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Internet Architecture Board (IAB) B. Carpenter
Request for Comments: 6709 B. Aboba, Ed.
Category: Informational S. Cheshire
ISSN: 2070-1721 September 2012
Design Considerations for Protocol Extensions
Abstract
This document discusses architectural issues related to the
extensibility of Internet protocols, with a focus on design
considerations. It is intended to assist designers of both base
protocols and extensions. Case studies are included. A companion
document, RFC 4775 (BCP 125), discusses procedures relating to the
extensibility of IETF protocols.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Architecture Board (IAB)
and represents information that the IAB has deemed valuable to
provide for permanent record. It represents the consensus of the
Internet Architecture Board (IAB). Documents approved for
publication by the IAB are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://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 to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Carpenter, et al. Informational [Page 1]
RFC 6709 Design Considerations for Extensions September 2012
Table of Contents
1. Introduction ....................................................3
1.1. Requirements Language ......................................4
2. Routine and Major Extensions ....................................4
2.1. What Constitutes a Major Extension? ........................4
2.2. When is an Extension Routine? ..............................6
3. Architectural Principles ........................................7
3.1. Limited Extensibility ......................................7
3.2. Design for Global Interoperability .........................8
3.3. Architectural Compatibility ...............................12
3.4. Protocol Variations .......................................13
3.5. Testability ...............................................16
3.6. Protocol Parameter Registration ...........................16
3.7. Extensions to Critical Protocols ..........................17
4. Considerations for the Base Protocol ...........................18
4.1. Version Numbers ...........................................19
4.2. Reserved Fields ...........................................22
4.3. Encoding Formats ..........................................23
4.4. Parameter Space Design ....................................23
4.5. Cryptographic Agility .....................................26
4.6. Transport .................................................27
4.7. Handling of Unknown Extensions ............................28
5. Security Considerations ........................................29
6. References .....................................................30
6.1. Normative References ......................................30
6.2. Informative References ....................................30
7. Acknowledgments ................................................35
8. IAB Members at the Time of Approval ............................35
Appendix A. Examples .............................................36
A.1. Already-Documented Cases ..................................36
A.2. RADIUS Extensions .........................................36
A.3. TLS Extensions ............................................39
A.4. L2TP Extensions ...........................................41
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1. Introduction
When developing protocols, IETF Working Groups (WGs) often include
mechanisms whereby these protocols can be extended in the future. It
is often a good principle to design extensibility into protocols; as
described in "What Makes for a Successful Protocol" [RFC5218], a
"wildly successful" protocol is one that becomes widely used in ways
not originally anticipated. Well-designed extensibility mechanisms
facilitate the evolution of protocols and help make it easier to roll
out incremental changes in an interoperable fashion. However, at the
same time, experience has shown that extensions carry the risk of
unintended consequences, such as interoperability issues, operational
problems, or security vulnerabilities.
The proliferation of extensions, even well-designed ones, can be
costly. As noted in "Simple Mail Transfer Protocol" [RFC5321]
Section 2.2.1:
Experience with many protocols has shown that protocols with few
options tend towards ubiquity, whereas protocols with many options
tend towards obscurity.
Each and every extension, regardless of its benefits, must be
carefully scrutinized with respect to its implementation,
deployment, and interoperability costs.
This is hardly a recent concern. "TCP Extensions Considered Harmful"
[RFC1263] was published in 1991. "Extend" or "extension" occurs in
the title of more than 400 existing Request for Comments (RFC)
documents. Yet, generic extension considerations have not been
documented previously.
The purpose of this document is to describe the architectural
principles of sound extensibility design, in order to minimize such
risks. Formal procedures for extending IETF protocols are discussed
in "Procedures for Protocol Extensions and Variations" BCP 125
[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.
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1.1. 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.
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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 (see Section 4.6), changing protocol
semantics, or defining new message types that may require
implementation changes in existing and deployed implementations
of the protocol, even if they do not want to make use of the new
functions. A base protocol that does not uniformly permit
"silent discard" of unknown extensions may automatically enter
this category, even for apparently minor extensions. Handling of
"unknown" extensions is discussed in more detail in Section 4.7.
