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Internet Engineering Task Force                                R. Winter
Internet-Draft                                                  M. Faath
Intended status: Informational                              F. Weisshaar
Expires: March 19, 2017          University of Applied Sciences Augsburg
                                                      September 15, 2016

Privacy considerations for IP broadcast and multicast protocol designers


   A number of application-layer protocols make use of IP broadcasts or
   multicast messages for functions such as local service discovery or
   name resolution.  Some of these functions can only be implemented
   efficiently using such mechanisms.  When using broadcasts or
   multicast messages, a passive observer in the same broadcast domain
   can trivially record these messages and analyze their content.
   Therefore, broadcast/multicast protocol designers need to take
   special care when designing their protocols.

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on March 19, 2017.

Copyright Notice

   Copyright (c) 2016 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

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   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
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Privacy considerations  . . . . . . . . . . . . . . . . . . .   3
     2.1.  Message frequency . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Persistent identifiers  . . . . . . . . . . . . . . . . .   4
     2.3.  Anticipate user behavior  . . . . . . . . . . . . . . . .   4
     2.4.  Consider potential correlation  . . . . . . . . . . . . .   5
     2.5.  Configurability . . . . . . . . . . . . . . . . . . . . .   5
   3.  Operational considerations  . . . . . . . . . . . . . . . . .   6
   4.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Other considerations  . . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Broadcast and multicast messages have a large (and to the sender
   unknown) receiver group by design.  Because of that, these two
   mechanisms are vital for a number of basic network functions such as
   auto-configuration.  Application developers use broadcast/multicast
   messages to implement things like local service or peer discovery and
   it appears that an increasing number of applications make use of it

   Using broadcast/multicast can become problematic if the information
   that is being distributed can be regarded as sensitive or when the
   information that is distributed by multiple of these protocols can be
   correlated in a way that sensitive data can be derived.  This is
   clearly true for any protocol, but broadcast/multicast is special in
   at least two respects:

   (a)  The aforementioned large receiver group, consisting of receivers
        unknown to the sender.  This makes eavesdropping trivial for
        anybody on a LAN without special privileges or a special
        location in the network and

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   (b)  encryption is more difficult when broadcasting/multicasting
        messages, leaving content of these messages in the clear and
        making it easier to spoof and replay these messages.

   Given the above, privacy protection for protocols based on broadcast
   or multicast communication is significantly more difficult compared
   to unicast communication and at the same time invading the privacy is
   much easier.

   Privacy considerations of IETF-specified protocols have received some
   attention in the recent past (e.g.  [RFC7721] or [RFC7819]).  There
   is also general guidance available for document authors on when and
   how to include a privacy considerations section in their documents
   and on how to evaluate the privacy implications of Internet protocols
   [RFC6973].  RFC6973 also describes potential threads to privacy in
   great detail and lists terminology that is also used in this

   In contrast to RFC6973, this document contains a number of privacy
   considerations especially for broadcast/multicast protocol designers
   that are intended to reduce the likelihood that a broadcast protocol
   can be misused to collect sensitive data about devices, users and
   groups of users on a LAN.  These considerations particularly apply to
   protocols designed outside the IETF for two reasons.  For one, non-
   standard protocols will likely not receive operational attention and
   support in making them more secure such as e.g.  DHCP snooping does
   for DHCP because they typically are not documented.  The other reason
   is that these protocols have been designed in isolation, where a set
   of considerations to follow is useful in the absence of a larger
   community providing feedback.  In particular, carelessly designed
   broadcast/multicast protocols can break privacy efforts at different
   layers of the protocol stack such as MAC address or IP address
   randomization [RFC4941].

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Privacy considerations

   There are a few obvious and a few not necessarily obvious things
   designers of broadcast/multicast protocols should consider in respect
   to the privacy implications of their protocol.  Most of these items
   are based on protocol behavior observed as part of experiments on
   operational networks [TRAC2016].

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2.1.  Message frequency

   Frequent broadcast/multicast traffic caused by an application can
   give user behavior and online times away.  This allows a passive
   observer to potentially deduce a user's current activity (e.g. a
   game) and it allows to create an online profile (i.e. times the user
   is on the network).  The higher the frequency of these messages, the
   more accurate this profile will be.  Given that broadcasts are only
   visible in the same broadcast domain, these messages also give the
   rough location of the user away (e.g. a campus or building).

