CoRE                                                        M.S. Lenders
Internet-Draft                                                 FU Berlin
Intended status: Standards Track                               C. Amsüss
Expires: 11 February 2022
                                                             C. Gündoğan
                                                            T.C. Schmidt
                                                             HAW Hamburg
                                                             M. Wählisch
                                                               FU Berlin
                                                          10 August 2021


                      DNS Queries over CoAPS (DoC)
                    draft-lenders-dns-over-coaps-00

Abstract

   This document defines a protocol for sending DNS messages over the
   DTLS-Secured Constrained Application Protocol (CoAPS).  Using the
   REST architecture specified in CoAP and the security features of
   DTLS, DNS over CoAPS provides encrypted DNS messages for constrained
   devices in the Internet of Things (IoT) based on common interfaces.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on TODO

   Source for this draft and an issue tracker can be found at
   https://github.com/anr-bmbf-pivot/draft-dns-over-coaps.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 11 February 2022.



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Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Selection of a DoC Server . . . . . . . . . . . . . . . . . .   4
     3.1.  URI Template Alternatives . . . . . . . . . . . . . . . .   4
   4.  Basic Message Exchange  . . . . . . . . . . . . . . . . . . .   4
     4.1.  DNS Queries in CoAP Requests  . . . . . . . . . . . . . .   4
       4.1.1.  Examples  . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  DNS Responses in CoAP Responses . . . . . . . . . . . . .   6
       4.2.1.  Response Codes and Handling DNS and CoAP errors . . .   6
       4.2.2.  Examples  . . . . . . . . . . . . . . . . . . . . . .   7
   5.  CoAP/CoRE Integration . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Proxies and caching . . . . . . . . . . . . . . . . . . .   8
     5.2.  OBSERVE (modifications)?  . . . . . . . . . . . . . . . .   9
     5.3.  OSCORE  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  URI template configuration  . . . . . . . . . . . . . . . . .   9
   7.  Considerations for Unencrypted Use  . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  12
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   This document defines DNS over CoAPS (DoC), a protocol to send DNS
   [RFC1035] queries and get DNS responses over the Constrained
   Application Protocol (CoAP) [RFC7252].  Each DNS query-response pair
   is mapped into a CoAP message exchange and secured by DTLS [RFC6347]
   to ensure message integrity and confidentiality.



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   The application use case of DoC is inspired by DNS over HTTPS
   [RFC8484] (DoH).  DoC, however, aims for the deployment in the
   constrained Internet of Things (IoT), which usually conflicts with
   the requirements introduced by HTTPS.

   To prevent TCP and HTTPS resource requirements, constrained IoT
   devices could use DNS over DTLS [RFC8094].  In contrast to DNS over
   DTLS, DoC utilizes CoAP features to mitigate drawbacks of datagram-
   based communication.  These features include: block-wise transfer,
   which solves the Path MTU problem of DNS over DTLS (see [RFC8094],
   section 5); CoAP proxies, which provide an additional level of
   caching; re-use of data structures for application traffic and DNS
   information, which saves memory on constrained devices.

                   - GET coaps://[2001::db8::1]/?dns=example.org
                  /- POST/FETCH coaps://[2001::db8::1]/
                 /
                CoAP request
   +--------+   [DNS query]   +--------+  DNS query   +--------+
   |  DoC   |---------------->|  DoC   |.............>|  DNS   |
   | Client |<----------------| Server |<.............| Server |
   +--------+  CoAP response  +--------+ DNS response +--------+
               [DNS response]

                      Figure 1: Basic DoC architecture

   The most important components of DoC can be seen in Figure 1: A DoC
   client tries to resolve DNS information by sending DNS queries
   carried within CoAP requests to a DoC server.  That DoC server may or
   may not resolve that DNS information itself by using other DNS
   transports with an upstream DNS server.  The DoC server then replies
   to the DNS queries with DNS responses carried within CoAP responses.

   TBD: additional feature sets of CoAP/CoRE

   *  resource directory for DoC service discovery,

   *  ...

2.  Terminology

   A server that provides the service specified in this document is
   called a "DoC server" to differentiate it from a classic "DNS
   server".  Correspondingly, a client using this protocol to retrieve
   the DNS information is called a "DoC client".

   The term "constrained nodes" is used as defined in [RFC7228].




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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Selection of a DoC Server

   A DoC client is configured with a URI Template [RFC6570].  This
   allows us to reuse configuration mechanisms provided for DoH.

   The URI Template SHOULD provide a variable "dns" so that GET requests
   can be used to retrieve the DNS information.  If the "dns" variable
   is not provided in the URI Template, GET requests can not be used for
   DoC exchanges.

   TBD:

   *  Support for more than one URI Template by DoC server.

