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rfc:rfc7073

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
 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 2013

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].

Borenstein & Kucherawy Standards Track [Page 2] RFC 7073 Email Identifiers Response Set November 2013

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").

Borenstein & Kucherawy Standards Track [Page 3] RFC 7073 Email Identifiers Response Set November 2013

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.

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 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

Borenstein & Kucherawy Standards Track [Page 5] RFC 7073 Email Identifiers Response Set November 2013

 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].

Borenstein & Kucherawy Standards Track [Page 6] RFC 7073 Email Identifiers Response Set November 2013

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.

Borenstein & Kucherawy Standards Track [Page 7] RFC 7073 Email Identifiers Response Set November 2013

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

Borenstein & Kucherawy Standards Track [Page 8]

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