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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DMARC Working Group S. Jones 3 Internet-Draft DMARC.org 4 Intended status: Informational J. Rae-Grant 5 Expires: December 22, 2017 Google 6 T. Adams 7 Paypal 8 K. Andersen, Ed. 9 LinkedIn 10 June 20, 2017 12 Recommended Usage of the Authenticated Received Chain (ARC) 13 draft-ietf-dmarc-arc-usage-02 15 Abstract 17 The Authentication Received Chain (ARC) provides a means to preserve 18 email authentication results and verify the identity of email message 19 handlers, each of which participates by inserting certain header 20 fields before passing the message on. But the specification does not 21 indicate how intermediaries and receivers should interpret or utilize 22 ARC. This document will provide guidance in these areas. 24 Status of This Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at http://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on December 22, 2017. 41 Copyright Notice 43 Copyright (c) 2017 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 59 2. How does ARC work? . . . . . . . . . . . . . . . . . . . . . 3 60 3. Guidance for Receivers/Validators . . . . . . . . . . . . . . 4 61 3.1. What is the significance of an intact ARC chain? . . . . 4 62 3.2. What exactly is an "intact" ARC chain? . . . . . . . . . 4 63 3.3. What is the significance of an invalid ("broken") ARC 64 chain? . . . . . . . . . . . . . . . . . . . . . . . . . 5 65 3.4. What does the absence of an ARC chain in a message mean? 5 66 3.5. What reasonable conclusions can you draw based upon 67 seeing lots of mail with ARC chains? . . . . . . . . . . 5 68 3.6. What if none of the intermediaries have been seen 69 previously? . . . . . . . . . . . . . . . . . . . . . . . 6 70 3.7. What about ARC chains where some intermediaries are known 71 and others are not? . . . . . . . . . . . . . . . . . . . 6 72 3.8. What should message handlers do when they detect 73 malicious content in messages where ARC is present? . . . 6 74 3.9. What feedback does a sender or domain owner get about ARC 75 when it is applied to their messages? . . . . . . . . . . 7 76 3.10. What prevents a malicious actor from removing the ARC 77 header fields, altering the content, and creating a new 78 ARC chain? . . . . . . . . . . . . . . . . . . . . . . . 7 79 4. Guidance for Intermediaries . . . . . . . . . . . . . . . . . 8 80 4.1. What is an Intermediary under ARC? . . . . . . . . . . . 8 81 4.2. What are the minimum requirements for an ARC 82 Intermediary? . . . . . . . . . . . . . . . . . . . . . . 8 83 4.2.1. More specifically a participating ARC intermediary 84 must do the following: . . . . . . . . . . . . . . . 8 85 4.3. Should every MTA be an ARC participant? . . . . . . . . . 8 86 4.4. What should an intermediary do in the case of an invalid 87 or "broken" ARC chain? . . . . . . . . . . . . . . . . . 9 88 4.5. What should I do in the case where there is no ARC chain 89 present in a message? . . . . . . . . . . . . . . . . . . 9 90 4.6. How could ARC affect my reputation as an intermediary? . 9 91 4.7. What can I do to influence my reputation as an 92 intermediary? . . . . . . . . . . . . . . . . . . . . . . 9 93 5. Guidance for Originators . . . . . . . . . . . . . . . . . . 10 94 5.1. Where can I find out more information? . . . . . . . . . 10 95 5.2. How/where can I test interoperabililty for my 96 implementation? . . . . . . . . . . . . . . . . . . . . . 10 98 5.3. How can ARC impact my email? . . . . . . . . . . . . . . 10 99 5.4. How can ARC impact my reputation as a message sender? . . 10 100 5.5. Can I tell intermediaries not to use ARC? . . . . . . . . 11 101 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 102 6.1. Normative References . . . . . . . . . . . . . . . . . . 11 103 6.2. Informative References . . . . . . . . . . . . . . . . . 11 104 6.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 12 105 Appendix A. GLOSSARY . . . . . . . . . . . . . . . . . . . . . . 12 106 Appendix B. References . . . . . . . . . . . . . . . . . . . . . 15 107 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 15 108 Appendix D. Comments and Feedback . . . . . . . . . . . . . . . 15 109 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 111 1. Introduction 113 [ARC] is intended to be used primarily by intermediaries, or message 114 handlers - those parties who may forward or resend messages, with or 115 without alterations, such that they will no longer pass the SPF, 116 DKIM, and/or [RFC7489] authentication mechanisms. In such cases ARC 117 may provide the final message recipient with useful information about 118 the original sender. 