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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for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-05) exists of draft-ietf-dnssd-privacy-00 == Outdated reference: A later version (-05) exists of draft-ietf-intarea-hostname-practice-00 -- Obsolete informational reference (is this intentional?): RFC 3315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: November 30, 2017 Conntac GmbH 6 F. Weisshaar 7 University of Applied Sciences Augsburg 8 May 29, 2017 10 Privacy considerations for IP broadcast and multicast protocol designers 11 draft-ietf-intarea-broadcast-consider-03 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 November 30, 2017. 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 . . . . . . . 3 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 . . . . . . . . . . . . . 6 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 . . . . . . . . . . . . . . . . . . . 10 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 and it appears that an increasing 86 number of applications make use of it [TRAC2016]. That is not 87 entirely surprising. As RFC 919 [RFC0919] puts it, "The use of 88 broadcasts [...] is a good base for many applications". Broadcast 89 and multicast functionality in a subnetwork are therefore important 90 as a lack thereof renders the protocols underlying these mechanisms 91 inoperable [RFC3819]. 93 Using broadcast/multicast can become problematic if the information 94 that is being distributed can be regarded as sensitive or when the 95 information that is distributed by multiple of these protocols can be 96 correlated in a way that sensitive data can be derived. This is 97 clearly true for any protocol, but broadcast/multicast is special in 98 at least two respects: 100 (a) The aforementioned large receiver group, consisting of receivers 101 unknown to the sender. This makes eavesdropping without special 102 privileges or a special location in the network trivial for 103 anybody in the broadcast/multicast domain. 105 (b) Encryption is more difficult when broadcast/multicast messages, 106 leaving content of these messages in the clear and making it 107 easier to spoof and replay them. 109 Given the above, privacy protection for protocols based on broadcast 110 or multicast communication is significantly more difficult compared 111 to unicast communication and at the same time invading the privacy is 112 much easier. 114 Privacy considerations of IETF-specified protocols have received some 115 attention in the recent past (e.g. RFC 7721 [RFC7721] or RFC 7919 116 [RFC7819]). There is also general guidance available for document 117 authors on when and how to include a privacy considerations section 118 in their documents and on how to evaluate the privacy implications of 119 Internet protocols [RFC6973]. RFC6973 also describes potential 120 threats to privacy in great detail and lists terminology that is also 121 used in this document. 123 In contrast to RFC6973, this document contains a number of privacy 124 considerations especially for broadcast/multicast protocol designers 125 that are intended to reduce the likelihood that a broadcast/multicast 126 protocol can be misused to collect sensitive data about devices, 127 users and groups of users on a broadcast/multicast domain. These 128 considerations particularly apply to protocols designed outside the 129 IETF for two reasons. For one, non-standard protocols will likely 130 not receive operational attention and support in making them more 131 secure such as e.g. DHCP snooping does for DHCP because they 132 typically are not documented. The other reason is that these 133 protocols have been designed in isolation, where a set of 134 considerations to follow is useful in the absence of a larger 135 community providing feedback. In particular, carelessly designed 136 broadcast/multicast protocols can break privacy efforts at different 137 layers of the protocol stack such as MAC address or IP address 138 randomization [RFC4941]. 140 1.1. Types and usage of broadcast and multicast 142 In IPv4, two major types of broadcast addresses exist, the limited 143 broadcast which is defined as all-ones (255.255.255.255, defined in 144 section 5.3.5.1 of [RFC1812]) and the directed broadcast with the 145 given network prefix of an IP address and the host part of all-ones 146 (defined in section 5.3.5.2. of [RFC1812]). Broadcast packets are 147 received by all nodes in a subnetwork. Limited broadcasts never 148 transit a router. The same is true for directed broadcasts by 149 default, but routers MAY provide an option to do this [RFC2644]. 150 IPv6 on the other hand does not provide broadcast addresses but 151 solely relies on multicast [RFC4291]. 153 In contrast to broadcast addresses, multicast addresses represent an 154 identifier for a set of interfaces that can be a set different from 155 all nodes in the subnetwork. All interfaces that are identified by a 156 given multicast address receive packets destined towards that address 157 and are called a multicast group. In both IPv4 and IPv6, multiple 158 pre-defined multicast addresses exist. The ones most relevant for 159 this document are the ones with subnet scope. For IPv4, an IP prefix 160 is reserved for this purpose called the Local Network Control Block 161 (224.0.0.0/24, defined in section 4 of [RFC5771]). For IPv6, the 162 relevant multicast addresses are the two All Nodes Addresses, which 163 every IPv6-capable host is required to recognize as identifying 164 itself (see section 2.7.1 of [RFC4291]). 