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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" ipr="trust200902" docName="draft-iab-privacy-partitioning-05" number="9614" category="info" submissionType="IAB" consensus="true" tocInclude="true" updates="" obsoletes="" sortRefs="true" symRefs="true" xml:lang="en" version="3">
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  <front>
    <title abbrev="Partitioning for Privacy">Partitioning as an Architecture for Privacy</title>
    <seriesInfo name="Internet-Draft" value="draft-iab-privacy-partitioning-05"/> name="RFC" value="9614"/>
    <author fullname="Mirja Kühlewind">
      <organization>Ericsson Research</organization> Kühlewind" initials="M." surname="Kühlewind">
      <address>
        <email>mirja.kuehlewind@ericsson.com</email>
      </address>
    </author>
    <author fullname="Tommy Pauly">
      <organization>Apple</organization> Pauly" initials="T." surname="Pauly">
      <address>
        <email>tpauly@apple.com</email>
      </address>
    </author>
    <author fullname="Christopher A. Wood">
      <organization>Cloudflare</organization> Wood" initials="C. A." surname="Wood">
      <address>
        <email>caw@heapingbits.net</email>
      </address>
    </author>

    <date year="2024" month="January" day="11"/>
    <keyword>Internet-Draft</keyword>
    <abstract>
      <?line 33?>

<t>This month="July"/>

<keyword>MASQUE</keyword>
<keyword>Privacy Pass</keyword>
<keyword>Oblivious HTTP</keyword>
<keyword>Proxy</keyword>

    <abstract><t>This document describes the principle of privacy
    partitioning, which selectively spreads data and communication across
    multiple parties as a means to improve privacy by separating user identity
    from user data.  This document describes emerging patterns in protocols to
    partition what data and metadata is revealed through protocol
    interactions, provides common terminology, and discusses how to analyze
    such models.</t>
    </abstract>
    <note removeInRFC="true">
      <name>Discussion Venues</name>
      <t>Discussion of this document takes place on the
    Internet Architecture Board Internet Engineering Task Force mailing list (iab@iab.org),
    which is archived at <eref target=""/>.</t>
      <t>Source for this draft and an issue tracker can be found at
    <eref target="https://github.com/intarchboard/draft-obliviousness"/>.</t>
    </note>

  </front>
  <middle>
    <?line 41?>

<section anchor="introduction">
      <name>Introduction</name>
      <t>Protocols such as TLS and IPsec provide a secure (authenticated and
      encrypted) channel between two endpoints over which endpoints transfer
      information. Encryption and authentication of data in transit are
      necessary to protect information from being seen or modified by parties
      other than the intended protocol participants. As such, this kind of
      security is necessary for ensuring that information transferred over
      these channels remains private.</t>
      <t>However, a secure channel between two endpoints is insufficient for
      the privacy of the endpoints themselves. In recent years, privacy
      requirements have expanded beyond the need to protect data in transit
      between two endpoints. Some examples of this expansion include:</t>
      <ul spacing="normal">
        <li>
          <t>A
        <li>A user accessing a service on a website might not consent to
        reveal their location, but if that service is able to observe the
        client's IP address, it can learn something about the user's
        location. This is problematic for privacy since the service can link
        user data to the user's location.</t>
        </li>
        <li>
          <t>A location.</li>
	<li>A user might want to be able to access content for which they are
	authorized, such as a news article, without needing to have article; but the news service might track
	which specific articles they
read on users access which articles, even if the user doesn't want their account being recorded.
	activity to be tracked. This is problematic for privacy since the
	service can link user activity to the user's account.</t>
        </li>
        <li>
          <t>A account.</li>
	<li>A client device that needs might need to upload metrics to an aggregation
	service might want to be
able to contribute data and in doing so allow the service to attribute the system without having their specific
	metrics contributions
attributed to them. that client device.  This is problematic for
	privacy since the service can link client contributions to the
	specific client.</t>
        </li> client.</li>

      </ul>
      <t>The commonality in these examples is that clients want to interact with or use a service
without exposing too much user-specific or identifying information to that service. In particular,
separating the user-specific identity information from user-specific data is necessary for
privacy. Thus, in order to protect user privacy, it is important to keep identity (who) and data
(what) separate.</t>
      <t>This document defines "privacy partitioning," sometimes also referred to as the "decoupling principle"
<xref target="DECOUPLING"/>, as the general technique used to separate the data
and metadata visible to various parties in network communication, with the aim of improving
user privacy. Although privacy partitioning cannot guarantee there is no link between user-specific
identity and user-specific data, when applied properly it helps ensure that user privacy violations
become more technically difficult to achieve over time.</t>
      <t>Several IETF working groups are working on protocols or systems that adhere to the principle
of privacy partitioning, including Oblivious HTTP Application Intermediation (OHAI), Multiplexed
Application Substrate over QUIC Encryption (MASQUE), Privacy Pass, and Privacy Preserving Measurement (PPM). This document summarizes
work in those groups and describes a framework for reasoning thinking about the resulting privacy posture of different
endpoints in practice.</t>
      <t>Privacy partitioning is particularly relevant as a tool for data
      minimization, which is described in <xref section="6.1"
      sectionFormat="of" target="RFC6973"/>. <xref target="RFC6973"/> provides
      guidance for privacy considerations in Internet protocols, along with a
      set of questions on how to evaluate the data minimization of a protocol
      in <xref section="7.1" sectionFormat="of" target="RFC6973"/>. Protocols
      that employ privacy partitioning ought to consider the questions in that
      section when evaluating their design, particularly with regard to how
      identifiers and data can be correlated by protocol participants and
      observers in each context that has been partitioned. Privacy
      partitioning can also be used as a way to separate identity providers
      from relying parties (see <xref section="6.1.4" sectionFormat="of"
      target="RFC6973"/>), as in the case of Privacy Pass (see Section <xref
      target="privacypass"/>).</t>
      <t>Privacy partitioning is not a panacea; applying it well requires
      holistic analysis of the system in question to determine whether or not
      partitioning as a tool, and as implemented, offers meaningful privacy
      improvements. See <xref target="limits"/> for more information.</t>
    </section>
    <section anchor="privacy-partitioning">
      <name>Privacy Partitioning</name>
      <t>For the purposes of user privacy, this document focuses on
      user-specific information. This might include any identifying
      information that is specific to a user, such as their email address or
      IP address, or any data about the user, such as their date of
      birth. Informally, the goal of privacy partitioning is to ensure that
      each party in a system beyond the user themselves only have has access to
      one type of user-specific information.</t>
      <t>This is a simple application of the principle of least privilege, wherein every party in
a system only has access to the minimum amount of information needed to fulfill their
function. Privacy partitioning advocates for this minimization by ensuring that protocols,
applications, and systems only reveal user-specific information to parties that need access
to the information for their intended purpose.</t>
      <t>Put simply, privacy partitioning aims to separate <em>who</em> someone is from <em>what</em> they do. In the
rest of this section, we describe how privacy partitioning can be used to achieve this goal.</t>
      <section anchor="privacy-contexts">
        <name>Privacy Contexts</name>
        <t>Each piece of user-specific information exists within some context, where a context
is abstractly defined as a set of data, metadata, and the entities that share access
to that information. In order to prevent the correlation of user-specific information across
contexts, partitions need to ensure that any single entity (other than the client itself)
does not participate in more than one context where the information is visible.</t>
        <t><xref target="RFC6973"/> discusses the importance of identifiers in reducing correlation as a way
of improving privacy:</t>
        <blockquote>
          <t>Correlation is the combination of various pieces of information
          related to an individual or that obtain that characteristic when combined... Correlation
          combined....</t>
	  <t>Correlation is closely related to identification.
          Internet protocols can facilitate correlation by allowing
          individuals' activities to be tracked and combined over time.</t> time....</t>
          <t>Pseudonymity is strengthened when less personal data can be
          linked to the pseudonym; when the same pseudonym is used less often
          and across fewer contexts; and when independently chosen pseudonyms
          are more frequently used for new actions (making them, from an
          observer's or attacker's perspective, unlinkable).</t>
        </blockquote>
        <t>Context separation is foundational to privacy partitioning and
        reducing correlation.  As an example, consider an unencrypted HTTP
        session over TCP, wherein the context includes both the content of the
        transaction as well as any metadata from the transport and IP headers;
        and the participants include the client, routers, other network
        middleboxes, intermediaries, and the server.
Middlboxes  Middleboxes or intermediaries
        might simply forward traffic, traffic or might terminate the traffic at any
        layer (such as terminating the TCP connection from the client and
        creating another TCP connection to the server). Regardless of how the
        middlebox interacts with the traffic, for the purposes of privacy
        partitioning, it is able to observe all of the data in the
        context.</t>
        <figure anchor="diagram-middlebox">
          <name>Diagram of a basic unencrypted client-to-server connection Basic Unencrypted Client-to-Server Connection with middleboxes</name> Middleboxes</name>
          <artset>
            <artwork type="svg"><svg xmlns="http://www.w3.org/2000/svg" version="1.1" height="176" width="560" viewBox="0 0 560 176" class="diagram" text-anchor="middle" font-family="monospace" font-size="13px" stroke-linecap="round">
                <path d="M 8,32 L 8,160" fill="none" stroke="black"/>
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                <g class="text">
                  <text x="48" y="52">Context</text>
                  <text x="88" y="52">A</text>
                  <text x="172" y="84">HTTP</text>
                  <text x="68" y="100">Client</text>
                  <text x="288" y="100">Middlebox</text>
                  <text x="492" y="100">Server</text>
                  <text x="168" y="116">TCP</text>
                  <text x="172" y="132">flow</text>
                </g>
              </svg>
            </artwork>
            <artwork type="ascii-art"><![CDATA[
+-------------------------------------------------------------------+
| Context A                                                         |
|  +--------+                +-----------+              +--------+  |
|  |        +------HTTP------+           +--------------+        |  |
|  | Client |                | Middlebox |              | Server |  |
|  |        +------TCP-------+           +--------------+        |  |
|  +--------+      flow      +-----------+              +--------+  |
|                                                                   |
+-------------------------------------------------------------------+
]]></artwork>
          </artset>
        </figure>