   *  DoC server identity, key exchange, ...

3.1.  URI Template Alternatives

   TBD:

   *  CRI [I-D.ietf-core-href] or CoRAL [I-D.ietf-core-coral]

4.  Basic Message Exchange

4.1.  DNS Queries in CoAP Requests

   A DoC client encodes a single DNS query in one or more CoAP request
   messages using either the CoAP GET, FETCH [RFC8132], or POST method.
   More than one CoAP request message MAY be used if the FETCH or POST
   method are used and block-wise transfer [RFC7959] is supported by the
   client.  If more than one CoAP request message is used to encode the
   DNS query, it must be chained together using the Block1 option in
   those CoAP requests.  To make use of the recovery mechanism of CoAP,
   the CoAP request SHOULD be carried in a Confirmable (CON) messages.

   For a POST or FETCH request the URI Template specified in Section 3
   is processed without any variables set.  For a GET request the URI
   Template is extended with the "dns" variable set to the content of
   the DNS query, encoded with "base64url" [RFC4648].






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   If new Content Formats are specified in the future, the specification
   MUST define the variable used in the URI Template with that new
   format.

   For POST and FETCH methods, the DNS query is included in the payloads
   of the CoAP request messages in the binary format as specified in
   [RFC1035].  The Content Format option MUST be included to indicate
   the message type as "application/dns-message".  Due to the lack of
   encoding requirements, both FETCH and POST methods are generally
   smaller than GET requests.

   A DoH server MUST implement both the GET and POST method and MAY
   implement the FETCH method.

   Using GET enables CoAP proxies en-route to the DoC server to cache a
   successful response.  However, as the DNS query is carried in the URI
   and thus in one of the URI-* options within a GET request, block-wise
   transfer can not be used with that method.  As a cache-friendly
   alternative, the FETCH method can be used, which is an extension to
   legacy CoAP, specified in [RFC8132].

   Requests of either method type SHOULD include an Accept option to
   indicate what type of content can be parsed in the response.  A
   client MUST be able to parse messages of Content Format "application/
   dns-message" regardless of the provided Accept option.  Messages of
   that Content Format are DNS responses in binary format as specified
   in [RFC1035].

   To simplify cache-key calculations at the CoAP proxies en-route, DoC
   clients using Content Formats that include the ID field from the DNS
   message, such as "application/dns-message", SHOULD use DNS ID 0 in
   every DNS query.  The CoAP message ID takes the same function on the
   CoAP layer.  Dedicated identification of DNS message exchanges on the
   wire is thus not necessary.

4.1.1.  Examples

   The following examples illustrate the usage of different CoAP
   messages to resolve "example.org.  IN AAAA" based on the URI template
   "coaps://[2001:db8::1]/{?dns}".  The CoAP body is encoded in
   "application/dns-message" Content Format.

   GET request:

   GET coaps://[2001::db8::1]/
   URI-Query: dns=AAABIAABAAAAAAAAB2V4YW1wbGUDb3JnAAAcAAE
   Accept: application/dns-message




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   POST request:

   POST coaps://[2001::db8::1]/
   Content-Format: application/dns-message
   Accept: application/dns-message
   Payload: 00 00 01 20 00 02 00 00 00 00 00 00 07 65 78 61 [binary]
            6d 70 6c 65 03 6f 72 67 00 00 1c 00 01 c0 0c 00 [binary]
            01 00 01                                        [binary]

   FETCH request:

   FETCH coaps://[2001::db8::1]/
   Content-Format: application/dns-message
   Accept: application/dns-message
   Payload: 00 00 01 20 00 02 00 00 00 00 00 00 07 65 78 61 [binary]
            6d 70 6c 65 03 6f 72 67 00 00 1c 00 01 c0 0c 00 [binary]
            01 00 01                                        [binary]

4.2.  DNS Responses in CoAP Responses

   This document specifies responses of Content Format "application/dns-
   message" which encodes the DNS response in the binary format,
   specified in [RFC1035].  For this type of responses, the Content
   Format option indicating the "application/dns-message" format MUST be
   included.  A DoC server MUST be able to parse requests of Content
   Format "application/dns-message".

   Each DNS query-response pair is mapped to a train of one or more of
   CoAP request-response pairs.  If supported, a DoC server MAY transfer
   the DNS response in more than one CoAP response using the Block2
   option [RFC7959].

4.2.1.  Response Codes and Handling DNS and CoAP errors

   A DNS response indicates either success or failure for the DNS query.
   As such, it is RECOMMENDED that CoAP responses that carry any valid
   DNS response, use a 2.xx Success response code.  GET and FETCH
   requests SHOULD be responded to with a 2.05 Content response.  POST
   requests SHOULD be responded to with a 2.01 Created response.

