idnits 2.17.1 draft-ietf-dmarc-arc-usage-01.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- ** The document seems to lack a Security Considerations section. ** The document seems to lack an IANA Considerations section. (See Section 2.2 of https://www.ietf.org/id-info/checklist for how to handle the case when there are no actions for IANA.) ** There are 7 instances of too long lines in the document, the longest one being 30 characters in excess of 72. -- The draft header indicates that this document obsoletes draft-jones-arc-usage-01, but the abstract doesn't seem to mention this, which it should. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- No information found for rfcdraft-jones-arc-usage-01 - is the name correct? -- The document date (December 27, 2016) is 2667 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- -- Looks like a reference, but probably isn't: '1' on line 540 -- Looks like a reference, but probably isn't: '2' on line 688 == Missing Reference: 'DMARC-INTEROP' is mentioned on line 595, but not defined == Unused Reference: 'RFC7601' is defined on line 504, but no explicit reference was found in the text == Unused Reference: 'RFC7960' is defined on line 516, but no explicit reference was found in the text == Unused Reference: 'ENHANCED-STATUS' is defined on line 523, but no explicit reference was found in the text == Unused Reference: 'OAR' is defined on line 528, but no explicit reference was found in the text ** Obsolete normative reference: RFC 7601 (Obsoleted by RFC 8601) Summary: 4 errors (**), 0 flaws (~~), 6 warnings (==), 5 comments (--). 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 Obsoletes: draft-jones-arc-usage-01 (if J. Rae-Grant 5 approved) Google 6 Intended status: Informational T. Adams 7 Expires: May 29, 2016 Paypal 8 K. Andersen, Ed. 9 LinkedIn 10 December 27, 2016 12 Recommended Usage of the Authenticated Received Chain (ARC) 13 draft-ietf-dmarc-arc-usage-01 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 May 29, 2017. 41 Copyright Notice 43 Copyright (c) 2016 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, . . . . . . . . . . . . . . . . . . . . . 7 78 4. Guidance for Intermediaries . . . . . . . . . . . . . . . . . 8 79 4.1. What is an Intermediary under ARC? . . . . . . . . . . . 8 80 4.2. What are the minimum requirements for an ARC 81 Intermediary? . . . . . . . . . . . . . . . . . . . . . . 8 82 4.2.1. More specifically a participating ARC intermediary 83 must do the following: . . . . . . . . . . . . . . . 8 84 4.3. Should every MTA be an ARC participant? . . . . . . . . . 8 85 4.4. What should an intermediary do in the case of an invalid 86 or "broken" ARC chain? . . . . . . . . . . . . . . . . . 9 87 4.5. What should I do in the case where there is no ARC chain 88 present in a message? . . . . . . . . . . . . . . . . . . 9 89 4.6. How could ARC affect my reputation as an intermediary? . 9 90 4.7. What can I do to influence my reputation as an 91 intermediary? . . . . . . . . . . . . . . . . . . . . . . 9 92 5. Guidance for Originators . . . . . . . . . . . . . . . . . . 10 93 5.1. Where can I find out more information? . . . . . . . . . 10 94 5.2. How/where can I test interoperabililty for my 95 implementation? . . . . . . . . . . . . . . . . . . . . . 10 96 5.3. How can ARC impact my email? . . . . . . . . . . . . . . 10 97 5.4. How can ARC impact my reputation as a message sender? . . 10 98 5.5. Can I tell intermediaries not to use ARC? . . . . . . . . 11 99 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 100 6.1. Normative References . . . . . . . . . . . . . . . . . . 11 101 6.2. Informative References . . . . . . . . . . . . . . . . . 11 102 6.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 12 103 Appendix A. GLOSSARY . . . . . . . . . . . . . . . . . . . . . . 12 104 Appendix B. References . . . . . . . . . . . . . . . . . . . . . 15 105 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 15 106 Appendix D. Comments and Feedback . . . . . . . . . . . . . . . 15 107 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 109 1. Introduction 111 [ARC] is intended to be used primarily by intermediaries, or message 112 handlers - those parties who may forward or resend messages, with or 113 without alterations, such that they will no longer pass the SPF, 114 DKIM, and/or [RFC7489] authentication mechanisms. In such cases ARC 115 may provide the final message recipient with useful information about 116 the original sender. 