Network Working Group M. Thomson
Internet-Draft Mozilla
Intended status: Informational July 30, 2019
Expires: January 31, 2020
Long-term Viability of Protocol Extension Mechanisms
draft-iab-use-it-or-lose-it-00
Abstract
The ability to change protocols depends on exercising the extension
and version negotiation mechanisms that support change. Protocols
that don't use these mechanisms can find that deploying changes can
be difficult and costly.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Implementations of Protocols are Imperfect . . . . . . . . . 3
2.1. Good Protocol Design is Not Itself Sufficient . . . . . . 3
2.2. Examples of Disuse . . . . . . . . . . . . . . . . . . . 4
2.3. Multi-Party Interactions and Middleboxes . . . . . . . . 5
3. Retaining Viable Protocol Evolution Mechanisms . . . . . . . 6
3.1. Examples of Active Use . . . . . . . . . . . . . . . . . 7
3.2. Dependency is Better . . . . . . . . . . . . . . . . . . 7
3.3. Unused Extension Points Become Unusable . . . . . . . . . 8
4. Defensive Design Principles for Protocols . . . . . . . . . . 9
4.1. Active Use . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Cryptography . . . . . . . . . . . . . . . . . . . . . . 9
4.3. Grease . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.4. Invariants . . . . . . . . . . . . . . . . . . . . . . . 11
4.5. Effective Feedback . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7. Informative References . . . . . . . . . . . . . . . . . . . 12
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
A successful protocol [SUCCESS] will change in ways that allow it to
continue to fulfill the 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
broadly.
This document examines the specific conditions that determine whether
protocol maintainers have the ability to design and deploy new or
modified protocols. Section 2 highlights some historical issues with
difficulties in transitions to new protocol features. Section 3
argues that ossified protocols are more difficult to update and
successful protocols make frequent use of new extensions and code-
points. Section 4 outlines several strategies that might aid in
ensuring that protocol changes remain possible over time.
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The experience that informs this document is predominantly at
"higher" layers of the network stack, in protocols that operate at
very large scale and Internet-scale applications. It is possible
that these conclusions are less applicable to protocol deployments
that have less scale and diversity, or operate under different
constraints.
2. Implementations of Protocols are Imperfect
A change to a protocol can be made extremely difficult to deploy if
there are bugs in implementations with which the new deployment needs
to interoperate. Bugs in the handling of new codepoints or
extensions can mean that instead of handling the mechanism as
designed, endpoints react poorly. This can manifest as abrupt
termination of sessions, errors, crashes, or disappearances of
endpoints and timeouts.
Interoperability with other implementations is usually highly valued,
so deploying mechanisms that trigger adverse reactions can be
untenable. Where interoperability is a competitive advantage, this
is true even if the negative reactions happen infrequently or only
under relatively rare conditions.
Deploying a change to a protocol could require implementations 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 deployed.
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.
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 design of a protocol extension point or
version negotiation capability is critical to the freedom that it
ultimately offers.
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RFC 6709 [EXTENSIBILITY] contains a great deal of well-considered
advice on designing for extension. It includes the following advice:
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.
This has proven to be insufficient in practice. Many protocols have
evidence of imperfect implementation of critical mechanisms of this
sort. 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 remedy:
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 an
engineering practice that values simplicity, which could result in
deferring implementation of version negotiation and extension
mechanisms. This leads to these mechanisms being uniquely affected
by this problem.
2.2. Examples of Disuse
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
extensibility.
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 proportions 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] has 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 extension that is used with
considerable success in other parts of the TLS protocol. The
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original design of SNI includes the ability to include multiple names
of different types.
What is telling in this case is that SNI was 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].
Ossified DNS code bases and systems resulted in fears that new
Resource Record Codes (RRCodes) would take years of software
propagation before new RRCodes could be used. The result for a long
time was heavily overloaded use of the TXT record, such as in the
Sender Policy Framework [SPF]. It wasn't until after the standard
mechanism for dealing with new RRCodes [RRTYPE] was considered widely
deployed that new RRCodes can be safely created and used.
As a counter example, the first version of the Simple Network
Management Protocol (SNMP) [SNMPv1] defines 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
actions.
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
simple.
2.3. Multi-Party Interactions and Middleboxes
Even the most superficially simple protocols can often involve more
actors than is immediately apparent. A two-party protocol has two
ends, but even at the endpoints of an interaction, protocol elements
can be passed on to other entities in ways that can affect protocol
operation.
One of the key challenges in deploying new features is ensuring
compatibility with all actors that could be involved in the protocol.
Protocols deployed without active measures against intermediation
will tend to become intermediated over time, as network operators
deploy middleboxes to perform some function on traffic
[PATH-SIGNALS]. In particular, one of the consequences of an
unencrypted protocol is that any element on path can interact with
the protocol. For example, HTTP was specifically designed with
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intermediation in mind, transparent proxies [HTTP] are not only
possible but sometimes advantageous, despite some significant
downsides. Consequently, transparent proxies for cleartext HTTP are
commonplace. The DNS protocol was designed with intermediation in
mind through its use of caching recursive resolvers [DNS]. What was
less anticipated was the forced spoofing of DNS records by many
middle-boxes such as those that inject authentication or pay-wall
mechanisms as an authentication and authorization check, which are
now prevalent in hotels, coffee shops and business networks.
