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Internet Architecture Board (IAB)                           B. CarpenterRequest for Comments: 6709                                 B. Aboba, Ed.Category: Informational                                      S. CheshireISSN: 2070-1721                                           September 2012Design Considerations for Protocol ExtensionsAbstract   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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6709.Copyright Notice   Copyright (c) 2012 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Carpenter, et al.             Informational                     [Page 1]

RFC 6709          Design Considerations for Extensions    September 2012Table of Contents1. Introduction ....................................................31.1. Requirements Language ......................................42. Routine and Major Extensions ....................................42.1. What Constitutes a Major Extension? ........................42.2. When is an Extension Routine? ..............................63. Architectural Principles ........................................73.1. Limited Extensibility ......................................73.2. Design for Global Interoperability .........................83.3. Architectural Compatibility ...............................123.4. Protocol Variations .......................................133.5. Testability ...............................................163.6. Protocol Parameter Registration ...........................163.7. Extensions to Critical Protocols ..........................174. Considerations for the Base Protocol ...........................184.1. Version Numbers ...........................................194.2. Reserved Fields ...........................................224.3. Encoding Formats ..........................................234.4. Parameter Space Design ....................................234.5. Cryptographic Agility .....................................264.6. Transport .................................................274.7. Handling of Unknown Extensions ............................285. Security Considerations ........................................296. References .....................................................306.1. Normative References ......................................306.2. Informative References ....................................307. Acknowledgments ................................................358. IAB Members at the Time of Approval ............................35Appendix A.  Examples .............................................36A.1. Already-Documented Cases ..................................36A.2. RADIUS Extensions .........................................36A.3. TLS Extensions ............................................39A.4. L2TP Extensions ...........................................41Carpenter, et al.             Informational                     [Page 2]

RFC 6709          Design Considerations for Extensions    September 20121.  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   [RFC4775].   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.Carpenter, et al.             Informational                     [Page 3]

RFC 6709          Design Considerations for Extensions    September 20121.1.  Requirements Language   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   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.Carpenter, et al.             Informational                     [Page 4]

RFC 6709          Design Considerations for Extensions    September 2012   Extension designers should examine their design for the following   issues:   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 (seeSection 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 inSection 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.Carpenter, et al.             Informational                     [Page 5]

RFC 6709          Design Considerations for Extensions    September 2012   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       extensions.   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 inSection 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   effects.   Examples of routine extensions include the Dynamic Host Configuration   Protocol (DHCP) vendor-specific option [RFC2132], Remote   Authentication Dial In User Service (RADIUS) Vendor-Specific   Attributes [RFC2865], the enterprise Object IDentifier (OID) tree for   Management Information Base (MIB) modules, and vendor Multipurpose   Internet Mail Extension (MIME) types.  Such extensions can safely be   made with minimal discussion.Carpenter, et al.             Informational                     [Page 6]

RFC 6709          Design Considerations for Extensions    September 2012   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       interoperability.   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.Carpenter, et al.             Informational                     [Page 7]

RFC 6709          Design Considerations for Extensions    September 2012   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 InteroperabilitySection 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.Carpenter, et al.             Informational                     [Page 8]

RFC 6709          Design Considerations for Extensions    September 2012   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   environments.   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],AppendixB:      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 meansCarpenter, et al.             Informational                     [Page 9]

RFC 6709          Design Considerations for Extensions    September 2012      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 inSection 4 of the 2010 update [RFC5727].  More generally, as   noted inSection 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   points.   Such values should be used with care and only for their stated   purpose: experiments and local use.  They are unsuitable for   Internet-wide use, since they may be used for conflicting purposes   and thereby cause interoperability failures.  Packets containing   experimental or local use values must not be allowed out of the   domain in which they are meaningful.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 beCarpenter, et al.             Informational                    [Page 10]

RFC 6709          Design Considerations for Extensions    September 2012      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      procedures.   However, as noted inAppendix 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      interoperability.Carpenter, et al.             Informational                    [Page 11]

RFC 6709          Design Considerations for Extensions    September 20123.2.3.  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   protocol.   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.Carpenter, et al.             Informational                    [Page 12]

RFC 6709          Design Considerations for Extensions    September 2012   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       capabilities.   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.  InCarpenter, et al.             Informational                    [Page 13]

RFC 6709          Design Considerations for Extensions    September 2012   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 inSection 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   [RFC5421]:      The reuse of previously assigned EAP Type Codes is incompatible      with EAP method negotiation as defined inRFC 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 choicesCarpenter, et al.             Informational                    [Page 14]

