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Versions: (draft-winfaa-intarea-broadcast-consider) 00 01 02 03 04 05 06 07 08 09 RFC 8386

Internet Engineering Task Force                                R. Winter
Internet-Draft                   University of Applied Sciences Augsburg
Intended status: Informational                                  M. Faath
Expires: November 30, 2017                                  Conntac GmbH
                                                            F. Weisshaar
                                 University of Applied Sciences Augsburg
                                                            May 29, 2017

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 like 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/
   multicast 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 November 30, 2017.

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

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Types and usage of broadcast and multicast  . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Privacy considerations  . . . . . . . . . . . . . . . . . . .   4
     2.1.  Message frequency . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Persistent identifiers  . . . . . . . . . . . . . . . . .   5
     2.3.  Anticipate user behavior  . . . . . . . . . . . . . . . .   6
     2.4.  Consider potential correlation  . . . . . . . . . . . . .   6
     2.5.  Configurability . . . . . . . . . . . . . . . . . . . . .   7
   3.  Operational considerations  . . . . . . . . . . . . . . . . .   8
   4.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Other considerations  . . . . . . . . . . . . . . . . . . . .   9
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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 or link-layer address lookup.  Also 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 [TRAC2016].  That is not
   entirely surprising.  As RFC 919 [RFC0919] puts it, "The use of
   broadcasts [...] is a good base for many applications".  Broadcast
   and multicast functionality in a subnetwork are therefore important
   as a lack thereof renders the protocols underlying these mechanisms
   inoperable [RFC3819].

   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

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   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 without special
        privileges or a special location in the network trivial for
        anybody in the broadcast/multicast domain.

   (b)  Encryption is more difficult when broadcast/multicast messages,
        leaving content of these messages in the clear and making it
        easier to spoof and replay them.

   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.  RFC 7721 [RFC7721] or RFC 7919
   [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
   threats to privacy in great detail and lists terminology that is also
   used in this document.

   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/multicast
   protocol can be misused to collect sensitive data about devices,
   users and groups of users on a broadcast/multicast domain.  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.  Types and usage of broadcast and multicast

   In IPv4, two major types of broadcast addresses exist, the limited
   broadcast which is defined as all-ones (, defined in
   section of [RFC1812]) and the directed broadcast with the

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   given network prefix of an IP address and the host part of all-ones
   (defined in section of [RFC1812]).  Broadcast packets are
   received by all nodes in a subnetwork.  Limited broadcasts never
   transit a router.  The same is true for directed broadcasts by
   default, but routers MAY provide an option to do this [RFC2644].
   IPv6 on the other hand does not provide broadcast addresses but
   solely relies on multicast [RFC4291].

   In contrast to broadcast addresses, multicast addresses represent an
   identifier for a set of interfaces that can be a set different from
   all nodes in the subnetwork.  All interfaces that are identified by a
   given multicast address receive packets destined towards that address
   and are called a multicast group.  In both IPv4 and IPv6, multiple
   pre-defined multicast addresses exist.  The ones most relevant for
   this document are the ones with subnet scope.  For IPv4, an IP prefix
   is reserved for this purpose called the Local Network Control Block
   (, defined in section 4 of [RFC5771]).  For IPv6, the
   relevant multicast addresses are the two All Nodes Addresses, which
   every IPv6-capable host is required to recognize as identifying
   itself (see section 2.7.1 of [RFC4291]).

   Typical usage of these addresses include local service discovery
   (e.g. mDNS [RFC6762] and LLMNR [RFC4795] make use of multicast),
   autoconfiguration (e.g.  DHCPv4 [RFC2131] uses broadcasts and DHCPv6
   [RFC3315] uses multicast addresses) and other vital network services
   such as address resolution or duplicate address detection.  But
   besides these core network functions, also applications make use of
   broadcast and multicast functionality, often implementing proprietary
   protocols.  In sum, these protocols distribute a diverse set of
   potentially privacy sensitive information to a large receiver group.

1.2.  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/multicasts
   are only visible in the same broadcast/multicast domain, these
   messages also give the rough location of the user away (e.g. a campus
   or building).