120 2. How does ARC work? 122 Consider a mailing list as an example, where the message submitter's 123 domain publishes a DMARC policy other than "p=none". The message is 124 received, a prefix is added to the RFC5322.Subject header field, some 125 text is appended to the message body, and the message is sent to list 126 members with the original RFC5322.From address intact. In this case 127 SPF may pass because the mailing list operator uses their own domain 128 in the RFC5321.MailFrom header field, but this domain will not match 129 the RFC5322.From address, thus the DMARC SPF result cannot be a 130 "pass." Any DKIM signature from the message submitter's domain will 131 be broken as the message body has been altered (and if included in 132 the signature, the RFC5322.Subject header field). Again, the DMARC 133 DKIM result cannot be a "pass." And if the mailing list operator 134 inserted an Authentication-Results header field it was most likely 135 stripped and/or replaced by the next message receiver. 137 If the mailing list implemented ARC, it would record the contents of 138 the Authentication-Results header field in the ARC-Authentication- 139 Results header field. It would then create an an ARC-Message- 140 Signature header field, which includes a cryptographic signature of 141 the message itself, and then an ARC-Seal header field, which includes 142 a cryptographic signature of a few key message header fields - 143 including the other ARC header fields. 145 Any subsequent system participating in ARC that was not performing 146 final delivery of the message within its ADMD boundaries would also 147 generate and insert ARC header fields whose signatures cover all ARC 148 header fields inserted into the message by previous message handlers. 149 Thus the information from any previous ARC participants, including 150 the ARC-Authentication-Results header field from the mailing list 151 operator, would be signed at each ADMD that handled the message. 153 When the message reaches the final receiving system, the SPF and DKIM 154 results will not satisfy the DMARC policy for the message author's 155 domain. However if the receiving system implements ARC then it can 156 check for and validate an ARC chain and verify that the contents of 157 the ARC-Authentication-Results header field were conveyed intact from 158 the mailing list operator. At that point the receiving system might 159 choose to use those authentication results in the decision of whether 160 or not to deliver the message, even though it failed to pass the 161 usual authentication checks. 163 3. Guidance for Receivers/Validators 165 3.1. What is the significance of an intact ARC chain? 167 An intact ARC chain conveys authentication results like SPF and DKIM 168 as observed by the first ARC participant. In cases where the message 169 no longer produces passing results for DKIM, SPF, or DMARC but an 170 intact ARC chain is present, the message receiver may choose to use 171 the contents of the ARC-Authentication-Results header field in 172 determining how to handle the message. 174 3.2. What exactly is an "intact" ARC chain? 176 Note that not all ADMDs will implement ARC, and receivers will see 177 messages where one or more non-participating ADMDs handled a message 178 before, after, or in between participating ADMDs. 180 An intact ARC chain is one where the ARC header fields that are 181 present can be validated, and in particular the ARC-Message-Signature 182 header field from the last ARC participant can still be validated. 183 This shows that, whether another ADMD handled the message after the 184 last ARC participant or not, the portions of the message covered by 185 that signature were not altered. If any non-participating ADMDs 186 handled the message between ARC intermediaries but did not alter the 187 message in a way that invalidated the most recent ARC-Message- 188 Signature present at that time, the chain would still be considered 189 intact by the next ARC participant, and recorded as such in the ARC- 190 Seal header field they insert. 192 Message receivers may make local policy decisions about whether to 193 use the contents of the ARC-Authentication-Results header field in 194 cases where a message no longer passes DKIM, DMARC, and/or SPF 195 checks. Whether an ARC chain is intact can be used to inform that 196 local policy decision. 