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We’re gonna need a bigger threat model

Slides IAB Workshop: Design Expectations vs Deployment Reality in Protocol Development (dedrws) Team
Title We’re gonna need a bigger threat model
Abstract Stephen Farrell, We’re gonna need a bigger threat model
State Active
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Last updated 2023-02-07


Network Working Group                                         S. Farrell
Internet-Draft                                    Trinity College Dublin
Intended status: Informational                               May 1, 2019
Expires: November 2, 2019

                 We're gonna need a bigger threat model


   We argue that an expanded threat model is needed for Internet
   protocol development as protocol endpoints can no longer be
   considered to be generally trustworthy for any general definition of

   This draft will be a submission to the DEDR IAB workshop.

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
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   This Internet-Draft will expire on November 2, 2019.

Copyright Notice

   Copyright (c) 2019 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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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Examples of deliberate adversarial behaviour in applications    4
     2.1.  Malware in curated application stores . . . . . . . . . .   4
     2.2.  Virtual private networks (VPNs) . . . . . . . . . . . . .   5
     2.3.  Compromised (home) networks . . . . . . . . . . . . . . .   5
     2.4.  Web browsers  . . . . . . . . . . . . . . . . . . . . . .   5
     2.5.  Web site policy deception . . . . . . . . . . . . . . . .   5
     2.6.  Tracking bugs in mail . . . . . . . . . . . . . . . . . .   6
     2.7.  Troll farms in online social networks . . . . . . . . . .   6
     2.8.  Smart televisions . . . . . . . . . . . . . . . . . . . .   6
     2.9.  So-called Internet of things  . . . . . . . . . . . . . .   7
   3.  Inadvertent adversarial behaviours  . . . . . . . . . . . . .   7
   4.  Possible directions for an expanded threat model  . . . . . .   8
     4.1.  Develop a BCP for privacy considerations  . . . . . . . .   8
     4.2.  Consider the user perspective . . . . . . . . . . . . . .   8
     4.3.  Consider ABuse-cases as well as use-cases . . . . . . . .   8
     4.4.  Re-consider protocol design "lore"  . . . . . . . . . . .   8
     4.5.  Isolation . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.6.  Transparency  . . . . . . . . . . . . . . . . . . . . . .   9
     4.7.  Minimise  . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.8.  Same-Origin Policy  . . . . . . . . . . . . . . . . . . .   9
     4.9.  Greasing  . . . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Informative References  . . . . . . . . . . . . . . . . .  10
     9.2.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  15
     A.1.  Changes from -01 to -02 . . . . . . . . . . . . . . . . .  15
     A.2.  Changes from -00 to -01 . . . . . . . . . . . . . . . . .  15
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   [[There's a github repo for this -- issues and PRs are welcome there.
   <> ]]

   [RFC3552], Section 3 defines an "Internet Threat Model" which has
   been commonly used when developing Internet protocols.  That assumes
   that "the end-systems engaging in a protocol exchange have not

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   themselves been compromised."  RFC 3552 is a formal part of of the
   IETF's process as it is also BCP72.

   Since RFC 3552 was written, we have seen a greater emphasis on
   considering privacy and [RFC6973] provides privacy guidance for
   protocol developers.  RFC 6973 is not a formal BCP, but appears to
   have been useful for protocol developers as it is referenced by 38
   later RFCs at the time of writing [1].

   BCP188, [RFC7258] subsequently recognised pervasive monitoring as a
   particular kind of attack and has also been relatively widely
   referenced (39 RFCs at the time of writing [2]).  To date, perhaps
   most documents referencing BCP188 have considered state-level or in-
   network adversaries.

   In this document, we argue that we need to epxand our threat model to
   acknowledge that today many applications are themselves rightly
   considered potential adversaries for at least some relevant actors.
   However, those (good) actors cannot in general refuse to communicate
   and will with non-negligible probability encounter applications that
   are adversarial.

   We also argue that not recognising this reality causes Internet
   protocol designs to sometimes fail to protect the systems and users
   who depend on those.

   Discussion related to expanding our concept of threat-model ought not
   (but perhaps inevitably will) involve discussion of weakening how
   confidentiality is provided in Internet protocols.  Whilst it may
   superficially seem to be the case that encouraging in-network
   interception could help with detection of adversarial application
   behaviours, such a position is clearly mistaken once one notes that
   adding middleboxes that can themselves be adversarial cannot be a
   solution to the problem of possibly encountering adversarial code on
   the network.  It is also the case that the IETF has rough consensus
   to provide better, and not weaker, security and privacy, which
   includes confidentiality services.  The IETF has maintained that
   consensus over three decades, despite repeated (and repetitive;-)
   debates on the topic.  That consensus is represented in [RFC2804],
   BCP 200 [RFC1984] and more latterly, the above-mentioned BCP 188 as
   well as in the numerous RFCs referencing those works.  The
   probability that discussion of expanding our threat model leads to a
   change in that rough consensus seems highly remote.

