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rfc:rfc8165

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

            Design Considerations for Metadata Insertion

Abstract

 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
 http://www.rfc-editor.org/info/rfc8165.

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
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Hardie Informational [Page 1] RFC 8165 Design Considerations for Metadata Insertion May 2017

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
 "Collaborator".

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

Hardie Informational [Page 2] RFC 8165 Design Considerations for Metadata Insertion May 2017

 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
 added.
 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
 broad).
 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

Hardie Informational [Page 3] RFC 8165 Design Considerations for Metadata Insertion May 2017

 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

Hardie Informational [Page 4] RFC 8165 Design Considerations for Metadata Insertion May 2017

 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.

Hardie Informational [Page 5] RFC 8165 Design Considerations for Metadata Insertion May 2017

8. References

8.1. Normative References

 [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
            FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
            <http://www.rfc-editor.org/info/rfc4949>.
 [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,
            <http://www.rfc-editor.org/info/rfc6973>.
 [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,
            <http://www.rfc-editor.org/info/rfc7624>.

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,
            <http://www.rfc-editor.org/info/rfc5389>.
 [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
            for DNS (EDNS(0))", STD 75, RFC 6891,
            DOI 10.17487/RFC6891, April 2013,
            <http://www.rfc-editor.org/info/rfc6891>.
 [RFC7239]  Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
            RFC 7239, DOI 10.17487/RFC7239, June 2014,
            <http://www.rfc-editor.org/info/rfc7239>.
 [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>.

Hardie Informational [Page 6] RFC 8165 Design Considerations for Metadata Insertion May 2017

 [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,
            <http://www.rfc-editor.org/info/rfc7871>.

Acknowledgements

 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

Hardie Informational [Page 7]

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