166 Typical usage of these addresses include local service discovery 167 (e.g. mDNS [RFC6762] and LLMNR [RFC4795] make use of multicast), 168 autoconfiguration (e.g. DHCPv4 [RFC2131] uses broadcasts and DHCPv6 169 [RFC3315] uses multicast addresses) and other vital network services 170 such as address resolution or duplicate address detection. But 171 besides these core network functions, also applications make use of 172 broadcast and multicast functionality, often implementing proprietary 173 protocols. In sum, these protocols distribute a diverse set of 174 potentially privacy sensitive information to a large receiver group. 176 1.2. Requirements Language 178 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 179 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 180 document are to be interpreted as described in RFC 2119 [RFC2119]. 182 2. Privacy considerations 184 There are a few obvious and a few not necessarily obvious things 185 designers of broadcast/multicast protocols should consider in respect 186 to the privacy implications of their protocol. Most of these items 187 are based on protocol behavior observed as part of experiments on 188 operational networks [TRAC2016]. 190 2.1. Message frequency 192 Frequent broadcast/multicast traffic caused by an application can 193 give user behavior and online times away. This allows a passive 194 observer to potentially deduce a user's current activity (e.g. a 195 game) and it allows to create an online profile (i.e. times the user 196 is on the network). The higher the frequency of these messages, the 197 more accurate this profile will be. Given that broadcasts/multicasts 198 are only visible in the same broadcast/multicast domain, these 199 messages also give the rough location of the user away (e.g. a campus 200 or building). 202 This behavior has e.g. been observed by a synchronization mechanism 203 of a popular application, where multiple messages have been sent per 204 minute via broadcast. Given this behavior, it is possible to record 205 a device's time on the network with a sub-minute accuracy given only 206 the traffic of this single application installed on the device. But 207 also services used for local name resolution in modern operating 208 systems utilize broadcast/multicast protocols (e.g. mDNS, LLMNR or 209 NetBIOS) to announce for example their shares regularly and allow a 210 tracking of the online time of a device. 212 If a protocol relies on frequent or periodic broadcast/multicast 213 messages, the frequency SHOULD be chosen conservatively, in 214 particular if the messages contain persistent identifiers (see next 215 subsection). Also, intelligent message suppression mechanisms such 216 as the ones employed in mDNS [RFC6762] SHOULD be implemented. The 217 lower the frequency of broadcast messages, the harder traffic 218 analysis and surveillance becomes. 220 2.2. Persistent identifiers 222 A few broadcast/multicast protocols observed in the wild make use of 223 persistent identifiers. This includes the use of host names or more 224 abstract persistent identifiers such as a UUID or similar. These 225 IDs, which e.g. identify the installation of a certain application 226 might not change across updates of the software and are therefore 227 extremely long lived. This allows a passive observer to track a user 228 precisely if broadcast/multicast messages are frequent. This is even 229 true in case the IP and/or MAC address changes. Such identifiers 230 also allow two different interfaces (e.g. WiFi and Ethernet) to be 231 correlated to the same device. If the application makes use of 232 persistent identifiers for multiple installations of the same 233 application for the same user, this even allows to infer that 234 different devices belong to the same user. 236 The aforementioned broadcast messages from a synchronization 237 mechanism of a popular application also included a persistent 238 identifier in every broadcast. This identifier did never change 239 after the application was installed and allowed to track a device 240 even when it changed its network interface or when it connected to a 241 different network. 243 If a broadcast/multicast protocol relies on IDs to be transmitted, it 244 SHOULD be considered if frequent ID rotations are possible in order 245 to make user tracking more difficult. Persistent IDs are considered 246 bad practice in general for broadcast and multicast communication as 247 persistent application layer IDs will make efforts on lower layers to 248 randomize identifiers (e.g. [I-D.huitema-6man-random-addresses]) 249 useless or at least much more difficult. 251 2.3. Anticipate user behavior 253 A large number of users name their device after themselves, either 254 using their first name, last name or both. Often a host name 255 includes the type, model or maker of a device, its function or 256 includes language specific information. Based on gathered data, this 257 appears currently to be prevalent user behavior [TRAC2016]. For 258 protocols using the host name as part of the messages, this clearly 259 will reveal personally identifiable information to everyone on the 260 local network. This information can also be used to mount more 261 sophisticated attacks, when e.g. the owner of a device is identified 262 (as an interesting target) or properties of the device are known 263 (e.g. known vulnerabilities). 