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RFC 5504 - Downgrading Mechanism for Email Address International


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Network Working Group                                   K. Fujiwara, Ed.
Request for Comments: 5504                                Y. Yoneya, Ed.
Category: Experimental                                              JPRS
                                                              March 2009

      Downgrading Mechanism for Email Address Internationalization

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   Traditional mail systems handle only ASCII characters in SMTP
   envelope and mail header fields.  The Email Address
   Internationalization (UTF8SMTP) extension allows UTF-8 characters in
   SMTP envelope and mail header fields.  To avoid rejecting
   internationalized email messages when a server in the delivery path
   does not support the UTF8SMTP extension, some sort of converting
   mechanism is required.  This document describes a downgrading
   mechanism for Email Address Internationalization.  Note that this is
   a way to downgrade, not tunnel.  There is no associated up-conversion
   mechanism, although internationalized email clients might use
   original internationalized addresses or other data when displaying or
   replying to downgraded messages.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................4
   3. New Header Fields Definition ....................................5
      3.1. Envelope Information Preservation Header Fields ............5
      3.2. Address Header Fields' Preservation Header Fields ..........6
      3.3. Unknown Header Fields' Preservation Header Fields ..........6
   4. SMTP Downgrading ................................................7
      4.1. Path Element Downgrading ...................................7
      4.2. ORCPT downgrading ..........................................8
   5. Email Header Fields Downgrading .................................8
      5.1. Downgrading Method for Each ABNF Element ...................8
           5.1.1. RECEIVED Downgrading ................................9
           5.1.2. UNSTRUCTURED Downgrading ............................9
           5.1.3. WORD Downgrading ....................................9
           5.1.4. COMMENT Downgrading .................................9
           5.1.5. MIME-VALUE Downgrading ..............................9
           5.1.6. DISPLAY-NAME Downgrading ............................9
           5.1.7. MAILBOX Downgrading .................................9
           5.1.8. ENCAPSULATION Downgrading ..........................10
           5.1.9. TYPED-ADDRESS Downgrading ..........................10
      5.2. Downgrading Method for Each Header Field ..................10
           5.2.1. Address Header Fields That Contain <address>s ......10
           5.2.2. Address Header Fields with Typed Addresses .........11
           5.2.3. Downgrading Non-ASCII in Comments ..................11
           5.2.4. Received Header Field ..............................11
           5.2.5. MIME Content Header Fields .........................12
           5.2.6. Non-ASCII in <unstructured> ........................12
           5.2.7. Non-ASCII in <phrase> ..............................12
           5.2.8. Other Header Fields ................................12
   6. MIME Body-Part Header Field Downgrading ........................12
   7. Security Considerations ........................................13
   8. Implementation Notes ...........................................14
      8.1. RFC 2047 Encoding .........................................14
      8.2. Trivial Downgrading .......................................15
      8.3. 7bit Transport Consideration ..............................15
   9. IANA Considerations ............................................16
   10. Acknowledgements ..............................................18
   11. References ....................................................18
      11.1. Normative References .....................................18
      11.2. Informative References ...................................19
   Appendix A.  Examples .............................................20
     A.1.  Downgrading Example 1 .....................................20
     A.2.  Downgrading Example 2 .....................................22

1.  Introduction

   Traditional mail systems, which are defined by [RFC5321] and
   [RFC5322], allow ASCII characters in SMTP envelope and mail header
   field values.  The UTF8SMTP extension ([RFC4952], [RFC5335], and
   [RFC5336]) allows UTF-8 characters in SMTP envelope and mail header
   field values.

   If an envelope address or header field contains non-ASCII characters,
   the message cannot be delivered unless every system in the delivery
   path supports UTF8SMTP.  This document describes a downgrading
   mechanism to avoid rejection of such messages when a server that does
   not support the UTF8SMTP extension is encountered.  This downgrading
   mechanism converts envelope and mail header fields to an all-ASCII
   representation.

   [RFC5335] allows UTF-8 characters to be used in mail header fields
   and MIME header fields.  The downgrading mechanism specified here
   converts mail header fields and MIME header fields to ASCII.

   This document does not change any protocols except by defining new
   header fields.  It describes the conversion method from the
   internationalized email envelopes/messages that are defined in
   [RFC4952], [RFC5335], and [RFC5336] to the traditional email
   envelopes/messages defined in [RFC5321] and [RFC5322].

   Section 3.2 of [RFC5336] defines when downgrading occurs.  If the
   SMTP client has a UTF8SMTP envelope or an internationalized message
   and the SMTP server doesn't support the UTF8SMTP extension, then the
   SMTP client MUST NOT send a UTF8SMTP envelope or an internationalized
   message to the SMTP server.  The section lists 4 choices in this
   case.  The fourth choice is downgrading, as described here.

