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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 MARF Working Group J. Falk 3 Internet-Draft Return Path 4 Updates: 5965 (if approved) M. Kucherawy, Ed. 5 Intended status: Standards Track Cloudmark 6 Expires: October 22, 2012 April 20, 2012 8 Creation and Use of Email Feedback Reports: An Applicability Statement 9 for the Abuse Reporting Format (ARF) 10 draft-ietf-marf-as-14 12 Abstract 14 RFC 5965 defines an extensible, machine-readable format intended for 15 mail operators to report feedback about received email to other 16 parties. This Applicability Statement describes common methods for 17 utilizing this format for reporting both abuse and authentication 18 failure events. Mailbox Providers of any size, mail sending 19 entities, and end users can use these methods as a basis to create 20 procedures that best suit them. Some related optional mechanisms are 21 also discussed. 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 http://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 October 22, 2012. 40 Copyright Notice 42 Copyright (c) 2012 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 47 (http://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 58 1.1. Discussion . . . . . . . . . . . . . . . . . . . . . . . . 3 59 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 60 3. Solicited and Unsolicited Reports . . . . . . . . . . . . . . 4 61 4. Generating And Handling Solicited Abuse Reports . . . . . . . 4 62 4.1. General Considerations for Feedback Providers . . . . . . 4 63 4.2. Where To Send Reports . . . . . . . . . . . . . . . . . . 5 64 4.3. What To Put In Reports . . . . . . . . . . . . . . . . . . 5 65 4.4. General Considerations for Feedback Consumers . . . . . . 5 66 4.5. What To Expect . . . . . . . . . . . . . . . . . . . . . . 6 67 4.6. What To Do With Reports . . . . . . . . . . . . . . . . . 6 68 5. Generating and Handling Unsolicited Abuse Reports . . . . . . 6 69 5.1. General Considerations . . . . . . . . . . . . . . . . . . 6 70 5.2. When To Generate Reports . . . . . . . . . . . . . . . . . 6 71 5.3. Where To Send Reports . . . . . . . . . . . . . . . . . . 7 72 5.4. What To Put In Reports . . . . . . . . . . . . . . . . . . 8 73 5.5. What To Do With Reports . . . . . . . . . . . . . . . . . 9 74 6. Generating Automatic Authentication Failure Reports . . . . . 9 75 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 76 8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 77 8.1. In Other Documents . . . . . . . . . . . . . . . . . . . . 11 78 8.2. Forgeries . . . . . . . . . . . . . . . . . . . . . . . . 11 79 8.3. Amplification Attacks . . . . . . . . . . . . . . . . . . 11 80 8.4. Automatic Generation . . . . . . . . . . . . . . . . . . . 11 81 8.5. Reporting Multiple Incidents . . . . . . . . . . . . . . . 12 82 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13 83 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 84 10.1. Normative References . . . . . . . . . . . . . . . . . . . 13 85 10.2. Informative References . . . . . . . . . . . . . . . . . . 14 86 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15 88 1. Introduction 90 The Abuse Reporting Format (ARF) was initially developed for two very 91 specific use cases. Initially, it was intended to be used for 92 reporting feedback between large email operators, or from large email 93 operators to end user network access operators, any of whom could be 94 presumed to have automated abuse-handling systems. Secondarily, it 95 is used by those same large mail operators to send those same reports 96 to other entities, including those involved in sending bulk email for 97 commercial purposes. In either case, the reports would be triggered 98 by direct end user action such as clicking on a "report spam" button 99 in their email client. 101 Though other uses for the ARF format defined in [RFC5965] have been 102 discussed (and may be documented similarly in the future), abuse 103 remains the primary application, with a small amount of adoption of 104 extensions that enable authentication failure reporting. 106 This Applicability Statement provides direction for using the Abuse 107 Reporting Format (ARF) in both contexts. It also includes some 108 statements about the use of ARF in conjunction with other email 109 technologies. 111 The purpose for reporting abusive messages is to stop recurrences. 