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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Missing Reference: 'CFWS' is mentioned on line 335, but not defined == Outdated reference: A later version (-12) exists of draft-ietf-eai-frmwrk-4952bis-10 == Outdated reference: A later version (-13) exists of draft-ietf-eai-rfc5335bis-11 == Outdated reference: A later version (-06) exists of draft-ietf-eai-rfc5337bis-dsn-02 -- Obsolete informational reference (is this intentional?): RFC 5504 (Obsoleted by RFC 6530) Summary: 2 errors (**), 0 flaws (~~), 5 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Email Address Internationalization K. Fujiwara 3 (EAI) JPRS 4 Internet-Draft July 11, 2011 5 Intended status: Standards Track 6 Expires: January 12, 2012 8 Post-delivery Message Downgrading for Internationalized Email Messages 9 draft-ietf-eai-popimap-downgrade-02.txt 11 Abstract 13 The Email Address Internationalization (UTF8SMTP) extension allows 14 UTF-8 characters in mail header fields. POP and IMAP servers support 15 internationalized email messages. If a POP/IMAP client does not 16 support Email Address Internationalization, POP/IMAP servers cannot 17 send Internationalized Email Headers to the client and cannot remove 18 the message. To avoid the situation, this document describes a 19 conversion mechanism for internationalized Email messages to be 20 traditional message format. 22 Status of This Memo 24 This Internet-Draft is submitted to IETF in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF), its areas, and its working groups. Note that 29 other groups may also distribute working documents as Internet- 30 Drafts. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 The list of current Internet-Drafts can be accessed at 38 http://www.ietf.org/ietf/1id-abstracts.txt. 40 The list of Internet-Draft Shadow Directories can be accessed at 41 http://www.ietf.org/shadow.html. 43 This Internet-Draft will expire on January 12, 2012. 45 Copyright Notice 47 Copyright (c) 2011 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (http://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 64 3. Updating RFC 5322 . . . . . . . . . . . . . . . . . . . . . . 4 65 4. New Header Fields Definition . . . . . . . . . . . . . . . . . 5 66 4.1. Unknown Header Fields' Preservation Header Fields . . . . 5 67 5. Email Header Fields Downgrading . . . . . . . . . . . . . . . 6 68 5.1. Downgrading Method for Each ABNF Element . . . . . . . . . 6 69 5.1.1. RECEIVED Downgrading . . . . . . . . . . . . . . . . . 7 70 5.1.2. UNSTRUCTURED Downgrading . . . . . . . . . . . . . . . 7 71 5.1.3. WORD Downgrading . . . . . . . . . . . . . . . . . . . 7 72 5.1.4. COMMENT Downgrading . . . . . . . . . . . . . . . . . 7 73 5.1.5. MIME-VALUE Downgrading . . . . . . . . . . . . . . . . 7 74 5.1.6. DISPLAY-NAME Downgrading . . . . . . . . . . . . . . . 7 75 5.1.7. GROUP Downgrading . . . . . . . . . . . . . . . . . . 7 76 5.1.8. MAILBOX Downgrading . . . . . . . . . . . . . . . . . 8 77 5.1.9. ENCAPSULATION Downgrading . . . . . . . . . . . . . . 8 78 5.1.10. TYPED-ADDRESS Downgrading . . . . . . . . . . . . . . 8 79 5.2. Downgrading Method for Each Header Field . . . . . . . . . 8 80 5.2.1. Address Header Fields That Contain
