RFC 8165

Internet Engineering Task Force (IETF)                         T. Hardie
Request for Comments: 8165                                      May 2017
Category: Informational
ISSN: 2070-1721

              Design Considerations for Metadata Insertion


   The IAB published RFC 7624 in response to several revelations of
   pervasive attacks on Internet communications.  This document
   considers the implications of protocol designs that associate
   metadata with encrypted flows.  In particular, it asserts that
   designs that share metadata only by explicit actions at the host are
   preferable to designs in which middleboxes insert metadata.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2017 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
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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................2
   3. Design Pattern ..................................................2
   4. Advice ..........................................................3
   5. Deployment Considerations .......................................4
   6. IANA Considerations .............................................5
   7. Security Considerations .........................................5
   8. References ......................................................6
      8.1. Normative References .......................................6
      8.2. Informative References .....................................6
   Acknowledgements ...................................................7
   Author's Address ...................................................7

1.  Introduction

   To minimize the risks associated with pervasive surveillance, it is
   necessary for the Internet technical community to address the
   vulnerabilities exploited in the attacks documented in [RFC7258] and
   the threats described in [RFC7624].  The goal of this document is to
   address a common design pattern that emerges from the increase in
   encryption: explicit association of metadata that would previously
   have been inferred from the plaintext protocol.

2.  Terminology

   This document makes extensive use of standard security and privacy
   terminology; see [RFC4949] and [RFC6973].  Readers should be familiar
   with the terms defined in [RFC6973], including "Eavesdropper",
   "Observer", "Initiator", "Intermediary", "Recipient", "Attack" (in a
   privacy context), "Correlation", "Fingerprint", "Traffic Analysis",
   and "Identifiability" (and related terms).  Readers should also be
   familiar with terms that are specific to the attacks discussed in
   [RFC7624], including "Pervasive Attack", "Passive Pervasive Attack",
   "Active Pervasive Attack", "Observation", "Inference", and

3.  Design Pattern

   One of the core mitigations for the loss of confidentiality in the
   presence of pervasive surveillance is data minimization, which limits
   the amount of data disclosed to those elements absolutely required to
   complete the relevant protocol exchange.  When data minimization is
   in effect, some information that was previously available may be
   removed from specific protocol exchanges.  The information may be
   removed explicitly (for example, by a browser suppressing cookies

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   during private modes) or by other means.  As noted in [RFC7624], some
   topologies that aggregate or alter the network path also act to
   reduce the ease with which metadata is available to eavesdroppers.

   In some cases, other actors within a protocol context will continue
   to have access to the information that has been thus withdrawn from
   specific protocol exchanges.  If those actors attach the information
   as metadata to those protocol exchanges, the confidentiality effect
   of data minimization is lost.

   Restoring information is particularly tempting at systems not
   primarily deployed to increase confidentiality.  A proxy providing
   compression, for example, may wish to restore the identity of the
   requesting party; similarly, a VPN system used to provide channel
   security may believe that the origin IP should be restored.  Actors
   considering restoring metadata may believe that they understand the
   relevant privacy considerations or believe that, because the primary
   purpose of the service was not privacy-related, none exist.  Examples
   of this design pattern include [RFC7239] and [RFC7871].

4.  Advice

   Avoid inserting metadata to restore information that would otherwise
   be unavailable to later participants in a protocol exchange.  It
   contributes to the overall loss of confidentiality for the Internet
   and trust in the Internet as a medium.  Do not add metadata to flows
   at intermediary devices unless a positive affirmation of approval for
   restoration has been received from the actor whose data will be

   Instead, design the protocol so that the actor can add such metadata
   themselves so that it flows end to end, rather than requiring the
   action of other parties.  In addition to improving privacy, this
   approach ensures consistent availability between the communicating
   parties, no matter what path is taken.  (Note that this document does
   not attempt to describe how an actor sets policies on providing this
   metadata, as the range of systems that might be implied is very

   As an example, RFC 7871 describes a method that had already been
   deployed and notes that it is unlikely that a clean-slate design
   would result in this mechanism.  If a clean-slate design were built
   to follow the advice in this document, that design would likely not
   use a core element of RFC 7871: rather than adding metadata at a
   proxy, it would provide facilities for end systems to add it to their
   initial queries.  In the case of RFC 7871, the relevant metadata is
   relatively easy for an end system to derive, as Session Traversal
   Utilities for NAT (STUN) [RFC5389] provides a method for learning the

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   reflexive transport address from which a client subnet could be
   derived.  This would allow clients to populate this data themselves,
   thus affirming their consent and providing data at a granularity with
   which they were comfortable.  As in RFC 7871, the addition of this
   data would require confirmation that the upstream DNS resolver
   understands what to do with it, but the same negotiation mechanism,
   an Extension Mechanisms for DNS (EDNS(0)) option [RFC6891], could be
   used.  Because of this negotiation, there would be a new variability
   in responses that would change the caching behavior for data supplied
   by participating servers.  This is not a major change from the
   current design, however, as the same considerations set out in
   Sections 7.3.2 and 7.5 of RFC 7871 would apply to client-supplied
   subnets as well as to proxy-supplied subnets.

