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'RFC2879') (Obsoleted by RFC 2879) ** Obsolete normative reference: RFC 2305 (Obsoleted by RFC 3965) Summary: 9 errors (**), 0 flaws (~~), 7 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group K. Toyoda, MGCS 3 Internet Draft D. Crocker, Brandenburg 4 draft-ietf-fax-esmtp-conneg-03.txt August 2002 5 Expires: January 2003 7 SMTP Service Extension 8 for Content Negotiation 10 STATUS OF THIS MEMO 12 This document is an Internet-Draft and is in full 13 conformance with all provisions of Section 10 of 14 RFC2026. Internet-Drafts are working documents of the 15 Internet Engineering Task Force (IETF), its areas, and 16 its working groups. Note that other groups may also 17 distribute working documents as Internet-Drafts. 19 Internet-Drafts are draft documents valid for a maximum 20 of six months and may be updated, replaced, or 21 obsoleted by other documents at any time. It is 22 inappropriate to use Internet-Drafts as reference 23 material or to cite them other than as "work in 24 progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt 29 The list of Internet-Draft Shadow Directories can be 30 accessed at 31 http://www.ietf.org/shadow.html. 33 COPYRIGHT NOTICE 35 Copyright (C) The Internet Society (2001). All Rights 36 Reserved. 38 ABSTRACT 40 This document defines a content negotiation service 41 extension for SMTP [ESMTP1, ESMTP2] whereby an SMTP 42 client may request information about content 43 capabilities of the target device or system that is 44 serviced by an SMTP server. The SMTP server may report 45 the target's content capabilities back to the client. 46 This process emulates a classic facsimile start-of- 47 session capabilities negotiation, although it can be used 48 for a broad range of email-based scenarios. This service 49 extension is primarily intended for "direct", one-hop, 50 originator/recipient SMTP transfers, although relayed 51 scenarios through multiple SMTP servers are permitted. 53 1. INTRODUCTION 55 When a data source and a receiver have interactive access to 56 each other, the receiver often informs the source of its 57 capabilities, to permit optimized performance or functionality for 58 the interaction. Classic telephone-based facsimile is an example, 59 as are voice over IP and ESMTP, among Internet applications. The 60 store-and-forward nature of Internet mail is usually assumed to 61 preclude such capabilities exchanges, although the sender in a 62 store-and-forward scenario could benefit from knowing precise 63 details about the receiver. In some configurations, direct 64 email-based interactions -- with the originating ESMTP client 65 and the destination ESMTP server able to have direct TCP 66 connect -- are possible, such as over an intranet. In addition 67 an end-to-end exchange can ESMTP for hop-by-hop enforcement. 69 This document defines an SMTP-based service extension [ESMTP1, 70 ESMTP2] for content negotiation, whereby an SMTP client may 71 request information about content capabilities of the target 72 device or system that is serviced by an SMTP server. The SMTP 73 server may report the target's content capabilities back to the 74 client. This process can be used to emulate a classic facsimile 75 start-of-session capabilities negotiation, as well as being used 76 for other email-based services. The extension is primarily 77 intended for "direct" SMTP transfers, although relayed scenarios 78 are permitted through a series of SMTP servers and are discussed 79 in Appendix B. 81 2. CONVENTIONS 83 In examples, "C:" and "S:" indicate lines sent by the 84 client and server respectively. 86 The key words "MUST", "MUST NOT", "SHOULD", "SHOULD 87 NOT", and "MAY" in this document are to be interpreted 88 as defined in "Key words for use in RFCs to Indicate 89 Requirement Levels" [KEYWORDS]. 91 3. CONTENT NEGOTIATION SERVICE EXTENSION 93 (1) The name of the SMTP service extension is 94 "Content_Negotiation" 96 (2) The EHLO keyword value associated with this extension 97 is "CONNEG" 99 (3) A parameter using the keyword "CONNEG" is added to the 100 RCPT command 102 (4) The server responds with a report of the content 103 capabilities of the device or system that embodies the 104 target RCPT address. 