198 So for example one message receiver may decide that, for messages 199 with an intact ARC chain where a DMARC evaluation does not pass, but 200 the ARC-Authentication-Results header field indicates a DKIM pass was 201 reported by the first ARC intermediary that matches the domain in the 202 RFC5322.From header field, it will override a DMARC "p=reject" 203 policy. Another message receiver may decide to do so for intact ARC 204 chains where the ARC-Authentication-Results header field indicates an 205 SPF pass. A third message receiver may use very different criteria, 206 according to their requirements, while a fourth may choose not to 207 take ARC information into account at all. 209 3.3. What is the significance of an invalid ("broken") ARC chain? 211 An ARC chain is not considered to be valid if the signatures in the 212 ARC-Seal header fields cannot be verified. For example the remote 213 server delivering the message to the local ADMD is not reflected in 214 any ARC header fields, perhaps because they have not implemented ARC, 215 but they modified the message such that ARC and DKIM signatures 216 already in the message were invalidated. 218 In such cases the ARC-Authentication-Results header field should not 219 have any influence on the disposition of the message. For example, a 220 message that fails under DMARC and has an invalid ARC chain would be 221 subject to that DMARC policy, which may cause it to be quarantined or 222 rejected. 224 3.4. What does the absence of an ARC chain in a message mean? 226 The absence of an ARC chain means nothing. ARC is intended to allow 227 a participating message handler to preserve certain authentication 228 results when a message is being forwarded and/or modified such that 229 the final recipient can evaluate this information. If they are 230 absent, there is nothing extra that ARC requires the final recipient 231 to do. 233 3.5. What reasonable conclusions can you draw based upon seeing lots of 234 mail with ARC chains? 236 With sufficient history, ARC can be used to augment DMARC 237 authentication policy (i.e. a message could fail DMARC, but validated 238 ARC information and therefore could be considered as validly 239 authenticated as reported by the first ARC participant). 241 If the validator does content analysis and reputation tracking, the 242 ARC participants in a message can be credited or discredited for good 243 or bad content. By analyzing different ARC chains involved in "bad" 244 messages, a validator might identify malicious participating 245 intermediaries. 247 With a valid chain and good reputations for all ARC participants, 248 receivers may choose to apply a "local policy override" to the DMARC 249 policy assertion for the domain authentication evaluation, depending 250 on the ARC-Authentication-Results header field value. Normal content 251 analysis should never be skipped. 253 3.6. What if none of the intermediaries have been seen previously? 255 This has no impact on the operation of ARC, as ARC is not a 256 reputation system. ARC conveys the results of other authentication 257 mechanisms such that the participating message handlers can be 258 positively identified. Final message recipients may or may not 259 choose to examine these results when messages fail other 260 authentication checks. They are more likely to override, say, a 261 failing DMARC result in the presence of an intact ARC chain where the 262 participating ARC message handlers have been observed to not convey 263 "bad" content in the past, and the initial ARC participant indicates 264 the message they received had passed authentication checks. 266 3.7. What about ARC chains where some intermediaries are known and 267 others are not? 269 Validators may choose to build reputation models for ARC message 270 handlers they have observed. Generally speaking it is more feasible 271 to accrue positive reputation to intermediaries when they 272 consistently send messages that are evaluated positively in terms of 273 content and ARC chains. When messages are received with ARC chains 274 that are not intact, it is very difficult identify which 275 intermediaries may have manipulated the message or injected bad 276 content. 278 3.8. What should message handlers do when they detect malicious content 279 in messages where ARC is present? 281 Message handlers should do what they normally do when they detect 282 malicious content in a message - hopefully that means quarantining or 283 discarding the message. ARC information should never make malicious 284 content acceptable. 286 In such cases it is difficult to determine where the malicious 287 content may have been injected. What ARC can do in such cases is 288 verify that a given intermediary or message handler did in fact 289 handle the message as indicated in the header fields. In such cases 290 a message recipient who maintains a reputation system about email 291 senders may wish to incorporate this information as an additional 292 factor in the score for the intermediaries and sender in question. 