Downgrading mechanism for Email Address Internationalization
draft-ietf-eai-downgrade-07.txt

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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 bouncing 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
2.  Terminology
3.  New header fields definition
    3.1.  Envelope information preservation headers
    3.2.  Address header field preservation headers
    3.3.  Unknown header fields preservation headers
4.  SMTP Downgrading
5.  Email header fields downgrading
    5.1.  Downgrading method for each header field
6.  MIME body part headers downgrading
7.  Security considerations
8.  Implementation notes
    8.1.  Trivial downgrading
    8.2.  7bit transport consideration
9.  IANA Considerations
10.  Acknowledgements
11.  Change History
    11.1.  draft-yoneya-ima-downgrade: Version 00
    11.2.  draft-yoneya-ima-downgrade: Version 01
    11.3.  draft-ietf-eai-downgrade: Version 00
    11.4.  draft-ietf-eai-downgrade: Version 01
    11.5.  draft-ietf-eai-downgrade: Version 02
    11.6.  draft-ietf-eai-downgrade: Version 03
    11.7.  draft-ietf-eai-downgrade: Version 04
    11.8.  draft-ietf-eai-downgrade: Version 05
    11.9.  draft-ietf-eai-downgrade: Version 06
    11.10.  draft-ietf-eai-downgrade: Version 07
12.  Normative References
Appendix A.  Examples
    A.1.  Downgrading example 1
    A.2.  Downgrading example 2
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

Traditional mail systems which are defined by [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.) and [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.) allow ASCII characters in SMTP envelope and mail header field values. The UTF8SMTP extension [RFC4952] (Klensin, J. and Y. Ko, “Overview and Framework for Internationalized Email,” July 2007.), [I‑D.ietf‑eai‑utf8headers] (Yeh, J., “Internationalized Email Headers,” February 2008.) and [I‑D.ietf‑eai‑smtpext] (Yao, J. and W. MAO, “SMTP extension for internationalized email address,” January 2008.) 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. To avoid bouncing such messages when a server is encountered which does not support the UTF8SMTP extension, this document describes a downgrading mechanism. Downgrading a message converts envelope and header fields to an all-ASCII representation.

[I‑D.ietf‑eai‑utf8headers] (Yeh, J., “Internationalized Email Headers,” February 2008.) 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 which are defined in [RFC4952] (Klensin, J. and Y. Ko, “Overview and Framework for Internationalized Email,” July 2007.) [I‑D.ietf‑eai‑utf8headers] (Yeh, J., “Internationalized Email Headers,” February 2008.) [I‑D.ietf‑eai‑smtpext] (Yao, J. and W. MAO, “SMTP extension for internationalized email address,” January 2008.) to the traditional email envelopes/messages which are defined in [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.) [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.).

[I‑D.ietf‑eai‑smtpext] (Yao, J. and W. MAO, “SMTP extension for internationalized email address,” January 2008.) section 2.2 defines when downgrading occurs. If the SMTP client has an UTF8SMTP envelope or an internationalized message and the SMTP server doesn't support the UTF8SMTP SMTP extension, then the SMTP client MUST NOT send a UTF8SMTP envelope or an internationalized message to the SMTP server. The section shows 4 choices. The fourth choice is downgrading, as described here.

Downgrading may be implemented in MUAs, MSAs, MTAs which act as the SMTP client, or in MDAs, POP servers, IMAP servers which 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 information as much as possible.

Downgrading in UTF8SMTP consists of the following four parts:

• New header fields definition
• SMTP downgrading
• Email header fields downgrading
• MIME header fields downgrading

In Section 3 (New header fields definition), many header fields starting with "downgraded" are introduced. They preserve the original envelope information and the original header fields.

The SMTP downgrading is described in Section 4 (SMTP Downgrading). It generates ASCII only envelope information from an UTF8SMTP envelope.

The Email header fields downgrading is described in Section 5 (Email header fields downgrading). It generates ASCII only header fields.

The MIME header fields are expanded in [I‑D.ietf‑eai‑utf8headers] (Yeh, J., “Internationalized Email Headers,” February 2008.). The MIME header fields downgrading is described in Section 6 (MIME body part headers downgrading). It generates ASCII only MIME header fields.