265 A popular operating system vendor includes the name the user chooses 266 for the user account during the installation process as part of the 267 host name of the device. The name of the operating system is also 268 included, revealing therefore two pieces of information, which can be 269 regarded as private information if the host name is used in 270 broadcast/multicast messages. 272 Where possible, the use of host names and other user provided 273 information in broadcast/multicast protocols SHOULD be avoided. If 274 only a persistent ID is needed, this can be generated. An 275 application might want to display the information it will broadcast 276 on the LAN at install/config time, so the user is at least aware of 277 the application's behavior. More host name considerations can be 278 found in [I-D.ietf-intarea-hostname-practice]. More information on 279 user participation can be found in RFC 6973 [RFC6973]. 281 2.4. Consider potential correlation 283 A large number of services and applications make use of the 284 broadcast/multicast mechanism. That means there are various sources 285 of information that are easily accessible by a passive observer. In 286 isolation, the information these protocols reveal might seem 287 harmless, but given multiple such protocols, it might be possible to 288 correlate this information. E.g. a protocol that uses frequent 289 messages including a UUID to identify the particular installation 290 does not give the identity of the user away. But a single message 291 including the user's host name might just do that and it can be 292 correlated using e.g. the MAC address of the device's interface. 294 In the experiments described in [TRAC2016], it was possible to 295 correlate frequently sent broadcast messages that included a unique 296 identifier with other broadcast/multicast messages containing 297 usernames (e.g. mDNS, LLMNR or NetBIOS), but also relationships to 298 other users. This allowed to reveal the real identity of the users 299 of many devices but it also gave some information about their social 300 environment away. 302 A broadcast protocol designer should be aware of the fact that even 303 if - in isolation - the information a protocol leaks seems harmless, 304 there might be ways to correlate that information with other 305 broadcast protocol information to reveal sensitive information about 306 a user. 308 2.5. Configurability 310 A lot of applications and services using broadcast/multicast 311 protocols do not include the means to declare "safe" environments 312 (e.g. based on the SSID of a WiFi network and the MAC addresses of 313 the access points). E.g. a device connected to a public WiFi will 314 likely broadcast the same information as when connected to the home 315 network. It would be beneficial if certain behavior could be 316 restricted to "safe" environments. 318 A popular operating system e.g. allows the user to specify the trust 319 level of the network the device connects to, which for example 320 restricts specific system services (using broadcast/multicast 321 messages for their normal operation) to be used in untrusted 322 networks. Such functionality could implemented as part of an 323 application. 325 An application developer making use of broadcasts/multicasts as part 326 of the application SHOULD make the broadcast feature, if possible, 327 configurable, so that potentially sensitive information does not leak 328 on public networks, where the thread to privacy is much larger. 330 3. Operational considerations 332 Besides changing end-user behavior, choosing sensible defaults as an 333 operating system vendor (e.g. for suggesting host names) and the 334 considerations for protocol designers mentioned in this document, 335 there are things that the network administrators/operators can do to 336 limit the above mentioned problems. 338 A feature not uncommonly found on access points e.g. is to filter 339 broadcast and multicast traffic. This will potentially break certain 340 applications or some of their functionality but will also protect the 341 users from potentially leaking sensitive information. 343 4. Summary 345 Increasingly, applications rely on broadcast and multicast messages. 346 For some, broadcasts/multicasts are the basis of their application 347 logic, others use broadcasts/multicasts to improve certain aspects of 348 the application but are fully functional in case broadcasts/ 349 multicasts fail. Irrespective of the role of broadcast and multicast 350 messages for the application, the designers of protocols that make 351 use of them should be very careful in their protocol design because 352 of the special nature of broad- and multicast. 