2. Changes to the basic architectural assumptions. This may include
architectural assumptions that are explicitly stated or those
that have been assumed by implementers. For example, this would
include adding a requirement for session state to a previously
stateless protocol.
3. New usage scenarios not originally intended or investigated.
This can potentially lead to operational difficulties when
deployed, even in cases where the "on-the-wire" format has not
changed. For example, the level of traffic carried by the
protocol may increase substantially, packet sizes may increase,
and implementation algorithms that are widely deployed may not
scale sufficiently or otherwise be up to the new task at hand.
For example, a new DNS Resource Record (RR) type that is too big
to fit into a single UDP packet could cause interoperability
problems with existing DNS clients and servers. Similarly, the
additional traffic that results from an extension to a routing
protocol could have a detrimental impact on the performance or
stability of implementations that do not implement the extension.
4. Changes to the extension model. Adverse impacts are very likely
if the base protocol contains an extension mechanism and the
proposed extension does not fit into the model used to create and
define that mechanism. Extensions that have the same properties
as those that were anticipated when an extension mechanism was
devised are much less likely to be disruptive than extensions
that don't fit the model. Also, changes to the extension model
itself (including changes limiting further extensibility) can
create interoperability problems.
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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 in Section 5.
8. Performance impact. An extension that impacts performance can
have adverse consequences, particularly if the performance of
existing deployments is affected.
2.2. When is an Extension Routine?
An extension may be considered 'routine' if it does not meet the
criteria for being considered a 'major' extension and if its handling
is opaque to the protocol itself (e.g., does not substantially change
the pattern of messages and responses). For this to apply, no
changes to the base protocol can be required, nor can changes be
required to existing and currently deployed implementations, unless
they make use of the extension. Furthermore, existing
implementations should not be impacted. This typically requires that
implementations be able to ignore 'routine' extensions without ill
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.
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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.
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As noted in "What Makes for a Successful Protocol" [RFC5218], "wildly
successful" protocols far exceed their original goals, in terms of
scale, purpose (being used in scenarios far beyond the initial
design), or both. This implies that all potential uses may not be
known at inception. As a result, extensibility mechanisms may need
to be revisited as additional use cases reveal themselves. However,
this does not imply that an initial design needs to take all
potential needs into account at inception.
3.2. Design for Global Interoperability
Section 3.1 of "Procedures for Protocol Extensions and Variations"
BCP 125 [RFC4775] notes:
According to its Mission Statement [RFC3935], the IETF produces
high quality, relevant technical and engineering documents,
including protocol standards. The mission statement goes on to
say that the benefit of these standards to the Internet "is in
interoperability - that multiple products implementing a standard
are able to work together in order to deliver valuable functions
to the Internet's users".
One consequence of this mission is that the IETF designs protocols
for the single Internet. The IETF expects its protocols to work
the same everywhere. Protocol extensions designed for limited
environments may be reasonable provided that products with these
extensions interoperate with products without the extensions.
Extensions that break interoperability are unacceptable when
products with and without the extension are mixed. It is the
IETF's experience that this tends to happen on the Internet even
when the original designers of the extension did not expect this
to happen.
Another consequence of this definition of interoperability is that
the IETF values the ability to exchange one product implementing a
protocol with another. The IETF often specifies mandatory-to-
implement functionality as part of its protocols so that there is
a core set of functionality sufficient for interoperability that
all products implement. The IETF tries to avoid situations where
protocols need to be profiled to specify which optional features
are required for a given environment, because doing so harms
interoperability on the Internet as a whole.
Since the global Internet is more than a collection of incompatible
protocols (or "profiles") for use in separate private networks,
implementers supporting extensions in shipping products or multi-site
experimental usage must assume that systems will need to interoperate
on the global Internet.
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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], Appendix
B:
The primary problem with the "X-" convention is that
unstandardized parameters have a tendency to leak into the
protected space of standardized parameters, thus introducing the
need for migration from the "X-" name to a standardized name.
