Network Working Group J. Klensin
Internet-Draft October 3, 2003
Expires: April 2, 2004
Internationalization of Email Addresses
draft-klensin-emailaddr-i18n-00.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
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
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
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definitions and use of Internet mail, and proposes a solution that is
likely to serve users and the network well for the long term.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. History, Context, and Design Constraints . . . . . . . . . . 4
2.1 MUAs, MTAs, addresses, and learning from MIME and ESMTP . . 4
2.2 An MUA-based Solution is Not Necessary . . . . . . . . . . . 6
2.2.1 Obtaining an Internationalized Email Address . . . . . . . . 7
2.2.2 Relay environment . . . . . . . . . . . . . . . . . . . . . 7
2.2.3 Internationalizing the Sender . . . . . . . . . . . . . . . 7
2.3 An MUA-based Solution is Unworkable . . . . . . . . . . . . 8
2.3.1 MX diversion . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3.2 Embedded commands . . . . . . . . . . . . . . . . . . . . . 8
2.4 Encoding the Whole Address String . . . . . . . . . . . . . 9
2.5 Looking back and looking forward . . . . . . . . . . . . . . 10
2.6 Summary of Design Issues . . . . . . . . . . . . . . . . . . 10
3. A Mail Transport-level Protocol . . . . . . . . . . . . . . 10
3.1 General Principles and Objectives . . . . . . . . . . . . . 10
3.2 Framework for the Internationalization Extension . . . . . . 11
3.3 The Address Internationalization Service Extension . . . . . 11
3.4 Extended Mailbox Address Syntax . . . . . . . . . . . . . . 12
3.5 Additional ESMTP Changes and Clarifications . . . . . . . . 13
3.5.1 The Initial SMTP Exchange . . . . . . . . . . . . . . . . . 13
3.5.2 Trace Fields . . . . . . . . . . . . . . . . . . . . . . . . 13
3.6 Protocol Loose Ends . . . . . . . . . . . . . . . . . . . . 13
3.6.1 Punycode in Domain Names? . . . . . . . . . . . . . . . . . 14
3.6.2 Local Character Codes in Local Parts? . . . . . . . . . . . 14
3.6.3 Restrictions on Characters in Local Part? . . . . . . . . . 14
3.6.4 Requirement for 8BITMIME? . . . . . . . . . . . . . . . . . 14
3.6.5 Message Header and Body Issues with MTA Approach? . . . . . 15
3.6.6 Variant Addresses (Aliases) in a Command Verb . . . . . . . 15
3.6.7 The Received field 'for' clause . . . . . . . . . . . . . . 15
4. Advice to Designers and Operators of Mail-receiving
Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5. Security considerations . . . . . . . . . . . . . . . . . . 16
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
Normative References . . . . . . . . . . . . . . . . . . . . 16
Informative References . . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . 19
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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. 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, indeed, must not, understand how the receiving one will
interpret an address.
The balance of this document explores these issues in more detail.
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. These two concepts 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].
In its present internet-draft form, the document contains a great
deal of explanatory material and rationale for the approach chosen.
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The actual protocol material appears almost entirely in Section 3,
especially Section 3.2 through Section 3.4. 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 skip the
reasoning and comparison to other alternatives in this document and
examine the protocol proposal should skip to those sections.
2. 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 that way 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. 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.
2.1 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
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
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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 approach
has been carefully examined by others [I-D.hoffman-imaa]. This
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, and
4. 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 approach appear to imagine.
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., [E]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 -- origin, relay, MX, gateway, 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
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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 on the example.com host
named "xyz%def". 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, "xyz%def"
might be a mailbox while "xyz%ghi" might be a route; "foo-baz" might
be a 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,
"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.
It is not even necessary to use a delimiter to support some forms or
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 is an
impossibility without major, incompatible, and retroactive changes in
how mail addressing is defined.
2.2 An MUA-based Solution is Not Necessary
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2.2.1 Obtaining an Internationalized Email Address
One of the classic arguments for an MUA-based approach (to
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 they 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.
2.2.2 Relay environment
As in many other areas with email, 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
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.
2.2.3 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-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
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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 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.
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.
