Internet Architecture Board (IAB) M. Thomson
Request for Comments: 9170
Category: Informational T. Pauly
ISSN: 2070-1721 December 2021
Long-Term Viability of Protocol Extension Mechanisms
The ability to change protocols depends on exercising the extension
and version-negotiation mechanisms that support change. This
document explores how regular use of new protocol features can ensure
that it remains possible to deploy changes to a protocol. Examples
are given where lack of use caused changes to be more difficult or
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 candidates for any level of Internet
Standard; see Section 2 of RFC 7841
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9170
Copyright (c) 2021 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
) in effect on the date of
publication of this document. Please review these documents
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to this document.
Table of Contents 1.
Imperfect Implementations Limit Protocol Evolution 2.1.
Good Protocol Design Is Not Itself Sufficient 2.2.
Disuse Can Hide Problems 2.3.
Multi-party Interactions and Middleboxes 3.
Active Use 3.1.
Dependency Is Better 3.2.
Version Negotiation 3.3.
Falsifying Active Use 3.4.
Examples of Active Use 3.5.
Restoring Active Use 4.
Complementary Techniques 4.1.
Fewer Extension Points 4.2.
Limiting Participation 4.4.
Effective Feedback 5.
Security Considerations 6.
IANA Considerations 7.
Informative References Appendix A
. Examples A.1.
IAB Members at the Time of Approval
A successful protocol [SUCCESS] needs to change in ways that allow it
to continue to fulfill the changing needs of its users. New use
cases, conditions, and constraints on the deployment of a protocol
can render a protocol that does not change obsolete.
Usage patterns and requirements for a protocol shift over time. In
response, implementations might adjust usage patterns within the
constraints of the protocol, the protocol could be extended, or a
replacement protocol might be developed. Experience with Internet-
scale protocol deployment shows that each option comes with different
costs. [TRANSITIONS] examines the problem of protocol evolution more
An extension point is a mechanism that allows a protocol to be
changed or enhanced. This document examines the specific conditions
that determine whether protocol maintainers have the ability to
design and deploy new or modified protocols via their specified
extension points. Section 2
highlights some historical examples of
difficulties in transitions to new protocol features. Section 3
argues that ossified protocols are more difficult to update and
describes how successful protocols make frequent use of new
extensions and code points. Section 4
outlines several additional
strategies that might aid in ensuring that protocol changes remain
possible over time.
The experience that informs this document is predominantly at
"higher" layers of the network stack, in protocols with limited
numbers of participants. Though similar issues are present in many
protocols that operate at scale, the trade-offs involved with
applying some of the suggested techniques can be more complex when
there are many participants, such as at the network layer or in
2. Imperfect Implementations Limit Protocol Evolution
It can be extremely difficult to deploy a change to a protocol if
implementations with which the new deployment needs to interoperate
do not operate predictably. Variation in how new code points or
extensions are handled can be the result of bugs in implementation or
specifications. Unpredictability can manifest as errors, crashes,
timeouts, abrupt termination of sessions, or disappearances of
The risk of interoperability problems can in turn make it infeasible
to deploy certain protocol changes. If deploying a new code point or
extension makes an implementation less reliable than others, even if
only in rare cases, it is far less likely that implementations will
adopt the change.
Deploying a change to a protocol could require implementations to fix
a substantial proportion of the bugs that the change exposes. This
can involve a difficult process that includes identifying the cause
of these errors, finding the responsible implementation(s),
coordinating a bug fix and release plan, contacting users and/or the
operator of affected services, and waiting for the fix to be
Given the effort involved in fixing problems, the existence of these
sorts of bugs can outright prevent the deployment of some types of
protocol changes, especially for protocols involving multiple parties
or that are considered critical infrastructure (e.g., IP, BGP, DNS,
or TLS). It could even be necessary to come up with a new protocol
design that uses a different method to achieve the same result.
This document only addresses cases where extensions are not
deliberately blocked. Some deployments or implementations apply
policies that explicitly prohibit the use of unknown capabilities.
