Skip to main content

Data minimization among protocol participants

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft whose latest revision state is "Expired".
Author Jari Arkko
Last updated 2023-03-13
RFC stream Internet Architecture Board (IAB)
Stream IAB state (None)
Consensus boilerplate Unknown
IAB shepherd (None)
Network Working Group                                           J. Arkko
Internet-Draft                                                  Ericsson
Intended status: Informational                            March 14, 2023
Expires: September 15, 2023

             Data minimization among protocol participants


   Communications security has been at the center of many security
   improvements in the Internet.  The goal has been to ensure that
   communications are protected against outside observers and attackers.
   Privacy has also been a key focus area, and understanding the privacy
   implications of new Internet technology is an important factor when
   IETF works on such technologies.  One key aspect of privacy is
   minimization of data disclosed to other parties.

   This document highlights the need for a particular focus with respect
   to data minimization.  Avoiding data leakage to outside parties is of
   course important, but it can also be necessary to limit it among the
   primary protocol participants.

   This is because is necessary to protect against endpoints that are
   compromised, malicious, or whose interests simply do not align with
   the interests of users.  It is important to consider the role of a
   participant and limit any data provided to it according to that role.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 15, 2023.

Arkko                  Expires September 15, 2023               [Page 1]
Internet-Draft              Data Minimization                 March 2023

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Types of information  . . . . . . . . . . . . . . . . . .   4
     2.2.  Dealing with compromise . . . . . . . . . . . . . . . . .   4
     2.3.  Related work  . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Communications security has been at the center of many security
   improvements on the Internet.  The goal has been to ensure that
   communications are protected against outside observers and attackers.

   This has been exemplified in many aspects of IETF efforts, in the
   threat models [RFC3552], concerns about surveillance [RFC7258], IAB
   statements [Confidentiality], and the introduction of encryption in
   many protocols [RFC9000], [RFC7858], [RFC8484].  This has been very
   successful.  Advances in protecting most of our communications with
   strong cryptographic has resulted in much improved security.  Work on
   these advances continues to be a key part of IETF's security effort.

   Privacy has also been at the center of many activities in the IETF.
   Improvements in communications security obviously have improved
   privacy as well, but the concept is broader.  Privacy and its impact
   on protocol development activities at IETF is discussed in [RFC6973],
   covering a number of topics, from understanding privacy threats to
   threat mitigation, including data minimization.

Arkko                  Expires September 15, 2023               [Page 2]
Internet-Draft              Data Minimization                 March 2023

   One key aspect of privacy is minimization of data disclosed to other

   This document highlights the need for a particular focus with respect
   to data minimization.  Avoiding data leakage to outside parties is of
   course important, but it can also be necessary to limit it among the
   primary protocol participants (and not just observers/attackers).  As
   RFC 6973 states:

      "Limiting the data collected by protocol elements to
       only what is necessary (collection limitation) is
       the most straightforward way to help reduce privacy
       risks associated with the use of the protocol."

   This document offers some further discussion and motivation for this.
   This document suggests that limiting the sharing of data to the
   protocol participants is a key technique in limiting the data
   collection mentioned above.  This document also suggests that what
   information is given to any other participant should depend on the
   role of that participant.

   The reason why this is important is that it is possible that
   endpoints are compromised, malicious, or have interests that do not
   align with the interests of users.  Even closed, managed networks may
   have compromised nodes, justifying careful consideration of what
   information is provided to different nodes in the network.  And in
   all networks, increased use of communication security means
   adversaries may resort to new avenues of attack.  New adversaries and
   risks have also arisen, e.g., due to increasing amount of information
   stored in various Internet services.  And in situations where
   interests do not align across the protocol participants, limiting
   data collection by a protocol participant itself - who is interested
   in data collection - may not be sufficient.

   Careful control of information is also useful for technology
   evolution.  For instance, allowing a party to unnecessarily collect
   or receive information may lead to a similar effect as described in
   [RFC8546] for protocols: regardless of initial expectations, over
   time unnecessary information will get used, leading to, for instance,
   ossification.  Systems end up depend on having access to exactly the
   same information as they had access to previously.  This makes it
   hard to change what information is provided or how it is provided.

2.  Recommendations

   The Principle of Least Privilege [PoLP] is applicable:

Arkko                  Expires September 15, 2023               [Page 3]
Internet-Draft              Data Minimization                 March 2023

     "Every program and every user of the system should operate
      using the least set of privileges necessary to complete the

   In this context, it is recommended that the protocol participants
   minimize the information they share.  I.e., they should provide only
   the information to each other that is necessary for the function that
   is expected to be performed by the other party.