s . . . . 9 81 5.2.2. Address Header Fields with Typed Addresses . . . . . . 9 82 5.2.3. Downgrading Non-ASCII in Comments . . . . . . . . . . 9 83 5.2.4. Received Header Field . . . . . . . . . . . . . . . . 10 84 5.2.5. MIME Content Header Fields . . . . . . . . . . . . . . 10 85 5.2.6. Non-ASCII in . . . . . . . . . . . . . 10 86 5.2.7. Non-ASCII in . . . . . . . . . . . . . . . . 10 87 5.2.8. Other Header Fields . . . . . . . . . . . . . . . . . 10 88 6. MIME Body-Part Header Field Downgrading . . . . . . . . . . . 11 89 7. Security Considerations . . . . . . . . . . . . . . . . . . . 11 90 8. Implementation Notes . . . . . . . . . . . . . . . . . . . . . 12 91 8.1. RFC 2047 Encoding . . . . . . . . . . . . . . . . . . . . 12 92 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 93 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13 94 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 95 11.1. Normative References . . . . . . . . . . . . . . . . . . . 13 96 11.2. Informative References . . . . . . . . . . . . . . . . . . 14 97 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 14 98 A.1. Downgrading Example . . . . . . . . . . . . . . . . . . . 14 100 1. Introduction 102 Traditional mail systems, which are defined by [RFC5322], allow ASCII 103 characters in mail header field values. The UTF8SMTP extension 104 ([I-D.ietf-eai-frmwrk-4952bis] and [I-D.ietf-eai-rfc5335bis] allows 105 UTF-8 characters in mail header field values. 107 If a header field contains non-ASCII characters, POP/IMAP servers 108 cannot send Internationalized Email Headers to the client and cannot 109 remove the message. This message downgrading mechanism converts mail 110 header fields to an all-ASCII representation. The POP/IMAP servers 111 can use the downgrading mechanism and send the Internationalized 112 Email message as a traditional form. 114 [I-D.ietf-eai-rfc5335bis] allows UTF-8 characters to be used in mail 115 header fields and MIME header fields. The message downgrading 116 mechanism specified here describes the conversion method from the 117 internationalized email messages that are defined in 118 [I-D.ietf-eai-frmwrk-4952bis], and [I-D.ietf-eai-rfc5335bis] to the 119 traditional email messages defined in [RFC5322]. 121 There is no good way to convert "From:" and "Sender:" header fields, 122 the draft need to update [RFC5322] to allow empty "From:" and 123 "Sender:" header fields and it is described in Section 3. 125 Message Downgrading may be implemented in POP server and IMAP server 126 only. 128 This document tries to define the message downgrading process 129 clearly. 131 Downgrading consists of the following four parts: 133 o Updating RFC 5322 135 o New header field definitions 137 o Email header field downgrading 139 o MIME header field downgrading 141 In Section 4 of this document, header fields starting with 142 "Downgraded-" are introduced. They preserve the original header 143 fields. 145 Email header field downgrading is described in Section 5. It 146 generates ASCII-only header fields. 148 MIME header fields are expanded in [I-D.ietf-eai-rfc5335bis]. MIME 149 header field downgrading is described in Section 6. It generates 150 ASCII-only MIME header fields. 152 Displaying downgraded messages that originally contained 153 internationalized header fields is out of scope of this document. A 154 POP/IMAP client which does not support UTF8 extension does not know 155 internationalized message format described in 156 [I-D.ietf-eai-rfc5335bis]. 158 2. Terminology 160 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 161 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 162 document are to be interpreted as described in RFC 2119 [RFC2119]. 164 All specialized terms used in this specification are defined in the 165 Email Address Internationalization (EAI) overview 166 [I-D.ietf-eai-frmwrk-4952bis], in the mail message specifications 167 [RFC5322], or in the MIME documents [RFC2045] [RFC2047] [RFC2183] 168 [RFC2231]. The terms "ASCII address", "internationalized email 169 address", "non-ASCII address", "i18mail address", "UTF8SMTP", 170 "message", and "mailing list" are used with the definitions from 171 [I-D.ietf-eai-frmwrk-4952bis]. 173 This document depends on [I-D.ietf-eai-rfc5335bis]. Key words used 174 in those documents are used in this document, too. 176 The term "non-ASCII" refers to a UTF-8 string that contains at least 177 one non-ASCII character. 