This is especially true of functions that seek to make security
guarantees, like firewalls.
The set of interoperable features in a protocol is often the subset
of its features that have some value to those implementing and
deploying the protocol. It is not always the case that future
extensibility is in that set.
2.1. Good Protocol Design Is Not Itself Sufficient
It is often argued that the careful design of a protocol extension
point or version-negotiation capability is critical to the freedom
that it ultimately offers. RFC 6709
[EXTENSIBILITY] contains a great deal of well-considered
advice on designing for extensions. It includes the following
| 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
There are a number of protocols for which this has proven to be
insufficient in practice. These protocols have imperfect
implementations of these mechanisms. Mechanisms that aren't used are
the ones that fail most often. The same paragraph from RFC 6709
acknowledges the existence of this problem but does not offer any
| 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.
Indeed, basic interoperability is considered critical early in the
deployment of a protocol. A desire to deploy can result in early
focus on a reduced feature set, which could result in deferring
implementation of version-negotiation and extension mechanisms. This
leads to these mechanisms being particularly affected by this
2.2. Disuse Can Hide Problems
There are many examples of extension points in protocols that have
been either completely unused or their use was so infrequent that
they could no longer be relied upon to function correctly. Appendix A
includes examples of disuse in a number of widely deployed
Even where extension points have multiple valid values, if the set of
permitted values does not change over time, there is still a risk
that new values are not tolerated by existing implementations. If
the set of values for a particular field of a protocol or the order
in which these values appear remains fixed over a long period, some
implementations might not correctly handle a new value when it is
introduced. For example, implementations of TLS broke when new
values of the signature_algorithms extension were introduced.
2.3. Multi-party Interactions and Middleboxes
One of the key challenges in deploying new features is ensuring
compatibility with all actors that could be involved in the protocol.
Even the most superficially simple protocols can often involve more
actors than is immediately apparent.
The design of extension points needs to consider what actions
middleboxes might take in response to a protocol change as well as
the effect those actions could have on the operation of the protocol.
Deployments of protocol extensions also need to consider the impact
of the changes on entities beyond protocol participants and
middleboxes. Protocol changes can affect the behavior of
applications or systems that don't directly interact with the
protocol, such as when a protocol change modifies the formatting of
data delivered to an application.
3. Active Use
The design of a protocol for extensibility and eventual replacement
[EXTENSIBILITY] does not guarantee the ability to exercise those
options. The set of features that enable future evolution need to be
interoperable in the first implementations and deployments of the
protocol. Implementation of mechanisms that support evolution is
necessary to ensure that they remain available for new uses, and
history has shown this occurs almost exclusively through active
Only by using the extension capabilities of a protocol is the
availability of that capability assured. "Using" here includes
specifying, implementing, and deploying capabilities that rely on the
extension capability. Protocols that fail to use a mechanism, or a
protocol that only rarely uses a mechanism, could lead to that
mechanism being unreliable.
Implementations that routinely see new values are more likely to
correctly handle new values. More frequent changes will improve the
likelihood that incorrect handling or intolerance is discovered and
rectified. The longer an intolerant implementation is deployed, the
more difficult it is to correct.
Protocols that routinely add new extensions and code points rarely
have trouble adding additional ones especially when the handling of
new versions or extensions are well defined. The definition of
mechanisms alone is insufficient; it is the assured implementation
and active use of those mechanisms that determines their
What constitutes "active use" can depend greatly on the environment
in which a protocol is deployed. The frequency of changes necessary
to safeguard some mechanisms might be slow enough to attract
ossification in another protocol deployment, while being excessive in
3.1. Dependency Is Better
The easiest way to guarantee that a protocol mechanism is used is to
make the handling of it critical to an endpoint participating in that
protocol. This means that implementations must rely on both the
existence of extension mechanisms and their continued, repeated
expansion over time.
For example, the message format in SMTP relies on header fields for
most of its functions, including the most basic delivery functions.
