Network Working Group B. Carpenter (ed)
Internet-Draft IBM
Intended status: Informational March 3, 2007
Expires: September 4, 2007
Design considerations for protocol extensions
draft-carpenter-extension-recs-01
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document discusses issues related to the extensibility of
Internet protocols, with a focus on the architectural design
considerations involved. Concrete case study examples are included.
It is intended to assist designers of both base protocols and
extensions.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Interoperability . . . . . . . . . . . . . . . . . . . . . . . 3
3. Specific Considerations for Robust Extensions . . . . . . . . 5
3.1. Checklist for Interoperability of Extensions . . . . . . . 5
3.2. Use of Registered Values . . . . . . . . . . . . . . . . . 6
3.3. When is an Extension Routine? . . . . . . . . . . . . . . 6
3.4. Specific Issues with Major Extensions . . . . . . . . . . 7
4. Considerations for the Base Protocol . . . . . . . . . . . . . 7
4.1. Version Numbers . . . . . . . . . . . . . . . . . . . . . 8
4.2. Reserved Fields . . . . . . . . . . . . . . . . . . . . . 8
4.3. Encoding Formats . . . . . . . . . . . . . . . . . . . . . 9
4.4. Avoiding Unnecessary Extensibility . . . . . . . . . . . . 9
4.5. Documenting Extensibility . . . . . . . . . . . . . . . . 9
5. Running Code Must Run Right . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
9. Change log [RFC Editor: please remove this section] . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 13
A.1. Already documented cases . . . . . . . . . . . . . . . . . 13
A.2. RADIUS Extensions . . . . . . . . . . . . . . . . . . . . 13
A.3. TLS Extensions . . . . . . . . . . . . . . . . . . . . . . 15
A.4. L2TP Extensions . . . . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 18
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1. Introduction
For the origins of this draft, please see the Acknowledgements
section. Authorship will be updated.
When an initial protocol design is extended, there is always a risk
of introducing interoperability defects, security defects, etc.,
along with the additional functionality. This risk is especially
high if the extension is performed by a different team than the
original designers, who may stray outside implicit design constraints
or assumptions. This document aims to describe technical
considerations for protocol extensions that will minimize such risks.
Although written in general terms, it is largely aimed at people
considering extending an IETF protocol.
This document is intended to be technical. Formal procedures for
extending IETF protocols are discussed in BCP 125 [RFC4775].
IETF protocols typically include mechanisms whereby they can be
extended in the future. It is of course a good principle to design
extensiblity into protocols; one common definition of a successful
protocol is one that becomes widely used in ways not originally
anticipated. Well-designed extensibility mechanisms facilitate the
evolution of protocols and help make it easier to roll out
incremental changes in an interoperable fashion. At the same time,
experience suggests that extensibility features should be limited to
what is clearly necessary when the protocol is developed and that any
later extensions should be done carefully and with a full
understanding of the base protocol, existing implementations, and
current operational practice.
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 300 existing RFCs. Yet generic extension
considerations have not been documented previously.
This document is intended to to assist designers of both base
protocols and extensions. A good extension design depends on a good
base protocol. Although Section 3 is aimed principally at extension
designers, and Section 4 at base protocol designers, readers are
advised to study the whole document, since the two sets of
considerations are closely linked.
2. Interoperability
Global interoperability is a primary goal of IETF protocol design.
Experience shows that software is often used outside the particular
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special environment it was originally intended for, so extensions
cannot be designed for an isolated environment. Designers of
extensions must assume the high likelihood of a system using the
extension having to interoperate with systems on the global Internet.
For this reason, 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. Consider for example a "private"
extension installed on a work computer which, being portable, is
sometimes installed on a home network or in a hotel. If the
"private" extension is incompatible with an unextended version of the
same protocol, problems will occur.
Another aspect is that that mechanisms included to allow the
extension of protocols should not be used to create incompatible
forks in development instead. Ideally, the protocol mechanisms for
extension and versioning should be sufficiently well described that
compatibility can be assessed on paper. Otherwise, when two
"private" extensions encounter each other on a public network,
unexpected interoperability problems may occur.