   Downgrading may be implemented in Mail User Agents (MUAs), Mail
   Submission Agents (MSAs), and Mail Transport Agents (MTAs) that act
   as SMTP clients.  It may also be implemented in Message Delivery
   Agents (MDAs), Post Office Protocol (POP) servers, and IMAP servers
   that store or offer UTF8SMTP envelopes or internationalized messages
   to non-UTF8SMTP-compliant systems, which include message stores.

   This document tries to define the downgrading process clearly and it
   preserves the original internationalized email information as much as
   possible.

   Downgrading in UTF8SMTP consists of the following four parts:

   o  New header field definitions
   o  SMTP downgrading
   o  Email header field downgrading
   o  MIME header field downgrading

   In Section 3 of this document, many header fields starting with
   "Downgraded-" are introduced.  They preserve the original envelope
   information and the original header fields.

   SMTP downgrading is described in Section 4.  It generates ASCII-only
   envelope information from a UTF8SMTP envelope.

   Email header field downgrading is described in Section 5.  It
   generates ASCII-only header fields.

   MIME header fields are expanded in [RFC5335].  MIME header field
   downgrading is described in Section 6.  It generates ASCII-only MIME
   header fields.

   Displaying downgraded messages that originally contained
   internationalized email addresses or internationalized header fields
   is described in an another document ([DISPLAY]).

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   All specialized terms used in this specification are defined in the
   Email Address Internationalization (EAI) overview [RFC4952], in the
   mail specifications [RFC5321] [RFC5322], or in the MIME documents
   [RFC2045] [RFC2047] [RFC2183] [RFC2231].  The terms "ASCII address",
   "internationalized email address", "non-ASCII address", "i18mail
   address", "UTF8SMTP", "message", and "mailing list" are used with the
   definitions from [RFC4952].

   This document depends on [RFC5335], [RFC5336], and [RFC5337].  Key
   words used in those documents are used in this document, too.

   The term "non-ASCII" refers to a UTF-8 string that contains at least
   one non-ASCII character.

   A "UTF8SMTP envelope" has email originator/recipient addresses
   expanded by [RFC5336] and [RFC5337].

   A "UTF8SMTP message" is an email message expanded by [RFC5335].

3.  New Header Fields Definition

   New header fields starting with "Downgraded-" are defined here to
   preserve those original envelope and mail header field values that
   contain UTF-8 characters.  During downgrading, one new "Downgraded-"
   header field is added for each original envelope or mail header field
   that cannot be passed as-is to a server that does not support
   UTF8SMTP.  The original envelope or mail header field is removed or
   rewritten.  Only those envelope and mail header fields that contain
   non-ASCII characters are affected.  The result of this process is a
   message that is compliant with existing email specifications
   [RFC5321] and [RFC5322].  The original internationalized information
   can be retrieved by examining the "Downgraded-" header fields that
   were added.

3.1.  Envelope Information Preservation Header Fields

   SMTP envelope downgraded information <downgraded-envelope-addr>
   consists of the original non-ASCII address and the downgraded all-
   ASCII address.  The ABNF [RFC5234] syntax is as follows:

   downgraded-envelope-addr = [FWS] "<" [ A-d-l ":" ] uMailbox
                              FWS "<" Mailbox ">" ">" [CFWS]

   <uMailbox> is defined in [RFC5336]; <Mailbox> and <A-d-l> are defined
   in Section 4.1.2 of [RFC5321].

   Two header fields, "Downgraded-Mail-From:" and "Downgraded-Rcpt-To:",
   are defined to preserve SMTP envelope downgraded information.  The
   header field syntax is specified as follows:

   fields             =/ downgradedmailfrom / downgradedrcptto

   downgradedmailfrom =  "Downgraded-Mail-From:" unstructured CRLF

   downgradedrcptto   =  "Downgraded-Rcpt-To:"   unstructured CRLF

   The unstructured content is downgraded-envelope-addr and treated as
   if it were unstructured, with [RFC2047] encoding (and charset UTF-8)
   as needed.

3.2.  Address Header Fields' Preservation Header Fields

   The address header fields' preservation header fields are defined to
   preserve the original header field.  Their value field holds the
   original header field value.  The header field syntax is specified as
   follows:

   fields                   =/ known-downgraded-headers ":"
                               unstructured CRLF

   known-downgraded-headers =  "Downgraded-" original-headers

   original-headers         =  "From" / "Sender" /
                               "To" / "Cc" / "Bcc" /
                               "Reply-To" /
                               "Resent-From" / "Resent-Sender" /
                               "Resent-To" / "Resent-Cc" /
                               "Resent-Bcc" / "Resent-Reply-To" /
                               "Return-Path" /
                               "Disposition-Notification-To"

   To preserve a header field in a "Downgraded-" header field:

   1.  Generate a new "Downgraded-" header field whose value is the
       original header field value.

   2.  Treat the generated header field content as if it were
       unstructured, and then apply [RFC2047] encoding with charset
       UTF-8 as necessary so that the result is ASCII.