118 2. How does ARC work? 120 Consider a mailing list as an example, where the message submitter's 121 domain publishes a DMARC policy other than "p=none". The message is 122 received, a prefix is added to the RFC5322.Subject header field, some 123 text is appended to the message body, and the message is sent to list 124 members with the original RFC5322.From address intact. In this case 125 SPF may pass because the mailing list operator uses their own domain 126 in the RFC5321.MailFrom header field, but this domain will not match 127 the RFC5322.From address, thus the DMARC SPF result cannot be a 128 "pass." Any DKIM signature from the message submitter's domain will 129 be broken as the message body has been altered (and if included in 130 the signature, the RFC5322.Subject header field). Again, the DMARC 131 DKIM result cannot be a "pass." And if the mailing list operator 132 inserted an Authentication-Results header field it was most likely 133 stripped and/or replaced by the next message receiver. 135 If the mailing list implemented ARC, it would record the contents of 136 the Authentication-Results header field in the ARC-Authentication- 137 Results header field. It would then create an an ARC-Message- 138 Signature header field, which includes a cryptographic signature of 139 the message itself, and then an ARC-Seal header field, which includes 140 a cryptographic signature of a few key message header fields - 141 including the other ARC header fields. 143 Any subsequent system participating in ARC that was not performing 144 final delivery of the message within its ADMD boundaries would also 145 generate and insert ARC header fields whose signatures cover all ARC 146 header fields inserted into the message by previous message handlers. 147 Thus the information from any previous ARC participants, including 148 the ARC-Authentication-Results header field from the mailing list 149 operator, would be signed at each ADMD that handled the message. 151 When the message reaches the final receiving system, the SPF and DKIM 152 results will not satisfy the DMARC policy for the message author's 153 domain. However if the receiving system implements ARC then it can 154 check for and validate an ARC chain and verify that the contents of 155 the ARC-Authentication-Results header field were conveyed intact from 156 the mailing list operator. At that point the receiving system might 157 choose to use those authentication results in the decision of whether 158 or not to deliver the message, even though it failed to pass the 159 usual authentication checks. 161 3. Guidance for Receivers/Validators 163 3.1. What is the significance of an intact ARC chain? 165 An intact ARC chain conveys authentication results like SPF and DKIM 166 as observed by the first ARC participant. In cases where the message 167 no longer produces passing results for DKIM, SPF, or DMARC but an 168 intact ARC chain is present, the message receiver may choose to use 169 the contents of the ARC-Authentication-Results header field in 170 determining how to handle the message. 172 3.2. What exactly is an "intact" ARC chain? 174 Note that not all ADMDs will implement ARC, and receivers will see 175 messages where one or more non-participating ADMDs handled a message 176 before, after, or in between participating ADMDs. 178 An intact ARC chain is one where the ARC header fields that are 179 present can be validated, and in particular the ARC-Message-Signature 180 header field from the last ARC participant can still be validated. 181 This shows that, whether another ADMD handled the message after the 182 last ARC participant or not, the portions of the message covered by 183 that signature were not altered. If any non-participating ADMDs 184 handled the message between ARC intermediaries but did not alter the 185 message in a way that invalidated the most recent ARC-Message- 186 Signature present at that time, the chain would still be considered 187 intact by the next ARC participant, and recorded as such in the ARC- 188 Seal header field they insert. 190 Message receivers may make local policy decisions about whether to 191 use the contents of the ARC-Authentication-Results header field in 192 cases where a message no longer passes DKIM, DMARC, and/or SPF 193 checks. Whether an ARC chain is intact can be used to inform that 194 local policy decision. 