        <t>Adding TLS encryption to the HTTP session is a simple partitioning
        technique that splits the previous context into two separate contexts: the contexts.
        The content of the transaction is now only visible to the client,
        TLS-terminating intermediaries, and server; server, while the metadata in
        transport and IP headers remain in the original context. In this
        scenario, without any further partitioning, the entities that
        participate in both contexts can allow the data in both contexts to be
        correlated.</t>
        <figure anchor="diagram-https">
          <name>Diagram of how adding encryption splits How Adding Encryption Splits the context Context into two</name> Two</name>
          <artset>
            <artwork type="svg"><svg xmlns="http://www.w3.org/2000/svg" version="1.1" height="304" width="560" viewBox="0 0 560 304" class="diagram" text-anchor="middle" font-family="monospace" font-size="13px" stroke-linecap="round">
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                <g class="text">
                  <text x="48" y="52">Context</text>
                  <text x="88" y="52">A</text>
                  <text x="68" y="100">Client</text>
                  <text x="280" y="100">HTTPS</text>
                  <text x="492" y="100">Server</text>
                  <text x="48" y="180">Context</text>
                  <text x="88" y="180">B</text>
                  <text x="68" y="228">Client</text>
                  <text x="176" y="228">TCP</text>
                  <text x="288" y="228">Middlebox</text>
                  <text x="492" y="228">Server</text>
                  <text x="180" y="244">flow</text>
                </g>
              </svg>
            </artwork>
            <artwork type="ascii-art"><![CDATA[
+-------------------------------------------------------------------+
| Context A                                                         |
|  +--------+                                           +--------+  |
|  |        |                                           |        |  |
|  | Client +-------------------HTTPS-------------------+ Server |  |
|  |        |                                           |        |  |
|  +--------+                                           +--------+  |
|                                                                   |
+-------------------------------------------------------------------+
| Context B                                                         |
|  +--------+                +-----------+              +--------+  |
|  |        |                |           |              |        |  |
|  | Client +-------TCP------+ Middlebox +--------------+ Server |  |
|  |        |       flow     |           |              |        |  |
|  +--------+                +-----------+              +--------+  |
|                                                                   |
+-------------------------------------------------------------------+
]]></artwork>
          </artset>
        </figure>
        <t>Another way to create a partition is to simply use separate
        connections. For example, to split two separate HTTP requests from one
        another, a client could issue the requests on separate TCP
        connections, each on a different network, network and at different times; times, and
        avoid including obvious identifiers like HTTP cookies across the
        requests.</t>
        <figure anchor="diagram-dualconnect">
          <name>Diagram of making separate connections Making Separate Connections to generate separate contexts</name> Generate Separate Contexts</name>
          <artset>
            <artwork type="svg"><svg xmlns="http://www.w3.org/2000/svg" version="1.1" height="304" width="560" viewBox="0 0 560 304" class="diagram" text-anchor="middle" font-family="monospace" font-size="13px" stroke-linecap="round">
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                  <text x="48" y="52">Context</text>
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                  <text x="288" y="228">Middlebox</text>
                  <text x="492" y="228">Server</text>
                  <text x="172" y="244">flow</text>
                  <text x="200" y="244">B</text>
                  <text x="288" y="244">B</text>
                </g>
              </svg>
            </artwork>
            <artwork type="ascii-art"><![CDATA[
+-------------------------------------------------------------------+
| Context A                                                         |
|  +--------+                +-----------+              +--------+  |
|  |        | IP A           |           |              |        |  |
|  | Client +-------TCP------+ Middlebox +--------------+ Server |  |
|  |        |      flow A    |     A     |              |        |  |
|  +--------+                +-----------+              +--------+  |
|                                                                   |
+-------------------------------------------------------------------+
| Context B                                                         |
|  +--------+                +-----------+              +--------+  |
|  |        | IP B           |           |              |        |  |
|  | Client +-------TCP------+ Middlebox +--------------+ Server |  |
|  |        |      flow B    |     B     |              |        |  |
|  +--------+                +-----------+              +--------+  |
|                                                                   |
+-------------------------------------------------------------------+
]]></artwork>
          </artset>
        </figure>
        <t>Using separate connections to create separate contexts can reduce or eliminate
the ability of specific parties to correlate activity across contexts. However,
any identifier at any layer that is common across different contexts can be
used as a way to correlate activity. Beyond IP addresses, many other factors
can be used together to create a fingerprint of a specific device (such as
MAC
Media Access Control (MAC) addresses, device properties, software properties and behavior, application state, etc).</t> etc.).</t>
      </section>
      <section anchor="context-separation">
        <name>Context Separation</name>
        <t>In order to define and analyze how various partitioning techniques work, the boundaries of what is
being partitioned need to be established. This is the role of context separation. In particular,
in order to prevent the correlation of user-specific information across contexts, partitions need
to ensure that any single entity (other than the client itself) does not participate in contexts
where both identifiers are visible.</t>
        <t>Context separation can be achieved in different ways, for example, over time, across network paths, based
on (en)coding, etc. The privacy-oriented protocols described in this document generally involve
more complex partitioning, but the techniques to partition communication contexts still employ the
same techniques:</t>
        <ol spacing="normal" type="1"><li>
            <t>Cryptographic
        <ul spacing="normal">
	  <li>Cryptographic protection, such as the use of encryption to
	  specific parties, allows partitioning of contexts between different
	  parties (those with the ability to remove cryptographic protections,
	  and those without).</t>
          </li>
          <li>
            <t>Connection without).</li>
          <li>Connection separation across time or space to allow
          partitioning of contexts for different application transactions over
          the network.</t>
          </li>
        </ol> network.</li>
        </ul>
        <t>These techniques are frequently used in conjunction for context separation. For example,
encrypting an HTTP exchange using TLS between the client and TLS-terminating server might prevent
a network middlebox that sees a client IP address from seeing the user account identifier,
but it doesn't prevent the TLS-terminating server from observing both identifiers and correlating
them. As such, preventing correlation requires separating contexts, such as by using proxying to
conceal a client's IP address that would otherwise be used as an identifier.</t>
      </section>
      <section anchor="approaches-to-partitioning">
        <name>Approaches to Partitioning</name>
        <t>While all of the partitioning protocols described in this document
        create separate contexts using cryptographic protection and/or
        connection separation, each one has a unique approach that results in
        different sets of contexts. Since many of these protocols are new, it
        is yet to be seen how each approach will be used at scale across the Internet,
        Internet and what new models will emerge in the future.</t>
        <t>There are multiple factors that lead to a diversity in approaches
        to partitioning, including:</t>
        <ul spacing="normal">
          <li>
            <t>Adding
          <li>Adding privacy partitioning to existing protocol ecosystems
          places requirements and constraints on how contexts are
          constructed. CONNECT-style proxying is intended to work with servers
          that are unaware of privacy contexts, requiring more intermediaries
          to provide strong separation guarantees.
Oblivious HTTP, on  On the other hand,
          Oblivious HTTP assumes servers that cooperate with context
separation, and thus
          separation and, thus, reduces the overall number of elements in the solution.</t>
          </li>
          <li>
            <t>Whether
          solution.</li>
          <li>Whether or not information exchange needs to happen
          bidirectionally in an interactive fashion determines how contexts
          can be separated. Some use cases, like metrics collection for PPM,
          can occur with whereby information flowing only flows from clients to servers, servers and
          can function even when clients are no longer connected.  Privacy
          Pass is an example of a case that can be either interactive or not,
          depending on whether tokens can be cached and reused. CONNECT-style
          proxying and Oblivious HTTP often requires require bidirectional and
          interactive communication.</t>
          </li>
          <li>
            <t>The communication.</li>
          <li>The degree to which contexts need to be partitioned depends in
          part on the client's threat models and level of trust in various
          protocol participants. For example, in Oblivious HTTP, clients allow
          relays to learn that clients are accessing a particular
          application-specific gateway. If clients do not trust relays with
          this information, they can instead use a multi-hop CONNECT-style
          proxy approach wherein no single party learns whether specific
          clients are accessing a specific application. This is the default
          trust model for systems like Tor, where multiple hops are used to
          drive down the probability of privacy violations due to collusion or
          related attacks.</t>
          </li> attacks.</li>
        </ul>
      </section>
    </section>