   However, it is not clear if the IETF will reach rough consensus on a
   description of such an expanded threat model.  We argue that ignoring
   this aspect of deployed reality may not bode well for Internet
   protocol development.

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   Absent such an expanded threat model, we expect to see more of a
   mismatch between expectaions and the deployment reality for some
   Internet protocols.

   This internet-draft is a submission to the IAB's DEDR workshop [3]
   and is not intended to become an RFC.

   We note that another author has independently proposed changes to the
   Internet threat model for related, but different, reasons,
   [I-D.arkko-arch-internet-threat-model] also as a submission to the
   DEDR workshop.

   We are saddened by, and apologise for, the somewhat dystopian
   impression that this document may impart - hopefully, there's a bit
   of hope at the end;-)

2.  Examples of deliberate adversarial behaviour in applications

   In this section we describe a few documented examples of deliberate
   adversarial behaviour by applications that could affect Internet
   protocol development.  The adversarial behaviours described below
   involve various kinds of attack, varying from simple fraud, to
   credential theft, surveillance and contributing to DDoS attacks.
   This is not intended to be a comprehensive nor complete survey, but
   to motivate us to consider deliberate adversarial behaviour by

   While we have these examples of deliberate adversarial behaviour,
   there are also many examples of application developers doing their
   best to protect the security and privacy of their users or customers.
   That's just the same as the case today where we need to consider in-
   network actors as potential adversaries despite the many examples of
   network operators who do act primarily in the best interests of their
   users.  So this section is not intended as a slur on all or some
   application developers.

2.1.  Malware in curated application stores

   Despite the best efforts of curators, so-called App-Stores frequently
   distribute malware of many kinds and one recent study [curated]
   claims that simple obfuscation enables malware to avoid detection by
   even sophisticated operators.  Given the scale of these deployments,
   ditribution of even a small percentage of malware-infected
   applictions can affect a huge number of people.

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2.2.  Virtual private networks (VPNs)

   Virtual private networks (VPNs) are supposed to hide user traffic to
   various degrees depending on the particular technology chosen by the
   VPN provider.  However, not all VPNs do what they say, some for
   example misrepresenting the countries in which they provide vantage
   points. [vpns]

2.3.  Compromised (home) networks

   What we normally might consider network devices such as home routers
   do also run applications that can end up being adversarial, for
   example running DNS and DHCP attacks from home routers targeting
   other devices in the home.  One study [home] reports on a 2011 attack
   that affected 4.5 million DSL modems in Brazil.  The absence of
   software update [RFC8240] has been a major cause of these issues and
   rises to the level that considering this as intentional behaviour by
   device vendors who have chosen this path is warranted.

2.4.  Web browsers

   Tracking of users in order to support advertising based business
   models is ubiquitous on the Internet today.  HTTP header fields (such
   as cookies) are commonly used for such tracking, as are structures
   within the content of HTTP responses such as links to 1x1 pixel
   images and (ab)use of Javascript APIs offered by browsers. [tracking]

   While some people may be sanguine about this kind of tracking, others
   consider this behaviour unwelcome, when or if they are informed that
   it happens, [attitude] though the evidence here seems somewhat harder
   to interpret and many studies (that we have found to date) involve
   small numbers of users.  Historically, browsers have not made this
   kind of tracking visible and have enabled it by default, though some
   recent browser versions are starting to enable visibility and
   blocking of some kinds of tracking.  Browsers are also increasingly
   imposing more stringent requirements on plug-ins for varied security

2.5.  Web site policy deception

   Many web sites today provide some form of privacy policy and terms of
   service, that are known to be mostly unread. [unread] This implies
   that, legal fiction aside, users of those sites have not in reality
   agreed to the specific terms published and so users are therefore
   highly exposed to being exploited by web sites, for example
   [cambridge] is a recent well-publicised case where a service provider
   abused the data of 87 million users via a partnership.  While many
   web site operators claim that they care deeply about privacy, it