   Information sharing may relate to different types of protocol
   exchanges, e.g., interaction of an endpoint with the network or with
   intermediaries.  Other documents address aspects related to networks
   ([RFC8546], [RFC8558], [I-D.iab-path-signals-collaboration]).
   Thomson [I-D.thomson-tmi] discusses the role intermediaries.
   Communications security largely addresses observers and outsider
   adversaries, and [RFC6973] discusses associated traffic analysis
   threats.  The focus in this document is on the primary protocol
   participants, such as a server in a client-server architecture or a
   service enables some kind of interaction among groups of users.

   As with communication security, we try to avoid providing too much
   information as it may be misused or leak through attacks.  The same
   principle applies not just to routers and potential attackers on
   path, but also many other services in the Internet, including servers
   that provide some function.

   Of course, participants may provide more information to each after
   careful consideration, e.g., information provided in exchange of some
   benefit, or to parties that are trusted by the participant.

2.1.  Types of information

   The use of identifiers has been extensively discussed in [RFC6973],

   Note that indirectly inferred information can also end up being
   shared, such as message arrival times or patterns in the traffic flow
   ([RFC6973]).  Information may also be obtained from fingerprinting
   the protocol participants, in an effort to identify unique endpoints
   or users ([RFC6973]).  Information may also be combined from multiple
   sources, e.g., websites and social media systems collaborating to
   identify visiting users [WP2021].

2.2.  Dealing with compromise

   Even with careful exposure of information, compromises may occur.  It
   is important to build defenses to protect information, even when some
   component in a system becomes compromised.  This may involve designs
   where no single party has all information such as with Oblivious DNS

Arkko                  Expires September 15, 2023               [Page 4]
Internet-Draft              Data Minimization                 March 2023

   [I-D.annee-dprive-oblivious-dns], [I-D.pauly-dprive-oblivious-doh] or
   HTTP [I-D.ietf-ohai-ohttp], cryptographic designs where a service
   such as with the recent IETF PPM effort [I-D.ietf-ppm-dap], service
   side encryption of data at rest, confidential computing, and other

   Protocols can ensure that forward secrecy is provided, so that the
   damage resulting from a compromise of keying material has limited

   On the client side, the client may trust that another party handles
   information appropriately, but take steps to ensure or verify that
   this is the case.  For instance, as discussed above, the client can
   encrypt a message only to the actual final recipient, even if the
   server holds the message before it is delivered.

   A corollary of the recommendation is that information should not be
   disclosed, stored, or routed in cleartext through services that do
   not need to have that information for the function they perform.

   For instance, a transport connection between two components of a
   system is not an end-to-end connection even if it encompasses all the
   protocol layers up to the application layer.  It is not end-to-end,
   if the information or control function it carries extends beyond
   those components.  For instance, just because an e-mail server can
   read the contents of an e-mail message do not make it a legitimate
   recipient of the e-mail.

   The general topic of ensuring that protocol mechanisms stays
   evolvable and workable is covered in [I-D.iab-use-it-or-lose-it].
   But the associated methods for reducing fingerprinting possibilities
   probably deserve further study [Fingerprinting] [AmIUnique].
   [I-D.wood-pearg-website-fingerprinting] discusses one aspect of this.

2.3.  Related work

   Cooper et al.  [RFC6973] discuss the general concept of privacy,
   including data minimization.  Section 6.1 in RFC 6973 is about data
   minimization, and there they provide the general statement quoted in
   Section 1.  That is exactly about what the present document is also
   about.  Section 7 in RFC 6973 is about guidelines to authors of
   specifications.  And Section 7.1 in turn asks the a number of
   questions that protocol designers can use to further analyse the
   impact of their design.  For data minimization the questions relate
   to identifiers, data, observers, and fingerprinting.  This includes,
   for instance, asking what information is exposed to which protocol
   entities, and if there are ways to limit such exposure.  For the

Arkko                  Expires September 15, 2023               [Page 5]
Internet-Draft              Data Minimization                 March 2023

   present document this is the particularly important question in RFC

   Observers.  Which information discussed in (a) and (b)
   is exposed to each other protocol entity (i.e., recipients,
   intermediaries, and enablers)?  Are there ways for protocol
   implementers to choose to limit the information shared with
   each entity?  Are there operational controls available to
   limit the information shared with each entity?

   These questions are in line with avoiding sharing information to a
   protocol participant unless it is needed for its role.  Therefore,
   this too is exactly what the present document is about.  It is clear
   from both the text and interviewing the authors of RFC 6973 that they
   believed in the importance of limiting data disclosure across all
   possible parties, from outsiders to the primary protocol
   participants, just as the author of the current document does.