3.3.  Unknown Header Fields' Preservation Header Fields

   The unknown header fields' preservation header fields are defined to
   encapsulate those original header fields that contain non-ASCII
   characters and are not otherwise provided for in this specification.
   The encapsulation header field name is the concatenation of
   "Downgraded-" and the original name.  The value field holds the
   original header field value.

   The header field syntax is specified as follows:

   fields     =/ unknown-downgraded-headers ":" unstructured CRLF

   unknown-downgraded-headers = "Downgraded-" original-header-field-name

   original-header-field-name = field-name

   field-name =  1*ftext

   ftext      =  %d33-57 /           ; Any character except
                 %d59-126            ;  controls, SP, and ":".

   To encapsulate a header field in a "Downgraded-" header field:

   1.  Generate a new "Downgraded-" header field whose value is the
       original header field value.

   2.  Treat the generated header field content as if it were
       unstructured, and then apply [RFC2047] encoding with charset
       UTF-8 as necessary so the result is ASCII.

   3.  Remove the original header field.

4.  SMTP Downgrading

   The targets of downgrading elements in an SMTP envelope are below:

   o  <reverse-path> of MAIL FROM command
   o  <forward-path> of RCPT TO command
   o  ORCPT parameter of RCPT TO command

   <reverse-path> and <forward-path> are described in [RFC5321] and
   [RFC5336].  The ORCPT parameter is described in [RFC3461] and
   [RFC5337].

4.1.  Path Element Downgrading

   Downgrading the <path> of MAIL FROM and RCPT TO commands uses the
   ALT-ADDRESS parameter defined in [RFC5336].  An SMTP command is
   downgradable if the <path> contains a non-ASCII address and the
   command has an ALT-ADDRESS parameter that specifies an ASCII address.
   Since only non-ASCII addresses are downgradable, specifying an ALT-
   ADDRESS value for an all-ASCII address is invalid for use with this
   specification, and no interpretation is assigned to it.  This
   restriction allows for future extension of the specification even
   though no such extensions are currently anticipated.

   Note that even if no downgrading is performed on the envelope,
   message header fields and message body MIME header fields that
   contain non-ASCII characters MUST be downgraded.  This is described
   in Sections 5 and 6.

   When downgrading, replace each <path> that contains a non-ASCII mail
   address with its specified alternative ASCII address, and preserve
   the original information using "Downgraded-Mail-From" and

   "Downgraded-Rcpt-To" header fields as defined in Section 3.  Before
   replacing, decode the ALT-ADDRESS parameter value because it is
   encoded as xtext [RFC3461].

   To avoid disclosing recipient addresses, the downgrading process MUST
   NOT add the "Downgraded-Rcpt-To:" header field if the SMTP
   downgrading targets multiple recipients.  See Section 7 for more
   details.

   As a result of the recipient address downgrading, the domain part of
   the recipient address prior to downgrading might be different from
   the domain part of the new recipient address.  If the result of
   address resolution for the domain part of the new recipient address
   contains the server at the connection destination of the SMTP session
   for the recipient address prior to downgrading, the SMTP connection
   is valid for the new recipient address.  Otherwise, the downgrading
   process MUST NOT send the downgraded message to the new recipient
   address via the connection and MUST try to send the downgraded
   message to the new recipient address.

4.2.  ORCPT downgrading

   The "RCPT TO" command can have an ORCPT parameter if the Delivery
   Status Notification (DSN) extension [RFC3461] is supported.  If the
   ORCPT parameter contains a "utf-8" type address and the address
   contains raw non-ASCII characters, the address MUST be converted to
   utf-8-addr-xtext form.  Those forms are described in [RFC5337] and
   clarified by successor documents such as [DSNBIS].

   Before converting to utf-8-addr-xtext form, remove xtext encoding.

5.  Email Header Fields Downgrading

   This section defines the conversion method to ASCII for each header
   field that may contain non-ASCII characters.

   [RFC5335] expands "Received:" header fields; [RFC5322] describes ABNF
   elements <mailbox>, <word>, <comment>, <unstructured>; [RFC2045]
   describes ABNF element <value>.

5.1.  Downgrading Method for Each ABNF Element

   Header field downgrading is defined below for each ABNF element.
   Downgrading an unknown header field is also defined as ENCAPSULATION
   downgrading.  Converting the header field terminates when no non-
   ASCII characters remain in the header field.