196 So for example one message receiver may decide that, for messages 197 with an intact ARC chain where a DMARC evaluation does not pass, but 198 the ARC-Authentication-Results header field indicates a DKIM pass was 199 reported by the first ARC intermediary that matches the domain in the RFC5322.From header field, it 200 will override a DMARC "p=reject" policy. Another message receiver 201 may decide to do so for intact ARC chains where the ARC- 202 Authentication-Results header field indicates an SPF pass. A third 203 message receiver may use very different criteria, according to their 204 requirements, while a fourth may choose not to take ARC information 205 into account at all. 207 3.3. What is the significance of an invalid ("broken") ARC chain? 209 An ARC chain is not considered to be valid if the signatures in the 210 ARC-Seal header fields cannot be verified. For example the remote 211 server delivering the message to the local ADMD is not reflected in 212 any ARC header fields, perhaps because they have not implemented ARC, 213 but they modified the message such that ARC and DKIM signatures 214 already in the message were invalidated. 216 In such cases the ARC-Authentication-Results header field should not 217 have any influence on the disposition of the message. For example, a 218 message that fails under DMARC and has an invalid ARC chain would be 219 subject to that DMARC policy, which may cause it to be quarantined or 220 rejected. 222 3.4. What does the absence of an ARC chain in a message mean? 224 The absence of an ARC chain means nothing. ARC is intended to allow 225 a participating message handler to preserve certain authentication 226 results when a message is being forwarded and/or modified such that 227 the final recipient can evaluate this information. If they are absent, 228 there is nothing extra that ARC requires the final recipient to do. 230 3.5. What reasonable conclusions can you draw based upon seeing lots of 231 mail with ARC chains? 233 With sufficient history, ARC can be used to augment DMARC 234 authentication policy (i.e. a message could fail DMARC, but validated ARC 235 information could be considered to show the message was authenticated as 236 reported by the first ARC participant). 238 If the validator does content analysis and reputation tracking, the 239 ARC participants in a message can be credited or discredited for good 240 or bad content. By analyzing different ARC chains involved in "bad" 241 messages, a validator might identify malicious participating 242 intermediaries. 244 With a valid chain and good reputations for all ARC participants, 245 receivers may choose to apply a "local policy override" to the DMARC 246 policy assertion for the domain authentication evaluation, depending 247 on the ARC-Authentication-Results header field value. Normal content 248 analysis should never be skipped. 250 3.6. What if none of the intermediaries have been seen previously? 252 This has no impact on the operation of ARC, as ARC is not a 253 reputation system. ARC conveys the results of other authentication 254 mechanisms such that the participating message handlers can be 255 positively identified. Final message recipients may or may not 256 choose to examine these results when messages fail other 257 authentication checks. They are more likely to override, say, a 258 failing DMARC result in the presence of an intact ARC chain where the 259 participating ARC message handlers have been observed to not convey 260 "bad" content in the past, and the initial ARC participant indicates 261 the message they received had passed authentication checks. 263 3.7. What about ARC chains where some intermediaries are known and 264 others are not? 266 Validators may choose to build reputation models for ARC message 267 handlers they have observed. Generally speaking it is more feasible 268 to accrue positive reputation to intermediaries when they 269 consistently send messages that are evaluated positively in terms of 270 content and ARC chains. When messages are received with ARC chains 271 that are not intact, it is very difficult identify which 272 intermediaries may have manipulated the message or injected bad 273 content. 275 3.8. What should message handlers do when they detect malicious content 276 in messages where ARC is present? 278 Message handlers should do what they normally do when they detect 279 malicious content in a message - hopefully that means quarantining or 280 discarding the message. ARC information should never make malicious 281 content acceptable. 283 In such cases it is difficult to determine where the malicious 284 content may have been injected. What ARC can do in such cases is 285 verify that a given intermediary or message handler did in fact 286 handle the message as indicated in the header fields. In such cases 287 a message recipient who maintains a reputation system about email 288 senders may wish to incorporate this information as an additional 289 factor in the score for the intermediaries and sender in question. 