RFC8386: Privacy Considerations for Protocols Relying on IP Broadcast or Multicast

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Internet Engineering Task Force (IETF)                         R. Winter
Request for Comments: 8386       University of Applied Sciences Augsburg
Category: Informational                                         M. Faath
ISSN: 2070-1721                                             Conntac GmbH
                                                            F. Weisshaar
                                 University of Applied Sciences Augsburg
                                                                May 2018

                       Privacy Considerations for
             Protocols Relying on IP Broadcast or Multicast


   A number of application-layer protocols make use of IP broadcast 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 broadcast or multicast
   messages, a passive observer in the same broadcast or multicast
   domain can trivially record these messages and analyze their content.
   Therefore, designers of protocols that make use of broadcast or
   multicast messages need to take special care when designing their

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 candidates 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

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Copyright Notice

   Copyright (c) 2018 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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   publication of this document.  Please review these documents
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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
      1.1. Types and Usage of Broadcast and Multicast .................4
      1.2. Requirements Language ......................................5
   2. Privacy Considerations ..........................................5
      2.1. Message Frequency ..........................................5
      2.2. Persistent Identifiers .....................................6
      2.3. Anticipate User Behavior ...................................6
      2.4. Consider Potential Correlation .............................7
      2.5. Configurability ............................................7
   3. Operational Considerations ......................................8
   4. Summary .........................................................8
   5. Other Considerations ............................................9
   6. IANA Considerations ............................................10
   7. Security Considerations ........................................10
   8. References .....................................................10
      8.1. Normative References ......................................10
      8.2. Informative References ....................................10
   Acknowledgments ...................................................13
   Authors' Addresses ................................................13

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
   autoconfiguration and link-layer address lookup.  Also, application
   developers use broadcast/multicast messages to implement things such
   as local service or peer discovery.  It appears that an increasing
   number of applications make use of it as suggested by experimental
   results obtained on campus networks, including the IETF meeting
   network [TRAC2016].  This trend is not entirely surprising.  As

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   [RFC919] puts it, "The use of broadcasts [...] is a good base for
   many applications".  Broadcast and multicast functionality in a
   subnetwork is therefore important because a lack thereof renders the
   protocols relying on these mechanisms inoperable [RFC3819].

   Using broadcast/multicast can become problematic if the information
   that is being distributed can be regarded as sensitive or if the
   information that is distributed by multiple 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 consists of receivers
        unknown to the sender.  This makes eavesdropping without special
        privileges or a special location in the network trivial for
        anybody in the same broadcast/multicast domain.

   (b)  Encryption is difficult when broadcast/multicast messages are
        used, because, for instance, a non-trivial key management
        protocol might be required.  When encryption is not used, the
        content of these messages is easily accessible, making it easy
        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, invasion of privacy
   is much easier.

   Privacy considerations for IETF-specified protocols have received
   some attention in the recent past (e.g., [RFC7721] and [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].  RFC 6973 also describes potential threats to
   privacy in great detail and lists terminology that is also used in
   this document.  In contrast to RFC 6973, this document contains a
   number of privacy considerations, especially for protocols that rely
   on broadcast/multicast, that are intended to reduce the likelihood
   that a broadcast- or multicast-based protocol can be misused to
   collect sensitive data about devices, users, and groups of users in a
   broadcast/multicast domain.

   The above-mentioned considerations particularly apply to protocols
   designed outside the IETF for two reasons.  First, non-standard
   protocols will likely not receive operational attention and support
   in making them more secure, e.g., what DHCP snooping does for DHCP.
   Because these protocols are typically not documented, network
   equipment does not provide similar features for them.  Second, these

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   protocols have been designed in isolation, where a set of
   considerations to follow is useful in the absence of a larger
   community providing feedback and expertise to improve the protocol.
   In particular, carelessly designed protocols that use broadcast/
   multicast can break privacy efforts at different layers of the
   protocol stack such as Media Access Control (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: limited
   broadcast and directed broadcast.  Section 5.3.5 of [RFC1812] defines
   limited broadcast as all-ones ( and defines directed
   broadcast as the given network prefix of an IP address and the local
   part of all-ones.  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 relies solely 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
   called the "Local Network Control Block" (, defined in
   Section 4 of [RFC5771]) is reserved for this purpose.  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 includes local service discovery
   (e.g., Multicast DNS (mDNS) [RFC6762] and Link-Local Multicast Name
   Resolution (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.  Aside
   from these core network functions, applications also 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,
   and the only requirement to be part of this receiver group is to be
   on the same subnetwork.