A deployment of SMTP cannot avoid including an implementation of
header field handling. In addition to this, the regularity with
which new header fields are defined and used ensures that deployments
frequently encounter header fields that they do not yet (and may
never) understand. An SMTP implementation therefore needs to be able
to both process header fields that it understands and ignore those
that it does not.
In this way, implementing the extensibility mechanism is not merely
mandated by the specification, it is crucial to the functioning of a
protocol deployment. Should an implementation fail to correctly
implement the mechanism, that failure would quickly become apparent.
Caution is advised to avoid assuming that building a dependency on an
extension mechanism is sufficient to ensure availability of that
mechanism in the long term. If the set of possible uses is narrowly
constrained and deployments do not change over time, implementations
might not see new variations or assume a narrower interpretation of
what is possible. Those implementations might still exhibit errors
when presented with new variations.
3.2. Version Negotiation
As noted in Section 2.1
, protocols that provide version-negotiation
mechanisms might not be able to test that feature until a new version
is deployed. One relatively successful design approach has been to
use the protocol selection mechanisms built into a lower-layer
protocol to select the protocol. This could allow a version-
negotiation mechanism to benefit from active use of the extension
point by other protocols.
For instance, all published versions of IP contain a version number
as the four high bits of the first header byte. However, version
selection using this field proved to be unsuccessful. Ultimately,
successful deployment of IPv6 over Ethernet [RFC2464
] required a
different EtherType from IPv4. This change took advantage of the
already diverse usage of EtherType.
Other examples of this style of design include Application-Layer
Protocol Negotiation ([ALPN]) and HTTP content negotiation
(Section 12 of [HTTP]).
This technique relies on the code point being usable. For instance,
the IP protocol number is known to be unreliable and therefore not
3.3. Falsifying Active Use
"Grease" was originally defined for TLS [GREASE] but has been adopted
by other protocols such as QUIC [QUIC]. Grease identifies lack of
use as an issue (protocol mechanisms "rusting" shut) and proposes
reserving values for extensions that have no semantic value attached.
The design in [GREASE] is aimed at the style of negotiation most used
in TLS, where one endpoint offers a set of options and the other
chooses the one that it most prefers from those that it supports. An
endpoint that uses grease randomly offers options, usually just one,
from a set of reserved values. These values are guaranteed to never
be assigned real meaning, so its peer will never have cause to
genuinely select one of these values.
More generally, greasing is used to refer to any attempt to exercise
extension points without changing endpoint behavior other than to
encourage participants to tolerate new or varying values of protocol
The principle that grease operates on is that an implementation that
is regularly exposed to unknown values is less likely to be
intolerant of new values when they appear. This depends largely on
the assumption that the difficulty of implementing the extension
mechanism correctly is as easy or easier than implementing code to
identify and filter out reserved values. Reserving random or
unevenly distributed values for this purpose is thought to further
discourage special treatment.
Without reserved greasing code points, an implementation can use code
points from spaces used for private or experimental use if such a
range exists. In addition to the risk of triggering participation in
an unwanted experiment, this can be less effective. Incorrect
implementations might still be able to identify these code points and
In addition to advertising bogus capabilities, an endpoint might also
selectively disable noncritical protocol elements to test the ability
of peers to handle the absence of certain capabilities.
This style of defensive design is limited because it is only
superficial. As greasing only mimics active use of an extension
point, it only exercises a small part of the mechanisms that support
extensibility. More critically, it does not easily translate to all
forms of extension points. For instance, highest mutually supported
version (HMSV) negotiation cannot be greased in this fashion. Other
techniques might be necessary for protocols that don't rely on the
particular style of exchange that is predominant in TLS.
Grease is deployed with the intent of quickly revealing errors in
implementing the mechanisms it safeguards. Though it has been
effective at revealing problems in some cases with TLS, the efficacy
of greasing isn't proven more generally. Where implementations are
able to tolerate a non-zero error rate in their operation, greasing
offers a potential option for safeguarding future extensibility.
