Internet DRAFT - draft-klensin-emailaddr-i18n
draft-klensin-emailaddr-i18n
Network Working Group J. Klensin
Internet-Draft July 18, 2005
Expires: January 19, 2006
Internationalization of Email Addresses
draft-klensin-emailaddr-i18n-03.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Internationalization of electronic mail addresses is, if anything,
more important than the already-completed effort for domain names.
In most of the contexts in which they are used, domain names can be
hidden within or as part of various types of references. Email
addresses, by contrast, are crucial: use of names of people or
organizations as, or as part of, the email local part is, for obvious
reasons, a well-established tradition on the network. Preventing
people from spelling their names correctly is, in the long term,
inexcusable. At the same time, email addresses pose a number of
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special problems -- they are more difficult than simple domain names
in some respects, but actually easier in others. This document
discusses the issues with internationalization of email addresses,
explains why some obvious approaches are incompatible with the
definitions and use of Internet mail, and proposes a solution -- for
both addressing and email internationalization more generally -- that
is likely to serve users and the network well for the long term.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Three Models for Transition . . . . . . . . . . . . . . . . . 5
2.1 No Infrastructure Changes . . . . . . . . . . . . . . . . 5
2.2 Transport-level Negotiation . . . . . . . . . . . . . . . 5
2.3 Replace SMTP and the Current Internet Mail Environment . . 6
2.4 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. History, Context, and Design Constraints . . . . . . . . . . . 7
3.1 The Presentation Issue . . . . . . . . . . . . . . . . . . 8
3.2 MUAs, MTAs, addresses, and learning from MIME and ESMTP . 8
3.3 An Encoded-address, MUA-transparent, Solution may
Eliminate an Important Opportunity . . . . . . . . . . . . 11
3.4 An MUA-only-based Solution is Not Necessary . . . . . . . 12
3.4.1 Obtaining an Internationalized Email Address . . . . . 12
3.4.2 Relay environment . . . . . . . . . . . . . . . . . . 13
3.4.3 Internationalizing the Sender . . . . . . . . . . . . 14
3.5 A Solution Based on MUA Changes Alone is Unworkable . . . 15
3.5.1 MX Diversion . . . . . . . . . . . . . . . . . . . . . 15
3.5.2 Embedded commands . . . . . . . . . . . . . . . . . . 15
3.6 Encoding the Whole Address String . . . . . . . . . . . . 15
3.7 Looking back and looking forward . . . . . . . . . . . . . 16
3.8 Summary of Design Issues and Tradeoffs . . . . . . . . . . 16
4. A Mail Transport-level Protocol . . . . . . . . . . . . . . . 17
4.1 General Principles and Objectives . . . . . . . . . . . . 17
4.2 Framework for the Internationalization Extension . . . . . 17
4.3 The Address Internationalization Service Extension . . . . 18
4.4 Extended Mailbox Address Syntax . . . . . . . . . . . . . 19
4.5 The ALT-ADDRESS parameter . . . . . . . . . . . . . . . . 19
4.6 Formation of the Alternate Address . . . . . . . . . . . . 20
4.7 Additional ESMTP Changes and Clarifications . . . . . . . 21
4.7.1 The Initial SMTP Exchange . . . . . . . . . . . . . . 21
4.7.2 Trace Fields . . . . . . . . . . . . . . . . . . . . . 21
5. Impact on the MUA and on Message Headers . . . . . . . . . . . 21
6. Bundling of Extensions and Options . . . . . . . . . . . . . . 22
7. Protocol Loose Ends . . . . . . . . . . . . . . . . . . . . . 23
7.1 Punycode in Domain Names? . . . . . . . . . . . . . . . . 23
7.2 Local Character Codes in Local Parts? . . . . . . . . . . 23
7.3 Restrictions on Characters in Local Part? . . . . . . . . 24
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7.4 Requirement for 8BITMIME? . . . . . . . . . . . . . . . . 24
7.5 Message Header and Body Issues with MTA Approach? . . . . 24
7.6 The Received field 'for' clause . . . . . . . . . . . . . 24
8. Internationalization and Full Localization . . . . . . . . . . 25
9. Advice to Designers and Operators of Mail-receiving Systems . 26
10. Internationalization Considerations . . . . . . . . . . . . 27
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . 27
12. Security considerations . . . . . . . . . . . . . . . . . . 27
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
14. An Appeal . . . . . . . . . . . . . . . . . . . . . . . . . 28
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
15.1 Normative References . . . . . . . . . . . . . . . . . . . 29
15.2 Informative References . . . . . . . . . . . . . . . . . . 29
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 31
Intellectual Property and Copyright Statements . . . . . . . . 32
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1. Introduction
Internationalization of electronic mail addresses is, if anything,
more important than the already-completed effort for domain names.
In most of the contexts in which they are used, domain names can be
hidden within, or as part of, various types of references or the
references themselves may be hidden. It also remains controversial
whether internationalization of domain names is actually necessary,
no matter how attractive and important it may appear at first glance.
Email addresses, by contrast, are crucial: use of names of people or
organizations as, or as part of, the email local part is, for obvious
reasons, a well-established tradition on the network. While the
characters permitted in domain name strings have always been somewhat
constrained so that they are not confused with syntax requirements of
present and future applications, preventing people from spelling
their names correctly is, in the long term, inexcusable. However,
while it is tempting to ignore them, email addresses pose a number of
special problems. Unlike domain names --and, consequently, the
domain part of an email address (after the last "@")-- the local part
(or mailbox name) is essentially unconstrained with regard to syntax
or the characters used. There are no special delimiters comparable
to the period used to separate domain name labels, there is no
standardized structure comparable to the domain name system's
hierarchy, and it has always been a firm protocol requirement that no
host other than the one to which final delivery is made is permitted
to parse or interpret the address (see section 2.3.10 of [RFC2821]).
In some respects, this makes things much more difficult: it is far
more difficult to know what behavior will cause existing systems to
cease working properly. In others, it actually makes them easier,
since the originating system is not required to understand how the
receiving one will interpret an address and indeed must not do so.
The balance of this document explores these issues in more detail.
1.1 Terminology
While much of the description here depends on the abstractions of
"Mail Transfer Agent" ("MTA") and "Mail User Agent" ("MUA"), it is
important to understand that those terms and the underlying concepts
postdate the design of the Internet's email architecture and the
"protocols on the wire" principle. That architecture, as it has
evolved, and the "wire" principle have prevented any strong and
standardized distinctions about how MTAs and MUAs interact on a given
origin or destination host (or even whether they are separate).
This document assumes a reasonable understanding of the protocols and
terminology of the most recent core email standards documented in RFC
2821 [RFC2821] and RFC 2822 [RFC2822].
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In its present internet-draft form, the document contains a great
deal of explanatory material and rationale for the approach chosen.
The actual protocol material appears almost entirely in Section 4,
especially Section 4.2 through Section 4.4, and in Section 5. If it
appears to be a candidate for standards-track publication, the
explanatory material, rationale, and most of the other background
materials should be removed to a separate document. Those who wish
to bypass the reasoning and comparison to other alternatives in this
document and examine the protocol proposal should skip to those
sections.