293 However reputation systems are very complex, and usually unique to 294 those organizations operating them, and therefore beyond the scope of 295 this document. 297 3.9. What feedback does a sender or domain owner get about ARC when it 298 is applied to their messages? 300 ARC itself does not include any mechanism for feedback or reporting. 301 It does however recommend that message receiving systems that use ARC 302 to augment their delivery decisions, who use DMARC and decide to 303 deliver a message because of ARC information, should include a 304 notation to that effect in their normal DMARC reports. These 305 notations would be easily identifiable by report processors, so that 306 senders and domain owners can see where ARC is being used to augment 307 the deliverability of their messages. 309 3.10. What prevents a malicious actor from removing the ARC header 310 fields, altering the content, and creating a new ARC chain? 312 ARC does not prevent a malicious actor from doing this. Nor does it 313 prevent a malicious actor from removing all but the first ADMD's ARC 314 header fields and altering the message, eliminating intervening 315 participants from the ARC chain. Or similar variations. 317 A valid ARC chain does not provide any automatic benefit. With an 318 intact ARC chain, the final message recipient may choose to use the 319 contents of the ARC-Authentication-Results header field in 320 determining how to handle the message. The decision to use the ARC- 321 Authentication-Results header field is dependent on evaluation of 322 those ARC intermediaries. 324 In the first case, the bad actor has succeeded in manipulating the 325 message but they have attached a verifiable signature identifying 326 themselves. While not an ideal situation, it is something they are 327 already able to do without ARC involved, but now a signature linked 328 to the domain responsible for the manipulation is present. 330 Additionally in the second case it is possible some negative 331 reputational impact might accrue to the first ARC participant left in 332 place until more messages reveal the pattern of activity by the bad 333 actor. But again, a bad actor can similarly manipulate a sequence of 334 RFC5322.Received header fields today without ARC, but with ARC that 335 bad actor has verifiably identified themselves. 337 4. Guidance for Intermediaries 339 4.1. What is an Intermediary under ARC? 341 In the context of ARC, an Intermediary is typically an Administrative 342 Management Domain [RFC5598] that is receiving a message, potentially 343 manipulating or altering it, and then passing it on to another ADMD 344 for delivery. Common examples of Intermediaries are mailing lists, 345 alumni or professional email address providers that forward messages 346 such as universities or professional organizations, et cetera. 348 4.2. What are the minimum requirements for an ARC Intermediary? 350 A participating ARC intermediary must validate the ARC chain on a 351 message it receives, if one is present. It then attaches its own ARC 352 seal and signature, including an indication if the chain failed to 353 validate upon receipt. 355 4.2.1. More specifically a participating ARC intermediary must do the 356 following: 358 1. Validate that the ARC chain, if one is already present in the 359 message, is intact and well-formed. 361 2. Validate that the most recent sender matches the last entry in 362 the ARC chain (if present). 364 3. Validate that the most recent sender's DKIM signature is 365 attached, and matches the reference to it in the ARC chain (if 366 present). 368 4. Generate a new ARC Signature and add it to the message according 369 to the ARC specification. 371 5. Generate a new ARC Seal and add it to the message according to 372 the ARC specification. 374 4.3. Should every MTA be an ARC participant? 376 Generally speaking, ARC is designed to operate at the ADMD level. 377 When a message is first received by an ADMD, the traditional 378 authentication results should be captured and preserved - this could 379 be the common case of creating an Authentication-Results header 380 field. But when it is determined that the message is being sent on 381 outside of that ADMD, that is when the ADMD should add itself to the 382 ARC chain - before sending the message outside of the ADMD. 384 Some organizations may operate multiple ADMDs, with more or less 385 independence between them. While they should make a determination 386 based on their specific circumstances, it may be useful and 387 appropriate to have one or both ADMDs be ARC participants. 