    <section anchor="a-survey-of-protocols-using-partitioning">
      <name>A Survey of Protocols using Using Partitioning</name>
      <t>The following section discusses current on-going work in the IETF
      that is applying privacy partitioning.</t>
      <section anchor="masque">
        <name>CONNECT Proxying and MASQUE</name>
        <t>HTTP forward proxies, when
        <t>When using encryption on the connection between the client and the
        proxy, HTTP forward proxies provide privacy partitioning by separating
        a connection into multiple segments.  When connections to targets via
        the proxy themselves are encrypted, the proxy cannot see the
        end-to-end content. HTTP has historically supported forward proxying
        for TCP-like streams via the CONNECT method. More recently, the
        Multiplexed Application Substrate over QUIC Encryption (MASQUE) working group
        Working Group has developed protocols to similarly proxy UDP <xref target="CONNECT-UDP"/>
        target="RFC9297"/> and IP packets <xref target="CONNECT-IP"/> target="RFC9484"/> based on
        tunneling.</t>
        <t>In a single-proxy setup, there is a tunnel connection between the
        client and proxy and an end-to-end connection that is tunnelled tunneled between
        the client and target. This setup, as shown in the figure below, <xref
        target="diagram-1hop"/>, partitions communication into:</t>
        <ul spacing="normal">
          <li>
            <t>a
          <li>a Client-to-Target encrypted context, which contains the
          end-to-end content within the TLS session to the target, such as
          HTTP content;</t>
          </li>
          <li>
            <t>a content;</li>
          <li>a Client-to-Target proxied context, which is the end-to-end data to
          exchanged with the target that is also visible to the proxy, such as a TLS session;</t>
          </li>
          <li>
            <t>a
          session;</li>
          <li>a Client-to-Proxy context, which contains the transport metadata
          between the client and the target, and the request to the proxy to
          open a connection to the target;</t> target; and </li>
          <li>
            <t>and a
	  <li>a Proxy-to-Target context, which for TCP and UDP proxying
	  contains any packet header information that is added or modified by
	  the proxy, e.g., the IP and TCP/UDP headers.</t>
          </li> headers.</li>
        </ul>
        <figure anchor="diagram-1hop">
          <name>Diagram of one-hop proxy contexts</name> One-Hop Proxy Contexts</name>
          <artset>
            <artwork type="svg"><svg xmlns="http://www.w3.org/2000/svg" version="1.1" height="560" width="560" viewBox="0 0 560 560" class="diagram" text-anchor="middle" font-family="monospace" font-size="13px" stroke-linecap="round">
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                <path d="M 32,64 L 32,128" fill="none" stroke="black"/>
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                <path d="M 456,256 L 528,256" fill="none" stroke="black"/>
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                <path d="M 32,320 L 104,320" fill="none" stroke="black"/>
                <path d="M 240,320 L 336,320" fill="none" stroke="black"/>
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                <path d="M 208,352 L 240,352" fill="none" stroke="black"/>
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                <path d="M 240,384 L 336,384" fill="none" stroke="black"/>
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                <path d="M 456,448 L 528,448" fill="none" stroke="black"/>
                <path d="M 336,480 L 352,480" fill="none" stroke="black"/>
                <path d="M 432,480 L 456,480" fill="none" stroke="black"/>
                <path d="M 240,512 L 336,512" fill="none" stroke="black"/>
                <path d="M 456,512 L 528,512" fill="none" stroke="black"/>
                <path d="M 8,544 L 552,544" fill="none" stroke="black"/>
                <g class="text">
                  <text x="84" y="52">Client-to-Target</text>
                  <text x="192" y="52">Encrypted</text>
                  <text x="264" y="52">Context</text>
                  <text x="68" y="100">Client</text>
                  <text x="272" y="100">HTTPS</text>
                  <text x="492" y="100">Target</text>
                  <text x="272" y="116">content</text>
                  <text x="84" y="180">Client-to-Target</text>
                  <text x="184" y="180">Proxied</text>
                  <text x="248" y="180">Context</text>
                  <text x="68" y="228">Client</text>
                  <text x="168" y="228">Proxied</text>
                  <text x="288" y="228">Proxy</text>
                  <text x="492" y="228">Target</text>
                  <text x="152" y="244">TLS</text>
                  <text x="188" y="244">flow</text>
                  <text x="80" y="308">Client-to-Proxy</text>
                  <text x="176" y="308">Context</text>
                  <text x="68" y="356">Client</text>
                  <text x="168" y="356">Transport</text>
                  <text x="288" y="356">Proxy</text>
                  <text x="164" y="372">flow</text>
                  <text x="80" y="436">Proxy-to-Target</text>
                  <text x="176" y="436">Context</text>
                  <text x="288" y="484">Proxy</text>
                  <text x="392" y="484">Transport</text>
                  <text x="492" y="484">Target</text>
                  <text x="388" y="500">flow</text>
                </g>
              </svg>
            </artwork>
            <artwork type="ascii-art"><![CDATA[
+-------------------------------------------------------------------+
| Client-to-Target Encrypted Context                                |
|  +--------+                                           +--------+  |
|  |        |                                           |        |  |
|  | Client +------------------HTTPS--------------------+ Target |  |
|  |        |                 content                   |        |  |
|  +--------+                                           +--------+  |
|                                                                   |
+-------------------------------------------------------------------+
| Client-to-Target Proxied Context                                  |
|  +--------+                +-----------+              +--------+  |
|  |        |                |           |              |        |  |
|  | Client +----Proxied-----+   Proxy   +--------------+ Target |  |
|  |        |    TLS flow    |           |              |        |  |
|  +--------+                +-----------+              +--------+  |
|                                                                   |
+-------------------------------------------------------------------+
| Client-to-Proxy Context                                           |
|  +--------+                +-----------+                          |
|  |        |                |           |                          |
|  | Client +---Transport----+   Proxy   |                          |
|  |        |     flow       |           |                          |
|  +--------+                +-----------+                          |
|                                                                   |
+-------------------------------------------------------------------+
| Proxy-to-Target Context                                           |
|                            +-----------+              +--------+  |
|                            |           |              |        |  |
|                            |   Proxy   +--Transport---+ Target |  |
|                            |           |    flow      |        |  |
|                            +-----------+              +--------+  |
|                                                                   |
+-------------------------------------------------------------------+
]]></artwork>
          </artset>
        </figure>
        <t>Using two (or more) proxies provides better privacy
        partitioning. In particular, with two proxies, each proxy sees the
        Client metadata, metadata but not the Target; the Target, the Target but not the Client
metadata;
        metadata, or neither.</t>
        <t>In addition to the contexts described above for the single proxy
        case, the two-hop proxy case shown in the figure below <xref target="diagram-2hop"/>
        changes the contexts in several ways:</t>
        <ul spacing="normal">
          <li>
            <t>the
          <li>the Client-to-Target proxied context only includes the second
          proxy (referred to here as "Proxy B");</t>
          </li>
          <li>
            <t>a B");</li>
          <li>a new Client-to-Proxy B context is added, which is the TLS
          session from the client to Proxy B that is also visible to the first
          proxy (referred to here as "Proxy A");</t>
          </li>
          <li>
            <t>the A");</li>
          <li>the contexts that see transport data only (TCP or UDP over IP)
          are separated out into three separate contexts, a Client-to-Proxy A
          context, a Proxy A-to-Proxy B context, and a Proxy B-to-Target context.</t>
          </li>
          context.</li>
         </ul>
        <figure anchor="diagram-2hop">
          <name>Diagram of two-hop proxy contexts</name> Two-Hop Proxy Contexts</name>
          <artset>
            <artwork type="svg"><svg xmlns="http://www.w3.org/2000/svg" version="1.1" height="816" width="560" viewBox="0 0 560 816" class="diagram" text-anchor="middle" font-family="monospace" font-size="13px" stroke-linecap="round">
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                <g class="text">
                  <text x="84" y="52">Client-to-Target</text>
                  <text x="192" y="52">Encrypted</text>
                  <text x="264" y="52">Context</text>
                  <text x="68" y="100">Client</text>
                  <text x="272" y="100">HTTPS</text>
                  <text x="492" y="100">Target</text>
                  <text x="272" y="116">content</text>
                  <text x="84" y="180">Client-to-Target</text>
                  <text x="184" y="180">Proxied</text>
                  <text x="248" y="180">Context</text>
                  <text x="68" y="228">Client</text>
                  <text x="216" y="228">Proxied</text>
                  <text x="360" y="228">Proxy</text>
                  <text x="492" y="228">Target</text>
                  <text x="200" y="244">TLS</text>
                  <text x="236" y="244">flow</text>
                  <text x="360" y="244">B</text>
                  <text x="80" y="308">Client-to-Proxy</text>
                  <text x="152" y="308">B</text>
                  <text x="192" y="308">Context</text>
                  <text x="68" y="356">Client</text>
                  <text x="216" y="356">Proxy</text>
                  <text x="360" y="356">Proxy</text>
                  <text x="216" y="372">A</text>
                  <text x="360" y="372">B</text>
                  <text x="80" y="436">Client-to-Proxy</text>
                  <text x="152" y="436">A</text>
                  <text x="192" y="436">Context</text>
                  <text x="68" y="484">Client</text>
                  <text x="216" y="484">Proxy</text>
                  <text x="216" y="500">A</text>
                  <text x="40" y="564">Proxy</text>
                  <text x="108" y="564">A-to-Proxy</text>
                  <text x="160" y="564">B</text>
                  <text x="200" y="564">Context</text>
                  <text x="216" y="612">Proxy</text>
                  <text x="360" y="612">Proxy</text>
                  <text x="216" y="628">A</text>
                  <text x="360" y="628">B</text>
                  <text x="40" y="692">Proxy</text>
                  <text x="112" y="692">B-to-Target</text>
                  <text x="192" y="692">Context</text>
                  <text x="360" y="740">Proxy</text>
                  <text x="492" y="740">Target</text>
                  <text x="360" y="756">B</text>
                </g>
              </svg>
            </artwork>
            <artwork type="ascii-art"><![CDATA[
+-------------------------------------------------------------------+
| Client-to-Target Encrypted Context                                |
|  +--------+                                           +--------+  |
|  |        |                                           |        |  |
|  | Client +------------------HTTPS--------------------+ Target |  |
|  |        |                 content                   |        |  |
|  +--------+                                           +--------+  |
|                                                                   |
+-------------------------------------------------------------------+
| Client-to-Target Proxied Context                                  |
|  +--------+                           +-------+       +--------+  |
|  |        |                           |       |       |        |  |
|  | Client +----------Proxied----------+ Proxy +-------+ Target |  |
|  |        |          TLS flow         |   B   |       |        |  |
|  +--------+                           +-------+       +--------+  |
|                                                                   |
+-------------------------------------------------------------------+
| Client-to-Proxy B Context                                         |
|  +--------+         +-------+         +-------+                   |
|  |        |         |       |         |       |                   |
|  | Client +---------+ Proxy +---------+ Proxy |                   |
|  |        |         |   A   |         |   B   |                   |
|  +--------+         +-------+         +-------+                   |
|                                                                   |
+-------------------------------------------------------------------+
| Client-to-Proxy A Context                                         |
|  +--------+         +-------+                                     |
|  |        |         |       |                                     |
|  | Client +---------+ Proxy |                                     |
|  |        |         |   A   |                                     |
|  +--------+         +-------+                                     |
|                                                                   |
+-------------------------------------------------------------------+
| Proxy A-to-Proxy B Context                                        |
|                     +-------+         +-------+                   |
|                     |       |         |       |                   |
|                     | Proxy +---------+ Proxy |                   |
|                     |   A   |         |   B   |                   |
|                     +-------+         +-------+                   |
|                                                                   |
+-------------------------------------------------------------------+
| Proxy B-to-Target Context                                         |
|                                       +-------+       +--------+  |
|                                       |       |       |        |  |
|                                       | Proxy +-------+ Target |  |
|                                       |   B   |       |        |  |
|                                       +-------+       +--------+  |
|                                                                   |
+-------------------------------------------------------------------+
]]></artwork>
          </artset>
        </figure>