112 The methods described in this document focus on automating abuse 113 reporting as much as practical, so as to minimize the work of a 114 site's abuse team. There are further reasons why abuse feedback 115 generation is worthwhile, such as instruction of mail filters or 116 reputation trackers, or to initiate investigations of particularly 117 egregious abuses. These other applications are not discussed in this 118 memo. 120 Further introduction to this topic may be found in [RFC6449]. 122 1.1. Discussion 124 [RFC Editor: please remove this section prior to publication.] 126 This document is being discussed within the IETF MARF Working Group, 127 on the marf@ietf.org mailing list. 129 2. Definitions 131 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 132 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 133 document are to be interpreted as described in [RFC2119], and are 134 intended to replace the Requirement Levels described in Section 3.3 135 of [RFC2026]. 137 Some of the terminology used in this document is taken from 138 [RFC5598]. 140 "Mailbox Provider" refers to an organization that accepts, stores, 141 and offers access to [RFC5322] messages ("email messages") for end 142 users. Such an organization has typically implemented SMTP 143 ([RFC5321]), and might provide access to messages through IMAP 144 ([RFC3501]), POP ([RFC1939]), a proprietary interface designed for 145 HTTP ([RFC2616]), or a proprietary protocol. 147 3. Solicited and Unsolicited Reports 149 The original application of [RFC5965], and still by far the most 150 common, is when two mail systems make a private agreement to exchange 151 abuse reports, usually reports due to recipients manually reporting 152 messages as spam. We refer to these as solicited reports. 154 Other uses for ARF involve such reports sent between parties that 155 don't know each other. These unsolicited reports are sent without 156 prior arrangement between the parties as to the context and meaning 157 of the reports, so the constraints on how these unsolicited reports 158 need to be structured such that the reports generated are likely to 159 be useful to the recipient, to what address(es) they can usefully be 160 sent, what issues the can be used to report, and how they can be 161 handled by the receiver of the report are very different. 163 The two cases are covered separately in following sections. 165 4. Generating And Handling Solicited Abuse Reports 167 [The numbered items in these subsections are not intended to be in a 168 paricular sequence. The numbers are here during document development 169 to make it easier to identify the items for discussion, and will be 170 removed before publication.] 172 4.1. General Considerations for Feedback Providers 174 1. A Mailbox Provider receives reports of abusive or unwanted mail 175 from its users, most often by providing a "report spam" button 176 (or similar nomenclature) in the MUA (Mail User Agent). The 177 method of transferring this message and any associated metadata 178 from the MUA to the Mailbox Provider's ARF processing system is 179 not defined by any standards document, but is discussed further 180 in Section 3.2 of [RFC6449]. Policy concerns related to the 181 collection of this data are discussed in Section 3.4 of 182 [RFC6449]. 183 2. To implement the recommendations of this memo, the reports are 184 formatted per [RFC5965], and transmitted as an email message 185 ([RFC5322]), typically using SMTP ([RFC5321]). 186 3. Ongoing maintenance of an ARF processing system is discussed in 187 Section 3.6 of [RFC6449]. 189 4.2. Where To Send Reports 191 1. The Mailbox Provider SHOULD NOT send reports to addresses that 192 have not explicitly requested them. A valid deviation might be 193 the result of local policy instructions. The process whereby 194 such parties may request the reports is discussed in Section 3.5 195 of [RFC6449]. 197 4.3. What To Put In Reports 199 1. The reports SHOULD use "Feedback-Type: abuse", for its type. 200 Although a Mailbox Provider generating the reports can use other 201 types appropriate to the nature of the abuse being reported, the 202 operator receiving the reports might not treat different feedback 203 types differently. 204 2. The following fields are optional in [RFC5965], but SHOULD be 205 used in this context when their corresponding values are 206 available: Original-Mail-From, Arrival-Date, Source-IP, Original- 207 Rcpt-To. Other optional fields can be included, as the 208 implementer feels is appropriate. 209 3. User-identifiable data MAY be obscured as described in 210 [I-D.IETF-MARF-REDACTION]. 