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1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Privacy Considerations

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

2.1.  Message Frequency

   Frequent broadcast/multicast traffic caused by an application can
   give away user behavior and online connection times.  This allows a
   passive observer to potentially deduce a user's current activity
   (e.g., a game) and to create an online profile (i.e., times the user
   is on the network).  This profile becomes more accurate as the
   frequency of messages and the time duration over which they are sent
   increases.  Given that broadcast/multicast messages are only visible
   in the same broadcast/multicast domain, these messages also give away
   the rough location of the user (e.g., a campus or building).

   This behavior has, for example, 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.  Also, services used for local name resolution in modern
   operating systems utilize broadcast- or multicast-based protocols
   (e.g., mDNS, LLMNR, or NetBIOS) to announce, for example, resources
   on a regular basis.  This also allows tracking of the online times 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
   Section 2.2).  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 passive traffic
   analysis and surveillance becomes.

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2.2.  Persistent Identifiers

   A few protocols that make use of broadcast/multicast messages
   observed in the wild also make use of persistent identifiers.  This
   includes the use of host names or more abstract persistent
   identifiers such as a Universally Unique Identifiers (UUIDs) or
   similar.  These IDs, which, for example, identify the installation of
   a certain application, might not change across updates of the
   software and can therefore be extremely long lived.  This allows a
   passive observer to track a user precisely if broadcast/multicast
   messages are frequent.  This is even true if the IP and/or MAC
   address changes.  Such identifiers also allow two different
   interfaces (e.g., Wi-Fi 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 a passive observer 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
   identifier in every broadcast.  This identifier never changed after
   the application was installed, which allowed for the tracking of a
   device even when it changed its network interface or when it
   connected to a different network.

   In general, persistent IDs are considered bad practice for broadcast
   and multicast communication, as persistent application-layer IDs will
   make efforts to randomize identifiers (e.g., [RANDOM-ADDR]) on lower
   layers useless.  When protocols that make use of broadcast/multicast
   need to make use of IDs, these IDs SHOULD be rotated frequently to
   make user tracking 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
   language-specific information.  Based on data gathered during
   experiments performed at IETF meetings and at a large campus network,
   this appears to be the currently 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, e.g., when the owner of a device is
   identified (as an interesting target) or properties of the device are
   known (e.g., known vulnerabilities).  Host names are also a type of
   persistent identifier; therefore, the considerations in Section 2.2

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   Some of the most commonly used operating systems include 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 can also be included, therefore revealing two pieces of
   information that 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 protocols making use of broadcast/multicast SHOULD be
   avoided.  An application might want to display the information it
   will broadcast on the LAN at install/config time, so that the user is
   at least aware of the application's behavior.  More host name
   considerations can be found in [RFC8117].  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.  For example, a protocol that uses
   frequent messages including a UUID to identify the particular
   installation does not give away the identity of the user.  However, a
   single message including the user's host name might do that, and it
   can be correlated using, for example, the MAC address of the device's

   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); this revealed relationships
   among users.  This allowed the real identity of the users of many
   devices to be revealed, and it also gave away some information about
   their social environment.

   A designer of a protocol that makes use of broadcast/multicast needs
   to be aware of the fact that even if the information a protocol leaks
   seems harmless in isolation, there might be ways to correlate that
   information with information from other protocols to reveal sensitive
   information about a user.

2.5.  Configurability

   A lot of applications and services relying on broadcast- or
   multicast-based protocols do not include the means to declare "safe"
   environments (e.g., based on the Service Set Identifier (SSID) of a

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   Wi-Fi network and the MAC addresses of the access points).  For
   example, a device connected to a public Wi-Fi network will likely
   broadcast the same information as when connected to the home network.
   It would be beneficial if certain behaviors could be restricted to
   "safe" environments.