   CoAP responses with non-successful response codes MUST NOT contain
   any payload and may only be used on errors in the CoAP layer or when
   a request does not fulfill the requirements of the DoC protocol.

   For consistency, communications errors with an upstream DNS server
   such as timeouts SHOULD be indicated with a SERVFAIL DNS response in
   a successful CoAP response.




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   A DoC client might try to repeat a non-successful exchange unless
   otherwise prohibited.  For instance, a FETCH request MUST NOT be
   repeated with a URI Template for which the DoC server already
   responded with a 4.05 Method Not Allowed, as the server might only
   implement legacy CoAP and does not support the FETCH method.  The DoC
   client might also elect to repeat a non-successful exchange with a
   different URI Template, for instance, when the response indicates an
   unsupported content format.

4.2.2.  Examples

   The following examples illustrate the replies to the query
   "example.org.  IN AAAA record", recursion turned on.  Successful
   responses carry one answer record including address
   2001:db8:1::1:2:3:4 and TTL 58719.

   A successful response to a GET or FETCH request:

   2.05 Content
   Content-Format: application/dns-message
   Max-Age: 58719
   Payload: 00 00 81 a0 00 01 00 01 00 00 00 00 07 65 78 61 [binary]
            6d 70 6c 65 03 6f 72 67 00 00 1c 00 01 c0 0c 00 [binary]
            1c 00 01 00 01 37 49 00 10 20 01 0d b8 00 01 00 [binary]
            00 00 01 00 02 00 03 00 04

   A successful response to a POST request uses a different response
   code:

   2.03 Created
   Content-Format: application/dns-message
   Max-Age: 58719
   Payload: 00 00 81 a0 00 01 00 01 00 00 00 00 07 65 78 61 [binary]
            6d 70 6c 65 03 6f 72 67 00 00 1c 00 01 c0 0c 00 [binary]
            1c 00 01 00 01 37 49 00 10 20 01 0d b8 00 01 00 [binary]
            00 00 01 00 02 00 03 00 04

   When a DNS error (SERVFAIL in this case) is noted in the DNS
   response, the CoAP request still indicates success:

   2.05 Content
   Content-Format: application/dns-message
   Payload: 00 00 81 a2 00 01 00 00 00 00 00 00 07 65 78 61 [binary]
            6d 70 6c 65 03 6f 72 67 00 00 1c 00 01          [binary]







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   When an error occurs on the CoAP layer, the DoC server SHOULD respond
   with an appropriate CoAP error, for instance "4.15 Unsupported
   Content-Format" if the Content Format option in the request was not
   set to "application/dns-message".

5.  CoAP/CoRE Integration

5.1.  Proxies and caching

   DoC exchanges may be cached by CoAP proxies and DNS caches en-route.
   It is desirable that DoC exchanges follow the same paradigm as all
   CoAP exchanges so they do not need any special handling by a CoAP
   cache implementation.

   Two requirements to a DoC exchange are necessary to that goal: First,
   the ID field of the DNS header SHOULD always be 0, when using the
   "application/dns-message" Content Format.  This allows for both GET
   URIs and FETCH payload to always have the same value for the same DNS
   query, and thus they do not interfere with cache key generation.
   Second, it is RECOMMENDED to set the Max-Age option of a response to
   the minimum TTL in the Answer section of a DNS response.  This
   prevents expired records unintentionally being served from a CoAP
   cache.

     DoC client           DoC proxy           DNS server
          |  CoAP req [rt 1]  |                    |
          |------------------>|  DNS query [rt 1]  |
          |                   |------------------->|
          |  CoAP req [rt 2]  |                    |
          |------------------>|      DNS resp      |
          |     CoAP resp     |<-------------------|
          |<------------------|                    |
          |                   |                    |

      Figure 2: CoAP retransmission (rt) is received before DNS query
                         could have been fulfilled.

   TBD:

   *  Responses that are not globally valid

   *  General CoAP proxy problem, but what to do when DoC server is a
      DNS proxy, response came not yet in but retransmission by DoC
      client was received (see Figure 2)

      -  send empty ACK (maybe move to best practices appendix
         (https://github.com/anr-bmbf-pivot/draft-dns-over-coaps/
         issues/6#issuecomment-895880206))



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   It is RECOMMENDED that servers set an ETag option on large responses
   (TBD: more concrete guidance) that have a short Max-Age relative to
   the expected clients' caching time.  Thus, clients that need to
   revalidate a response can do so using the established ETag mechanism.
   With responses large enough to be fragmented, it's best practice for
   servers to set an ETag anyway.

5.2.  OBSERVE (modifications)?

   *  TBD

   *  DoH has considerations on Server Push to deliver additional,
      potentially outstanding requests + response to the DoC client for
      caching

   *  OBSERVE does not include the request it would have been generated
      from ==> cannot be cached without corresponding request having
      been send over the wire.