106 4. CONNEG PARAMETER TO RCPT 108 4.1 Parameter 110 Keyword: 112 CONNEG 114 Argument: 116 REQUIRED 117 The client requires support for the capability. If the 118 target does not support the CONNEG parameter, the target 119 MUST reject the RCPT command with a 504 reply. 121 If the target can not support the capability due to a 122 temporary problem, it MUST reject the RCPT command with 123 a 404 reply. 125 OPTIONAL 126 The client requests the target to use the capability. If 127 the target can not support the capability at this time, the 128 target MUST process the address and message as if the 129 requested CONNEG capabilities had not been specified. 131 If the argument does not exist, the default is "REQUIRED". 133 When a capability is REQUIRED by the client but can not 134 currently be supported by the target, an error response will 135 have significant performance impact to overall SMTP 136 processing. Use of the OPTIONAL parameter will ensure high 137 SMTP performance. 139 4.2 Client Action 141 If the server issued a 250-CONNEG, as part of its 142 EHLO response for the current session, the client MAY 143 issue the CONNEG parameter with RCPT. 145 If the client issues the CONNEG parameter with 146 RCPT, then it MUST honor the capabilities specified 147 in the CONNEG RCPT reply, and transform data that is 148 sent, so that the server can accept the data. The 149 client SHOULD transform the data to the "highest" level 150 of capability of the server. 152 If the server rejects the RCPT command with a 404 reply, the 153 client may later reissue the RCPT with the CONNEG parameter 154 in a different SMTP session. 156 If the server returns an EHLO 250 code without CONNEG 157 capabilities, the client MUST NOT issue a CONNEG parameter 158 with RCPT. 160 Methods of using of this option with multiple addressees, for 161 the same content, are discussed in Appendix A. 163 4.3 Server Action 165 If the client specifies "CONNEG=REQUIRED" in the RCPT, 166 but the server does not support the CONNEG parameter, the server 167 MUST reject the RCPT command with a 504 reply. 169 If the server supports the CONNEG parameter, but can not return 170 the recepient's capability temporarily, the server MUST reject the 171 RCPT command with a 404 reply. For example, if the server gets 172 the capability information from a directory, but its connection is 173 offline, the server MUST reject the RCPT command with a 404 174 reply. 176 If the client specifies "CONNEG=OPTIONAL" in the RCPT, 177 but the server does not support the CONNEG parameter or can not 178 return the recipient's capability temporarily, the server MUST 179 process the address and message as if the requested CONNEG 180 capabilities had not been specified. 182 Regardless of the value of the parameter, if the server does 183 support the CONNEG parameter and the address is acceptable, 184 then it MUST issue a 250 reply, followed by the capabilities 185 of the server that is specified by the RCPT address. 186 Successful responses to CONNEG RCPT requests will always be 187 multiple SMTP lines. The first line is the normal RCPT response, 188 and subsequent lines beginning with the exact string 189 "250-CONNEG " and "250 CONNEG " are the CONNEG responses. 190 The last line begins with "250 CONNEG ". 192 If the SMTP server supports ENHANCEDSTATUSCODES [RFC1893], the 193 response strings for a success are "250-2.1.5 CONNEG" and 194 "250 2.1.5 CONNEG". The response strings for indicating a 195 permanent failure are "504-5.3.3 CONNEG" and "504 5.3.3 CONNEG". 196 The response strings for a temporary failure are "404-4.3.3 197 CONNEG" and "404 5.3.3 CONNEG". 199 All CONNEG-capable clients and CONNEG-capable servers MUST 200 be able to successfully process CONNEG lines that are up to 512 201 characters long, as required by RFC2821. If the length of CONNEG 202 lines is greater than 512 characters, the server MUST insert 203 line breaks and make next CONNEG line. 205 The contents of the capability listing MUST conform to 206 the specifications in "Content Feature Schema for Internet Fax". 