An example of what might go wrong is misuse of the X- mail headers
originally defined in SMTP [RFC0822]. X-anything was a valid mail
header; but it had no defined meaning in the standard. Suppose a
mail implementation assigns specifc semantics to X-anything that
causes it to take specific action, such as discarding a message as
spam. If it encounters a message from a different implementation
that assigns different semantics, it may take quite inappropriate
action, such as discarding a valid message. Thus, relying on the
implied semantics of an X- header automatically creates a risk of
operational failures. X- headers were removed from [RFC2822]. Even
when they are assigned semantics, as in [RFC4356], great care must be
taken that implementations do not rely on such semantics in messages
that have crossed the open Internet.
Thus we observe that 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.
Additionally, it should be noted that protocol variations -
specifications that look very similar to the original but are
actually different - are even more harmful to interoperability than
extensions. In general, such variations should be avoided. If they
cannot be avoided, as many of the following considerations as
possible should be applied, to minimize the damage to
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interoperability.
3. Specific Considerations for Robust Extensions
This section makes explicit some design considerations based on the
community's experience with extensibility mechanisms.
3.1. Checklist for Interoperability of Extensions
Good interoperability stems from a number of factors, including:
1. having a well-written spec, that makes clear and precise what an
implementor needs to implement and what impact each individual
operation (e.g., a message sent to a peer) will have when
invoked.
2. learning lessons from deployment, including understanding what
current implementations do and how a proposed extension will
interact with deployed systems.
3. having an adequate transition or coexistence story for deploying
the new extension. What impact will the proposed extension have
on implementations that do not understand it? Is there a way to
negotiate or determine the capabilities of a peer?
4. being architecturally compatible with the base protocol. For
example, does the extension make use of features as envisioned by
the original protocol designers, or is a new mechanism being
invented?
5. respecting underlying architectural or security assumptions
(including those that may not be well-documented, those that may
have arison as a result of operational experience, or those that
only became understood after the original protocol was
published).
6. will the proposed extension (or its proposed usage) operationally
stress existing implementations or the underlying protocol itself
if widely deployed?
7. some protocols have become critical components of the Internet
infrastructure. Does the proposed extension (or its proposed
usage) have the potential for negatively impacting such
infrastructure to the point where explicit steps would be
appropriate to isolate existing uses from new ones?
8. does the proposed extension extend the data model in a major way?
Does the extension fundamentally change basic assumptions about
data handling within the protocol? For example, do the
extensions reverse the flow of data, allow formerly static
parameters to be changed on the fly, add new data types or change
assumptions relating to the frequency of reads/writes?
9. can the extended protocol negotiate with an unextended partner to
find a common subset of useful functions?
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3.2. Use of Registered Values
An extension is often likely to make use of additional values added
to an existing IANA registry (in many cases, simply by adding a new
"TLV" (type-length-value) field). To avoid conflicting usage of the
same value, it is essential that all new values are properly
registered by the applicable procedures. See BCP 26, [RFC2434] for
the general rules, and individual protocol RFCs, and the IANA web
site, for specfic rules and registries. 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
TCP (or UDP) registered port numbers. This can lead to a client for
one service attempting to communicate with a server for another
service. Numerous cases could be cited, but not without embarassing
specific implementors.
In some cases, it may be appropriate to use values designated as
"experimental" or "local use" in early implementations of an
extension. For example, [RFC4727] discusses experimental values for
IP and transport headers, and [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.
3.3. When is an Extension Routine?
An extension may be considered relatively routine if it basically
amounts to a new data element that is opaque to the protocol itself
(i.e. does not substantially change the pattern of messages and
responses).
For this to apply, the protocol must have been designed to carry
opaque data elements, so that no changes to the underlying base
protocol are needed to carry a new type of data. Moreover, no
changes are required to existing and currently deployed
implementations of the underlying protocol unless they want to make
use of the new data type. Using the existing protocol to carry a new
type of opaque data should not impact existing implementations or
cause operational problems. Typically this means that the protocol
is specified to silently discard unknown data elements.
