Internet Engineering Task Force                                R. Winter
Internet-Draft                                                  M. Faath
Intended status: Informational                              F. Weisshaar
Expires: April 21, 2016          University of Applied Sciences Augsburg
                                                        October 19, 2015

    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

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   This Internet-Draft will expire on April 21, 2016.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Design considerations . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Message frequency . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Persistent identifiers  . . . . . . . . . . . . . . . . .   3
     2.3.  Anticipate user behaviour . . . . . . . . . . . . . . . .   4
     2.4.  Rember - You are not alone  . . . . . . . . . . . . . . .   4
     2.5.  Configurability . . . . . . . . . . . . . . . . . . . . .   4
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Appendix A.  Additional Stuff . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   Broadcast and multicast messages have a large 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 really, but broadcast/multicast is
   special in two respects: a) the aforementioned large receiver group
   which makes it trivial for anybody on a LAN to collect the
   information without special priviledges or a special location in the
   network and b) encryption is more difficult when broadcasting/
   multicasting messages.  This draft documents a number of design
   considerations for broadcast/multicast protocol designers that are
   intended to reduce the likelyhood that a broadcast protocol can be
   misused to collect sensitive data about devices, users and groups of
   users on a LAN.

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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.  Design considerations

   There are a few obvious and a few not necessarily obvious things
   designers of broadcast/multicast protocols should consider.  Most of
   these items are based on protocol behaviour observed as part of an
   experiment on an operational network.

2.1.  Message frequency

   Frequent broadcast/multicast traffic caused by an application can
   give user behaviour and online times away.  This allows a passive
   observer to potentially decuct 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 persisten identifiers.

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
   e.g.  identify the installation of a certain application and might
   not change across updates of the software.  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 persitent 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 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.

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2.3.  Anticipate user behaviour

   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
   language specific information.  Based on gathered data, this appears
   to currently be prevalent user behaviour.  For protocols using the
   hostname as part of the messages, this clearly will reveal personally
   identifiable information to everyone on the local network.

   Where possible, the use of hostnames 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 behaviour.

2.4.  Rember - You are not alone

   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 the protocol's information 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 protocols do not
   include the means to declare "safe" environments (e.g. based on the
   SSID of a WiFi network).  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 behaviour could be
   restricted to "safe" environments.

   An application developer making use of broadcasts as part of the
   application should make the broadcast feature, if possible,
   configurable, so that potentially sensitive information does not leak
   on public networks.

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3.  IANA Considerations

   This memo includes no request to IANA.

4.  Security Considerations


5.  Normative References

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

Appendix A.  Additional Stuff

   This becomes an Appendix.

Authors' Addresses

   Rolf Winter
   University of Applied Sciences Augsburg


   Michael Faath
   University of Applied Sciences Augsburg


   Fabian Weisshaar
   University of Applied Sciences Augsburg


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