5.1.1.  RECEIVED Downgrading

   If the header field name is "Received:" and the FOR clause contains a
   non-ASCII address, remove the FOR clause from the header field.
   Other parts (not counting <comment>s) should not contain non-ASCII
   values.

5.1.2.  UNSTRUCTURED Downgrading

   If the header field has an <unstructured> field that contains non-
   ASCII characters, apply [RFC2047] encoding with charset UTF-8.

5.1.3.  WORD Downgrading

   If the header field has any <word> fields that contain non-ASCII
   characters, apply [RFC2047] encoding with charset UTF-8.

5.1.4.  COMMENT Downgrading

   If the header field has any <comment> fields that contain non-ASCII
   characters, apply [RFC2047] encoding with charset UTF-8.

5.1.5.  MIME-VALUE Downgrading

   If the header field has any <value> elements defined by [RFC2045] and
   the elements contain non-ASCII characters, encode the <value>
   elements according to [RFC2231] with charset UTF-8 and leave the
   language information empty.  If the <value> element is <quoted-
   string> and it contains <CFWS> outside the DQUOTE, remove the <CFWS>
   before this conversion.

5.1.6.  DISPLAY-NAME Downgrading

   If the header field has any <address> (<mailbox> or <group>) elements
   and they have <display-name> elements that contain non-ASCII
   characters, encode the <display-name> elements according to [RFC2047]
   with charset UTF-8.  DISPLAY-NAME downgrading is the same algorithm
   as WORD downgrading.

5.1.7.  MAILBOX Downgrading

   The <mailbox> elements have no equivalent format for non-ASCII
   addresses.  If the header field has any <mailbox> elements that
   contain non-ASCII characters, preserve the header field in the
   corresponding "Downgraded-" header field, which is defined in
   Section 3.2, and rewrite each <mailbox> element to ASCII-only format.
   The <mailbox> element that contains non-ASCII characters is one of
   three formats.

   o  [ Display-name ] "<" Utf8-addr-spec 1*FCS "<" Addr-spec ">>"

         Rewrite it as:
         [ Display-name ] "<" Addr-spec ">"

   o  [ Display-name ] "<" Utf8-addr-spec ">"
   o  Utf8-addr-spec

         Rewrite both as:
         [ Display-name ] "Internationalized Address " Encoded-word
         " Removed:;"
         where the <Encoded-word> is the original <Utf8-addr-spec>
         encoded according to [RFC2047].

5.1.8.  ENCAPSULATION Downgrading

   If the header field contains non-ASCII characters and is such that no
   rule is given above, encapsulate it in a "Downgraded-" header field
   as described in Section 3.3 as a last resort.

   Applying this procedure to "Received:" header field is prohibited.

5.1.9.  TYPED-ADDRESS Downgrading

   If the header field contains <utf-8-type-addr> and the <utf-8-type-
   addr> contains raw non-ASCII characters, it is in utf-8-address form.
   Convert it to utf-8-addr-xtext form as described in Section 4.2.
   COMMENT downgrading is also performed in this case.  If the address
   type is unrecognized and the header field contains non-ASCII
   characters, then fall back to using ENCAPSULATION downgrading on the
   entire header field.

5.2.  Downgrading Method for Each Header Field

   Header fields are listed in [RFC4021].  This section describes the
   downgrading method for each header field.

   If the whole mail header field does not contain non-ASCII characters,
   email header field downgrading is not required.  Each header field's
   downgrading method is described below.

5.2.1.  Address Header Fields That Contain <address>s

   From:
   Sender:
   To:
   Cc:
   Bcc:

   Reply-To:
   Resent-From:
   Resent-Sender:
   Resent-To:
   Resent-Cc:
   Resent-Bcc:
   Resent-Reply-To:
   Return-Path:
   Disposition-Notification-To:

   If the header field contains <mailbox> elements that contain non-
   ASCII addresses, preserve the header field in a "Downgraded-" header
   field before the conversion.  Then perform COMMENT downgrading,
   DISPLAY-NAME downgrading, and MAILBOX downgrading.

5.2.2.  Address Header Fields with Typed Addresses

   Original-Recipient:
   Final-Recipient:

   If the header field contains non-ASCII characters, perform TYPED-
   ADDRESS downgrading.

5.2.3.  Downgrading Non-ASCII in Comments

   Date:
   Message-ID:
   Resent-Message-ID:
   In-Reply-To:
   References:
   Resent-Date:
   Resent-Message-ID:
   MIME-Version:
   Content-ID:
   Content-Transfer-Encoding:
   Content-Language:
   Accept-Language:
   Auto-Submitted:

   These header fields do not contain non-ASCII characters except in
   comments.  If the header field contains UTF-8 characters in comments,
   perform COMMENT downgrading.