However, this relies on there being a sufficient proportion of
participants that are willing to invest the effort and tolerate the
risk of interoperability failures.
3.4. Examples of Active Use
Header fields in email [SMTP], HTTP [HTTP], and SIP [SIP] all derive
from the same basic design, which amounts to a list of name/value
pairs. There is no evidence of significant barriers to deploying
header fields with new names and semantics in email and HTTP as
clients and servers generally ignore headers they do not understand
or need. The widespread deployment of SIP back-to-back user agents
(B2BUAs), which generally do not ignore unknown fields, means that
new SIP header fields do not reliably reach peers. This does not
necessarily cause interoperability issues in SIP but rather causes
features to remain unavailable until the B2BUA is updated. All three
protocols are still able to deploy new features reliably, but SIP
features are deployed more slowly due to the larger number of active
participants that need to support new features.
As another example, the attribute-value pairs (AVPs) in Diameter
[DIAMETER] are fundamental to the design of the protocol. Any use of
Diameter requires exercising the ability to add new AVPs. This is
routinely done without fear that the new feature might not be
These examples show extension points that are heavily used are also
being relatively unaffected by deployment issues preventing addition
of new values for new use cases.
These examples show that a good design is not required for success.
On the contrary, success is often despite shortcomings in the design.
For instance, the shortcomings of HTTP header fields are significant
enough that there are ongoing efforts to improve the syntax
3.5. Restoring Active Use
With enough effort, active use can be used to restore capabilities.
Extension Mechanisms for DNS ([EDNS]) was defined to provide
extensibility in DNS. Intolerance of the extension in DNS servers
resulted in a fallback method being widely deployed (see
Section 6.2.2 of [EDNS]). This fallback resulted in EDNS being
disabled for affected servers. Over time, greater support for EDNS
and increased reliance on it for different features motivated a flag
day [DNSFLAGDAY] where the workaround was removed.
The EDNS example shows that effort can be used to restore
capabilities. This is in part because EDNS was actively used with
most resolvers and servers. It was therefore possible to force a
change to ensure that extension capabilities would always be
available. However, this required an enormous coordination effort.
A small number of incompatible servers and the names they serve also
became inaccessible to most clients.
4. Complementary Techniques
The protections to protocol evolution that come from active use
) can be improved through the use of other defensive
techniques. The techniques listed here might not prevent
ossification on their own, but they can make active use more
4.1. Fewer Extension Points
A successful protocol will include many potential types of
extensions. Designing multiple types of extension mechanisms, each
suited to a specific purpose, might leave some extension points less
heavily used than others.
Disuse of a specialized extension point might render it unusable. In
contrast, having a smaller number of extension points with wide
applicability could improve the use of those extension points. Use
of a shared extension point for any purpose can protect rarer or more
Both extensions and core protocol elements use the same extension
points in protocols like HTTP [HTTP] and DIAMETER [DIAMETER]; see Section 3.4
Documenting aspects of the protocol that cannot or will not change as
extensions or new versions are added can be a useful exercise. Section 2.2
] defines invariants as:
| Invariants are core properties that are consistent across the
| network and do not change over extremely long time-scales.
Understanding what aspects of a protocol are invariant can help guide
the process of identifying those parts of the protocol that might
change. [QUIC-INVARIANTS] and Section 9.3 of [TLS13] are both
examples of documented invariants.
As a means of protecting extensibility, a declaration of protocol
invariants is useful only to the extent that protocol participants
are willing to allow new uses for the protocol. A protocol that
declares protocol invariants relies on implementations understanding
and respecting those invariants. If active use is not possible for
all non-invariant parts of the protocol, greasing (Section 3.3
be used to improve the chance that invariants are respected.
Protocol invariants need to be clearly and concisely documented.
Including examples of aspects of the protocol that are not invariant,
such as Appendix A
of [QUIC-INVARIANTS], can be used to clarify
4.3. Limiting Participation
Reducing the number of entities that can participate in a protocol or
limiting the extent of participation can reduce the number of
entities that might affect extensibility. Using TLS or other
cryptographic tools can therefore reduce the number of entities that
can influence whether new features are usable.