In this document, an address is "all-ASCII" if every character in the
address is in the ASCII character repertoire [ASCII]; an address is
"non-ASCII" if any character is not in the ASCII character
repertoire.
The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED",
and "MAY" in this document are to be interpreted as described in RFC
2119 [RFC2119].
2. Three Models for Transition
Almost every attempt to extend the Internet mail system to support
new formats leads quickly to a controversy about how to implement
such changes and fit them into the existing system. The proposals
tend to fall into three categories:
2.1 No Infrastructure Changes
Avoid any more infrastructure changes than are absolutely necessary.
Instead, use sometimes-elaborate coding and other "tricks" to embed
the new facilities in the old ones, even if those facilities will
look very odd to those whose client and user interface systems have
not been upgraded. MIME, which has been quite successful, is the
most-cited example of this approach, although it is worth remembering
that it was initially quite unpopular because it exposed many users
to rather opaque codings. It continues to be criticized as
institutionalizing incompatibility rather than providing good
interoperability. Specificially on the latter observation, MIME
conformance does nothing to prevent delivery of a message to a user
that the user has no capability of decoding and reading.
2.2 Transport-level Negotiation
Use a transport-level negotiation model of some variety to ensure
that the recipient machine can and will accept the format and
structure of the message and options being sent. Variations on this
approach include ensuring that the message can be delivered, but not
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that it can be read. This approach has been taken in situations in
which, e.g., sending the message without such acceptance could result
in actual information loss (as distinct from "mere" information
inaccessibility for the MIME case). ESMTP ([RFC1651], [RFC2821])
provides the framework for this approach; options such as 8BITMIME
[RFC1652] are clear examples of cases in which the approach is
necessary.
2.3 Replace SMTP and the Current Internet Mail Environment
Start a conversation about discarding more or less everything and
moving toward a "next generation" of Internet mail in the hope of a
huge gain in elegance, capability, or other functions.
2.4 Analysis
Of these three, only the second appears to be plausible for
internationalization of email local parts.
The third --a new mail system, replacing SMTP and possibly the mail
header and MIME models-- is the most easily discarded as a
possibility. Despite many brief bursts of enthusiasm, development of
standards by other organizations, and well-funded proprietary
products, proposed SMTP replacements have tended to not go anywhere:
standard Internet mail, with or without extensions, is sufficiently
well-established (and entrenched) as an interoperable interchange
standard that historical proposed "next generation" alternatives have
been forced, by the marketplace, to interoperate with and,
ultimately, to yield to, it. Those that did not make a serious
attempt to interoperate with Internet mail have largely disappeared.
There is no reason to believe that a new set of proposals will fare
any better.
In considering the other two approaches, it is important to note that
none of the fundamental transitions have ever been, or will ever be,
easy, quick, and without side-effects. While it is easy, safe, and
fairly painless to add a new media type to MIME, the MIME framework
itself provided a very unpleasant user experience until mail user
agents (MUAs) and related software were upgraded. Similarly, while
most ESMTP extensions can be added at a relatively low level of pain,
the infrastructure upgrades needed to accommodate the extension
framework itself (and hence any of the extensions) were significant,
especially in the case of MTA software that had been operating
smoothly, without maintenance or upgrades, for years.
For internationalization, the important question is whether the
transition properties are "more like MIME" (i.e., can be accomplished
at an MUA-MUA level, without the transport system getting involved)
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or whether the changes required are significant enough to require
transport negotiation (or a "next generation mail" environment). It
is the position of this document that the complex of changes needed
to make Internet email fully internationalized requires, if not "next
generation mail", at least the type of position that characterized
the original, "framework", MIME and ESMTP deployments: "if you want
any of these features, you are going to implement and accept a
package of changes; if that is not feasible, you need to stay with
ASCII, or ASCII-encoded, addresses and headers until you are ready to
upgrade". The alternatives are just too complex, and therefore
problem-prone, in terms of combinations of alternate forms, options,
transitions, upgrading and downgrading, and so on to preserve a high
level of interoperability.
The sections that follow discuss the motivation and implications of
this conclusion in more detail before moving on to a specific
proposal.
3. History, Context, and Design Constraints
Several key issues in how email works and is handled impose
significant constraints on the solution space. Email is often used
as a transport mechanism for information that will be acted on by
computers, not merely read by people. While the approach is not
common, some of the systems that use it as a computer-computer
communication medium encode routing, processing, or validation
information into the envelope address fields. More commonly,
recipient systems use special address formats to encode local routing
or priority information. In recent years, some of these addressing
techniques have become important anti-spam tools for some users and
communities. Most of these techniques have a long history. Most or
all of them conform to email standards and practices that, in turn,
go back to the first uses of email on the ARPANet. Backward-
compatibility --not damaging the interoperability of standards-
conforming programs that are now deployed and working correctly--
makes it inappropriate to make decisions by conducting user surveys
and concluding that "not too many" people will be hurt. Any new
system must preserve existing practices and flexibilities unless
there are overwhelming reasons -- e.g., an absence of plausible
alternatives -- to not do so.
Historically, when one of these approaches has required that the
email address local part be partitioned into components that are then
interpreted differently or in some special sequence, the information
has been organized according to some lexical convention, typically
either based on one or more delimiters or on some sort of position
and length notation (or a mixture of the two for different purposes).
Either may be applied left-to-right or right-to-left and, again, we
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have a history of each, including the notorious "!a!b!c!d%e%f" local
parts, which can be interpreted as
o A single mailbox name, "!a!b!c!d%e%f"
o A routing instruction to send the address "!b!c!d%e%f" to host "a"
o A routing instruction to send the address "!a!b!c!d%e" to host "f"
or some combinations of those interpretations.
Because the correct interpretation can only be known at the
destination host, attempts by intermediate hosts, or even the
originating user, to interpret the structure of the address string
cause serious problems with mail reliability. Worse, because the
organization of the system(s) that make up the destination host
cannot, in general, be known to the sender, approaches that assuming
decoding from some coded form at some particular order in the process
of receiving and delivering the message will cause some fraction of
systems that are now fully conformant to Internet mail standards to
fail to properly handle the address.
3.1 The Presentation Issue
Before continuing, it is important to note that any
internationalization system, regardless of how it is implemented at
the protocol level, will require changes at the user interface level
if it is to function in a way that end users consider reasonable.
Unless addresses are presented to the user in familiar characters and
formats, the user's perception will be, not of internationalization
and behavior that is user and culturally friendly, but of a
relatively hostile environment. One thing we have almost certainly
learned from nearly forty years of experience with email is that
users strongly prefer email addresses that closely resemble names or
initials to those involving, e.g., user ID numbers or complex coding
that makes the local part appear as gibberish. Indeed, that
principle --of wanting local parts to appear intelligible-- is
arguably the entire reason for wanting to internationalize these
addresses. Otherwise, any identifier would suffice whether it had
mnemonic value to users or not. If a user sees "xn--fltstrm-5wa1o"
(a punycode form) or "F=E4ltstr=F6m" (the MIME quoted-printable
form), rather than the correctly-written localized string, the result
is almost certain to be unhappiness.
