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Versions: 00 01 rfc6462                                    Informational
Network Working Group                                          A. Cooper
Internet-Draft                                                       CDT
Intended status: Informational                           October 9, 2011
Expires: April 11, 2012

               Report from the Internet Privacy Workshop


   On December 8-9, 2010, the IAB co-hosted an Internet privacy workshop
   with the W3C, ISOC, and MIT's Computer Science and Artificial
   Intelligence Laboratory.  The workshop revealed some of the
   fundamental challenges in designing, deploying, and analyzing
   privacy-protective Internet protocols and systems.  Although workshop
   participants and the community as a whole are still far from
   understanding how best to systematically address privacy within
   Internet standards development, workshop participants identified a
   number of potential next steps.  For the IETF, these included the
   creation of a privacy directorate to review Internet drafts, further
   work on documenting privacy considerations for protocol developers,
   and a number of exploratory efforts concerning fingerprinting and
   anonymized routing.  Potential action items for the W3C included
   investigating the formation of a privacy interest group and
   formulating guidance about fingerprinting, referrer headers, data
   minimization in APIs, usability, and general considerations for non-
   browser-based protocols.

   Note that this document is a report on the proceedings of the
   workshop.  The views and positions documented in this report are
   those of the workshop participants and do not necessarily reflect the
   views of the IAB, W3C, ISOC or MIT CSAIL.

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."

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   This Internet-Draft will expire on April 11, 2012.

Copyright Notice

   Copyright (c) 2011 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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Workshop Overview  . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Technical Discussion . . . . . . . . . . . . . . . . . . .  5
     2.2.  SDO Discussion . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Design Challenges  . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Ease of Fingerprinting . . . . . . . . . . . . . . . . . .  7
     3.2.  Information Leakage  . . . . . . . . . . . . . . . . . . .  8
     3.3.  Differentiating Between First and Third Parties  . . . . .  8
     3.4.  Lack of Transparency and User Awareness  . . . . . . . . .  9
   4.  Deployment and Analysis Challenges . . . . . . . . . . . . . . 10
     4.1.  Generative Protocols vs. Contextual Threats  . . . . . . . 10
     4.2.  Tension Between Privacy Protection and Usability . . . . . 12
     4.3.  Interaction Between Business, Legal and Technical
           Incentives . . . . . . . . . . . . . . . . . . . . . . . . 12
       4.3.1.  Role of Regulation . . . . . . . . . . . . . . . . . . 13
       4.3.2.  P3P: A Case Study of the Importance of Incentives  . . 14
   5.  Conclusions and Next Steps . . . . . . . . . . . . . . . . . . 15
     5.1.  IETF Outlook . . . . . . . . . . . . . . . . . . . . . . . 15
     5.2.  W3C Outlook  . . . . . . . . . . . . . . . . . . . . . . . 16
     5.3.  Other Future Work  . . . . . . . . . . . . . . . . . . . . 16
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 17
   Appendix A.  Workshop Materials  . . . . . . . . . . . . . . . . . 19
   Appendix B.  Workshop Participants . . . . . . . . . . . . . . . . 19
   Appendix C.  Accepted Position Papers  . . . . . . . . . . . . . . 22
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25

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1.  Introduction

   On December 8-9, 2010, the IAB co-hosted a workshop with the W3C,
   ISOC, and MIT's Computer Science and Artificial Intelligence
   Laboratory (CSAIL) about Internet privacy [Workshop].  The workshop
   was organized to help the Internet community gain some understanding
   of what it means for Internet-based systems to respect privacy, how
   such systems have been or could be designed, how the relationship
   between the web and the broader Internet impacts privacy, and what
   specific work the IETF and/or the W3C might pursue to address
   Internet privacy.  An overview of topics discussed at the workshop is
   provided in Section 2.

   The workshop discussions revealed the complexity and broad-based
   nature of privacy on the Internet.  Across numerous different
   applications, a number of fundamental design challenges appear again
   and again, the increasing ease of user/device/application
   fingerprinting, unforeseen information leakage, difficulties in
   distinguishing first parties from third parties, complications
   arising from system dependencies, and the lack of transparency and
   user awareness of privacy risks and tradeoffs (see Section 3).
   Workshop participants also identified a number of barriers to
   successful deployment and analysis of privacy-minded protocols and
   systems, including the difficulty of using generic protocols and
   tools to defend against context-specific threats, the tension between
   privacy protection and usability, and the difficulty of navigating
   between business, legal and individual incentives (see Section 4).

   Privacy challenges far outnumber solutions, but the workshop
   identified a number of concrete preliminary steps that standards
   organizations can take to help ensure respect for user privacy in the
   design of future standards and systems.  For the IETF, these included
   the creation of a privacy directorate to review Internet drafts,
   further work on documenting privacy considerations for protocol
   developers, and initiating a number of exploratory efforts concerning
   fingerprinting and anonymized routing.  Potential action items for
   the W3C included investigating the formation of a privacy interest
   group and formulating guidance about fingerprinting, referrer
   headers, data minimization in APIs, usability, and general
   considerations for non-browser-based protocols.  These next steps and
   workshop outcomes are discussed in Section 5.