179 A "UTF8SMTP message" is an email message expanded by 180 [I-D.ietf-eai-rfc5335bis]. 182 3. Updating RFC 5322 184 "From:" header field or "Sender:" header field may contain non-ASCII 185 addresses in internationalized Email messages. These non-ASCII 186 addresses are not allowed in [RFC5322]. The draft proposes that the 187 pop/imap downgrading uses syntax and encodes non-ASCII 188 addresses into with empty described in 189 Section 5. 191 This specification redefines "From:", "Sender:", "Resent-From:" and 192 "Resent-Sender:" header fields defined in Section 3.6.2 and 3.6.6 of 193 [RFC5322] to allow in the header fields. 195 from = "From:" address-list CRLF 196 resent-from = "Resent-From:" address-list CRLF 197 sender = "Sender:" address CRLF 198 resent-sender = "Resent-Sender:" address CRLF 200 [[Note in Draft: There are still outstanding questions about the use 201 of group syntax that the WG should resolve, or confirm that it is 202 willing to live with and figure out how to describe in the document, 203 at IETF 81. They include 205 1. RFC 5322 does not allow group syntax in From and Sender header 206 fields. Existing MUAs may become very confused when they see 207 group syntax in originator fields. 209 2. Use of group syntax in this way will essentially make it 210 impossible to reply to a message. 212 3. "Reply-To:" header field allows the group syntax in [RFC5322]. 213 Is it correct ? 215 4. The ABNF syntax here is not yet complete. 217 5. Should the document explicitly recommend the use of comments, 218 possibly with encoded words, to document the original non-ASCII 219 mailboxes? 221 Suggestion made to the WG for more in depth discussion. ]] 223 4. New Header Fields Definition 225 New header fields starting with "Downgraded-" are defined here to 226 preserve those mail header field values that contain UTF-8 227 characters. During downgrading, one new "Downgraded-" header field 228 is added for each mail header field that cannot be passed as-is to a 229 POP/IMAP client that does not support UTF8 extension. The original 230 mail header field is removed or rewritten. Only those mail header 231 fields that contain non-ASCII characters are affected. The result of 232 this process is a message that is compliant with existing email 233 specifications [RFC5322]. The original internationalized information 234 can be retrieved by examining the "Downgraded-" header fields that 235 were added. 237 4.1. Unknown Header Fields' Preservation Header Fields 239 The unknown header fields' preservation header fields are defined to 240 encapsulate those original header fields that contain non-ASCII 241 characters and are not otherwise provided for in this specification. 243 The encapsulation header field name is the concatenation of 244 "Downgraded-" and the original name. The value field holds the 245 original header field value. 247 The header field syntax is specified as follows: 249 fields =/ unknown-downgraded-headers ":" unstructured CRLF 251 unknown-downgraded-headers = "Downgraded-" original-header-field-name 253 original-header-field-name = field-name 255 field-name = 1*ftext 257 ftext = %d33-57 / ; Any character except 258 %d59-126 ; controls, SP, and ":". 260 To encapsulate a header field in a "Downgraded-" header field: 262 1. Generate a new "Downgraded-" header field whose value is the 263 original header field value. 265 2. Treat the generated header field content as if it were 266 unstructured, and then apply [RFC2047] encoding with charset 267 UTF-8 as necessary so the result is ASCII. 269 3. Remove the original header field. 271 5. Email Header Fields Downgrading 273 This section defines the conversion method to ASCII for each header 274 field that may contain non-ASCII characters. 