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Examples of routine extensions include the DHC vendor-specific
option, the enterprise OID tree for MIB modules, vendor MIME types,
and some classes of (non-critical) certification extensions. Such
extensions can safely be made with minimal discussion and are usually
indicated by having IANA Considerations that allow assignments of
code points with minimal overhead (e.g., First Come First Served)
[RFC2434].
3.4. Specific Issues with Major Extensions
Major extensions may have some or all of the following
characteristics, each of which leads to a risk of interoperability
failure:
1. Change or extend the way in which the basic underlying protocol
works, e.g., by changing the semantics of existing PDUs or
defining new message types that may require implementation
changes in existing and deployed implementations of the
protocols, even if they do not want to make use of the new
functions or data types. (Note that a base protocol without a
"silent discard" rule for unknown data elements may automatically
enter this category, even for apparently minor extensions.)
2. Change basic architectural assumptions about the protocol that
have been an assumed part of the protocol and its
implementations. For example, add a requirement for session
state to a previously stateless protocol.
3. Lead to new uses of the protocol in ways not originally intended
or investigated, potentially leading to operational and other
difficulties when deployed, even in cases where the "on-the-wire"
format has not changed. For example, the overall quantity of
traffic the protocol is expected to carry might go up
substantially, typical packet sizes may increase compared to
existing deployments, simple implementation algorithms that are
widely deployed may not scale sufficiently or otherwise be up to
the new task at hand, etc. A specific example would be a new DNS
RR type that is too big to fit into a single UDP packet.
Each of these leads directly to a need to pay extremely close
attention to backward compatibility with implementations and
deployments of the unextended protocol, and to the risk of
inadvertent introduction of security or operational exposures.
Extension designers should examine their design for the above three
issues.
4. Considerations for the Base Protocol
Ideally, the document that defines a base protocol's extension
mechanisms will include guidance to future extension writers that
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help them use extension mechanisms properly. It may also be possible
to define classes of extensions that need little or no review, while
other classes need wide review. The details will necessarily be
technology-specific.
4.1. Version Numbers
Any mechanism for extension by versioning must include provisions to
ensure interoperability, or at least clean failure modes. Imagine
someone creating a protocol and using a "version" field and
populating it with a value (1, let's say), but giving no information
about what would happen when a new version number appears in it.
That's bad protocol design and description; it should be clear what
the expectation is and how you test it. For example, stating that
1.X must be compatible with any version 1 code, but version 2 or
greater is not expected to be compatible, has different implications
than stating that version 1 must be a proper subset of version 2.
An interesting example here is ROHC (Robust Header Compression).
ROHCv1 [RFC3095] supports a certain set of profiles for compression
algorithms. But experience has shown that these profiles have
limitations, so the ROHC WG is developing ROHCv2. A ROHCv1
implementation will not contain code for the ROHCv2 profiles. As the
ROHC WG charter said at the time of writing:
...It should be noted that the v2
profiles will thus not be compatible with the original (ROHCv1)
profiles, which means less complex ROHC implementations can be
realized by not providing support for ROHCv1 (over links not yet
supporting ROHC, or by shifting out support for ROHCv1 in the long
run). Profile support is agreed through the ROHC channel negotiation,
which is part of the ROHC framework and thus not changed by ROHCv2.
Thus in this case both backwards-compatible and backwards-
incompatible deployments are possible. The important point is a
clearly thought out approach to the question of operational
compatibility.
4.2. Reserved Fields
Protocols commonly include one or more "reserved" fields, clearly
intended for future extensions. It is good practice to specify the
value to be inserted in such a field by the sender (typically zero)
and the action to be taken by the receiver when seeing some other
value (typically no action). If this is not done, future
implementation of new values in the reserved field may break old
software. Similarly, protocols should carefully specify how
receivers should react to unknown TLVs etc., such that failures occur
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only when that is truly the desired result.
