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<pre>Internet Engineering Task Force (IETF) N. Borenstein
Request for Comments: 7073 Mimecast
Category: Standards Track M. Kucherawy
ISSN: 2070-1721 November 2013
<span class="h1">A Reputation Response Set for Email Identifiers</span>
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 <a href="./rfc5741#section-2">Section 2 of RFC 5741</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="http://www.rfc-editor.org/info/rfc7073">http://www.rfc-editor.org/info/rfc7073</a>.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
(<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) 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.
<span class="grey">Borenstein & Kucherawy Standards Track [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc7073">RFC 7073</a> Email Identifiers Response Set November 2013</span>
Table of Contents
<a href="#section-1">1</a>. Introduction ....................................................<a href="#page-2">2</a>
<a href="#section-2">2</a>. Terminology and Definitions .....................................<a href="#page-2">2</a>
<a href="#section-2.1">2.1</a>. Key Words ..................................................<a href="#page-2">2</a>
<a href="#section-2.2">2.2</a>. Email Definitions ..........................................<a href="#page-2">2</a>
<a href="#section-2.3">2.3</a>. Other Definitions ..........................................<a href="#page-3">3</a>
<a href="#section-3">3</a>. Discussion ......................................................<a href="#page-3">3</a>
<a href="#section-3.1">3.1</a>. Assertions .................................................<a href="#page-3">3</a>
<a href="#section-3.2">3.2</a>. Response Set Extensions ....................................<a href="#page-4">4</a>
<a href="#section-3.3">3.3</a>. Identifiers ................................................<a href="#page-4">4</a>
<a href="#section-3.4">3.4</a>. Query Extensions ...........................................<a href="#page-5">5</a>
<a href="#section-4">4</a>. IANA Considerations .............................................<a href="#page-5">5</a>
<a href="#section-4.1">4.1</a>. Registration of 'email-id' Reputation Application ..........<a href="#page-5">5</a>
<a href="#section-5">5</a>. Security Considerations .........................................<a href="#page-6">6</a>
<a href="#section-6">6</a>. References ......................................................<a href="#page-7">7</a>
<a href="#section-6.1">6.1</a>. Normative References .......................................<a href="#page-7">7</a>
<a href="#section-6.2">6.2</a>. Informative References .....................................<a href="#page-7">7</a>
<a href="#appendix-A">Appendix A</a>. Positive vs. Negative Assertions .......................<a href="#page-8">8</a>
<a href="#appendix-B">Appendix B</a>. Acknowledgments ........................................<a href="#page-8">8</a>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
This document specifies a response set for describing the reputation
of an email identifier. A "response set" in this context is defined
in [<a href="./rfc7070" title=""An Architecture for Reputation Reporting"">RFC7070</a>] 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
[<a href="./rfc7071" title=""A Media Type for Reputation Interchange"">RFC7071</a>]. 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.
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Terminology and Definitions</span>
This section defines terms used in the rest of the document.
<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>. Key Words</span>
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 [<a href="#ref-KEYWORDS" title=""Key words for use in RFCs to Indicate Requirement Levels"">KEYWORDS</a>].
<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>. Email Definitions</span>
Commonly used definitions describing entities in the email
architecture are defined and discussed in [<a href="#ref-EMAIL-ARCH">EMAIL-ARCH</a>].
<span class="grey">Borenstein & Kucherawy Standards Track [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc7073">RFC 7073</a> Email Identifiers Response Set November 2013</span>
<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>. Other Definitions</span>
Other terms of importance in this document are defined in [<a href="./rfc7070" title=""An Architecture for Reputation Reporting"">RFC7070</a>],
the base document for the reputation services work.
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Discussion</span>
The expression of reputation about an email identifier requires
extensions of the base set defined in [<a href="./rfc7070" title=""An Architecture for Reputation Reporting"">RFC7070</a>]. This document
defines and registers some common assertions about an entity found in
a piece of [<a href="#ref-MAIL" title=""Internet Message Format"">MAIL</a>].
<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>. Assertions</span>
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 [<a href="#ref-IODEF-PHISHING">IODEF-PHISHING</a>])
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").
<span class="grey">Borenstein & Kucherawy Standards Track [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc7073">RFC 7073</a> Email Identifiers Response Set November 2013</span>
<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>. Response Set Extensions</span>
The "email-id" reputation application recognizes the following
OPTIONAL extensions to the basic response set defined in [<a href="./rfc7071" title=""A Media Type for Reputation Interchange"">RFC7071</a>]:
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 [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>] signature in
the message
ipv4: The IPv4 address of the client
ipv6: The IPv6 address of the client
<a href="./rfc5321">rfc5321</a>.helo: The <a href="./rfc5321">RFC5321</a>.HELO value used by the client (see
[<a href="#ref-SMTP" title=""Simple Mail Transfer Protocol"">SMTP</a>])
<a href="./rfc5321">rfc5321</a>.mailfrom: The <a href="./rfc5321">RFC5321</a>.MailFrom value of the envelope of
the message (see [<a href="#ref-SMTP" title=""Simple Mail Transfer Protocol"">SMTP</a>])
<a href="./rfc5322">rfc5322</a>.from: The <a href="./rfc5322">RFC5322</a>.From field of the message (see [<a href="#ref-MAIL" title=""Internet Message Format"">MAIL</a>])
spf: The domain name portion of the identifier (<a href="./rfc5321">RFC5321</a>.MailFrom
or <a href="./rfc5321">RFC5321</a>.HELO) verified by [<a href="#ref-SPF" title=""Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1"">SPF</a>]
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.