2.  Workshop Overview

   The workshop explored both current technical challenges to protecting
   privacy and the ways in which standards organizations can help to
   address those challenges.  Links to workshop materials are listed in

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   Appendix A.

2.1.  Technical Discussion

   The workshop explored privacy challenges in three different technical
   domains: at the network level, at the browser level, and with respect
   to cross-site data exchanges.  Example technologies were highlighted
   in each area to motivate the discussion.

   At the network level, participants discussed IP address hiding in
   mobility protocols, privacy extensions for IPv6 addressing [RFC4941],
   and onion routing.  Discussion about the Tor project [Tor] was
   particularly insightful.  Tor is a circuit-based, low-latency
   communication service designed to anonymize protocols that run over
   TCP.  End hosts participating in a Tor exchange choose a path through
   the network and build a circuit in which each "onion router" in the
   path knows its predecessor and successor, but no other nodes in the
   circuit.  Each onion router in the path unwraps and decrypts received
   information before relaying it downstream.

   For Tor to provide anonymity guarantees, Tor nodes need to be able to
   strip out information elements that can be used to re-identify users
   over time.  For example, web technologies such as cookies, large
   portions of JavaScript, and almost all browser plug-ins (including
   Flash) need to be disabled in order to maintain Tor's privacy
   properties during web use, significantly hampering usability.

   At the browser level, the discussion focused first on experiences
   with "private browsing" modes.  Private browsing puts a browser into
   a temporary session where no information about the user's browsing
   session is stored locally after the session ends.  The goal is to
   protect the user's browsing behavior from others who may make use of
   the same browser on the same machine.  Private browsing is not
   designed to protect the user from being tracked by malware (e.g.,
   keyloggers), remote servers, employers, or governments, but there is
   some evidence that users fail to understand the distinction between
   protection from snooping among users who share a device and these
   other forms of tracking.  The specific protections offered by private
   browsing modes also vary from browser to browser, creating privacy
   loopholes in some cases.

   The browser discussion also addressed proposals for "Do Not Track"
   (DNT) technologies to be built into browsers to provide users with a
   simple way to opt out of web tracking.  At the time of the workshop,
   various different technical proposals had been designed to offer
   users the ability to indicate their preference to opt out or to block
   communication to certain web sites altogether.  The discussions at
   the workshop illustrated a lack of agreement about what type of

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   tracking is acceptable, which technical mechanisms would be best
   suited for different scenarios, and how the mechanisms would interact
   with other aspects of privacy protection (such as notices to users).

   The cross-site data-sharing discussion focused on current uses of
   OAuth (with Facebook Connect, for example).  While improvements have
   been made in obtaining user consent to sharing data between sites,
   challenges remain with regard to data minimization, ease-of-use,
   hidden sharing of data, and centralization of identity information.

2.2.  SDO Discussion

   Participants discussed past experiences in approaching privacy within
   the IETF and the W3C. Individual protocol efforts within the IETF
   have sought to address certain privacy threats over the years.
   Protocol designers have taken steps to reduce the potential for
   identifiability associated with protocol usage, such as in the IPv6
   privacy extensions case [RFC4941].  Protocols architected to rely on
   intermediaries have sought to minimize the user data exposed in
   transit, most notably in SIP [RFC3323].  Protocol architectures used
   in inter-personal exchange have sought to give users granular control
   over their information, including presence [RFC2778] and geolocation
   information [RFC3693].  Efforts to square privacy with usability are
   ongoing; the ALTO working group [ALTO], for example, is working out
   how to balance the needs of users and network operators to share data
   with each other about content preferences and network topologies
   against legitimate concerns about revealing too much of either kind
   of information.

   The IETF also has experience to draw on in building a culture of
   security awareness.  Beginning with [RFC1543], all RFCs were required
   to contain a security considerations section.  But that simple
   mandate did not immediately translate into the extensive security
   consciousness that permeates the IETF today.  Over many years and
   with much effort invested, a more systematic approach to security has
   evolved that makes use of a variety of tools and resources: the
   security area itself, guidelines to RFC authors about security
   considerations [RFC3552], the security directorate, security advisors
   assigned to individual working groups, security tutorials at IETF
   meetings, and so on.

   The W3C likewise has a number of past efforts to draw on.  One of the
   earliest large-scale standards efforts aimed at improving web privacy
   was the Platform for Privacy Preferences [P3P].  The idea behind P3P
   was to have web sites provide machine-readable privacy policies that
   browsers could vet and possibly override according to the user's
   preference.  The P3P policy expression language was robust enough to
   allow sites to make complex assertions about how they intended to

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   make use of data related to users, but market developments have
   created a number of challenges with deployed policies.

   More recent work at the W3C centered around the appropriateness of
   various privacy features to be included in the Geolocation API
   [Geolocation], which gives web sites a way to access the user's
   precise location.  The API requires that implementations obtain user
   consent before accessing location information and allow users to
   revoke that consent, but decisions about retention, secondary use,
   and data minimization are left up to individual web sites and
   applications.  The geolocation effort and the P3P experience both
   raise questions about how to navigate usability, regulation, business
   incentives, and other aspects that normally lie outside the scope of
   SDO work.