   From a protocol perspective, in other words, this approach would be a
   minor change from RFC 7871, would be as fully featured, and would
   provide better privacy properties than the on-path update mechanism
   RFC 7871 provides.  The next section examines why, despite this,
   deployment considerations have sometimes trumped cleaner designs.

5.  Deployment Considerations

   There are a few common tensions associated with the deployment of
   systems that restore metadata.  The first is the trade-off in speed
   of deployment for different actors.  The Forwarded HTTP Extension in
   [RFC7239] provides an example of this.  When used with a proxy, it
   restores information related to the original requesting party, thus
   allowing a responding server to tailor responses according to the
   original party's region, network, or other characteristics associated
   with the identity.  It would, of course, be possible for the
   originating client to add this data itself, after using STUN
   [RFC5389] or a similar mechanism to first determine the information
   to declare.  This would require, however, full specification and
   adoption of this mechanism by the end systems.  It would not be
   available at all during this period and would thereafter be limited
   to systems that have been upgraded to include it.  The long tail of
   browser deployments indicates that many systems might go without
   upgrades for a significant period of time.  The proxy infrastructure,
   in contrast, is commonly under more active management and represents
   a much smaller number of elements; this impacts both the general
   deployment difficulty and the number of systems that the origin
   server must trust.

   The second common tension is between metadata minimization and the
   desire to tailor content responses.  For origin servers whose content
   is common across users, the loss of metadata may have limited impact
   on the system's functioning.  For other systems, which commonly
   tailor content by region or network, the loss of metadata may imply a

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   loss of functionality.  Where the user desires this functionality,
   restoration can commonly be achieved by the use of other identifiers
   or login procedures.  Where the user does not desire this
   functionality, but it is a preference of the server or a third party,
   adjustment is more difficult.  At the extreme, content blocking by
   network origin may be a regulatory requirement.  Trusting a network
   intermediary to provide accurate data is, of course, fragile in this
   case, but it may be a part of the regulatory framework.

   There are also tensions with latency of operation.  For example,
   where the end system does not initially know the information that
   would be added by on-path devices, it must engage the protocol
   mechanisms to determine it.  Determining a public IP address to
   include in a locally supplied header might require a STUN exchange,
   and the additional latency of this exchange discourages deployment of
   host-based solutions.  To minimize this latency, engaging those
   mechanisms may need to be done in parallel with or in advance of the
   core protocol exchanges with which this metadata would be supplied.

   These tensions do not change the basic recommendation, but they
   suggest that the parties who are introducing encryption and data
   minimization for existing protocols consider carefully whether the
   work also implies introducing mechanisms for the end-to-end
   provisioning of metadata when a user has actively consented to
   provide it.

6.  IANA Considerations

   This document makes no request of IANA.

7.  Security Considerations

   This memorandum describes a design pattern emerging from responses to
   the attacks described in [RFC7258].  Continued use of this design
   pattern, which uses mid-flow devices to restore metadata, lowers the
   impact of mitigations to that attack.

   Note that some emergency service recipients, notably PSAPs (Public
   Safety Answering Points) may prefer data provided by a network to
   data provided by an end system, because an end system could use false
   data to attack others or consume resources.  While this has the
   consequence that the data available to the PSAP is often more coarse
   than that available to the end system, the risk of false data being
   provided involves a risk to the lives of those targeted.

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8.  References

8.1.  Normative References

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,

   [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,

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <http://www.rfc-editor.org/info/rfc7258>.

   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C., and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624,
              DOI 10.17487/RFC7624, August 2015,

8.2.  Informative References

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,

   [RFC7239]  Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
              RFC 7239, DOI 10.17487/RFC7239, June 2014,

   [RFC7687]  Farrell, S., Wenning, R., Bos, B., Blanchet, M., and H.
              Tschofenig, "Report from the Strengthening the Internet
              (STRINT) Workshop", RFC 7687, DOI 10.17487/RFC7687,
              December 2015, <http://www.rfc-editor.org/info/rfc7687>.

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   [RFC7871]  Contavalli, C., van der Gaast, W., Lawrence, D., and W.
              Kumari, "Client Subnet in DNS Queries", RFC 7871,
              DOI 10.17487/RFC7871, May 2016,


   This document is derived in part from the work initially done on the
   perpass mailing list and at the STRINT workshop [RFC7687].  The text
   was originally developed by the IAB's Privacy and Security Program
   before submission to the IETF.  The document also benefited from an
   extensive review by Mohamed Boucadair.

Author's Address

   Ted Hardie

   Email: ted.ietf@gmail.com

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