idnits 2.17.1 draft-ietf-intarea-broadcast-consider-05.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (October 25, 2017) is 2375 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-05) exists of draft-ietf-dnssd-privacy-00 == Outdated reference: A later version (-05) exists of draft-ietf-intarea-hostname-practice-00 -- Obsolete informational reference (is this intentional?): RFC 3315 (Obsoleted by RFC 8415) -- Obsolete informational reference (is this intentional?): RFC 4941 (Obsoleted by RFC 8981) Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Engineering Task Force R. Winter 3 Internet-Draft University of Applied Sciences Augsburg 4 Intended status: Informational M. Faath 5 Expires: April 28, 2018 Conntac GmbH 6 F. Weisshaar 7 University of Applied Sciences Augsburg 8 October 25, 2017 10 Privacy considerations for IP broadcast and multicast protocol designers 11 draft-ietf-intarea-broadcast-consider-05 13 Abstract 15 A number of application-layer protocols make use of IP broadcasts or 16 multicast messages for functions like local service discovery or name 17 resolution. Some of these functions can only be implemented 18 efficiently using such mechanisms. When using broadcasts or 19 multicast messages, a passive observer in the same broadcast/ 20 multicast domain can trivially record these messages and analyze 21 their content. Therefore, broadcast/multicast protocol designers 22 need to take special care when designing their protocols. 24 Status of This Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at http://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on April 28, 2018. 41 Copyright Notice 43 Copyright (c) 2017 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 59 1.1. Types and usage of broadcast and multicast . . . . . . . 4 60 1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4 61 2. Privacy considerations . . . . . . . . . . . . . . . . . . . 4 62 2.1. Message frequency . . . . . . . . . . . . . . . . . . . . 5 63 2.2. Persistent identifiers . . . . . . . . . . . . . . . . . 5 64 2.3. Anticipate user behavior . . . . . . . . . . . . . . . . 6 65 2.4. Consider potential correlation . . . . . . . . . . . . . 7 66 2.5. Configurability . . . . . . . . . . . . . . . . . . . . . 7 67 3. Operational considerations . . . . . . . . . . . . . . . . . 8 68 4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 69 5. Other considerations . . . . . . . . . . . . . . . . . . . . 9 70 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 71 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 72 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 73 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 74 9.1. Normative References . . . . . . . . . . . . . . . . . . 10 75 9.2. Informative References . . . . . . . . . . . . . . . . . 10 76 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 78 1. Introduction 80 Broadcast and multicast messages have a large (and to the sender 81 unknown) receiver group by design. Because of that, these two 82 mechanisms are vital for a number of basic network functions such as 83 auto-configuration or link-layer address lookup. Also application 84 developers use broadcast/multicast messages to implement things like 85 local service or peer discovery. It appears that an increasing 86 number of applications make use of it as suggested by experimental 87 results obtained on campus networks including the IETF meeting 88 network [TRAC2016]. This trend is not entirely surprising. As RFC 89 919 [RFC0919] puts it, "The use of broadcasts [...] is a good base 90 for many applications". Broadcast and multicast functionality in a 91 subnetwork are therefore important as a lack thereof renders the 92 protocols underlying these mechanisms inoperable [RFC3819]. 94 Using broadcast/multicast can become problematic if the information 95 that is being distributed can be regarded as sensitive or when the 96 information that is distributed by multiple of these protocols can be 97 correlated in a way that sensitive data can be derived. This is 98 clearly true for any protocol, but broadcast/multicast is special in 99 at least two respects: 101 (a) The aforementioned large receiver group, consisting of receivers 102 unknown to the sender. This makes eavesdropping without special 103 privileges or a special location in the network trivial for 104 anybody in the broadcast/multicast domain. 106 (b) Encryption is more difficult when broadcast/multicast messages, 107 leaving content of these messages in the clear and making it 108 easier to spoof and replay them. 110 Given the above, privacy protection for protocols based on broadcast 111 or multicast communication is significantly more difficult compared 112 to unicast communication and at the same time invading the privacy is 113 much easier. 