212 4.4. General Considerations for Feedback Consumers 214 1. ARF report streams are established proactively between Feedback 215 Providers and Feedback Consumers. Recommendations for preparing 216 to make that request are discussed in Section 4.1 of [RFC6449]. 217 2. Operators MUST be able to accept ARF [RFC5965] reports as email 218 messages [RFC5322] over SMTP [RFC5321]. These and other types of 219 email messages that can be received are discussed in Section 4.2 220 of [RFC6449]. 221 3. Recipients of feedback reports that are part of formal feedback 222 arrangements have to be capable of handling large volumes of 223 reports. This could require automation of report processing. 224 Discussion of this can be found in Section 4.4 of [RFC6449]. 226 4.5. What To Expect 228 1. An automated report processing system MUST accept all Feedback- 229 Types defined in [RFC5965] or extensions to it. However, Mailbox 230 Providers might only make use of the "abuse" Feedback-Type. 231 Therefore, report receivers might be required to do additional 232 analysis to separate different types of abuse reports after 233 receipt if they do not have prior specific knowledge of the 234 sender of the report. 235 2. Reports receivers MUST accept reports that have obscured their 236 user-identifiable data as described in [I-D.IETF-MARF-REDACTION]. 237 That document also discusses the handling of such reports. This 238 technique is also discussed in Section 4.4 of [RFC6449]. 240 4.6. What To Do With Reports 242 1. Section 4.3 of [RFC6449] discusses actions that mail operators 243 might take upon receiving a report (or multiple reports). 245 5. Generating and Handling Unsolicited Abuse Reports 247 [The numbered items in these subsections are not intended to be in a 248 paricular sequence. The numbers are here during document development 249 to make it easier to identify the items for discussion, and will be 250 removed before publication.] 252 5.1. General Considerations 254 1. It is essential for report recipients to be capable of throttling 255 reports being sent to avoid damage to their own installations. 256 Therefore, Feedback Providers MUST provide a way for report 257 recipients to request that no further reports be sent. 258 Unfortunately, no standardized mechanism for such requests exists 259 to date, and all existing mechanisms for meeting this requirement 260 are out-of-band. 261 2. Message authentication is generally a good idea, but it is 262 especially important to encourage credibility of and thus 263 response to unsolicited reports. Therefore, as with any other 264 message, Feedback Providers sending unsolicited reports SHOULD 265 send reports that will pass Sender Policy Framework ([RFC4408]) 266 and/or DomainKeys Identified Mail ([RFC6376]) checks. 268 5.2. When To Generate Reports 270 1. Handling of unsolicited reports has a significant cost to the 271 receiver. Senders of unsolicited reports, especially those 272 sending large volumes of them automatically SHOULD NOT send 273 reports that cannot be used as a basis for action by the 274 recipient, whether this is due to the report being sent about an 275 incident that is not abuse-related, the report being sent to an 276 email address that won't result in action, or the content or 277 format of the report being hard for the recipient to read or use. 278 2. Feedback Providers SHOULD NOT report all mail sent from a 279 particular sender merely because some of it is determined to be 280 abusive. 281 3. Mechanical reports of mail that "looks like" spam, based solely 282 on the results of inline content analysis tools, SHOULD NOT be 283 sent since, because of their subjective nature, they are unlikely 284 to provide a basis for the recipient to take action. Complaints 285 generated by end users about mail that is determined by them to 286 be abusive, or mail delivered to "spam trap" or "honeypot" 287 addresses, are far more likely to be accurate and MAY be sent. 288 4. If a Feedback Provider applies SPF to arriving messages, a report 289 SHOULD NOT be generated to the RFC5321.MailFrom domain if the SPF 290 evaluation produced a "Fail", "SoftFail", "TempError" or 291 "PermError" report, as no reliable assertion or assumption can be 292 made that use of the domain was authorized. A valid exception 293 would be specific knowledge that the SPF result is not definitive 294 for that domain under those circumstances (for example, a message 295 that is also DKIM-signed by the same domain, and that signature 296 validates). 