5.2.4.  Received Header Field

   Received:

   Perform COMMENT downgrading and RECEIVED downgrading.

5.2.5.  MIME Content Header Fields

   Content-Type:
   Content-Disposition:

   Perform MIME-VALUE downgrading and COMMENT downgrading.

5.2.6.  Non-ASCII in <unstructured>

   Subject:
   Comments:
   Content-Description:

   Perform UNSTRUCTURED downgrading.

5.2.7.  Non-ASCII in <phrase>

   Keywords:

   Perform WORD downgrading.

5.2.8.  Other Header Fields

   For all other header fields that contain non-ASCII characters, are
   user-defined, and are missing from this document or future defined
   header fields, perform ENCAPSULATION downgrading.

   If the software understands the header field's structure and a
   downgrading algorithm other than ENCAPSULATION is applicable, that
   software SHOULD use that algorithm; ENCAPSULATION downgrading is used
   as a last resort.

   Mailing list header fields (those that start in "List-") are part of
   this category.

6.  MIME Body-Part Header Field Downgrading

   MIME body-part header fields may contain non-ASCII characters
   [RFC5335].  This section defines the conversion method to ASCII-only
   header fields for each MIME header field that contains non-ASCII
   characters.  Parse the message body's MIME structure at all levels
   and check each MIME header field to see whether it contains non-ASCII
   characters.  If the header field contains non-ASCII characters in the
   header field value, the header field is a target of the MIME body-
   part header field's downgrading.  Each MIME header field's
   downgrading method is described below.  COMMENT downgrading, MIME-
   VALUE downgrading, and UNSTRUCTURED downgrading are described in
   Section 5.

   Content-ID:
      The "Content-ID:" header field does not contain non-ASCII
      characters except in comments.  If the header field contains UTF-8
      characters in comments, perform COMMENT downgrading.

   Content-Type:

      Content-Disposition:  Perform MIME-VALUE downgrading and COMMENT
                            downgrading.

      Content-Description:  Perform UNSTRUCTURED downgrading.

7.  Security Considerations

   A downgraded message's header fields contain ASCII characters only.
   But they still contain MIME-encapsulated header fields that contain
   non-ASCII UTF-8 characters.  Furthermore, the body part may contain
   UTF-8 characters.  Implementations parsing Internet messages need to
   accept UTF-8 body parts and UTF-8 header fields that are MIME-
   encoded.  Thus, this document inherits the security considerations of
   MIME-encoded header fields ([RFC2047] and [RFC3629]).

   Rewriting header fields increases the opportunities for undetected
   spoofing by malicious senders.  However, rewritten header fields are
   preserved into Downgraded-* header fields, and parsing Downgraded-*
   header fields enables the detection of spoofing caused by
   downgrading.

   Addresses that do not appear in the message header fields may appear
   in the RCPT commands to an SMTP server for a number of reasons.
   Copying information from the envelope into the header fields risks
   inadvertent information disclosure (see [RFC5321] and Section 4 of
   this document).  Mitigating inadvertent information disclosure is
   also discussed in these locations.

   The techniques described here invalidate methods that depend on
   digital signatures over the envelope or any part of the message,
   which includes the top-level header fields and body-part header
   fields.  Depending on the specific message being downgraded, the
   following techniques are likely to break: DomainKeys Identified Mail
   (DKIM), and possibly S/MIME and Pretty Good Privacy (PGP).  The two
   obvious mitigations are to stick to 7-bit transport when using these
   techniques (as most/all of them presently require) or to make sure to
   have UTF8SMTP end-to-end when needed.

   Many gateways and servers on the Internet will discard header fields
   with which they are not familiar.  To the extent to which the
   downgrade procedures depend on new header fields (e.g.,

   "Downgraded-") to avoid information loss, the risk of having those
   header fields dropped and subsequent implications must be identified.
   In particular, if the "Downgraded-" header fields are dropped, there
   is no possibility of reconstructing the original information at any
   point (before, during, or after delivery).  Such gateways violate
   [RFC2979] and can be upgraded to correct the problem.

   Even though the information is not lost, the original message cannot
   be perfectly reconstructed because some downgrading methods remove
   information (see Sections 5.1.1 and 5.1.5).  Hence, downgrading is a
   one-way process.

   While information in any email header field should usually be treated
   with some suspicion, current email systems commonly employ various
   mechanisms and protocols to make the information more trustworthy.
   Currently, information in the new Downgraded-* header fields is
   usually not inspected by these mechanisms, and may be even less
   trustworthy than the traditional header fields.  Note that the
   Downgraded-* header fields could have been inserted with malicious
   intent (and with content unrelated to the traditional header fields).

