idnits 2.17.1 draft-ietf-intarea-broadcast-consider-09.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 (March 13, 2018) is 2229 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for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-05) exists of draft-ietf-dnssd-privacy-00 == Outdated reference: A later version (-15) exists of draft-ietf-mboned-ieee802-mcast-problems-01 -- Obsolete informational reference (is this intentional?): RFC 3315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: September 14, 2018 Conntac GmbH 6 F. Weisshaar 7 University of Applied Sciences Augsburg 8 March 13, 2018 10 Privacy considerations for protocols relying on IP broadcast and 11 multicast 12 draft-ietf-intarea-broadcast-consider-09 14 Abstract 16 A number of application-layer protocols make use of IP broadcasts or 17 multicast messages for functions such as local service discovery or 18 name resolution. Some of these functions can only be implemented 19 efficiently using such mechanisms. When using broadcasts or 20 multicast messages, a passive observer in the same broadcast/ 21 multicast domain can trivially record these messages and analyze 22 their content. Therefore, designers of protocols that make use of 23 broadcast/multicast messages need to take special care when designing 24 their protocols. 26 Status of This Memo 28 This Internet-Draft is submitted in full conformance with the 29 provisions of BCP 78 and BCP 79. 31 Internet-Drafts are working documents of the Internet Engineering 32 Task Force (IETF). Note that other groups may also distribute 33 working documents as Internet-Drafts. The list of current Internet- 34 Drafts is at http://datatracker.ietf.org/drafts/current/. 36 Internet-Drafts are draft documents valid for a maximum of six months 37 and may be updated, replaced, or obsoleted by other documents at any 38 time. It is inappropriate to use Internet-Drafts as reference 39 material or to cite them other than as "work in progress." 41 This Internet-Draft will expire on September 14, 2018. 43 Copyright Notice 45 Copyright (c) 2018 IETF Trust and the persons identified as the 46 document authors. All rights reserved. 48 This document is subject to BCP 78 and the IETF Trust's Legal 49 Provisions Relating to IETF Documents 50 (http://trustee.ietf.org/license-info) in effect on the date of 51 publication of this document. Please review these documents 52 carefully, as they describe your rights and restrictions with respect 53 to this document. Code Components extracted from this document must 54 include Simplified BSD License text as described in Section 4.e of 55 the Trust Legal Provisions and are provided without warranty as 56 described in the Simplified BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 61 1.1. Types and usage of broadcast and multicast . . . . . . . 4 62 1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4 63 2. Privacy considerations . . . . . . . . . . . . . . . . . . . 5 64 2.1. Message frequency . . . . . . . . . . . . . . . . . . . . 5 65 2.2. Persistent identifiers . . . . . . . . . . . . . . . . . 5 66 2.3. Anticipate user behavior . . . . . . . . . . . . . . . . 6 67 2.4. Consider potential correlation . . . . . . . . . . . . . 7 68 2.5. Configurability . . . . . . . . . . . . . . . . . . . . . 7 69 3. Operational considerations . . . . . . . . . . . . . . . . . 8 70 4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 71 5. Other considerations . . . . . . . . . . . . . . . . . . . . 9 72 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 73 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 74 8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 75 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 76 9.1. Normative References . . . . . . . . . . . . . . . . . . 10 77 9.2. Informative References . . . . . . . . . . . . . . . . . 10 78 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 80 1. Introduction 82 Broadcast and multicast messages have a large (and to the sender 83 unknown) receiver group by design. Because of that, these two 84 mechanisms are vital for a number of basic network functions such as 85 auto-configuration or link-layer address lookup. Also application 86 developers use broadcast/multicast messages to implement things such 87 as local service or peer discovery. It appears that an increasing 88 number of applications make use of it as suggested by experimental 89 results obtained on campus networks including the IETF meeting 90 network [TRAC2016]. This trend is not entirely surprising. As 91 [RFC0919] puts it, "The use of broadcasts [...] is a good base for 92 many applications". Broadcast and multicast functionality in a 93 subnetwork are therefore important as a lack thereof renders the 94 protocols relying on these mechanisms inoperable [RFC3819]. 