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   seems prudent to assume that some (or most?) do not in fact care
   about user privacy, or at least not in ways with which many of their
   users would agree.  And of course, today's web sites are actually
   mostly fairly complex web applications and are no longer static sets
   of HTML files, so calling these "web sites" is perhaps a misnomer,
   but considered as web applications, that may for example link in
   advertising networks, it seems clear that many exist that are

2.6.  Tracking bugs in mail

   Some mail user agents (MUAs) render HTML content by default (with a
   subset not allowing that to be turned off, perhaps particularly on
   mobile devices) and thus enable the same kind of adversarial tracking
   seen on the web.  Attempts at such intentional tracking are also seen
   many times per day by email users - in one study [mailbug] the
   authors estimated that 62% of leakage to third parties was
   intentional, for example if leaked data included a hash of the
   recipient email address.

2.7.  Troll farms in online social networks

   Online social network applications/platforms are well-known to be
   vulnerable to troll farms, sometimes with tragic consequences, [4]
   where organised/paid sets of users deliberately abuse the application
   platform for reasons invisible to a normal user.  For-profit
   companies building online social networks are well aware that subsets
   of their "normal" users are anything but.  In one US study, [troll]
   sets of troll accounts were roughly equally distributed on both sides
   of a controversial discussion.  While Internet protocol designers do
   sometimes consider sybil attacks [sybil], arguably we have not
   provided mechanisms to handle such attacks sufficiently well,
   especially when they occur within walled-gardens.  Equally, one can
   make the case that some online social networks, at some points in
   their evolution, appear to have prioritised counts of active users so
   highly that they have failed to invest sufficient effort for
   detection of such troll farms.

2.8.  Smart televisions

   There have been examples of so-called "smart" televisions spying on
   their owners without permission [5] and one survey of user attitudes
   [smarttv] found "broad agreement was that it is unacceptable for the
   data to be repurposed or shared" although the level of user
   understanding may be questionable.  What is clear though is that such
   devices generally have not provided controls for their owners that
   would allow them to meaningfully make a decision as to whether or not
   they want to share such data.

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2.9.  So-called Internet of things

   Many so-called Internet of Things (IoT) devices ("so-called" as all
   devices were already things:-) have been found extremely deficient
   when their security and privacy aspects were analysed, for example
   children's toys. [toys] While in some cases this may be due to
   incompetence rather than being deliberately adversarial behaviour,
   the levels of incompetence frequently seen imply that it is valid to
   consider such cases as not being accidental.

3.  Inadvertent adversarial behaviours

   Not all adversarial behaviour by applications is deliberate, some is
   likely due to various levels of carelessness (some quite
   understandable, others not) and/or due to erroneous assumptions about
   the environments in which those applications (now) run.
   We very briefly list some such cases:

   o  Application abuse for command and control, for example, use of IRC
      or apache logs for malware command and control [6]

   o  Carelessly leaky buckets [7], for example, lots of Amazon S3 leaks
      showing that careless admins can too easily cause application
      server data to become available to adversaries

   o  Virtualisation exposing secrets, for example, Meltdown and Spectre
      [8] and similar side-channels

   o  Compromised badly-maintained web sites, that for example, have led
      to massive online databases of passwords [9]

   o  Supply-chain attacks, for example, the Target attack [10]

   o  Breaches of major service providers, that many of us might have
      assumed would be sufficiently capable to be the best large-scale
      "Identity providers", for example:

      *  3 billion accounts: yahoo [11]

      *  "up to 600M" account passwords stored in clear: facebook [12]

      *  many millions at risk: telcos selling location data [13]

      *  50 million accounts: facebook [14]

      *  14 million accounts: verizon [15]

      *  "hundreds of thousands" of accounts: google [16]

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      *  unknown numbers, some email content exposed: microsoft [17]

   o  Breaches of smaller service providers: Too many to enumerate,

4.  Possible directions for an expanded threat model

   As we believe useful conclusions in this space require community
   consensus, we won't offer definitive descriptions of an expanded
   threat model but we will call out some potential directions that
   could be explored at the DEDR workshop and thereafter, if there is
   interest in this topic.

4.1.  Develop a BCP for privacy considerations

   It may be time for the IETF to develop a BCP for privacy
   considerations, possibly starting from [RFC6973].

4.2.  Consider the user perspective

   [I-D.nottingham-for-the-users] argues that, in relevant cases where
   there are conflicting requirements, the "IETF considers end users as
   its highest priority concern."  Doing so seems consistent with the
   expanded threat model being argued for here, so may indicate that a
   BCP in that space could also be useful.

