REPUTE Working Group N. Borenstein
Internet-Draft Mimecast
Intended status: Standards Track M. Kucherawy
Expires: February 28, 2014
A. Sullivan, Ed.
Dyn, Inc.
August 27, 2013
A Model for Reputation Reporting
draft-ietf-repute-model-08
Abstract
This document describes a general architecture for a reputation-based
service and a model for requesting reputation-related data over the
Internet, where "reputation" refers to predictions or expectations
about an entity or an identifier such as a domain name. The document
roughly follows the recommendations of RFC4101 for describing a
protocol model.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 28, 2014.
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
Borenstein, et al. Expires February 28, 2014 [Page 1]
Internet-Draft Reputation Reporting Model August 2013
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. High-Level Architecture . . . . . . . . . . . . . . . . . . . 5
4. Terminology and Definitions . . . . . . . . . . . . . . . . . 7
4.1. Response Set . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Reputon . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Information Represented in a Response Set . . . . . . . . . . 8
6. Information Flow in the Reputation Query Protocol . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 9
8.1. Data In Transit . . . . . . . . . . . . . . . . . . . . . 9
8.2. Aggregation . . . . . . . . . . . . . . . . . . . . . . . 9
8.3. Collection Of Data . . . . . . . . . . . . . . . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9.1. Biased Reputation Agents . . . . . . . . . . . . . . . . . 10
9.2. Malformed Messages . . . . . . . . . . . . . . . . . . . . 11
9.3. Further Discussion . . . . . . . . . . . . . . . . . . . . 11
10. Informative References . . . . . . . . . . . . . . . . . . . . 11
Appendix A. Public Discussion . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Borenstein, et al. Expires February 28, 2014 [Page 2]
Internet-Draft Reputation Reporting Model August 2013
1. Introduction
Historically, many Internet protocols have operated between
unauthenticated entities. For example, an email message's author
field (From) [MAIL] can contain any display name or address and is
not verified by the recipient or other agents along the delivery
path. Similarly, a sending email server using [SMTP] trusts that the
[DNS] has led it to the intended receiving server. Both kinds of
trust are easily betrayed, opening the operation to subversion of
some kind, which leads to spam, phishing, and other attacks.
In recent years, explicit identity authentication mechanisms have
begun to see wider deployment. For example, the [DKIM] protocol
permits associating a validated identifier to a message. This
association is cryptographically strong, and is an improvement over
the prior state of affairs, but it does not distinguish between
identifiers of good actors and bad. Even when it is possible to
validate the domain name in an author field (e.g.
"trustworthy.example.com" in "john.doe@trustworthy.example.com")
there is no basis for knowing whether it is associated with a good
actor worthy of trust. As a practical matter, both bad actors and
good adopt basic authentication mechanisms like DKIM. In fact, bad
actors tend to adopt them even more rapidly than the good actors do
in the hope that some receivers will confuse identity authentication
with identity assessment. The former merely means that the name is
being used by its owner or their agent, while the latter makes a
statement about the quality of the owner.
With the advent of these authentication protocols, it is possible to
statisfy the requirement for a mechanism by which mutually trusted
parties can exchange assessment information about other actors. For
these purposes, we may usefully define "reputation" as "the
estimation in which an identifiable actor is held, especially by the
community or the Internet public generally". We may call an
aggregation of individual assessments "reputation input."
While the need for reputation services has been perhaps especially
clear in the email world, where abuses are commonplace, other
Internet services are coming under attack and may have a similar
need. For instance, a reputation mechanism could be useful in rating
the security of web sites, the quality of service of an Internet
Service Provider (ISP), or an Application Service Provider (ASP).
More generally, there are many different opportunities for use of
reputation services, such as customer satisfaction at e-commerce
sites, and even things unrelated to Internet protocols, such as
plumbers, hotels, or books. Just as human beings traditionally rely
on the recommendations of trusted parties in the physical world, so
too they can be expected to make use of such reputation services in a
Borenstein, et al. Expires February 28, 2014 [Page 3]
Internet-Draft Reputation Reporting Model August 2013
variety of applications on the Internet.
A full trust architecture encompasses a range of actors and
activities, to enable an end-to-end service for creating, exchanging,
and consuming trust-related information. One component of that is a
query mechanism, to permit retrieval of a reputation. Not all such
reputation services will need to convey the same information. Some
need only produce a basic rating, while others need to provide
underlying detail. This is akin to the difference between check
approval versus a credit report.