276 [I-D.ietf-eai-rfc5335bis] expands "Received:" header fields; 277 [RFC5322] describes ABNF elements , , , 278 ; [RFC2045] describes ABNF element . 280 5.1. Downgrading Method for Each ABNF Element 282 Header field downgrading is defined below for each ABNF element. 283 Downgrading an unknown header field is also defined as ENCAPSULATION 284 downgrading. Converting the header field terminates when no non- 285 ASCII characters remain in the header field. 287 5.1.1. RECEIVED Downgrading 289 If the header field name is "Received:" and the FOR clause contains a 290 non-ASCII address, remove the FOR clause from the header field. 291 Other parts (not counting s) should not contain non-ASCII 292 values. 294 5.1.2. UNSTRUCTURED Downgrading 296 If the header field has an field that contains non- 297 ASCII characters, apply [RFC2047] encoding with charset UTF-8. 299 5.1.3. WORD Downgrading 301 If the header field has any fields that contain non-ASCII 302 characters, apply [RFC2047] encoding with charset UTF-8. 304 5.1.4. COMMENT Downgrading 306 If the header field has any fields that contain non-ASCII 307 characters, apply [RFC2047] encoding with charset UTF-8. 309 5.1.5. MIME-VALUE Downgrading 311 If the header field has any elements defined by [RFC2045] and 312 the elements contain non-ASCII characters, encode the 313 elements according to [RFC2231] with charset UTF-8 and leave the 314 language information empty. If the element is and it contains outside the DQUOTE, remove the 316 before this conversion. 318 5.1.6. DISPLAY-NAME Downgrading 320 If the header field has any
( or ) elements 321 and they have elements that contain non-ASCII 322 characters, encode the elements according to [RFC2047] 323 with charset UTF-8. DISPLAY-NAME downgrading is the same algorithm 324 as WORD downgrading. 326 5.1.7. GROUP Downgrading 328 is defined in Section 3.4 of [RFC5322]. The elements 329 may contain s which contain non-ASCII addresses. 331 If the header field has any elements which contain 332 elements that contain non-ASCII addresses, rewrite each 333 element as 335 "Internationalized_Address_Removed" display-name ENCODED_WORD ":;" [CFWS] 337 where the is the original encoded 338 according to [RFC2047]. 340 5.1.8. MAILBOX Downgrading 342 The elements have no equivalent format for non-ASCII 343 addresses. If the header field has any elements that 344 contain non-ASCII characters, rewrite each element to 345 ASCII-only format. The element that contains non-ASCII 346 characters is one of two formats. 348 [ Display-name ] "<" Utf8-addr-spec ">" 350 Utf8-addr-spec 352 Rewrite both as: 353 [ Display-name ] "Internationalized Address " Encoded-word 354 " Removed:;" 355 where the is the original encoded 356 according to [RFC2047]. 358 5.1.9. ENCAPSULATION Downgrading 360 If the header field contains non-ASCII characters and is such that no 361 rule is given above, encapsulate it in a "Downgraded-" header field 362 as described in Section 4.1 as a last resort. 364 Applying this procedure to "Received:" header field is prohibited. 366 5.1.10. TYPED-ADDRESS Downgrading 368 If the header field contains and the contains raw non-ASCII characters, it is in utf-8-address form. 370 Convert it to utf-8-addr-xtext form. Those forms are described in 371 [I-D.ietf-eai-rfc5337bis-dsn]. COMMENT downgrading is also performed 372 in this case. If the address type is unrecognized and the header 373 field contains non-ASCII characters, then fall back to using 374 ENCAPSULATION downgrading on the entire header field. 376 5.2. Downgrading Method for Each Header Field 378 Header fields are listed in [RFC4021]. This section describes the 379 downgrading method for each header field. 381 If the whole mail header field does not contain non-ASCII characters, 382 email header field downgrading is not required. Each header field's 383 downgrading method is described below. 385 5.2.1. Address Header Fields That Contain