3.2 MUAs, MTAs, addresses, and learning from MIME and ESMTP
The development and deployment of MIME [RFC2045] provided a number of
important lessons for the community about how to design extensions
and enhanced features without harm to the installed and conforming
email system. Perhaps the most important of these was that it is
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easier, and often more expedient, to make changes that have impact
only on mail user agents. If it is possible to make changes that way
--generally changes that involve only message headers and the message
body or body parts-- users who need particular features can switch to
user agents that support them or press for those features in the user
agents they have already selected. Even in the worst case in which
support for features the user considers critical is not readily
available, it is possible, with proper user agent design, to save the
entire message to a file and then use stand-alone software to
interpret the information and perform the desired functions.
Providing these functions in the message headers and body permits
them to be moved opaquely through the mail transport system, thus
avoiding any requirement to modify originating or delivery MTAs or
intermediate relays. In practice, the user may have little control
over those systems. Since changes to them typically impacts large
numbers of users, those who are responsible for them are often
reluctant to make changes in response to the needs of a few users.
It is hence reasonable to conclude that, if it is feasible to support
address internationalization strictly at the MUA level, keeping the
internationalized addresses opaque to the transport system, that is a
more desirable approach than requiring MTA changes. The MUA-only
approach has been carefully examined by others (see, e.g., the
obsolete Internet Draft [Hoffman-IMAA]) and proposed more recently as
a temporary measure in [JET-IMA].
The present document argues that
1. Addressing is a fundamental MTA-level function,
2. Some of the complexities encountered when trying to encode
addresses so as to avoid MTA interactions are symptoms that
attempting to "hide" the MTA function so that it can be handled
by MUAs is not an architecturally desirable approach,
3. The restrictions on email uses and syntax required to provide
internationalization at MUA level are unnecessarily risky, and
almost certainly damaging, to deployed email infrastructure,
4. If internationalization is to be plausible, it is critical that
addressing information be represented in essentially the same way
in the message envelope (i.e., the SMTP command structure) and
the message body (i.e., both message headers and, where feasible,
message text). Different encodings in different places,
especially ones that are copied back and forth, will make both
mail system maintainers and operators and end users very unhappy.
and
5. MTA-level solutions are feasible, architecturally more elegant,
and perhaps not as difficult to deploy in relevant communities as
the strongest advocates of the MUA-only approach appear to
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imagine. See Section 3.8 for additional discussion on this
point.
The decision as to what to do in message bodies and formats (e.g.,
[RFC2822] and MIME [RFC2045]) and what to handle in message
transport (i.e., in extended SMTP) is critical because, as discussed
below, the level at which something is handled is both determined by,
and determines, how information is appropriately encoded. This
decision ultimately depends on the application of two principles:
1. If body content is opaque, anything still visible to transport
requires transport negotiation.
2. Anything an MTA -- be it origin, relay, MX, gateway, or delivery
-- needs to understand or process must be handled as part of mail
transport. The discussion below might be titled "why the MTA
must get involved".
Whether mail addresses meet these criteria, and hence must be
comprehensible in transport, depends on how much the sending MUA
needs to know to construct, and the delivery MTA needs to know to
deliver, a message. Traditionally, we have kept the former knowledge
level at zero: if a sender produces "!a!b!c@example.com" in response
to information that it is a valid address, it still does not know
whether this is a "bang path" or a slightly-perverse name for a
single mailbox. Is "xyz%def@example.com" a specification for routing
to mailbox "xyz" on host "def" or a mailbox named "xyz%def" on the
example.com host? Are "foo+bar@..." or "foo-baz@..." subaddresses
"bar" and "baz" for the mailbox "foo", or are they simple addresses?
Is "jjoneschem@labs.example.com" a local mailbox on that host or an
instruction to route mail to "jjones" in the chemistry department?
Under the rules established in [RFC0821] and [RFC1123], as summarized
and updated in [RFC2821], all of those decisions are up to
"example.com", its MX alternatives, or hosts in that domain, and they
may make very local decisions about them. For example, even within
the same domain (on the same apparent host), "xyz%def" might be a
mailbox while "xyz%ghi" might contain a route; "foo-baz" might
represent and address and subaddress while "foo-blog" might be a
mailbox.
The sender cannot, in the general case, know.
Worse, while non-alphanumeric characters like "+", "-", and "%" have
been used in these examples, delimiters for subaddresses, implicit
routing, embedded commands, and so on are, again, up to the
destination MTA and its interpretations. "X" might be as good a
delimiter as "+". It might even be a better one in some
applications. And, since local-parts are defined as case-sensitive,
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"x" might be a normal address character in the same address in which
"X" was an important delimiter.
Of course, in a completely non-ASCII environment, it would make sense
to substitute characters from the local script for "+", "-", "%",
and so on. If one wants a string completely in local language (i.e.,
non-ASCII) characters, then there may be no desire to break that
convention in order to use an ASCII delimiter (see Section 8 for
additional discussion of this issue).
It is not even necessary to use a delimiter to support some forms of
subaddressing or local routing. Suppose an organization adopted the
convention that externally-visible email address local parts were
structured as, e.g., a three-letter department code, followed by a
five-letter code representing the individual, optionally followed by
a code representing a project. Many organizations use just such
systems and there is no way (and no need) for an email sender to
understand the system or whether it is actually used for mail routing
internally.
Consequently, the idea of a sender breaking an address up into its
component parts and encoding those parts separately, or even just
doing an encoding in sections that preserves the positions of the
delimiters (as measured from the left) is an impossibility without
major, incompatible, and retroactive changes in how mail addressing
is defined. Conversely, if the sender encodes the entire address, or
the entire local part, without understanding the structure of the
address in the same way that the target system does, it is likely
that important information will be lost or, possibly, the message
will be mis-delivered.
3.3 An Encoded-address, MUA-transparent, Solution may Eliminate an
Important Opportunity
The principle above that addresses should have the same form in
headers and in envelopes leads quickly to a reasoning path that
argues for representation of most or all mail headers in some form of
Unicode, including, but not limited to, those headers that explicitly
list addresses. Several proposals have been outlined for doing this;
perhaps the best developed (at the time the first version of this
document was written) was the "UTF-8-HEADERS" proposal
[Hoffman.UTF-8]. Like this proposal, it requires envelope (SMTP
extension) negotiation to protect the headers that are encoded in
UTF-8. This proposal differs from that one by putting somewhat more
reliance on envelope facilities to prevent what its author considers
a number of layering and interaction problems, most of them arising
from the proposed "Address-map:" headers of that UTF-8-HEADERS
proposal.
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3.4 An MUA-only-based Solution is Not Necessary
3.4.1 Obtaining an Internationalized Email Address
One of the classic arguments for an approach based on MUA changes
only (for international addresses or anything else) is that users
will be able to install and use solutions on their own, even if the
administrators of their systems are unenthused about the particular
function or extension and delay, or decline, to install it. That
argument was certainly true for MIME, especially in the presence of
the capability to store messages as files and apply post-MUA tools.