   If an internationalized MUA would simply try to "upgrade" the message
   for display purposes (that is, display the information in the
   Downgraded-* header fields instead of the traditional header fields),
   the effectiveness of the deployed mechanisms and protocols is likely
   to be reduced, and the user may be exposed to additional risks.  More
   guidance on how to display downgraded messages is given in [DISPLAY].

   Concerns about the trustworthiness of the Downgraded-* header fields
   are not limited to displaying and replying in MUAs, and should be
   carefully considered before using such header fields for other
   purposes as well.

   See the "Security Considerations" section in [RFC4952] for more
   discussion.

8.  Implementation Notes

8.1.  RFC 2047 Encoding

   While [RFC2047] has a specific algorithm to deal with whitespace in
   adjacent encoded words, there are a number of deployed
   implementations that fail to implement the algorithm correctly.  As a
   result, whitespace behavior is somewhat unpredictable in practice
   when multiple encoded words are used.  While RFC 5322 states that
   implementations SHOULD limit lines to not more than 78 characters,
   implementations MAY choose to allow overly long encoded words in

   order to work around faulty [RFC2047] implementations.
   Implementations that choose to do so SHOULD have an optional
   mechanism to limit line length to 78 characters.

8.2.  Trivial Downgrading

   Downgrading is an alternative to avoid the rejection of messages that
   require UTF8SMTP support by a server that does not provide such
   support.  Implementing the full specification of this document is
   desirable, but a partial implementation is also possible.

   If a partial downgrading implementation confronts an unsupported
   downgrading target, the implementation MUST NOT send the message to a
   server that does not support UTF8SMTP.  Instead, it MUST either
   reject the message or generate a notification of non-deliverability.

   A partial downgrading, trivial downgrading, is discussed.  It does
   not support non-ASCII addresses in SMTP envelope and address header
   fields, unknown header field downgrading, or the MIME body-part
   header field downgrading.  It supports:

   o  some simple header field downgrading: Subject
   o  comments and display name downgrading: From, To, Cc
   o  trace header field downgrading: Received

   Otherwise, the downgrading fails.

   Trivial downgrading targets mail messages that are generated by
   UTF8SMTP-aware MUAs and contain non-ASCII characters in comments,
   display names, and unstructured parts without using non-ASCII email
   addresses.  These mail messages usually do not contain non-ASCII
   email addresses in the SMTP envelope and its header fields.  But it
   is not deliverable via a UTF8SMTP-unaware SMTP server.  Implementing
   full specification downgrading may be hard, but trivial downgrading
   saves mail messages without using non-ASCII addresses.

8.3.  7bit Transport Consideration

   The SMTP client may encounter a SMTP server that does not support the
   8BITMIME SMTP extension [RFC1652].  The server does not support
   "8bit" or "binary" data.  Implementers need to consider converting
   "8bit" data to "base64" or "quoted-printable" encoded form and adjust
   the "Content-Transfer-Encoding" header field accordingly.  If the
   body contains multiple MIME parts, this conversion MUST be performed
   for each MIME part.

9.  IANA Considerations

   IANA has registered the following header fields in the Permanent
   Message Header Field registry, in accordance with the procedures set
   out in [RFC3864].

   Header field name:  Downgraded-Mail-From
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Rcpt-To
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-From
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Sender
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-To
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Cc
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Bcc
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Reply-To
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Resent-From
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Resent-Sender
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Resent-To
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Resent-Cc
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Resent-Bcc
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Resent-Reply-To
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Return-Path
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Disposition-Notification-To
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Furthermore, IANA is requested to refuse registration of all field
   names that start with "Downgraded-".  For unknown header fields, use
   the downgrading method described in Section 3.3 to avoid conflicts
   with existing IETF activity (Email Address Internationalization).

10.  Acknowledgements

   Significant comments and suggestions were received from John Klensin,
   Harald Alvestrand, Chris Newman, Randall Gellens, Charles Lindsey,
   Marcos Sanz, Alexey Melnikov, Frank Ellermann, Edward Lewis, S.
   Moonesamy, and JET members.

11.  References

11.1.  Normative References

   [RFC1652]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
              Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
              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.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2183]  Troost, R., Dorner, S., and K. Moore, "Communicating
              Presentation Information in Internet Messages: The
              Content-Disposition Header Field", RFC 2183, August 1997.

   [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
              Word Extensions:
              Character Sets, Languages, and Continuations", RFC 2231,
              November 1997.

   [RFC2979]  Freed, N., "Behavior of and Requirements for Internet
              Firewalls", RFC 2979, October 2000.

   [RFC3461]  Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
              Extension for Delivery Status Notifications (DSNs)",
              RFC 3461, January 2003.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [RFC4021]  Klyne, G. and J. Palme, "Registration of Mail and MIME
              Header Fields", RFC 4021, March 2005.