        <t>Forward proxying, such as the modes of proxying in the protocols
        developed in MASQUE, uses both encryption (via TLS) and separation of
        connections (via proxy hops that see only the next hop) to achieve
        privacy partitioning.</t>
      </section>
      <section anchor="oblivious-http-and-dns">
        <name>Oblivious HTTP and DNS</name>
        <t>Oblivious HTTP <xref target="OHTTP"/>, target="RFC9458"/>, developed in the Oblivious
        HTTP Application Intermediation (OHAI) working group, Working Group, adds per-message
        encryption to HTTP exchanges through a relay system. Clients send
        requests through an Oblivious Relay, which cannot read message
        contents, to an Oblivious Gateway, which can decrypt the messages but
        cannot communicate directly with the client or observe client metadata
        like an IP address.  Oblivious HTTP relies on Hybrid Public Key
        Encryption <xref target="HPKE"/> target="RFC9180"/> to perform encryption.</t>
        <t>Oblivious HTTP uses both encryption and separation of connections
        to achieve privacy partitioning.</t>
        <ul spacing="normal">
          <li>
            <t>End-to-end
          <li>End-to-end messages are encrypted between the Client and
          Gateway. The content of these inner messages are visible to the
          Client, Gateway, and Target. This is the Client-to-Target context.</t>
          </li>
          <li>
            <t>The
          context.</li>
          <li>The encrypted messages exchanged between the Client and Gateway
          are visible to the Relay, but the Relay cannot decrypt the messages.
          This is the Client-to-Gateway context.</t>
          </li>
          <li>
            <t>The context.</li>
          <li>The transport (such as TCP and TLS) connections between the
          Client, Relay, and Gateway form two separate contexts: a
          Client-to-Relay context and a Relay-to-Gateway context. It is
          important to note that the Relay-to-Gateway connection can be a
          single connection, even if the Relay has many separate Clients. This
          provides better anonymity by making the pseudonym presented by the
          Relay to be shared across many Clients.</t>
          </li> Clients.</li>
        </ul>
        <figure anchor="diagram-ohttp">
          <name>Diagram of Oblivious HTTP contexts</name> Contexts</name>
          <artset>
            <artwork type="svg"><svg xmlns="http://www.w3.org/2000/svg" version="1.1" height="560" width="560" viewBox="0 0 560 560" class="diagram" text-anchor="middle" font-family="monospace" font-size="13px" stroke-linecap="round">
                <path d="M 8,32 L 8,544" fill="none" stroke="black"/>
                <path d="M 32,64 L 32,128" fill="none" stroke="black"/>
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                <g class="text">
                  <text x="84" y="52">Client-to-Target</text>
                  <text x="184" y="52">Context</text>
                  <text x="68" y="100">Client</text>
                  <text x="368" y="100">Gateway</text>
                  <text x="492" y="100">Target</text>
                  <text x="88" y="180">Client-to-Gateway</text>
                  <text x="192" y="180">Context</text>
                  <text x="68" y="228">Client</text>
                  <text x="216" y="228">Relay</text>
                  <text x="368" y="228">Gateway</text>
                  <text x="80" y="308">Client-to-Relay</text>
                  <text x="176" y="308">Context</text>
                  <text x="68" y="356">Client</text>
                  <text x="216" y="356">Relay</text>
                  <text x="84" y="436">Relay-to-Gateway</text>
                  <text x="184" y="436">Context</text>
                  <text x="216" y="484">Relay</text>
                  <text x="368" y="484">Gateway</text>
                </g>
              </svg>
            </artwork>
            <artwork type="ascii-art"><![CDATA[
+-------------------------------------------------------------------+
| Client-to-Target Context                                          |
|  +--------+                           +---------+     +--------+  |
|  |        |                           |         |     |        |  |
|  | Client +---------------------------+ Gateway +-----+ Target |  |
|  |        |                           |         |     |        |  |
|  +--------+                           +---------+     +--------+  |
|                                                                   |
+-------------------------------------------------------------------+
| Client-to-Gateway Context                                         |
|  +--------+         +-------+         +---------+                 |
|  |        |         |       |         |         |                 |
|  | Client +---------+ Relay +---------+ Gateway |                 |
|  |        |         |       |         |         |                 |
|  +--------+         +-------+         +---------+                 |
|                                                                   |
+-------------------------------------------------------------------+
| Client-to-Relay Context                                           |
|  +--------+         +-------+                                     |
|  |        |         |       |                                     |
|  | Client +---------+ Relay |                                     |
|  |        |         |       |                                     |
|  +--------+         +-------+                                     |
|                                                                   |
+-------------------------------------------------------------------+
| Relay-to-Gateway Context                                          |
|                     +-------+         +---------+                 |
|                     |       |         |         |                 |
|                     + Relay +---------+ Gateway |                 |
|                     |       |         |         |                 |
|                     +-------+         +---------+                 |
|                                                                   |
+-------------------------------------------------------------------+
]]></artwork>
          </artset>
        </figure>
        <t>Oblivious DNS over HTTPS (ODoH) <xref target="ODOH"/> target="RFC9230"/> applies the same
        principle as Oblivious HTTP, HTTP but operates on DNS messages only. As a
        precursor to the more generalized Oblivious HTTP, it relies on the
        same HPKE cryptographic primitives, primitives and can be analyzed in the same
        way.</t>
      </section>
      <section anchor="privacypass">
        <name>Privacy Pass</name>
        <t>Privacy Pass is an architecture <xref target="PRIVACYPASS"/>
        target="RFC9576"/> and a set of protocols
        being developed in the Privacy Pass working group Working Group that allows clients
        to present proof of verification in an anonymous and unlinkable fashion,
        fashion via tokens. These tokens originally were originally designed as a way to
        prove that a client had solved a CAPTCHA, but they can be applied to other
        types of user or device attestation checks as well. In Privacy Pass,
        clients interact with an attester and issuer for the purposes of
        issuing a token, and clients then interact with an origin server to
        redeem said token.</t>
        <t>In Privacy Pass, privacy partitioning is achieved with cryptographic protection (in the form of blind
signature protocols or similar) and separation of connections across two contexts:
a "redemption context" between clients and origins (servers that request and receive tokens), and an
"issuance context" between clients, attestation servers, and token issuance servers. The cryptographic
protection ensures that information revealed during the issuance context is separated from information
revealed during the redemption context.</t>
        <figure anchor="diagram-privacypass">
          <name>Diagram of contexts Contexts in Privacy Pass</name>
          <artset>
            <artwork type="svg"><svg xmlns="http://www.w3.org/2000/svg" version="1.1" height="304" width="560" viewBox="0 0 560 304" class="diagram" text-anchor="middle" font-family="monospace" font-size="13px" stroke-linecap="round">
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                <g class="text">
                  <text x="60" y="52">Redemption</text>
                  <text x="136" y="52">Context</text>
                  <text x="68" y="100">Origin</text>
                  <text x="220" y="100">Client</text>
                  <text x="52" y="180">Issuance</text>
                  <text x="120" y="180">Context</text>
                  <text x="220" y="228">Client</text>
                  <text x="356" y="228">Attester</text>
                  <text x="492" y="228">Issuer</text>
                </g>
              </svg>
            </artwork>
            <artwork type="ascii-art"><![CDATA[
+-------------------------------------------------------------------+
| Redemption Context                                                |
|  +--------+         +--------+                                    |
|  |        |         |        |                                    |
|  | Origin +---------+ Client |                                    |
|  |        |         |        |                                    |
|  +--------+         +--------+                                    |
|                                                                   |
+-------------------------------------------------------------------+
| Issuance Context                                                  |
|                     +--------+      +----------+      +--------+  |
|                     |        |      |          |      |        |  |
|                     | Client +------+ Attester +------+ Issuer |  |
|                     |        |      |          |      |        |  |
|                     +--------+      +----------+      +--------+  |
|                                                                   |
+-------------------------------------------------------------------+
]]></artwork>
          </artset>
        </figure>

        <t>Since the redemption context and issuance context are separate
        connections that involve separate entities, they can also be further
        decoupled by running those parts of the protocols at different times.
        Clients can fetch tokens through the issuance context early, early and cache
        the tokens
to for later use in redemption contexts.  This can aid in
        partitioning identifiers and data.</t>
        <t><xref target="PRIVACYPASS"/> target="RFC9576"/> describes
        different deployment models for which entities operate origins,
        attesters, and issuers; in some models, they are all separate
        entities, but and in others, others they can be operated by the same entity. The
        model impacts the effectiveness of partitioning, and some models (such
        as when all three are operated by the same entity) only provide
        effective partitioning when the timing of connections on the two
        contexts are not correlated, correlated and when the client uses different
        identifiers (such as different IP addresses) on each context.</t>
      </section>

      <section anchor="privacy-preserving-measurement">
        <name>Privacy Preserving Measurement</name>
        <t>The Privacy Preserving Measurement (PPM) working group Working Group is chartered
        to develop protocols and systems that help a data aggregation or
        collection server (or multiple, multiple non-colluding servers) compute
        aggregate values without learning the value of any one client's
        individual measurement. The Distributed Aggregation Protocol (DAP) is the
        primary working item of the group.</t>
        <t>At a high level, DAP uses a combination of cryptographic protection
        (in the form of secret sharing amongst non-colluding servers) to
        establish two contexts: an contexts:</t>
	<ul spacing="normal">