298 5.3. Where To Send Reports 300 1. Rather than generating feedback reports themselves, MUAs SHOULD 301 make abuse reports back to their mailbox providers so that they 302 can generate and send ARF messages on behalf of end users (see 303 Section 3.2 of [RFC6449]). This allows centralized processing 304 and tracking of reports, and provides training input to filtering 305 systems. There is, however, no standard mechanism for this 306 signaling between MUAs and mailbox providers to trigger abuse 307 reports. 308 2. Feedback Providers SHOULD NOT send reports to recipients that are 309 uninvolved or only peripherally involved. For example, they 310 SHOULD NOT send reports to the operator of every Autonomous 311 System in the path between the apparent originating system and 312 the operator generating the report. Instead, they need to send 313 reports to recipients that are both responsible for the messages 314 and are able to do something about them. 315 3. Deciding where to send an unsolicited report will typically rely 316 on heuristics. Abuse addresses in WHOIS ([RFC3912]) records of 317 the IP address relaying the subject message and/or of the domain 318 name found in the results of a PTR ("reverse lookup") query on 319 that address are likely reasonable candidates, as is the 320 abuse@domain role address (see [RFC2142]) of related domains. 322 Unsolicited reports SHOULD NOT be sent to email addresses that 323 are not clearly intended to handle abuse reports. Legitimate 324 candidates include those found in WHOIS records or on a web site 325 that either are explicitly described as an abuse contact, or are 326 of the form "abuse@domain". 327 4. Where an abusive message is authenticated using a domain-level 328 authentication technology such as DKIM ([RFC6376]) or SPF 329 ([RFC4408]), the domain that has been verified by the 330 authentication mechanism is often a reasonable candidate for 331 receiving feedback about the message. For DKIM, though, while 332 the authenticated domain has some responsibility for the mail 333 sent, it can be a poor contact point for abuse issues (for 334 example, it could represent the message's author but not its 335 sender, it could identify the bad actor responsible for the 336 message, or it could refer to a domain that cannot receive mail 337 at all). 338 5. Often, unsolicited reports will have no meaning if sent to abuse 339 reporting addresses belonging to the abusive parties themselves. 340 In fact, it is possible that such reports might reveal 341 information about complainants. Reports SHOULD NOT be sent to 342 such addresses if they can be identified beforehand, except where 343 the abusive party is known to be responsive to such reports. 345 5.4. What To Put In Reports 347 1. Reports SHOULD use "Feedback-Type: abuse", but can use other 348 types as appropriate. However, the Mailbox Provider generating 349 the reports cannot assume that the operator receiving the reports 350 will treat different Feedback-Types differently. 351 2. Reports SHOULD include the following optional fields whenever 352 their corresponding values are available and applicable to the 353 report: Original-Mail-From, Arrival-Date, Source-IP, Original- 354 Rcpt-To. Other optional fields can be included, as the 355 implementer feels is appropriate. 356 3. Experience suggests use of ARF is advisable in most contexts. 357 Automated recipient systems can handle abuse reports sent in ARF 358 format at least as well as any other format such as plain text, 359 with or without a copy of the message attached. That holds even 360 for systems that did not request ARF format reports, assuming 361 such reports are generated considering the possibility of 362 recipients that don't use automated ARF parsing. Anyone sending 363 unsolicited reports in ARF format can legitimately presume that 364 some recipients will only be able to access the human readable 365 (first, text/plain) part of it, and SHOULD include all 366 information needed also in this part. Further, they SHOULD 367 ensure that the report is readable when viewed as plain text, to 368 give low-end ticketing systems as much assistance as possible. 369 In extreme cases, failure to take these steps may result in the 370 report being discarded or ignored. 372 5.5. What To Do With Reports 374 1. Receivers of unsolicited reports can take advantage of the 375 standardized parts of the ARF format to automate processing. 