96 Using broadcast/multicast can become problematic if the information 97 that is being distributed can be regarded as sensitive or when the 98 information that is distributed by multiple of these protocols can be 99 correlated in a way that sensitive data can be derived. This is 100 clearly true for any protocol, but broadcast/multicast is special in 101 at least two respects: 103 (a) The aforementioned large receiver group, consisting of receivers 104 unknown to the sender. This makes eavesdropping without special 105 privileges or a special location in the network trivial for 106 anybody in the same broadcast/multicast domain. 108 (b) Encryption is difficult when broadcast/multicast messages are 109 used, for instance because a non-trivial key management protocol 110 might be required. When encryption is not used, the content of 111 these messages is easily accessible, making it easy to spoof and 112 replay them. 114 Given the above, privacy protection for protocols based on broadcast 115 or multicast communication is significantly more difficult compared 116 to unicast communication and at the same time invading the privacy is 117 much easier. 119 Privacy considerations of IETF-specified protocols have received some 120 attention in the recent past (e.g. [RFC7721] or [RFC7819]). There 121 is also general guidance available for document authors on when and 122 how to include a privacy considerations section in their documents 123 and on how to evaluate the privacy implications of Internet protocols 124 [RFC6973]. RFC6973 also describes potential threats to privacy in 125 great detail and lists terminology that is also used in this 126 document. In contrast to RFC6973, this document contains a number of 127 privacy considerations especially for protocols that rely on 128 broadcast/multicast, intended to reduce the likelihood that a 129 broadcast/multicast protocol can be misused to collect sensitive data 130 about devices, users and groups of users in a broadcast/multicast 131 domain. 133 The above mentioned considerations particularly apply to protocols 134 designed outside the IETF - for two reasons. For one, non-standard 135 protocols will likely not receive operational attention and support 136 in making them more secure, e.g. what DHCP snooping does for DHCP. 137 But because these protocols are typically not documented, network 138 equipment does not provide similar features for them. The other 139 reason is that these protocols have been designed in isolation, where 140 a set of considerations to follow is useful in the absence of a 141 larger community providing feedback and expertise to improve the 142 protocol. In particular, carelessly designed protocols that use 143 broadcast/multicast can break privacy efforts at different layers of 144 the protocol stack such as MAC address or IP address randomization 145 [RFC4941]. 147 1.1. Types and usage of broadcast and multicast 149 In IPv4, two major types of broadcast addresses exist, the limited 150 broadcast which is defined as all-ones (255.255.255.255, defined in 151 section 5.3.5.1 of [RFC1812]) and the directed broadcast with the 152 given network prefix of an IP address and the host part of all-ones 153 (defined in section 5.3.5.2. of [RFC1812]). Broadcast packets are 154 received by all nodes in a subnetwork. Limited broadcasts never 155 transit a router. The same is true for directed broadcasts by 156 default, but routers may provide an option to do this [RFC2644]. 157 IPv6 on the other hand does not provide broadcast addresses but 158 solely relies on multicast [RFC4291]. 160 In contrast to broadcast addresses, multicast addresses represent an 161 identifier for a set of interfaces that can be a set different from 162 all nodes in the subnetwork. All interfaces that are identified by a 163 given multicast address receive packets destined towards that address 164 and are called a multicast group. In both IPv4 and IPv6, multiple 165 pre-defined multicast addresses exist. The ones most relevant for 166 this document are the ones with subnet scope. For IPv4, an IP prefix 167 is reserved for this purpose called the Local Network Control Block 168 (224.0.0.0/24, defined in section 4 of [RFC5771]). For IPv6, the 169 relevant multicast addresses are the two All Nodes Addresses, which 170 every IPv6-capable host is required to recognize as identifying 171 itself (see section 2.7.1 of [RFC4291]). 