	  <li>an "upload context" between clients and non-colluding
	  aggregation servers (in which the servers are separated into
	  "Helper" and "Leader" roles) wherein aggregation servers possibly
	  learn client identity but nothing about their individual measurement reports, and
a
	  reports; and</li>
	  <li>a "collect context" wherein a collector learns
	  aggregate measurement results and nothing about individual client data.</t>
	  data.</li>
	</ul>
        <figure anchor="pa-topology">
          <name>Diagram of contexts Contexts in DAP</name>
          <artset>
            <artwork type="svg"><svg xmlns="http://www.w3.org/2000/svg" version="1.1" height="224" width="488" viewBox="0 0 488 224" class="diagram" text-anchor="middle" font-family="monospace" font-size="13px" stroke-linecap="round">
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                <g class="text">
                  <text x="44" y="52">Upload</text>
                  <text x="104" y="52">Context</text>
                  <text x="352" y="52">Collect</text>
                  <text x="416" y="52">Context</text>
                  <text x="236" y="84">Helper</text>
                  <text x="60" y="132">Client</text>
                  <text x="392" y="132">Collector</text>
                  <text x="236" y="180">Leader</text>
                </g>
              </svg>
            </artwork>
            <artwork type="ascii-art"><![CDATA[
+-------------------------------------+--------------------+
| Upload Context                      | Collect Context    |
|                     +------------+  |                    |
|              +----->|   Helper   |  |                    |
| +--------+   |      +------------+  |                    |
| |        +---+             ^        |   +-----------+    |
| | Client |                 |        |   | Collector |    |
| |        +---+             v        |   +-----+-----+    |
| +--------+   |      +------------+  |         |          |
|              +----->|   Leader   |<-----------+          |
|                     +------------+  |                    |
+-------------------------------------+--------------------+
]]></artwork>
          </artset>
        </figure>
      </section>
    </section>
    <section anchor="applying-privacy-partitioning">
      <name>Applying Privacy Partitioning</name>
      <t>Applying privacy partitioning to an existing or new system or
      protocol requires the following steps:</t>
      <ol spacing="normal" type="1"><li>
          <t>Identify type="1">
	<li>Identify the types of information used or exposed in a system or
	protocol, some of which can be used to identify a user or correlate to
	other contexts.</t>
        </li>
        <li>
          <t>Partition contexts.</li>
        <li>Partition data to minimize the amount of user-identifying or
        correlatable information in any given context to only include what is
        necessary for that
context, context and prevent the sharing of data across
        contexts wherever possible.</t>
        </li> possible.</li>
      </ol>
      <t>The most impactful types of information to partition are (a)
      user-identifying information, such as user identifiers (including
      account names or IP addresses) that can be linked and (b)
      non-user-identifying information (including content a user generates or
      accesses), which can be often sensitive when combined with a user
      identifier.</t>
      <t>In this section, we discuss considerations for partitioning these types of information.</t>
      <section anchor="user-identifying-information">
        <name>User-Identifying Information</name>
        <t>User data can itself be user-identifying, in which case it should
        be treated as an identifier.  For example, Oblivious DoH and Oblivious
        HTTP partition the client IP address and client request data into
        separate contexts, thereby ensuring that no entity beyond the client
        can observe both. Collusion across contexts could reverse this partitioning, but can also promote
        partitioning and cause non-user-identifying information to user-identifying. become
        user-identifying information.  For example, in CONNECT proxy systems that use
        QUIC, the QUIC connection ID is inherently non-user-identifying since
        it is generated randomly (<xref section="5.1" sectionFormat="comma" target="QUIC"/>). sectionFormat="of"
        target="RFC9000"/>). However, if combined with another context that
        has user-identifying information such as the client IP address, the
        QUIC connection ID can become user-identifying information.</t>
        <t>Some information is innate to client user-agents, including details
        of the network location and implementation of protocols in hardware
        and software, and network location. software.  This information can be used to construct
        user-identifying information, which is a process sometimes referred to
        as fingerprinting. "fingerprinting".  Depending on the application and system
        constraints, users may not be able to prevent fingerprinting in
        privacy contexts.  As a result, fingerprinting information, when
        combined with non-user-identifying user data, could promote turn that
        otherwise innocuous user data to into user-identifying information.</t>
      </section>
      <section anchor="selecting-client-identifiers">
        <name>Selecting Client Identifiers</name>
        <t>The selection of client identifiers used in the contexts used for
        privacy partitioning has a large impact on the effectiveness of
        partitioning. Identifier selection can either undermine or improve the
        value of partitioning. Generally, each context involves some form of
        client identifier, which might be directly associated with a client identity,
        identity but can also be a pseudonym or a random one-time
        identifier.</t>
        <t>Using the same client identifier across multiple contexts can
        partly or wholly undermine the effectiveness of partitioning, partitioning by
        allowing the various contexts to be linked back to the same client.
        For example, if a client uses proxies as described in <xref
        target="masque"/> to separate connections, connections but uses the same email
        address to authenticate to two servers in different contexts, those
        actions can be linked back to the same client. While this does not
        undermine all of the partitioning achieved through proxying (the
        contexts along the network path still cannot correlate the client
        identity and what servers are being accessed), the overall effect of
        partitioning is diminished.</t>
        <t>When possible, using per-context unique client identifiers provides better partitioning properties.
For example, a client can use a unique email address as an account identifier with each different
server it needs to log into. The same approach can apply across many layers, as seen with
per-network MAC address randomization <xref target="I-D.ietf-madinas-mac-address-randomization"/>, use of
multiple temporary IP addresses across connections and over time <xref target="RFC8981"/>, and use of
unique per-subscription identifiers for HTTP Web Push <xref target="RFC8030"/>.</t>
      </section>
      <section anchor="incorrect-or-incomplete-partitioning">
        <name>Incorrect or Incomplete Partitioning</name>
        <t>Privacy partitioning can be applied incorrectly or
        incompletely. Contexts may contain more user-identifying information
        than desired, or some information in a context may be more
        user-identifying than intended. Moreover, splitting user-identifying
        information over multiple contexts has to be done with care, as
        creating more contexts can increase the number of entities that need
        to be trusted to not collude.  Nevertheless, partitions can help
        improve the client's privacy posture when applied carefully.</t>
        <t>Evaluating and qualifying the resulting privacy of a system or
        protocol that applies privacy partitioning depends on the contexts
        that exist and the types of user-identifying information in each
        context. Such evaluation is helpful for identifying ways in which
        systems or protocols can improve their privacy posture. For example,
        consider DNS-over-HTTPS DNS over HTTPS <xref target="DOH"/>, target="RFC8484"/>, which produces a
        single context which that contains both the client IP address and client
        query. One application of privacy partitioning results in ODoH, which
        produces two contexts, one with the client IP address and the other
        with the client query.</t>
      </section>
      <section anchor="selecting-information-within-a-context">
        <name>Selecting Information Within within a Context</name>
        <t>Recognizing potential applications of privacy partitioning requires
        identifying the contexts in use, the information exposed in a context,
        and the intent of information exposed in a context. Unfortunately,
        determining what information to include in a given context is a
        non-trivial task. In principle, the information contained in a context
        should be fit for purpose. As such, new systems or protocols developed
        should aim to ensure that all information exposed in a context serves
        as few purposes as possible. Designing with this principle from the
        start helps mitigate issues that arise if users of the system or
        protocol inadvertently ossify on the information available in
        contexts. Legacy systems that have ossified on information available
        in contexts may be difficult to change in practice. As an example,
        many existing anti-abuse systems depend on some client identifier identifier,
        such as the client IP address, coupled with client data, data to provide
        value. Partitioning contexts in these systems such that they no longer
        determine the client identity requires new solutions to the anti-abuse
        problem.</t>
      </section>
    </section>
    <section anchor="limits">
      <name>Limits of Privacy Partitioning</name>
      <t>Privacy partitioning aims to increase user privacy, though though, as
      stated, it is merely one of possibly many architectural tools that help
      manage privacy risks. Understanding the limits of its benefits requires
      a more comprehensive analysis of the system in question.  Such analysis
      also helps determine whether or not the tool has been applied
      correctly. In particular, the value of privacy partitioning depends on
      numerous factors, including, though not limited to:</t>
      <ul spacing="normal">
        <li>
          <t>Non-collusion to, the following: </t>
<ul><li>non-collusion
      across contexts; and</t>
        </li>
        <li>
          <t>The contexts and</li> <li>the type of information exposed in each context.</t>
        </li>
      </ul>
      context.</li></ul>
      <t>We elaborate on each below.</t> in the following sections.</t>
      <section anchor="violations-by-collusion">
        <name>Violations by Collusion</name>
        <t>Privacy partitions ensure that only the client, i.e., the entity which
        that is responsible for partitioning, can independently link all
        user-specific information. No other entity individually knows how to
        link all the user-specific information as long as they do not collude
        with each other across contexts. Thus, non-collusion is a fundamental
        requirement for privacy partitioning to offer meaningful privacy for end-users.
        end users. In particular, the trust relationships that users have with
        different parties affect the resulting impact on the user's
        privacy.</t>
        <t>As an example, consider OHTTP, Oblivious HTTP (OHTTP), wherein the Oblivious Relay knows
        the client identity but not the client data, and the Oblivious Gateway
        knows the client data but not the client identity.  If the Oblivious
        Relay and Gateway collude, they can link client identity and data
        together for each request and response transaction by simply observing
        requests in transit.</t>
        <t>It is not currently possible to guarantee with technical protocol
        measures that two entities are not colluding. Even if two entities do
        not collude directly, if both entities reveal information to other
        parties, it will not be possible to guarantee that the information
        won't be combined. However, there are some mitigations that can be
        applied to reduce the risk of collusion happening in practice:</t>
        <ul spacing="normal">
          <li>
            <t>Policy
          <li>Policy and contractual agreements between entities involved in
          partitioning to disallow logging or sharing of data, along with
          auditing to validate that the policies are being followed.  For
          cases where logging is required (such as for service operation),
          such logged data should be minimized and anonymized to prevent it
          from being useful for collusion.</t>
          </li>
          <li>
            <t>Protocol collusion.</li>
          <li>Protocol requirements to make collusion or data sharing more difficult.</t>
          </li>
          <li>
            <t>Adding
          difficult.</li>
          <li>Adding more partitions and contexts, contexts to make it increasingly
          difficult to collude with enough parties to recover identities.</t>
          </li> identities.</li>
        </ul>
      </section>
      <section anchor="violations-by-insufficient-or-incorrect-partitioning">
        <name>Violations by Insufficient or Incorrect Partitioning</name>
        <t>Insufficient or incorrect application of privacy partitioning can
        lessen or negate benefits to users.  In particular, it is possible to
        apply partitioning in a way that is either insufficient or incorrect
        for meaningful privacy. For example, partitioning at one layer in the
        stack can fail to account for linkable information at different layers
        in the stack. Privacy violations can stem from partitioning failures
        in a multitude of ways, some of which are described below.</t> in the following
        sections.</t>
        <section anchor="violations-from-application-information">
          <name>Violations from Application Information</name>
          <t>Partitioning at the network layer can be insufficient when the
          application layer fails to properly partition. As an example,
          consider OHTTP used for the purposes of hiding client-identifying
          information for a browser telemetry system. It is entirely possible
          for reports in such a telemetry system to contain both
          client-specific telemetry data, such as information about their
          specific browser instance, as well as client-identifying
          information, such as the client's email address, location, or IP
          address. Even though OHTTP separates the client IP address from the
          server via a relay, the server can still learn this directly from
          the client's telemetry report.</t>
        </section>
        <section anchor="violations-from-network-information">
          <name>Violations from Network Information</name>
          <t>It is also possible to inadequately partition at the network
          layer. As an example, consider both TLS Encrypted Client Hello (ECH)
          <xref target="I-D.ietf-tls-esni"/> and VPNs. ECH uses cryptographic
          protection (encryption) to hide information from unauthorized
          parties, but both clients and servers (two entities) can link
          user-specific data to a user-specific identifier (IP address).
          Similarly, while VPNs hide identifiers from end servers, the VPN
          server can still see the identifiers of both the client and
          server. Applying privacy partitioning would advocate for at least
          two additional entities to avoid revealing both identity (who) and
          user actions (what) from each involved party.</t>
        </section>
        <section anchor="violations-from-side-channels">
          <name>Violations from Side Channels</name>
          <t>Beyond the information that is intentionally revealed by applying
          privacy partitioning, it is also possible for the information to be
          unintentionally revealed through side channels. For example, in the
          two-hop proxy arrangement described in <xref target="masque"/>,
          Proxy A sees and proxies TLS data between the client and Proxy
          B. While it does not directly learn information that Proxy B sees,
          it does learn information through metadata, such as the timing and
          size of encrypted data being proxied. Traffic analysis could be
          exploited to learn more information from such metadata, including,
          in some cases, application data that Proxy A was never meant to
          see. Although privacy partitioning does not obviate such attacks, it
          does increase the cost necessary to carry them out in practice. See
          <xref target="security-considerations"/> for more discussion on this
          topic.</t>
        </section>
        <section anchor="identifying-partitions">
          <name>Identifying Partitions</name>
          <t>While straightforward violations of user privacy that stem from
          insufficient partitioning may seem straightforward to mitigate, it
          remains an open problem to rigorously determine what information
          needs to be partitioned for meaningful
privacy, privacy and to implement it
          in a way that achieves the desired properties. In essence, it is
          difficult to determine whether a certain set of information reveals
          "too much" about a specific user, and it is similarly challenging to
          determine whether or not an implementation of partitioning works as
          intended. There is ample evidence of data being assumed "private" or
          "anonymous" but, in hindsight, winds up revealing too much
          information such that it allows one to link back to individual
          clients; see <xref target="DataSetReconstruction"/> and <xref
          target="CensusReconstruction"/> for more examples of this in the
          real world.</t>
        </section>
      </section>
    </section>
    <section anchor="partitioning-impacts">
      <name>Partitioning Impacts</name>
      <t>Applying privacy partitioning to communication protocols leads to a
      substantial change in communication patterns.  For example, instead of
      sending traffic directly to a service, essentially all user traffic is
      routed through a set of intermediaries, possibly adding more end-to-end
      round trips in the process (depending on the system and protocol). This
      has a number of practical implications, described below.</t>
      <ol spacing="normal" type="1"><li>
          <t>Service spacing="normal">
	<li><t>Service operational or management challenges. challenges: Information that
	is traditionally usually passively observed in the network or metadata that
	has been unintentionally revealed to the service provider cannot will no
	longer be used anymore available; for e.g., example, this can impact existing security
	procedures such as application rate limiting or DDoS mitigation.
However,
	Current network management techniques deployed at present often rely on
	information that is exposed by
most typical traffic but without any guarantees that the information is accurate.  </t>
          <t>
Privacy lacks guarantees
	or accuracy.</t>
	<t>Privacy partitioning provides an opportunity for improvements in
	these management techniques by enabling active exchange of information
	with each entity in a privacy-preserving way and requesting exactly
	the information needed for a specific task or function rather than
	relying on the assumption that
are information derived from a limited set of unintentionally
	revealed information which that cannot be guaranteed to be
present available and may disappear at any time
	be removed in future.</t>
        </li>
        <li>
          <t>Varying the future.</t></li>

