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
draft-ietf-repute-model-02
Abstract
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
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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. "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.
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
information."
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
domainrep@ietf.org mailing list. See
https://www.ietf.org/mailman/listinfo/domainrep.
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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
Cloudmark
128 King St., 2nd Floor
San Francisco, CA 94107
USA
Email: superuser@gmail.com
Andrew Sullivan (editor)
Dyn, Inc.
150 Dow St.
Manchester, NH 03101
USA
Email: asullivan@dyn.com
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