3.  Design Challenges

   Workshop discussions surfaced a number of key issues that can make
   designing privacy-sensitive protocols and systems difficult: the
   increasing ease of user/device/application fingerprinting, unforeseen
   information leakage, difficulties in distinguishing first parties
   from third parties, complications arising from system dependencies,
   and the lack of transparency and user awareness of privacy risks and

3.1.  Ease of Fingerprinting

   Internet applications and protocols now share so many unique
   identifiers and other bits of information as part of their ordinary
   operation that it is becoming increasingly easy for remote nodes to
   create unique device or application fingerprints and re-identify the
   same devices or applications over time [Panopticlick].  Hardware
   identifiers, IP addresses, transport protocol parameters, cookies,
   other forms of web storage, and a vast array of browser-based
   information may be routinely shared as users browse the web.  The
   ease of fingerprinting presents a significant challenge for any
   application that seeks to guarantee anonymity or unlinkability (such
   as [Tor], which uses onion routing to strip out data that identifies
   communications endpoints).

   In many cases the information that can be used to fingerprint a
   device was not originally shared for that purpose; identifiers and
   other information are provided to support some other functionality
   (like IP addresses being shared in order to route packets), and may
   incidentally be used to fingerprint.  This complicates the task of
   preventing fingerprinting because each application or protocol likely
   needs its own identifiers and information to function.  Furthermore,

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   some services are increasingly coming to rely on fingerprinting in
   order to detect fraud or provide customized content, for example.
   Finding privacy-friendly substitutes for fingerprinting will only
   become more difficult as these services become more entrenched (see
   Section 4.3).

   The space of fingerprinting mitigations requires further exploration.
   For example, workshop participants discussed the use of Javascript
   queries to obtain a browser's (often highly unique) font list, and
   the tradeoffs associated with browsers instead (or additionally)
   supporting some small subset of fonts in order to reduce browser
   identifiability.  As with many other privacy features, such a
   restriction presents a tradeoff between privacy and usability, and in
   the case of fingerprinting writ large, it may be difficult to find
   consensus about which mitigations appropriately balance both values.
   As a first step, the IETF may consider documenting the fingerprinting
   implications for widely used IETF protocols (TCP, HTTP, SIP, etc.).

3.2.  Information Leakage

   Internet protocols and services tend to leak information in ways that
   were not foreseen at design time, as explored during the IETF 77
   technical plenary [IETF77] and in recent research
   [Krishnamurthy07][Krishnamurthy09].  For example, the HTTP referrer
   header [RFC2616] (misspelled in the original specification as
   "Referer") provides a way for a web site to obtain the URI of the
   resource that referred the user to the site.  Referrer headers
   provide valuable insights to web sites about where their users come
   from, but they can also leak sensitive information (search terms or
   user IDs, for example) because URI strings on the web often contain
   this information.  The infrastructure of an individual web site is
   often designed solely with a view to making the site itself function
   properly, and embedding search terms or other user-specific
   information in URIs may serve that goal, but when those URIs leak out
   to other sites via a referrer header, it creates the potential for
   third parties to use and abuse the data contained therein.

   The use of URIs for authentication of identity or capabilities can be
   susceptible to the same kinds of problems.  Relying on a "possession
   model" where any user in possession of an authentication or
   capability URI can gain access to a resource is only suitable in
   situations with some means of control over URI distribution, and can
   lead to wide leakage when used on the open web.

3.3.  Differentiating Between First and Third Parties

   Distinguishing between "first-party" interactions and "third-party"
   interactions is important for understanding the implications of data

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   collection, sharing and use that take place during the normal course
   of web use.  Unfortunately, the traditional meanings of these
   concepts do not always clearly match up with user expectations or
   evolving web technologies.  Traditionally, the term "first party" has
   been used to refer to the domain of a web site to which a user agent
   directs an explicit request on behalf of a user.  The term "third
   party" has been used to refer to the domain of a web resource that a
   user agent requests as a result of a first-party request, with the
   third-party resource hosted at a different domain from the first-
   party domain.

   This distinction between first-party and third-party domains is in
   part a result of long-standing user agent practices for handling HTTP
   cookies.  Typically, HTTP cookies are returned only to the origin
   server that set them [RFC6265].  Cookies set from first-party domains
   may not be read by third-party domains and vice versa.  In some
   cases, cookies set from first-party domains that contain subdomains
   are accessible by all subdomains of the first-party domain.  The
   distinction between first-party domains and third-party domains is
   reflected in browser-based cookie controls: major web browsers all
   offer distinct first-party cookie settings and third-party cookie

   However, a user's perception or expectation of the difference between
   a "first party" and a "third party" may not fall neatly within these
   distinctions.  Users may expect that content hosted on a first-party
   subdomain, but provided or used by a third party, would be treated as
   third-party content, but browsers often treat it as first-party
   content.  Conversely, when third-party content appears from a source
   with which the user has an established relationship -- such as the
   Facebook "Like" button or other social widgets -- users may consider
   their interaction with that content to be a desirable first-party
   interaction, even though the content is hosted on a third-party

   Handling these expectations programmatically is difficult since the
   same identifier structures (domains, subdomains) can correlate to
   different user expectations in different contexts.  On the other
   hand, prompting users to express a preference about what kinds of
   data collection and use should be allowable by each party encountered
   on the web is not practical.  Web and browser developers are actively
   seeking novel ways to address this challenge, but there are few
   clear-cut solutions.