   If a protocol relies on frequent or periodic broadcast/multicast
   messages, the frequency SHOULD be chosen conservatively, in
   particular if the messages contain persistent identifiers (see next
   subsection).  Also, intelligent message suppression mechanisms such
   as the ones employed in mDNS [RFC6762] SHOULD be implemented.  The
   lower the frequency of broadcast messages, the harder traffic
   analysis and surveillance becomes.

2.2.  Persistent identifiers

   A few broadcast/multicast protocols observed in the wild make use of
   persistent identifiers.  This includes the use of hostnames or more
   abstract persistent identifiers such as a UUID or similar.  These
   IDs, which e.g. identify the installation of a certain application
   might not change across updates of the software and are therefore
   extremely long lived.  This allows a passive observer to track a user
   precisely if broadcast/multicast messages are frequent.  This is even
   true in case the IP and/or MAC address changes.  Such identifiers
   also allow two different interfaces (e.g.  WiFi and Ethernet) to be
   correlated to the same device.  If the application makes use of
   persistent identifiers for multiple installations of the same
   application for the same user, this even allows to infer that
   different devices belong to the same user.

   If a broadcast/multicast protocol relies on IDs to be transmitted, it
   SHOULD be considered if frequent ID rotations are possible in order
   to make user tracking more difficult.  Persistent IDs are considered
   bad practice in general for broadcast and multicast communication as
   persistent application layer IDs will make efforts on lower layers to
   randomize identifiers (e.g.  [I-D.huitema-6man-random-addresses])
   useless or at least much more difficult.

2.3.  Anticipate user behavior

   A large number of users name their device after themselves, either
   using their first name, last name or both.  Often a hostname includes
   the type, model or maker of a device, its function or includes

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   language specific information.  Based on gathered data, this appears
   currently to be prevalent user behavior [TRAC2016].  For protocols
   using the hostname as part of the messages, this clearly will reveal
   personally identifiable information to everyone on the local network.
   This information can also be used to mount more sophisticated
   attacks, when e.g. the owner of a device is identified (as an
   interesting target) or properties of the device are known (e.g. known

   Where possible, the use of hostnames and other user provided
   information in broadcast/multicast protocols SHOULD be avoided.  If
   only a persistent ID is needed, this can be generated.  An
   application might want to display the information it will broadcast
   on the LAN at install/config time, so the user is at least aware of
   the application's behavior.  More hostname considerations can be
   found in [I-D.ietf-intarea-hostname-practice].  More information on
   user participation can be found in [RFC6973].

2.4.  Consider potential correlation

   A large number of services and applications make use of the
   broadcast/multicast mechanism.  That means there are various sources
   of information that are easily accessible by a passive observer.  In
   isolation, the information these protocols reveal might seem
   harmless, but given multiple such protocols, it might be possible to
   correlate this information.  E.g.  a protocol that uses frequent
   messages including a UUID to identify the particular installation
   does not give the identity of the user away.  But a single message
   including the user's hostname might just do that and it can be
   correlated using e.g. the MAC address of the device's interface.

   A broadcast protocol designer should be aware of the fact that even
   if - in isolation - the information a protocol leaks seems harmless,
   there might be ways to correlate that information with other
   broadcast protocol information to reveal sensitive information about
   a user.

2.5.  Configurability

   A lot of applications and services using broadcast/multicast
   protocols do not include the means to declare "safe" environments
   (e.g. based on the SSID of a WiFi network and MAC addresses of access
   points).  E.g. a device connected to a public WiFi will likely
   broadcast the same information as when connected to the home network.
   It would be beneficial if certain behavior could be restricted to
   "safe" environments.

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   An application developer making use of broadcasts/multicasts as part
   of the application SHOULD make the broadcast feature, if possible,
   configurable, so that potentially sensitive information does not leak
   on public networks, where the thread to privacy is much larger.

3.  Operational considerations

   Besides changing end-user behavior, choosing sensible defaults as an
   operating system vendor (e.g. for suggesting host names) and the
   considerations for protocol designers mentioned in this document,
   there are things that the network administrators/operators can do to
   limit the above mentioned problems.

   A feature not uncommonly found on access points e.g. is to filter
   broadcast and multicast traffic.  This will potentially break certain
   applications or some of their functionality but will also protect the
   users from potentially leaking sensitive information.