   This behavior has e.g. been observed by a synchronization mechanism
   of a popular application, where multiple messages have been sent per
   minute via broadcast.  Given this behavior, it is possible to record
   a device's time on the network with a sub-minute accuracy given only
   the traffic of this single application installed on the device.  But
   also services used for local name resolution in modern operating
   systems utilize broadcast/multicast protocols (e.g. mDNS, LLMNR or
   NetBIOS) to announce for example their shares regularly and allow a
   tracking of the online time of a device.

   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 host names 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.

   The aforementioned broadcast messages from a synchronization
   mechanism of a popular application also included a persistent

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   identifier in every broadcast.  This identifier did never change
   after the application was installed and allowed to track a device
   even when it changed its network interface or when it connected to a
   different network.

   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 host name
   includes the type, model or maker of a device, its function or
   includes language specific information.  Based on gathered data, this
   appears currently to be prevalent user behavior [TRAC2016].  For
   protocols using the host name 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 vulnerabilities).

   A popular operating system vendor includes the name the user chooses
   for the user account during the installation process as part of the
   host name of the device.  The name of the operating system is also
   included, revealing therefore two pieces of information, which can be
   regarded as private information if the host name is used in
   broadcast/multicast messages.

   Where possible, the use of host names 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 host name considerations can be
   found in [I-D.ietf-intarea-hostname-practice].  More information on
   user participation can be found in RFC 6973 [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

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   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 host name might just do that and it can be
   correlated using e.g. the MAC address of the device's interface.

   In the experiments described in [TRAC2016], it was possible to
   correlate frequently sent broadcast messages that included a unique
   identifier with other broadcast/multicast messages containing
   usernames (e.g. mDNS, LLMNR or NetBIOS), but also relationships to
   other users.  This allowed to reveal the real identity of the users
   of many devices but it also gave some information about their social
   environment away.

   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 the MAC addresses of
   the 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.

   A popular operating system e.g. allows the user to specify the trust
   level of the network the device connects to, which for example
   restricts specific system services (using broadcast/multicast
   messages for their normal operation) to be used in untrusted
   networks.  Such functionality could implemented as part of an

   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.

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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/multicasts to improve certain aspects of
   the application but are fully functional in case broadcasts/
   multicasts 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 implement 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.  Also, for some protocols
      privacy extensions are being specified, which can be used if
      implemented.  E.g. for DNS-SD privacy extensions are documented in

   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, Joe Touch and Stephane Bortzmeyer
   for their valuable input to this document.

   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

   It should be noted that certain applications could make use of
   existing mechanisms to protect multicast traffic such as the ones
   defined in [RFC5374].  Examples of such applications can be found in
   Appendix A. of [RFC5374].  Given the required infrastructure and
   assumptions about these applications and the security infrastructure,
   many applications will not be able to make use of such mechanisms.

9.  References

9.1.  Normative References

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

9.2.  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. Kaiser, "Privacy Extensions for DNS-
              SD", draft-ietf-dnssd-privacy-00 (work in progress),
              October 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.

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   [RFC0919]  Mogul, J., "Broadcasting Internet Datagrams", STD 5, RFC
              919, DOI 10.17487/RFC0919, October 1984,

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, DOI 10.17487/RFC2131, March 1997,

   [RFC2644]  Senie, D., "Changing the Default for Directed Broadcasts
              in Routers", BCP 34, RFC 2644, DOI 10.17487/RFC2644,
              August 1999, <http://www.rfc-editor.org/info/rfc2644>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <http://www.rfc-editor.org/info/rfc3315>.

   [RFC3819]  Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, DOI 10.17487/RFC3819, July 2004,

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <http://www.rfc-editor.org/info/rfc4291>.

   [RFC4795]  Aboba, B., Thaler, D., and L. Esibov, "Link-local
              Multicast Name Resolution (LLMNR)", RFC 4795, DOI
              10.17487/RFC4795, January 2007,

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

   [RFC5374]  Weis, B., Gross, G., and D. Ignjatic, "Multicast
              Extensions to the Security Architecture for the Internet
              Protocol", RFC 5374, DOI 10.17487/RFC5374, November 2008,

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   [RFC5771]  Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
              IPv4 Multicast Address Assignments", BCP 51, RFC 5771, DOI
              10.17487/RFC5771, March 2010,

   [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,

   [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
   Conntac GmbH

   Email: faath@conntac.net

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   Fabian Weisshaar
   University of Applied Sciences Augsburg

   Email: fabian.weisshaar@hs-augsburg.de

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