173 Typical usage of these addresses include local service discovery 174 (e.g. Multicast DNS (mDNS) [RFC6762] and Link-Local Multicast Name 175 Resolution (LLMNR) [RFC4795] make use of multicast), 176 autoconfiguration (e.g. DHCPv4 [RFC2131] uses broadcasts and DHCPv6 177 [RFC3315] uses multicast addresses) and other vital network services 178 such as address resolution or duplicate address detection. But 179 besides these core network functions, also applications make use of 180 broadcast and multicast functionality, often implementing proprietary 181 protocols. In sum, these protocols distribute a diverse set of 182 potentially privacy sensitive information to a large receiver group 183 and to be part of this receiver group, the only requirement is to be 184 on same subnetwork. 186 1.2. Requirements Language 188 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 189 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 190 document are to be interpreted as described in [RFC2119]. 192 2. Privacy considerations 194 There are a few obvious and a few not necessarily obvious things 195 designers of protocols utilizing broadcast/multicast should consider 196 in respect to the privacy implications of their protocol. Most of 197 these items are based on protocol behavior observed as part of 198 experiments on operational networks [TRAC2016]. 200 2.1. Message frequency 202 Frequent broadcast/multicast traffic caused by an application can 203 give away user behavior and online connection times. This allows a 204 passive observer to potentially deduce a user's current activity 205 (e.g. a game) and it allows to create an online profile (i.e. times 206 the user is on the network). The higher the frequency of these 207 messages and the duration of time these messages are sent, the more 208 accurate this profile will be. Given that broadcasts/multicasts are 209 only visible in the same broadcast/multicast domain, these messages 210 also give the rough location of the user away (e.g. a campus or 211 building). 213 This behavior has e.g. been observed by a synchronization mechanism 214 of a popular application, where multiple messages have been sent per 215 minute via broadcast. Given this behavior, it is possible to record 216 a device's time on the network with a sub-minute accuracy given only 217 the traffic of this single application installed on the device. But 218 also services used for local name resolution in modern operating 219 systems utilize broadcast/multicast protocols (e.g. mDNS, LLMNR or 220 NetBIOS) to announce for example resources regularly which also allow 221 tracking the online time of a device. 223 If a protocol relies on frequent or periodic broadcast/multicast 224 messages, the frequency SHOULD be chosen conservatively, in 225 particular if the messages contain persistent identifiers (see next 226 subsection). Also, intelligent message suppression mechanisms such 227 as the ones employed in mDNS [RFC6762] SHOULD be implemented. The 228 lower the frequency of broadcast messages, the harder passive traffic 229 analysis and surveillance becomes. 231 2.2. Persistent identifiers 233 A few protocols that make use of broadcast/multicast messages 234 observed in the wild make use of persistent identifiers. This 235 includes the use of host names or more abstract persistent 236 identifiers such as a universally unique identifiers (UUID) or 237 similar. These IDs, which e.g. identify the installation of a 238 certain application might not change across updates of the software 239 and can therefore be extremely long lived. This allows a passive 240 observer to track a user precisely if broadcast/multicast messages 241 are frequent. This is even true in case the IP and/or MAC address 242 changes. Such identifiers also allow two different interfaces (e.g. 243 WiFi and Ethernet) to be correlated to the same device. If the 244 application makes use of persistent identifiers for multiple 245 installations of the same application for the same user, this even 246 allows to infer that different devices belong to the same user. 248 The aforementioned broadcast messages from a synchronization 249 mechanism of a popular application also included a persistent 250 identifier in every broadcast. This identifier never changed after 251 the application was installed and it allowed to track a device even 252 when it changed its network interface or when it connected to a 253 different network. 255 Persistent IDs are considered bad practice in general for broadcast 256 and multicast communication, as persistent application layer IDs will 257 make efforts on lower layers to randomize identifiers (e.g. 258 [I-D.huitema-6man-random-addresses]) useless. When protocols that 259 make use of broadcast/multicast need to make use of IDs, these IDs 260 SHOULD be rotated frequently to make user tracking more difficult. 