376 Independent of the sender of the report, they can improve 377 processing by separating valid from invalid reports by, for 378 example, looking for references to IP address ranges, domains, 379 and mailboxes for which the recipient organization is responsible 380 in the copy of the reported message, and by correlating multiple 381 reports of similar messages to identify bulk senders. 382 2. Per Section 4.4 of [RFC6449], a network service provider MAY use 383 ARF data for automated forwarding of feedback messages to the 384 originating customer. 385 3. Published abuse mailbox addresses SHOULD NOT reject non-ARF 386 messages based solely on the format, as generation of ARF 387 messages can occasionally be unavailable or not applicable. 388 Deviation from this requirement could be done due to local policy 389 decisions regarding other message criteria. 390 4. Although [RFC6449] suggests that replying to feedback is not 391 useful, in the case of receipt of ARF reports where no feedback 392 arrangement has been established, a non-automated reply might be 393 desirable to indicate what action resulted from the complaint, 394 heading off more severe filtering by the Feedback Provider. In 395 addition, using an address that cannot receive replies precludes 396 any requests for additional information, and increases the 397 likelihood that further reports will be discarded or blocked. 398 Thus, a Feedback Provider sending unsolicited reports SHOULD NOT 399 generate reports for which a reply cannot be received. Where an 400 unsolicited report results in the establishment of contact with a 401 responsible and responsive party, this can be saved for future 402 complaint handling and possible establishment of a formal 403 (solicited) feedback arrangement. See Section 3.5 of [RFC6449] 404 for a discussion of establishment of feedback arrangements. 406 6. Generating Automatic Authentication Failure Reports 408 [These numbered items are not intended to be in a paricular sequence. 409 The numbers are here during document development to make it easier to 410 identify the items for discussion, and will be removed before 411 publication.] 413 There are some cases where report generation is caused by automation 414 rather than user request. A specific example of this is reporting, 415 using the ARF format (or extensions to it), of messages that fail 416 particular message authentication checks. Examples of this include 418 [I-D.IETF-MARF-DKIM-REPORTING] and [I-D.IETF-MARF-SPF-REPORTING]. 419 The considerations presented below apply in those cases. 421 The applicability statement for this use case is somewhat smaller as 422 many of the issues associated with abuse reports are not relevant to 423 reports about authentication failures. 425 1. Automatic feedback generators MUST select recipients based on 426 data provided by the report recipient. In particular, recipients 427 MUST NOT be selected using heuristics. 428 2. If the message under evaluation by the Verifier is an ARF 429 ([RFC5965]) message, a report MUST NOT be automatically 430 generated. 431 3. The message for a new report sent via SMTP MUST be constructed so 432 as to avoid amplification attacks, deliberate or otherwise. The 433 envelope sender address of the report MUST be chosen so that 434 these reports will not generate mail loops. Similar to Section 2 435 of [RFC3464], the envelope sender address of the report MUST be 436 chosen to ensure that no feedback reports will be issued in 437 response to the report itself. Therefore, when an SMTP 438 transaction is used to send a report, the MAIL FROM command 439 SHOULD use the NULL reverse-path, i.e., "MAIL FROM:<>". An 440 exception to this would be the use of a reverse-path selected 441 such that SPF checks on the report will pass; in such cases, the 442 operator will need to make provisions to avoid the amplification 443 attack or mail loop via other means. 444 4. Reports SHOULD use "Feedback-Type: auth-failure", but MAY use 445 other types as appropriate. However, the Mailbox Provider 446 generating the reports cannot assume that the operator receiving 447 the reports will treat different Feedback-Types differently. 448 5. These reports SHOULD include the following optional fields, 449 although they are optional in [RFC5965], whenever their 450 corresponding values are available: Original-Mail-From, Arrival- 451 Date, Source-IP, Original-Rcpt-To. Other optional fields can be 452 included, as the implementer feels is appropriate. 454 7. IANA Considerations 456 [RFC Editor: please remove this section prior to publication.] 