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

All specialized terms used in this specification are defined in the EAI overview [RFC4952] (Klensin, J. and Y. Ko, “Overview and Framework for Internationalized Email,” July 2007.) or in [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.)[RFC2822] (Resnick, P., “Internet Message Format,” April 2001.), MIME documents [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.) [RFC2047] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.) [RFC2183] (Troost, R., Dorner, S., and K. Moore, “Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field,” August 1997.) [RFC2231] (Freed, N. and K. Moore, “MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations,” November 1997.). The terms "ASCII address", "internationalized email address", "non-ASCII address", "i18mail address", "UTF8SMTP", "message" and "mailing list" are used with the definitions from [RFC4952] (Klensin, J. and Y. Ko, “Overview and Framework for Internationalized Email,” July 2007.) document.

This document depends on [I‑D.ietf‑eai‑smtpext] (Yao, J. and W. MAO, “SMTP extension for internationalized email address,” January 2008.), [I‑D.ietf‑eai‑utf8headers] (Yeh, J., “Internationalized Email Headers,” February 2008.), and [I‑D.ietf‑eai‑dsn] (Newman, C. and A. Melnikov, “Internationalized Delivery Status and Disposition Notifications,” January 2008.). Key words used in these document are used in this document, too.

The term "non-ASCII" is an UTF-8 string which contains at least one non-ASCII character.

An "UTF8SMTP envelope" has Email originator/recipient addresses expanded by [I‑D.ietf‑eai‑smtpext] (Yao, J. and W. MAO, “SMTP extension for internationalized email address,” January 2008.) and [I‑D.ietf‑eai‑dsn] (Newman, C. and A. Melnikov, “Internationalized Delivery Status and Disposition Notifications,” January 2008.).

An "UTF8SMTP message" is Email messages expanded by [I‑D.ietf‑eai‑utf8headers] (Yeh, J., “Internationalized Email Headers,” February 2008.).

3.  New header fields definition

New header fields starting with "Downgraded-" are defined here to preserve those original envelope and header values which contain UTF-8 characters. During downgrading, one new "Downgraded-" header field is added for each original envelope or header field which cannot be passed as-is to a server which does not support UTF8SMTP. The original envelope or header field is removed or rewritten. Only those envelope and header fields which contain non-ASCII characters are affected. The result of this process is a message which is compliant with existing email specifications [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.) and [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.). The original internationalized information can be retrieved by examining the "Downgraded-" header fields which were added. Even though the information is not lost, the original message cannot be perfectly reconstructed. Hence, downgrading is a one-way process. However, an internationalized client might use the information in the "Downgraded-" header fields when processing a downgraded message, for example, such as displaying or composing a reply.

3.1.  Envelope information preservation headers

Two headers "Downgraded-Mail-From:" and "Downgraded-Rcpt-To:" are defined to preserve SMTP envelope downgraded information. SMTP envelope downgraded information consists of the original non-ASCII address and the downgraded all-ASCII address. The header field syntax is specified as follows:

fields      =/ downgradedmailfrom / downgradedrcptto
downgradedmailfrom = "Downgraded-Mail-From:" [FWS] "<" uPath ">" 1*[FWS]
                                     "<" Mailbox ">" [FWS] CRLF
downgradedrcptto = "Downgraded-Rcpt-To:" [FWS] "<" uPath ">" 1*[FWS]
                                     "<" Mailbox ">" [FWS] CRLF

Original non-ASCII address <uPath> is defined in [I‑D.ietf‑eai‑smtpext] (Yao, J. and W. MAO, “SMTP extension for internationalized email address,” January 2008.); it is treated as unstructured in this header and it is encoded according to [RFC2047] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.). <Mailbox> is defined in [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.), section 4.1.2.

3.2.  Address header field preservation headers

The address header fields preservation headers are defined to preserve the original header field. Their value field holds the original header field value. Any original header field value is treated as an <unstructured> and it MUST be encoded according to [RFC2047] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.) with charset='UTF-8'. The header field syntax is specified as follows:

fields     =/ known-downgraded-headers ":" unstructued CRLF
known-downgraded-headers =  "Downgraded-" original-headers
original-headers = "From" / "To" / "Cc" / "Bcc" /
                   "Sender" / "Reply-To" /
                   "Resent-From" / "Resent-Sender" /
                   "Resent-To" / "Resent-Cc" / "Return-Path"

Preserving a header field in a downgraded header field is defined as:

1. Generate new downgraded header field whose value is the original header field value.
2. Encode the generated header according to [RFC2047] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.) with charset='UTF-8'.

3.3.  Unknown header fields preservation headers

The unknown header fields preservation headers are defined to encapsulate those original header field which contains non-ASCII characters and are not otherwise provided for in the 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.