262 2.3. Anticipate user behavior 264 A large number of users name their device after themselves, either 265 using their first name, last name or both. Often a host name 266 includes the type, model or maker of a device, its function or it 267 includes language specific information. Based on data gathered 268 during experiments performed at IETF meetings and at a large campus 269 network, this appears currently to be prevalent user behavior 270 [TRAC2016]. For protocols using the host name as part of the 271 messages, this clearly will reveal personally identifiable 272 information to everyone on the local network. This information can 273 also be used to mount more sophisticated attacks, when e.g. the owner 274 of a device is identified (as an interesting target) or properties of 275 the device are known (e.g. known vulnerabilities). Host names are 276 also a type of persistent identifier and therefore the considerations 277 in Section 2.2 apply. 279 Some of the most commonly used operating systems include the name the 280 user chooses for the user account during the installation process as 281 part of the host name of the device. The name of the operating 282 system can also be included, revealing therefore two pieces of 283 information, which can be regarded as private information if the host 284 name is used in broadcast/multicast messages. 286 Where possible, the use of host names and other user-provided 287 information in protocols making use of broadcast/multicast SHOULD be 288 avoided. An application might want to display the information it 289 will broadcast on the LAN at install/config time, so the user is at 290 least aware of the application's behavior. More host name 291 considerations can be found in [RFC8117]. More information on user 292 participation can be found in [RFC6973]. 294 2.4. Consider potential correlation 296 A large number of services and applications make use of the 297 broadcast/multicast mechanism. That means there are various sources 298 of information that are easily accessible by a passive observer. In 299 isolation, the information these protocols reveal might seem 300 harmless, but given multiple such protocols, it might be possible to 301 correlate this information. E.g. a protocol that uses frequent 302 messages including a UUID to identify the particular installation 303 does not give the identity of the user away. But a single message 304 including the user's host name might just do that and it can be 305 correlated using e.g. the MAC address of the device's interface. 307 In the experiments described in [TRAC2016], it was possible to 308 correlate frequently sent broadcast messages that included a unique 309 identifier with other broadcast/multicast messages containing 310 usernames (e.g. mDNS, LLMNR or NetBIOS), but also relationships to 311 other users. This allowed to reveal the real identity of the users 312 of many devices but it also gave some information about their social 313 environment away. 315 A designer of a protocol that makes use of broadcast/multicast needs 316 to be aware of the fact that even if - in isolation - the information 317 a protocol leaks seems harmless, there might be ways to correlate 318 that information with information from other protocols to reveal 319 sensitive information about a user. 321 2.5. Configurability 323 A lot of applications and services relying on broadcast/multicast 324 protocols do not include the means to declare "safe" environments 325 (e.g. based on the SSID of a WiFi network and the MAC addresses of 326 the access points). E.g. a device connected to a public WiFi will 327 likely broadcast the same information as when connected to the home 328 network. It would be beneficial if certain behavior could be 329 restricted to "safe" environments. 331 A popular operating system e.g. allows the user to specify the trust 332 level of the network the device connects to, which for example 333 restricts specific system services (using broadcast/multicast 334 messages for their normal operation) to be used in trusted networks 335 only. Such functionality could implemented as part of an 336 application. 338 An application developer making use of broadcasts/multicasts as part 339 of the application SHOULD make the broadcast feature, if possible, 340 configurable, so that potentially sensitive information does not leak 341 on public networks, where the threat to privacy is much larger. 343 3. Operational considerations 345 Besides changing end-user behavior, choosing sensible defaults as an 346 operating system vendor (e.g. for suggesting host names) and the 347 considerations for protocol designers mentioned in this document, 348 there is something that the network administrators/operators can do 349 to limit the above mentioned problems. 