389 4.4. What should an intermediary do in the case of an invalid or 390 "broken" ARC chain? 392 In general terms, a participating ARC intermediary will note that an 393 ARC chain was present and invalid, or broken, when it attaches its 394 own ARC seal and signature. However the fact that the ARC chain was 395 invalid should have no impact on whether and how the message is 396 delivered. 398 4.5. What should I do in the case where there is no ARC chain present 399 in a message? 401 A participating ARC intermediary receiving a message with no ARC 402 chain, and which will be delivered outside its ADMD, should start an 403 ARC chain according to the ARC specification. This will include 404 capturing the normal email authentication results for the 405 intermediary (SPF, DKIM, DMARC, etc), which will be conveyed as part 406 of the ARC chain. 408 4.6. How could ARC affect my reputation as an intermediary? 410 Message receivers often operate reputation systems, which build a 411 behavioral profile of various message handlers and intermediaries. 412 The presence or absence of ARC is yet another data point that may be 413 used as an input to such reputation systems. Messages deemed to have 414 good content may provide a positive signal for the intermediaries 415 that handled it, while messages with bad content may provide a 416 negative signal for the those intermediaries. Intact and valid ARC 417 elements may amplify or attenuate such signals, depending on the 418 circumstances. 420 Reputation systems are complex and usually specific to a given 421 message receiver, and a meaningful discussion of such a broad topic 422 is beyond the scope of this document. 424 4.7. What can I do to influence my reputation as an intermediary? 426 Today it is extremely simple for a malicious actor to construct a 427 message that includes your identity as an intermediary, even though 428 you never handled the message. It is possible that an intermediary 429 implementing ARC on all traffic it handles might receive some 430 reputational benefit by making it easier to detect when their 431 involvement in conveying bad traffic has been "forged." 432 As mentioned previously reputation systems are very complex and 433 usually specific to a given message receiver, and a meaningful 434 discussion of such a broad topic is beyond the scope of this 435 document. 437 5. Guidance for Originators 439 5.1. Where can I find out more information? 441 Please join the arc-discuss list at arc-discuss@dmarc.org 442 [1][mailto:arc-discuss@dmarc.org]. 444 To discuss the IETF spec itself, please join the dmarc working group 445 at [https://datatracker.ietf.org/wg/dmarc]. 447 5.2. How/where can I test interoperabililty for my implementation? 449 The arc-discuss list is the best place to stay in touch with work in 450 progress. 452 5.3. How can ARC impact my email? 454 Prior to ARC, certain DMARC policies on a domain would cause messages 455 using those domains in the RFC5322.From field, and which pass through 456 certain kinds of intermediaries (mailing lists, forwarding services), 457 to fail authentication checks at the message receiver. As a result 458 these messages might not be delivered to the intended recipient. 460 ARC seeks to provide these so-called "indirect mailflows" with a 461 means to preserve email authentication results as recorded by 462 participating intermediaries. Message receivers may accept validated 463 ARC information to supplement the information that DMARC provides, 464 potentially deciding to deliver the message even though a DMARC check 465 did not pass. 467 The net result for domain owners and senders is that ARC may allow 468 messages routed through participating ARC intermediaries to be 469 delivered, even though those messages would not have been delivered 470 in the absence of ARC. 472 5.4. How can ARC impact my reputation as a message sender? 474 Message receivers often operate reputation systems, which build a 475 behavioral profile of various message senders (and perhaps 476 intermediaries). The presence or absence of ARC is yet another data 477 point that may be used as an input to such reputation systems. 478 Messages deemed to have good content may provide a positive signal 479 for the sending domain and the intermediaries that handled it, while 480 messages with bad content may provide a negative signal for the 481 sending domain and the intermediaries that handled it. Intact and 482 valid ARC elements may amplify or attenuate such signals, depending 483 on the circumstances. 