290 However reputation systems are very complex, and usually unique to 291 those organizations operating them, and therefore beyond the scope of 292 this document. 294 3.9. What feedback does a sender or domain owner get about ARC when it 295 is applied to their messages? 297 ARC itself does not include any mechanism for feedback or reporting. 298 It does however recommend that message receiving systems that use ARC 299 to augment their delivery decisions, who use DMARC and decide to 300 deliver a message because of ARC information, should include a 301 notation to that effect in their normal DMARC reports. These 302 notations would be easily identifiable by report processors, so that 303 senders and domain owners can see where ARC is being used to augment 304 the deliverability of their messages. 306 3.10. What prevents a malicious actor from removing the ARC header 307 fields, altering the content, and creating a new ARC chain? 309 ARC does not prevent a malicious actor from doing this. Nor does it 310 prevent a malicious actor from removing all but the first ADMD's ARC 311 header fields and altering the message, eliminating intervening 312 participants from the ARC chain. Or similar variations. 314 A valid ARC chain does not provide any automatic benefit. With an 315 intact ARC chain, the final message recipient may choose to use the 316 contents of the ARC-Authentication-Results header field in 317 determining how to handle the message. The decision to use the ARC- 318 Authentication-Results header field is dependent on evaluation of 319 those ARC intermediaries. 321 In the first case, the bad actor has succeeded in manipulating the 322 message but they have attached a verifiable signature identifying 323 themselves. While not an ideal situation, it is something they are 324 already able to do without ARC involved, but now a signature linked to the 325 domain responsible for the manipulation is present. 327 Additionally in the second case it is possible some negative 328 reputational impact might accrue to the first ARC participant left in 329 place until more messages reveal the pattern of activity by the bad 330 actor. But again, a bad actor can similarly manipulate a sequence of 331 RFC5322.Received header fields today without ARC, but with ARC that 332 bad actor has verifiably identified themselves. 334 4. Guidance for Intermediaries 336 4.1. What is an Intermediary under ARC? 338 In the context of ARC, an Intermediary is typically an Administrative 339 Management Domain [RFC5598] that is receiving a message, potentially 340 manipulating or altering it, and then passing it on to another ADMD 341 for delivery. Common examples of Intermediaries are mailing lists, 342 alumni or professional email address providers that forward messages 343 such as universities or professional organizations, et cetera. 345 4.2. What are the minimum requirements for an ARC Intermediary? 347 A participating ARC intermediary must validate the ARC chain on a 348 message it receives, if one is present. It then attaches its own ARC 349 seal and signature, including an indication if the chain failed to 350 validate upon receipt. 352 4.2.1. More specifically a participating ARC intermediary must do the 353 following: 355 1. Validate that the ARC chain, if one is already present in the 356 message, is intact and well-formed. 358 2. Validate that the most recent sender matches the last entry in 359 the ARC chain (if present). 361 3. Validate that the most recent sender's DKIM signature is 362 attached, and matches the reference to it in the ARC chain (if 363 present). 365 4. Generate a new ARC Signature and add it to the message according 366 to the ARC specification. 368 5. Generate a new ARC Seal and add it to the message according to 369 the ARC specification. 371 4.3. Should every MTA be an ARC participant? 373 Generally speaking, ARC is designed to operate at the ADMD level. 374 When a message is first received by an ADMD, the traditional 375 authentication results should be captured and preserved - this could 376 be the common case of creating an Authentication-Results header 377 field. But when it is determined that the message is being sent on 378 outside of that ADMD, that is when the ADMD should add itself to the 379 ARC chain - before sending the message outside of the ADMD. 381 Some organizations may operate multiple ADMDs, with more or less 382 independence between them. While they should make a determination 383 based on their specific circumstances, it may be useful and 384 appropriate to have one or both ADMDs be ARC participants. 