351 A feature commonly found on access points e.g. is to manage/filter 352 broadcast and multicast traffic. This will potentially break certain 353 applications or some of their functionality but will also protect the 354 users from potentially leaking sensitive information. Wireless 355 access points often provide finer-grained control beyond a simple on/ 356 off switch for well-known protocols or provide mechanisms to manage 357 broadcast/multicast traffic intelligently using e.g. proxies (see 358 [I-D.ietf-mboned-ieee802-mcast-problems]). These mechanisms however 359 only work on standardized protocols. 361 4. Summary 363 Increasingly, applications rely on protocols that send and receive 364 broadcast and multicast messages. For some, broadcasts/multicasts 365 are the basis of their application logic, others use broadcasts/ 366 multicasts to improve certain aspects of the application but are 367 fully functional in case broadcasts/multicasts fail. Irrespective of 368 the role of broadcast and multicast messages for the application, the 369 designers of protocols that make use of them should be very careful 370 in their protocol design because of the special nature of broadcast 371 and multicast. 373 It is not always possible to implement certain functionality via 374 unicast, but in case a protocol designer chooses to rely on 375 broadcast/multicast, the following should be carefully considered: 377 o IETF-specified protocols, such as mDNS [RFC6762], SHOULD be used 378 if possible as operational support might exist to protect against 379 the leakage of private information. Also, for some protocols 380 privacy extensions are being specified, which can be used if 381 implemented. E.g. for DNS-SD privacy extensions are documented in 382 [I-D.ietf-dnssd-privacy] 384 o Using user-specified information inside broadcast/multicast 385 messages SHOULD be avoided, as users will often use personal 386 information or other information aiding attackers, in particular 387 if the user is unaware about how that information is being used 389 o The use of persistent IDs in messages SHOULD be avoided, as this 390 allows user tracking, correlation and potentially has a 391 devastating effect on other privacy protection mechanisms 393 o If one really must design a new protocol relying on broadcast/ 394 multicast and cannot use an IETF-specified protocol, then: 396 * the protocol SHOULD be very conservative in how frequently it 397 sends messages as an effort in data minimization 399 * it SHOULD make use of mechanisms implemented in IETF-specified 400 protocols that can be helpful in privacy protection such as 401 message suppression in mDNS 403 * it SHOULD be designed in a way that information sent in 404 broadcast/multicast messages cannot be correlated with 405 information from other protocols using broadcast/multicast 407 * it SHOULD be possible to let the user configure "safe" 408 environments if possible (e.g. based on the SSID) to minimize 409 the risk of information leakage (e.g. a home network as opposed 410 to a public Wifi) 412 5. Other considerations 414 Besides privacy implications, frequent broadcasting also represents a 415 performance problem. In particular in certain wireless technologies 416 such as 802.11, broadcast and multicast are transmitted at a much 417 lower rate (the lowest common denominator rate) compared to unicast 418 and therefore have a much bigger impact on the overall available 419 airtime [I-D.ietf-mboned-ieee802-mcast-problems]. Further, it will 420 limit the ability for devices to go to sleep if frequent broadcasts 421 are being sent. A similar problem in respect to Router 422 Advertisements is addressed in 423 [I-D.ietf-v6ops-reducing-ra-energy-consumption]. In that respect 424 broadcasts/multicast can be used for another class of attacks that is 425 not related to privacy. The potential impact on network performance 426 should nevertheless be considered when designing a protocol that 427 makes use of broadcast/multicast. 429 6. Acknowledgments 431 We would like to thank Eliot Lear, Joe Touch and Stephane Bortzmeyer 432 for their valuable input to this document. 434 This work was partly supported by the European Commission under grant 435 agreement FP7-318627 mPlane. Support does not imply endorsement. 437 7. IANA Considerations 439 This memo includes no request to IANA. 441 8. Security Considerations 443 This document deals with privacy-related considerations of broadcast- 444 and multicast-based protocols. It contains advice for designers of 445 such protocols to minimize the leakage of privacy-sensitive 446 information. The intent of the advice is to make sure that 447 identities will remain anonymous and user tracking will be made 448 difficult. 