115 Privacy considerations of IETF-specified protocols have received some 116 attention in the recent past (e.g. RFC 7721 [RFC7721] or RFC 7819 117 [RFC7819]). There is also general guidance available for document 118 authors on when and how to include a privacy considerations section 119 in their documents and on how to evaluate the privacy implications of 120 Internet protocols [RFC6973]. RFC6973 also describes potential 121 threats to privacy in great detail and lists terminology that is also 122 used in this document. In contrast to RFC6973, this document 123 contains a number of privacy considerations especially for broadcast/ 124 multicast protocol designers that are intended to reduce the 125 likelihood that a broadcast/multicast protocol can be misused to 126 collect sensitive data about devices, users and groups of users on a 127 broadcast/multicast domain. 129 The above mentioned considerations particularly apply to protocols 130 designed outside the IETF for two reasons. For one, non-standard 131 protocols will likely not receive operational attention and support 132 in making them more secure such as e.g. DHCP snooping does for DHCP 133 because they typically are not documented. The other reason is that 134 these protocols have been designed in isolation, where a set of 135 considerations to follow is useful in the absence of a larger 136 community providing feedback. In particular, carelessly designed 137 broadcast/multicast protocols can break privacy efforts at different 138 layers of the protocol stack such as MAC address or IP address 139 randomization [RFC4941]. 141 1.1. Types and usage of broadcast and multicast 143 In IPv4, two major types of broadcast addresses exist, the limited 144 broadcast which is defined as all-ones (255.255.255.255, defined in 145 section 5.3.5.1 of [RFC1812]) and the directed broadcast with the 146 given network prefix of an IP address and the host part of all-ones 147 (defined in section 5.3.5.2. of [RFC1812]). Broadcast packets are 148 received by all nodes in a subnetwork. Limited broadcasts never 149 transit a router. The same is true for directed broadcasts by 150 default, but routers MAY provide an option to do this [RFC2644]. 151 IPv6 on the other hand does not provide broadcast addresses but 152 solely relies on multicast [RFC4291]. 154 In contrast to broadcast addresses, multicast addresses represent an 155 identifier for a set of interfaces that can be a set different from 156 all nodes in the subnetwork. All interfaces that are identified by a 157 given multicast address receive packets destined towards that address 158 and are called a multicast group. In both IPv4 and IPv6, multiple 159 pre-defined multicast addresses exist. The ones most relevant for 160 this document are the ones with subnet scope. For IPv4, an IP prefix 161 is reserved for this purpose called the Local Network Control Block 162 (224.0.0.0/24, defined in section 4 of [RFC5771]). For IPv6, the 163 relevant multicast addresses are the two All Nodes Addresses, which 164 every IPv6-capable host is required to recognize as identifying 165 itself (see section 2.7.1 of [RFC4291]). 167 Typical usage of these addresses include local service discovery 168 (e.g. mDNS [RFC6762] and LLMNR [RFC4795] make use of multicast), 169 autoconfiguration (e.g. DHCPv4 [RFC2131] uses broadcasts and DHCPv6 170 [RFC3315] uses multicast addresses) and other vital network services 171 such as address resolution or duplicate address detection. But 172 besides these core network functions, also applications make use of 173 broadcast and multicast functionality, often implementing proprietary 174 protocols. In sum, these protocols distribute a diverse set of 175 potentially privacy sensitive information to a large receiver group. 177 1.2. Requirements Language 179 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 180 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 181 document are to be interpreted as described in RFC 2119 [RFC2119]. 183 2. Privacy considerations 185 There are a few obvious and a few not necessarily obvious things 186 designers of broadcast/multicast protocols should consider in respect 187 to the privacy implications of their protocol. Most of these items 188 are based on protocol behavior observed as part of experiments on 189 operational networks [TRAC2016]. 191 2.1. Message frequency 193 Frequent broadcast/multicast traffic caused by an application can 194 give away user behavior and online connection times. This allows a 195 passive observer to potentially deduce a user's current activity 196 (e.g. a game) and it allows to create an online profile (i.e. times 197 the user is on the network). The higher the frequency of these 198 messages and the duration of time these messages are sent, the more 199 accurate this profile will be. Given that broadcasts/multicasts are 200 only visible in the same broadcast/multicast domain, these messages 201 also give the rough location of the user away (e.g. a campus or 202 building). 204 This behavior has e.g. been observed by a synchronization mechanism 205 of a popular application, where multiple messages have been sent per 206 minute via broadcast. Given this behavior, it is possible to record 207 a device's time on the network with a sub-minute accuracy given only 208 the traffic of this single application installed on the device. But 209 also services used for local name resolution in modern operating 210 systems utilize broadcast/multicast protocols (e.g. mDNS, LLMNR or 211 NetBIOS) to announce for example their shares regularly and allow a 212 tracking of the online time of a device. 