But it does not seem to apply for email addresses. In general, users
cannot create email accounts or aliases controlling delivery of
messages from external systems. Those accounts and aliases must be
created by system administrators responsible for the mail servers.
If those administrators are not sympathetic to internationalized
mailbox names, such names will not exist on the receiving system.
Having apparatus to send those names through the protocols will be
essentially useless: a message that bounces because the relevant
account or mailbox does not exist will bounce equally well whether
the target address is in ASCII or in some other script and whether or
not the receiving MTA is required to explicitly agree to access
internationalized addresses. Conversely, if the administrators of
the mail system host are sympathetic to internationalization, it is
reasonable to expect that appropriate software can and will be
installed at the MTA level.
An apparent important exception to the position taken in the above
paragraph arises for subscription, often free, email services such as
those operated under the "Hotmail", "Juno", "Netscape", and "Yahoo"
names. Some of these systems permit users to select their own names
(local parts) through an automated process. If the user creates a
mailbox using an encoded name, users with MUAs that support the
encoding will be able to send mail using a name in the user's
preferred characters if, of course, the user or MTA somehow knows the
encoding is being used. But the user cannot know what capabilities
the correspondents will have available, and hence must give out both
the name in local characters and the encoded form. This may turn out
to be necessary, but is unlikely to be considered desirable. Also,
if the user has presentation software that recognizes the coding
conventions, then he or she will be able to see the original-language
names in incoming messages and may not know what names to pass on to
those who lack such presentation software. And, more important, many
users of such systems access them through "web mail" interfaces,
using standard (or at least common) browsers. The issues in getting
those browsers upgraded to automatically recognize and decode special
encoding forms may be as difficult, or more difficult, than those
associated with convincing a system administrator to install a
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special address or upgraded MTA.
Consider this practice from a user point of view. First, the domain
names for these systems --as compared to institutional mail systems--
will generally continue to be in ASCII, so the goal of an email
address that is entirely or predominantly in the user's language will
be unattainable. If the domain names are non-ASCII (i.e., are IDNA
encodings of non-ASCII strings), it is reasonable to assume that an
operator who would choose such a name would be willing to
internationalize its MTA. Second, such systems are most often
accessed through web-based interfaces where most email header
information appears to the user browser as running text. Because an
email local part can, today, take on the form of almost any ASCII
string, it is not reasonable to expect that a browser, even one with
some localized functions, will be able to accurately detect an
embedded, specially-coded, mailbox local part and correctly decode
and render it. Heuristics based on detection of an at-sign ("@")
will, of course, work for many cases, but will also produce a certain
number of false positives, perhaps destroying URLs or examples in the
text. After all, the "@" symbol has been around since long before
there was email. It is worth noting that any recognition and
decoding of local parts using a local encoding relies on heuristics
that may fail: all such strings are historically-valid email local
parts, and, unlike the DNS situation, it is impossible to conduct a
reliable survey to determine that no one is using any particular
encoding form, especially if the encoding indicator appears embedded
in the local part string, rather than as a prefix. By contrast, if
the MTA sees a Unicode string, and Unicode strings are placed in
message headers and message bodies as needed, the transition may be
more difficult, but no long-term user confusion or exposure to ugly
encodings will be necessary.
To be very specific about this, if the local part of an address is
encoded, e.g., with some ACE form as suggested in [JET-IMA], there is
no practical way for the receiving system to know to decode it,
rather than treat it as an ordinary mailbox name, unless it is
notified via a SMTP envelope extension.
3.4.2 Relay environment
As in many other areas with email, many of the difficulties with an
MTA-based model for internationalization of addresses arise, not when
the originating MTA communicates directly with the delivery MTA, but
when relay MTAs are involved. If the both the sending and receiving
systems support internationalized addresses, it is still possible
that an intermediate relay will not do so, forcing mail to bounce
that could be delivered if there were a direct connection between
sender and receiver. But, as with the installation of email
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addresses on a system, relays do not get inserted in the mail path by
accident. If internationalized addresses are important to the
destination host, its administrators will chose lower-preference MX
hosts or other relays that can support internationalized addresses.
3.4.3 Internationalizing the Sender
If we assume a destination host that can accept, and properly handle,
an internationalized address, and we assume that any MX-designated
intermediaries for that host will be chosen to be similarly capable,
one situation is left in which it would be advantageous to have an
MUA-only-based solution. If a originating/ sending system is not
capable of generating or sending an internationalized address, but
the prospective receiving system is, it would be good to enable the
originating user to generate and somehow send to the relevant
address.
This is a real issue, and deserves some serious consideration. But
it seems better to find a good temporary, transitional, mechanism for
it than to permanently burden the email system with an uncomfortable
mechanism just to accommodate this case. One example of a
transitional mechanism might be to use encapsulation, i.e., ESMTP
tunneling over MIME [RFC2442], to route the address and message to a
friendly gateway host that would unpack the message and transmit it
using this specification. Other examples, less attractive at first
glance but still plausible, would include defining and using small
variations on the message encapsulation mechanisms that are integral
to MIME [RFC2046], or the more complex encapsulation designed for
HTML [RFC2557], to accomplish the same purpose.
The one transitional option that is not plausible is to simply send
an encoded string without envelope modification. "Just send ACE" has
the same unfortunate properties as the "just send eight" system
proposed when MIME was adopted: in both cases, even if the message or
address are not damaged in transit, the recipient will not have
sufficient information available to be able to accurately determine
what it has received and, hence, whether or not to decode it.
So, a user with an MUA that has the capability to handle an
internationalized address, but who does not have access to an
originating MTA with the capabilities defined here, may be given
access to a reasonable transition strategy until the needed
capabilities are available. Note that this does not require an open
relay, since all of the user authentication capabilities of ESMTP
[RFC2554] and SUBMIT [RFC2476] would be available. One can even
imagine a service with a per-message charging system, which would
presumably encourage rapid upgrading.
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3.5 A Solution Based on MUA Changes Alone is Unworkable
The difficulties identified in the examples above are, perhaps
obviously, not the only ones. Other issues arise with intermediate
MX relay and gateway hosts, commands embedded in local parts, and
special formats used in gateways to other environments, among other
cases. Some of those additional cases are described briefly below.
3.5.1 MX Diversion
If the domain part of an email address is associated with several MX
records and the mail is delivered to one of them that is not the best
preference host, subsequent mail processing between that intermediate
host and the ultimately destination one is not required to use SMTP.
If, instead, it performs some gateway function, it may need to
inspect or alter the local part to determine how to route and deliver
the message. If the local part were encoded in some fashion that
prevented that inspection process, and the MTA was not aware that it
needed to apply special techniques, mail delivery might well fail.
3.5.2 Embedded commands
In addition to the address forms with special syntax or semantics
described elsewhere, systems have been developed that embed commands
in address local parts. These might, of course, use entirely
different syntax constructions and formats than are typical in
conventional addresses and, in an internationalized environment,
might reasonably use character coding conventions that are neither
ASCII nor Unicode-based.