4.  Summary

   Increasingly, applications rely on broadcast and multicast messages.
   For some, broadcasts/multicasts are the basis of their application
   logic, others use broadcasts to improve certain aspects of the
   application but are fully functional in case broadcasts fail.
   Irrespective of the role of broadcast and multicast messages for the
   application, the designers of protocols that make use of them should
   be very careful in their protocol design because of the special
   nature of broad- and multicast.

   It is not always possible to implememt certain functionality via
   unicast, but in case a protocol designer chooses to rely on
   broadcast/multicast, the following should be carefully considered:

   o  IETF-specified protocols, such as mDNS [RFC6762], should be used
      if possible as operational support might exist to protect against
      the leakage of private information

   o  Avoid using user-specified information inside broadcast/multicast
      messages as users will often use personal information or other
      information aiding attackers, in particular if the user is unaware
      about how that information is being used

   o  Avoid persistent IDs in messages as this allows user tracking,
      correlation and potentially has a devastating effect on other
      privacy protection mechanisms

   o  If you really must use a broadcast/multicast protocol and cannot
      use an IETF-specified protocol, then:

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      *  Be very conservative in how frequently you send messages as an
         effort in data minimization

      *  Seek advice from IETF-specifies protocols such as message
         suppression in mDNS

      *  Try to design the protocol in a way that the information cannot
         be correlated with other information in broadcast/multicast

      *  Let the user configure safe environments if possible (e.g.
         based on the SSID)

   [Note: discussions on this document should be take place on the
   Intarea mailing list of the IETF.  Subscription:
   https://www.ietf.org/mailman/listinfo/int-area, Mailing list archive:

5.  Other considerations

   Besides the privacy implications of frequent broadcasting, it also
   represents a performance problem.  In particular in certain wireless
   technologies such as 802.11, broadcast and multicast are transmitted
   at a much lower rate (the lowest common denominator rate) compared to
   unicast and therefore have a much bigger impact on the overall
   available airtime.  Further, it will limit the ability for devices to
   go to sleep if frequent broadcasts are being sent.  A similar problem
   in respect to Router Advertisements is addressed in
   [I-D.ietf-v6ops-reducing-ra-energy-consumption].  In that respect
   broadcasts can be used for another class of attacks that not related
   to privacy.  The potential impact on network performance should
   nevertheless be considered by broadcast protocol designers.

6.  Acknowledgments

   We would like to thank Eliot Lear and Stephane Bortzmeyer for their

   This work was partly supported by the European Commission under grant
   agreement FP7-318627 mPlane.  Support does not imply endorsement.

7.  IANA Considerations

   This memo includes no request to IANA.

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8.  Security Considerations

   This document deals with privacy-related considerations of broadcast-
   and multicast-based protocols.  It contains advice for designers of
   such protocols to minimize the leakage of privacy-sensitive
   information.  The intent of the advice is to make sure that
   identities will remain anonymous and user tracking will be made

9.  Informative References

              Huitema, C., "Implications of Randomized Link Layers
              Addresses for IPv6 Address Assignment", draft-huitema-
              6man-random-addresses-03 (work in progress), March 2016.

              Huitema, C. and D. Thaler, "Current Hostname Practice
              Considered Harmful", draft-ietf-intarea-hostname-
              practice-00 (work in progress), October 2015.

              Yourtchenko, A. and L. Colitti, "Reducing energy
              consumption of Router Advertisements", draft-ietf-v6ops-
              reducing-ra-energy-consumption-03 (work in progress),
              November 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 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,

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   [RFC7721]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              RFC 7721, DOI 10.17487/RFC7721, March 2016,

   [RFC7819]  Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy
              Considerations for DHCP", RFC 7819, DOI 10.17487/RFC7819,
              April 2016, <http://www.rfc-editor.org/info/rfc7819>.

              Faath, M., Weisshaar, F., and R. Winter, "How Broadcast
              Data Reveals Your Identity and Social Graph", 7th
              International Workshop on TRaffic Analysis and
              Characterization IEEE TRAC 2016, September 2016.

Authors' Addresses

   Rolf Winter
   University of Applied Sciences Augsburg

   Email: rolf.winter@hs-augsburg.de

   Michael Faath
   University of Applied Sciences Augsburg

   Email: michael.faath@hs-augsburg.de

   Fabian Weisshaar
   University of Applied Sciences Augsburg

   Email: fabian.weisshaar@hs-augsburg.de

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