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Internet Engineering Task Force (IETF)                     N. BorensteinRequest for Comments: 7073                                      MimecastCategory: Standards Track                                   M. KucherawyISSN: 2070-1721                                            November 2013A Reputation Response Set for Email IdentifiersAbstract   This document defines a response set for describing assertions a   reputation service provider can make about email identifiers, for use   in generating reputons.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7073.Copyright Notice   Copyright (c) 2013 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Borenstein & Kucherawy       Standards Track                    [Page 1]

RFC 7073             Email Identifiers Response Set        November 2013Table of Contents1. Introduction ....................................................22. Terminology and Definitions .....................................22.1. Key Words ..................................................22.2. Email Definitions ..........................................22.3. Other Definitions ..........................................33. Discussion ......................................................33.1. Assertions .................................................33.2. Response Set Extensions ....................................43.3. Identifiers ................................................43.4. Query Extensions ...........................................54. IANA Considerations .............................................54.1. Registration of 'email-id' Reputation Application ..........55. Security Considerations .........................................66. References ......................................................76.1. Normative References .......................................76.2. Informative References .....................................7Appendix A. Positive vs. Negative Assertions .......................8Appendix B. Acknowledgments ........................................81.  Introduction   This document specifies a response set for describing the reputation   of an email identifier.  A "response set" in this context is defined   in [RFC7070] and is used to describe assertions a reputation service   provider can make about email identifiers as well as metadata that   can be included in such a reply beyond the base set specified there.   An atomic reputation response is called a "reputon", defined in   [RFC7071].  That document also defines a media type to contain a   reputon for transport, and creates a registry for reputation   applications and the interesting parameters of each.2.  Terminology and Definitions   This section defines terms used in the rest of the document.2.1.  Key Words   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [KEYWORDS].2.2.  Email Definitions   Commonly used definitions describing entities in the email   architecture are defined and discussed in [EMAIL-ARCH].Borenstein & Kucherawy       Standards Track                    [Page 2]

RFC 7073             Email Identifiers Response Set        November 20132.3.  Other Definitions   Other terms of importance in this document are defined in [RFC7070],   the base document for the reputation services work.3.  Discussion   The expression of reputation about an email identifier requires   extensions of the base set defined in [RFC7070].  This document   defines and registers some common assertions about an entity found in   a piece of [MAIL].3.1.  Assertions   The "email-id" reputation application recognizes the following   assertions:   abusive:  The subject identifier is associated with sending or      handling email of a personally abusive, threatening, or otherwise      harassing nature   fraud:  The subject identifier is associated with the sending or      handling of fraudulent email, such as "phishing" (some good      discussion on this topic can be found in [IODEF-PHISHING])   invalid-recipients:  The subject identifier is associated with      delivery attempts to nonexistent recipients   malware:  The subject identifier is associated with the sending or      handling of malware via email   spam:  The subject identifier is associated with the sending or      handling of unwanted bulk email   For all assertions, the "rating" scale is linear: a value of 0.0   means there is no data to support the assertion, a value of 1.0 means   all accumulated data support the assertion, and the intervening   values have a linear relationship (i.e., a score of "x" is twice as   strong of an assertion as a value of "x/2").Borenstein & Kucherawy       Standards Track                    [Page 3]

RFC 7073             Email Identifiers Response Set        November 20133.2.  Response Set Extensions   The "email-id" reputation application recognizes the following   OPTIONAL extensions to the basic response set defined in [RFC7071]:   email-id-identity:  A token indicating the source of the identifier;      that is, where the subject identifier was found in the message.      This MUST be one of:      dkim: The signing domain, i.e., the value of the "d=" tag, found            on a valid DomainKeys Identified Mail [DKIM] signature in            the message      ipv4: The IPv4 address of the client      ipv6: The IPv6 address of the clientrfc5321.helo:  TheRFC5321.HELO value used by the client (see            [SMTP])rfc5321.mailfrom:  TheRFC5321.MailFrom value of the envelope of            the message (see [SMTP])rfc5322.from:  TheRFC5322.From field of the message (see [MAIL])      spf:  The domain name portion of the identifier (RFC5321.MailFrom            orRFC5321.HELO) verified by [SPF]   sources:  A token relating a count of the number of sources of data      that contributed to the reported reputation.  This is in contrast      to the "sample-size" parameter, which indicates the total number      of reports across all reporting sources.   A reply that does not contain the "identity" or "sources" extensions   is making a non-specific statement about how the reputation returned   was developed.  A client can use or ignore such a reply at its   discretion.3.3.  Identifiers   In evaluating an email message on the basis of reputation, there can   be more than one identifier in the message needing to be validated.   For example, a message may have different email addresses in theRFC5321.MailFrom parameter and theRFC5322.From header field.  TheRFC5321.Helo identifier will obviously be different.  Consequently,   the software evaluating the email message may need to query for the   reputation of more than one identifier.Borenstein & Kucherawy       Standards Track                    [Page 4]