4.3. Encoding Formats
Using widely-supported encoding formats leads to better
interoperability and easier extensibility. An excellent example is
the SNMP SMI. Guidelines exist for defining the MIB objects that
SNMP carries [RFC4181]. Also, multiple textual conventions have been
published, so that MIB designers do not have to reinvent the wheel
when they need a commonly encountered construct. For example, the
"Textual Conventions for Internet Network Addresses" [RFC4001] can be
used by any MIB designer needing to define objects containing IP
addresses, thus ensuring consistency as the body of MIBs is extended.
4.4. Avoiding Unnecessary Extensibility
Protocols that permit easy extensions may have the perverse side
effect of making it easy to construct incompatible extensions.
Consequently, protocols should not be made more extensible than
clearly necessary at inception, and the process for defining new
extensibility mechanisms should ensure that adequate review of
proposed extensions will take place before widespread adoption. In
practice, this means First Come First Served [RFC2434] and similar
policies that allow routine extensions should be used sparingly, as
they imply minimal or no review. In particular, they should be
limited to cases, such as allowing new opaque data elements, that are
unlikely to cause protocol failures.
In order to increase the likelihood that routine extensions are truly
routine, protocol documents should provide guidelines explaining how
they should be performed. 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.
4.5. Documenting Extensibility
Some protocol components are designed with the specific intention of
allowing extensibility. These should be clearly identified, with
specific and complete instructions on how to extend them, including
the process for adequate review of extension proposals: do they need
community review and if so how much and by whom? For example, the
definition of additional data formats that can be carried opaquely
may require no review, while the addition of new protocol message
types might require a Standards Track action. Guidance on writing
appropriate IANA Considerations text may be found in [RFC2434].
In a number of cases, there is a need for explicit guidance relating
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to extensions beyond what is encapsulated in the IANA considerations
section of the base specification. The usefulness of [RFC4181] would
appear to suggest that protocols whose data model is likely to be
widely extended (particularly using vendor-specific elements) need a
Design Guidelines document specifically addressing extensions.
5. Running Code Must Run Right
Experience shows that it is insufficient to correctly specify
extensibility and backwards compatibility in an RFC. It is also of
importance that every implementation must fully respect the
compatibility mechanisms; if not, non-interoperable pairs of
implementations may arise. The TLS case study below shows how
important this may be.
6. Security Considerations
An extension must not introduce new security risks without also
providing adequate counter-measures, and in particular it must not
inadvertently defeat security measures in the unextended protocol.
Thus, the security analysis for an extension needs to be as thorough
as for the original protocol - effectively it needs to be a
regression analysis to check that the extension doesn't inadvertently
invalidate the original security model.
This analysis may be simple (e.g. adding an extra opaque data element
is unlikely to create a new risk) or quite complex (e.g. adding a
handshake to a previously stateless protocol may create a completely
new opportunity for an attacker).
7. IANA Considerations
This draft requires no action by IANA.
8. Acknowledgements
This document is heavily based on an earlier draft under a different
title by Scott Bradner and Thomas Narten.
That draft stated: The initial version of this document was put
together by the IESG in 2002. Since then, it has been reworked in
response to feedback from John Loughney, Henrik Levkowetz, Mark
Townsley, Randy Bush, Bernard Aboba and others.
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Valuable comments and suggestions on the current form of the document
were made by Jari Arkko, Ted Hardie, Loa Andersson, Eric Rescorla,
Bernard Aboba, Pekka Savola, and Leslie Daigle.
The text on TLS experience was contributed by Yngve Pettersen.
This document was produced using the xml2rfc tool [RFC2629].
9. Change log [RFC Editor: please remove this section]
draft-carpenter-extension-recs-01: 2007-03-04. Updated according to
comments, especially the wording about TLS, added various specific
examples.
draft-carpenter-extension-recs-00: original version, 2006-10-12.
Derived from draft-iesg-vendor-extensions-02.txt dated 2004-06-04 by
focussing on architectural issues; the more procedural issues in that
draft were moved to RFC 4775.
