Movatterモバイル変換

Internet Engineering Task Force (IETF)                         T. HardieRequest for Comments: 8165                                      May 2017Category: InformationalISSN: 2070-1721Design Considerations for Metadata InsertionAbstract   The IAB publishedRFC 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; seeSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 2017Table of Contents1. Introduction ....................................................22. Terminology .....................................................23. Design Pattern ..................................................24. Advice ..........................................................35. Deployment Considerations .......................................46. IANA Considerations .............................................57. Security Considerations .........................................58. References ......................................................68.1. Normative References .......................................68.2. Informative References .....................................6   Acknowledgements ...................................................7   Author's Address ...................................................71.  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 cookiesHardie                        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 ofRFC 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 ofRFC 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 theHardie                        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 inRFC 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   Sections7.3.2 and7.5 ofRFC 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 fromRFC 7871, would be as fully featured, and would   provide better privacy properties than the on-path update mechanismRFC 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 aHardie                        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 20178.  References8.1.  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.comHardie                        Informational                     [Page 7]