use
   in RFCs to Indicate Requirement Levels" [RFC2119], 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 Sections 3 and 4 of the Key Words document [RFC2119],
   recommendations cannot be removed from profiles without serious

      [T]here 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 Section 5 of the Key Words document
   [RFC2119], implementations that 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 specify
   extensibility and backward compatibility correctly 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
   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 extension behavior.

3.6.  Protocol Parameter Registration

   As noted in Section 3.2 of "Procedures for Protocol Extensions and
   Variations" BCP 125 [RFC4775]:

      An extension is often likely to make use of additional values
      added to an existing IANA registry....  It is essential that such
      new values are properly registered by the applicable procedures,
      including expert review where applicable....  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.

   Before creating a new protocol parameter registry, existing
   registries should be examined to determine whether one of them can be
   used instead (see http://www.iana.org/protocols/).

   To avoid conflicting usage of the same registry value, as well as to
   prevent potential difficulties in determining and transferring
   parameter ownership, it is essential that all new values are

   registered.  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.  Another
   common case is the use of unregistered URI schemes.  Numerous cases
   could be cited, but not without embarrassing specific implementers.
   For general rules, see the IANA Considerations guidelines document
   [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.
   Where self-allocation is prevalent, the information contained within
   registries may become inaccurate, particularly when third parties are
   prohibited from updating entries so as to improve accuracy.  In these
   situations, it is important to consider whether registration
   processes need to be changed to support the role of a registry as
   "documentation of how the Internet is operating".

3.7.  Extensions to Critical Protocols

   Some protocols (such as the Domain Name System (DNS), the Border
   Gateway Protocol (BGP), and the Hypertext Transfer Protocol (HTTP))
   or algorithms (such as congestion control) have become critical
   components of the Internet infrastructure.  A critical component is
   one whose failure can lead to Internet-wide reliability and security
   issues or performance degradation.  When such protocols or algorithms
   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, unforeseen issues may result when adding it to a critical
   protocol.  For example, both IS-IS and OSPF support opaque Link State
   Advertisements (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 "Revised Error Handling for BGP UPDATE Messages"

   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.  To
   promote interoperability, designers should:

   1.  Ensure a well-written base protocol specification.  Does the base
       protocol specification make clear what an implementer 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 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 (or differently-extended) peer to find a common
       subset of useful functions?

   3.  Respect 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?

   4.  Minimize 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?

   5.  Provide guidance on data model extensions.  Is there a document
       that explains when a protocol extension is routine and when it
       represents a major change?

       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?

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.
   This would be a bad protocol design and description; it should be
   clear what the expectation is and how it can be tested.  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 Section 1 of the
   MIME specification [RFC1341]:

      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, the 1992 MIME specification [RFC1341] provided little
   guidance on versioning behavior, or even the format of the MIME-
   Version header, which was specified to contain "text".  The 1993
   update [RFC1521] 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 the 1993 update [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

   An example of a protocol with a better versioning scheme is ROHC
   (Robust Header Compression).  ROHCv1 [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 backward-compatible and backward-
   incompatible deployments are possible.  The important point is to
   have a clearly thought out approach to the question of operational

   In the past, protocols have utilized a variety of strategies for
   versioning, each with its own benefits and drawbacks in terms of
   capability and complexity of implementation:

   1.  No versioning support.  This approach is exemplified by the
       Extensible Authentication Protocol (EAP) [RFC3748] as well as the
       Remote Authentication Dial In User Service (RADIUS) protocol
       [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

   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 the 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 for lower versions.  This is the approach taken by "Port-
       Based Network 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; and implementations supporting higher
       versions need to refrain from mandating new features when
       encountering legacy implementations.

   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

       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 that 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

   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
   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 Internationalized
   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 arise relating to collation,
   valid code points, encoding, normalization, and comparison, which
   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, "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.

4.4.  Parameter Space Design

   In some protocols, the parameter space either has no specified limit
   (e.g., Header field names) or is sufficiently large that it is
   unlikely to be exhausted.  In other protocols, the parameter space is
   limited 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 [PEAP].

   b.  A small parameter space (e.g., 8 bits or less) along with a First
       Come, First Served (FCFS) allocation policy [RFC5226].  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 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 resale of protocol

      For example, a few years after publication of PPP EAP [RFC2284] in
      1998, it became clear that the combination of an FCFS allocation
      policy [RFC5226] 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, Section 6.2 of the 2004
      update [RFC3748] 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, Section 6.2 of the 2004
      EAP specification [RFC3748] 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; this may require a new
      version of the protocol (such as was required for IPv6).  Where
      vendor-specific parameter support is available, this may be
      achieved by allocating a 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

   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.

      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 the 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.  Section 3 of "Guidance for Authentication,
   Authorization, and Accounting (AAA) Key Management" BCP 132 [RFC4962]
   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
   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
       efficiency; and

   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

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

   b.  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.