<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>. Identifiers</span>
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
<a href="./rfc5321">RFC5321</a>.MailFrom parameter and the <a href="./rfc5322">RFC5322</a>.From header field. The
<a href="./rfc5321">RFC5321</a>.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.
<span class="grey">Borenstein & Kucherawy Standards Track [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc7073">RFC 7073</a> Email Identifiers Response Set November 2013</span>
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.
<span class="h3"><a class="selflink" id="section-3.4" href="#section-3.4">3.4</a>. Query Extensions</span>
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
<a href="#section-3.2">Section 3.2</a>.
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. IANA Considerations</span>
This memo presents one action for IANA, namely the registration of
the reputation application "email-id".
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. Registration of 'email-id' Reputation Application</span>
This section registers the "email-id" reputation application, as per
the IANA Considerations section of [<a href="./rfc7071" title=""A Media Type for Reputation Interchange"">RFC7071</a>]. 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: [<a href="./rfc7073">RFC7073</a>]
o Status: current
<span class="grey">Borenstein & Kucherawy Standards Track [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc7073">RFC 7073</a> Email Identifiers Response Set November 2013</span>
o Subject: A string appropriate to the identifier of interest (see
<a href="#section-3.2">Section 3.2</a> of this document)
o Application-specific query parameters:
identity: (current) as defined in <a href="#section-3.4">Section 3.4</a> of this document
o Application-specific assertions:
abusive: (current) as defined in <a href="#section-3.1">Section 3.1</a> of this document
fraud: (current) as defined in <a href="#section-3.1">Section 3.1</a> of this document
invalid-recipients: (current) as defined in <a href="#section-3.1">Section 3.1</a> of this
document
malware: (current) as defined in <a href="#section-3.1">Section 3.1</a> of this document
spam: (current) as defined in <a href="#section-3.1">Section 3.1</a> of this document
o Application-specific response set extensions:
identity: (current) as defined in <a href="#section-3.2">Section 3.2</a> of this document
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Security Considerations</span>
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 [<a href="#ref-CONSIDERATIONS" title="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">CONSIDERATIONS</a>].
<span class="grey">Borenstein & Kucherawy Standards Track [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
<span class="grey"><a href="./rfc7073">RFC 7073</a> Email Identifiers Response Set November 2013</span>
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. References</span>
<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>. Normative References</span>
[<a id="ref-DKIM">DKIM</a>] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
<a href="./rfc6376">RFC 6376</a>, September 2011.
[<a id="ref-EMAIL-ARCH">EMAIL-ARCH</a>]
Crocker, D., "Internet Mail Architecture", <a href="./rfc5598">RFC 5598</a>, July
2009.
[<a id="ref-KEYWORDS">KEYWORDS</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
[<a id="ref-RFC7070">RFC7070</a>] Borenstein, N., Kucherawy, M., and A. Sullivan, "An
Architecture for Reputation Reporting", <a href="./rfc7070">RFC 7070</a>, November
2013.
[<a id="ref-RFC7071">RFC7071</a>] Borenstein, N. and M. Kucherawy, "A Media Type for
Reputation Interchange", <a href="./rfc7071">RFC 7071</a>, November 2013.
[<a id="ref-SMTP">SMTP</a>] Klensin, J., "Simple Mail Transfer Protocol", <a href="./rfc5321">RFC 5321</a>,
October 2008.
[<a id="ref-SPF">SPF</a>] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1", <a href="./rfc4408">RFC</a>
<a href="./rfc4408">4408</a>, April 2006.
<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>. Informative References</span>
[<a id="ref-CONSIDERATIONS">CONSIDERATIONS</a>]
Kucherawy, M., "Operational Considerations Regarding
Reputation Services", Work in Progress, May 2013.
[<a id="ref-IODEF-PHISHING">IODEF-PHISHING</a>]
Cain, P. and D. Jevans, "Extensions to the IODEF-Document
Class for Reporting Phishing", <a href="./rfc5901">RFC 5901</a>, July 2010.
[<a id="ref-MAIL">MAIL</a>] Resnick, P., Ed., "Internet Message Format", <a href="./rfc5322">RFC 5322</a>,
October 2008.
<span class="grey">Borenstein & Kucherawy Standards Track [Page 7]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-8" ></span>
<span class="grey"><a href="./rfc7073">RFC 7073</a> Email Identifiers Response Set November 2013</span>
<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>. Positive vs. Negative Assertions</span>
[<a id="ref-CONSIDERATIONS">CONSIDERATIONS</a>] 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.
<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>. Acknowledgments</span>
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]
</pre>
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