[PATH-SIGNALS] also recommends the use of encryption and integrity
protection to limit participation. For example, encryption is used
by the QUIC protocol [QUIC] to limit the information that is
available to middleboxes and integrity protection prevents
4.4. Effective Feedback
While not a direct means of protecting extensibility mechanisms,
feedback systems can be important to discovering problems.
The visibility of errors is critical to the success of techniques
like grease (see Section 3.3
). The grease design is most effective
if a deployment has a means of detecting and reporting errors.
Ignoring errors could allow problems to become entrenched.
Feedback on errors is more important during the development and early
deployment of a change. It might also be helpful to disable
automatic error recovery methods during development.
Automated feedback systems are important for automated systems, or
where error recovery is also automated. For instance, connection
failures with HTTP alternative services [ALT-SVC] are not permitted
to affect the outcome of transactions. An automated feedback system
for capturing failures in alternative services is therefore necessary
for failures to be detected.
How errors are gathered and reported will depend greatly on the
nature of the protocol deployment and the entity that receives the
report. For instance, end users, developers, and network operations
each have different requirements for how error reports are created,
managed, and acted upon.
Automated delivery of error reports can be critical for rectifying
deployment errors as early as possible, as seen in [DMARC] and
5. Security Considerations
Many of the problems identified in this document are not the result
of deliberate actions by an adversary but more the result of
mistakes, decisions made without sufficient context, or simple
neglect, i.e., problems therefore not the result of opposition by an
adversary. In response, the recommended measures generally assume
that other protocol participants will not take deliberate action to
prevent protocol evolution.
The use of cryptographic techniques to exclude potential participants
is the only strong measure that the document recommends. However,
authorized protocol peers are most often responsible for the
identified problems, which can mean that cryptography is insufficient
to exclude them.
The ability to design, implement, and deploy new protocol mechanisms
can be critical to security. In particular, it is important to be
able to replace cryptographic algorithms over time [AGILITY]. For
example, preparing for the replacement of weak hash algorithms was
made more difficult through misuse [HASH].
6. IANA Considerations
This document has no IANA actions.
7. Informative References
[AGILITY] Housley, R., "Guidelines for Cryptographic Algorithm
Agility and Selecting Mandatory-to-Implement Algorithms",
BCP 201, RFC 7696
, DOI 10.17487/RFC7696
, November 2015,
[ALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301
, DOI 10.17487/RFC7301
July 2014, <https://www.rfc-editor.org/info/rfc7301
[ALT-SVC] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838
, DOI 10.17487/RFC7838
April 2016, <https://www.rfc-editor.org/info/rfc7838
[DIAMETER] Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
Ed., "Diameter Base Protocol", RFC 6733
, October 2012,
[DMARC] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489
, DOI 10.17487/RFC7489
, March 2015,
"DNS Flag Day 2019", May 2019,
[EDNS] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891
, April 2013,
[EXT-TCP] Honda, M., Nishida, Y., Raiciu, C., Greenhalgh, A.,
Handley, M., and H. Tokuda, "Is it still possible to
extend TCP?", IMC '11: Proceedings of the 2011 ACM SIGCOMM
conference on Internet measurement conference,
DOI 10.1145/2068816.2068834, November 2011,
Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709
, September 2012,
[GREASE] Benjamin, D., "Applying Generate Random Extensions And
Sustain Extensibility (GREASE) to TLS Extensibility", RFC 8701
, DOI 10.17487/RFC8701
, January 2020,
[HASH] Bellovin, S. and E. Rescorla, "Deploying a New Hash
Algorithm", Proceedings of NDSS, 2006,
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", Work in Progress, Internet-Draft,
draft-ietf-httpbis-semantics-19, September 2021,
Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941
, DOI 10.17487/RFC8941
, February 2021,
[HTTP11] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1", Work in Progress, Internet-Draft, draft-
ietf-httpbis-messaging-19, September 2021,
Kario, H., "Re: [TLS] Thoughts on Version Intolerance",
July 2016, <https://mailarchive.ietf.org/arch/msg/tls/
[MPTCP] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", RFC 8684
, DOI 10.17487/RFC8684
Raiciu, C., Paasch, C., Barre, S., Ford, A., Honda, M.,
Duchene, F., Bonaventure, O., and M. Handley, "How Hard
Can It Be? Designing and Implementing a Deployable
Multipath TCP", April 2012,
Barik, R., Welzl, M., Fairhurst, G., Elmokashfi, A.,
Dreibholz, T., and S. Gjessing, "On the usability of
transport protocols other than TCP: A home gateway and
internet path traversal study", Computer Networks, Vol.