RFC 7073             Email Identifiers Response Set        November 2013   The purpose of including the identity in the reply is to expose to   the client the context in which the identifier was extracted from the   message under evaluation.  In particular, several of the items listed   are extracted verbatim from the message and have not been subjected   to any kind of validation, while a domain name present in a valid   DKIM signature has some more reliable semantics associated with it.   Computing or using reputation information about unauthenticated   identifiers has substantially reduced value, but can sometimes be   useful when combined.  For example, a reply that indicates a message   contained one of these low-value identifiers with a high "spam"   rating might not be worthy of notice, but a reply that indicates a   message contained several of them could be grounds for suspicion.   A client interested in checking these weaker identifiers would issue   a query about each of them using the same assertion (e.g., "spam"),   and then collate the results to determine which ones and how many of   them came back with ratings indicating content of concern, and take   action accordingly.  For stronger identifiers, decisions can   typically be made based on a few or even just one of them.3.4.  Query Extensions   A query within this application can include the OPTIONAL query   parameter "identity" to indicate which specific identity is of   interest to the query.  Legal values are the same as those listed inSection 3.2.4.  IANA Considerations   This memo presents one action for IANA, namely the registration of   the reputation application "email-id".4.1.  Registration of 'email-id' Reputation Application   This section registers the "email-id" reputation application, as per   the IANA Considerations section of [RFC7071].  The registration   parameters are as follows:   o  Application symbolic name: email-id   o  Short description: Evaluates DNS domain names or IP addresses      found in email identifiers   o  Defining document: [RFC7073]   o  Status: currentBorenstein & Kucherawy       Standards Track                    [Page 5]

RFC 7073             Email Identifiers Response Set        November 2013   o  Subject: A string appropriate to the identifier of interest (seeSection 3.2 of this document)   o  Application-specific query parameters:      identity:  (current) as defined inSection 3.4 of this document   o  Application-specific assertions:      abusive:  (current) as defined inSection 3.1 of this document      fraud:  (current) as defined inSection 3.1 of this document      invalid-recipients:  (current) as defined inSection 3.1 of this            document      malware:  (current) as defined inSection 3.1 of this document      spam: (current) as defined inSection 3.1 of this document   o  Application-specific response set extensions:      identity:  (current) as defined inSection 3.2 of this document5.  Security Considerations   This document is primarily an IANA action and doesn't describe any   protocols or protocol elements that might introduce new security   concerns.   Security considerations relevant to email and email authentication   can be found in most of the documents listed in the References   sections below.  Information specific to use of reputation services   can be found in [CONSIDERATIONS].Borenstein & Kucherawy       Standards Track                    [Page 6]

RFC 7073             Email Identifiers Response Set        November 20136.  References6.1.  Normative References   [DKIM]     Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,RFC 6376, September 2011.   [EMAIL-ARCH]              Crocker, D., "Internet Mail Architecture",RFC 5598, July              2009.   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC7070]  Borenstein, N., Kucherawy, M., and A. Sullivan, "An              Architecture for Reputation Reporting",RFC 7070, November              2013.   [RFC7071]  Borenstein, N. and M. Kucherawy, "A Media Type for              Reputation Interchange",RFC 7071, November 2013.   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol",RFC 5321,              October 2008.   [SPF]      Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)              for Authorizing Use of Domains in E-Mail, Version 1",RFC4408, April 2006.6.2.  Informative References   [CONSIDERATIONS]              Kucherawy, M., "Operational Considerations Regarding              Reputation Services", Work in Progress, May 2013.   [IODEF-PHISHING]              Cain, P. and D. Jevans, "Extensions to the IODEF-Document              Class for Reporting Phishing",RFC 5901, July 2010.   [MAIL]     Resnick, P., Ed., "Internet Message Format",RFC 5322,              October 2008.Borenstein & Kucherawy       Standards Track                    [Page 7]

RFC 7073             Email Identifiers Response Set        November 2013Appendix A.  Positive vs. Negative Assertions   [CONSIDERATIONS] some current theories about reputation, namely that   it will possibly have more impact to develop positive reputations and   focus on giving preferential treatment to content or sources that   earn those.  However, the assertions defined in this document are all   clearly negative in nature.   In effect, this document is recording current use of reputation and   of this framework in particular.  It is expected that, in the future,   the application being registered here will be augmented, and other   applications registered, that focus more on positive assertions   rather than negative ones.Appendix B.  Acknowledgments   The authors wish to acknowledge the contributions of the following to   this specification: Scott Hollenbeck, Scott Kitterman, Peter Koch,   John Levine, Danny McPherson, S. Moonesamy, Doug Otis, and David F.   Skoll.Authors' Addresses   Nathaniel Borenstein   Mimecast   203 Crescent St., Suite 303   Waltham, MA  02453   USA   Phone: +1 781 996 5340   EMail: nsb@guppylake.com   Murray S. Kucherawy   270 Upland Drive   San Francisco, CA  94127   USA   EMail: superuser@gmail.comBorenstein & Kucherawy       Standards Track                    [Page 8]