An overall reckoning of goodness versus badness can be defined
generically, but specific applications are likely to want to describe
reputations for multiple attributes: an e-commerce site might be
rated on price, speed of delivery, customer service, etc., and might
receive very different ratings on each. Therefore, the model defines
a generic query mechanism and basic format for reputation retrieval,
but allows extensions for each application.
Omitted from this model is the means by which a reputation-reporting
agent goes about collecting such data and the method for creating an
evaluation. The mechanism defined here merely enables asking a
question and getting an answer; the remainder of an overall service
provided by such a reputation agent is specific to the implementation
of that service and is out of scope here.
2. Overview
The basic premise of this reputation system involves a client that is
seeking to evaluate content based on an identifier associated with
the content, and a reputation service provider that collects,
aggregates, and makes available for consumption, scores based on the
collected data. Typically client and service operators enter into
some kind of agreement during which some parameters are exchanged
such as the location at which the reputation service can be reached,
the nature of the reputation data being offered, possibly some client
authentication details, and the like.
Upon receipt of some content the client operator wishes to evaluate
(an Internet message, for example), the client extracts from the
content one or more identifiers of interest to be evaluated.
Examples of this include the domain name found in the From: field of
a message, or the domain name extracted from a valid DomainKeys
Identified Mail (DKIM) signature.
Next, the goal is to ask the reputation service provider what the
reputation of the extracted identifier is. The query will contain
Borenstein, et al. Expires February 28, 2014 [Page 4]
Internet-Draft Reputation Reporting Model August 2013
the identifier to be evaluated and possibly some context-specific
information (such as to establish the context of the query, e.g., an
email message) or client-specific information. The client typically
folds the data in the response into whatever local evaluation logic
it applies to decide what disposition the content deserves.
3. High-Level Architecture
A reputation mechanism functions as a component of an overall
service. A current example is that of an email system that uses
DomainKeys Identified Mail (DKIM; see [DKIM]) to affix a stable
identifier to a message and then uses that as a basis for evaluation:
+-------------+ +------------+
| Author | | Recipient |
+-------------+ +------------+
| ^
V |
+-------------+ +------------+
| MSA | | MDA |
+-------------+ +------------+
| ^
| |
| +------------+
| | Handling |
| | Filter |
| +------------+
| ^
| |
| +------------+ +------------+
| | Reputation |<=====>| Identifier |
| | Service | | Assessor |
| +------------+ +------------+
| ^
V |
+----------------------------------------------------------+
| +------------+ Responsible Identifier +------------+ |
| | Identifier |. . . . . . . . . . . . . .>| Identifier | |
| | Signer | | Verifier | |
| +------------+ DKIM Service +------------+ |
+----------------------------------------------------------+
| ^
V |
+-------------+ /~~~~~~~~~~\ +------+-----+
| MTA |----->( other MTAs )------>| MTA |
+-------------+ \~~~~~~~~~~/ +------------+
Borenstein, et al. Expires February 28, 2014 [Page 5]
Internet-Draft Reputation Reporting Model August 2013
Figure 1: Actors in a Trust Sequence Using DKIM
(See [EMAIL-ARCH] for a general description of the Internet messaging
architecture.) In this figure, the solid lines indicate the flow of
a message; the dotted line indicates transfer of validated
identifiers within the message content; and the double line shows the
query and response of the reputation information.
Here, the DKIM Service provides one or more stable identifiers that
is the basis for the reputation query. On receipt of a message from
an MTA, the DKIM Service provides a (possibly empty) set of validated
identifiers -- domain names, in this case -- which are the subjects
of reputation queries made by the Identity Assessor. The Identity
Assessor queries a Reputation Service to determine the reputation of
the provided identifiers, and delivers the identifiers and their
reputations to the Handling Filter. The Handling Filter makes a
decision about whether and how to deliver the message to the
recipient based on these and other inputs about the message, possibly
including evaluation mechansisms in addition to DKIM.
This document outlines the reputation query and response mechanism.
It provides the following definitions:
o Vocabulary for the current work and work of this type;
o The types and content of queries that can be supported;
o The extensible range of response information that can be provided;
o A query/response protocol;
o Query/response transport conventions.