s 387 From: 388 Sender: 389 To: 390 Cc: 391 Bcc: 392 Reply-To: 393 Resent-From: 394 Resent-Sender: 395 Resent-To: 396 Resent-Cc: 397 Resent-Bcc: 398 Resent-Reply-To: 399 Return-Path: 400 Disposition-Notification-To: 402 If the header field contains elements that contain non-ASCII 403 addresses, perform COMMENT downgrading, DISPLAY-NAME downgrading, and 404 GROUP downgrading. 406 If the header field contains elements that contain non- 407 ASCII addresses, perform COMMENT downgrading, DISPLAY-NAME 408 downgrading, and MAILBOX downgrading. 410 5.2.2. Address Header Fields with Typed Addresses 412 Original-Recipient: 413 Final-Recipient: 415 If the header field contains non-ASCII characters, perform TYPED- 416 ADDRESS downgrading. 418 5.2.3. Downgrading Non-ASCII in Comments 420 Date: 421 Message-ID: 422 Resent-Message-ID: 423 In-Reply-To: 424 References: 426 Resent-Date: 427 Resent-Message-ID: 428 MIME-Version: 429 Content-ID: 430 Content-Transfer-Encoding: 431 Content-Language: 432 Accept-Language: 433 Auto-Submitted: 435 These header fields do not contain non-ASCII characters except in 436 comments. If the header field contains UTF-8 characters in comments, 437 perform COMMENT downgrading. 439 5.2.4. Received Header Field 441 Received: 443 Perform COMMENT downgrading and RECEIVED downgrading. 445 5.2.5. MIME Content Header Fields 447 Content-Type: 448 Content-Disposition: 450 Perform MIME-VALUE downgrading and COMMENT downgrading. 452 5.2.6. Non-ASCII in 454 Subject: 455 Comments: 456 Content-Description: 458 Perform UNSTRUCTURED downgrading. 460 5.2.7. Non-ASCII in 462 Keywords: 464 Perform WORD downgrading. 466 5.2.8. Other Header Fields 468 For all other header fields that contain non-ASCII characters, are 469 user-defined, and are missing from this document or future defined 470 header fields, perform ENCAPSULATION downgrading. 472 If the software understands the header field's structure and a 473 downgrading algorithm other than ENCAPSULATION is applicable, that 474 software SHOULD use that algorithm; ENCAPSULATION downgrading is used 475 as a last resort. 477 Mailing list header fields (those that start in "List-") are part of 478 this category. 480 6. MIME Body-Part Header Field Downgrading 482 MIME body-part header fields may contain non-ASCII characters 483 [I-D.ietf-eai-rfc5335bis]. This section defines the conversion 484 method to ASCII-only header fields for each MIME header field that 485 contains non-ASCII characters. Parse the message body's MIME 486 structure at all levels and check each MIME header field to see 487 whether it contains non-ASCII characters. If the header field 488 contains non-ASCII characters in the header field value, the header 489 field is a target of the MIME body-part header field's downgrading. 490 Each MIME header field's downgrading method is described below. 491 COMMENT downgrading, MIME-VALUE downgrading, and UNSTRUCTURED 492 downgrading are described in Section 5. 494 Content-ID: 495 The "Content-ID:" header field does not contain non-ASCII 496 characters except in comments. If the header field contains UTF-8 497 characters in comments, perform COMMENT downgrading. 499 Content-Type: 501 Content-Disposition: Perform MIME-VALUE downgrading and COMMENT 502 downgrading. 504 Content-Description: Perform UNSTRUCTURED downgrading. 506 7. Security Considerations 508 A downgraded message's header fields contain ASCII characters only. 509 But they still contain MIME-encapsulated header fields that contain 510 non-ASCII UTF-8 characters. Furthermore, the body part may contain 511 UTF-8 characters. Implementations parsing Internet messages need to 512 accept UTF-8 body parts and UTF-8 header fields that are MIME- 513 encoded. Thus, this document inherits the security considerations of 514 MIME-encoded header fields ([RFC2047] and [RFC3629]). 