A Reputation Response Set for Email Identifiers
RFC 7073

Internet Engineering Task Force (IETF)                     N. Borenstein
Request for Comments: 7073                                      Mimecast
Category: Standards Track                                   M. Kucherawy
ISSN: 2070-1721                                            November 2013

            A Reputation Response Set for Email Identifiers

Abstract

   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 in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://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 to BCP 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1. Introduction ....................................................2
   2. Terminology and Definitions .....................................2
      2.1. Key Words ..................................................2
      2.2. Email Definitions ..........................................2
      2.3. Other Definitions ..........................................3
   3. Discussion ......................................................3
      3.1. Assertions .................................................3
      3.2. Response Set Extensions ....................................4
      3.3. Identifiers ................................................4
      3.4. Query Extensions ...........................................5
   4. IANA Considerations .............................................5
      4.1. Registration of 'email-id' Reputation Application ..........5
   5. Security Considerations .........................................6
   6. References ......................................................7
      6.1. Normative References .......................................7
      6.2. Informative References .....................................7
   Appendix A. Positive vs. Negative Assertions .......................8
   Appendix B. Acknowledgments ........................................8

1.  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].

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2.3.  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").

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3.2.  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 client

      rfc5321.helo:  The RFC5321.HELO value used by the client (see
            [SMTP])

      rfc5321.mailfrom:  The RFC5321.MailFrom value of the envelope of
            the message (see [SMTP])

      rfc5322.from:  The RFC5322.From field of the message (see [MAIL])

      spf:  The domain name portion of the identifier (RFC5321.MailFrom
            or RFC5321.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 the
   RFC5321.MailFrom parameter and the RFC5322.From header field.  The
   RFC5321.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.

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   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 in
   Section 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: current

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   o  Subject: A string appropriate to the identifier of interest (see
      Section 3.2 of this document)

   o  Application-specific query parameters:

      identity:  (current) as defined in Section 3.4 of this document

   o  Application-specific assertions:

      abusive:  (current) as defined in Section 3.1 of this document

      fraud:  (current) as defined in Section 3.1 of this document

      invalid-recipients:  (current) as defined in Section 3.1 of this
            document

      malware:  (current) as defined in Section 3.1 of this document

      spam: (current) as defined in Section 3.1 of this document

   o  Application-specific response set extensions:

      identity:  (current) as defined in Section 3.2 of this document

5.  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].

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6.  References

6.1.  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", RFC
              4408, 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.

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Appendix 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.com

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