It provides an extremely simple query/response model that can be
carried over a variety of transports, including the Domain Name
System. (Although not typically thought of as a 'transport', the DNS
provides generic capabilities and can be thought of as a mechanism
for transporting queries and responses that have nothing to do with
Internet addresses, such as is one with a DNS BlockList [DNSBL].)
Each specification for Repute transport is independent of any other
specification. A diagram of the basic query service is found in
Figure 2.
Borenstein, et al. Expires February 28, 2014 [Page 6]
Internet-Draft Reputation Reporting Model August 2013
+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . +
. Reputation Service .
. +------------+ .
. | Reputation | .
. | Database | .
. +------------+ .
. | .
. V .
. +-----------+ Query +----------+ .
. | |. . . . . . . . . . . . . .>| | .
. | Client | | Server | .
. | |< . . . . . . . . . . . . . | | .
. +-----+-----+ Response +----------+ .
. ^ ^ .
+ . . . | . . . . . . . . . . . . . . . . . . . | . . . . +
V |
+-----------+ +-----------+ |
| Transport |<-------------->| Transport |<---+
+-----------+ DNS +-----------+
TCP
UDP
...
Figure 2: Basic Reputation Query Service
The precise syntaxes of both the query and response are application-
specific. An application of the model defines the parameters
available to queries of that type, and also defines the data returned
in response to any query.
4. Terminology and Definitions
This section defines terms used in the rest of the document.
4.1. Response Set
A "Response Set" comprises those data that are returned in response
to a reputation query about a particular entity. The types of data
are specific to an application; the data returned in the evaluation
of email senders would be different than the reputation data returned
about a movie or a baseball player.
Response Sets have symbolic names, and these have to be registered
with IANA, in the Reputation Applications Registry, to prevent name
collisions. IANA registries are created in a separate document.
Each definition of a Response Set also needs to define its registry
entry.
Borenstein, et al. Expires February 28, 2014 [Page 7]
Internet-Draft Reputation Reporting Model August 2013
4.1.1. Assertions and Ratings
One of the key properties of a Response Set is called an Assertion.
Assertions are claims made about the subject of a reputation query.
For example, one might assert that a particular restaurant serves
good food. In the context of this model, the assertion would be
"serves good food".
Assertions are coupled with a numeric value called a Rating, which is
an indication of how much the party generating the Response Set
agrees with the assertion being made. For example, with the above
Assertion, a rating of 1.0 indicates strong agreement, while a rating
of 0.0 indicates no support for the assertion.
4.2. Reputon
A "reputon" is an object that comprises the basic response to a
reputation query. It contains the response set relevant to the
subject of the query. Its specific encoding is left to documents
that implement this model.
5. Information Represented in a Response Set
The basic information to be represented in the protocol is fairly
simple, and includes the following:
o the identity of the entity providing the reputation information;
o the identity of the entity being rated;
o the application context for the query (e.g., email address
evaluation);
o the overall rating score for that entity;
o the level of confidence in the accuracy of that rating; and
o the number of data points underlying that score.
Beyond this, arbitrary amounts of additional information might be
provided for specific uses of the service. The entire collection is
the Response Set for that application. The query/response protocol
defines a syntax for representing such Response Sets, but each
application defines its own Response Set. Thus, the basic information
also includes the name of the application for which the reputation
data is being expressed.
Borenstein, et al. Expires February 28, 2014 [Page 8]
Internet-Draft Reputation Reporting Model August 2013
Each application requires its own specification of the Response Set.
For example, a specification might be needed for a reputation
Response Set for an "email-sending-domain"; the Response Set might
include information on how often spam was received from that domain.
Additional documents define a [MIME] type for reputation data, and
protocols for exchanging such data.
6. Information Flow in the Reputation Query Protocol
The basic Response Set could be wrapped into a new MIME media type
[MIME] or a DNS RR, and transported accordingly. It also could be
the integral payload of a purpose-built protocol. For a basic
request/response scenario, one entity (the Client) will ask a second
entity (the Server) for reputation data about a third entity (the
Target), and the second entity will respond with that data.
An application might benefit from an extremely lightweight mechanism,
supporting constrained queries and responses, while others might need
to support larger and more complex responses.
7. IANA Considerations
This document presents no actions for IANA.
[RFC Editor: Please remove this section prior to publication.]
