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Internet Engineering Task Force (IETF)                         R. WinterRequest for Comments: 8386       University of Applied Sciences AugsburgCategory: Informational                                         M. FaathISSN: 2070-1721                                             Conntac GmbH                                                            F. Weisshaar                                 University of Applied Sciences Augsburg                                                                May 2018Privacy Considerations forProtocols Relying on IP Broadcast or MulticastAbstract   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   protocols.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; seeSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8386.Winter, et al.                Informational                     [Page 1]

RFC 8386       Broadcast/Multicast Privacy Considerations       May 2018Copyright Notice   Copyright (c) 2018 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (https://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Table of Contents1. Introduction ....................................................21.1. Types and Usage of Broadcast and Multicast .................41.2. Requirements Language ......................................52. Privacy Considerations ..........................................52.1. Message Frequency ..........................................52.2. Persistent Identifiers .....................................62.3. Anticipate User Behavior ...................................62.4. Consider Potential Correlation .............................72.5. Configurability ............................................73. Operational Considerations ......................................84. Summary .........................................................85. Other Considerations ............................................96. IANA Considerations ............................................107. Security Considerations ........................................108. References .....................................................108.1. Normative References ......................................108.2. Informative References ....................................10   Acknowledgments ...................................................13   Authors' Addresses ................................................131.  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.  AsWinter, et al.                Informational                     [Page 2]

RFC 8386       Broadcast/Multicast Privacy Considerations       May 2018   [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 toRFC 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, theseWinter, et al.                Informational                     [Page 3]

RFC 8386       Broadcast/Multicast Privacy Considerations       May 2018   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 (255.255.255.255) 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" (224.0.0.0/24, defined inSection 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 (seeSection 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.Winter, et al.                Informational                     [Page 4]

RFC 8386       Broadcast/Multicast Privacy Considerations       May 20181.2.  Requirements Language   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and   "OPTIONAL" in this document are to be interpreted as described inBCP 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 (seeSection 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.Winter, et al.                Informational                     [Page 5]

RFC 8386       Broadcast/Multicast Privacy Considerations       May 20182.2.  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 inSection 2.2   apply.Winter, et al.                Informational                     [Page 6]

RFC 8386       Broadcast/Multicast Privacy Considerations       May 2018   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   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); 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 aWinter, et al.                Informational                     [Page 7]

RFC 8386       Broadcast/Multicast Privacy Considerations       May 2018   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   application.   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   larger.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.Winter, et al.                Informational                     [Page 8]

RFC 8386       Broadcast/Multicast Privacy Considerations       May 2018   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 devicesWinter, et al.                Informational                     [Page 9]

RFC 8386       Broadcast/Multicast Privacy Considerations       May 2018   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/   multicast.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   difficult.   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 inAppendix 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.  References8.1.  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,              <https://www.rfc-editor.org/info/rfc2119>.   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase inRFC2119 Key Words",BCP 14,RFC 8174, DOI 10.17487/RFC8174,              May 2017, <https://www.rfc-editor.org/info/rfc8174>.8.2.  Informative References   [DNSSD-PRIV]              Huitema, C. and D. Kaiser, "Privacy Extensions for DNS-              SD", Work in Progress,draft-ietf-dnssd-privacy-04, April              2018.Winter, et al.                Informational                    [Page 10]

RFC 8386       Broadcast/Multicast Privacy Considerations       May 2018   [MCAST-CONS]              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.   [RANDOM-ADDR]              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,              <https://www.rfc-editor.org/info/rfc919>.   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",RFC 1812, DOI 10.17487/RFC1812, June 1995,              <https://www.rfc-editor.org/info/rfc1812>.   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",RFC 2131, DOI 10.17487/RFC2131, March 1997,              <https://www.rfc-editor.org/info/rfc2131>.   [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,              <https://www.rfc-editor.org/info/rfc3819>.   [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,              <https://www.rfc-editor.org/info/rfc4795>.Winter, et al.                Informational                    [Page 11]

RFC 8386       Broadcast/Multicast Privacy Considerations       May 2018   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy              Extensions for Stateless Address Autoconfiguration in              IPv6",RFC 4941, DOI 10.17487/RFC4941, September 2007,              <https://www.rfc-editor.org/info/rfc4941>.   [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,              <https://www.rfc-editor.org/info/rfc5374>.   [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,              <https://www.rfc-editor.org/info/rfc5771>.   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS",RFC 6762,              DOI 10.17487/RFC6762, February 2013,              <https://www.rfc-editor.org/info/rfc6762>.   [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,              <https://www.rfc-editor.org/info/rfc6973>.   [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,              <https://www.rfc-editor.org/info/rfc7721>.   [RFC7772]  Yourtchenko, A. and L. Colitti, "Reducing Energy              Consumption of Router Advertisements",BCP 202,RFC 7772,              DOI 10.17487/RFC7772, February 2016,              <https://www.rfc-editor.org/info/rfc7772>.   [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,              <https://www.rfc-editor.org/info/rfc8117>.Winter, et al.                Informational                    [Page 12]

RFC 8386       Broadcast/Multicast Privacy Considerations       May 2018   [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.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.Authors' Addresses   Rolf Winter   University of Applied Sciences Augsburg   Augsburg   Germany   Email: rolf.winter@hs-augsburg.de   Michael Faath   Conntac GmbH   Augsburg   Germany   Email: faath@conntac.net   Fabian Weisshaar   University of Applied Sciences Augsburg   Augsburg   Germany   Email: fabian.weisshaar@hs-augsburg.deWinter, et al.                Informational                    [Page 13]