Any original header field value is treated as an unstructured value and it MUST be encoded according to [RFC2047] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.) with charset='UTF-8'. The header field syntax is specified as follows:

fields     =/ unknown-downgraded-headers ":" unstructued 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
                                                ;  ":".

Encapsulating a header field in a "Downgraded-" header field is defined as:

1. Generate new "Downgraded-" header whose value is the original header field value.
2. Encode the generated header field value according to [RFC2047] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.) with charset='UTF-8'. To preserve space characters, the whole header field value which include space characters SHOULD be encoded.
3. Remove the original header field.

4.  SMTP Downgrading

Target of downgrading elements in SMTP envelope are below:

• MAIL FROM:
• RCPT TO:
• ORCPT parameter

Downgrading the SMTP envelope uses ALT-ADDRESS parameter defined in [I‑D.ietf‑eai‑smtpext] (Yao, J. and W. MAO, “SMTP extension for internationalized email address,” January 2008.). An address is downgradable if the address is non-ASCII address and has ASCII address specified by ALT-ADDRESS parameter. 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 Section 5 (Email header fields downgrading) and Section 6 (MIME body part headers downgrading).

When downgrading, replace each non-ASCII mail address in the envelope 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 (New header fields definition). Before replacing, decode the ALT-ADDRESS parameter value because it is encoded as xtext [RFC3461] (Moore, K., “Simple Mail Transfer Protocol (SMTP) Service Extension for Delivery Status Notifications (DSNs),” January 2003.).

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

The "RCPT TO" command may have an ORCPT parameter when the recipient address is downgraded. The ORCPT parameter is used for DSN [RFC3461] (Moore, K., “Simple Mail Transfer Protocol (SMTP) Service Extension for Delivery Status Notifications (DSNs),” January 2003.). If the ORCPT parameter contains an "utf-8" address and the address contains non-ASCII characters, the ORCPT parameter MUST be converted to utf-8-addr-xtext form or utf-8-addr-unitext form which are described in [I‑D.ietf‑eai‑dsn] (Newman, C. and A. Melnikov, “Internationalized Delivery Status and Disposition Notifications,” January 2008.).

As a result of the recipient address replacement, the domain part of the original recipient address may not equal to 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 original recipient address, 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.

5.  Email header fields downgrading

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

[I‑D.ietf‑eai‑utf8headers] (Yeh, J., “Internationalized Email Headers,” February 2008.) expands Received: header fields, [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.) ABNF elements <mailbox>, <word>, <comment>, <unstructured>, [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.) ABNF element <value>.

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.

RECEIVED downgrading:

If the header field name is "Received:" and the FOR clause contains a non-ASCII addresses, remove the FOR clause from the header field. The other part does not contain non-ASCII values.

UNSTRUCTURED downgrading:

If the header field has an <unstructured> field which contains non-ASCII characters, encode the field according to [RFC2047] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.) with charset='UTF-8'. To preserve space characters, the whole header field value which include space characters SHOULD be encoded.

WORD downgrading:

If the header field has any <word> fields which contains non-ASCII characters, encode the fields according to [RFC2047] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.) with charset='UTF-8'.

COMMENT downgrading:

If the header field has any <comment> fields which contains non-ASCII characters, encode the fields according to [RFC2047] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.) with charset='UTF-8'.

MIME-VALUE downgrading:

If the header field has any <value> elements defined by [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.) and the elements contain non-ASCII characters, encode the <value> elements by [RFC2231] (Freed, N. and K. Moore, “MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations,” November 1997.) with charset='UTF-8' and the Language information empty. If the <value> element is <quoted-string> and it contains <CFWS> outside the DQUOTE, remove the <CFWS> before this conversion.

DISPLAY-NAME downgrading:

If the header field has any <mailbox> elements and they have <display-name> elements which contain non-ASCII characters, encode the <display-name> elements according to [RFC2047] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.) with charset='UTF-8'.

MAILBOX downgrading:

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

• [ Display-name ] "<" Utf8-addr-spec 1*FCS "<" Addr-spec ">>"
  
  Rewrite it as
  
  [ Display-name ] "<" Addr-spec ">"
  
  
• [ Display-name ] "<" Utf8-addr-spec ">"
• 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] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.).

ENCAPSULATION downgrading:

if the header field contains non-ASCII characters and for which no rule is given above, encapsulate it in a Downgraded header field described in Section 3.3 (Unknown header fields preservation headers) as a last resort.