A number of specialized applications of email do require, or
recommend, specific syntax in the local part. These are identified,
not to indicate that they are the only cases (they are not) but to
reinforce the point that one must be quite cautious in doing anything
that makes global assumptions about local part syntax and significant
characters. These applications include local part explicit routing
with the "percent hack" [RFC1123], gateways to and from X.400
environments [RFC2156], and gateways to fax systems [RFC3192].
3.6 Encoding the Whole Address String
Much of the above demonstrates why selective encoding of parts of the
local-part string is not practical, will exclude many important
cases, or will subject users to permanent use of the crytpic encoded
forms. Why, then, not encode the entire string and insist that the
delivery MTA recognize the presence of an encoded form and do
whatever decoding is needed before it does other processing? There
are three major reasons to approach the problem this way:
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1. Any change in address syntax interpretation is likely to be a
major, incompatible, change, since we do not now impose any
restrictions on how an MTA is organized or even on how, or
whether, the MTA, MUA, and other delivey-related functions are
actually divided up on a given host. Converting user agents to
handle international forms of addresses in a way that does not
produce user astonishment is likely to be a major undertaking,
regardless of what is done to the protocols and at what level.
2. Imposing a requirement that MTAs "understand" local-parts so that
they can be partially decoded as part of mail routing would seem
to defeat the main goal of encoding internationalized strings
into a compact ASCII-compatible form, i.e., to keep MTAs from
needing to understand the extended naming system
3. We potentially have three different encodings of an
internationalized string: the one used by the MTA, the one used
by the MUA, and the one seen by the user through applications
software or the operating system's display interface. Having all
three of these identical or closely compatible is desirable from
the standpoint of user understanding and debugging. Having them
different can cause many "interesting" problems, e.g., having to
return an error message that uses different coding, and hence
might represent an entirely different string, than the string the
user put into the process.
Instead, it would seem sensible to move from a straightforward
encoding of mail addresses in ASCII to a straightforward encoding in
Unicode via UTF-8 [RFC2277], imposing only those restrictions on the
characters in the local part that are implied by Unicode itself.
3.7 Looking back and looking forward
Another principle is implied by some of the discussion above.
Internationalization measures for the Internet will be with us for as
long as there are multiple languages and scripts in the world, i.e.,
probably forever. If a satisfactory long-term solution can be found,
and a reasonable transition strategy can be defined for it, it is
much better to optimize for the long term. The alternative of making
things more difficult or less functional forever -- for the
transport, the MUA, and/or the user interface system -- in order to
save some small effort in transition, or even to make the transition
a few months faster, represents a very poor tradeoff.
3.8 Summary of Design Issues and Tradeoffs
Each of the above subsections describes a strong case for continuing
to treat addressing as an MTA function, opaque except at the end
systems. The main alternative is to rely on the sending system being
able to understand the addressing system of the target host, and any
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relays accessed through MX relays, potentially needing to be able to
remove IDN encoding ("punycode" or otherwise) in order to determine
how to process or route the message. That alternative violates a
long-standing and important design principle of Internet email,
complicates a number of other cases, and does not offer sufficient
transition advantages to be worth any of those difficulties.
The protocol proposed here takes a giant step toward true
internationalization of electronic mail, providing a good functional
approximation to what we might have done several decades ago had
Unicode and the necessary understanding been available. It does not
go as far as one could imagine going in providing address forms that
would be compatible with local styles and models all over the world.
The issues in considering, and taking, those extra steps are
discussed in Section 8.
4. A Mail Transport-level Protocol
4.1 General Principles and Objectives
1. Whatever encoding is used should apply to the whole address and
be directly compatible with software used at the user interface.
2. An SMTP relay must either recognize the format explicitly,
agreeing to do so via an ESMTP option, select and use an ASCII-
only address, or bounce the message so that the sender can make
another plan.
3. In the interest of interoperability, charsets other than UTF-8 or
punycode are strongly discouraged. If a mail environment chooses
to use them anyway in the local part, interpretation at the "what
does this mean" level is the responsibility of the receiving MTA.
4.2 Framework for the Internationalization Extension
The following service extension is defined:
1. The name of the SMTP service extension is "Address
Internationalization";
2. The EHLO keyword value associated with this extension is "I18N";
3. No parameter values are defined for this EHLO keyword value. In
order to permit future (although unanticipated) extensions, the
EHLO response MUST NOT contain any parameters for that keyword.
If a parameter appears, the SMTP client that is conformant to
this version of this specification MUST treat the ESMTP response
as if the I18N keyword did not appear.
4. An optional parameter is added to the SMTP MAIL and RCPT
commands. This parameter is named ALT-ADDRESS. It requires an
argument that may be useful in "downgrading" (see Section 4.5) as
a substitute for the internationalized (UTF-8 coded) address.
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This all-ASCII address MAY incorporate the IDNA "punycode" form
if the domain name is internationalized. No algorithmic
transformation is specified for the local-part; in the general
case, it may identify a completely separate mailbox from the one
identified in the primary command argument.
5. No additional SMTP verbs are defined by this extension.
Most of the remainder of this memo specifies how support for the
extension affects the behavior of an SMTP client and server and what
message header changes it implies.
4.3 The Address Internationalization Service Extension
In the absence of this extension, SMTP clients and servers are
constrained to using only those addresses permitted by RFC 2821. The
local parts of those addresses may be made up of any ASCII
characters, although certain of them must be quoted as specified
there. It is notable in an internationalization context that there
is a long history on some systems of using overstruck ASCII
characters (a character, a backspace, and another character) within a
quoted string to approximate non-ASCII characters. This form of
internationalization should be phased out as this extension becomes
widely deployed but backward-compatibility considerations require
that it continue to be supported.
An SMTP Server that announces this extension MUST be prepared to
accept a UTF-8 string [RFC3629] in any position in which RFC 2821
specifies that a "mailbox" may appear. That string must be parsed
only as specified in RFC 2821, i.e., by separating the mailbox into
source route, local part and domain part, using only the characters
colon (U+003A), comma (U+002C), and at-sign (U+0040) as specified
there. Once isolated by this parsing process, the local part MUST be
treated as opaque unless the SMTP Server is the final delivery MTA.
Any domain names that are to be looked up in the DNS MUST be
processed into punycode form as specified in IDNA [RFC3490] unless
they are already in that form. Any domain names that are to be
compared to local strings SHOULD be checked for validity and then
MUST be compared as specified in IDNA.
An SMTP Client that receives the I18N extension keyword MAY transmit
a mailbox name as an internationalized string in UTF-8 form. It MAY
transmit the domain part of that string in either punycode (derived
from the IDNA process) or UTF-8 form but, if it sends the domain in
UTF-8, it SHOULD first verify that the string is valid for a domain
name according to IDNA rules. As required by RFC 2821, it MUST not
attempt to parse, evaluate, or transform the local part in any way.