458 This document has no IANA actions. 460 8. Security Considerations 461 8.1. In Other Documents 463 Implementers are strongly urged to review, at a minimum, the Security 464 Considerations sections of [RFC5965] and [RFC6449]. 466 8.2. Forgeries 468 Feedback Providers that relay user complaints directly, rather than 469 by reference to a stored message (e.g., IMAP or POP), could be duped 470 into sending a complaint about a message that the complaining user 471 never actually received, as an attack on the purported originator of 472 the falsified message. Feedback Providers need to be resilient to 473 such attack methods. 475 Also, these reports may be forged as easily as ordinary Internet 476 electronic mail. User agents and automatic mail handling facilities 477 (such as mail distribution list exploders) that wish to make 478 automatic use of reports of any kind should take appropriate 479 precautions to minimize the potential damage from denial-of-service 480 attacks. 482 Perhaps the simplest means of mitigating this threat is to assert 483 that these reports should themselves be signed with something like 484 DKIM or authorized by SPF. On the other hand, if there is a problem 485 with the DKIM infrastructure at the Verifier, signing DKIM failure 486 reports may produce reports that aren't trusted or even accepted by 487 their intended recipients. Similar issues could exist with SPF 488 evaluation. Use of both technologies can mitigate this risk to a 489 degree. 491 8.3. Amplification Attacks 493 Failure to comply with the recommendations regarding selection of the 494 envelope sender can lead to amplification denial-of-service attacks. 496 8.4. Automatic Generation 498 ARF ([RFC5965]) reports have historically been generated individually 499 as a result of some kind of human request, such as someone clicking a 500 "Report Abuse" button in a mail reader. In contrast, the mechanisms 501 described in some extension documents (i.e., 502 [I-D.IETF-MARF-DKIM-REPORTING] and [I-D.IETF-MARF-SPF-REPORTING]) are 503 focused around automated reporting. This obviously implies the 504 potential for much larger volumes or frequency of messages, and thus 505 greater mail system load (both for Feedback Providers and report 506 receivers). 508 Those mechanisms are primarily intended for use in generating reports 509 to aid implementers of DKIM ([RFC6376]), ADSP ([RFC5617]), and SPF 510 ([RFC4408]), and other related protocols during development and 511 debugging. They are not generally intended for prolonged forensic 512 use, specifically because of these load concerns. However, extended 513 use is possible by ADMDs that want to keep a close watch for fraud or 514 infrastructure problems. It is important to consider the impact of 515 doing so on both Feedback Providers and the requesting ADMDs. 517 A sender requesting these reports can cause its mail servers to be 518 overwhelmed if it sends out signed messages whose signatures fail to 519 verify for some reason, provoking a large number of reports from 520 Feedback Providers. Similarly, a Feedback Provider could be 521 overwhelmed by a large volume of messages requesting reports whose 522 signatures fail to validate, as those now need to send reports back 523 to the signer. 525 Limiting the rate of generation of these messages may be appropriate 526 but threatens to inhibit the distribution of important and possibly 527 time-sensitive information. 529 In general ARF feedback loop terms, it is often suggested that 530 Feedback Providers only create these (or any) ARF reports after an 531 out-of-band arrangement has been made between two parties. These 532 extension mechanisms then become ways to adjust parameters of an 533 authorized abuse report feedback loop that is configured and 534 activated by private agreement rather than starting to send them 535 automatically based solely on data found in the messages, which may 536 have unintended consequences. 538 8.5. Reporting Multiple Incidents 540 If it is known that a particular host generates abuse reports upon 541 certain incidents, an attacker could forge a high volume of messages 542 that will trigger such a report. The recipient of the report could 543 then be innundated with reports. This could easily be extended to a 544 distributed denial-of-service attack by finding a number of report- 545 generating servers. 