   For example, a popular operating system 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 trusted
   networks only.  Such functionality could be implemented as part of an

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

3.  Operational Considerations

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

   A feature commonly found on access points is the ability to manage/
   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.
   Wireless access points often provide finer-grained control beyond a
   simple on/off switch for well-known protocols or provide mechanisms
   to manage broadcast/multicast traffic intelligently using, for
   example, proxies (see [MCAST-CONS]).  However, these mechanisms only
   work on standardized protocols.

4.  Summary

   Increasingly, applications rely on protocols that send and receive
   broadcast and multicast messages.  For some, broadcast/multicast
   messages are the basis of their application logic; others use
   broadcast/multicast messages to improve certain aspects of the
   application but are fully functional in case broadcast/multicast
   messages 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 broadcast and multicast.

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   It is not always possible to implement certain functionality via
   unicast, but if 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; these can be used if
      implemented.  For example, for DNS-SD, privacy extensions are
      documented in [DNSSD-PRIV].

   o  Using user-specified information inside broadcast/multicast
      messages SHOULD be avoided, as users will often use personal
      information or other information that aids attackers, in
      particular if the user is unaware about how that information is
      being used.

   o  The use of persistent IDs in messages SHOULD be avoided, as this
      allows user tracking and correlation, and it potentially has a
      devastating effect on other privacy-protection mechanisms.

   o  If one must design a new protocol relying on broadcast/multicast
      and cannot use an IETF-specified protocol, then:

      *  the protocol SHOULD be very conservative in how frequently it
         sends messages as an effort in data minimization,

      *  it SHOULD make use of mechanisms implemented in IETF-specified
         protocols that can be helpful in privacy protection, such as
         message suppression in mDNS,

      *  it SHOULD be designed in such a way that information sent in
         broadcast/multicast messages cannot be correlated with
         information from other protocols using broadcast/multicast, and

      *  it SHOULD be possible to let the user configure "safe"
         environments if possible (e.g., based on the SSID) to minimize
         the risk of information leakage (e.g., a home network as
         opposed to a public Wi-Fi network).

5.  Other Considerations

   Besides privacy implications, frequent broadcasting 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 [MCAST-CONS].  Further, it will limit the ability for devices

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   to go to sleep if frequent broadcasts are being sent.  A similar
   problem in respect to Router Advertisements is addressed in
   [RFC7772].  In that respect, broadcast/multicast can be used for
   another class of attacks that is not related to privacy.  The
   potential impact on network performance should nevertheless be
   considered when designing a protocol that makes use of broadcast/

6.  IANA Considerations

   This document has no IANA actions.

7.  Security Considerations

   This document deals with privacy-related considerations for
   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

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

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

8.2.  Informative References

              Huitema, C. and D. Kaiser, "Privacy Extensions for DNS-
              SD", Work in Progress, draft-ietf-dnssd-privacy-04, April

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              Perkins, C., McBride, M., Stanley, D., Kumari, W., and J.
              Zuniga, "Multicast Considerations over IEEE 802 Wireless
              Media", Work in Progress, draft-ietf-mboned-ieee802-mcast-
              problems-01, February 2018.

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

   [RFC919]   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, <https://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, <https://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, <https://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,

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   [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,

   [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,

   [RFC7772]  Yourtchenko, A. and L. Colitti, "Reducing Energy
              Consumption of Router Advertisements", BCP 202, RFC 7772,
              DOI 10.17487/RFC7772, February 2016,

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

   [RFC8117]  Huitema, C., Thaler, D., and R. Winter, "Current Hostname
              Practice Considered Harmful", RFC 8117,
              DOI 10.17487/RFC8117, March 2017,

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   [TRAC2016] Faath, M., Weisshaar, F., and R. Winter, "How Broadcast
              Data Reveals Your Identity and Social Graph", Wireless
              Communications and Mobile Computing Conference
              (IWCMC), International Workshop on TRaffic Analysis and
              Characterization (TRAC), DOI 10.1109/IWCMC.2016.7577084,
              September 2016.


   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.

Authors' Addresses

   Rolf Winter
   University of Applied Sciences Augsburg

   Email: rolf.winter@hs-augsburg.de

   Michael Faath
   Conntac GmbH

   Email: faath@conntac.net

   Fabian Weisshaar
   University of Applied Sciences Augsburg

   Email: fabian.weisshaar@hs-augsburg.de

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