207 [RFC2879] 209 5. SYNTAX 211 Command with "CONNEG": 212 "RCPT TO:" ("" / "" 213 / Forward-Path) (SP "CONNEG =" ("REQUIRED" / 214 "OPTIOANL") CRLF 216 Reply: 217 ( ("250-" CRLF) *("250-CONNEG" capability CRLF) 218 ("250 CONNEG" capability CRLF) )/ 219 ( ("250-2.1.5" CRLF) 220 *("250-2.1.5 CONNEG" capability CRLF) 221 ("250 2.1.5 CONNEG" capability CRLF) )/ 222 ("504" CRLF) / 223 ("504 5.3.3" CRLF) / 224 ("404" CRLF) / 225 ("404 4.3.3" CRLF) / 227 capability = <> 229 6. EXAMPLES 231 6.1 Success Response 233 An example of ESMTP sequence with successful RCPT response 235 S: 220 ifax1.jp IFAX 237 C: EHLO ifax1.jp 239 S: 250-ifax1.jp 240 S: 250-DSN 241 S: 250 CONNEG 243 C: MAIL FROM: 245 S: 250 sender ok 247 C: RCPT TO: CONNEG = REQUIRED 249 S: 250- recipient ok 250 S: 250 CONNEG (&(image-file-structure=TIFF-minimal) 251 S: (MRC-mode=0)(color=Binary)(|(&(dpi=204) 252 S: (dpi-xyratio=[204/98,204/196]) )(&(dpi=200) 253 S: (dpi-xyratio=[200/100,1]) )(&(dpi=400) 254 S: (dpi-xyratio=1) ) )(|(image-coding=[MH,MR,MMR]) 255 S: (&(image-coding=JBIG)(image-coding-constraint=JBIG-T85) 256 S: (JBIG-stripe-size=128) ) )(paper-size=[letter,A4,B4]) 257 S: (ua-media=stationery) ) 259 C: DATA 261 S: 354 okay, send data 263 C: <> 272 S: 250 message accepted 274 C: QUIT 276 S: 221 goodbye 278 6.2 Long Response 280 An example of successful RCPT response when the length of 281 capability is greater than 512 characters. 283 S: 250-2.1.5 recipient ok 284 S: 250-2.1.5 CONNEG (&(image-file-structure=TIFF-minimal) ... 285 S: 250-2.1.5 CONNEG ..... 286 S: 250 2.1.5 CONNEG (color=Binary) 288 An example of succssful RCPT response when CONNEG-capable 289 server supports ENHANCEDSTATUSCODES. 291 S: 250-2.1.5 recipient ok 292 S: 250 2.1.5 CONNEG (&(image-file-structure=TIFF-minimal) 293 S: (MRC-mode=0)(color=Binary)(|(&(dpi=204) 294 S: (dpi-xyratio=[204/98,204/196]) )(&(dpi=200) 295 S: (dpi-xyratio=[200/100,1]) )(&(dpi=400) 296 S: (dpi-xyratio=1) ) )(|(image-coding=[MH,MR,MMR]) 297 S: (&(image-coding=JBIG)(image-coding-constraint=JBIG-T85) 298 S: (JBIG-stripe-size=128) ) )(paper-size=[letter,A4,B4]) 299 S: (ua-media=stationery) ) 301 6.3 Failure Response 303 An example of ESMTP sequence with parmanent failure RCPT 304 response. 306 S: 220 ifax1.jp IFAX 308 C: EHLO ifax1.jp 310 S: 250-ifax1.jp 311 S: 250-DSN 313 C: MAIL FROM: 315 S: 250 sender ok 317 C: RCPT TO: CONNEG = REQUIRED 319 S: 504 recipient ok 321 C: QUIT 323 S: 221 goodbye 325 6.4 Temporary Failure Response 327 An example of an ESMTP sequence with temporary failure RCPT 328 response when the value of parameter is "REQUIRED": 330 S: 220 ifax1.jp IFAX 332 C: EHLO ifax1.jp 334 S: 250-ifax1.jp 335 S: 250-DSN 336 S: 250 CONNEG 338 C: MAIL FROM: 340 S: 250 sender ok 342 C: RCPT TO: CONNEG = REQUIRED 344 S: 404 recipient ok 346 C: QUIT 348 S: 221 goodbye 349 . 350 . 351 . 352 retry according to implementation 354 6.5 Temporary Failure with Optional handling 356 An example of an ESMTP sequence with temporary failure RCPT 357 response when the value of parameter is "OPTIONAL": 359 S: 220 ifax1.jp IFAX 361 C: EHLO ifax1.jp 363 S: 250-ifax1.jp 364 S: 250-DSN 365 S: 250 CONNEG 367 C: MAIL FROM: 369 S: 250 sender ok 371 C: RCPT TO: CONNEG = OPTIONAL 373 S: 250 recipient ok 375 C: DATA 377 S: 354 okay, send data 379 C: <> 384 S: 250 message accepted 386 C: QUIT 388 S: 221 goodbye 390 7. IANA CONSIDERATIONS 392 On publicatiom of this document by the RFC Editor, the IANA 393 shall register the Content_Negotiation ESMTP extension defined 394 in section 3. 396 8. SECURITY CONSIDERATIONS 398 This ESMTP option calls for a respondent to disclose 399 its capabilities. Mechanisms for determining the 400 requestor's authenticated identity are outside the 401 scope of this specification. It is intended that this 402 mechanism permit disclosure of public information; 403 hence there is no particular need for security 404 measures. 406 However there is nothing to prevent disclosure of 407 sensitive information that should receive restricted 408 distribution. It is, therefore, the responsibility of 409 the disclosing ESMTP server to determine whether 410 additional security measures should be applied to the 411 use of this ESMTP option. 413 A man-in-the-middle attack might change the capabilities reported 414 for a given recipient. For example: Suppose the sender knows the 415 recipient has the ability to view color documents so they mark 416 some things in red in what is otherwise a black and white 417 document. But someone interferes with the returned capabilities, 418 indicating that the recipient only supports black and white. The 419 document is duly downgraded, with the result that the recipient 420 doesn't see what the sender marked. 