3.4.  Lack of Transparency and User Awareness

   There is no question that users lack a full understanding of how
   their information is being used and what the tradeoffs are between

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   having their data collected and accessing services at little or no
   cost.  Much of the tracking that takes place on the web is passive
   and invisible to users.  Most companies disclose their data usage
   practices in written privacy policies, but these policies are rarely
   read, difficult to understand, and often fail to disclose salient
   details (such as data retention lifetimes).  Even when web tracking
   is associated with some visual indication -- a highly targeted Gmail
   ad or the Facebook "Like" button, for example -- users often do not
   realize that it is occurring.

   Efforts abound to attempt to present information about data
   collection and usage in a more digestible way.  P3P was one early
   effort, but because it sought to support the expression of the vast
   expanse of potential policies that companies may have, it developed
   more complexity than the average user (or user interface) could
   sustain.  More recent efforts have focused on using a limited set of
   icons to represent policies or provide an indication that tracking is
   taking place.

4.  Deployment and Analysis Challenges

   Workshop participants identified a number of barriers to both
   deployment of privacy-protecting technologies and the analysis of the
   privacy properties of technological systems.  These included the
   difficulty of using generic protocols and tools to defend against
   context-specific threats, the tension between privacy protection and
   usability, and the difficulty of navigating between business, legal
   and individual incentives.

4.1.  Generative Protocols vs. Contextual Threats

   Privacy is not a binary state.  Rather than operating either entirely
   in private or entirely in public, individuals experience privacy
   contextually, resulting in differing requirements for privacy
   protection depending on the circumstance and the individual.  On the
   Internet, the contextual nature of privacy means that threats against
   it can vary depending on the deployment scenario, the usage scenario,
   the capabilities of different attackers, and the level of concern
   that different kinds of attackers generate among different users.

   Addressing the full waterfront of privacy threats within generic
   protocols and tools is largely intractable.  As a result, existing
   privacy features developed at the network and application layers have
   taken more targeted approaches.  For example, privacy extensions for
   stateless address autoconfiguration in IPv6 [RFC4941] support
   addresses constructed dynamically rather than generating addresses
   based on interface MAC addresses, which for most users are persistent

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   and unchangeable unique identifiers that could be used for long-term
   tracking.  While IPv6 privacy extensions provide important protection
   against tracking and re-identification by remote endpoints, they do
   not prevent -- and were not meant to prevent -- all parties from
   being able to associate an IP address with a particular user.  ISPs
   and governments still have means to make such associations, and
   remote endpoints have many other mechanisms at their disposal to
   attempt to identify users persistently, albeit without using IPv6

   This kind of experience with developing privacy tools shows that
   designing privacy features into systems and protocols requires a
   clear understanding of the scope of the threats they are designed to
   address.  This scope is currently being debated in discussion about
   developing "do not track" (DNT) mechanisms for the web and other
   online contexts.  A number of different approaches have been
   proposed, including browser functionality to retain opt-out cookies,
   an HTTP header that expresses the user's preference not to be
   tracked, and a browser-based block list mechanism that prevents the
   browser from communicating with tracking sites (for an overview, see
   [I-D.cooper-web-tracking-opt-outs]).  Regardless of the approach,
   these mechanisms function based on some understanding of which
   "tracking" users should be able to control, which in turn is based on
   some notion of the threats presented by different kinds of tracking
   conducted by different kinds of entities on the web.  Should DNT
   mechanisms apply to sites with which the user already has an
   established relationship?  Or sites that use only aggregate, non-
   individualized data?  Does tracking for fraud prevention or
   customization present different threats than tracking for advertising
   or marketing purposes?  The answers to these questions will dictate
   DNT design choices.

   The space of privacy threats on the Internet may appear particularly
   broad from a protocol design perspective because many of the
   protocols in widest use are designed generically to support a variety
   of applications and functionality.  HTTP, for example, is used for a
   wider variety of purposes than its original designers likely
   anticipated; it is unsurprising that some of these purposes include
   obtaining and using data about web users in ways that may be privacy-
   infringing.  It is unreasonable to ask protocol designers to mitigate
   the potential privacy risks of every possible deployment that may
   result from a particular protocol design; the key questions are about
   how the responsibility for protecting against privacy intrusion
   should be split between protocols, APIs, applications, and services
   and which kinds of privacy features can best be implemented in each

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4.2.  Tension Between Privacy Protection and Usability

   The workshop discussions highlighted the tension between providing
   privacy protections and maintaining usability.  Tor [Tor] provides
   some salient examples of this tradeoff.  Tor seeks to provide
   protection against network surveillance, but by lengthening the
   routing path it may significantly increase round-trip time.  Tor
   obscures endpoint IP addresses, thus it also interferes with IP-based
   geolocation.  Web browsing using Tor is particularly challenging, as
   much of Javascript, most browser plug-ins, and a number of other
   browser-based features need to be blocked or overridden in order to
   meet Tor's anonymity requirements.  With Tor, privacy clearly comes
   at a price.