   However, if there's something to be wished, RFC 6973 could perhaps
   been more explicit:

   o  It should be stated that information should not even be shared
      with a participant if it is not necessary for the expected role of
      that participant.  Yet, RFC 6973 says "Limiting the data collected
      by protocol elements to only what is necessary (collection
      limitation)".  But when there are potentially conflicting
      interests among the protocol participants, expecting a participant
      to limit its data collection itself seems insufficient.  What we
      need to do is to not even give that that participant the
      information.  Interviewing the authors of the RFC, this what the
      intent of the text was, but the text isn't explicit about it.

   o  Similarly, the Section 7.1 guidance is merely questions, not
      recommendations about limiting to the necessary information.

   o  The examples in Section 6 are largely related to cases where some
      information is relayed to some parties but not others.  For
      instance, the anonymity and identity confidentiality examples are
      about withholding identity from some parties such as the other
      endpoint of a call or outsiders observing an authentication
      exchange.  But they still disclose an identity to a party running
      a service.  This is of course necessary in many situations, but it
      would be useful to provide examples where information is withheld

   These are of course nuances that may change in a future revision of
   RFC 6973.

Arkko                  Expires September 15, 2023               [Page 6]
Internet-Draft              Data Minimization                 March 2023

   In the years after publishing [RFC6973] there has also been a number
   of documents on specific issues, such as one DNS Query Name
   Minimization [RFC7816], general DNS privacy advice including data
   minimization [RFC9076], advice for DHCP clients for minimizing
   information in requests they send to DHCP servers [RFC7844] (along
   with general privacy considerations of DHCP [RFC7819] [RFC7824]).
   These are on the topic of limiting information sent by one primary
   protocol particiant (client) to another (server).

   Hardie [RFC8558] discusses path signals, i.e., messages to or from
   on-path elements to endpoints.  In the past, path signals were often
   implicit, e.g., network nodes interpreting in a particular way
   transport protocol headers originally intended for end-to-end
   consumption.  The document recommends a principle that implicit
   signals should be avoided and that explicit signals be used instead,
   and only when the signal's originator intends that it be used by the
   network elements on the path.

   Arkko, Kuhlewind, Pauly, and Hardie
   [I-D.iab-path-signals-collaboration] discuss the same topic, and
   extend the RFC 8558 principles with recommendations to ensure minimum
   set of parties, minimum information, and consent.

   Kuehlewind, Pauly, and Wood [I-D.iab-privacy-partitioning] discuss
   the concept of privacy partitioning: how information can be split and
   carefully shared in ways where no individual party beyond the client
   requesting a service has full picture of who is asking and what is
   being asked.  This is of course a highly relevant technique, and
   should be a part of the data minimization toolkit.

   Thomson [I-D.thomson-tmi] discusses the role intermediaries in the
   Internet architecture, at different layers of the stack.  For
   instance, a router is an intermediary, some parts of DNS
   infrastructure can be intermediaries, messaging gateways are
   intermediaries.  Thomson discusses when intermediaries are or are not
   an appropriate tool, and presents a number of principles relating to
   the use of intermediaries, e.g., deliberate selection of protocol
   participants or limiting the capabilities or information exposure
   related to the intermediaries.

   Trammel and Kuehlewind [RFC8546] discuss the concept of a "wire
   image" of a protocol.  This is an abstraction of the information
   available to an on-path non-participant in a networking protocol.  It
   relates to the topic of non-participants interpreting information
   that is available to them in the "wire image" (or associated timing
   and other indirect information).  The issues are largely the same
   even for participants.  Even proper protocol participants may start

Arkko                  Expires September 15, 2023               [Page 7]
Internet-Draft              Data Minimization                 March 2023

   to use information available to them, regardless of whether it was
   intended to that participant or simply relayed through them.

3.  Acknowledgements

   The author would like to thank the participants of various IAB
   workshops and programs, and IETF discussion list contributors for
   interesting discussions in this area.  The author would in particular
   like to acknowledge the significant contributions of Martin Thomson,
   Nick Doty, Alissa Cooper, Stephen Farrell, Mark McFadden, John
   Mattsson, Chris Wood, Dominique Lazanski, Eric Rescorla, Russ
   Housley, Robin Wilton, Mirja Kuehlewind, Tommy Pauly, Jaime Jimenez
   and Christian Huitema.

   This work has been influenced by [RFC6973], [RFC8980],
   [I-D.farrell-etm] [I-D.arkko-arch-internet-threat-model-guidance],

4.  Informative References

              INRIA, ., "Am I Unique?", , 2020.

              The Internet Architecture Board, ., "IAB Statement on
              Internet Confidentiality",
              iab-statement-on-internet-confidentiality/ , November

              Laperdrix, P., Bielova, N., Baudry, B., and G. Avoine,
              "Browser Fingerprinting: A survey", arXiv:1905.01051v2
              [cs.CR] 4 Nov 2019 , November 2019.