485 Reputation systems are complex and usually specific to a given 486 message receiver, and a meaningful discussion of such a broad topic 487 is beyond the scope of this document. 489 5.5. Can I tell intermediaries not to use ARC? 491 At present there is no way for a message sender to request that 492 intermediaries not employ ARC. 494 6. References 496 6.1. Normative References 498 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 499 DOI 10.17487/RFC5321, October 2008, 500 . 502 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 503 DOI 10.17487/RFC5322, October 2008, 504 . 506 [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, 507 DOI 10.17487/RFC5598, July 2009, 508 . 510 [RFC7601] Kucherawy, M., "Message Header Field for Indicating 511 Message Authentication Status", RFC 7601, 512 DOI 10.17487/RFC7601, August 2015, 513 . 515 6.2. Informative References 517 [ARC] Andersen, K., Rae-Grant, J., Long, B., Adams, T., and S. 518 Jones, "Authenticated Received Chain (ARC) Protocol", June 519 2017, . 522 [ENHANCED-STATUS] 523 "IANA SMTP Enhanced Status Codes", n.d., 524 . 527 [OAR] Chew, M. and M. Kucherawy, "Original-Authentication- 528 Results Header Field", February 2012, 529 . 532 [RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based 533 Message Authentication, Reporting, and Conformance 534 (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015, 535 . 537 [RFC7960] Martin, F., Ed., Lear, E., Ed., Draegen. Ed., T., Zwicky, 538 E., Ed., and K. Andersen, Ed., "Interoperability Issues 539 between Domain-based Message Authentication, Reporting, 540 and Conformance (DMARC) and Indirect Email Flows", 541 RFC 7960, DOI 10.17487/RFC7960, September 2016, 542 . 544 6.3. URIs 546 [1] mailto:arc-discuss@dmarc.org 548 Appendix A. GLOSSARY 550 ADMD Administrative Management Domain as used in [RFC5598] and 551 similar references refers to a single entity operating one or more 552 computers within one or more domain names under said entity's 553 control. One example might be a small company with a single 554 server, handling email for that company's domain. Another example 555 might be a large university, operating many servers that fulfill 556 different roles, all handling email for several different domains 557 representing parts of the university. 559 ARC ARC is an acronym: Authentication Results Chain - see also [ARC] 561 ARC-Seal An [RFC5322] message header field formed in compliance with 562 the ARC specification. It includes certain content from all prior 563 ARC participants, if there are any. 565 ARC-Message-Signature (also abbreviated as "AMS") An [RFC5322] 566 message header field formed in compliance with the [ARC] 567 specification. It includes certain content about the message as 568 it was received and manipulated by the intermediary who inserted 569 it. 571 ARC-Authentication-Results (also abbreviated as "AAR") An [RFC5322] 572 message header field formed in compliance with the [ARC] 573 specification. It includes certain content about the message as 574 it was received by the intermediary. 576 Authentication Results Chain (ARC) A system that allows a Message 577 Receiver to identify Intermediaries or Message Handlers who have 578 conveyed a particular message. For more information see the 579 Abstract of this document, or refer to [ARC]. 581 Domain Naming System Block List (DNSBL) This is a system widely used 582 in email filtering services whereby information about the past 583 activity of a set of hosts or domains indicates that messages 584 should not be accepted from them, or at least should be subject to 585 greater scrutiny before being accepted. Common examples would be 586 SpamCop, Spamhaus.org, SORBS, etc. 588 Email Service Provider (ESP) An Email Service Provider is typically 589 a vendor or partner firm that sends mail on behalf of another 590 company. They may use email addresses in Internet domains 591 belonging to the client or partner firm in various [RFC5321] 592 fields or [RFC5322] message header fields of the messages they 593 send on their behalf. 595 Intermediary In the context of [ARC], an Intermediary is typically 596 an Administrative Management Domain (per [RFC5598]) that is 597 receiving a message, potentially manipulating or altering it, and 598 then passing it on to another ADMD for delivery. Also see 599 [RFC7960] for more information and discussion. Common examples of 600 Intermediaries are mailing lists, alumni or professional email 601 address providers like universities or professional organizations, 602 et cetera. 