5.1.  Downgrading method for each header field

Header fields are listed in [RFC4021] (Klyne, G. and J. Palme, “Registration of Mail and MIME Header Fields,” March 2005.). 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.

• Address header fields which contain <mailbox>s
  
  

  From:

  Sender:

  Reply-To:

  To:

  Cc:

  Bcc:

  Resent-From:

  Resent-Sender:

  Resent-To:

  Resent-Cc:

  Return-Path:

  
  
  If the header field contains <mailbox> elements which contains non-ASCII addresses, preserve the header field in a downgraded header before the conversion. Then perform COMMENT downgrading, DISPLAY-NAME downgrading and MAILBOX downgrading.
  
  
• Downgrading Non-ASCII in comments
  
  

  Date:

  Message-ID:

  In-Reply-To:

  References:

  Resent-Date:

  Resent-Message-ID:

  MIME-Version:

  Content-ID:

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

  Received:

  
  
  perform COMMENT downgrading and perform RECEIVED downgrading.
  
  
• MIME Content header fields
  
  

  Content-Type:

  Content-Disposition:

  
  
  Perform MIME-VALUE downgrading.
  
  
• Non-ASCII in <unstructured>
  
  

  Subject:

  Comments:

  Content-Description:

  
  
  Perform UNSTRUCTURED downgrading.
  
  
• Non-ASCII in <unstructured> or <phrase>
  
  

  Keywords:

  
  
  Perform WORD downgrading.
  
  
• Other header fields
  
  All other header fields which contains non-ASCII characters are user-defined, missing from this draft or future defined header fields. Perform ENCAPSULATION downgrading as a last resort.
  
  

6.  MIME body part headers downgrading

MIME body part header fields may contain non-ASCII characters [I‑D.ietf‑eai‑utf8headers] (Yeh, J., “Internationalized Email Headers,” February 2008.). This section defines the conversion method to ASCII only header fields for each MIME header field which contains non-ASCII characters. Parse the message body's MIME structure for all levels and check each MIME header field whether it contains non-ASCII characters. If the header field contains non-ASCII characters in the header value, the header is a target of the MIME body part headers downgrading. Each MIME header field's downgrading method is described below. COMMENT downgrading, MIME-VALUE downgrading, UNSTRUCTURED downgrading are described in Section 5 (Email header fields downgrading).

Content-ID:

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

Content-Type:

Content-Disposition:

Perform MIME-VALUE 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 which contains non-ASCII UTF-8 characters. And more, the body part may contain UTF-8 message. The recipients need to accept UTF-8 mail body and UTF-8 header fields which is MIME encoded.
• Rewriting headers increases the opportunities for undetected spoofing.
  
  
• Recipients addresses can be undisclosed if those addresses are listed on Bcc or group address. Those undisclosed addresses are used only in the Envelope. Copying information from the Envelope into headers risks inadvertent information disclosure (not just about addresses). See Section 4 (SMTP Downgrading) for instructions on recipient address handling.
  
  
• The techniques described here invalidates methods that depend on digital signatures over the envelope or any part of the message which includes the top-level header or body part headers. Depending on the specific message being downgraded, DKIM especially, but also possibly S/MIME, PGP, and similar techniques are all likely to break.
  
  
• Many gateways and servers on the Internet will discard headers with which they are not familiar. To the extent to which the downgrade procedures depend on new headers (e.g., "Downgraded") to avoid information loss, then the risk of having those headers dropped and its implications must be identified. In particular, it appears to me that, if the Downgraded headers are dropped, there is no possibility of reconstructing the original information at any point (before, during, or after delivery).
  
  

See "Security considerations" section in [RFC4952] (Klensin, J. and Y. Ko, “Overview and Framework for Internationalized Email,” July 2007.) for more discussion.

8.  Implementation notes

8.1.  Trivial downgrading

Downgrading is an alternative to rejection for delivering messages which require UTF8SMTP support to a server which does not provide this. 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 server which does not support UTF8SMTP. Instead, it MUST 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 fields downgrading, the MIME body part headers downgrading. It supports

• some simple header fields downgrading: Subject
• comments and display name downgrading: From, To, CC
• trace header field downgrading: Received

Otherwise, the downgrading fails.

Trivial downgrading targets mail messages which are generated by UTF8SMTP aware MUAs and contain non-ASCII characters in comments, display names, unstructured parts without using non-ASCII E-mail addresses. This E-mail message does not contain non-ASCII addresses in SMTP Envelope and its header fields. But it is not deliverable via UTF8SMTP un-aware MTA. Implementing full spec downgrading may be hard, but trivial downgrading saves mail messages without using non-ASCII addresses.

