REPUTE Working Group                                       N. Borenstein
Internet-Draft                                                  Mimecast
Intended status: Informational                              M. Kucherawy
Expires: December 17, 2012                                     Cloudmark
                                                        A. Sullivan, Ed.
                                                               Dyn, Inc.
                                                           June 15, 2012

                    A Model for Reputation Reporting


   This document describes a general architecture for a reputation-based
   service and a model for the exchange of reputation information on the
   Internet.  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
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   This Internet-Draft will expire on December 17, 2012.

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   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  High-Level Architecture . . . . . . . . . . . . . . . . . . . . 4
   3.  Terminology and Definitions . . . . . . . . . . . . . . . . . . 6
     3.1.  Response Set  . . . . . . . . . . . . . . . . . . . . . . . 6
   4.  Information Represented in a Response Set . . . . . . . . . . . 6
   5.  Information Flow in the Protocol  . . . . . . . . . . . . . . . 7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . . . 8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
     8.1.  Biased Reputation Agents  . . . . . . . . . . . . . . . . . 8
     8.2.  Malformed Messages  . . . . . . . . . . . . . . . . . . . . 9
   9.  Informative References  . . . . . . . . . . . . . . . . . . . . 9
   Appendix A.  Public Discussion  . . . . . . . . . . . . . . . . . . 9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 9

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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, stronger identity 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. "" in
   "") 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

   The added requirement -- which can usefully be undertaken only in the
   presence of such stronger identity validation -- is 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

   While the need for reputation information has been perhaps most 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 Application Service Provider (ASP); 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 information in a variety of applications on

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   the Internet.

   A full trust architecture encompasses a range of actors and
   activities, to enable an end-to-end service for creating and
   consuming trust-related information.  One component of that is a
   query mechanism, to permit retrieval of reputation information.  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, a model defines a
   generic query mechanism and basic format for reputation information,
   but allows extensions for each application.

   Omitted from this model is the means by which an reputation-reporting
   agent goes about collecting such data and the mechanism 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.  High-Level Architecture

   A reputation mechanism functions as a component of a service, such as
   that depicted in Figure 1 of [RFC5863], reproduced here:

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     +------+------+                            +------+------+
     |   Author    |                            |  Recipient  |
     +------+------+                            +------+------+
            |                                          ^
            |                                          |
            |                                   +------+------+
            |                                -->|  Handling   |<--
            |                                -->|   Filter    |<--
            |                                   +-------------+
            |                                          ^
            V                  Responsible             |
     +-------------+           Identifier       +------+------+
     | Responsible |. .       . . . . . . . . .>|  Identity   |
     |  Identity   |  .       .                 |  Assessor   |
     +------+------+  .       .                 +-------------+
            |         V       .                       ^ ^
            V         .       .                       | |
   +------------------.-------.--------------------+  | |
   | +------+------+  . . . > .   +-------------+  |  | |  +-----------+
   | | Identifier  |              | Identifier  +--|--+ +--+ Assessment|
   | |   Signer    +------------->| Validator   |  |       | Databases |
   | +-------------+              +-------------+  |       +-----------+
   |                 DKIM Service                  |

         Figure 1: RFC5683 'Actors in a Trust Sequence Using DKIM'

   Here, the reputation mechanism is shown only as a query by an
   Identity Assessor, made to Assessment Databases.

   This memo outlines the 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

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   addresses.)  Each specification for Repute transport is independent
   of any other specification.  A diagram of the basic query service is
   found in Figure 2.

        +-----------+         Query              +----------+
        |           +. . . . . . . . . . . . . .>|          |
        |  Client   |                            |  Server  |
        |           <. . . . . . . . . . . . . . +          |
        +-----+-----+         Response           +-------+--+
              |                                     ^    |
              V                                     |    |
        +------+----+                +-----------+  |    | Response
        | Transport |--------------->| Transport |--+    | Set
        +-----------+    DNS         +-----------+       |
                         TCP                             V
                         UDP                      +------------+
                         ...                      | Reputation |
                                                  | Database   |

                 Figure 2: Basic Reputation Query Service

   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.

3.  Terminology and Definitions

   This section defines terms used in the rest of the document.

3.1.  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 to prevent name collisions.  IANA registries are created in
   a separate memo.  Each definition of a Response Set needs to define
   its registry entry.

4.  Information Represented in a Response Set

   The basic information to be represented in the protocol is fairly

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   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 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.

   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.

5.  Information Flow in the 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 applications might benefit from an extremely lightweight
   mechanism, supporting constrained queries and responses, while others
   might need to support larger and more complex responses.

6.  IANA Considerations

   This memo presents no actions for IANA.

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7.  Privacy Considerations

   Some kinds of reputation data are sensitive, and should not be shared
   publicly.  For applications that have such sensitivity, it is
   imperative to pick a transport that will provide the required
   authentication and authorization mechanisms in order to secure
   communication and deliver responses correctly according to the
   proferred credentials.  Such transport questions are the province of
   the application definitions.

8.  Security Considerations

   This memo 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.

8.1.  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 reputation
   information 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 memo.

   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
   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.

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8.2.  Malformed Messages

   Both clients and servers of reputation systems need to be resistant
   to attacks that involve malformed messages, deliberate or otherwise.
   Failure to do so creates an opportunity for a denial-of-service.

9.  Informative References

   [DKIM]     Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
              J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
              Signatures", RFC 4871, May 2007.

   [DNS]      Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [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.

   [RFC5863]  Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker,
              "DomainKeys Identified Mail (DKIM) Development,
              Deployment, and Operations", RFC 5863, May 2010.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 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 memos takes place on the mailing list.  See

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Authors' Addresses

   Nathaniel Borenstein
   203 Crescent St., Suite 303
   Waltham, MA  02453

   Phone: +1 781 996 5340

   Murray S. Kucherawy
   128 King St., 2nd Floor
   San Francisco, CA  94107


   Andrew Sullivan (editor)
   Dyn, Inc.
   150 Dow St.
   Manchester, NH  03101


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