604 Mail/Message Transfer Agent (MTA) This refers to software that sends 605 and receives email messsages across a network with other MTAs. 606 Often run on dedicated servers, common examples are Exim, 607 Microsoft Exchange, Postfix, and Sendmail. 609 Mailflow A group of messages that share features in common. Typical 610 examples would be all messages sent by a given Message Sender to a 611 Message Receiver, related to a particular announcement, a given 612 mailing list, et cetera. 614 Malicious Actor A Malicious Actor is a party, often an Intermediary, 615 that will take actions that seek to exploit or defraud the 616 ultimate recipient of the message, or subvert the network controls 617 and infrastructure of the Message Receiver. Typical examples 618 would be a spammer who forges content or attributes of a message 619 in order to evade anti-spam measures, or an entity that adds an 620 attachment containing a virus to a message. 622 Message Handler A Message Handler is another name for an 623 Intermediary. 625 Message Receiver In the transmission of an email message from one 626 ADMD to another, this is the organization receiving the message on 627 behalf of the intended recipient or end user. The Message 628 Receiver may do this because the intended recipient is an employee 629 or member of the organization, or because the end user utilizes 630 email services provided by the Message Receiver (Comcast, GMail, 631 Yahoo, QQ, et cetera). 633 Message Sender In the transmission of an email message from one ADMD 634 to another, this is the organization sending the message on behalf 635 of the Originator or end user. 637 Originator This refers to the author of a given email message. In 638 different contexts it may refer to the end-user writing the 639 message, or the ADMD providing email services to that end-user. 641 Reputation In the larger context of email hygiene - blocking spam 642 and malicious messages - reputation generally refers to a wide 643 variety of techniques and mechanisms whereby a message receiver 644 uses the past actions of a sending host or domain to influence the 645 handling of messages received from them in the future. One of the 646 classic examples would be a Spamhaus-style DNSBL, where individual 647 IP addresses will be blocked from sending messages because they've 648 been identified as being bad actors. Very large message receivers 649 may build and maintain their own reputation systems of this kind, 650 whereas other organizations might choose to use commercial 651 products or free services. 653 Reputation Service Provider A Reputation Service Provider would be a 654 source of reputation information about a message sender. In this 655 context, the DNSBL services offered by Spamhaus would allow them 656 to be referred to as an RPS. Many spam and virus filtering 657 vendors incorporate similar functionality into their services. 659 Request For Comment (RFC) RFCs are memoranda that "contain technical 660 and organizational notes about the Internet." Created and managed 661 by the Internet Engineering Task Force (IETF), they are de facto 662 standards for various methods of communicating or collaborating 663 over the Internet. 665 RFC5321 - Simple Mail Transfer Protocol This document describes the 666 protocol used to transfer email messages between Message Transfer 667 Agents (MTA) over a network. Link: [RFC5321] 669 RFC5322 - Internet Message Format This document describes the format 670 of Internet email messages, including both the header fields 671 within the message and various types of content within the message 672 body. Link: [RFC5322] 674 Validator A Message Receiver that attempts to validate the ARC chain 675 in a message. 677 Appendix B. References 679 Appendix C. Acknowledgements 681 This draft is the work of OAR-Dev Group. 683 The authors thanks the entire OAR-Dev group for the ongoing help, 684 innumerable diagrams and discussions from all the participants, 685 especially: Alex Brotman, Brandon Long, Dave Crocker, Elizabeth 686 Zwicky, Franck Martin, Greg Colburn, J. Trent Adams, John Rae-Grant, 687 Mike Hammer, Mike Jones, Steve Jones, Terry Zink, Tim Draegen. 689 Appendix D. Comments and Feedback 691 Please address all comments, discussions, and questions to the dmarc 692 working group at [https://datatracker.ietf.org/wg/dmarc]. 694 Authors' Addresses 696 Steven Jones 697 DMARC.org 699 Email: smj@crash.com 701 John Rae-Grant 702 Google 704 Email: johnrg@google.com 706 J. Trent Adams 707 Paypal 709 Email: trent.adams@paypal.com 711 Kurt Andersen (editor) 712 LinkedIn 713 2029 Stierlin Ct. 714 Mountain View, California 94043 715 USA 717 Email: kurta@linkedin.com