547 The incident count referenced in ARF ([RFC5965]) provides a limited 548 form of mitigation. The host generating reports can elect to send 549 reports only periodically, with each report representing a number of 550 identical or nearly-identical incidents. One might even do something 551 inverse-exponentially, sending reports for each of the first ten 552 incidents, then every tenth incident up to 100, then every 100th 553 incident up to 1000, etc., until some period of relative quiet after 554 which the limitation resets. 556 The use of this for "nearly-identical" incidents in particular causes 557 a degradation in reporting quality, however. If for example a large 558 number of pieces of spam arrive from one attacker, a reporting agent 559 could decide only to send a report about a fraction of those 560 messages. While this averts a flood of reports to a system 561 administrator, the precise details of each incident are similarly not 562 sent. 564 Other rate limiting provisions might be considered, including 565 detection of a temporary failure response from the report destination 566 and thus halting report generation to that destination for some 567 period, or simply imposing or negotiating a hard limit on the number 568 of reports to be sent to a particular receiver in a given time frame. 570 9. Acknowledgements 572 The author and editor wish to thank Steve Atkins, John Levine, Shmuel 573 Metz, S. Moonesamy, and Alessandro Vesely for their contributions to 574 this memo. 576 All of the Best Practices referenced by this document are found in 577 [RFC6449], written within the Collaboration Committee of the 578 Messaging Anti-Abuse Working Group (MAAWG). 580 Finally, the original author wishes to thank the doctors and staff at 581 the University of Texas MD Anderson Cancer Center for doing what they 582 do. 584 10. References 586 10.1. Normative References 588 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 589 Requirement Levels", BCP 14, RFC 2119, March 1997. 591 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 592 October 2008. 594 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 595 October 2008. 597 [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, 598 July 2009. 600 [RFC5965] Shafranovich, Y., Levine, J., and M. Kucherawy, "An 601 Extensible Format for Email Feedback Reports", RFC 5965, 602 August 2010. 604 10.2. Informative References 606 [I-D.IETF-MARF-DKIM-REPORTING] 607 Kucherawy, M., "Extensions to DKIM for Failure Reporting", 608 draft-ietf-marf-dkim-reporting (work in progress), 609 January 2012. 611 [I-D.IETF-MARF-REDACTION] 612 Falk, JD. and M. Kucherawy, Ed., "Redaction of Potentially 613 Sensitive Data from Mail Abuse Reports", 614 draft-ietf-marf-redaction (work in progress), March 2011. 616 [I-D.IETF-MARF-SPF-REPORTING] 617 Kitterman, S., "SPF Authentication Failure Reporting using 618 the Abuse Report Format", draft-ietf-marf-spf-reporting 619 (work in progress), January 2012. 621 [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 622 STD 53, RFC 1939, May 1996. 624 [RFC2026] Bradner, S., "The Internet Standards Process -- Revision 625 3", BCP 9, RFC 2026, October 1996. 627 [RFC2142] Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND 628 FUNCTIONS", RFC 2142, May 1997. 630 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 631 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 632 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 634 [RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format 635 for Delivery Status Notifications", RFC 3464, 636 January 2003. 638 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 639 4rev1", RFC 3501, March 2003. 641 [RFC3912] Daigle, L., "WHOIS Protocol Specification", RFC 3912, 642 September 2004. 644 [RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) 645 for Authorizing Use of Domains in E-Mail, Version 1", 646 RFC 4408, April 2006. 648 [RFC5617] Allman, E., Fenton, J., Delany, M., and J. Levine, 649 "DomainKeys Identified Mail (DKIM) Author Domain Signing 650 Practices (ADSP)", RFC 5617, August 2009. 652 [RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys 653 Identified Mail (DKIM) Signatures", RFC 6376, 654 September 2011. 656 [RFC6449] Falk, J., "Complaint Feedback Loop Operational 657 Recommendations", RFC 6449, November 2011. 659 Authors' Addresses 661 J.D. Falk 662 Return Path 663 100 Mathilda Street, Suite 100 664 Sunnyvale, CA 94089 665 USA 667 Email: ietf@cybernothing.org 668 URI: http://www.returnpath.net/ 670 M. Kucherawy (editor) 671 Cloudmark 672 128 King St., 2nd Floor 673 San Francisco, CA 94107 674 US 676 Email: msk@cloudmark.com