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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 EMAILCORE J.C. Klensin, Ed. 3 Internet-Draft 4 Intended status: Standards Track K. Murchison, Ed. 5 Expires: 7 February 2022 Fastmail 6 E. Sam, Ed. 7 6 August 2021 9 Applicability Statement for IETF Core Email Protocols 10 draft-ietf-emailcore-as-03 12 Abstract 14 Electronic mail is one of the oldest Internet applications that is 15 still in very active use. While the basic protocols and formats for 16 mail transport and message formats have evolved slowly over the 17 years, events and thinking in more recent years have supplemented 18 those core protocols with additional features and suggestions for 19 their use. This Applicability Statement describes the relationship 20 among many of those protocols and provides guidance and makes 21 recommendations for the use of features of the core protocols. 23 Status of This Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at https://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on 7 February 2022. 40 Copyright Notice 42 Copyright (c) 2021 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 47 license-info) in effect on the date of publication of this document. 48 Please review these documents carefully, as they describe your rights 49 and restrictions with respect to this document. Code Components 50 extracted from this document must include Simplified BSD License text 51 as described in Section 4.e of the Trust Legal Provisions and are 52 provided without warranty as described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 57 2. Applicability of Some SMTP Provisions . . . . . . . . . . . . 3 58 2.1. Handling of the Domain Argument to the EHLO Command . . . 3 59 2.2. Use of Address Literals . . . . . . . . . . . . . . . . . 4 60 2.3. Use of Addresses in Top-Level Domains . . . . . . . . . . 4 61 3. Applicability of Message Format Provisions . . . . . . . . . 4 62 3.1. Use of Empty Quoted Strings . . . . . . . . . . . . . . . 4 63 4. MIME and Its Implications . . . . . . . . . . . . . . . . . . 5 64 5. Other Stuff . . . . . . . . . . . . . . . . . . . . . . . . . 5 65 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5 66 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 67 8. Security Considerations . . . . . . . . . . . . . . . . . . . 6 68 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 69 9.1. Normative References . . . . . . . . . . . . . . . . . . 6 70 9.2. Informative References . . . . . . . . . . . . . . . . . 6 71 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 7 72 A.1. Changes from draft-klensin-email-core-as-00 (2020-03-30) to 73 draft-ietf-emailcore-as-00 . . . . . . . . . . . . . . . 7 74 A.2. Changes from draft-ietf-emailcore-as-00 (2020-10-06) to 75 -01 . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 76 A.3. Changes from draft-ietf-emailcore-as-01 (2021-04-09) to 77 -02 . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 78 A.4. Changes from draft-ietf-emailcore-as-02 (2021-08-06) to 79 -03 . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 80 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 82 1. Introduction 84 In its current form, this draft is a placeholder and beginning of an 85 outline for the Applicability Statement that has been discussed as a 86 complement for proposed revisions of the base protocol specifications 87 for SMTP [RFC5321] (being revised as [I-D.ietf-emailcore-rfc5321bis]) 88 and Internet Message Format [RFC5322] (being revised as 89 [I-D.ietf-emailcore-rfc5322bis]). Among other things, it is expected 90 to capture topics that a potential WG concludes are important but 91 that should not become part of those core documents. 93 As discussed in [RFC2026], 94 "An Applicability Statement specifies how, and under what 95 circumstances, one or more TSs may be applied to support a 96 particular Internet capability." 98 That form of a standards track document is appropriate because one of 99 the roles of such a document is to explain the relationship among 100 technical specifications, describe how they are used together, and 101 make statements about what is "required, recommended, or elective". 103 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 104 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 105 document are to be interpreted as described in [RFC2119] and 106 [RFC8174]. 