Migration, in turn, introduces interoperability issues (and
sometimes security issues) because older implementations will
support only the "X-" name and newer implementations might support
only the standardized name. To preserve interoperability, newer
implementations simply support the "X-" name forever, which means
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that the unstandardized name has become a de facto standard (thus
obviating the need for segregation of the name space into
standardized and unstandardized areas in the first place).
As a result, the notion of "X-" headers from the 1982 Internet
Message Format standard [RFC822] was removed when the specification
was updated in 2001 [RFC2822]. Within SIP, the guidance published in
2002 regarding "P-" headers [RFC3427] was deprecated eight years
later in Section 4 of the 2010 update [RFC5727]. More generally, as
noted in Section 1 of the "X-" prefix deprecation document [RFC6648]:
This document generalizes from the experience of the email and SIP
communities by doing the following:
1. Deprecates the "X-" convention for newly defined parameters in
application protocols, including new parameters for
established protocols. This change applies even where the
"X-" convention was only implicit, and not explicitly
provided, such as was done for email in [RFC822].
3.2.2. Local Use
Values designated as "experimental" or "local use" are only
appropriate in limited circumstances such as in early implementations
of an extension restricted to a single site.
For example, "Experimental Values in IPv4, IPv6, ICMPv4, ICMPv6, UDP,
and TCP Headers" [RFC4727] discusses experimental values for IP and
transport headers, and "Definition of the Differentiated Services
Field (DS Field) in the IPv4 and IPv6 Headers" [RFC2474] defines
experimental/local use ranges for differentiated services code
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 be
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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 in Appendix B of the "X-" prefix deprecation
document [RFC6648], assigning a parameter block for experimental use
is only necessary when the parameter pool is limited:
"Assigning Experimental and Testing Numbers Considered Useful" ...
implies that the "X-" prefix is also useful for experimental
parameters. However, BCP 82 addresses the need for protocol
numbers when the pool of such numbers is strictly limited (e.g.,
DHCP options) or when a number is absolutely required even for
purely experimental purposes (e.g., the Protocol field of the IP
header). In almost all application protocols that make use of
protocol parameters (including email headers, media types, HTTP
headers, vCard parameters and properties, URNs, and LDAP field
names), the name space is not limited or constrained in any way,
so there is no need to assign a block of names for private use or
experimental purposes....
Therefore, it appears that segregating the parameter space into a
standardized area and a unstandardized area has few, if any,
benefits and has at least one significant cost in terms of
interoperability.
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3.2.3. Multi-Site Experiments
Where an experiment is undertaken among a diverse set of experimental
sites connected via the global Internet, the use of "experimental" or
"local use" code points is inadvisable. This might include, for
example, sites that take a prototype implementation of some protocol
and use that both within their site but, importantly, among the full
set of other sites interested in that protocol. In such a situation,
it is impractical and probably impossible to coordinate the
de-confliction of "experimental" code points. Section 4.1 of the
IANA Considerations guidelines document [RFC5226] notes:
For private or local use ... No attempt is made to prevent
multiple sites from using the same value in different (and
incompatible) ways.... assignments are not generally useful for
broad interoperability. It is the responsibility of the sites
making use of the Private Use range to ensure that no conflicts
occur (within the intended scope of use).
The Host Identity Protocol (HIP) [RFC5201] and the Locator/ID
Separation Protocol [LISP] are examples where a set of experimental
sites are collaborating among themselves, but not necessarily in a
tightly coordinated way. Both HIP and LISP have dealt with this by
having unique non-experimental code points allocated to HIP and LISP,
respectively, at the time of publication of their respective
Experimental RFCs.
3.3. Architectural Compatibility
Since protocol extension mechanisms may impact interoperability, it
is important that they be architecturally compatible with the base
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.
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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. In
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general, such variations should be avoided. Causes of protocol
variations include incompatible protocol extensions, uncoordinated
protocol development, and poorly designed "profiles".
Designing a protocol for extensibility may have the perverse side
effect of making it easy to construct incompatible variations.
Protocol extension mechanisms should not be used to create
incompatible forks in development. An extension may lead to
interoperability failures unless the extended protocol correctly
supports all mandatory and optional features of the unextended base
protocol, and implementations of the base protocol operate correctly
in the presence of the extensions. In addition, it is necessary for
an extension to interoperate with other extensions.