RFC 6709          Design Considerations for Extensions    September 2012       (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       specification).   While approach a) is potentially harmful, approach b) may be   beneficial.   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 inSection 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 Sections3 and4 of the Key Words document [RFC2119],   recommendations cannot be removed from profiles without serious   consideration:      [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      course.   Even the removal of optional features and requirements can have   consequences.  As noted inSection 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.)Carpenter, et al.             Informational                    [Page 15]

RFC 6709          Design Considerations for Extensions    September 20123.5.  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 inSection 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 (seehttp://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 areCarpenter, et al.             Informational                    [Page 16]

RFC 6709          Design Considerations for Extensions    September 2012   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 nodesCarpenter, et al.             Informational                    [Page 17]

RFC 6709          Design Considerations for Extensions    September 2012   that don't understand the LSA.  Within Interior Gateway Protocols   (IGPs), support for opaque LSAs has proven useful without introducing   instability.   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"   [ERROR-HANDLING].   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?Carpenter, et al.             Informational                    [Page 18]

RFC 6709          Design Considerations for Extensions    September 2012   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 inSection 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.Carpenter, et al.             Informational                    [Page 19]

RFC 6709          Design Considerations for Extensions    September 2012   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      specification....      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   decoration.   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   compatibility.Carpenter, et al.             Informational                    [Page 20]

RFC 6709          Design Considerations for Extensions    September 2012   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       approach.   2.  Highest mutually supported version (HMSV).  In this approach,       implementations exchange the version numbers of the highest       version each supports, with the negotiation agreeing on the       highest mutually supported protocol version.  This approach       implicitly assumes that later versions provide improved       functionality and that advertisement of a particular version       number implies support for all lower version numbers.  Where       these assumptions are invalid, this approach breaks down,       potentially resulting in interoperability problems.  An example       of this issue occurs in 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.Carpenter, et al.             Informational                    [Page 21]

RFC 6709          Design Considerations for Extensions    September 2012   5.  Min/max versioning.  This approach is similar to HMSV, but       without the implied obligation for clients and servers to support       all versions back to version 1, in perpetuity.  It allows clients       and servers to cleanly drop support for early versions when those       versions become so old that they are no longer relevant and no       longer required.  In this approach, the client initiating the       connection reports the highest and lowest protocol versions it       understands.  The server reports back the chosen protocol       version:       a.  If the server understands one or more versions in the           client's range, it reports back the highest mutually           understood version.       b.  If there is no mutual version, then the server reports back           some version that it does understand (selected as described           below).  The connection is then typically dropped by client           or server, but reporting this version number first helps           facilitate useful error messages at the client end:           *  If there is no mutual version, and the server speaks any              version higher than client max, it reports the lowest              version it speaks 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              <min>".   Protocols generally do not need any version-negotiation mechanism   more complicated than the mechanisms described here.  The nature of   protocol version-negotiation mechanisms is that, by definition, they   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   correctly.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 otherCarpenter, et al.             Informational                    [Page 22]

RFC 6709          Design Considerations for Extensions    September 2012   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   [ID-COMPARISON][PRECIS-STATEMENT][PRECIS-FRAMEWORK].   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 aCarpenter, et al.             Informational                    [Page 23]

RFC 6709          Design Considerations for Extensions    September 2012       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   include:   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      parameters.      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 demandCarpenter, et al.             Informational                    [Page 24]

RFC 6709          Design Considerations for Extensions    September 2012      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      implementation".   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      changes.   Parameter reclamation      In order to gain time, it may be necessary to reclaim unused      parameters.  However, it may not be easy to determine whether a      parameter that has been allocated is in use or not, particularly      if the entity that obtained the allocation no longer exists or has      been acquired (possibly multiple times).   Parameter transfer      When all the above mechanisms have proved infeasible and parameter      exhaustion looms in the near future, enabling the transfer of      ownership of protocol parameters can be considered as a means for      improving allocation efficiency.  However, enabling transfer of      parameter ownership can be far from simple if the parameter      allocation process was not originally designed to enable title      searches and ownership transfers.      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 registryCarpenter, et al.             Informational                    [Page 25]

RFC 6709          Design Considerations for Extensions    September 2012      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      employed.   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 inCarpenter, et al.             Informational                    [Page 26]

RFC 6709          Design Considerations for Extensions    September 2012   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   transports:   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   issues:   a.  NAT traversal concerns that were more easily addressed with UDP       transport;   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.Carpenter, et al.             Informational                    [Page 27]

RFC 6709          Design Considerations for Extensions    September 2012   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   variants.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   discarded".   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 inAppendix A.2 and inSection 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].Carpenter, et al.             Informational                    [Page 28]

RFC 6709          Design Considerations for Extensions    September 2012   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   algorithm.Carpenter, et al.             Informational                    [Page 29]