If the I18N SMTP extension is not offered by the Server, the SMTP
Client MUST not transmit an internationalized address. Instead, it
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MUST either return the message to the user as undeliverable or
replace it, either using the ASCII-only address specified with the
ALT-ADDRESS parameter or using some process (such as a directory
lookup) outside the scope of this specification, with a local-part
that conforms to the syntax rules of RFC 2821.
4.4 Extended Mailbox Address Syntax
RFC 2821, section 4.1.2, defines the syntax of a mailbox as
Mailbox = Local-part "@" Domain
Local-part = Dot-string / Quoted-string
; MAY be case-sensitive
Dot-string = Atom *("." Atom)
Atom = 1*atext
Quoted-string = DQUOTE *qcontent DQUOTE
Domain = (sub-domain 1*("." sub-domain)) / address-literal
sub-domain = Let-dig [Ldh-str]
(see that document for productions and definitions not provided here
-- their details are not important to understanding this
specification). The key changes made by this specification are,
informally, to
o Change the definition of "sub-domain" to permit either the
definition above or a UTF-8 (or other, see Section 7.1) string
representing a DNS label that is conformant with IDNA [RFC3490].
That sub-domain string MUST NOT contain the characters "@" or ".".
o Change the definition of "Atom" to permit either the definition
above or a UTF-8 (or other, see Section 7.3) string. That string
MUST NOT contain any of the ASCII characters (either graphics or
controls) that are not permitted in "atext"; it is otherwise
unrestricted.
4.5 The ALT-ADDRESS parameter
If the I18N extension is offered, the syntax of the SMTP MAIL and
RCPT commands is extended to support the optional "ALT-ADDRESS"
parameter, which takes one argument. Its syntax is
"alt-address=" Mailbox
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where "Mailbox" is strictly according to the (unextended) syntax
specified in RFC 2821. If the receiving SMTP server is required to
send the message on to a system that does not support this extension
or the one for UTF-8 headers (see Section 6, it MAY substitute the
specified address for the internationalized one to permit the message
to be delivered to a mailbox specified by the sender. If UTF-8
headers are not supported, it SHOULD also encapsulate the message as
described in Section 3.4.3. An SMTP server that cannot forward an
internationalized message using the addressing and UTF-8 header
extensions MUST either
o Substitute all-ASCII addresses and encapsulate the message, or
o "Bounce" the message, either rejecting it during the SMTP
transaction or returning it, as specified in RFC 2821.
Under normal circumstances, the final delivery SMTP server should be
configured so that the two mailbox names point to the same physical
store. However, just as case-matching for traditional ASCII local-
parts is not a requirement of the unmodified SMTP protocol, mapping
of these two mailbox names is not a requirement here: sites for whom
other issues outweigh the potential confusion should configure their
systems as they find appropriate.
While further analysis is required, it will probably be desirable to
extend the "Return-path:" trace header to include this parameter when
it is provided; doing so would increase the odds that an error
message could be properly routed in a number of edge and transition
cases.
4.6 Formation of the Alternate Address
It is tempting to want to form the alternate, all-ASCII, address
algorithmically so that, e.g., the sending MUA could compute it
without any user input other than the native-form address. Such a
computation could, for example, be the ACE transformation
contemplated by [JET-IMA]. In the general case, this is not possible
for the same reason that the use of an ASCII-compatible form without
option negotiation is not feasible: the sending system cannot know
the way in which the receiving one parses and interprets address
local parts.
However, for the range of simple cases in which the local part really
is atomic and represents a simple mailbox without subaddresses or
other internal structure, it would probably be helpful to have a
convention that the destination host could use to derive the
alternate address, such as a standard ACE-style encoding. One can
imagine prohibiting the use of alternate addresses that used selected
ACE prefix except as an ACE-introducer so that the sending MUA, upon
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receiving an reply or bounce on the alternate address could at least
produce a helpful error message for the user.
4.7 Additional ESMTP Changes and Clarifications
The mail transport process involves addresses ("mailboxes") and
domain names in contexts in addition to the MAIL and RCPT commands
and extended alternatives to them. In general, the rule is that,
when RFC 2821 specifies a mailbox, this document expects UTF-8 to be
used for the entire string; when RFC 2821 specifies a domain name,
the name should be in punycode form if its raw form is non-ASCII.
The following subsections list and discuss all of the relevant cases.
[[Note in draft: I hope]]
4.7.1 The Initial SMTP Exchange
When an SMTP or ESMTP connection is opened, the server sends a
"banner" response consisting of the 220 reply code and some
information. The client then sends the EHLO command. Since the
client cannot know whether the server supports internationalized
addresses until after it receives the response from EHLO, any domain
names that appear in this dialogue, or in responses to EHLO, must be
in hostname form, i.e., internationalized ones must be in punycode
form.
4.7.2 Trace Fields
Internationalized domain names in Received fields should be
transmitted in Unicode form. Addresses in "for" clauses need further
examination and might be treated differently depending on whether
8BITMIME is a requirement for internationalized addresses (See
Section 6. The reasoning in the introductory portion of Section 5
strongly suggests that these addresses be in Unicode form, rather
than some specialized encoding, but a counterargument is that users
do not look at Received fields and, if there is a standard encoding
available that is completely interoperable and information-
preserving, it should be used for both domain names and addresses
(perhaps in a comment or other supplemental information).
5. Impact on the MUA and on Message Headers
In addition to the trace fields ("Received" headers), mentioned
above, there are many other places in MUAs or in user presentation in
which email addresses or domain names appear. Each one, whether the
conventional From, To, or Cc header fields, or Message-IDs, or In-
Reply-To fields that may contain addresses or domain names, or in
message bodies or elsewhere, must be examined from an
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internationalization perspective. The user will expect to see
mailbox and domain names in local characters, and to see them
consistently: a situation in which an address is coded one way in a
"From" field, another way in a signature line in the body of a the
message, and, apparently arbitrarily, in one or the other of those
forms in Return-Path, Received, or reference fields, will create
confusion and frustration. Variations on that problem will exist
with any internationalization method, whether transport or MUA-only
in structure. Perhaps, if we have to live with it for a short time
as a transition activity, that is worthwhile. But the only practical
way to avoid it, in both the medium and the longer term, is to have
the encodings used in transport be as nearly as possible the same as
the encodings used in message headers and message bodies.
There appears to be a very strong case for concluding that the point
at which we internationalize email local parts is the point that we
should simply shift email headers to a full internationalized form,
presumably using UTF-8 rather than ASCII. The transition to that
model might involve support for legacy systems by extending the
encoding models of [RFC2045] and [RFC2231] to cover address, and
address-related, fields within headers but our target should be fully
internationalized headers, as discussed elsewhere in this document.
6. Bundling of Extensions and Options
An ongoing concern about any extensions to SMTP is that they will
combine to create a situation with enough possible combinations to
require profiling and the consequent increased complexity and
negative impact on interoperability. Reducing the number of
different options and combinations of them is therefore a useful
goal. In this particular case, we have
1. This proposal, to permit UTF-8 addresses and a alternate
addresses when it is not possible to reliably transmit or use the
UTF-8 ones.