As noted in Section 1 of "Uncoordinated Protocol Development
Considered Harmful" [RFC5704], incompatible forks in development can
result from the uncoordinated adaptation of a protocol, parameter, or
code point:
In particular, the IAB considers it an essential principle of the
protocol development process that only one SDO maintains design
authority for a given protocol, with that SDO having ultimate
authority over the allocation of protocol parameter code-points
and over defining the intended semantics, interpretation, and
actions associated with those code-points.
Note that problems can occur even when one Standards Development
Organization (SDO) maintains design authority, if protocol parameter
code points are reused. As an example, EAP-FAST [RFC5421][RFC5422]
reused previously assigned Extensible Authentication Protocol (EAP)
type codes. As described in the IESG note in the EAP-FAST document
[RFC5421]:
The reuse of previously assigned EAP Type Codes is incompatible
with EAP method negotiation as defined in RFC 3748.
3.4.1. Profiles
Profiling is a common technique for improving interoperability within
a target environment or set of scenarios. Generally speaking, there
are two approaches to profiling:
a) Removal or downgrading of normative requirements (thereby
creating potential interoperability problems).
b) Elevation of normative requirement levels (such as from a
MAY/SHOULD to a MUST). This can be done in order to improve
interoperability by narrowing potential implementation choices
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(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 in Section 6 of "Key words for use
in RFCs to Indicate Requirement Levels" [RFC2119], imperatives are to
be used with care, and as a result, their removal within a profile is
likely to result in serious consequences:
Imperatives of the type defined in this memo must be used with
care and sparingly. In particular, they MUST only be used where
it is actually required for interoperation or to limit behavior
which has potential for causing harm (e.g., limiting
retransmissions) For example, they must not be used to try to
impose a particular method on implementors where the method is not
required for interoperability.
As noted in Sections 3 and 4 of the Key Words document [RFC2119],
recommendations cannot be removed from profiles without serious
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 in Section 5 of the Key Words document
[RFC2119], implementations that do not support optional features
still retain the obligation to ensure interoperation with
implementations that do:
An implementation which does not include a particular option MUST
be prepared to interoperate with another implementation which does
include the option, though perhaps with reduced functionality. In
the same vein an implementation which does include a particular
option MUST be prepared to interoperate with another
implementation which does not include the option (except, of
course, for the feature the option provides.)
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3.5. Testability
Experience has shown that it is insufficient merely to specify
extensibility and backward compatibility correctly in an RFC. It is
also important that implementations respect the compatibility
mechanisms; if not, non-interoperable pairs of implementations may
arise. The TLS case study (Appendix A.3) shows how important this
can be.
In order to determine whether protocol extension mechanisms have been
properly implemented, testing is required. However, for this to be
possible, test cases need to be developed. If a base protocol
document specifies extension mechanisms but does not utilize them or
provide examples, it may not be possible to develop effective test
cases based on the base protocol specification alone. As a result,
base protocol implementations may not be properly tested, and non-
compliant extension behavior may not be detected until these
implementations are widely deployed.
To encourage correct implementation of extension mechanisms, base
protocol specifications should clearly articulate the expected
behavior of extension mechanisms and should include examples of
correct extension behavior.
3.6. Protocol Parameter Registration
As noted in Section 3.2 of "Procedures for Protocol Extensions and
Variations" BCP 125 [RFC4775]:
An extension is often likely to make use of additional values
added to an existing IANA registry.... It is essential that such
new values are properly registered by the applicable procedures,
including expert review where applicable.... Extensions may even
need to create new IANA registries in some cases.
Experience shows that the importance of this is often
underestimated during extension design; designers sometimes assume
that a new codepoint is theirs for the asking, or even simply for
the taking.
Before creating a new protocol parameter registry, existing
registries should be examined to determine whether one of them can be
used instead (see http://www.iana.org/protocols/).
To avoid conflicting usage of the same registry value, as well as to
prevent potential difficulties in determining and transferring
parameter ownership, it is essential that all new values are
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registered. If this is not done, there is nothing to prevent two
different extensions picking the same value. When these two
extensions "meet" each other on the Internet, failure is inevitable.