108 2. Applicability of Some SMTP Provisions 110 Over the years since RFC 5321 was published in October 2008, usage of 111 SMTP has evolved, machines and network speeds have increased, and the 112 frequency with which SMTP senders and receivers have to be prepared 113 to deal with systems that are disconnected from the Internet for long 114 periods or that require many hops to reach has decreased. During the 115 same period, the IETF has become much more sensitive to privacy and 116 security issues and the need to be more resistant or robust against 117 spam and other attacks. In addition SMTP (and Message Format) 118 extensions have been introduced that are expected to evolve the 119 Internet's mail system to better accommodate environments in which 120 Basic Latin Script is not the norm. 122 This section describes adjustments that may be appropriate for SMTP 123 under various circumstances and discusses the applicability of other 124 protocols that represent newer work or that are intended to deal with 125 relatively newer issues. 127 2.1. Handling of the Domain Argument to the EHLO Command 129 If the "Domain" argument to the EHLO command does not have an address 130 record in the DNS that matches the IP address of the client, the SMTP 131 server may refuse any mail from the client as part of established 132 anti-abuse practice. Operational experience has demonstrated that 133 the lack of a matching address record for the the domain name 134 argument is at best an indication of a poorly-configured MTA, and at 135 worst that of an abusive host. 137 2.2. Use of Address Literals 139 The "address-literal" ABNF non-terminal is used in various places in 140 [I-D.ietf-emailcore-rfc5321bis] grammar however, for SMTP connections 141 over the public internet, an "address-literal" as the argument to 142 EHLO command or the "Domain" part of the "Mailbox" argument to the 143 MAIL FROM command is quite likely to result in the message being 144 rejected as a matter of policy at many sites, since they are deemed 145 to be signs of at best a misconfigured server, and at worst either a 146 compromised host or a server that's intentionally configured to hide 147 its identity. 149 2.3. Use of Addresses in Top-Level Domains 151 While addresses in top-level domains (TLDs) are syntactically valid, 152 mail to these addresses has never worked reliably. A handful of 153 country code TLDs have top level MX records but they have never been 154 widely used nor well supported. In 2013 [RFC7085] found 18 TLDs with 155 MX records, which dropped to 17 in 2021 despite many new TLDs having 156 been added. 158 Mail sent to addresses with single label domains has typically 159 expected the address to be an abbreviation to be completed by a 160 search list, so mail to bob@sales would be completed to 161 bob@sales.example.com. This shortcut has led to unfortunate 162 consequnces; in one famous case, in 1991 when the .CS domain was 163 added to the root, mail in computer science departments started to 164 fail as mail to bob@cs was now treated as mail to Czechoslovakia. 165 Hence, for reliable service, mail SHOULD NOT use addreses that 166 contain single label domains. 168 3. Applicability of Message Format Provisions 170 This section describes adjustments to the Internet Message Format 171 that may be appropriate under various circumstances. 173 3.1. Use of Empty Quoted Strings 175 The "quoted-string" ABNF non-terminal is used in various places in 176 rfc5322bis grammar. While it allows for empty quoted string, such 177 construct is going to cause interoperability issues when used in 178 certain header fields. In particular, use of empty quoted strings is 179 NOT RECOMMENDED in "received-token" (a component of a Received header 180 field), "keywords" (a component of a Keywords header field) and 181 "local-part" (left hand side of email addresses). Use of empty 182 quoted strings is in particular problematic in the "local-part". For 183 example, all of the following email addresses are non interoperable: 185 "".bar@example.com 187 foo.""@example.net 189 ""@example.com 191 Use of empty quoted strings is fine in "display-name". 193 4. MIME and Its Implications 195 When the work leading to the original version of the MIME 196 specification was completed in 1992 [RFC1341], the intention was that 197 it be kept separate from the specification for basic mail headers in 198 RFC 822 [RFC0822]. That plan was carried forward into RFC 822's 199 successors, [RFC2822] and [RFC5322] and the successors of that 200 original MIME specification including [RFC2045]. The decision to do 201 so was different from the one made for SMTP, for which the core 202 specification was changed to allow for the extension mechanism 203 [RFC1425] which was then incorporated into RFC 5321 and its 204 predecessor [RFC2821]. 206 Various uses of MIME have become nearly ubiquitous in contemporary 207 email while others may have fallen into disuse or been repurposed 208 from the intent of their original design. 210 It may be appropriate to make some clear statements about the 211 applicability of MIME and its features. 213 5. Other Stuff 215 It is fairly clear that there will be things that do not fit into the 216 sections outlined above. As one example, if the IETF wants to say 217 something specific about signatures over headers or what (non-trace) 218 headers may reasonably be altered in transit, that may be more 219 appropriate to other sections than to any of the three suggested 220 above. 