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

   Mandating support for the initially selected transport protocol while
   designating additional transport protocols 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 the 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 or not it supports the extension.  This can lead to
   operational security issues that are difficult to detect and correct,
   as noted in Appendix A.2 and in Section 2.5 of "Common Remote
   Authentication Dial In User Service (RADIUS) Implementation Issues
   and Suggested Fixes" [RFC5080].

   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 ignoring the
   unknown extension and proceeding with the remainder of the message.
   This can be accomplished via a "Mandatory" bit in a TLV-based
   protocol such as the Layer 2 Tunneling Protocol (L2TP) [RFC2661], or
   a "Require" or "Proxy-Require" header in a text-based protocol such
   as SIP [RFC3261] or HTTP [RFC2616].

   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
   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 the Internet Key Exchange Protocol
   Version 2 (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 countermeasures; 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.  References

6.1.  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., Ed., 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.

6.2.  Informative References

              Scudder, J., Chen, E., Mohapatra, P., and K. Patel,
              "Revised Error Handling for BGP UPDATE Messages", Work in
              Progress, June 2012.

              Thaler, D., "Issues in Identifier Comparison for Security
              Purposes", Work in Progress, August 2012.

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

   [LISP]     Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol (LISP)", Work in Progress,
              May 2012.

   [PEAP]     Palekar, A., Simon, D., Salowey, J., Zhou, H., Zorn, G.,
              and S. Josefsson, "Protected EAP Protocol (PEAP) Version
              2", Work in Progress, October 2004.

              Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
              Preparation and Comparison of Internationalized Strings in
              Application Protocols", Work in Progress, August 2012.

              Blanchet, M. and A. Sullivan, "Stringprep Revision and
              PRECIS Problem Statement", Work in Progress, August 2012.

              TEXT MESSAGES", STD 11, RFC 822, August 1982.

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

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

   [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.

   [RFC2058]  Rigney, C., Rubens, A., Simpson, W., and S. Willens,
              "Remote Authentication Dial In User Service (RADIUS)", RFC
              2058, January 1997.

   [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.

   [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

   [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., Ed., "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 2000.

   [RFC2882]  Mitton, D., "Network Access Servers Requirements: Extended
              RADIUS Practices", RFC 2882, July 2000.

   [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)", RFC 3427, December 2002.

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

   [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.

   [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.
              Levkowetz, Ed., "Extensible Authentication Protocol
              (EAP)", RFC 3748, June 2004.

   [RFC3935]  Alvestrand, H., "A Mission Statement for the IETF", BCP
              95, 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., Ed., "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 2006.

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

   [RFC4929]  Andersson, L., Ed., and A. Farrel, Ed., "Change Process
              for Multiprotocol Label Switching (MPLS) and Generalized
              MPLS (GMPLS) Protocols and Procedures", BCP 129, RFC 4929,
              June 2007.

   [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.

   [RFC5201]  Moskowitz, R., Nikander, P., Jokela, P., Ed., and T.
              Henderson, "Host Identity Protocol", RFC 5201, April 2008.

   [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", BCP 145, RFC 5405, November

   [RFC5421]  Cam-Winget, N. and H. Zhou, "Basic Password Exchange
              within the Flexible Authentication via Secure Tunneling
              Extensible Authentication Protocol (EAP-FAST)", RFC 5421,
              March 2009.

   [RFC5422]  Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou,
              "Dynamic Provisioning Using Flexible Authentication via
              Secure Tunneling Extensible Authentication Protocol (EAP-
              FAST)", RFC 5422, March 2009.

   [RFC5704]  Bryant, S., Ed., Morrow, M., Ed., and IAB, "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 2010.

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

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

   [RFC6648]  Saint-Andre, P., Crocker, D., and M. Nottingham,
              "Deprecating the "X-" Prefix and Similar Constructs in
              Application Protocols", BCP 178, RFC 6648, June 2012.

7.  Acknowledgments

   This document is heavily based on an earlier draft by Scott Bradner
   and Thomas Narten, other parts of which were eventually published as
   RFC 4775.

   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, Ran Atkinson, Stewart Bryant, Leslie
   Daigle, Alan DeKok, Roy Fielding, Phillip Hallam-Baker, Ted Hardie,
   Alfred Hoenes, John Klensin, Barry Leiba, Eric Rescorla, Adam Roach,
   and Pekka Savola.  The text on TLS experience was contributed by
   Yngve Pettersen.

8.  IAB Members at the Time of Approval

   Bernard Aboba
   Jari Arkko
   Marc Blanchet
   Ross Callon
   Alissa Cooper
   Spencer Dawkins
   Joel Halpern
   Russ Housley
   David Kessens
   Danny McPherson
   Jon Peterson
   Dave Thaler
   Hannes Tschofenig

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.  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.

   From the beginning, uses of the RADIUS protocol extended beyond the
   scope of the original protocol definition (and beyond the scope of
   the RADIUS Working Group charter).  In addition to rampant self-
   allocation within the limited RADIUS standard attribute space,
   vendors defined their own RADIUS commands.  This led to the rapid
   proliferation of vendor-specific protocol variants.  To this day,
   many common implementation practices have not been documented.  For
   example, authentication server implementations are often typically
   based on a Data Dictionary, enabling addition of Attributes without
   requiring code changes.  Yet, the concept of a Data Dictionary is not
   mentioned in the RADIUS specification [RFC2865].