        <li><t>Varying performance effects and costs. costs:
	Depending on how context separation is done, privacy
	partitioning may affect application performance. As an example,
	Privacy Pass introduces an entire end-to-end round trip to issue a
	token before it can be redeemed, thereby decreasing performance. In
	contrast, while systems like CONNECT proxying may seem like they would regress
	reduce performance, oftentimes the highly optimized nature of
	proxy-to-proxy paths leads to improved performance.  </t>
          <t>
Performance may also push back against the desire to performance.</t>
        <t>Reduced performance can be a reason that protocols and deployments
        will not apply privacy partitioning.  For example, HTTPS connection
        reuse <xref (<xref section="9.1.1" sectionFormat="comma" target="HTTP2"/> sectionFormat="of" target="RFC9113"/>)
        allows clients to use an existing HTTPS session created for one origin
        to interact with different origins (provided that the original origin
        is authoritative for these alternative origins). Reusing connections
        saves the cost of connection establishment, establishment but means that the server
        can now link the client's activity with these two or more origins
        together. Applying privacy partitioning would prevent this, while but
        typically at the cost of less performance.  </t>
          <t>
In performance.</t>
        <t>In general, while performance and privacy tradeoffs trade-offs are often cast
        as a zero-sum game, in practice this is often not the case. The
        relationship between privacy and performance varies depending on a
        number of related factors, such as application characteristics,
        network path properties, and so on.</t>
        </li>
        <li>
          <t>Increased
        <li><t>Increased attack surface. surface:
	Even in the event that information is adequately partitioned
	across non-colluding parties, the resulting effects on the end-user end user
	may not always be positive. For example, using OHTTP as a basis for
	illustration, consider a hypothetical scenario where the Oblivious
	Gateway has an implementation flaw that causes all of its decrypt
	requests to be inappropriately logged to in a public or otherwise
	compromised location. Moreover, assume that the Target Resource for
	which these requests are destined does not have such an implementation
	flaw. Applications which that use OHTTP with this flawed Oblivious Gateway
	to interact with the Target Resource risk their user request
	information being made public, albeit in a way that is decoupled from
	user identifying information, whereas applications that do not use
	OHTTP to interact with the Target Resource do not risk this type of disclosure.</t>
        </li>
        <li>
          <t>Centralization.
	disclosure.</t></li>
        <li><t>Centralization: Depending on the protocol and system, as well
        as the desired privacy properties, the use of partitioning may
        inherently force centralization to a selected set of trusted
        participants.  As an example, the impact of OHTTP on end-user privacy
        generally increases proportionally to the number of users that exist
        behind a given Oblivious Relay. That is, the probability of an
        Oblivious Gateway determining the client associated with a request
        forwarded through an Oblivious Relay decreases as the number of
        possible clients behind the Oblivious Relay increases. This tradeoff trade-off
        encourages the centralization of the Oblivious Relays.</t>
        </li> Relays.</t></li>
      </ol>
    </section>
    <section anchor="security-considerations">
      <name>Security Considerations</name>
      <t><xref target="limits"/> discusses some of the limitations of privacy
      partitioning in practice, practice and advocates for holistic analysis to
      understand the extent to which privacy partitioning offers meaningful
      privacy improvements.  Applied correctly, partitioning helps improve an
      end-user's privacy posture, thereby making violations harder to do via
      technical, social, or policy means. For example, side channels such as
      traffic analysis <xref target="I-D.irtf-pearg-website-fingerprinting"/>
      or timing analysis are still possible and can allow an unauthorized
      entity to learn information about a context they are not a participant
      of.  Proposed mitigations for these types of attacks, e.g., padding
      application traffic or generating fake traffic, can be very expensive
      and are therefore not typically applied in practice.  Nevertheless,
      privacy partitioning moves the threat vector from one that has direct
      access to user-specific information to one which that requires more effort,
      e.g., computational resources, to violate end-user privacy.</t>
    </section>
    <section anchor="iana-considerations">
      <name>IANA Considerations</name>
      <t>This document has no IANA actions.</t>
    </section>
  </middle>

  <back>

    <displayreference target="RFC9297" to="CONNECT-UDP"/>
    <displayreference target="RFC9484" to="CONNECT-IP"/>
    <displayreference target="RFC9458" to="OHTTP"/>
    <displayreference target="RFC9180" to="HPKE"/>
    <displayreference target="RFC9230" to="ODOH"/>
    <displayreference target="RFC9000" to="QUIC"/>
    <displayreference target="I-D.ietf-madinas-mac-address-randomization" to="RANDOM-MAC"/>
    <displayreference target="RFC8484" to="DOH"/>
    <displayreference target="I-D.ietf-tls-esni" to="TLS-ESNI"/>
    <displayreference target="RFC9113" to="HTTP2"/>
    <displayreference target="I-D.irtf-pearg-website-fingerprinting" to="FINGERPINT"/>

    <references anchor="sec-informative-references">
      <name>Informative References</name>

      <reference anchor="CensusReconstruction" target="https://www.census.gov/data/academy/webinars/2021/disclosure-avoidance-series/simulated-reconstruction-abetted-re-identification-attack-on-the-2010-census.html">
        <front>
          <title>The Census Bureau's Simulated Reconstruction-Abetted
          Re-identification Attack on the 2010 Census</title>
          <author>
            <organization/>
            <organization>United States Consensus Bureau</organization>
          </author>
          <date>n.d.</date>
          <date month="May" year="2021"/>
        </front>
      </reference>