A surprisingly common case of this is misappropriation of assigned
Transmission Control Protocol (TCP) (or User Datagram Protocol (UDP))
registered port numbers. This can lead to a client for one service
attempting to communicate with a server for another service. Another
common case is the use of unregistered URI schemes. Numerous cases
could be cited, but not without embarrassing specific implementers.
For general rules, see the IANA Considerations guidelines document
[RFC5226], and for specific rules and registries, see the individual
protocol specification RFCs and the IANA web site.
While in theory a "Standards Track" or "IETF Consensus" parameter
allocation policy may be instituted to encourage protocol parameter
registration or to improve interoperability, in practice, problems
can arise if the procedures result in so much delay that requesters
give up and "self-allocate" by picking presumably unused code points.
Where self-allocation is prevalent, the information contained within
registries may become inaccurate, particularly when third parties are
prohibited from updating entries so as to improve accuracy. In these
situations, it is important to consider whether registration
processes need to be changed to support the role of a registry as
"documentation of how the Internet is operating".
3.7. Extensions to Critical Protocols
Some protocols (such as the Domain Name System (DNS), the Border
Gateway Protocol (BGP), and the Hypertext Transfer Protocol (HTTP))
or algorithms (such as congestion control) have become critical
components of the Internet infrastructure. A critical component is
one whose failure can lead to Internet-wide reliability and security
issues or performance degradation. When such protocols or algorithms
are extended, the potential exists for negatively impacting the
reliability and security of the global Internet.
As a result, special care needs to be taken with these extensions,
such as taking explicit steps to isolate existing uses from new ones.
For example, this can be accomplished by requiring the extension to
utilize a different port or multicast address or by implementing the
extension within a separate process, without access to the data and
control structures of the base protocol.
Experience has shown that even when a mechanism has proven benign in
other uses, unforeseen issues may result when adding it to a critical
protocol. For example, both IS-IS and OSPF support opaque Link State
Advertisements (LSAs), which are propagated by intermediate nodes
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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?
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4. Minimize impact on critical infrastructure. For a protocol that
represents a critical element of Internet infrastructure, it is
important to explain when it is appropriate to isolate new uses
of the protocol from existing ones.
For example, is it explained when a proposed extension (or usage)
has the potential for negatively impacting critical
infrastructure to the point where explicit steps would be
appropriate to isolate existing uses from new ones?
5. Provide guidance on data model extensions. Is there a document
that explains when a protocol extension is routine and when it
represents a major change?
For example, is it clear when a data model extension represents a
major versus a routine change? Are there guidelines describing
when an extension (such as a new data type) is likely to require
a code change within existing implementations?
4.1. Version Numbers
Any mechanism for extension by versioning must include provisions to
ensure interoperability, or at least clean failure modes. Imagine
someone creating a protocol and using a "version" field and
populating it with a value (1, let's say), but giving no information
about what would happen when a new version number appears in it.
This would be a bad protocol design and description; it should be
clear what the expectation is and how it can be tested. For example,
stating that 1.X must be compatible with any version 1 code, but
version 2 or greater is not expected to be compatible, has different
implications than stating that version 1 must be a proper subset of
version 2.
An example of an under-specified versioning mechanism is provided by
the MIME-Version header, originally defined in "MIME (Multipurpose
Internet Mail Extensions)" [RFC1341]. As noted in Section 1 of the
MIME specification [RFC1341]:
A MIME-Version header field ... uses a version number to declare a
message to be conformant with this specification and allows mail
processing agents to distinguish between such messages and those
generated by older or non-conformant software, which is presumed
to lack such a field.
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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.
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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.
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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 other
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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 a
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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 demand
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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 registry
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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 in
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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.
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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 in Appendix A.2 and in Section 2.5 of "Common Remote
Authentication Dial In User Service (RADIUS) Implementation Issues
and Suggested Fixes" [RFC5080].
In order to ensure that a recipient supports an extension, a
recipient encountering an unknown extension may be required to
explicitly reject it and to return an error, rather than ignoring the
unknown extension and proceeding with the remainder of the message.