8.2.  7bit transport consideration

The SMTP client may confront a SMTP server which does not support the 8BITMIME SMTP extension [RFC1652] (Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. Crocker, “SMTP Service Extension for 8bit-MIMEtransport,” July 1994.). The server does not support "8bit" or "binary" data. Implementers need to consider to replace "8bit" data to be "base64" or "quoted-printable" encoded form and reflect it to "Content-Transfer-Encoding" header field. If the body contains multiple MIME parts, this conversion must be performed for each MIME part according to [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.).

9.  IANA Considerations

IANA is requested to register the following header fields in the Permanent Message Header Field Repository, in accordance with the procedures set out in [RFC3864] (Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields,” September 2004.).

Header field name:

Downgraded-Mail-From

Applicable protocol:

mail

Status:

experimental

Author/change controller:

IETF

Specification document(s):

This document (Section 3 (New header fields definition))

Header field name:

Downgraded-Rcpt-To

Applicable protocol:

mail

Status:

experimental

Author/change controller:

IETF

Specification document(s):

This document (Section 3 (New header fields definition))

Header field name:

Downgraded-From

Applicable protocol:

mail

Status:

experimental

Author/change controller:

IETF

Specification document(s):

This document (Section 3 (New header fields definition))

Header field name:

Downgraded-Sender

Applicable protocol:

mail

Status:

experimental

Author/change controller:

IETF

Specification document(s):

This document (Section 3 (New header fields definition))

Header field name:

Downgraded-To

Applicable protocol:

mail

Status:

experimental

Author/change controller:

IETF

Specification document(s):

This document (Section 3 (New header fields definition))

Header field name:

Downgraded-Cc

Applicable protocol:

mail

Status:

experimental

Author/change controller:

IETF

Specification document(s):

This document (Section 3 (New header fields definition))

Header field name:

Downgraded-Reply-To

Applicable protocol:

mail

Status:

experimental

Author/change controller:

IETF

Specification document(s):

This document (Section 3 (New header fields definition))

Header field name:

Downgraded-Bcc

Applicable protocol:

mail

Status:

experimental

Author/change controller:

IETF

Specification document(s):

This document (Section 3 (New header fields definition))

Header field name:

Downgraded-Resent-From

Applicable protocol:

mail

Status:

experimental

Author/change controller:

IETF

Specification document(s):

This document (Section 3 (New header fields definition))

Header field name:

Downgraded-Resent-To

Applicable protocol:

mail

Status:

experimental

Author/change controller:

IETF

Specification document(s):

This document (Section 3 (New header fields definition))

Header field name:

Downgraded-Resent-Cc

Applicable protocol:

mail

Status:

experimental

Author/change controller:

IETF

Specification document(s):

This document (Section 3 (New header fields definition))

Header field name:

Downgraded-Resent-Sender

Applicable protocol:

mail

Status:

experimental

Author/change controller:

IETF

Specification document(s):

This document (Section 3 (New header fields definition))

Header field name:

Downgraded-Return-Path

Applicable protocol:

mail

Status:

experimental

Author/change controller:

IETF

Specification document(s):

This document (Section 3 (New header fields definition))

And more, IANA is requested to reserve all the field names that start by "Downgraded-" for unknown header fields downgrading described in Section 3.3 (Unknown header fields preservation headers), in accordance with the procedures set out in [RFC3864] (Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields,” September 2004.).

Header field name:

Names which starts by "Downgraded-"

Applicable protocol:

mail

Status:

experimental

Author/change controller:

IETF

Specification document(s):

This document (Section 3 (New header fields definition))

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.  Change History

This section is used for tracking the update of this document. Will be removed after finalize.

11.1.  draft-yoneya-ima-downgrade: Version 00

• Initial version
• Followed draft-yeh-ima-utf8headers-00 and draft-yao-smtpext-00

11.2.  draft-yoneya-ima-downgrade: Version 01

• Document structure was changed
• Followed draft-yeh-ima-utf8headers-01 and draft-yao-smtpext-02
• Downgrading requirements were added
• SMTP DATA encapsulation method was proposed
• Downgrading examples was provided

11.3.  draft-ietf-eai-downgrade: Version 00

• Followed draft-yeh-ima-utf8headers-01 and draft-ietf-eai-smtpext-00
• No header downgrading method was proposed
• Header encapsulation method was proposed