      <reference anchor="DECOUPLING">
        <front>
          <title>The decoupling principle: a practical privacy framework</title>
          <author fullname="Paul Schmitt" initials="P." surname="Schmitt">
            <organization>University of Hawai'i</organization>
          </author>
          <author fullname="Jana Iyengar" initials="J." surname="Iyengar">
            <organization>Fastly</organization>
          </author>
          <author fullname="Christopher Wood" initials="C." surname="Wood">
            <organization>Cloudflare</organization>
          </author>
          <author fullname="Barath Raghavan" initials="B." surname="Raghavan">
            <organization>USC</organization>
          </author>
          <date month="November" year="2022"/>
        </front>
        <seriesInfo name="Proceedings
        <refcontent>Proceedings of the 21st ACM Workshop on Hot Topics in" value="Networks"/>
        <seriesInfo name="DOI" value="10.1145/3563766.3564112"/>
        <refcontent>ACM</refcontent>
      </reference>
      <reference anchor="RFC6973">
        <front>
          <title>Privacy Considerations for Internet Protocols</title>
          <author fullname="A. Cooper" initials="A." surname="Cooper"/>
          <author fullname="H. Tschofenig" initials="H." surname="Tschofenig"/>
          <author fullname="B. Aboba" initials="B." surname="Aboba"/>
          <author fullname="J. Peterson" initials="J." surname="Peterson"/>
          <author fullname="J. Morris" initials="J." surname="Morris"/>
          <author fullname="M. Hansen" initials="M." surname="Hansen"/>
          <author fullname="R. Smith" initials="R." surname="Smith"/>
          <date month="July" year="2013"/>
          <abstract>
            <t>This document offers guidance for developing privacy considerations for inclusion in protocol specifications. It aims to make designers, implementers, and users of Internet protocols aware of privacy-related design choices. It suggests that whether any individual RFC warrants a specific privacy considerations section will depend on the document's content.</t>
          </abstract>
        </front>
        <seriesInfo name="RFC" value="6973"/>
        <seriesInfo name="DOI" value="10.17487/RFC6973"/>
      </reference>
      <reference anchor="CONNECT-UDP">
        <front>
          <title>HTTP Datagrams and the Capsule Protocol</title>
          <author fullname="D. Schinazi" initials="D." surname="Schinazi"/>
          <author fullname="L. Pardue" initials="L." surname="Pardue"/>
          <date month="August" year="2022"/>
          <abstract>
            <t>This document describes HTTP Datagrams, a convention for conveying multiplexed, potentially unreliable datagrams inside an HTTP connection.</t>
            <t>In HTTP/3, HTTP Datagrams can be sent unreliably using the QUIC DATAGRAM extension. When the QUIC DATAGRAM frame is unavailable or undesirable, HTTP Datagrams can be sent using the Capsule Protocol, which is a more general convention for conveying data in HTTP connections.</t>
            <t>HTTP Datagrams and the Capsule Protocol are intended for use by HTTP extensions, not applications.</t>
          </abstract>
        </front>
        <seriesInfo name="RFC" value="9297"/> Networks</refcontent>
        <seriesInfo name="DOI" value="10.17487/RFC9297"/> value="10.1145/3563766.3564112"/>
      </reference>
      <reference anchor="CONNECT-IP">
        <front>
          <title>Proxying IP in HTTP</title>
          <author fullname="Tommy Pauly" initials="T." surname="Pauly">
            <organization>Apple Inc.</organization>
          </author>
          <author fullname="David Schinazi" initials="D." surname="Schinazi">
            <organization>Google LLC</organization>
          </author>
          <author fullname="Alex Chernyakhovsky" initials="A." surname="Chernyakhovsky">
            <organization>Google LLC</organization>
          </author>
          <author fullname="Mirja Kühlewind" initials="M." surname="Kühlewind">
            <organization>Ericsson</organization>
          </author>
          <author fullname="Magnus Westerlund" initials="M." surname="Westerlund">
            <organization>Ericsson</organization>
          </author>
          <date day="28" month="April" year="2023"/>
          <abstract>
            <t>This document describes how to proxy IP packets in HTTP.  This protocol is similar to UDP proxying in HTTP but allows transmitting arbitrary IP packets.  More specifically, this document defines a protocol that allows an HTTP client to create an IP tunnel through an HTTP server that acts

      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6973.xml"/>
      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9297.xml"/>
      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9484.xml"/>
      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9458.xml"/>
      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9180.xml"/>
      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9230.xml"/>

<!-- [I-D.ietf-privacypass-architecture] Published as an IP proxy.  This document updates RFC 9298.
            </t>
          </abstract>
        </front>
        <seriesInfo name="Internet-Draft" value="draft-ietf-masque-connect-ip-13"/>
      </reference>
      <reference anchor="OHTTP">
        <front>
          <title>Oblivious HTTP</title>
          <author fullname="Martin Thomson" initials="M." surname="Thomson">
            <organization>Mozilla</organization>
          </author>
          <author fullname="Christopher A. Wood" initials="C. A." surname="Wood">
            <organization>Cloudflare</organization>
          </author>
          <date day="25" month="August" year="2023"/>
          <abstract>
            <t>   This document describes Oblivious HTTP, a protocol for forwarding
   encrypted HTTP messages.  Oblivious HTTP allows a client to make
   multiple requests to an origin server without that server being able
   to link those requests to the client or to identify the requests as
   having come from the same client, while placing only limited trust in
   the nodes used to forward the messages.

            </t>
          </abstract>
        </front>
        <seriesInfo name="Internet-Draft" value="draft-ietf-ohai-ohttp-10"/>
      </reference>
      <reference anchor="HPKE">
        <front>
          <title>Hybrid Public Key Encryption</title>
          <author fullname="R. Barnes" initials="R." surname="Barnes"/>
          <author fullname="K. Bhargavan" initials="K." surname="Bhargavan"/>
          <author fullname="B. Lipp" initials="B." surname="Lipp"/>
          <author fullname="C. Wood" initials="C." surname="Wood"/>
          <date month="February" year="2022"/>
          <abstract>
            <t>This document describes a scheme for hybrid public key encryption (HPKE). This scheme provides a variant of public key encryption of arbitrary-sized plaintexts for a recipient public key. It also includes three authenticated variants, including one that authenticates possession of a pre-shared key and two optional ones that authenticate possession of a key encapsulation mechanism (KEM) private key. HPKE works for any combination of an asymmetric KEM, key derivation function (KDF), and authenticated encryption with additional data (AEAD) encryption function. Some authenticated variants may not be supported by all KEMs. We provide instantiations of the scheme using widely used and efficient primitives, such 9576-->
      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9576.xml"/>

      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9000.xml"/>

<!-- [I-D.ietf-madinas-mac-address-randomization] [QUIC] IESG state: RFC Ed
     Queue as Elliptic Curve Diffie-Hellman (ECDH) key agreement, HMAC-based key derivation function (HKDF), and SHA2.</t>
            <t>This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF.</t>
          </abstract>
        </front>
        <seriesInfo name="RFC" value="9180"/>
        <seriesInfo name="DOI" value="10.17487/RFC9180"/>
      </reference>
      <reference anchor="ODOH">
        <front>
          <title>Oblivious DNS over HTTPS</title>
          <author fullname="E. Kinnear" initials="E." surname="Kinnear"/>
          <author fullname="P. McManus" initials="P." surname="McManus"/>
          <author fullname="T. Pauly" initials="T." surname="Pauly"/>
          <author fullname="T. Verma" initials="T." surname="Verma"/>
          <author fullname="C.A. Wood" initials="C.A." surname="Wood"/>
          <date month="June" year="2022"/>
          <abstract>
            <t>This document describes a protocol that allows clients to hide their IP addresses from DNS resolvers via proxying encrypted DNS over HTTPS (DoH) messages. This improves privacy of DNS operations by not allowing any one server entity to be aware of both the client IP address and the content of DNS queries and answers.</t>
            <t>This experimental protocol has been developed outside the IETF and is published here 7/30/24; updated to guide implementation, ensure interoperability among implementations, and enable wide-scale experimentation.</t>
          </abstract>
        </front>
        <seriesInfo name="RFC" value="9230"/>
        <seriesInfo name="DOI" value="10.17487/RFC9230"/>
      </reference>
      <reference anchor="PRIVACYPASS">
        <front>
          <title>The Privacy Pass Architecture</title>
          <author fullname="Alex Davidson" initials="A." surname="Davidson">
            <organization>LIP</organization>
          </author>
          <author fullname="Jana Iyengar" initials="J." surname="Iyengar">
            <organization>Fastly</organization>
          </author>
          <author fullname="Christopher A. Wood" initials="C. A." surname="Wood">
            <organization>Cloudflare</organization>
          </author>
          <date day="25" month="September" year="2023"/>
          <abstract>
            <t>   This document specifies the Privacy Pass architecture and
   requirements for its constituent protocols used for authorization
   based on privacy-preserving authentication mechanisms.  It describes
   the conceptual model of Privacy Pass and its protocols, its security
   and privacy goals, practical deployment models, and recommendations
   for each deployment model that helps ensure the desired security and
   privacy goals are fulfilled.

            </t>
          </abstract>
        </front>
        <seriesInfo name="Internet-Draft" value="draft-ietf-privacypass-architecture-16"/>
      </reference>
      <reference anchor="QUIC">
        <front>
          <title>QUIC: A UDP-Based Multiplexed and Secure Transport</title>
          <author fullname="J. Iyengar" initials="J." role="editor" surname="Iyengar"/>
          <author fullname="M. Thomson" initials="M." role="editor" surname="Thomson"/>
          <date month="May" year="2021"/>
          <abstract>
            <t>This document defines the core of the QUIC transport protocol. QUIC provides applications with flow-controlled streams for structured communication, low-latency connection establishment, and network path migration. QUIC includes security measures that ensure confidentiality, integrity, and availability in a range of deployment circumstances. Accompanying documents describe the integration of TLS for key negotiation, loss detection, long version because missing editor role
     and an exemplary congestion control algorithm.</t>
          </abstract>
        </front>
        <seriesInfo name="RFC" value="9000"/>
        <seriesInfo name="DOI" value="10.17487/RFC9000"/>
      </reference> not showing initials correctly -->
      <reference anchor="I-D.ietf-madinas-mac-address-randomization"> anchor="I-D.ietf-madinas-mac-address-randomization" target="https://datatracker.ietf.org/doc/html/draft-ietf-madinas-mac-address-randomization-12">
	<front>
	  <title>Randomized and Changing MAC Address state of affairs</title>
	  <author initials="JC." surname="Zuniga" fullname="Juan-Carlos Zúñiga" initials="J. C." surname="Zúñiga"> Zuniga">
	    <organization>CISCO</organization>
	  </author>
	  <author initials="CJ." surname="Bernardos" fullname="Carlos J. Bernardos" initials="C. J." surname="Bernardos"> role="editor">
	    <organization>Universidad Carlos III de Madrid</organization>
	  </author>
	  <author fullname="Amelia Andersdotter" initials="A." surname="Andersdotter"> surname="Andersdotter" fullname="Amelia Andersdotter">
	    <organization>Safespring AB</organization>
	  </author>
	  <date day="11" month="January" year="2024"/>
          <abstract>
            <t>   Internet privacy has become a major concern over the past few years.
   Users are becoming more aware that their online activity leaves a
   vast digital footprint, that communications are not always properly
   secured, and that their location and actions can be easily tracked.
   One of the main factors for the location tracking issue is the wide
   use of long-lasting identifiers, such as MAC addresses.