386 4.4. What should an intermediary do in the case of an invalid or 387 "broken" ARC chain? 389 In general terms, a participating ARC intermediary will note that an 390 ARC chain was present and invalid, or broken, when it attaches its 391 own ARC seal and signature. However the fact that the ARC chain was 392 invalid should have no impact on whether and how the message is 393 delivered. 395 4.5. What should I do in the case where there is no ARC chain present 396 in a message? 398 A participating ARC intermediary receiving a message with no ARC 399 chain, and which will be delivered outside its ADMD, should start an 400 ARC chain according to the ARC specification. This will include 401 capturing the normal email authentication results for the 402 intermediary (SPF, DKIM, DMARC, etc), which will be conveyed as part 403 of the ARC chain. 405 4.6. How could ARC affect my reputation as an intermediary? 407 Message receivers often operate reputation systems, which build a 408 behavioral profile of various message handlers and intermediaries. 409 The presence or absence of ARC is yet another data point that may be 410 used as an input to such reputation systems. Messages deemed to have 411 good content may provide a positive signal for the intermediaries 412 that handled it, while messages with bad content may provide a 413 negative signal for the those intermediaries. Intact and valid ARC 414 elements may amplify or attenuate such signals, depending on the 415 circumstances. 417 Reputation systems are complex and usually specific to a given 418 message receiver, and a meaningful discussion of such a broad topic 419 is beyond the scope of this document. 421 4.7. What can I do to influence my reputation as an intermediary? 423 Today it is extremely simple for a malicious actor to construct a 424 message that includes your identity as an intermediary, even though 425 you never handled the message. It is possible that an intermediary 426 implementing ARC on all traffic it handles might receive some 427 reputational benefit by making it easier to detect when their 428 involvement in conveying bad traffic has been "forged." 429 As mentioned previously reputation systems are very complex and 430 usually specific to a given message receiver, and a meaningful 431 discussion of such a broad topic is beyond the scope of this 432 document. 434 5. Guidance for Originators 436 5.1. Where can I find out more information? 438 Please join the arc-discuss list at arc-discuss@dmarc.org 439 [1][mailto:arc-discuss@dmarc.org]. 441 5.2. How/where can I test interoperabililty for my implementation? 443 The arc-discuss list is the best place to stay in touch with work in 444 progress. 446 5.3. How can ARC impact my email? 448 Prior to ARC, certain DMARC policies on a domain would cause messages 449 using those domains in the RFC5322.From field, and which pass through 450 certain kinds of intermediaries (mailing lists, forwarding services), 451 to fail authentication checks at the message receiver. As a result 452 these messages might not be delivered to the intended recipient. 454 ARC seeks to provide these so-called "indirect mailflows" with a 455 means to preserve email authentication results as recorded by 456 participating intermediaries. Message receivers may accept validated ARC 457 information to supplement the information that DMARC provides, 458 potentially deciding to deliver the message even though a DMARC check 459 did not pass. 461 The net result for domain owners and senders is that ARC may allow 462 messages routed through participating ARC intermediaries to be 463 delivered, even though those messages would not have been delivered 464 in the absence of ARC. 466 5.4. How can ARC impact my reputation as a message sender? 468 Message receivers often operate reputation systems, which build a 469 behavioral profile of various message senders (and perhaps 470 intermediaries). The presence or absence of ARC is yet another data 471 point that may be used as an input to such reputation systems. 472 Messages deemed to have good content may provide a positive signal 473 for the sending domain and the intermediaries that handled it, while 474 messages with bad content may provide a negative signal for the 475 sending domain and the intermediaries that handled it. Intact and 476 valid ARC elements may amplify or attenuate such signals, depending 477 on the circumstances. 