2. Several proposals to permit the use of UTF-8 (or other non-ASCII
encodings) in mail headers. See, e.g., [Hoffman.UTF-8]
3. Several proposals to protect those UTF-8 headers with an SMTP
extension. See, again, e.g., [Hoffman.UTF-8]
4. The established "8BITMIME" extension [RFC1652] that permits
message bodies to be transmitted in 8 bit form, rather than
requiring a content transfer encoding to force them into 7 bit
form. Given the difficulties that would occur if UTF-8 (or other
8 bit) headers were significantly encoded, this extension is all
but required for the UTF-8 header extensions to function
properly.
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5. Possibly also some type of "get envelope components out of the
headers" proposal, such as [Klensin.envelope]
In the interest of compatability and interoperability, it is
suggested that all of these extensions be bundled, with only one EHLO
keyword for all of them other than 8BITMIME, and with that new
keyword implying (and requiring support for) all of the listed
functions. This requires MTA implementers who wish to support these
features to do a slightly larger job, but avoids the interoperability
costs of excessive incrementalism. Put differently,
internationalization, taken seriously, is a large issue and a large
job, and these appear to be the pieces needed to get the job done
7. Protocol Loose Ends
These issues should be resolved, and this section eliminated, before
the document is considered complete.
7.1 Punycode in Domain Names?
It is not clear whether the flexibility of being able to pass domain
names in punycode, as well as UTF-8, form is needed. If it is not,
it should be eliminated as excess complexity.
7.2 Local Character Codes in Local Parts?
There are some reasons for permitting local-parts to be written in
locally-used character codes, i.e., in other than the UTF-8 encoding
of UNICODE. This could be done by tagging the local part in a
fashion similar to the technique of [RFC2047] but without encoding
the local part string itself. It clearly increases flexibility, and
the mailbox part can be defined as a simple octet string (as it
essentially is in the sections above). We can reasonably expect that
some systems, operating in local environments, will use local
character codes no matter what we specify. On the other hand, having
an application presented with an octet (or bit) string and not
knowing what charset is involved would wreak havoc on any attempt to
intelligently display local parts: if one cannot know the character
coding being used, then it is not possible to accurately decode the
characters and display appropriate character glyphs.
Use of local coding also implies an encoding for the local part
different from that for the domain part -- any MTA in the path must
be able to resolve the domain part into something that can be looked
up in the DNS and resolved and that, in turn, requires a globally-
known encoding.
On the other hand, if local codings can be avoided entirely, it will
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considerably reduce complexity and "opportunities" for systems to not
interoperate.
7.3 Restrictions on Characters in Local Part?
This specification is extremely liberal about what can be included in
a UTF-8 string that represents a local-part. In return, it
effectively prohibits the use of quoted strings, or quoted
characters, in non-ASCII local parts. Quoted strings and characters
in local parts have, in general, been nothing but trouble and there
appears to be no reason to carry that trouble forward into an
internationalized world (and the much greater complexity that quoting
in that environment might imply). There may also be a strong case
for applying restrictions, e.g., by use of a stringprep [RFC3454]
profile that would eliminate particularly problematic characters
while not forcing, e.g., even an approximation to case-mapping
(remember that ASCII local-parts are inherently case sensitive, even
though local systems are encouraged to not take advantage of that
feature).
7.4 Requirement for 8BITMIME?
This extension is carefully defined to be independent of "8BITMIME".
However, given the length of time 8BITMIME has been around, the
amount of deployment of it that exists, and the rather low likelihood
that any MTA implementer in his or her right mind will go to the
trouble of implementing this extension without also implementing
8BITMIME, it may be sensible to permit this extension only if
8BITMIME also appears or is combinated with it as suggested in
Section 6
7.5 Message Header and Body Issues with MTA Approach?
By viewing i18n addresses as an MTA problem, this document may not
address all of the interesting 2822/MIME and MUA implementation and
presentation style issues.
In particular, if both this extension and 8BITMIME are in use, is it
sensible to drop the requirement for RFC 2047/ 2231 encoding of
personal name fields? And, whether or not that requirement is
dropped, is the MUA description of Section 5 adequate?
7.6 The Received field 'for' clause
Decide what to do about the value of the "for" clause in Received
fields. See Section 4.7.2.
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8. Internationalization and Full Localization
Whenever one considers a new protocol, or revision of an existing
one, for internationalization or other aspects of support for an
improved user interface, important tradeoffs arise. These tradeoffs
can be described in several ways, e.g.,
o Simplicity versus localization capability
o User convenience, especially within a particular area or culture
versus global interoperability
o and so on
Maximum global interoperability is obtained by confining a protocol
to an very limited number of characters, ideally ones that are easily
distinguishable by people. The historical choice in this regard has
been the 26 upper-case ASCII letters, plus digits, plus a very small
number of special characters. It is probably no coincidence that
these characters (with different, bit-minimizing, encodings) are the
normal ones in early telegraphy and subsequent Telex character sets.
But, as soon as users start looking at these characters, the
complaints start to appear: text in all-upper-case is ugly, people
should be able to write their names as they normally do and not in
some transliterated or variant form, people should be able to
communicate in their own languages using their own character sets,
and so on. Ultimately, not only are the characters used in writing
at issue; so are the structures for constructing, e.g., command
sequences, with different preferences typically reflecting the
grammatical structures of different languages. With sufficient
ingenuity, all of these requirements can be accommodated, but only
typically at the cost of global interoperability or at the expense of
convenient use by people outside the locality or cultural group.
Email addresses illustrate this problem at its most difficult. They
are seen and used by end users and there has been little success in
hiding the forms that are actually used in the protocols. Worldwide,
most communication is almost certainly among people who share
languages and cultural assumptions, not in situations in which global
interoperability is important (and where it is important that global
interoperability be convenient and very reliable). On the other
hand, situations and communications that require global
interoperability are still common and are commercially and
intellectually important.
So the question is how far should one go. It is clearly important
and sensible to accommodate local character sets, and to do so in a
way that creates maximum convenience and attractive user interfaces
in the long term. But, as pointed out in passing in Section 4.3,
RFC2821 still requires the ASCII at-sign character to divide the
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local part from the domain name. If even lexical support for the
long-deprecated source routes is to be provided, comma and colon must
also be preserved and supported. This implies that a mailbox name
that is completely in some character script other than ASCII is
impossible without further changes to the email protocols. In
addition, the ordering implied by the "local-part@domain"
construction, usually read in English as "local part at domain",
seems quite strange and foreign in some other languages and cultures.
It is interesting that X.400 avoided this delimiter and ordering
problem entirely by using Distinguished Names in which the various
elements of an address were explicitly identified. But, when
Distinguished Names appear at the presentation layer or above, they
appear with the various fields identified by tags which are,
themselves, keywords that use a very restricted set of ASCII
(actually ISO 646 or IA5) characters.