173, pp. 107211, DOI 10.1016/j.comnet.2020.107211, May
Hardie, T., Ed., "Transport Protocol Path Signals", RFC 8558
, DOI 10.17487/RFC8558
, April 2019,
[QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000
, May 2021,
Thomson, M., "Version-Independent Properties of QUIC", RFC 8999
, DOI 10.17487/RFC8999
, May 2021,
[RAv4] Katz, D., "IP Router Alert Option", RFC 2113
, February 1997,
[RAv6] Partridge, C. and A. Jackson, "IPv6 Router Alert Option", RFC 2711
, DOI 10.17487/RFC2711
, October 1999,
] Postel, J., "Internet Protocol", STD 5, RFC 791
, September 1981,
] Deering, S., "Host extensions for IP multicasting", STD 5, RFC 1112
, DOI 10.17487/RFC1112
, August 1989,
] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464
, DOI 10.17487/RFC2464
, December 1998,
] Bryant, S., Ed., Morrow, M., Ed., and IAB, "Uncoordinated
Protocol Development Considered Harmful", RFC 5704
, November 2009,
[RRTYPE] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597
, DOI 10.17487/RFC3597
[SIP] 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,
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321
, October 2008,
Margolis, D., Brotman, A., Ramakrishnan, B., Jones, J.,
and M. Risher, "SMTP TLS Reporting", RFC 8460
, September 2018,
[SNI] Langley, A., "[TLS] Accepting that other SNI name types
will never work.", March 2016,
[SNMPv1] Case, J., Fedor, M., Schoffstall, M., and J. Davin,
"Simple Network Management Protocol (SNMP)", RFC 1157
, May 1990,
[SPF] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208
, April 2014,
[SUCCESS] Thaler, D. and B. Aboba, "What Makes for a Successful
Protocol?", RFC 5218
, DOI 10.17487/RFC5218
, July 2008,
[TCP] Postel, J., "Transmission Control Protocol", STD 7, RFC 793
, DOI 10.17487/RFC0793
, September 1981,
[TFO] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413
, DOI 10.17487/RFC7413
, December 2014,
[TLS-EXT] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066
, January 2011,
[TLS12] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246
, August 2008,
[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446
, DOI 10.17487/RFC8446
, August 2018,
Thaler, D., Ed., "Planning for Protocol Adoption and
Subsequent Transitions", RFC 8170
, DOI 10.17487/RFC8170
May 2017, <https://www.rfc-editor.org/info/rfc8170
This appendix contains a brief study of problems in a range of
Internet protocols at different layers of the stack.
Ossified DNS code bases and systems resulted in new Resource Record
Codes (RRCodes) being unusable. A new code point would take years of
coordination between implementations and deployments before it could
be relied upon. Consequently, use of the TXT record was overloaded
in order to avoid the effort and delays involved in allocating new
code points; this approach was used in the Sender Policy Framework
[SPF] and other protocols.
It was not until after the standard mechanism for dealing with new
RRCodes [RRTYPE] was considered widely deployed that new RRCodes
could be safely created and used.
HTTP has a number of very effective extension points in addition to
the aforementioned header fields. It also has some examples of
extension points that are so rarely used that it is possible that
they are not at all usable.