354 It is not always possible to implement certain functionality via 355 unicast, but in case a protocol designer chooses to rely on 356 broadcast/multicast, the following should be carefully considered: 358 o IETF-specified protocols, such as mDNS [RFC6762], SHOULD be used 359 if possible as operational support might exist to protect against 360 the leakage of private information. Also, for some protocols 361 privacy extensions are being specified, which can be used if 362 implemented. E.g. for DNS-SD privacy extensions are documented in 363 [I-D.ietf-dnssd-privacy] 365 o Avoid using user-specified information inside broadcast/multicast 366 messages as users will often use personal information or other 367 information aiding attackers, in particular if the user is unaware 368 about how that information is being used 370 o Avoid persistent IDs in messages as this allows user tracking, 371 correlation and potentially has a devastating effect on other 372 privacy protection mechanisms 374 o If you really must use a broadcast/multicast protocol and cannot 375 use an IETF-specified protocol, then: 377 * Be very conservative in how frequently you send messages as an 378 effort in data minimization 380 * Seek advice from IETF-specifies protocols such as message 381 suppression in mDNS 383 * Try to design the protocol in a way that the information cannot 384 be correlated with other information in broadcast/multicast 385 messages 387 * Let the user configure safe environments if possible (e.g. 388 based on the SSID) 390 [Note: discussions on this document should be take place on the 391 Intarea mailing list of the IETF. Subscription: 392 https://www.ietf.org/mailman/listinfo/int-area, Mailing list archive: 393 https://www.ietf.org/mail-archive/web/int-area/current/maillist.html] 395 5. Other considerations 397 Besides the privacy implications of frequent broadcasting, it also 398 represents a performance problem. In particular in certain wireless 399 technologies such as 802.11, broadcast and multicast are transmitted 400 at a much lower rate (the lowest common denominator rate) compared to 401 unicast and therefore have a much bigger impact on the overall 402 available airtime. Further, it will limit the ability for devices to 403 go to sleep if frequent broadcasts are being sent. A similar problem 404 in respect to Router Advertisements is addressed in 405 [I-D.ietf-v6ops-reducing-ra-energy-consumption]. In that respect 406 broadcasts can be used for another class of attacks that not related 407 to privacy. The potential impact on network performance should 408 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 [RFC0919] Mogul, J., "Broadcasting Internet Datagrams", STD 5, RFC 469 919, DOI 10.17487/RFC0919, October 1984, 470 . 472 [RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers", 473 RFC 1812, DOI 10.17487/RFC1812, June 1995, 474 . 476 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 477 2131, DOI 10.17487/RFC2131, March 1997, 478 . 480 [RFC2644] Senie, D., "Changing the Default for Directed Broadcasts 481 in Routers", BCP 34, RFC 2644, DOI 10.17487/RFC2644, 482 August 1999, . 484 [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, 485 C., and M. Carney, "Dynamic Host Configuration Protocol 486 for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 487 2003, . 489 [RFC3819] Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D., 490 Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. 491 Wood, "Advice for Internet Subnetwork Designers", BCP 89, 492 RFC 3819, DOI 10.17487/RFC3819, July 2004, 493 . 495 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 496 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 497 2006, . 499 [RFC4795] Aboba, B., Thaler, D., and L. Esibov, "Link-local 500 Multicast Name Resolution (LLMNR)", RFC 4795, DOI 501 10.17487/RFC4795, January 2007, 502 . 504 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy 505 Extensions for Stateless Address Autoconfiguration in 506 IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, 507 . 509 [RFC5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast 510 Extensions to the Security Architecture for the Internet 511 Protocol", RFC 5374, DOI 10.17487/RFC5374, November 2008, 512 . 514 [RFC5771] Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for 515 IPv4 Multicast Address Assignments", BCP 51, RFC 5771, DOI 516 10.17487/RFC5771, March 2010, 517 . 519 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, 520 DOI 10.17487/RFC6762, February 2013, 521 . 523 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., 524 Morris, J., Hansen, M., and R. Smith, "Privacy 525 Considerations for Internet Protocols", RFC 6973, DOI 526 10.17487/RFC6973, July 2013, 527 . 529 [RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy 530 Considerations for IPv6 Address Generation Mechanisms", 531 RFC 7721, DOI 10.17487/RFC7721, March 2016, 532 . 534 [RFC7819] Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy 535 Considerations for DHCP", RFC 7819, DOI 10.17487/RFC7819, 536 April 2016, . 538 [TRAC2016] 539 Faath, M., Weisshaar, F., and R. Winter, "How Broadcast 540 Data Reveals Your Identity and Social Graph", 7th 541 International Workshop on TRaffic Analysis and 542 Characterization IEEE TRAC 2016, September 2016. 544 Authors' Addresses 546 Rolf Winter 547 University of Applied Sciences Augsburg 548 Augsburg 549 DE 551 Email: rolf.winter@hs-augsburg.de 553 Michael Faath 554 Conntac GmbH 555 Augsburg 556 DE 558 Email: faath@conntac.net 559 Fabian Weisshaar 560 University of Applied Sciences Augsburg 561 Augsburg 562 DE 564 Email: fabian.weisshaar@hs-augsburg.de