   Even less aggressive privacy features may come with usability
   tradeoffs.  One example is the blocking of HTTP referrer headers for
   privacy protection reasons.  Some sites provide a customized
   experience to users based on the referring page, which means that
   disabling referrer headers, as some browsers allow users to do, may
   sacrifice user experience features on certain sites.  Part of the
   challenge is the level of nuance involved in making decisions about
   privacy -- how can users be made to understand the privacy tradeoffs
   of blocking HTTP referrer headers, for example, when the effects of
   doing so will vary from site to site, or when there is limited UI
   space to communicate the tradeoffs?  Even seemingly simple privacy
   controls like private browsing are not well understood.

   The feature set that implementors choose to make available is often
   reflective of the tension between usability and privacy.  For
   example, SIP supports S/MIME [RFC3261] to secure SIP request bodies,
   but given its user experience impact, few implementations include
   S/MIME support.  Although usability challenges are generally thought
   of as user-level issues that are out of scope for the IETF, to the
   extent that they trickle down into implementation decisions, they are
   highly relevant.

   Although workshop participants reached few firm conclusions about how
   to tackle usability issues arising from privacy features, the group
   agreed that it may be beneficial for the W3C to do some more thinking
   in this area, possibly toward the end of including usability
   considerations in individual specifications.  The challenge with such
   an effort will be to provide useful guidance without being overly
   prescriptive about how implementations should be designed.

4.3.  Interaction Between Business, Legal and Technical Incentives

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4.3.1.  Role of Regulation

   The Internet has sustained commercial content for decades.  Many
   services are offered at little or no cost in exchange for being able
   to sell advertising or collect user data (or both).  As the
   commercial value of the web in particular has exploded in recent
   years, the paradigm for regulating privacy has also begun to change,
   albeit more slowly.

   At the dawn of the commercial Internet, few web sites had written
   privacy policies that explained what they did with user data.  Under
   regulatory pressure, sites began to document their data collection
   and usage practices in publicly posted policies.  These policies
   quickly became lengthy legal documents that commercial sites could
   use to limit their liability, often by disclosing every possible
   practice that the site might engage in, rather than informing users
   about the salient practices of relevance to them.

   Because so many businesses are fueled by user data, any move to give
   users greater control over their data -- whether by better informing
   them about its use or providing tools and settings -- often requires
   the force of regulatory influence to succeed.  In recent years,
   regulatory authorities have put pressure on companies to improve
   their privacy disclosures by making them simpler, more concise, more
   prominent, and more accessible (see the 2010 Federal Trade Commission
   privacy report [FTC]).  Certain companies and industry sectors have
   responded by developing privacy icons, using short notices in
   addition to privacy policies, and making the language they use to
   describe privacy practices more accessible and easier to understand.

   Regulators play an important role in shaping incentive structures.
   Companies often seek a balance between acting to limit their
   liability and pushing the envelope with respect to uses of consumer
   data.  If regulators take a strong stand against certain practices --
   as, for example, European legislators have against cookies being set
   without user consent [Directive] -- legitimate businesses will feel
   compelled to comply.  But where there is regulatory uncertainty,
   business responses may differ according to different market
   strategies.  The variety of potential responses to the emerging
   discussion about mechanisms to control web tracking demonstrate this
   variation: some businesses will embrace support for enhanced user
   control, others may restrict their offerings or charge fees if they
   are unable to track users, and still others may elect to circumvent
   any new mechanisms put in place.  The absence of regulatory pressure
   tends to make the line between "good" and "bad" actors less evident.

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4.3.2.  P3P: A Case Study of the Importance of Incentives

   That absence of regulatory pressure revealed itself in the case of
   P3P. The first version of P3P was standardized in the early 2000's,
   when legalistic privacy policies were the norm and users had only
   elementary controls over the data collected about them on the web.
   P3P challenged that paradigm by providing a way for web sites to
   express machine-readable privacy policies for browsers to vet and
   possibly override according to the user's preference.  The P3P policy
   expression language was designed to allow sites to make complex
   assertions about how they intended to make use of data related to

   The designers of Internet Explorer 6 made a crucial decision to only
   allow sites to use third-party cookies if they had installed adequate
   P3P policies.  To avoid having their cookies blocked, most commercial
   sites adopted some P3P policy, although many sites merely cut and
   pasted from the example policies provided by the W3C. Today, large
   numbers of sites are misrepresenting their privacy practices in their
   P3P policies, but little has been done in response [Policies], and
   browser support for P3P outside of IE is limited.