479 Reputation systems are complex and usually specific to a given 480 message receiver, and a meaningful discussion of such a broad topic 481 is beyond the scope of this document. 483 5.5. Can I tell intermediaries not to use ARC? 485 At present there is no way for a message sender to request that 486 intermediaries not employ ARC. 488 6. References 490 6.1. Normative References 492 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 493 DOI 10.17487/RFC5321, October 2008, 494 . 496 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 497 DOI 10.17487/RFC5322, October 2008, 498 . 500 [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, 501 DOI 10.17487/RFC5598, July 2009, 502 . 504 [RFC7601] Kucherawy, M., "Message Header Field for Indicating 505 Message Authentication Status", RFC 7601, 506 DOI 10.17487/RFC7601, August 2015, 507 . 509 6.2. Informative References 511 [ARC] Andersen, K., Rae-Grant, J., Long, B., Adams, T., and S. 512 Jones, "Authenticated Received Chain (ARC) Protocol", December 513 2016, . 516 [RFC7960] 517 Martin, F., Lear, E., Draegen, T., Zwicky, E., and K. 518 Andersen, "Interoperability Issues Between DMARC and 519 Indirect Email Flows", June 2016, 520 . 523 [ENHANCED-STATUS] 524 "IANA SMTP Enhanced Status Codes", n.d., 525 . 528 [OAR] Chew, M. and M. Kucherawy, "Original-Authentication- 529 Results Header Field", February 2012, 530 . 533 [RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based 534 Message Authentication, Reporting, and Conformance 535 (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015, 536 . 538 6.3. URIs 540 [1] mailto:arc-discuss@dmarc.org 542 [2] mailto:arc-discuss@dmarc.org 544 Appendix A. GLOSSARY 546 ADMD Administrative Management Domain as used in [RFC5598] and 547 similar references refers to a single entity operating one or more 548 computers within one or more domain names under said entity's 549 control. One example might be a small company with a single 550 server, handling email for that company's domain. Another example 551 might be a large university, operating many servers that fulfill 552 different roles, all handling email for several different domains 553 representing parts of the university. 555 ARC ARC is an acronym: Authentication Results Chain - see also [ARC] 557 ARC-Seal An [RFC5322] message header field formed in compliance with 558 the ARC specification. It includes certain content from all prior 559 ARC participants, if there are any. 561 ARC-Message-Signature (also abbreviated as "AMS") An [RFC5322] 562 message header field formed in compliance with the [ARC] 563 specification. It includes certain content about the message as 564 it was received and manipulated by the intermediary who inserted 565 it. 567 ARC-Authentication-Results (also abbreviated as "AAR") An [RFC5322] 568 message header field formed in compliance with the [ARC] 569 specification. It includes certain content about the message as 570 it was received by the intermediary. 572 Authentication Results Chain (ARC) A system that allows a Message 573 Receiver to identify Intermediaries or Message Handlers who have 574 conveyed a particular message. For more information see the 575 Abstract of this document, or refer to [ARC]. 577 Domain Naming System Block List (DNSBL) This is a system widely used 578 in email filtering services whereby information about the past 579 activity of a set of hosts or domains indicates that messages 580 should not be accepted from them, or at least should be subject to 581 greater scrutiny before being accepted. Common examples would be 582 SpamCop, Spamhaus.org, SORBS, etc. 584 Email Service Provider (ESP) An Email Service Provider is typically 585 a vendor or partner firm that sends mail on behalf of another 586 company. They may use email addresses in Internet domains 587 belonging to the client or partner firm in various [RFC5321] 588 fields or [RFC5322] message header fields of the messages they 589 send on their behalf. 591 Intermediary In the context of [ARC], an Intermediary is typically 592 an Administrative Management Domain (per [RFC5598]) that is 593 receiving a message, potentially manipulating or altering it, and 594 then passing it on to another ADMD for delivery. Also see 595 [DMARC-INTEROP] for more information and discussion. Common 596 examples of Intermediaries are mailing lists, alumni or 597 professional email address providers like universities or 598 professional organizations, et cetera. 