Extension points in HTTP that might be unwise to use include the
extension point on each chunk in the chunked transfer coding
(Section 7.1 of [HTTP11]), the ability to use transfer codings other
than the chunked coding, and the range unit in a range request
(Section 14 of [HTTP]).
The version field in IP was rendered useless when encapsulated over
Ethernet, requiring a new EtherType with IPv6 [RFC2464
], due in part
to Layer 2 devices making version-independent assumptions about the
structure of the IPv4 header.
Protocol identifiers or code points that are reserved for future use
can be especially problematic. Reserving values without attributing
semantics to their use can result in diverse or conflicting semantics
being attributed without any hope of interoperability. An example of
this is the 224/3 address space in IPv4 that [RFC0791
without assigning any semantics. [RFC1112
] partially reclaimed that
reserved address space for use in multicast (224/4), but the
remaining address space (240/4) has not been successfully reclaimed
for any purpose.
For protocols that can use negotiation to attribute semantics to
values, it is possible that unused code points can be reclaimed for
active use, though this requires that the negotiation include all
protocol participants. For something as fundamental as addressing,
negotiation is difficult or even impossible, as all nodes on the
network path plus potential alternative paths would need to be
IP Router Alerts [RAv4][RAv6] use IP options or extension headers to
indicate that data is intended for consumption by the next-hop router
rather than the addressed destination. In part, the deployment of
router alerts was unsuccessful due to the realities of processing IP
packets at line rates, combined with bad assumptions in the protocol
design about these performance constraints. However, this was not
exclusively down to design problems or bugs, as the capability was
also deliberately blocked at some routers.
As a counter example, the first version of the Simple Network
Management Protocol (SNMP) [SNMPv1] states that unparseable or
unauthenticated messages are simply discarded without response:
| It then verifies the version number of the SNMP message. If there
| is a mismatch, it discards the datagram and performs no further
When SNMP versions 2, 2c, and 3 came along, older agents did exactly
what the protocol specifies. Deployment of new versions was likely
successful because the handling of newer versions was both clear and
Extension points in TCP [TCP] have been rendered difficult to use
largely due to middlebox interactions; see [EXT-TCP].
For instance, multipath TCP ([MPTCP]) can only be deployed
opportunistically; see [MPTCP-HOW-HARD]. Since MPTCP is a protocol
enhancement that doesn't impair the connection if it is blocked,
network path intolerance of the extension only results in the
multipath functionality becoming unavailable.
In comparison, the deployment of TCP Fast Open ([TFO]) critically
depends on extension capability being widely available. Though very
few network paths were intolerant of the extension in absolute terms,
TCP Fast Open could not be deployed as a result.
Transport Layer Security (TLS) [TLS12] provides examples of where a
design that is objectively sound fails when incorrectly implemented.
TLS provides examples of failures in protocol version negotiation and
Version negotiation in TLS 1.2 and earlier uses the "Highest mutually
supported version (HMSV)" scheme exactly as it is described in
[EXTENSIBILITY]. However, clients are unable to advertise a new
version without causing a non-trivial proportion of sessions to fail
due to bugs in server and middlebox implementations.
Intolerance to new TLS versions is so severe [INTOLERANCE] that TLS
1.3 [TLS13] abandoned HMSV version negotiation for a new mechanism.
The server name indication (SNI) [TLS-EXT] in TLS is another
excellent example of the failure of a well-designed extensibility
point. SNI uses the same technique for extensions that is used
successfully in other parts of the TLS protocol. The original design
of SNI anticipated the ability to include multiple names of different
SNI was originally defined with just one type of name: a domain name.
No other type has ever been standardized, though several have been
proposed. Despite an otherwise exemplary design, SNI is so
inconsistently implemented that any hope for using the extension
point it defines has been abandoned [SNI].
IAB Members at the Time of Approval
Internet Architecture Board members at the time this document was
approved for publication were:
Toerless Eckert, Wes Hardaker, Mirja Kühlewind, Eliot Lear, Mark
Nottingham, and Brian Trammell made significant contributions to this