   While theories abound to explain the current status of P3P
   implementations, there is no doubt that the relationship between
   regulatory and commercial incentives played a significant role.  The
   P3P policy expression language provided support for companies to be
   able to express in granular detail how they handle user data, but the
   companies had little reason to do so, preferring to protect
   themselves from the liability associated with revealing potentially
   unsavory practices.  In theory the threat of regulatory backlash
   could have served as an incentive to publish accurate P3P policies,
   but at the time of P3P's release, there was little regulatory
   interest in moving beyond long, legalistic privacy policies.  Even
   today, regulators are reluctant to bring enforcement actions against
   companies with misleading policies, perhaps because their own
   incentive structure compels them to focus on other, more prominent

   The P3P experience is instructive in general for attempts at crafting
   privacy features that require the active participation of both ends
   of a communication.  Actors that are meant to articulate their own
   privacy preferences, whether they be companies or individuals,
   require incentives to do so, as do those that are meant to process
   and react to such preferences.  For example, the IETF's GEOPRIV
   architecture allows for expression of user preferences about location
   information [RFC4119].  While users may have more incentive to
   disclose their privacy preferences than companies did in the P3P
   case, successful use of the GEOPRIV model will require endpoints that

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   consume location information to abide by those preferences, and in
   certain contexts -- commercial or employment-related, for example --
   they may be unwilling, or regulatory pressure may be required to spur
   a change in practice.

   It is clearly not the prerogative of Internet protocol developers to
   seek to change existing incentive structures.  But acknowledging what
   motivates businesses, individuals, and regulators is crucial to
   determining whether new privacy technologies will succeed or fail.

5.  Conclusions and Next Steps

5.1.  IETF Outlook

   The workshop demonstrated that the understanding of how to address
   privacy within the Internet standards community is nascent.  The IETF
   faces particular challenges because IETF protocols generally do not
   mandate implementation styles or pre-conceive particular deployment
   contexts, making the space of potential privacy threats attributable
   to any single protocol difficult to foresee at protocol design time.

   Workshop participants nonetheless outlined a number of potential next
   steps.  Work has already begun to attempt to provide guidance to
   protocol designers about the privacy impact of their specifications
   [I-D.morris-privacy-considerations].  In refining this guidance, many
   of the questions raised at the workshop will need to be confronted,
   including those about how to properly model privacy threats against
   generic protocols, how to anticipate privacy risks that have been
   exposed in the previous design efforts, and how to document risks
   that are more difficult to foresee and mitigate.  Workshop
   participants acknowledged that developing such guidance is likely
   necessary if document authors are expected to incorporate "Privacy
   Considerations" sections in their documents, but even with guidance
   this is likely to be an uphill battle for many authors for some time
   to come.

   As preliminary steps, those with privacy expertise may seek to apply
   the current guidance to existing IETF protocols.  The security area
   directors have also created a privacy directorate where privacy
   reviews of documents coming before the IESG are being conducted.

   Participants also expressed an interest in further pursuing a number
   of the technical topics discussed at the workshop, including lessons
   learned from the experience of Tor and the fingerprinting
   implications of HTTP, TCP, SIP, and other IETF protocols.  These and
   other efforts may be explored within the IRTF in addition to or in
   lieu of the IETF.

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5.2.  W3C Outlook

   The W3C is likewise in a position of seeking a more comprehensive
   approach to privacy within the SDO.  Because the work of the W3C
   operates within a more defined scope than that of the IETF -- namely,
   the web -- the questions before the W3C tend to lie more in the space
   of distinguishing between what can appropriately be accomplished
   within W3C specifications and what should be left to individual
   implementations, a theme that repeated itself again and again at the

   To further develop its approach to privacy, the W3C will investigate
   an interest group to discuss privacy topics.  Some potential topics
   that emerged from the workshop include the fingerprinting impact of
   W3C protocols, data minimization in APIs, dealing with referrer
   header privacy leakage, developing privacy considerations for non-
   browser-based protocols, and developing usability considerations as
   part of specification design.

5.3.  Other Future Work

   The workshop covered a number of topics that may deserve further
   exploration in the IETF, the W3C, and the privacy community at large.
   These include development of privacy terminology; articulation of
   privacy threat models; analysis and experimentation with "do not
   track" mechanisms for the web; work on cross-site data sharing,
   correlation, and linkability in web and non-web contexts; and
   investigation of policy expression languages.

6.  Acknowledgements

   Thanks to Bernard Aboba, Nick Doty and Hannes Tschofenig for their
   early reviews.

7.  IANA Considerations

   This memo includes no requests of IANA.

8.  Security Considerations

   Workshop participants discussed security aspects related to privacy,
   acknowledging that while much of the standards community may have
   once viewed most relevant privacy concerns as being encompassed by
   security considerations, there is a growing realization of privacy
   threats that lie outside the security realm.  These include concerns

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   related to data minimization, identifiability, and secondary use.
   Earlier security work provided minimal provision for privacy
   protection (e.g., the definition of "privacy" in [RFC2828] and some
   guidance about private information in [RFC3552]).

9.  Informative References

   [ALTO]     IETF, "Application-Layer Traffic Optimization",
               http://datatracker.ietf.org/wg/alto/charter/, 2011.

              European Parliament and Council of the European Union,
              "Directive 2009/136/EC of the European Parliament and of
              the Council",  http://eur-lex.europa.eu/LexUriServ/
              September 2010.

   [FTC]      Federal Trade Commission Staff, "A Preliminary FTC Staff
              Report on Protecting Consumer Privacy in an Era of Rapid
              Change: A Proposed Framework for Businesses and
              December 2010.

              Popescu, A., Ed., "Geolocation API Specification",
              September 2010.