Middleboxes are also protocol participants, to the degree that they
are able to observe and act in ways that affect the protocol. The
degree to which a middlebox participates varies from the basic
functions that a router performs to full participation. For example,
a SIP back-to-back user agent (B2BUA) [B2BUA] can be very deeply
involved in the SIP protocol.
This phenomenon appears at all layers of the protocol stack, even
when protocols are not designed with middlebox participation in mind.
TCP's [TCP] extension points have been rendered difficult to use,
largely due to middlebox interactions, as experience with Multipath
TCP [MPTCP] and Fast Open [TFO] has shown. IP's version field was
rendered useless when encapsulated over Ethernet, requring a new
ethertype with IPv6 [RFC2462], due in part to layer 2 devices making
version-independent assumptions about the structure of the IPv4
header.
By increasing the number of different actors involved in any single
protocol exchange, the number of potential implementation bugs that a
deployment needs to contend with also increases. In particular,
incompatible changes to a protocol that might be negotiated between
endpoints in ignorance of the presence of a middlebox can result in a
middlebox interfering in negative and unexpected ways.
Unfortunately, middleboxes can considerably increase the difficulty
of deploying new versions or other changes to a protocol.
3. Retaining Viable Protocol Evolution Mechanisms
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. Implementations of mechanisms that support evolution is
necessary to ensure that they remain available for new uses, and
history has shown this occurs almost exclusively under active
mechanism use.
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The conditions for retaining the ability to evolve a design is most
clearly evident in the protocols that are known to have viable
version negotiation or extension points. The definition of
mechanisms alone is insufficient; it's the assured implementation
through active use of those mechanisms that determines the existence
of freedom. Protocols that routinely add new extensions and code
points rarely have trouble adding additional ones, especially when
the handling of new versions or extension is well defined.
3.1. Examples of Active Use
For example, header fields in email [SMTP], HTTP [HTTP] and SIP [SIP]
all derive from the same basic design, which amounts to a list 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 can ignore headers they do not understand
or need. The widespread deployment of SIP B2BUAs means that new SIP
header fields do not reliably reach peers, however, which doesn't
necessarily cause interoperability issues but rather causes feature
deployment issues due to the lack of option passing Section 2.3.
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
successfully deployed.
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 also confirm the case that good design does not
guarantee 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 [HTTP-HEADERS].
Only by using a protocol's extension capabilities does it ensure the
availability of that capability. Protocols that fail to use a
mechanism, or a protocol that only rarely uses a mechanism, may
suffer an inability to rely on that mechanism.
3.2. Dependency is Better
The best way to guarantee that a protocol mechanism is used is to
make the understanding of it critical to an endpoint participating in
that protocol. This means that implementations must rely on both the
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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 it does 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.
3.3. Unused Extension Points Become Unusable
In contrast, 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.
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 [HTTP], the ability to use transfer codings
other than the chunked coding, and the range unit in a range request
[HTTP-RANGE].
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 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
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when new values of the signature_algorithms extension were
introduced.
Codepoints that are reserved for future use can be especially
problematic. Reserving codepoints 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 "class E" address space in IPv4 [RFC0988], which was reserved
without assigning any semantics. For protocols that can use
negotiation to attribute semantics to codepoints, it is possible that
unused codepoints can be reclaimed for active use, though this
requires that the negotiation include all protocol participants.
4. Defensive Design Principles for Protocols
There are several potential approaches that can provide some measure
of protection against a protocol deployment becoming resistant to
future changes.
4.1. Active Use
As discussed in Section 3, the most effective defense against
ossification of protocol extension points is active use.
Implementations are most likely to be tolerant of new values if they
depend on being able to frequently use new values. Failing that,
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.
What active use means 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
others. There are currently no firm guidelines for new protocol
development.
4.2. Cryptography
Cryptography can be used to reduce the number of middlebox entities
that can participate in a protocol. Using tools like TLS ensures
that only authorized participants are able to influence whether a new
protocol feature is used.
Permitting fewer protocol participants reduces the number of
implementations that can prevent a new mechanism from being deployed.
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As recommended in [PATH-SIGNALS], use of encryption and integrity
protection can be used to limit participation.
For example, the QUIC protocol [QUIC] adopts both encryption and
integrity protection. Encryption is used to carefully control what
information is exposed to middleboxes. For those fields that are not
encrypted, QUIC uses integrity protection to prevent modification.
4.3. Grease
"Grease" [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 the client offers a set of options and the server
chooses the one that it most prefers from those that it supports. A
client 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 the server 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
elements.
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 not significantly more effort 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 codepoints, 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 correctly identify these code
points and ignore them.
In addition to advertising bogus capabilities, an endpoint might also
selectively disable non-critical protocol elements to test the
ability of peers to handle the absence of certain capabilities.