RFC 6709          Design Considerations for Extensions    September 20126.  References6.1.  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",BCP125,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   [ERROR-HANDLING]              Scudder, J., Chen, E., Mohapatra, P., and K. Patel,              "Revised Error Handling for BGP UPDATE Messages", Work in              Progress, June 2012.   [ID-COMPARISON]              Thaler, D., "Issues in Identifier Comparison for Security              Purposes", Work in Progress, August 2012.   [IEEE-802.1X]              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.   [PRECIS-FRAMEWORK]              Saint-Andre, P. and M. Blanchet, "PRECIS Framework:              Preparation and Comparison of Internationalized Strings in              Application Protocols", Work in Progress, August 2012.   [PRECIS-STATEMENT]              Blanchet, M. and A. Sullivan, "Stringprep Revision and              PRECIS Problem Statement", Work in Progress, August 2012.Carpenter, et al.             Informational                    [Page 30]

RFC 6709          Design Considerations for Extensions    September 2012   [RFC822]   Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET              TEXT MESSAGES", STD 11,RFC 822, August 1982.   [RFC1263]  O'Malley, S. and L. Peterson, "TCP Extensions Considered              Harmful",RFC 1263, October 1991.   [RFC1341]  Borenstein, N. and N. Freed, "MIME (Multipurpose Internet              Mail Extensions): Mechanisms for Specifying and Describing              the Format of Internet Message Bodies",RFC 1341, June              1992.   [RFC1521]  Borenstein, N. and N. Freed, "MIME (Multipurpose Internet              Mail Extensions) Part One: Mechanisms for Specifying and              Describing the Format of Internet Message Bodies",RFC1521, September 1993.   [RFC2058]  Rigney, C., Rubens, A., Simpson, W., and S. Willens,              "Remote Authentication Dial In User Service (RADIUS)",RFC2058, 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              1998.   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext              Transfer Protocol -- HTTP/1.1",RFC 2616, June 1999.   [RFC2661]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,              G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",RFC 2661, August 1999.   [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)",RFC2671, August 1999.   [RFC2822]  Resnick, P., Ed., "Internet Message Format",RFC 2822,              April 2001.Carpenter, et al.             Informational                    [Page 31]

RFC 6709          Design Considerations for Extensions    September 2012   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,              "Remote Authentication Dial In User Service (RADIUS)",RFC2865, 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              2003.   [RFC3588]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.              Arkko, "Diameter Base Protocol",RFC 3588, September 2003.   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record              (RR) Types",RFC 3597, September 2003.   [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",BCP95,RFC 3935, October 2004.Carpenter, et al.             Informational                    [Page 32]

RFC 6709          Design Considerations for Extensions    September 2012   [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",BCP132,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.Carpenter, et al.             Informational                    [Page 33]

RFC 6709          Design Considerations for Extensions    September 2012   [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              2008.   [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,RFC5727, March 2010.   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,              "Internet Key Exchange Protocol Version 2 (IKEv2)",RFC5996, 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.Carpenter, et al.             Informational                    [Page 34]

RFC 6709          Design Considerations for Extensions    September 20127.  Acknowledgments   This document is heavily based on an earlier draft by Scott Bradner   and Thomas Narten, other parts of which were eventually published asRFC 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 TschofenigCarpenter, et al.             Informational                    [Page 35]

RFC 6709          Design Considerations for Extensions    September 2012Appendix 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.Carpenter, et al.             Informational                    [Page 36]

RFC 6709          Design Considerations for Extensions    September 2012      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   problems.   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 inSection 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 modestCarpenter, et al.             Informational                    [Page 37]

RFC 6709          Design Considerations for Extensions    September 2012      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      space.   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,Section2.5 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].Carpenter, et al.             Informational                    [Page 38]

RFC 6709          Design Considerations for Extensions    September 2012A.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"   [RFC4366]:      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   extensions.   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.Carpenter, et al.             Informational                    [Page 39]

RFC 6709          Design Considerations for Extensions    September 2012   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.Carpenter, et al.             Informational                    [Page 40]

RFC 6709          Design Considerations for Extensions    September 2012   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   extensions.   L2TP also provides for vendor-specific AVPs.  Because everything in   L2TP is encoded using AVPs, it would be easy to define vendor-   specific AVPs that would be considered major extensions.   L2TP also provides for a "mandatory" bit in AVPs.  Recipients of L2TP   messages containing AVPs 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.Carpenter, et al.             Informational                    [Page 41]

RFC 6709          Design Considerations for Extensions    September 2012Authors' 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   USA   EMail: bernard_aboba@hotmail.com   Stuart Cheshire   Apple Inc.   1 Infinite Loop   Cupertino, CA 95014   USA   EMail: cheshire@apple.comCarpenter, et al.             Informational                    [Page 42]