              Cooper, A. and H. Tschofenig, "Overview of Universal Opt-
              Out Mechanisms for Web Tracking",
              draft-cooper-web-tracking-opt-outs-00 (work in progress),
              March 2011.

              Aboba, B., Morris, J., Peterson, J., and H. Tschofenig,
              "Privacy Considerations for Internet Protocols",
              draft-morris-privacy-considerations-03 (work in progress),
              March 2011.

   [IETF77]   Krishnamurthy, B., "Privacy Leakage on the Internet",
              March 2010.

              Krishnamurthy, B., Malandrino, D., and C. Wills,

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              "Measuring privacy loss and the impact of privacy
              protection in web browsing. In Proceedings of the
              Symposium on Usable Privacy and Security, pages 52-63,
              Pittsburgh, PA USA, July 2007. ACM International
              Conference Proceedings Series.",
               http://www.cs.wpi.edu/~cew/papers/soups07.pdf, July 2007.

              Krishnamurthy, B. and C. Wills, "Privacy diffusion on the
              web: A longitudinal perspective. In Proceedings of the
              World Wide Web Conference, pages 541-550, Madrid, Spain,
              April 2009",  http://www.cs.wpi.edu/~cew/papers/www09.pdf,
              April 2009.

   [P3P]      Wenning, R., Ed. and M. Schunter, Ed., "The Platform for
              Privacy Preferences 1.1 (P3P1.1) Specification",
               http://www.w3.org/TR/P3P11/, November 2006.

              Electronic Frontier Foundation, "Panopticlick", 2011,

              Leon, P., Cranor, L., McDonald, A., and R. McGuire, "Token
              Attempt: The Misrepresentation of Website Privacy Policies
              through the Misuse of P3P Compact Policy Tokens",  http://
              tr_cylab10014.html, September 2010.

   [RFC1543]  Postel, J., "Instructions to RFC Authors", RFC 1543,
              October 1993.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2778]  Day, M., Rosenberg, J., and H. Sugano, "A Model for
              Presence and Instant Messaging", RFC 2778, February 2000.

   [RFC2828]  Shirey, R., "Internet Security Glossary", RFC 2828,
              May 2000.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3323]  Peterson, J., "A Privacy Mechanism for the Session

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              Initiation Protocol (SIP)", RFC 3323, November 2002.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              July 2003.

   [RFC3693]  Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and
              J. Polk, "Geopriv Requirements", RFC 3693, February 2004.

   [RFC4119]  Peterson, J., "A Presence-based GEOPRIV Location Object
              Format", RFC 4119, December 2005.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, September 2007.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.

   [Tor]      The Tor Project, Inc., "Tor", 2011,

              IAB, W3C, ISOC, MIT CSAIL, "Internet Privacy Workshop
              2010", 2011, <http://www.iab.org/activities/workshops/

Appendix A.  Workshop Materials

      Main page: http://www.iab.org/activities/workshops/

      Slides: http://www.iab.org/activities/workshops/

      Minutes: http://www.iab.org/activities/workshops/

      Position papers: http://www.iab.org/activities/workshops/

Appendix B.  Workshop Participants

      Fu-Ming Shih, MIT

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      Ian Jacobi, MIT

      Steve Woodrow, MIT

      Nick Mathewson, The Tor Project

      Peter Eckersley, Electronic Frontier Foundation

      John Klensin, IAB

      Oliver Hanka, Technical University Munich

      Alan Mislove, Northeastern University

      Ashkan Soltani, FTC

      Sam Hartman, Painless Security

      Kevin Trilli, TRUSTe

      Dorothy Gellert, InterDigital

      Aaron Falk, Raytheon - BBN Technologies

      Sean Turner, IECA

      Wei-Yeh Lee, NAVTEQ

      Chad McClung, The Boeing Company

      Jan Seedorf, NEC

      Dave Crocker, Brandenburg InternetWorking

      Lorrie Cranor, Carnegie Mellon University

      Noah Mendelsohn, W3C TAG Chair

      Stefan Winter, RESTENA

      Craig Wittenberg, Microsoft

      Bernard Aboba, IAB/Microsoft

      Heather West, Google

      Blaine Cook, British Telecom

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      Kasey Chappelle, Vodafone Group

      Russ Housley, IETF Chair/Vigil Security, LLC

      Daniel Appelquist, Vodafone R&D

      Olaf Kolkman, IAB Chair

      Jon Peterson, IAB/NeuStar, Inc.

      Balachander Krishnamurthy, AT&T Labs--Research

      Marc Linsner, Cisco Systems

      Jorge Cuellar, Siemens AG

      Arvind Narayanan, Stanford University

      Eric Rescorla, Skype

      Cullen Jennings, Cisco

      Christine Runnegar, Internet Society

      Alissa Cooper, Center for Democracy & Technology

      Jim Fenton, Cisco

      Oshani Seneviratne, MIT

      Lalana Kagal, MIT

      Fred Carter, Information & Privacy Commissioner of Ontario, Canada

      Frederick Hirsch, Nokia

      Benjamin Heitmann, DERI, NUI Galway, Ireland

      John Linn, RSA, The Security Division of EMC

      Paul Trevithick, Azigo

      Ari Schwartz, National Institute of Standards and Technology

      David Evans, University of Cambridge

      Nick Doty, UC Berkeley, School of Information

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      Sharon Paradesi, MIT

      Jonathan Mayer, Stanford University

      David Maher, Intertrust

      Brett McDowell, PayPal

      Leucio Antonio Cutillo, Eurecom

      Susan Landau, Radcliffe Institute for Advanced Study, Harvard

      Dave Crocker, Brandenburg InternetWorking

      Christopher Soghoian, FTC In-house Technologist, Center for
      Applied Cybersecurity Research, Indiana University