11.4.  draft-ietf-eai-downgrade: Version 01

• Followed draft-ietf-eai-utf8headers-00
• Header conversion and encapsulation method was merged
• Header conversion method was defined in detail

11.5.  draft-ietf-eai-downgrade: Version 02

• Followed draft-ietf-eai-utf8headers-01 and draft-ietf-eai-smtpext-01
• Specification about algorithmic generated address is removed
• No header downgrading method was removed
• SMTP DATA encapsulation method was removed

11.6.  draft-ietf-eai-downgrade: Version 03

• Followed draft-ietf-eai-utf8headers-03 and draft-ietf-eai-smtpext-03
• Downgraded: and Envelope-Downgraded: headers definition was added
• Mail header fields downgrading method was refined
• Examples in Appendix A were refined

11.7.  draft-ietf-eai-downgrade: Version 04

• Followed draft-ietf-eai-utf8headers-06, draft-ietf-eai-smtpext-07 and draft-ietf-eai-dsn-02
• Downgrading requirements and conditions were moved to Introduction.
• Descriptions about upgrading were removed.
• SPF and DKIM discussion were removed.
• Added many header fields downgrading.
• Allow address literal rewriting without alternate ASCII address in header fields.
• Added MIME body part headers downgrading.
• Added ORCPT downgrading.

11.8.  draft-ietf-eai-downgrade: Version 05

• fixed examples
  • ALT-ADDRESS parameter mistake
  • RFC2047(x) notation was changed to encoded-word format
• Added implementation consideration section and trivial downgrading
• Downgraded: and Envelope-Downgraded: headers are separated for each original headers.
• Removed list-* header fields downgrading
• Changed the way of writing the header field downgrading section

11.9.  draft-ietf-eai-downgrade: Version 06

• Moved decoding downgraded messages as a separate document
• Added a text to UNSTRUCTURED downgrading
• Added "replacing SMTP connection" if necessary to SMTP downgrading.
• fixed examples

11.10.  draft-ietf-eai-downgrade: Version 07

• Fixed some typos
• Added a text about 7bit transport

12. Normative References

Appendix A.  Examples

A.1.  Downgrading example 1

This section shows an SMTP Downgrading example. Consider a mail message.

• The sender address is "NON-ASCII-local@example.com" which is non-ASCII address. Its ASCII alternative is "ASCII-local@example.com" and its display-name is "DISPLAY-local".
• The "To" address is "NON-ASCII-remote1@example.net" which is non-ASCII address. Its ASCII alternative is "ASCII-remote1@example.net" and its display-name is "DISPLAY-remote1".
• The "CC" address is non-ASCII address "NON-ASCII-remote2@example.org" without alternative ASCII address. Its display-name is "DISPLAY-remote2".
• Three display-names contains non-ASCII characters.
• The Subject header is "NON-ASCII-SUBJECT" which contains non-ASCII characters.
• assume To: recipient's MTA (example.net) does not support UTF8SMTP.
• assume Cc: recipient's MTA (example.org) supports UTF8SMTP.

The example SMTP envelope/message is shown in Figure 1 (Original envelope/message (example 1)). In this example, the To: recipient's session is focused.

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

In this example, there are two SMTP recipients, one is To:, the other is CC:. The SMTP downgrading treats To: session downgrading. Figure 2 (SMTP Downgraded envelope/message (example 1)) shows 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

After SMTP downgrading, header fields downgrading is performed. Final downgraded message is shown in Figure 3 (Downgraded message (example 1)). Return-Path header 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

A.2.  Downgrading example 2

In many cases, the sender wants to use non-ASCII address, the recipient does not support UTF8SMTP and does not have non-ASCII address.

• The sender address is "NON-ASCII-local@example.com" which is non-ASCII address. Its ASCII alternative is "ASCII-local@example.com". It has a display-name "DISPLAY-local" which contains non-ASCII characters.
• The "To" address is "ASCII-remote1@example.net" which is ASCII only. It has a display-name "DISPLAY-remote1" which contains non-ASCII characters.
• The Subject header is "NON-ASCII-SUBJECT" which contains non-ASCII characters.

The second example envelope/message is shown in Figure 4 (Original message (example 2)).

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

In this example, SMTP session is downgradable. Figure 5 (SMTP Downgraded envelope/message (example 2)) shows 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

After SMTP downgrading, header fields downgrading is performed. The downgraded example is shown in Figure 6 (Downgraded message (example 2)).

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

Authors' Addresses

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