In principle at least, the protocol extensions proposed here could be
further extended to specify a separator character to distinguish
local part from domain name and the order in which those names
occurred. For example, the MAIL and RCPT commands could be extended
with parameters like
SEPARATOR="UTF-8-character" ORDER-RL to identify a form consisting of
the domain name followed by the local part, separated by the
designated character
But, while this would not impose particularly heavy burdens on SMTP
processors, it would be a potential nightmare for users, who would
have no way to accurately identify the components of an email
address, at least without significant out-of-band information. In
addition, going that far would almost certainly touch off the debate,
again, as to whether domain names should be presented in little-
endian or big-endian order -- an issue that is, again, culturally
sensitive as to which one feels most natural.
It is not clear how far one should go, and the community should
consider the issue very carefully.
9. Advice to Designers and Operators of Mail-receiving Systems
As discussed above, in the historical Internet email context, the
interpretation and permitted syntax for an email local-part is
entirely the responsibility of the receiving system. Systems can get
themselves into trouble and, more particularly, can seriously
restrict the number and type of users who can send mail to their
users, by poor choices of format and syntax. For example, general
advice to system designers has long included "treat addresses in a
case-independent fashion" and "do not use addresses that require
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quoting" in order to increase the odds that remote users will be able
to properly compose and transmit intended addresses. In a way, that
advice is an extreme generalization of the "receiver" side of the
robustness principle: being generous in what one accepts implies
accepting as many plausible variations of an address local-part
string as possible and designing the strict forms of those strings to
facilitate differentiation when it is appropriate.
As one moves toward internationalization of local parts, an expanded
version of these principles is useful and may be even more
appropriate, even though it is neither necessary nor desirable to
turn those principles into protocol requirements. For example, a
receiving host should normally consider any string that would match
under nameprep rules --or perhaps any string that would match under a
stringprep profile that provides more matching and exclusions than
nameprep-- as matching for local-part purposes. An even more
"liberal" receiving host might use some sort of variant tables for
its script(s) of interest to further expand the matching rules.
But, whatever extended matching rules the local host adopts, those
rules are a property of that host. Senders should continue to be
conservative about what they send, and relays should continue to
avoid presumptions about their understanding of the content of local-
parts. Receiving systems that have reason to adopt more restricted
syntax rules, or interpretations of matching, should continue to be
able to do so.
10. Internationalization Considerations
This entire specification addresses issues in internationalization
and especially the boundaries between internationalization and
locationalization and between network protocols and client/user
interface actions.
11. IANA Considerations
This specification does not contemplate any IANA registrations or
other actions.
12. Security considerations
Any expansion of permitted characters and encoding forms in email
addresses raises the risk, however slight, of misdirected or
undeliverable mail. The problem is worsened if address information
is carried in local character sets and must be converted to some
standard form. Any conversion of character sets may also be
problematic for digitally-signed information. Modulo those concerns,
the ideas proposed here do not introduce new security issues.
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Since email addresses are often transcribed from business cards and
notes on paper, they are subject to problems arising from confusable
characters. Those problems are somewhat reduced if the domain
associated with the mailbox is unambiguous and supports a relatively
small number of mailboxes whose names follow local system
conventions; they are increased with very large mail systems in which
users can freely select their own addresses.
13. Acknowledgements
The author acknowledges the contributions and comments of Dave
Crocker in a personal conversation, and the efforts of a private
discussion group, led by Paul Hoffman and Adam Costello, to develop
an MUA-only solution to this problem. The author had hoped that
effort would succeed, since the idea of requiring transport changes
to support internationalization (or any other new function) is
unattractive and should be avoided when possible. Difficulties that
group has encountered in properly defining a number of boundary
conditions, including appropriate delimiters for permitting internal
parsing of the local part and problems with right-to-left characters
and substrings, have led to the conclusion that it is time to get a
specific, transport-based, approach on the table. That conclusion
has been reinforced by increasing interest in the IETF in more
radical changes to the mail system, starting with extensions to
permit mail headers to be written in UTF-8. While the ideas leading
to the "IMAA" and other drafts have inspired several of the
properties of this proposal, their authors are, of course, not
responsible for the result and will probably disagree with it.
Comments from Adam Costello on the first public draft were
particularly helpful, and James Seng identified some
internationalization issues that had not been addressed in the
previous version.
14. An Appeal
The author received a number of favorable comments on the general
principles and design discussed in early drafts of this
specification. He is not, however, able to continue its development
as a one-person, or even one-person with occasional comments from
others, basis. In particular, he has almost no resources for
developing MTA, MUA, or presentation code to test and demonstrate the
concepts and details outlined above; without such resources, this
approach will, inevitably, fail sooner or later. So those who
consider the idea attractive should think about, and develop, ways to
join with the author in design team and development efforts.
15. References
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15.1 Normative References
[Hoffman.UTF-8]
Hoffman, P., "SMTP Service Extensions for Transmission of
Headers in UTF-8 Encoding", draft-hoffman-utf8headers-00
(work in progress), December 2003.
[Klensin.envelope]
Klensin, J., "A Cleaner SMTP Envelope for Internet Mail",
draft-klensin-email-envelope-00 (work in progress),
January 2004.
[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10,
RFC 821, August 1982.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC1651] Klensin, J., Freed, N., Rose, M., Stefferud, E., and E.
Crocker, "SMTP Service Extensions", RFC 1651, July 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels'", RFC 2119, March 1997.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)",
RFC 3491, March 2003.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629, November 2003.
15.2 Informative References
[Hoffman-IMAA]
Hoffman, P. and A. Costello, "Internationalizing Mail
Addresses in Applications (IMAA)", draft-hoffman-imaa-03
(work in progress), October 2003.
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[JET-IMA] Yao, J. and J. Yeh, "Internationalized eMail Address
(IMA)", draft-lee-jet-ima-00 (work in progress),
June 2005.
[RFC1652] Klensin, J., Freed, N., Rose, M., Stefferud, E., and E.
Crocker, "SMTP Service Extensions", RFC 1652, July 1994.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
[RFC2056] Denenberg, R., Kunze, J., and D. Lynch, "Uniform Resource
Locators for Z39.50", RFC 2056, November 1996.
[RFC2156] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay):
Mapping between X.400 and RFC 822/MIME", RFC 2156,
January 1998.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Word Extensions: Character Sets, Languages, and
Continuations", RFC 2231, November 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC2442] Freed, N., Newman, D., and Hoy, M., "The Batch SMTP Media
Type", RFC 2442, November 1998.
[RFC2476] Gellens, R. and J. Klensin, "Message Submission",
RFC 2476, December 1998.
[RFC2554] Myers, J., "SMTP Service Extension for Authentication",
RFC 2554, March 1999.
[RFC2557] Palme, F., Hopmann, A., Shelness, N., and E. Stefferud,
"MIME Encapsulation of Aggregate Documents, such as HTML
(MHTML)", RFC 2557, March 1999.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
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[RFC3192] Allocchio, C., "Minimal FAX address format in Internet
Mail", RFC 3192, October 2001.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
Author's Address
John C Klensin
1770 Massachusetts Ave, #322
Cambridge, MA 02140
USA
Phone: +1 617 491 5735
Email: john-ietf@jck.com
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