214 If a protocol relies on frequent or periodic broadcast/multicast 215 messages, the frequency SHOULD be chosen conservatively, in 216 particular if the messages contain persistent identifiers (see next 217 subsection). Also, intelligent message suppression mechanisms such 218 as the ones employed in mDNS [RFC6762] SHOULD be implemented. The 219 lower the frequency of broadcast messages, the harder traffic 220 analysis and surveillance becomes. 222 2.2. Persistent identifiers 224 A few broadcast/multicast protocols observed in the wild make use of 225 persistent identifiers. This includes the use of host names or more 226 abstract persistent identifiers such as a universally unique 227 identifiers (UUID) or similar. These IDs, which e.g. identify the 228 installation of a certain application might not change across updates 229 of the software and are therefore extremely long lived. This allows 230 a passive observer to track a user precisely if broadcast/multicast 231 messages are frequent. This is even true in case the IP and/or MAC 232 address changes. Such identifiers also allow two different 233 interfaces (e.g. WiFi and Ethernet) to be correlated to the same 234 device. If the application makes use of persistent identifiers for 235 multiple installations of the same application for the same user, 236 this even allows to infer that different devices belong to the same 237 user. 239 The aforementioned broadcast messages from a synchronization 240 mechanism of a popular application also included a persistent 241 identifier in every broadcast. This identifier did never change 242 after the application was installed and allowed to track a device 243 even when it changed its network interface or when it connected to a 244 different network. 246 Persistent IDs are considered bad practice in general for broadcast 247 and multicast communication, as persistent application layer IDs will 248 make efforts on lower layers to randomize identifiers (e.g. 249 [I-D.huitema-6man-random-addresses]) useless or at least much more 250 difficult. When broadcast/multicast protocols need to make use of 251 IDs, frequent rotations of these IDs SHOULD be considered to make 252 user tracking more difficult. 254 2.3. Anticipate user behavior 256 A large number of users name their device after themselves, either 257 using their first name, last name or both. Often a host name 258 includes the type, model or maker of a device, its function or 259 includes language specific information. Based on data gathered 260 during experiments performed at IETF meetings and at a large campus 261 network, this appears currently to be prevalent user behavior 262 [TRAC2016]. For protocols using the host name as part of the 263 messages, this clearly will reveal personally identifiable 264 information to everyone on the local network. This information can 265 also be used to mount more sophisticated attacks, when e.g. the owner 266 of a device is identified (as an interesting target) or properties of 267 the device are known (e.g. known vulnerabilities). 269 Some of the most commonly used operating systems include the name the 270 user chooses for the user account during the installation process as 271 part of the host name of the device. The name of the operating 272 system can also be included, revealing therefore two pieces of 273 information, which can be regarded as private information if the host 274 name is used in broadcast/multicast messages. 276 Where possible, the use of host names and other user provided 277 information in broadcast/multicast protocols SHOULD be avoided. If 278 only a persistent ID is needed, this can be generated randomly. An 279 application might want to display the information it will broadcast 280 on the LAN at install/config time, so the user is at least aware of 281 the application's behavior. More host name considerations can be 282 found in [I-D.ietf-intarea-hostname-practice]. More information on 283 user participation can be found in RFC 6973 [RFC6973]. 285 2.4. Consider potential correlation 287 A large number of services and applications make use of the 288 broadcast/multicast mechanism. That means there are various sources 289 of information that are easily accessible by a passive observer. In 290 isolation, the information these protocols reveal might seem 291 harmless, but given multiple such protocols, it might be possible to 292 correlate this information. E.g. a protocol that uses frequent 293 messages including a UUID to identify the particular installation 294 does not give the identity of the user away. But a single message 295 including the user's host name might just do that and it can be 296 correlated using e.g. the MAC address of the device's interface. 