              Edmundson, A., Schmitt, P., Feamster, N., and A. Mankin,
              "Oblivious DNS - Strong Privacy for DNS Queries", draft-
              annee-dprive-oblivious-dns-00 (work in progress), July
              2018, <

              Arkko, J. and S. Farrell, "Internet Threat Model
              Guidance", draft-arkko-arch-internet-threat-model-
              guidance-00 (work in progress), July 2021,

Arkko                  Expires September 15, 2023               [Page 8]
Internet-Draft              Data Minimization                 March 2023

              Farrell, S., "We're gonna need a bigger threat model",
              draft-farrell-etm-03 (work in progress), July 2019,

              Arkko, J., Hardie, T., Pauly, T., and M. Kuehlewind,
              "Considerations on Application - Network Collaboration
              Using Path Signals", draft-iab-path-signals-
              collaboration-03 (work in progress), February 2023,

              Kuehlewind, M., Pauly, T., and C. Wood, "Partitioning as
              an Architecture for Privacy", draft-iab-privacy-
              partitioning-01 (work in progress), March 2023,

              Thomson, M. and T. Pauly, "Long-Term Viability of Protocol
              Extension Mechanisms", draft-iab-use-it-or-lose-it-04
              (work in progress), October 2021,

              Thomson, M. and C. Wood, "Oblivious HTTP", draft-ietf-
              ohai-ohttp-07 (work in progress), March 2023,

              Geoghegan, T., Patton, C., Rescorla, E., and C. Wood,
              "Distributed Aggregation Protocol for Privacy Preserving
              Measurement", draft-ietf-ppm-dap-04 (work in progress),
              March 2023, <

              Lazanski, D., "An Internet for Users Again", draft-
              lazanski-smart-users-internet-00 (work in progress), July
              2019, <

Arkko                  Expires September 15, 2023               [Page 9]
Internet-Draft              Data Minimization                 March 2023

              Kinnear, E., McManus, P., Pauly, T., Verma, T., and C.
              Wood, "Oblivious DNS over HTTPS", draft-pauly-dprive-
              oblivious-doh-11 (work in progress), February 2022,

              Thomson, M., "Principles for the Involvement of
              Intermediaries in Internet Protocols", draft-thomson-
              tmi-04 (work in progress), September 2022,

              Goldberg, I., Wang, T., and C. Wood, "Network-Based
              Website Fingerprinting", draft-wood-pearg-website-
              fingerprinting-00 (work in progress), November 2019,

   [PoLP]     Saltzer, J. and M. Schroader, "The Protection of
              Information in Computer Systems", Fourth ACM Symposium on
              Operating System Principles , October 1975.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003, <https://www.rfc-

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013, <https://www.rfc-

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <>.

   [RFC7816]  Bortzmeyer, S., "DNS Query Name Minimisation to Improve
              Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,

   [RFC7819]  Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy
              Considerations for DHCP", RFC 7819, DOI 10.17487/RFC7819,
              April 2016, <>.

Arkko                  Expires September 15, 2023              [Page 10]
Internet-Draft              Data Minimization                 March 2023

   [RFC7824]  Krishnan, S., Mrugalski, T., and S. Jiang, "Privacy
              Considerations for DHCPv6", RFC 7824,
              DOI 10.17487/RFC7824, May 2016, <https://www.rfc-

   [RFC7844]  Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
              Profiles for DHCP Clients", RFC 7844,
              DOI 10.17487/RFC7844, May 2016, <https://www.rfc-

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

   [RFC8546]  Trammell, B. and M. Kuehlewind, "The Wire Image of a
              Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
              2019, <>.

   [RFC8558]  Hardie, T., Ed., "Transport Protocol Path Signals",
              RFC 8558, DOI 10.17487/RFC8558, April 2019,

   [RFC8980]  Arkko, J. and T. Hardie, "Report from the IAB Workshop on
              Design Expectations vs. Deployment Reality in Protocol
              Development", RFC 8980, DOI 10.17487/RFC8980, February
              2021, <>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021, <https://www.rfc-

   [RFC9076]  Wicinski, T., Ed., "DNS Privacy Considerations", RFC 9076,
              DOI 10.17487/RFC9076, July 2021, <https://www.rfc-

   [WP2021]   Fowler, Geoffrey., "There's no escape from Facebook, even
              if you don't use it", Washington Post , August 2021.

Arkko                  Expires September 15, 2023              [Page 11]
Internet-Draft              Data Minimization                 March 2023

Author's Address

   Jari Arkko
   Valitie 1B


Arkko                  Expires September 15, 2023              [Page 12]