516 Rewriting header fields increases the opportunities for undetected 517 spoofing by malicious senders. However, rewritten header fields are 518 preserved into Downgraded-* header fields, and parsing Downgraded-* 519 header fields enables the detection of spoofing caused by 520 downgrading. 522 The techniques described here invalidate methods that depend on 523 digital signatures over any part of the message, which includes the 524 top-level header fields and body-part header fields. Depending on 525 the specific message being downgraded, the following techniques are 526 likely to break: DomainKeys Identified Mail (DKIM), and possibly 527 S/MIME and Pretty Good Privacy (PGP). The two obvious mitigations 528 are to stick to 7-bit transport when using these techniques (as most/ 529 all of them presently require) or to make sure to have UTF8SMTP end- 530 to-end when needed. 532 While information in any email header field should usually be treated 533 with some suspicion, current email systems commonly employ various 534 mechanisms and protocols to make the information more trustworthy. 535 Currently, information in the new Downgraded-* header fields is 536 usually not inspected by these mechanisms, and may be even less 537 trustworthy than the traditional header fields. Note that the 538 Downgraded-* header fields could have been inserted with malicious 539 intent (and with content unrelated to the traditional header fields). 541 See the "Security Considerations" section in 542 [I-D.ietf-eai-frmwrk-4952bis] for more discussion. 544 8. Implementation Notes 546 8.1. RFC 2047 Encoding 548 While [RFC2047] has a specific algorithm to deal with whitespace in 549 adjacent encoded words, there are a number of deployed 550 implementations that fail to implement the algorithm correctly. As a 551 result, whitespace behavior is somewhat unpredictable in practice 552 when multiple encoded words are used. While RFC 5322 states that 553 implementations SHOULD limit lines to not more than 78 characters, 554 implementations MAY choose to allow overly long encoded words in 555 order to work around faulty [RFC2047] implementations. 556 Implementations that choose to do so SHOULD have an optional 557 mechanism to limit line length to 78 characters. 559 9. IANA Considerations 561 IANA is requested to refuse registration of all field names that 562 start with "Downgraded-". For unknown header fields, use the 563 downgrading method described in Section 4.1 to avoid conflicts with 564 existing IETF activity (Email Address Internationalization). 566 10. Acknowledgements 568 This document draws heavily from the experimental in-transit message 569 downgrading procedure described in RFC 5504 [RFC5504]. The 570 contribution of the co-author of that earlier document, Y. Yoneya, 571 are gratefully acknowledged. 573 11. References 575 11.1. Normative References 577 [I-D.ietf-eai-frmwrk-4952bis] Klensin, J. and Y. Ko, "Overview and 578 Framework for Internationalized 579 Email", 580 draft-ietf-eai-frmwrk-4952bis-10 (work 581 in progress), September 2010. 583 [I-D.ietf-eai-rfc5335bis] Yang, A., Steele, S., and N. Freed, 584 "Internationalized Email Headers", 585 draft-ietf-eai-rfc5335bis-11 (work in 586 progress), July 2011. 588 [I-D.ietf-eai-rfc5337bis-dsn] Hansen, T., Newman, C., and A. 589 Melnikov, "Internationalized Delivery 590 Status and Disposition Notifications", 591 draft-ietf-eai-rfc5337bis-dsn-02 (work 592 in progress), April 2011. 594 [RFC2045] Freed, N. and N. Borenstein, 595 "Multipurpose Internet Mail Extensions 596 (MIME) Part One: Format of Internet 597 Message Bodies", RFC 2045, 598 November 1996. 