422 An indirect exposure can occur when the report of a capability 423 implies use of specific software. If that software is known to 424 have security weaknesses, the capabilities report effectively 425 advertises the associated opportunity to exploit the security 426 weakness. 428 For target SMTP servers that require security mechanisms to be in 429 force at the start of the session, the target SHOULD refrain from 430 including the CONNEG parameter in an EHLO response until the 431 requisite security mechanisms are in force. 433 For digitally signed content, the use of this option poses a 434 special challenge. Digitally signing content relies on that 435 content to be in a particular form. Use of this option changes 436 that form. Hence an SMTP client that uses this option on 437 digitally signed content MUST be able to recompute the digital 438 signature of the content. 440 9. ACKNOWLEDGEMENTS 442 Graham Klyne provided useful suggestions to an earlier 443 draft. 445 10. NORMATIVE REFERENCES 447 [ESMTP1] Klensin, J., Freed, N., Rose, M., Stefferud, 448 E. and D. Crocker, "SMTP Service Extensions", 449 RFC 1869, November 1995 451 [ESMTP2] Klensin, J., "Simple Mail Transfer Protocol", 452 RFC 2821, April 2001. 454 [RFC2879] McIntyre, L. and G. Klyne, "Content Feature 455 Schema for Internet Fax", RFC 2531, August 456 2000 458 [RFC2305] Toyoda, K., Ohno, H., Murai, J. and D. Wing, 459 "A Simple Mode of Facsimile Using Internet Mail", 460 RFC 2305, March 1998. 462 11. AUTHORS' ADDRESSES 464 Kiyoshi Toyoda 465 Matsushita Graphic Communication Systems,Inc 466 2-3-8 Shimomeguro, Meguro-Ku 467 Tokyo 153 JAPAN 469 +81.3.5434.7161 470 ktoyoda@rdmg.mgcs.mei.co.jp 472 Dave Crocker 473 Brandenburg InternetWorking 474 675 Spruce Drive 475 Sunnyvale, CA 94086 USA 477 +1.408.246.8253 478 dcrocker@brandenburg.com 480 12. FULL COPYRIGHT STATEMENT 482 Copyright (C) The Internet Society (2001). All Rights 483 Reserved. 485 This document and translations of it may be copied and 486 furnished to others, and derivative works that comment 487 on or otherwise explain it or assist in its 488 implementation may be prepared, copied, published and 489 distributed, in whole or in part, without restriction 490 of any kind, provided that the above copyright notice 491 and this paragraph are included on all such copies and 492 derivative works. However, this document itself may 493 not be modified in any way, such as by removing the 494 copyright notice or references to the Internet Society 495 or other Internet organizations, except as needed for 496 the purpose of developing Internet standards in which 497 case the procedures for copyrights defined in the 498 Internet Standards process must be followed, or as 499 required to translate it into languages other than 500 English. 502 The limited permissions granted above are perpetual and 503 will not be revoked by the Internet Society or its 504 successors or assigns. 506 This document and the information contained herein is 507 provided on an "AS IS" basis and THE INTERNET SOCIETY 508 AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL 509 WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT 510 LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION 511 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 512 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A 513 PARTICULAR PURPOSE. 515 APPENDIX 517 A. USAGE FOR MULTIPLE RECIPIENTS 519 ESMTP permits specification of multiple recipients for the same 520 content. This option specified here can produce information that 521 different recipients have different content capabilities. How 522 this differential information is used will depend upon the 523 requirements of the sender. This specification does not mandate 524 particular choices. Three scenarios are possible: 526 (1) Single Recipient 528 For each recipient, conduct a separate ESMTP session. This 529 ensures that each content transfer can be tailored to the 530 capabilities of each recipient. This also consumes the largest 531 amount of bandwidth and the largest number of cross-network SMTP 532 protocol exchanges 534 (2) Common Content 536 For a single ESMTP session, issue RCPT commands that obtain 537 content capabilities information for each recipient. With the 538 DATA command, send the best content form that can be processed by 539 ALL of the recipients. Some recipients will receive content that 540 is below their best capabilities. However this approach also 541 consumes the least bandwidth and has the fewest cross-network 542 protocol exchanges 544 (3) Partial Batching 546 This scenario begins the same as the "Common Content" scenario. 