This can be accomplished via a "Mandatory" bit in a TLV-based
protocol such as the Layer 2 Tunneling Protocol (L2TP) [RFC2661], or
a "Require" or "Proxy-Require" header in a text-based protocol such
as SIP [RFC3261] or HTTP [RFC2616].
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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.
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6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4775] Bradner, S., Carpenter, B., Ed., and T. Narten,
"Procedures for Protocol Extensions and Variations", BCP
125, RFC 4775, December 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
6.2. Informative References
[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", RFC
1521, September 1993.
[RFC2058] Rigney, C., Rubens, A., Simpson, W., and S. Willens,
"Remote Authentication Dial In User Service (RADIUS)", RFC
2058, January 1997.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC2284] Blunk, L. and J. Vollbrecht, "PPP Extensible
Authentication Protocol (EAP)", RFC 2284, March 1998.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474, December
1998.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
RFC 2661, August 1999.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
2671, August 1999.
[RFC2822] Resnick, P., 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)", RFC
2865, June 2000.
[RFC2882] Mitton, D., "Network Access Servers Requirements: Extended
RADIUS Practices", RFC 2882, July 2000.
[RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
Compression (ROHC): Framework and four profiles: RTP, UDP,
ESP, and uncompressed", RFC 3095, July 2001.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3427] Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J.,
and B. Rosen, "Change Process for the Session Initiation
Protocol (SIP)", RFC 3427, December 2002.
[RFC3575] Aboba, B., "IANA Considerations for RADIUS (Remote
Authentication Dial In User Service)", RFC 3575, July
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", BCP
95, 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", BCP
132, RFC 4962, July 2007.
[RFC5080] Nelson, D. and A. DeKok, "Common Remote Authentication
Dial In User Service (RADIUS) Implementation Issues and
Suggested Fixes", RFC 5080, December 2007.
[RFC5201] Moskowitz, R., Nikander, P., Jokela, P., Ed., and T.
Henderson, "Host Identity Protocol", RFC 5201, April 2008.
[RFC5218] Thaler, D. and B. Aboba, "What Makes For a Successful
Protocol?", RFC 5218, July 2008.
[RFC5225] Pelletier, G. and K. Sandlund, "RObust Header Compression
Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and
UDP-Lite", RFC 5225, April 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
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, RFC
5727, March 2010.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
5996, September 2010.
[RFC6055] Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on
Encodings for Internationalized Domain Names", RFC 6055,
February 2011.
[RFC6158] DeKok, A., Ed., and G. Weber, "RADIUS Design Guidelines",
BCP 158, RFC 6158, March 2011.
[RFC6648] Saint-Andre, P., Crocker, D., and M. Nottingham,
"Deprecating the "X-" Prefix and Similar Constructs in
Application Protocols", BCP 178, RFC 6648, June 2012.
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RFC 6709 Design Considerations for Extensions September 2012
7. Acknowledgments
This document is heavily based on an earlier draft by Scott Bradner
and Thomas Narten, other parts of which were eventually published as
RFC 4775.
That draft stated: "The initial version of this document was put
together by the IESG in 2002. Since then, it has been reworked in
response to feedback from John Loughney, Henrik Levkowetz, Mark
Townsley, Randy Bush and others."
Valuable comments and suggestions on the current form of the document
were made by Loa Andersson, Ran Atkinson, Stewart Bryant, Leslie
Daigle, Alan DeKok, Roy Fielding, Phillip Hallam-Baker, Ted Hardie,
Alfred Hoenes, John Klensin, Barry Leiba, Eric Rescorla, Adam Roach,
and Pekka Savola. The text on TLS experience was contributed by
Yngve Pettersen.
8. IAB Members at the Time of Approval
Bernard Aboba
Jari Arkko
Marc Blanchet
Ross Callon
Alissa Cooper
Spencer Dawkins
Joel Halpern
Russ Housley
David Kessens
Danny McPherson
Jon Peterson
Dave Thaler
Hannes Tschofenig
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Appendix A. Examples
This section discusses some specific examples as case studies.