222 6. Acknowledgments 224 The Emailcore group arose out of discussions on the ietf-smtp group 225 over changes and additions that should be made to the core email 226 protocols. It was agreed upon that it was time to create a working 227 group that would fix many potential errors and opportunities for 228 misunderstandings within the RFCs. 230 7. IANA Considerations 232 This memo includes no requests to or actions for IANA. The IANA 233 registries associated with the protocol specifications it references 234 are specified in their respective documents. 236 8. Security Considerations 238 All drafts are required to have a security considerations section and 239 this one eventually will. 241 ... To be supplied ... 243 9. References 245 9.1. Normative References 247 [RFC2026] Bradner, S., "The Internet Standards Process -- Revision 248 3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996, 249 . 251 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 252 Extensions (MIME) Part One: Format of Internet Message 253 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 254 . 256 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 257 Requirement Levels", BCP 14, RFC 2119, 258 DOI 10.17487/RFC2119, March 1997, 259 . 261 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 262 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 263 May 2017, . 265 9.2. Informative References 267 [I-D.ietf-emailcore-rfc5321bis] 268 Klensin, J. C., "Simple Mail Transfer Protocol", Work in 269 Progress, Internet-Draft, draft-ietf-emailcore-rfc5321bis- 270 03, 10 July 2021, . 273 [I-D.ietf-emailcore-rfc5322bis] 274 Resnick, P. W., "Internet Message Format", Work in 275 Progress, Internet-Draft, draft-ietf-emailcore-rfc5322bis- 276 01, 29 March 2021, . 279 [RFC0822] Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET 280 TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822, 281 August 1982, . 283 [RFC1341] Borenstein, N. and N. Freed, "MIME (Multipurpose Internet 284 Mail Extensions): Mechanisms for Specifying and Describing 285 the Format of Internet Message Bodies", RFC 1341, 286 DOI 10.17487/RFC1341, June 1992, 287 . 289 [RFC1425] Klensin, J., Freed, N., Ed., Rose, M., Stefferud, E., and 290 D. Crocker, "SMTP Service Extensions", February 1993, 291 . 293 [RFC2821] Klensin, J., Ed., "Simple Mail Transfer Protocol", 294 RFC 2821, DOI 10.17487/RFC2821, April 2001, 295 . 297 [RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822, 298 DOI 10.17487/RFC2822, April 2001, 299 . 301 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 302 DOI 10.17487/RFC5321, October 2008, 303 . 305 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 306 DOI 10.17487/RFC5322, October 2008, 307 . 309 [RFC7085] Levine, J. and P. Hoffman, "Top-Level Domains That Are 310 Already Dotless", RFC 7085, DOI 10.17487/RFC7085, December 311 2013, . 313 Appendix A. Change Log 315 RFC Editor: Please remove this appendix before publication. 317 A.1. Changes from draft-klensin-email-core-as-00 (2020-03-30) to draft- 318 ietf-emailcore-as-00 320 * Change of filename, metadata, and date to reflect transition to WG 321 document for new emailcore WG. No other substantive changes 323 A.2. Changes from draft-ietf-emailcore-as-00 (2020-10-06) to -01 325 * Added co-authors (list is in alphabetical order for the present). 327 * Updated references to 5321bis and 5322bis. 329 * Added note at top, "This version is provided as a document 330 management convenience to update the author list and make an un- 331 expired version available to the WG. There are no substantive 332 changes from the prior version", which should be removed for 333 version -02. 335 A.3. Changes from draft-ietf-emailcore-as-01 (2021-04-09) to -02 337 * Added new editors and also added some issues the emailcore group 338 will be dealing with. 340 * Added reference to RFC 6648. 342 A.4. Changes from draft-ietf-emailcore-as-02 (2021-08-06) to -03 344 * Moved discussion of address-literals (issue #1) and domain names 345 in EHLO (issue #19) under SMTP Provisions section 347 * Moved discussion of empty quoted-strings under Message Format 348 Provisions section 350 * Added text on use of addresses in TLDs (issue #50) 352 * Marked all authors as editors. 354 * Miscellaneous editorial changes. 356 Authors' Addresses 358 John C Klensin (editor) 359 1770 Massachusetts Ave, Ste 322 360 Cambridge, MA 02140 361 United States of America 363 Phone: +1 617 245 1457 364 Email: john-ietf@jck.com 366 Kenneth Murchison (editor) 367 Fastmail US LLC 368 1429 Walnut Street - Suite 1201 369 Philadelphia, PA 19102 370 United States of America 372 Email: murch@fastmailteam.com 373 E Sam (editor) 375 Email: winshell64@gmail.com