10. References
10.1. Normative References
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 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.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[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,
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ESP, and uncompressed", RFC 3095, July 2001.
[RFC3427] Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J.,
and B. Rosen, "Change Process for the Session Initiation
Protocol (SIP)", BCP 67, RFC 3427, December 2002.
[RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 3546, June 2003.
[RFC3932] Alvestrand, H., "The IESG and RFC Editor Documents:
Procedures", BCP 92, RFC 3932, October 2004.
[RFC4001] Daniele, M., Haberman, B., Routhier, S., and J.
Schoenwaelder, "Textual Conventions for Internet Network
Addresses", RFC 4001, February 2005.
[RFC4181] Heard, C., "Guidelines for Authors and Reviewers of MIB
Documents", BCP 111, RFC 4181, September 2005.
[RFC4356] Gellens, R., "Mapping Between the Multimedia Messaging
Service (MMS) and Internet Mail", RFC 4356, January 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.
[RFC4775] Bradner, S., Carpenter, B., and T. Narten, "Procedures for
Protocol Extensions and Variations", BCP 125, RFC 4775,
December 2006.
10.2. Informative References
[I-D.andersson-rtg-gmpls-change]
Andersson, L. and A. Farrel, "Change Process for
Multiprotocol Label Switching (MPLS) and Generalized MPLS
(GMPLS) Protocols and Procedures",
draft-andersson-rtg-gmpls-change-08 (work in progress),
March 2007.
[RFC1263] O'Malley, S. and L. Peterson, "TCP Extensions Considered
Harmful", RFC 1263, October 1991.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
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[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
RFC 2661, August 1999.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC3575] Aboba, B., "IANA Considerations for RADIUS (Remote
Authentication Dial In User Service)", RFC 3575,
July 2003.
[RFC3735] Hollenbeck, S., "Guidelines for Extending the Extensible
Provisioning Protocol (EPP)", RFC 3735, March 2004.
[RFC4485] Rosenberg, J. and H. Schulzrinne, "Guidelines for Authors
of Extensions to the Session Initiation Protocol (SIP)",
RFC 4485, May 2006.
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]
The (G)MPLS change process (mainly procedural)
[I-D.andersson-rtg-gmpls-change]
LDAP extensions[RFC4521]
EPP extensions[RFC3735]
DNS extensions[RFC2671]
It is relatively common for MIBs, which are all in effect
extensions of the SMI data model, to be defined or extended
outside the IETF. BCP 111 [RFC4181] offers detailed guidance for
authors and reviewers.
A.2. RADIUS Extensions
The RADIUS [RFC2865] protocol was designed to be extensible via
addition of Attributes to a Data Dictionary on the server, without
requiring code changes. However, this extensibility model assumed
that Attributes would conform to a limited set of data types and that
vendor extensionns would be limited to use by vendors in situations
in which interoperability was not required. Subsequent developments
have stretched those assumptions.
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[RFC2865] Section 6.2 defines a mechanism for Vendor-Specific
extensions (Attribute 26), and states that use:
"... 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 SDOs to define their extensions to the RADIUS protocol.
This has caused a number of problems. Since the VSA mechanism was
not designed for interoperability, VSAs do not contain a "mandatory"
bit. As a result, RADIUS clients and servers may not know whether it
is safe to ignore unknown attributes. For example, [RFC2865] Section
5 states:
"A RADIUS server MAY ignore Attributes with an unknown Type. A
RADIUS client MAY ignore Attributes with an unknown Type."
However, in the case where the VSAs pertain to security (e.g.
Filters) it may not be safe to ignore them, since [RFC2865] also
states:
"A NAS that does not implement a given service MUST NOT implement the
RADIUS attributes for that service. For example, a NAS that is
unable to offer ARAP service MUST NOT implement the RADIUS attributes
for ARAP. A NAS MUST treat a RADIUS access-accept authorizing an
unavailable service as an access-reject instead."