   There have been several initiatives at the IETF and the IEEE 802
   standards committees to overcome some of these privacy issues.  This
   document provides an overview of these activities, with the intention
   to inform the technical community about them, and help coordinate
   between present and future standardization activities.

            </t>
          </abstract>
        </front>
        <seriesInfo name="Internet-Draft" value="draft-ietf-madinas-mac-address-randomization-10"/>
      </reference>
      <reference anchor="RFC8981">
        <front>
          <title>Temporary Address Extensions for Stateless Address Autoconfiguration in IPv6</title>
          <author fullname="F. Gont" initials="F." surname="Gont"/>
          <author fullname="S. Krishnan" initials="S." surname="Krishnan"/>
          <author fullname="T. Narten" initials="T." surname="Narten"/>
          <author fullname="R. Draves" initials="R." surname="Draves"/>
          <date month="February" year="2021"/>
          <abstract>
            <t>This document describes an extension to IPv6 Stateless Address Autoconfiguration that causes hosts to generate temporary addresses with randomized interface identifiers for each prefix advertised with autoconfiguration enabled. Changing addresses over time limits the window of time during which eavesdroppers and other information collectors may trivially perform address-based network-activity correlation when the same address is employed for multiple transactions by the same host. Additionally, it reduces the window of exposure of a host as being accessible via an address that becomes revealed as a result of active communication. This document obsoletes RFC 4941.</t>
          </abstract>
        </front>
        <seriesInfo name="RFC" value="8981"/>
        <seriesInfo name="DOI" value="10.17487/RFC8981"/>
      </reference>
      <reference anchor="RFC8030">
        <front>
          <title>Generic Event Delivery Using HTTP Push</title>
          <author fullname="M. Thomson" initials="M." surname="Thomson"/>
          <author fullname="E. Damaggio" initials="E." surname="Damaggio"/>
          <author fullname="B. Raymor" initials="B." role="editor" surname="Raymor"/>
          <date month="December" year="2016"/>
          <abstract>
            <t>This document describes a simple protocol for the delivery of real- time events to user agents. This scheme uses HTTP/2 server push.</t>
          </abstract>
        </front>
        <seriesInfo name="RFC" value="8030"/>
        <seriesInfo name="DOI" value="10.17487/RFC8030"/>
      </reference>
      <reference anchor="DOH">
        <front>
          <title>DNS Queries over HTTPS (DoH)</title>
          <author fullname="P. Hoffman" initials="P." surname="Hoffman"/>
          <author fullname="P. McManus" initials="P." surname="McManus"/>
          <date month="October" year="2018"/>
          <abstract>
            <t>This document defines a protocol for sending DNS queries and getting DNS responses over HTTPS. Each DNS query-response pair is mapped into an HTTP exchange.</t>
          </abstract>
        </front>
        <seriesInfo name="RFC" value="8484"/>
        <seriesInfo name="DOI" value="10.17487/RFC8484"/>
      </reference>
      <reference anchor="I-D.ietf-tls-esni">
        <front>
          <title>TLS Encrypted Client Hello</title>
          <author fullname="Eric Rescorla" initials="E." surname="Rescorla">
            <organization>RTFM, Inc.</organization>
          </author>
          <author fullname="Kazuho Oku" initials="K." surname="Oku">
            <organization>Fastly</organization>
          </author>
          <author fullname="Nick Sullivan" initials="N." surname="Sullivan">
            <organization>Cloudflare</organization>
          </author>
          <author fullname="Christopher A. Wood" initials="C. A." surname="Wood">
            <organization>Cloudflare</organization>
          </author>
          <date day="9" month="October" year="2023"/>
          <abstract>
            <t>   This document describes a mechanism in Transport Layer Security (TLS)
   for encrypting a ClientHello message under a server public key.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Source for this draft and an issue tracker can be found at
   https://github.com/tlswg/draft-ietf-tls-esni
   (https://github.com/tlswg/draft-ietf-tls-esni).

            </t>
          </abstract> day="28" year="2024"/>
	</front>
	<seriesInfo name="Internet-Draft" value="draft-ietf-tls-esni-17"/> value="draft-ietf-madinas-mac-address-randomization-12"/>
      </reference>

      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8981.xml"/>
      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8030.xml"/>
      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8484.xml"/>

      <!-- [I-D.ietf-tls-esni] IESG state: I-D Exists 7/30/2024 -->
      <xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-ietf-tls-esni.xml"/>

      <reference anchor="DataSetReconstruction">
        <front>
          <title>Robust De-anonymization of Large Sparse Datasets</title>
          <author fullname="Arvind Narayanan" initials="A." surname="Narayanan">
            <organization/>
          </author>
          <author fullname="Vitaly Shmatikov" initials="V." surname="Shmatikov">
            <organization/>
          </author>
          <date month="May" year="2008"/>
        </front>
        <seriesInfo name="2008 IEEE
        <refcontent>IEEE Symposium on Security and Privacy (sp" value="2008)"/> Privacy</refcontent>
        <seriesInfo name="DOI" value="10.1109/sp.2008.33"/>
        <refcontent>IEEE</refcontent>
      </reference>
      <reference anchor="HTTP2">
        <front>
          <title>HTTP/2</title>
          <author fullname="M. Thomson" initials="M." role="editor" surname="Thomson"/>
          <author fullname="C. Benfield" initials="C." role="editor" surname="Benfield"/>
          <date month="June" year="2022"/>
          <abstract>
            <t>This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP), referred to

      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9113.xml"/>

      <!-- [I-D.irtf-pearg-website-fingerprinting] IESG state: Expired as HTTP version 2 (HTTP/2). HTTP/2 enables a more efficient use of network resources and a reduced latency by introducing field compression and allowing multiple concurrent exchanges on 7/30/24-->
      <xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-irtf-pearg-website-fingerprinting.xml"/>

    </references>
<section numbered="false" anchor="iab-members">
      <name>IAB Members at the same connection.</t>
            <t>This document obsoletes RFCs 7540 and 8740.</t>
          </abstract>
        </front>
        <seriesInfo name="RFC" value="9113"/>
        <seriesInfo name="DOI" value="10.17487/RFC9113"/>
      </reference>
      <reference anchor="I-D.irtf-pearg-website-fingerprinting">
        <front>
          <title>Network-Based Website Fingerprinting</title>
          <author fullname="Ian Goldberg" initials="I." surname="Goldberg">
            <organization>University of Waterloo</organization>
          </author>
          <author fullname="Tao Wang" initials="T." surname="Wang">
            <organization>HK University Time of Science and Technology</organization>
          </author>
          <author fullname="Christopher A. Wood" initials="C. A." surname="Wood">
            <organization>Cloudflare</organization>
          </author>
          <date day="8" month="September" year="2020"/>
          <abstract>
            <t>   The IETF is well on its way to protecting connection metadata with
   protocols such as DNS-over-TLS and DNS-over-HTTPS, and work-in-
   progress towards encrypting the TLS SNI.  However, more work is
   needed to protect traffic metadata, especially in Approval</name>
<t>Internet Architecture Board members at the context of web
   traffic.  In time this document, we survey Website Fingerprinting attacks,
   which are a class of attacks that use machine learning techniques to
   attack web privacy, and highlight metadata leaks used by said
   attacks.  We also survey proposed mitigations for such leakage and
   discuss their applicability to IETF protocols such as TLS, QUIC, and
   HTTP.  We endeavor to show that Website Fingerprinting attacks are a
   serious problem that affect all Internet users, and we pose open
   problems and directions document was
   approved for future research in this area.

            </t>
          </abstract>
        </front>
        <seriesInfo name="Internet-Draft" value="draft-irtf-pearg-website-fingerprinting-01"/>
      </reference>
    </references>
    <?line 834?> publication were:</t>
      <ul empty="true" spacing="compact" bare="false" indent="3">
        <li>
          <t><contact fullname="Dhruv Dhody"/></t>
        </li>
        <li>
          <t><contact fullname="Lars Eggert"/></t>
        </li>
        <li>
          <t><contact fullname="Wes Hardaker"/></t>
        </li>
        <li>
          <t><contact fullname="Cullen Jennings"/></t>
        </li>
        <li>
          <t><contact fullname="Mallory Knodel"/></t>
        </li>
        <li>
          <t><contact fullname="Suresh Krishnan"/></t>
        </li>
        <li>
          <t><contact fullname="Mirja Kühlewind"/></t>
        </li>
        <li>
          <t><contact fullname="Tommy Pauly"/></t>
        </li>
        <li>
          <t><contact fullname="Alvaro Retana"/></t>
        </li>
        <li>
          <t><contact fullname="David Schinazi"/></t>
        </li>
        <li>
          <t><contact fullname="Christopher Wood"/></t>
        </li>
        <li>
          <t><contact fullname="Qin Wu"/></t>
        </li>
        <li>
          <t><contact fullname="Jiankang Yao"/></t>
        </li>
      </ul>
</section>

<section numbered="false" anchor="acknowledgments">
      <name>Acknowledgments</name>
      <t>We would like to thank Martin Thomson, Eliot Lear, Mark Nottingham, Niels <contact fullname="Martin Thomson"/>, <contact
      fullname="Eliot Lear"/>, <contact fullname="Mark Nottingham"/>, <contact
      fullname="Niels ten Oever, Vittorio Bertola,
Antoine Fressancourt, Cullen Jennings, and Dhruv Dhody Oever"/>, <contact fullname="Vittorio Bertola"/>,
      <contact fullname="Antoine Fressancourt"/>, <contact fullname="Cullen
      Jennings"/>, and <contact fullname="Dhruv Dhody"/> for their reviews and
      feedback.</t>
    </section>
  </back>
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