450 It should be noted that certain applications could make use of 451 existing mechanisms to protect multicast traffic such as the ones 452 defined in [RFC5374]. Examples of such applications can be found in 453 Appendix A. of [RFC5374]. Given the required infrastructure and 454 assumptions about these applications and the security infrastructure, 455 many applications will not be able to make use of such mechanisms. 457 9. References 459 9.1. Normative References 461 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 462 Requirement Levels", BCP 14, RFC 2119, March 1997. 464 9.2. Informative References 466 [I-D.huitema-6man-random-addresses] 467 Huitema, C., "Implications of Randomized Link Layers 468 Addresses for IPv6 Address Assignment", draft-huitema- 469 6man-random-addresses-03 (work in progress), March 2016. 471 [I-D.ietf-dnssd-privacy] 472 Huitema, C. and D. Kaiser, "Privacy Extensions for DNS- 473 SD", draft-ietf-dnssd-privacy-00 (work in progress), 474 October 2016. 476 [I-D.ietf-mboned-ieee802-mcast-problems] 477 Perkins, C., McBride, M., Stanley, D., Kumari, W., and J. 478 Zuniga, "Multicast Considerations over IEEE 802 Wireless 479 Media", draft-ietf-mboned-ieee802-mcast-problems-01 (work 480 in progress), February 2018. 482 [I-D.ietf-v6ops-reducing-ra-energy-consumption] 483 Yourtchenko, A. and L. Colitti, "Reducing energy 484 consumption of Router Advertisements", draft-ietf-v6ops- 485 reducing-ra-energy-consumption-03 (work in progress), 486 November 2015. 488 [RFC0919] Mogul, J., "Broadcasting Internet Datagrams", STD 5, RFC 489 919, DOI 10.17487/RFC0919, October 1984, 490 . 492 [RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers", 493 RFC 1812, DOI 10.17487/RFC1812, June 1995, 494 . 496 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 497 2131, DOI 10.17487/RFC2131, March 1997, 498 . 500 [RFC2644] Senie, D., "Changing the Default for Directed Broadcasts 501 in Routers", BCP 34, RFC 2644, DOI 10.17487/RFC2644, 502 August 1999, . 504 [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, 505 C., and M. Carney, "Dynamic Host Configuration Protocol 506 for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 507 2003, . 509 [RFC3819] Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D., 510 Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. 511 Wood, "Advice for Internet Subnetwork Designers", BCP 89, 512 RFC 3819, DOI 10.17487/RFC3819, July 2004, 513 . 515 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 516 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 517 2006, . 519 [RFC4795] Aboba, B., Thaler, D., and L. Esibov, "Link-local 520 Multicast Name Resolution (LLMNR)", RFC 4795, DOI 521 10.17487/RFC4795, January 2007, 522 . 524 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy 525 Extensions for Stateless Address Autoconfiguration in 526 IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, 527 . 529 [RFC5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast 530 Extensions to the Security Architecture for the Internet 531 Protocol", RFC 5374, DOI 10.17487/RFC5374, November 2008, 532 . 534 [RFC5771] Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for 535 IPv4 Multicast Address Assignments", BCP 51, RFC 5771, DOI 536 10.17487/RFC5771, March 2010, 537 . 539 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, 540 DOI 10.17487/RFC6762, February 2013, 541 . 543 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., 544 Morris, J., Hansen, M., and R. Smith, "Privacy 545 Considerations for Internet Protocols", RFC 6973, DOI 546 10.17487/RFC6973, July 2013, 547 . 549 [RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy 550 Considerations for IPv6 Address Generation Mechanisms", 551 RFC 7721, DOI 10.17487/RFC7721, March 2016, 552 . 554 [RFC7819] Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy 555 Considerations for DHCP", RFC 7819, DOI 10.17487/RFC7819, 556 April 2016, . 558 [RFC8117] Huitema, C., Thaler, D., and R. Winter, "Current Hostname 559 Practice Considered Harmful", RFC 8117, DOI 10.17487/ 560 RFC8117, March 2017, . 563 [TRAC2016] 564 Faath, M., Weisshaar, F., and R. Winter, "How Broadcast 565 Data Reveals Your Identity and Social Graph", 7th 566 International Workshop on TRaffic Analysis and 567 Characterization IEEE TRAC 2016, September 2016. 569 Authors' Addresses 571 Rolf Winter 572 University of Applied Sciences Augsburg 573 Augsburg 574 DE 576 Email: rolf.winter@hs-augsburg.de 578 Michael Faath 579 Conntac GmbH 580 Augsburg 581 DE 583 Email: faath@conntac.net 585 Fabian Weisshaar 586 University of Applied Sciences Augsburg 587 Augsburg 588 DE 590 Email: fabian.weisshaar@hs-augsburg.de