547 Content capabilities information is obtained during a single ESMTP 548 session, with all of the RCPT commands issued together. The 549 difference for this scenario is that the client SMTP then 550 terminates the session with an RSET and begins one or more new, 551 separate sessions. Addressees are processed in batches, according 552 to the similarity of their capabilities. This option balances 553 consumption of bandwidth with optimization of content, at the cost 554 of a terminated session. 556 B. SCENARIOS FOR RELAYING 558 ESMTP is a direct transfer mechanism, using a single TCP 559 connection. It supports Internet mail store-and-forward through 560 such characteristics as global addressing and ESMTP enforcement of 561 global features. Use of the ESMTP Conneg option is 562 straightforward when the originating ESMTP client is able to 563 directly connect to the destination ESMTP server. The nature of 564 this option is to return address-specific information that will 565 affect content transmission. Hence the use of this option in the 566 presence of the in-direct effects of store-and-forward is not 567 obvious. 569 This Appendix discusses some styles of use for the ESMTP Conneg 570 option when messages are being relayed. The Appendix is intended 571 only to provide discussion. It is neither intended to be 572 exhaustive nor restrictive. Other scenarios are likely and 573 encouraged. 575 B.1 Relay Server Modes 577 The key challenge for use of the ESMTP Conneg option is the 578 requirement the receiving ESMTP server be able to return 579 capabilities on behalf of a target addressee. This requirement 580 translates into two, basic styles of operation for the receiving 581 ESMTP server: 583 (1) Server Knows Addressee Capabilities 585 Here the receiving server has the necessary details about an 586 addressee, at the time the sending server issues the RCPT 587 response. Typically, this information will be obtained through a 588 direct, real-time query mechanism, either to a directory 589 containing addressee information or to a service run directly on 590 behalf of the individual addressee and possibly on the addressee's 591 system. 593 One form of query to the addressee's system is to stack a 594 cascading sequence of ESMTP sessions and RCPT commands 595 together, all the way to the destination ESMTP server. Although 596 theoretically possible, this attempt to turn a multi-hop scenario 597 into a real-time, pseudo-direct query is not practical. It 598 will most likely result in response delays for the RCPT command 599 that are not acceptable, particularly over the public Internet. 601 (2) Server Ensures Conversion 603 Fundamental to the use of a capabilities exchange is the 604 requirement that the receiver of capabilities information be able 605 to convert content into a form that is more capable than the 606 default form, when the receiver indicates that it can support the 607 superior form. That is, use of this option presumes that the 608 sender is holding a "more capable" form of the content and will 609 map it to a "less capable form" if the receiver does not support 610 the superior form. 612 Hence another style of relaying configuration is to have a relay 613 SMTP respond to an RCPT capability query by indicating that it 614 supports the most capable form. The sender will pass the best 615 version it can. The receiver has then taken responsibility for 616 performing later conversions, as necessary, to the next hop in the 617 sequence. That is, the receiving ESMTP server inherits the same 618 level of responsibility already being held by the sending ESMTP 619 client. 