600 [RFC2047] Moore, K., "MIME (Multipurpose 601 Internet Mail Extensions) Part Three: 602 Message Header Extensions for Non- 603 ASCII Text", RFC 2047, November 1996. 605 [RFC2119] Bradner, S., "Key words for use in 606 RFCs to Indicate Requirement Levels", 607 BCP 14, RFC 2119, March 1997. 609 [RFC2183] Troost, R., Dorner, S., and K. Moore, 610 "Communicating Presentation 611 Information in Internet Messages: The 612 Content-Disposition Header Field", 613 RFC 2183, August 1997. 615 [RFC2231] Freed, N. and K. Moore, "MIME 616 Parameter Value and Encoded Word 617 Extensions: Character Sets, Languages, 618 and Continuations", RFC 2231, 619 November 1997. 621 [RFC3629] Yergeau, F., "UTF-8, a transformation 622 format of ISO 10646", STD 63, 623 RFC 3629, November 2003. 625 [RFC4021] Klyne, G. and J. Palme, "Registration 626 of Mail and MIME Header Fields", 627 RFC 4021, March 2005. 629 [RFC5322] Resnick, P., Ed., "Internet Message 630 Format", RFC 5322, October 2008. 632 11.2. Informative References 634 [RFC5504] Fujiwara, K. and Y. Yoneya, 635 "Downgrading Mechanism for Email 636 Address Internationalization", 637 RFC 5504, March 2009. 639 Appendix A. Examples 641 A.1. Downgrading Example 643 This appendix shows an message downgrading example. Consider a 644 received mail message where: 646 o The sender address is a non-ASCII address, 647 "NON-ASCII-local@example.com". Its display-name is "DISPLAY- 648 local". 650 o The "To:" header field contains two non-ASCII addresses, 651 "NON-ASCII-remote1@example.net" and 652 "NON-ASCII-remote2@example.com" Its display-names are "DISPLAY- 653 remote1" and "DISPLAY-remote2". 655 o The "Cc:" header field contains a non-ASCII address, 656 "NON-ASCII-remote3@example.org". Its display-name is "DISPLAY- 657 remote3". 659 o Four display names contain non-ASCII characters. 661 o The Subject header field is "NON-ASCII-SUBJECT", which contains 662 non-ASCII characters. 664 o There is an unknown header field "X-Unknown-Header" which contains 665 non-ASCII characters. 667 Return-Path: 668 Received: from ... by ... for 669 Received: from ... by ... for 670 From: DISPLAY-local 671 To: DISPLAY-remote1 , 672 DISPLAY-remote2 673 Cc: DISPLAY-remote3 674 Subject: NON-ASCII-SUBJECT 675 Date: DATE 676 Message-Id: MESSAGE_ID 677 Mime-Version: 1.0 678 Content-Type: text/plain; charset="UTF-8" 679 Content-Transfer-Encoding: 8bit 680 X-Unknown-Header: NON-ASCII-CHARACTERS 682 MAIL_BODY 684 Figure 1: Received message in a mail drop 686 The downgraded message is shown in Figure 2. "Return-Path:", 687 "From:", "To:" and "Cc:" header fields are rewritten. "X-Unknown- 688 Header:" is encapsulated as "Downgraded-X-Unknown-Header:". 690 Return-Path: Internationalized address 691 =?UTF-8?Q?NON-ASCII-local@example.com?= removed:; 692 Received: from ... by ... 693 Received: from ... by ... 694 From: =?UTF-8?Q?DISPLAY-local?= Internationalized address 695 =?UTF-8?Q?NON-ASCII-local@example.com?= removed:; 696 To: =?UTF-8?Q?DISPLAY-remote1?= Internationalized address 697 =?UTF-8?Q?NON-ASCII-remote1@example.net?= removed:;, 698 =?UTF-8?Q?DISPLAY-remote2?= Internationalized address 699 =?UTF-8?Q?NON-ASCII-remote2@example.com?= removed:;, 700 Cc: =?UTF-8?Q?DISPLAY-remote3?= Internationalized address 701 =?UTF-8?Q?NON-ASCII-remote3@example.org?= removed:; 702 Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?= 703 Date: DATE 704 Message-Id: MESSAGE_ID 705 Mime-Version: 1.0 706 Content-Type: text/plain; charset="UTF-8" 707 Content-Transfer-Encoding: 8bit 708 Downgraded-X-Unknown-Header: =?UTF-8?Q?NON-ASCII-CHARACTERS?= 710 MAIL_BODY 712 Figure 2: Downgraded message 714 Author's Address 716 Kazunori Fujiwara 717 Japan Registry Services Co., Ltd. 718 Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda 719 Chiyoda-ku, Tokyo 101-0065 720 Japan 722 Phone: +81 3 5215 8451 723 EMail: fujiwara@jprs.co.jp