600 Mail/Message Transfer Agent (MTA) This refers to software that sends 601 and receives email messsages across a network with other MTAs. 602 Often run on dedicated servers, common examples are Exim, 603 Microsoft Exchange, Postfix, and Sendmail. 605 Mailflow A group of messages that share features in common. Typical 606 examples would be all messages sent by a given Message Sender to a 607 Message Receiver, related to a particular announcement, a given 608 mailing list, et cetera. 610 Malicious Actor A Malicious Actor is a party, often an Intermediary, 611 that will take actions that seek to exploit or defraud the 612 ultimate recipient of the message, or subvert the network controls 613 and infrastructure of the Message Receiver. Typical examples 614 would be a spammer who forges content or attributes of a message 615 in order to evade anti-spam measures, or an entity that adds an 616 attachment containing a virus to a message. 618 Message Handler A Message Handler is another name for an 619 Intermediary. 621 Message Receiver In the transmission of an email message from one 622 ADMD to another, this is the organization receiving the message on 623 behalf of the intended recipient or end user. The Message 624 Receiver may do this because the intended recipient is an employee 625 or member of the organization, or because the end user utilizes 626 email services provided by the Message Receiver (Comcast, GMail, 627 Yahoo, QQ, et cetera). 629 Message Sender In the transmission of an email message from one ADMD 630 to another, this is the organization sending the message on behalf 631 of the Originator or end user. 633 Originator This refers to the author of a given email message. In 634 different contexts it may refer to the end-user writing the 635 message, or the ADMD providing email services to that end-user. 637 Reputation In the larger context of email hygiene - blocking spam 638 and malicious messages - reputation generally refers to a wide 639 variety of techniques and mechanisms whereby a message receiver 640 uses the past actions of a sending host or domain to influence the 641 handling of messages received from them in the future. One of the 642 classic examples would be a Spamhaus-style DNSBL, where individual 643 IP addresses will be blocked from sending messages because they've 644 been identified as being bad actors. Very large message receivers 645 may build and maintain their own reputation systems of this kind, 646 whereas other organizations might choose to use commercial 647 products or free services. 649 Reputation Service Provider A Reputation Service Provider would be a 650 source of reputation information about a message sender. In this 651 context, the DNSBL services offered by Spamhaus would allow them 652 to be referred to as an RPS. Many spam and virus filtering 653 vendors incorporate similar functionality into their services. 655 Request For Comment (RFC) RFCs are memoranda that "contain technical 656 and organizational notes about the Internet." Created and managed 657 by the Internet Engineering Task Force (IETF), they are de facto 658 standards for various methods of communicating or collaborating 659 over the Internet. 661 RFC5321 - Simple Mail Transfer Protocol This document describes the 662 protocol used to transfer email messages between Message Transfer 663 Agents (MTA) over a network. Link: [RFC5321] 665 RFC5322 - Internet Message Format This document describes the format 666 of Internet email messages, including both the header fields 667 within the message and various types of content within the message 668 body. Link: [RFC5322] 670 Validator A Message Receiver that attempts to validate the ARC chain 671 in a message. 673 Appendix B. References 675 Appendix C. Acknowledgements 677 This draft is the work of OAR-Dev Group. 679 The authors thanks the entire OAR-Dev group for the ongoing help, 680 innumerable diagrams and discussions from all the participants, 681 especially: Alex Brotman, Brandon Long, Dave Crocker, Elizabeth 682 Zwicky, Franck Martin, Greg Colburn, J. Trent Adams, John Rae-Grant, 683 Mike Hammer, Mike Jones, Steve Jones, Terry Zink, Tim Draegen. 685 Appendix D. Comments and Feedback 687 Please address all comments, discussions, and questions to arc- 688 discuss@dmarc.org [2][mailto:arc-discuss@dmarc.org]. 690 Authors' Addresses 692 Steven Jones 693 DMARC.org 695 Email: smj@crash.com 697 John Rae-Grant 698 Google 700 Email: johnrg@google.com 702 J. Trent Adams 703 Paypal 705 Email: trent.adams@paypal.com 707 Kurt Andersen (editor) 708 LinkedIn 709 2029 Stierlin Ct. 710 Mountain View, California 94043 711 USA 713 Email: kurta@linkedin.com