4.3.  Consider ABuse-cases as well as use-cases

   Protocol developers and those implementing and deploying Internet
   technologies are typically most interested in a few specific use-
   cases for which they need solutions.  Expanding our threat model to
   include adversarial application behaviours [abusecases] seems likely
   to call for significant attention to be paid to potential abuses of
   whatever new or re-purposed technology is being considered.

4.4.  Re-consider protocol design "lore"

   It could be that this discussion demonstrates that it is timely to
   reconsider some protocol design "lore" as for example is done in
   [I-D.iab-protocol-maintenance].  More specifically, protocol
   extensibility mechanisms may inadvertently create vectors for abuse-
   cases, given that designers cannot fully analyse their impact at the
   time a new protocol is defined or standardised.  One might conclude
   that a lack of extensibility could be a virtue for some new
   protocols, in contrast to earlier assumptions.  As pointed out by one
   commenter though, people can find ways to extend things regardless,
   if they feel the need.

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4.5.  Isolation

   Sophisticated users can sometimes deal with adversarial behaviours in
   applications by using different instances of those applications, for
   example, differently configured web browsers for use in different
   contexts.  Applications (including web browsers) and operating
   systems are also building in isolation via use of different processes
   or sandboxing.  Protocol artefacts that relate to uses of such
   isolation mechanisms might be worth considering.  To an extent, the
   IETF has in practice already recognised some of these issues as being
   in-scope, e.g.  when considering the linkability issues with
   mechanisms such as TLS session tickets, or QUIC connection

4.6.  Transparency

   Certificate transparency (CT) [RFC6962] has been an effective
   countermeasure for X.509 certificate mis-issuance, which used be a
   known application layer misbehaviour in the public web PKI.  While
   the context in which CT operates is very constrained (essentially to
   the public CAs trusted by web browsers), similar approaches could be
   useful for other protocols or technologies.

   In addition, legislative requirements such as those imposed by the
   GDPR for subject access to data [18] could lead to a desire to handle
   internal data structures and databases in ways that are reminiscent
   of CT, though clearly with significant authorisation being required
   and without the append-only nature of a CT log.

4.7.  Minimise

   As recommended in [RFC6973] data minimisation and additional
   encryption are likely to be helpful - if applications don't ever see
   data, or a cleartext form of data, then they should have a harder
   time misbehaving.  Similarly, not adding new long-term identifiers,
   and not exposing existing ones, would seem helpful.

4.8.  Same-Origin Policy

   The Same-Origin Policy (SOP) [RFC6454] perhaps already provides an
   example of how going beyond the RFC 3552 threat model can be useful.
   Arguably, the existence of the SOP demonstrates that at least web
   browsers already consider the 3552 model as being too limited.
   (Clearly, differentiating between same and not-same origins
   implicitly assumes that some origins are not as trustworthy as

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4.9.  Greasing

   The TLS protocol [RFC8446] now supports the use of GREASE
   [I-D.ietf-tls-grease] as a way to mitigate on-path ossification.
   While this technique is not likely to prevent any deliberate
   misbehaviours, it may provide a proof-of-concept that network
   protocol mechanisms can have impact in this space, if we spend the
   time to try analyse the incentives of the various parties.

5.  Conclusions

   At this stage we don't think it approriate to claim that any strong
   conclusion can be reached based on the above.  We do however, claim
   that the is a topic that could be worth discussion at the DEDR
   workshop and elsewhere.

6.  Security Considerations

   This draft is all about security, and privacy.

   Encryption is one of the most effective tools in countering network
   based attackers and will also have a role in protecting against
   adversarial applications.  However, today many existing tools for
   countering adversarial applications assume they can inspect network
   traffic to or from potentially adversarial applications.  These facts
   of course cause tensions (e.g. see [RFC8404]).  Expanding our threat
   model could possibly help reduce some of those tensions, if it leads
   to the development of protocols that make exploitation harder or more
   transparent for adversarial applications.

7.  IANA Considerations

   There are no IANA considerations.

8.  Acknowledgements

   We'll happily ack anyone who's interested enough to read and comment
   on this.  With no implication that they agree with some or all of the
   above, thanks to Jari Arkko, Carsten Bormann, Christian Huitema and
   Daniel Kahn Gillmor for comments on an earlier version of the text.