   [RFC4952]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 4952, July 2007.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [RFC5335]  Abel, Y., "Internationalized Email Headers", RFC 5335,
              September 2008.

   [RFC5336]  Yao, J. and W. Mao, "SMTP Extension for Internationalized
              Email Addresses", RFC 5336, September 2008.

   [RFC5337]  Newman, C. and A. Melnikov, "Internationalized Delivery
              Status and Disposition Notifications", RFC 5337,
              September 2008.

11.2.  Informative References

   [DISPLAY]  Fujiwara, K., "Displaying Downgraded Messages for Email
              Address Internationalization", Work in Progress,
              March 2009.

   [DSNBIS]   Newman, C. and A. Melnikov, "Internationalized Delivery
              Status and Disposition Notifications", Work in Progress,
              December 2008.

Appendix A.  Examples

A.1.  Downgrading Example 1

   This appendix shows an SMTP downgrading example.  Consider a mail
   message where:

   o  The sender address is "NON-ASCII-local@example.com", which is a
      non-ASCII address.  Its ASCII alternative is
      "ASCII-local@example.com" and its display-name is "DISPLAY-local".

   o  The "To:" address is "NON-ASCII-remote1@example.net", which is a
      non-ASCII address.  Its ASCII alternative is
      "ASCII-remote1@example.net" and its display-name is "DISPLAY-
      remote1".

   o  The "Cc:" address is a non-ASCII address,
      "NON-ASCII-remote2@example.org", without an alternative ASCII
      address.  Its display-name is "DISPLAY-remote2".

   o  Three display names contain non-ASCII characters.

   o  The Subject header field is "NON-ASCII-SUBJECT", which contains
      non-ASCII characters.

   o  Assume the "To:" recipient's MTA (example.net) does not support
      UTF8SMTP.

   o  Assume the "Cc:" recipient's MTA (example.org) supports UTF8SMTP.

   The first example SMTP envelope/message is shown in Figure 1.  In
   this example, the "To:" recipient's session is the focus.

   MAIL FROM: <NON-ASCII-local@example.com>
               ALT-ADDRESS=ASCII-local@example.com
   RCPT TO: <NON-ASCII-remote1@example.net>
             ALT-ADDRESS=ASCII-remote1@example.net
   RCPT TO: <NON-ASCII-remote2@example.org>
   -------------------------------------------------------------
   Message-Id: MESSAGE_ID
   Mime-Version: 1.0
   Content-Type: text/plain; charset="UTF-8"
   Content-Transfer-Encoding: 8bit
   Subject: NON-ASCII-SUBJECT
   From: DISPLAY-local <NON-ASCII-local@example.com
    <ASCII-local@example.com>>
   To: DISPLAY-remote1 <NON-ASCII-remote1@example.net
    <ASCII-remote1@example.net>>
   Cc: DISPLAY-remote2 <NON-ASCII-remote2@example.org>
   Date: DATE

   MAIL_BODY

              Figure 1: Original envelope/message (example 1)

   In this example, there are two SMTP recipients; one is "To:", the
   other is "Cc:".  The SMTP downgrading uses To: session downgrading.
   Figure 2 shows an SMTP downgraded example.

   MAIL FROM: <ASCII-local@example.com>
   RCPT TO: <ASCII-remote1@example.net>
   -------------------------------------------------------------
   Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com_?=
    =?UTF-8?Q?<ASCII-local@example.com>>?=
   Downgraded-Rcpt-To: =?UTF-8?Q?<NON-ASCII-remote1@example.net_?=
    =?UTF-8?Q?<ASCII-remote1@example.net>>?=
   Message-Id: MESSAGE_ID
   Mime-Version: 1.0
   Content-Type: text/plain; charset="UTF-8"
   Content-Transfer-Encoding: 8bit
   Subject: NON-ASCII-SUBJECT
   From: DISPLAY-local <NON-ASCII-local@example.com
    <ASCII-local@example.com>>
   To: DISPLAY-remote1 <NON-ASCII-remote1@example.net
    <ASCII-remote1@example.net>>
   Cc: DISPLAY-remote2 <NON-ASCII-remote2@example.org>
   Date: DATE

   MAIL_BODY

          Figure 2: SMTP downgraded envelope/message (example 1)

   After SMTP downgrading, header field downgrading is performed.  The
   final downgraded message is shown in Figure 3.  A Return-Path header
   field will be added by the final destination MTA.