   As noted in "Extended RADIUS Practices" [RFC2882], Section 1:

      The RADIUS Working Group was formed in 1995 to document the
      protocol of the same name, and was chartered to stay within a set
      of bounds for dial-in terminal servers.  Unfortunately the real
      world of Network Access Servers (NASes) hasn't stayed that small
      and simple, and continues to evolve at an amazing rate.

      This document shows some of the current implementations on the
      market have already outstripped the capabilities of the RADIUS
      protocol.  A quite a few features have been developed completely
      outside the protocol.  These features use the RADIUS protocol
      structure and format, but employ operations and semantics well
      beyond the RFC documents.

   The limited set of data types defined in the RADIUS specification
   [RFC2865] led to subsequent documents defining new data types.  Since
   new data types are typically defined implicitly as part of defining a
   new attribute and because RADIUS client and server implementations
   differ in their support of these additional specifications, there is
   no definitive registry of RADIUS data types, and data type support
   has been inconsistent.  To catalog commonly implemented data types as
   well as to provide guidance for implementers and attribute designers,
   Section 2.1 of "RADIUS Design Guidelines" [RFC6158] includes advice
   on basic and complex data types.  Unfortunately, these guidelines
   [RFC6158] were published in 2011, 14 years after the RADIUS protocol
   was first documented [RFC2058] in 1997.

   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

   One issue concerns the data model for VSAs.  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.  Since this enables VSAs to be
   defined that 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
   Section 2.4 of "RADIUS Design Guidelines" BCP 158 [RFC6158]:

      RADIUS attributes can often be developed within the vendor space
      without loss (and possibly even with gain) in functionality.  As a
      result, translation of RADIUS attributes developed within the
      vendor space into the standard space may provide only modest

      benefits, while accelerating the exhaustion of the standard space.
      We do not expect that all RADIUS attribute specifications
      requiring interoperability will be developed within the IETF, and
      allocated from the standard space.  A more scalable approach is to
      recognize the flexibility of the vendor space, while working
      toward improvements in the quality and availability of RADIUS
      attribute specifications, regardless of where they are developed.

      It is therefore NOT RECOMMENDED that specifications intended
      solely for use by a vendor or SDO be translated into the standard

   Another issue is how implementations should handle unknown VSAs.
   Section 5.26 of the RADIUS specification [RFC2865] states:

      Servers not equipped to interpret the vendor-specific information
      sent by a client MUST ignore it (although it may be reported).
      Clients which do not receive desired vendor-specific information
      SHOULD make an attempt to operate without it, although they may do
      so (and report they are doing so) in a degraded mode.

   However, since VSAs do not contain a "mandatory" bit, RADIUS clients
   and servers may not know whether it is safe to ignore unknown VSAs.
   For example, in the case where VSAs pertain to security (e.g.,
   Filters), it may not be safe to ignore them.  As a result, Section
   2.5 of "Common Remote Authentication Dial In User Service (RADIUS)
   Implementation Issues and Suggested Fixes" [RFC5080] includes the
   following caution:

      To avoid misinterpretation of service requests encoded within
      VSAs, RADIUS servers SHOULD NOT send VSAs containing service
      requests to RADIUS clients that are not known to understand them.
      For example, a RADIUS server should not send a VSA encoding a
      filter without knowledge that the RADIUS client supports the VSA.

   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) [RFC5226], this permitted 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) Version 2 Protocol was developed by
   Netscape to be used to secure online transactions on the Internet.
   It was later replaced by SSLv3, also developed by Netscape.  SSLv3
   was then further developed by the IETF as the Transport Layer
   Security (TLS) 1.0 [RFC2246].

   The SSLv3 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"

      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  SSLv2 and SSLv3/TLS are not compatible.  It is possible to use
         SSLv2 protocol messages to initiate an SSLv3/TLS connection,
         but it is not possible to communicate with an SSLv2
         implementation using SSLv3/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 SSLv2 problem has forced SSLv3 and TLS clients to continue to use
   SSLv2 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.

   SSLv3 and TLS also permitted extension of the Client Hello and Server
   Hello handshake messages.  This functionality was fully defined by
   the introduction of TLS extensions, which make it possible to add new
   functionality to the handshake, such as the name of the server the
   client is connecting to, request certificate status information, and
   indicate Certificate Authority support, maximum record length, etc.
   Several of these extensions also introduce new handshake messages.

   It has turned out that many SSLv3 and TLS implementations that do not
   support TLS extensions did not ignore the unknown extensions, as
   required by the protocol specifications, but instead failed to
   establish connections.  Since several of the implementations behaving
   in this manner are used by high-profile Internet sites, such as
   online banking sites, 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

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

   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 that 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.

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 (editor)
   PMB 606
   15600 NE 8th Street, Suite B1
   Bellevue, WA 98008

   EMail: bernard_aboba@hotmail.com

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

   EMail: cheshire@apple.com


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