298 In the experiments described in [TRAC2016], it was possible to 299 correlate frequently sent broadcast messages that included a unique 300 identifier with other broadcast/multicast messages containing 301 usernames (e.g. mDNS, LLMNR or NetBIOS), but also relationships to 302 other users. This allowed to reveal the real identity of the users 303 of many devices but it also gave some information about their social 304 environment away. 306 A broadcast protocol designer should be aware of the fact that even 307 if - in isolation - the information a protocol leaks seems harmless, 308 there might be ways to correlate that information with other 309 broadcast protocol information to reveal sensitive information about 310 a user. 312 2.5. Configurability 314 A lot of applications and services using broadcast/multicast 315 protocols do not include the means to declare "safe" environments 316 (e.g. based on the SSID of a WiFi network and the MAC addresses of 317 the access points). E.g. a device connected to a public WiFi will 318 likely broadcast the same information as when connected to the home 319 network. It would be beneficial if certain behavior could be 320 restricted to "safe" environments. 322 A popular operating system e.g. allows the user to specify the trust 323 level of the network the device connects to, which for example 324 restricts specific system services (using broadcast/multicast 325 messages for their normal operation) to be used in untrusted 326 networks. Such functionality could implemented as part of an 327 application. 329 An application developer making use of broadcasts/multicasts as part 330 of the application SHOULD make the broadcast feature, if possible, 331 configurable, so that potentially sensitive information does not leak 332 on public networks, where the threat to privacy is much larger. 334 3. Operational considerations 336 Besides changing end-user behavior, choosing sensible defaults as an 337 operating system vendor (e.g. for suggesting host names) and the 338 considerations for protocol designers mentioned in this document, 339 there are things that the network administrators/operators can do to 340 limit the above mentioned problems. 342 A feature not uncommonly found on access points e.g. is to filter 343 broadcast and multicast traffic. This will potentially break certain 344 applications or some of their functionality but will also protect the 345 users from potentially leaking sensitive information. 347 4. Summary 349 Increasingly, applications rely on broadcast and multicast messages. 350 For some, broadcasts/multicasts are the basis of their application 351 logic, others use broadcasts/multicasts to improve certain aspects of 352 the application but are fully functional in case broadcasts/ 353 multicasts fail. Irrespective of the role of broadcast and multicast 354 messages for the application, the designers of protocols that make 355 use of them should be very careful in their protocol design because 356 of the special nature of broad- and multicast. 358 It is not always possible to implement certain functionality via 359 unicast, but in case a protocol designer chooses to rely on 360 broadcast/multicast, the following should be carefully considered: 362 o IETF-specified protocols, such as mDNS [RFC6762], SHOULD be used 363 if possible as operational support might exist to protect against 364 the leakage of private information. Also, for some protocols 365 privacy extensions are being specified, which can be used if 366 implemented. E.g. for DNS-SD privacy extensions are documented in 367 [I-D.ietf-dnssd-privacy] 369 o Using user-specified information inside broadcast/multicast 370 messages SHOULD be avoided, as users will often use personal 371 information or other information aiding attackers, in particular 372 if the user is unaware about how that information is being used 374 o The use of persistent IDs in messages SHOULD be avoided, as this 375 allows user tracking, correlation and potentially has a 376 devastating effect on other privacy protection mechanisms 378 o If you really must use a broadcast/multicast protocol and cannot 379 use an IETF-specified protocol, then: 381 * You SHOULD be very conservative in how frequently you send 382 messages as an effort in data minimization 384 * You SHOULD seek advice from IETF-specified protocols such as 385 message suppression in mDNS 387 * You SHOULD try to design the protocol in a way that the 388 information cannot be correlated with other information in 389 broadcast/multicast messages 391 * You SHOULD let the user configure safe environments if possible 392 (e.g. based on the SSID) 394 5. Other considerations 396 Besides privacy implications, frequent broadcasting also represents a 397 performance problem. In particular in certain wireless technologies 398 such as 802.11, broadcast and multicast are transmitted at a much 399 lower rate (the lowest common denominator rate) compared to unicast 400 and therefore have a much bigger impact on the overall available 401 airtime [I-D.perkins-intarea-multicast-ieee802]. Further, it will 402 limit the ability for devices to go to sleep if frequent broadcasts 403 are being sent. A similar problem in respect to Router 404 Advertisements is addressed in 405 [I-D.ietf-v6ops-reducing-ra-energy-consumption]. In that respect 406 broadcasts/multicast can be used for another class of attacks that 407 not related to privacy. The potential impact on network performance 408 should nevertheless be considered by broadcast protocol designers. 410 6. Acknowledgments 412 We would like to thank Eliot Lear, Joe Touch and Stephane Bortzmeyer 413 for their valuable input to this document. 