621 B.2 Some End-to-End Scenarios 623 This section suggests methods of using ESMTP Conneg for achieving 624 an end-to-end service that uses knowledge of recipient 625 capabilities for modifying the content or the handling of the 626 content. This section is intended only to explore possible 627 scenarios. Others are feasible and likely. The choice of scenario 628 to support will depend upon particular service policies chosen for 629 a relay. This specification provides no constraint or guidance 630 about which policies to choose. 632 (1) Basic Tranfer betweeen Organizations 634 Again noting that alternative configurations and support 635 environments are permitted, a simple example of combining the two 636 modes of receiving server style can be helpful. The scenario to 637 consider is sending from one organization's email system to 638 another organization's email system, across the Internet. 640 This scenario presumes two, independent email services, one for 641 each organization. In this simple example, the user has a local 642 ESMTP server and it talks with the organization's Internet email 643 gateway. Hence there are four ESMTP servers. The originating 644 server takes the message from the originating user. It then 645 relays the message to the originating organization's ESMTP 646 gateway. This gateway, in turn, relays the message to the 647 destination organization's gateway, which finally delivers it to 648 the recipient's server. 650 In this simple example, the originating ESMTP server includes a 651 CONNEG option on the RCPT command to the originating 652 organization's Internet gateway and the gateway always responds 653 that it supports the highest capabilities. It is then given the 654 most capable form of the content. 656 The originating gateway then performs a RCPT CONNEG exchange with 657 the destination organization's gateway. This gateway can operate 658 in the same "Ensure Conversion" mode as the originating gateway, 659 or it can perform a real-time query about the addressee's 660 capabilities. The former mode defers resolving recipient 661 capabilities until the final step. The real-time query requires 662 both that the query mechanism be timely, in order to avoid RCPT 663 response delays, and that it be accurate, with correct information 664 about the addressee. 666 The final exchange is, of course, "direct". The recipient's ESMTP 667 server is presumed to have easy access to the necessary 668 capabilities information. 670 (2) Integration with Multipart/Alternative 672 Basic support for Conneg will typically involve a Conneg client 673 that has a single, high-quality version of content and then maps 674 is "down" to the best quality that can be supported by the 675 responding Conneg server. That is, the sending system will do a 676 conversion. In another mode of operation the Conneg client 677 already holds a number of different mappings of content at 678 different levels of quality. The client will use the Conneg 679 response to choose the content quality that is "closest" to the 680 capabilities of the receiver. 682 MIME's Multipart/Alternative provides a means for a content 683 originator to send a fixed set of content quality choices. An 684 ESMTP relay in the sequence can then choose to use Conneg as a 685 means of selecting one of the alternatives to transmit, rather 686 than transmitting the full set. Such a scenario is particularly 687 useful when the communication path changes from high-bandwidth to 688 low. 690 (3) Content Staging on Retrieval Server 692 Rather than converting content to a lower quality, an ESMTP Conneg 693 client might choose to use a Conneg response for choosing not to 694 send the content directly. It might, instead, remove the content 695 from the message and store the content on a retrieval server (for 696 access through HTTP, FTP or the like.) It can then put a citation 697 into the message, which points to the stored content. 699 For the retrieval server, access security, as well as life-time 700 management of the content on the retrieval server, should be 701 considered. A full discussion of these considerations is out of 702 the scope of this memo." 704 (4) Alternate Addressing 706 If an ESMTP Conneg client has access to multiple addresses for the 707 same recipient, it might use Conneg to determine which is most 708 capabable for particular content and send the content to that one. 710 This scenario requires a number of infrastucture features for 711 which there currently are no standards.