A.1. Already-Documented Cases
There are certain documents that specify a change process or describe
extension considerations for specific IETF protocols:
The SIP change process [RFC3427], [RFC4485], [RFC5727]
The (G)MPLS change process (mainly procedural) [RFC4929]
LDAP extensions [RFC4521]
EPP extensions [RFC3735]
DNS extensions [RFC2671][RFC3597]
SMTP extensions [RFC5321]
It is relatively common for MIBs, which are all in effect extensions
of the SMI data model, to be defined or extended outside the IETF.
BCP 111 [RFC4181] offers detailed guidance for authors and reviewers.
A.2. RADIUS Extensions
The RADIUS [RFC2865] protocol was designed to be extensible via
addition of Attributes. This extensibility model assumed that
Attributes would conform to a limited set of data types and that
vendor extensions would be limited to use by vendors in situations in
which interoperability was not required. Subsequent developments
have stretched those assumptions.
From the beginning, uses of the RADIUS protocol extended beyond the
scope of the original protocol definition (and beyond the scope of
the RADIUS Working Group charter). In addition to rampant self-
allocation within the limited RADIUS standard attribute space,
vendors defined their own RADIUS commands. This led to the rapid
proliferation of vendor-specific protocol variants. To this day,
many common implementation practices have not been documented. For
example, authentication server implementations are often typically
based on a Data Dictionary, enabling addition of Attributes without
requiring code changes. Yet, the concept of a Data Dictionary is not
mentioned in the RADIUS specification [RFC2865].
As noted in "Extended RADIUS Practices" [RFC2882], Section 1:
The RADIUS Working Group was formed in 1995 to document the
protocol of the same name, and was chartered to stay within a set
of bounds for dial-in terminal servers. Unfortunately the real
world of Network Access Servers (NASes) hasn't stayed that small
and simple, and continues to evolve at an amazing rate.
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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 in
Section 2.4 of "RADIUS Design Guidelines" BCP 158 [RFC6158]:
RADIUS attributes can often be developed within the vendor space
without loss (and possibly even with gain) in functionality. As a
result, translation of RADIUS attributes developed within the
vendor space into the standard space may provide only modest
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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, Section
2.5 of "Common Remote Authentication Dial In User Service (RADIUS)
Implementation Issues and Suggested Fixes" [RFC5080] includes the
following caution:
To avoid misinterpretation of service requests encoded within
VSAs, RADIUS servers SHOULD NOT send VSAs containing service
requests to RADIUS clients that are not known to understand them.
For example, a RADIUS server should not send a VSA encoding a
filter without knowledge that the RADIUS client supports the VSA.
In addition to extending RADIUS by use of VSAs, SDOs have also
defined new values of the Service-Type attribute in order to create
new RADIUS commands. Since the RADIUS specification [RFC2865]
defined Service-Type values as being allocated First Come, First
Served (FCFS) [RFC5226], this permitted new RADIUS commands to be
allocated without IETF review. This oversight has since been fixed
in "IANA Considerations for RADIUS" [RFC3575].
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A.3. TLS Extensions
The Secure Sockets Layer (SSL) Version 2 Protocol was developed by
Netscape to be used to secure online transactions on the Internet.
It was later replaced by SSLv3, also developed by Netscape. SSLv3
was then further developed by the IETF as the Transport Layer
Security (TLS) 1.0 [RFC2246].
The SSLv3 protocol was not explicitly specified to be extended. Even
TLS 1.0 did not define an extension mechanism explicitly. However,
extension "loopholes" were available. Extension mechanisms were
finally defined in "Transport Layer Security (TLS) Extensions"
[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.
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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.
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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.
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Authors' Addresses
Brian Carpenter
Department of Computer Science
University of Auckland
PB 92019
Auckland, 1142
New Zealand
EMail: brian.e.carpenter@gmail.com
Bernard Aboba (editor)
PMB 606
15600 NE 8th Street, Suite B1
Bellevue, WA 98008
USA
EMail: bernard_aboba@hotmail.com
Stuart Cheshire
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
USA
EMail: cheshire@apple.com
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