8. Privacy Considerations
8.1. Data In Transit
Some kinds of reputation data are sensitive, and should not be shared
publicly. For cases that have such sensitivity, it is imperative to
protect the information from unauthorized access and viewing. The
model described here neither suggests nor precludes any particular
transport mechanism for the data. However, for the purpose of
illustration, a reputation service that operates over HTTP might
employ any of its well-known mechanisms to solve these problems,
which include OpenPGP [OPENPGP], Transport Layer Security [TLS], and
S/MIME [SMIME].
8.2. Aggregation
The data that are collected as input to a reputation calculation are
in essence a statement by one party about the actions or output of
another. What one party says about another is often meant to be kept
Borenstein, et al. Expires February 28, 2014 [Page 9]
Internet-Draft Reputation Reporting Model August 2013
in confidence. Accordingly, steps often need to be taken to secure
the submission of these input data to a reputation service provider.
Moreover, although the aggregated reputation is the product provided
by this service, its inadvertent exposure can have undesirable
effects. Just as the collection of data about a subject needs due
consideration to privacy and security, so too does the output and
storage of whatever aggregation the service provider applies.
8.3. Collection Of Data
The basic notion of collection and storage of reputation data is
obviously a privacy issue in that the opinions of one party about
another are likely to be sensitive. Inadvertent or unauthorized
exposure of those data can lead to personal or commercial damage.
9. Security Considerations
This document introduces an overall protocol model, but no
implementation details. As such, the security considerations
presented here are very high-level. The detailed analyses of the
various specific components of the protocol can be found the
documents that instantiate this model.
9.1. Biased Reputation Agents
As with [VBR], an agent seeking to make use of a reputation reporting
service is placing some trust that the service presents an unbiased
"opinion" of the object about which reputation is being returned.
The result of trusting the data is, presumably, to guide action taken
by the reputation client. It follows, then, that bias in the
reputation service can adversely affect the client. Clients
therefore need to be aware of this possibility and the effect it
might have. For example, a biased system returning a reputation
about a DNS domain found in email messages could result in the
admission of spam, phishing or malware through a mail gateway (by
rating the domain name more favourably than warranted) or could
result in the needless rejection or delay of mail (by rating the
domain more unfavourably than warranted). As a possible mitigation
strategy, clients might seek to interact only with reputation
services that offer some disclosure of the computation methods for
the results they return. Such disclosure and evaluation is beyond
the scope of the present document.
Similarly, a client placing trust in the results returned by such a
service might suffer if the service itself is compromised, returning
biased results under the control of an attacker without the knowledge
Borenstein, et al. Expires February 28, 2014 [Page 10]
Internet-Draft Reputation Reporting Model August 2013
of the agency providing the reputation service. This might result
from an attack on the data being returned at the source, or from a
man-in-the-middle attack. Protocols, therefore, need to be designed
so as to be as resilient against such attacks as possible.
9.2. Malformed Messages
Both clients and servers of reputation systems need to be resistant
to attacks that involve malformed messages, deliberate or otherwise.
Malformations can be used to confound clients and servers alike in
terms of identifying the party or parties responsible for the content
under evaluation. This can result in delivery of undesirable or even
dangerous content.
9.3. Further Discussion
Numerous other topics related to use and management of reputation
systems can be found in [I-D.REPUTE-CONSIDERATIONS].
10. Informative References
[DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", RFC 6376,
September 2011.
[DNS] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[DNSBL] Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
February 2010.
[EMAIL-ARCH]
Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009.
[I-D.REPUTE-CONSIDERATIONS]
Kucherawy, M., "Operational Considerations Regarding
Reputation Services", draft-ietf-repute-considerations
(work in progress), November 2012.
[MAIL] Resnick, P., "Internet Message Format", RFC 5322,
October 2008.
[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
Borenstein, et al. Expires February 28, 2014 [Page 11]
Internet-Draft Reputation Reporting Model August 2013
[OPENPGP] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
[SMIME] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2: Message
Specification", RFC 5751, January 2010.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[VBR] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
Reference", RFC 5518, April 2009.
Appendix A. Public Discussion
Public discussion of this suite of documents takes place on the
domainrep@ietf.org mailing list. See
https://www.ietf.org/mailman/listinfo/domainrep.
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, et al. Expires February 28, 2014 [Page 12]
Internet-Draft Reputation Reporting Model August 2013
Andrew Sullivan (editor)
Dyn, Inc.
150 Dow St.
Manchester, NH 03101
USA
Email: asullivan@dyn.com
Borenstein, et al. Expires February 28, 2014 [Page 13]