Return-Path: <ASCII-local@example.com>
Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com_?=
 =?UTF-8?Q?<ASCII-local@example.com>>?=
Downgraded-Rcpt-To: =?UTF-8?Q?<NON-ASCII-remote1@example.net_?=
 =?UTF-8?Q?<ASCII-remote1@example.net>>?=
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?=
From: =?UTF-8?Q?DISPLAY-local?= <ASCII-local@example.com>
Downgraded-From: =?UTF-8?Q?DISPLAY-local_<NON-ASCII-local@example.com_?=
 =?UTF-8?Q?<ASCII-local@example.com>>?=
To: =?UTF-8?Q?DISPLAY-remote1?= <ASCII-remote1@example.net>
Downgraded-To: =?UTF-8?Q?DISPLAY-remote1_?=
 =?UTF-8?Q?<NON-ASCII-remote1@example.net_<ASCII-remote1@example.net>>?=
Cc: =?UTF-8?Q?DISPLAY-remote2?= Internationalized address
 =?UTF-8?Q?NON-ASCII-remote2@example.org?= removed:;
Downgraded-Cc: =?UTF-8?Q?DISPLAY-remote2_?=
 =?UTF-8?Q?<NON-ASCII-remote2@example.org>?=
Date: DATE

MAIL_BODY

                 Figure 3: Downgraded message (example 1)

A.2.  Downgrading Example 2

   In many cases, the sender wants to use a non-ASCII address and the
   recipient is a traditional mail user.  The SMTP server handing mail
   for the recipient and/or the recipient's MUA does not support
   UTF8SMTP extension.  Consider a mail message where:

   o  The sender address is "NON-ASCII-local@example.com", which is a
      non-ASCII address.  Its ASCII alternative is
      "ASCII-local@example.com".  It has a display-name "DISPLAY-local",
      which contains non-ASCII characters.

   o  The "To:" address is "ASCII-remote1@example.net", which is ASCII-
      only.  It has a display-name, "DISPLAY-remote1", which contains
      non-ASCII characters.

   o  The "Subject:" header field is "NON-ASCII-SUBJECT", which contains
      non-ASCII characters.

   The second example envelope/message is shown in Figure 4.

   MAIL From: <NON-ASCII-local@example.com>
               ALT-ADDRESS=ASCII-local@example.com
   RCPT TO: <ASCII-remote1@example.net>
   -------------------------------------------------------------
   Message-Id: MESSAGE_ID
   Mime-Version: 1.0
   Content-Type: text/plain; charset="UTF-8"
   Content-Transfer-Encoding: 8bit
   Subject: NON-ASCII-SUBJECT
   From: DISPLAY-local <NON-ASCII-local@example.com
    <ASCII-local@example.com>>
   To: DISPLAY-remote1 <ASCII-remote1@example.net>
   Date: DATE

   MAIL_BODY

                  Figure 4: Original message (example 2)

   In this example, SMTP session is downgradable.  Figure 5 shows an
   SMTP downgraded envelope/message.

   MAIL From: <ASCII-local@example.com>
   RCPT TO: <ASCII-remote1@example.net>
   -------------------------------------------------------------
   Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com_?=
    ?=UTF8?Q?<ASCII-local@example.com>>?=
   Message-Id: MESSAGE_ID
   Mime-Version: 1.0
   Content-Type: text/plain; charset="UTF-8"
   Content-Transfer-Encoding: 8bit
   Subject: NON-ASCII-SUBJECT
   From: DISPLAY-local <NON-ASCII-local@example.com
    <ASCII-local@example.com>>
   To: DISPLAY-remote1 <ASCII-remote1@example.net>
   Date: DATE

   MAIL_BODY

          Figure 5: SMTP downgraded envelope/message (example 2)

   After SMTP downgrading, header field downgrading is performed.  The
   downgraded example is shown in Figure 6.

Return-Path: <ASCII-local@example.com>
Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com_?=
 =?UTF8?Q?<ASCII-local@example.com>>?=
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?=
Downgraded-From: =?UTF-8?Q?DISPLAY-local_<NON-ASCII-local@example.com_?=
 =?UTF-8?Q?<ASCII-local@example.com>>?=
From: =?UTF-8?Q?DISPLAY-local?= <ASCII-local@example.com>
To: =?UTF-8?Q?DISPLAY-remote1?= <ASCII-remote1@example.net>
Date: DATE

MAIL_BODY

                 Figure 6: Downgraded message (example 2)

Authors' Addresses

   Kazunori Fujiwara (editor)
   Japan Registry Services Co., Ltd.
   Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
   Chiyoda-ku, Tokyo  101-0065
   Japan

   Phone: +81 3 5215 8451
   EMail: fujiwara@jprs.co.jp

   Yoshiro Yoneya (editor)
   Japan Registry Services Co., Ltd.
   Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
   Chiyoda-ku, Tokyo  101-0065
   Japan

   Phone: +81 3 5215 8451
   EMail: yone@jprs.co.jp

 

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