      Trent Adams, Internet Society

      Thomas Roessler, W3C

      Karen O'Donoghue, ISOC

      Hannes Tschofenig, IAB/Nokia Siemens Networks

      Lucy Elizabeth Lynch, Internet Society

      Karen Sollins, MIT

      Tim Berners-Lee, W3C

Appendix C.  Accepted Position Papers

   1.   Addressing the privacy management crisis in online social
        networks by Krishna Gummadi, Balachander Krishnamurthy, and Alan

   2.   Thoughts on Adding "Privacy Considerations" to Internet Drafts
        by Alissa Cooper, and John Morris

   3.   Toward Objective Global Privacy Standards by Ari Schwartz

   4.   SocialKeys: Transparent Cryptography via Key Distribution over
        Social Networks by Arvind Narayanan

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   5.   Web Crawlers and Privacy: The Need to Reboot Robots.txt by
        Arvind Narayanan and Pete Warden

   6.   I Know What You Will Do Next Summer by Balachander Krishnamurthy

   7.   An architecture for privacy-enabled user profile portability on
        the Web of Data by Benjamin Heitmann and Conor Hayes

   8.   Addressing Identity on the Web by Blaine Cook

   9.   Protection-by-Design: Enhancing ecosystem capabilities to
        protect personal information by Jonathan Fox and Brett McDowell

   10.  Privacy-preserving identities for a safer, more trusted internet
        by Christian Paquin

   11.  Why Private Browsing Modes Do Not Deliver Real Privacy by
        Christopher Soghoian

   12.  Incentives for Privacy by Cullen Jennings

   13.  Joint Privacy Workshop: Position Comments by D. Crocker by Dave

   14.  Using properties of physical phenomena and information flow
        control to manage privacy by David Evans and David M. Eyers

   15.  Privacy Approaches for Internet Video Advertising by Dave Maher

   16.  Privacy on the Internet by Dorothy Gellert

   17.  Can We Have a Usable Internet Without User Trackability? by Eric

   18.  Privacy by Design: The 7 Foundational Principles --
        Implementation and Mapping of Fair Information Practices by Fred
        Carter and Ann Cavoukian

   19.  Internet Privacy Workshop Position Paper: Privacy and Device
        APIs by Frederick Hirsch

   20.  Position Paper for Internet Privacy Workshop by Heather West

   21.  I 'like' you, but I hate your apps by Ian Glazer

   22.  Privicons: A approach to communicating privacy preferences
        between Users by E. Forrest and J. SchallabAP.ck

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   23.  Privacy Preservation Techniques to establish Trustworthiness for
        Distributed, Inter-Provider Monitoring by J. Seedorf, S.
        Niccolini, A. Sarma, B. Trammell, and G. Bianchi

   24.  Trusted Intermediaries as Privacy Agents by Jim Fenton

   25.  Protocols are for sharing by John Kemp

   26.  On Technology and Internet Privacy by John Linn

   27.  Do Not Track: Universal Web Tracking Opt-out by Jonathan Mayer
        and Arvind Narayanan

   28.  Location Privacy Protection Through Obfuscation by Jorge Cuellar

   29.  Everything we thought we knew about privacy is wrong by Kasey
        Chappelle and Dan Appelquist

   30.  TRUSTe Position Paper by Kevin Trilli

   31.  Position Paper: Incentives for Adoption of Machine-Readable
        Privacy Notices by Lorrie Cranor

   32.  Facilitate, don't mandate by Ari Rabkin, Nick Doty and Deirdre
        K. Mulligan

   33.  Location Privacy in Next Generation Internet Architectures by
        Oliver Hanka

   34.  HTTPa: Accountable HTTP by Oshani Seneviratne and Lalana Kagal

   35.  Personal Data Service by Paul Trevithick

   36.  Several Pressing Problems in Hypertext Privacy by Peter

   37.  Adding Privacy in Existing Security Systems by Sam Hartman

   38.  Mobility and Privacy by S. Brim, M. Linsner, B. McLaughlin, and
        K. Wierenga

   39.  Saveface: Save George's faces in Social Networks where Contexts
        Collapse by Fuming Shih and Sharon Paradesi

   40.  eduroam -- a world-wide network access roaming consortium on the
        edge of preserving privacy vs. identifying users by Stefan

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   41.  Effective Device API Privacy: Protecting Everyone (Not Just the
        User) by Susan Landau

   42.  Safebook: Privacy Preserving Online Social Network by L. Antonio
        Cutillo, R. Molva, and M. Onen

Author's Address

   Alissa Cooper
   1634 I Street NW, Suite 1100
   Washington, DC  20006

   Email: acooper@cdt.org
   URI:   http://www.cdt.org/

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