Since it was not envisaged that multi-vendor VSA implementations
would need to interoperate, [RFC2865] does not define the data model
for VSAs, and allows multiple subattributes to be included within a
single Attribute of type 26. However, this enables VSAs to be
defined which would not be supportable by current implementations if
placed within the standard RADIUS attribute space. This has caused
problems in standardizing widely deployed VSAs.
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 [RFC2865] defined Service-Type values as
being allocated First Come, First Served (FCFS), this essentially
enabled new RADIUS commands to be allocated without IETF review.
This oversight has since been fixed in [RFC3575].
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A.3. TLS Extensions
The Secure Sockets Layer (SSL) v2 protocol was developed by Netscape
to be used to secure online transactions on the Internet. It was
later replaced by SSL v3, also developed by Netscape. SSL v3 was
then further developed by the IETF as the Transport Layer Security
(TLS) protocol.
The SSL v3 protocol was not explicitly specified to be extended.
Even TLS 1.0 did not define an extension mechanism explicitly.
However, extension "loopholes" were available. Extension mechanisms
were finally defined in [RFC3546]:
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 SSL v2 and SSL v3/TLS are not compatible. It is possible to use
SSL v2 protocol messages to intiate a SSL v3/TLS connection, but
it is not possible to communicate with a SSL v2 implementation
using SSL v3/TLS protocol messages.
o There are implementations that refuse to accept handshakes using
newer versions of the protocol than they support.
o There are other implementations that accept newer versions, but
have implemented the version rollback protection clumsily.
The SSL v2 problem has forced clients to use SSL v3 and TLS clients
to continue to use SSL v2 Client Hellos for their initial handshake
with almost all servers until 2006, much longer than would have been
desirable, in order to interoperate with old servers.
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The problem with incorrect handling of newer versions has also forced
many clients to actually disable the newer protocol versions, either
by default, or by automatically disabling the functionality, to be
able to connect to such servers. Effectively, this means that the
version rollback protection in SSL and TLS is non-existent if talking
to a fatally compromised older version.
SSL v3 and TLS also permitted expansion of the Client Hello and
Server Hello handshake messages. This functionality was fully
defined by the introduction of TLS Extensions, which makes it
possible to add new functionality to the handshake, such as the name
of the server the client is connecting to, request certificate status
information, indicate Certificate Authority support, maximum record
length, etc. Several of these extensions also introduces new
handshake messages.
It has turned out that many SSL v3 and TLS implementations that do
not support TLS Extensions, did not, as specified in the protocols,
ignore the unknown extensions, but instead failed to establish
connections.
Several of the implementations behaving in this manner are used by
high profile Internet sites, such as online banking sites, and this
has caused a significant delay in the deployment of clients
supporting TLS Extensions, and several of the clients that have
enabled support are using heuristics that allow them to disable the
functionality when they detect a problem.
Looking forward, the protocol version problem, in particular, can
cause future security problems for the TLS protocol. The strength of
the Digest algorithms (MD5 and SHA-1) used by SSL and and TLS is
weakening, and work is underway to define TLS 1.2 which will permit
new methods to be used in the protocol instead of the currently used
methods. 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).
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.
A.4. L2TP Extensions
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
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message type. Thus, extensions relating to Message Type AVPs would
likely be considered major extensions.
L2TP also provides for Vendor-Specific AVPs. Because everything in
L2TP is encoded using AVPs, it would be easy to define vendor-
specific AVPs that would be considered major extensions.
L2TP also provides for a "mandatory" bit in AVPs. Recipients of L2TP
messages containing AVPs they do not understand but that have the
mandatory bit set, are expected to reject the message and terminate
the tunnel or session the message refers to. This leads to
interesting interoperability issues, because a sender can include a
vendor-specific AVP with the M-bit set, which then cause 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 above concerns have been a
problem in practice. At the time of this writing, the authors are
unaware of the existance of vendor-specific AVPs that also set the
M-bit.
Author's Address
Brian Carpenter (ed)
IBM
8 Chemin de Blandonnet
1214 Vernier,
Switzerland
Email: brc@zurich.ibm.com
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