2.3 An MUA-based Solution is Unworkable
The examples given 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.
2.3.1 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, the receiving host 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.
2.3.2 Embedded commands
In addition to the address forms with special syntax or semantics
described elsewhere, systems have been developed that embed commands
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in address local parts. These might, of course, use entirely
different syntax parts 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].
2.4 Encoding the Whole Address String
Much of the above demonstrates why selective encoding of parts of the
local-part string is not practical. 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:
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 and MUA 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
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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.
2.5 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 in order to save
some small effort in transition, of even to make the transition a few
months faster, represents a very poor tradeoff.
2.6 Summary of Design Issues
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
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.
3. A Mail Transport-level Protocol
3.1 General Principles and Objectives
1. Whatever encoding 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, or bounce the message so
that the sender can make another plan.
3. If any charset other than UTF-8 or punycode is permitted and used
for the local part, its interpretation at the "what does this
mean" level is the responsibility of the receiving MTA.
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3.2 Framework for the Internationalization Extension
The following service extension is defined:
1. the name of the SMTP service extension is "Internationalized
Addresses";
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. 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. no parameters are added to any SMTP command.
[[Note in draft: A variation on this is probably excess
complexity, rather than a good tradeoff, but should be considered
in terms of whether it would be a good transitional aid. It would
be possible to permit an optional parameter on the MAIL and RCPT
commands that would specify an all-ASCII address to be used if an
MTA (SMTP Sender) encounters an SMTP Receiver that does not
support this extension. Such a parameter might be called
"AddressVariant" or even just "alias". It would be especially
useful in error handling if used on the MAIL command. ]]
5. no additional SMTP verbs are defined by this extension.
The remainder of this memo specifies how support for the extension
affects the behavior of an SMTP client and server.
3.3 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 over struck ASCII
characters (a character, a backspace, and another character) within a
quoted string to approximate non-ASCII characters. This form of
internationalization should probably 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
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accept a UTF-8 string [RFC2279] 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
MUST either return the message to the user as undeliverable or
replace it, using some process outside the scope of this
specification such as a directory lookup, with a local-part that
conforms to the syntax rules of RFC 2821.
3.4 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
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-- 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 3.6.1) string
representing a 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 3.6.3) 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.
3.5 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, UTF-8 is used for the entire
string; when it 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]]
3.5.1 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.
3.5.2 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.
3.6 Protocol Loose Ends
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These issues should be resolved, and this section eliminated, before
the document is considered complete.
3.6.1 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.
3.6.2 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. 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.
3.6.3 Restrictions on Characters in Local Part?
The 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. Those 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
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).
3.6.4 Requirement for 8BITMIME?
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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 without also implementing 8BITMIME, it
may be sensible to permit this extension only if 8BITMIME also
appears.
3.6.5 Message Header and Body Issues with MTA Approach?
By viewing i18n addresses as an MTA problem, this document does not
address a number of interesting 2822/MIME 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?
3.6.6 Variant Addresses (Aliases) in a Command Verb
A determination should be made as to whether a parameter to the MAIL
and RCPT commands that would specify an alternate, ASCII-only,
address is desirable and the text in Section 3.2, item 4, corrected
accordingly.
3.6.7 The Received field 'for' clause
Decide what to do about the value of the "for" clause in Received
fields. See Section 3.5.2.
4. 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
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
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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
an expanded stringprep protocol-- 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.
5. 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.
6. 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. While their ideas
have inspired several of the properties of this proposal they are, of
course, not responsible for the result and will probably disagree
with it.
Normative References
[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
821, August 1982.
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[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
[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.
Informative References
[I-D.hoffman-imaa]
Hoffman, P. and A. Costello, "Internationalizing Mail
Addresses in Applications (IMAA)", draft-hoffman-imaa-02
(work in progress), August 2003.
[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.
[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.
[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.
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[RFC2476] Gellens, R. and J. Klensin, "Message Submission", RFC
2476, December 1998.
[RFC2554] Myers, J., "SMTP Service Extension for Authentication",
RFC 2554, March 1999.
[RFC2556] Bradner, S., "OSI connectionless transport services on top
of UDP Applicability Statement for Historic Status", RFC
2556, 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.
[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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