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rfc:rfc5504

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.

Fujiwara & Yoneya Experimental [Page 1] RFC 5504 UTF8SMTP Downgrade March 2009

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

Fujiwara & Yoneya Experimental [Page 2] RFC 5504 UTF8SMTP Downgrade March 2009

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.

Fujiwara & Yoneya Experimental [Page 3] RFC 5504 UTF8SMTP Downgrade March 2009

 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.

Fujiwara & Yoneya Experimental [Page 4] RFC 5504 UTF8SMTP Downgrade March 2009

 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.

Fujiwara & Yoneya Experimental [Page 5] RFC 5504 UTF8SMTP Downgrade March 2009

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

Fujiwara & Yoneya Experimental [Page 6] RFC 5504 UTF8SMTP Downgrade March 2009

 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

Fujiwara & Yoneya Experimental [Page 7] RFC 5504 UTF8SMTP Downgrade March 2009

 "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.

Fujiwara & Yoneya Experimental [Page 8] RFC 5504 UTF8SMTP Downgrade March 2009

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.

Fujiwara & Yoneya Experimental [Page 9] RFC 5504 UTF8SMTP Downgrade March 2009

 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:

Fujiwara & Yoneya Experimental [Page 10] RFC 5504 UTF8SMTP Downgrade March 2009

 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.

Fujiwara & Yoneya Experimental [Page 11] RFC 5504 UTF8SMTP Downgrade March 2009

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.

Fujiwara & Yoneya Experimental [Page 12] RFC 5504 UTF8SMTP Downgrade March 2009

 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.,

Fujiwara & Yoneya Experimental [Page 13] RFC 5504 UTF8SMTP Downgrade March 2009

 "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

Fujiwara & Yoneya Experimental [Page 14] RFC 5504 UTF8SMTP Downgrade March 2009

 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.

Fujiwara & Yoneya Experimental [Page 15] RFC 5504 UTF8SMTP Downgrade March 2009

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)

Fujiwara & Yoneya Experimental [Page 16] RFC 5504 UTF8SMTP Downgrade March 2009

 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)

Fujiwara & Yoneya Experimental [Page 17] RFC 5504 UTF8SMTP Downgrade March 2009

 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.

Fujiwara & Yoneya Experimental [Page 18] RFC 5504 UTF8SMTP Downgrade March 2009

 [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.

Fujiwara & Yoneya Experimental [Page 19] RFC 5504 UTF8SMTP Downgrade March 2009

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.

Fujiwara & Yoneya Experimental [Page 20] RFC 5504 UTF8SMTP Downgrade March 2009

 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)

Fujiwara & Yoneya Experimental [Page 21] RFC 5504 UTF8SMTP Downgrade March 2009

 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.

Fujiwara & Yoneya Experimental [Page 22] RFC 5504 UTF8SMTP Downgrade March 2009

 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)

Fujiwara & Yoneya Experimental [Page 23] RFC 5504 UTF8SMTP Downgrade March 2009

 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

Fujiwara & Yoneya Experimental [Page 24]

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