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This style of defensive design is limited because it is only
superficial. 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, 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.
4.4. Invariants
Documenting aspects of the protocol that cannot or will not change as
extensions or new versions are added can be a useful exercise.
Understanding what aspects of a protocol are invariant can help guide
the process of identifying those parts of the protocol that might
change.
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. Like with greasing,
protocol participants could still purposefully block the deployment
of new features. A protocol that declares protocol invariants relies
on implementations understanding and respecting those invariants.
Protocol invariants need to be clearly and concisely documented.
Including examples of aspects of the protocol that are not invariant,
such as the appendix of [QUIC-INVARIANTS], can be used to clarify
intent.
4.5. Effective Feedback
While not a direct means of protecting extensibility mechanisms,
feedback systems can be important to discovering problems.
Visibility of errors is critical to the success of techniques like
grease (see Section 4.3). 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.
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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, such as seen in [DMARC] and
[SMTP-TLS-Reporting].
5. Security Considerations
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 replacement of weak hash algorithms was made
more difficult through misuse [HASH].
6. IANA Considerations
This document makes no request of IANA.
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,
<https://www.rfc-editor.org/info/rfc7696>.
[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>.
[B2BUA] Kaplan, H. and V. Pascual, "A Taxonomy of Session
Initiation Protocol (SIP) Back-to-Back User Agents",
RFC 7092, DOI 10.17487/RFC7092, December 2013,
<https://www.rfc-editor.org/info/rfc7092>.
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[DIAMETER]
Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
Ed., "Diameter Base Protocol", RFC 6733,
DOI 10.17487/RFC6733, October 2012,
<https://www.rfc-editor.org/info/rfc6733>.
[DMARC] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>.
[DNS] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[EXTENSIBILITY]
Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709,
DOI 10.17487/RFC6709, September 2012,
<https://www.rfc-editor.org/info/rfc6709>.
[GREASE] Benjamin, D., "Applying GREASE to TLS Extensibility",
draft-ietf-tls-grease-02 (work in progress), January 2019.
[HASH] Bellovin, S. and E. Rescorla, "Deploying a New Hash
Algorithm", Proceedings of NDSS '06 , 2006,
<https://www.cs.columbia.edu/~smb/papers/new-hash.pdf>.
[HTTP] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[]
Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-10 (work in progress),
April 2019.
[HTTP-RANGE]
Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
RFC 7233, DOI 10.17487/RFC7233, June 2014,
<https://www.rfc-editor.org/info/rfc7233>.
[INTOLERANCE]
Kario, H., "Re: [TLS] Thoughts on Version Intolerance",
July 2016, <https://mailarchive.ietf.org/arch/msg/tls/
bOJ2JQc3HjAHFFWCiNTIb0JuMZc>.
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[MPTCP] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
<https://www.rfc-editor.org/info/rfc6824>.
[PATH-SIGNALS]
Hardie, T., "Transport Protocol Path Signals", draft-iab-
path-signals-03 (work in progress), January 2019.
[QUIC] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-20 (work
in progress), April 2019.
[QUIC-INVARIANTS]
Thomson, M., "Version-Independent Properties of QUIC",
draft-ietf-quic-invariants-06 (work in progress), July
2019.
[RFC0988] Deering, S., "Host extensions for IP multicasting",
RFC 988, DOI 10.17487/RFC0988, July 1986,
<https://www.rfc-editor.org/info/rfc988>.
[RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, DOI 10.17487/RFC2462,
December 1998, <https://www.rfc-editor.org/info/rfc2462>.
[RRTYPE] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
2003, <https://www.rfc-editor.org/info/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,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[SMTP] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[SMTP-TLS-Reporting]
Margolis, D., Brotman, A., Ramakrishnan, B., Jones, J.,
and M. Risher, "SMTP TLS Reporting", RFC 8460,
DOI 10.17487/RFC8460, September 2018,
<https://www.rfc-editor.org/info/rfc8460>.
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[SNI] Langley, A., "Accepting that other SNI name types will
never work", March 2016,
<https://mailarchive.ietf.org/arch/msg/
tls/1t79gzNItZd71DwwoaqcQQ_4Yxc>.
[SNMPv1] Case, J., Fedor, M., Schoffstall, M., and J. Davin,
"Simple Network Management Protocol (SNMP)", RFC 1157,
DOI 10.17487/RFC1157, May 1990,
<https://www.rfc-editor.org/info/rfc1157>.
[SPF] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
<https://www.rfc-editor.org/info/rfc7208>.
[SUCCESS] Thaler, D. and B. Aboba, "What Makes for a Successful
Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,
<https://www.rfc-editor.org/info/rfc5218>.
[TCP] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[TFO] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.
[TLS-EXT] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.
[TLS12] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[TRANSITIONS]
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>.
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Acknowledgments
Wes Hardaker, Mirja Kuehlewind, Mark Nottingham, and Brian Trammell
made significant contributions to this document.
Author's Address
Martin Thomson
Mozilla
Email: mt@lowentropy.net
Thomson Expires January 31, 2020 [Page 16]