415 This work was partly supported by the European Commission under grant 416 agreement FP7-318627 mPlane. Support does not imply endorsement. 418 7. IANA Considerations 420 This memo includes no request to IANA. 422 8. Security Considerations 424 This document deals with privacy-related considerations of broadcast- 425 and multicast-based protocols. It contains advice for designers of 426 such protocols to minimize the leakage of privacy-sensitive 427 information. The intent of the advice is to make sure that 428 identities will remain anonymous and user tracking will be made 429 difficult. 431 It should be noted that certain applications could make use of 432 existing mechanisms to protect multicast traffic such as the ones 433 defined in [RFC5374]. Examples of such applications can be found in 434 Appendix A. of [RFC5374]. Given the required infrastructure and 435 assumptions about these applications and the security infrastructure, 436 many applications will not be able to make use of such mechanisms. 438 9. References 440 9.1. Normative References 442 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 443 Requirement Levels", BCP 14, RFC 2119, March 1997. 445 9.2. Informative References 447 [I-D.huitema-6man-random-addresses] 448 Huitema, C., "Implications of Randomized Link Layers 449 Addresses for IPv6 Address Assignment", draft-huitema- 450 6man-random-addresses-03 (work in progress), March 2016. 452 [I-D.ietf-dnssd-privacy] 453 Huitema, C. and D. Kaiser, "Privacy Extensions for DNS- 454 SD", draft-ietf-dnssd-privacy-00 (work in progress), 455 October 2016. 457 [I-D.ietf-intarea-hostname-practice] 458 Huitema, C. and D. Thaler, "Current Hostname Practice 459 Considered Harmful", draft-ietf-intarea-hostname- 460 practice-00 (work in progress), October 2015. 462 [I-D.ietf-v6ops-reducing-ra-energy-consumption] 463 Yourtchenko, A. and L. Colitti, "Reducing energy 464 consumption of Router Advertisements", draft-ietf-v6ops- 465 reducing-ra-energy-consumption-03 (work in progress), 466 November 2015. 468 [I-D.perkins-intarea-multicast-ieee802] 469 Perkins, C., Stanley, D., Kumari, W., and J. Zuniga, 470 "Multicast Considerations over IEEE 802 Wireless Media", 471 draft-perkins-intarea-multicast-ieee802-03 (work in 472 progress), July 2017. 474 [RFC0919] Mogul, J., "Broadcasting Internet Datagrams", STD 5, RFC 475 919, DOI 10.17487/RFC0919, October 1984, 476 . 478 [RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers", 479 RFC 1812, DOI 10.17487/RFC1812, June 1995, 480 . 482 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 483 2131, DOI 10.17487/RFC2131, March 1997, 484 . 486 [RFC2644] Senie, D., "Changing the Default for Directed Broadcasts 487 in Routers", BCP 34, RFC 2644, DOI 10.17487/RFC2644, 488 August 1999, . 490 [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, 491 C., and M. Carney, "Dynamic Host Configuration Protocol 492 for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 493 2003, . 495 [RFC3819] Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D., 496 Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. 497 Wood, "Advice for Internet Subnetwork Designers", BCP 89, 498 RFC 3819, DOI 10.17487/RFC3819, July 2004, 499 . 501 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 502 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 503 2006, . 505 [RFC4795] Aboba, B., Thaler, D., and L. Esibov, "Link-local 506 Multicast Name Resolution (LLMNR)", RFC 4795, DOI 507 10.17487/RFC4795, January 2007, 508 . 510 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy 511 Extensions for Stateless Address Autoconfiguration in 512 IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, 513 . 515 [RFC5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast 516 Extensions to the Security Architecture for the Internet 517 Protocol", RFC 5374, DOI 10.17487/RFC5374, November 2008, 518 . 520 [RFC5771] Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for 521 IPv4 Multicast Address Assignments", BCP 51, RFC 5771, DOI 522 10.17487/RFC5771, March 2010, 523 . 525 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, 526 DOI 10.17487/RFC6762, February 2013, 527 . 529 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., 530 Morris, J., Hansen, M., and R. Smith, "Privacy 531 Considerations for Internet Protocols", RFC 6973, DOI 532 10.17487/RFC6973, July 2013, 533 . 535 [RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy 536 Considerations for IPv6 Address Generation Mechanisms", 537 RFC 7721, DOI 10.17487/RFC7721, March 2016, 538 . 540 [RFC7819] Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy 541 Considerations for DHCP", RFC 7819, DOI 10.17487/RFC7819, 542 April 2016, . 544 [TRAC2016] 545 Faath, M., Weisshaar, F., and R. Winter, "How Broadcast 546 Data Reveals Your Identity and Social Graph", 7th 547 International Workshop on TRaffic Analysis and 548 Characterization IEEE TRAC 2016, September 2016. 550 Authors' Addresses 552 Rolf Winter 553 University of Applied Sciences Augsburg 554 Augsburg 555 DE 557 Email: rolf.winter@hs-augsburg.de 559 Michael Faath 560 Conntac GmbH 561 Augsburg 562 DE 564 Email: faath@conntac.net 565 Fabian Weisshaar 566 University of Applied Sciences Augsburg 567 Augsburg 568 DE 570 Email: fabian.weisshaar@hs-augsburg.de