9.  References

9.1.  Informative References

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              McDermott, J. and C. Fox, "Using abuse case models for
              security requirements analysis", IEEE Annual Computer
              Security Applications Conference (ACSAC'99) 1999, 1999,

              Chanchary, F. and S. Chiasson, "User Perceptions of
              Sharing, Advertising, and Tracking", Symposium on Usable
              Privacy and Security (SOUPS) 2015, 2015,

              Isaak, J. and M. Hanna, "User Data Privacy: Facebook,
              Cambridge Analytica, and Privacy Protection",
              Computer 51.8 (2018): 56-59, 2018,

   [curated]  Hammad, M., Garcia, J., and S. MaleK, "A large-scale
              empirical study on the effects of code obfuscations on
              Android apps and anti-malware products", ACM International
              Conference on Software Engineering 2018, 2018,

              Arkko, J., "Changes in the Internet Threat Model", draft-
              arkko-arch-internet-threat-model-00 (work in progress),
              April 2019.

              Thomson, M., "The Harmful Consequences of the Robustness
              Principle", draft-iab-protocol-maintenance-02 (work in
              progress), March 2019.

              Benjamin, D., "Applying GREASE to TLS Extensibility",
              draft-ietf-tls-grease-02 (work in progress), January 2019.

              Nottingham, M., "The Internet is for End Users", draft-
              nottingham-for-the-users-07 (work in progress), March

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   [mailbug]  Englehardt, S., Han, J., and A. Narayanan, "I never signed
              up for this! Privacy implications of email tracking",
              Proceedings on Privacy Enhancing Technologies 2018.1
              (2018): 109-126., 2018,

   [RFC1984]  IAB and IESG, "IAB and IESG Statement on Cryptographic
              Technology and the Internet", BCP 200, RFC 1984,
              DOI 10.17487/RFC1984, August 1996,

   [RFC2804]  IAB and IESG, "IETF Policy on Wiretapping", RFC 2804,
              DOI 10.17487/RFC2804, May 2000,

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

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,

   [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, <>.

   [RFC8240]  Tschofenig, H. and S. Farrell, "Report from the Internet
              of Things Software Update (IoTSU) Workshop 2016",
              RFC 8240, DOI 10.17487/RFC8240, September 2017,

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   [RFC8404]  Moriarty, K., Ed. and A. Morton, Ed., "Effects of
              Pervasive Encryption on Operators", RFC 8404,
              DOI 10.17487/RFC8404, July 2018,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [smarttv]  Malkin, N., Bernd, J., Johnson, M., and S. Egelman, ""What
              Can't Data Be Used For?" Privacy Expectations about Smart
              TVs in the U.S.", European Workshop on Usable Security
              (Euro USEC) 2018, 2018, <

   [sybil]    Viswanath, B., Post, A., Gummadi, K., and A. Mislove, "An
              analysis of social network-based sybil defenses", ACM
              SIGCOMM Computer Communication Review 41(4), 363-374.
              2011, 2011,

   [toys]     Chu, G., Apthorpe, N., and N. Feamster, "Security and
              Privacy Analyses of Internet of Things Childrens' Toys",
              IEEE Internet of Things Journal 6.1 (2019): 978-985.,
              2019, <>.

              Ermakova, T., Fabian, B., Bender, B., and K. Klimek, "Web
              Tracking-A Literature Review on the State of Research",
               Proceedings of the 51st Hawaii International Conference
              on System Sciences, 2018,

   [troll]    Stewart, L., Arif, A., and K. Starbird, "Examining trolls
              and polarization with a retweet network", ACM Workshop on
              Misinformation and Misbehavior Mining on the Web 2018,
              2018, <

   [unread]   Obar, J. and A. Oeldorf-Hirsch, "The biggest lie on the
              internet: Ignoring the privacy policies and terms of
              service policies of social networking services",
              Information, Communication and Society (2018): 1-20, 2018,

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   [vpns]     Khan, M., DeBlasio, J., Voelker, G., Snoeren, A., Kanich,
              C., and N. Vallina-Rodrigue, "An empirical analysis of the
              commercial VPN ecosystem", ACM Internet Measurement
              Conference 2018 (pp. 443-456), 2018,

9.2.  URIs
















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Appendix A.  Change Log

   This isn't gonna end up as an RFC, but may as well be tidy...

A.1.  Changes from -01 to -02

   o  Oops - got an RFC number wrong in reference

A.2.  Changes from -00 to -01

   o  Made a bunch more edits and added more references

   o  I had lots of typos (as always:-)

   o  cabo: PR#1 fixed more typos and noted extensbility danger

Author's Address

   Stephen Farrell
   Trinity College Dublin


Farrell                 Expires November 2, 2019               [Page 15]