   *  If use case exists: extend OBSERVE with option that contains
      "promised" request (see [RFC7540], section 8.2)?

   *  Other caveat: clients can't cache, only proxys so value needs to
      be evaluated

   *  Potential use case: [RFC8490] Section 4.1.2

5.3.  OSCORE

   *  TBD

   *  With OSCORE DTLS might not be required

6.  URI template configuration

   *  TBD

   *  Maybe out-of-scope?

   *  DHCP and RA options to deliver?  [I-D.peterson-doh-dhcp]

   *  CoRE-RD [I-D.ietf-core-resource-directory] (...; can not express
      URI templates)

   *  When no actual templating is involved: regular resource discovery
      ("rt=core.dns"?) through .well-known/core





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7.  Considerations for Unencrypted Use

   *  TBD

   *  DTLS-transport should be used

   *  Non-DTLS can have benefits: Blockwise-transfer for IEEE 802.15.4,
      additional layer of caching, ...

8.  Security Considerations

   TODO Security

9.  IANA Considerations

   IANA is requested to assign CoAP Content-Format ID for the DNS
   message media type in the "CoAP Content-Formats" sub-registry, within
   the "CoRE Parameters" registry [RFC7252], corresponding the
   "application/dns-message" media type from the "Media Types" registry:

   Media-Type: application/dns-message

   Encoding: -

   Id: TBD

   Reference: [TBD-this-spec]

10.  References

10.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/rfc/rfc1035>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/rfc/rfc4648>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/rfc/rfc6347>.



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   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <https://www.rfc-editor.org/rfc/rfc6570>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7252>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/rfc/rfc7959>.

   [RFC8132]  van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
              FETCH Methods for the Constrained Application Protocol
              (CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
              <https://www.rfc-editor.org/rfc/rfc8132>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

10.2.  Informative References

   [I-D.ietf-core-coral]
              Hartke, K., "The Constrained RESTful Application Language
              (CoRAL)", Work in Progress, Internet-Draft, draft-ietf-
              core-coral-03, 9 March 2020,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              coral-03>.

   [I-D.ietf-core-href]
              Bormann, C. and H. Birkholz, "Constrained Resource
              Identifiers", Work in Progress, Internet-Draft, draft-
              ietf-core-href-06, 25 July 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              href-06>.

   [I-D.ietf-core-resource-directory]
              Amsüss, C., Shelby, Z., Koster, M., Bormann, C., and P. V.
              D. Stok, "CoRE Resource Directory", Work in Progress,
              Internet-Draft, draft-ietf-core-resource-directory-28, 7
              March 2021, <https://datatracker.ietf.org/doc/html/draft-
              ietf-core-resource-directory-28>.





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   [I-D.peterson-doh-dhcp]
              Peterson, T., "DNS over HTTP resolver announcement Using
              DHCP or Router Advertisements", Work in Progress,
              Internet-Draft, draft-peterson-doh-dhcp-01, 21 October
              2019, <https://datatracker.ietf.org/doc/html/draft-
              peterson-doh-dhcp-01>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/rfc/rfc7228>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/rfc/rfc7540>.

   [RFC8094]  Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
              Transport Layer Security (DTLS)", RFC 8094,
              DOI 10.17487/RFC8094, February 2017,
              <https://www.rfc-editor.org/rfc/rfc8094>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/rfc/rfc8484>.

   [RFC8490]  Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
              Lemon, T., and T. Pusateri, "DNS Stateful Operations",
              RFC 8490, DOI 10.17487/RFC8490, March 2019,
              <https://www.rfc-editor.org/rfc/rfc8490>.

Appendix A.  Change Log

   TBD:

   *  Reconsider usage of GET/POST (https://github.com/anr-bmbf-pivot/
      draft-dns-over-coaps/issues/2)?

   *  Request text duplication (https://github.com/anr-bmbf-pivot/draft-
      dns-over-coaps/issues/4)

   *  TTL vs. Max-Age (https://github.com/anr-bmbf-pivot/draft-dns-over-
      coaps/issues/5)

Acknowledgments

   TODO acknowledge.




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Authors' Addresses

   Martine Sophie Lenders
   Freie Universität Berlin

   Email: m.lenders@fu-berlin.de


   Christian Amsüss

   Email: christian@amsuess.com


   Cenk Gündoğan
   HAW Hamburg

   Email: cenk.guendogan@haw-hamburg.de


   Thomas C. Schmidt
   HAW Hamburg

   Email: t.schmidt@haw-hamburg.de


   Matthias Wählisch
   Freie Universität Berlin

   Email: m.waehlisch@fu-berlin.de






















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