Internet DRAFT - draft-ietf-core-dns-over-coap
draft-ietf-core-dns-over-coap
CoRE M. S. Lenders
Internet-Draft FU Berlin
Intended status: Standards Track C. Amsüss
Expires: 14 September 2023
C. Gündoğan
Huawei Technologies
T. C. Schmidt
HAW Hamburg
M. Wählisch
FU Berlin
13 March 2023
DNS over CoAP (DoC)
draft-ietf-core-dns-over-coap-02
Abstract
This document defines a protocol for sending DNS messages over the
Constrained Application Protocol (CoAP). These CoAP messages are
protected by DTLS-Secured CoAP (CoAPS) or Object Security for
Constrained RESTful Environments (OSCORE) to provide encrypted DNS
message exchange for constrained devices in the Internet of Things
(IoT).
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Constrained RESTful
Environments Working Group mailing list (core@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/core/.
Source for this draft and an issue tracker can be found at
https://github.com/core-wg/draft-dns-over-coap.
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/.
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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 14 September 2023.
Copyright Notice
Copyright (c) 2023 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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Selection of a DoC Server . . . . . . . . . . . . . . . . . . 4
4. Basic Message Exchange . . . . . . . . . . . . . . . . . . . 4
4.1. The "application/dns-message" Content-Format . . . . . . 5
4.2. DNS Queries in CoAP Requests . . . . . . . . . . . . . . 5
4.2.1. Request Format . . . . . . . . . . . . . . . . . . . 5
4.2.2. Support of CoAP Caching . . . . . . . . . . . . . . . 5
4.2.3. Examples . . . . . . . . . . . . . . . . . . . . . . 5
4.3. DNS Responses in CoAP Responses . . . . . . . . . . . . . 6
4.3.1. Response Codes and Handling DNS and CoAP errors . . . 6
4.3.2. Support of CoAP Caching . . . . . . . . . . . . . . . 6
4.3.3. Examples . . . . . . . . . . . . . . . . . . . . . . 7
5. CoAP/CoRE Integration . . . . . . . . . . . . . . . . . . . . 7
5.1. DNS Push . . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Considerations for Unencrypted Use . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8.1. New "application/dns-message" Content-Format . . . . . . 9
8.2. New "core.dns" Resource Type . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Reference Implementations . . . . . . . . . . . . . 12
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Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 12
B.1. Since draft-ietf-core-dns-over-coap-01 . . . . . . . . . 12
B.2. Since draft-ietf-core-dns-over-coap-00 . . . . . . . . . 12
B.3. Since draft-lenders-dns-over-coap-04 . . . . . . . . . . 12
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
This document defines DNS over CoAP (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. Each CoAP message is secured
by DTLS [RFC9147] or Object Security for Constrained RESTful
Environments (OSCORE) [RFC8613] to ensure message integrity and
confidentiality.
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.
To prevent resource requirements of DTLS or TLS on top of UDP (e.g.,
introduced by DNS over QUIC [RFC9250]), DoC allows for lightweight
end-to-end payload encryption based on OSCORE.
. FETCH coaps://[2001:db8::1]/
/
/
CoAP request
+------+ [DNS query] +------+ DNS query .---------------.
| DoC |---------------->| DoC |--- --- --- --->| DNS |
|Client|<----------------|Server|<--- --- --- ---| Infrastructure |
+------+ CoAP response +------+ DNS response '---------------'
[DNS response]
\ /\ /
'-----DNS over CoAP----' '--DNS over UDP/HTTPS/QUIC/...--'
Figure 1: Basic DoC architecture
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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 is a
DNS client (i.e., a stub or recursive resolver) that resolves DNS
information by using other DNS transports such as DNS over UDP
[RFC1035], DNS over HTTPS [RFC8484], or DNS over QUIC [RFC9250] when
communicating with the upstream DNS infrastructure. Using that
information, the DoC server then replies to the queries of the DoC
client with DNS responses carried within CoAP responses.
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". A DoC server acts either as a DNS stub resolver [RFC8499]
or a DNS recursive resolver [RFC8499].
A client using the service specified in this document to retrieve the
DNS information is called a "DoC client".
The term "constrained nodes" is used as defined in [RFC7228].
The terms "CoAP payload" and "CoAP body" are used as defined in
[RFC7959], Section 2.
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
In this document, it is assumed that the DoC client knows the DoC
server and the DNS resource at the DoC server. Possible options
could be manual configuration of a URI [RFC3986] or CRI
[I-D.ietf-core-href], or automatic configuration, e.g., using a CoRE
resource directory [RFC9176], DHCP or Router Advertisement options
[I-D.ietf-add-dnr]. Automatic configuration SHOULD only be done from
a trusted source.
When discovering the DNS resource through a link mechanism that
allows describing a resource type (e.g., the Resource Type Attribute
in [RFC6690]), the resource type "core.dns" can be used to identify a
generic DNS resolver that is available to the client.
4. Basic Message Exchange
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4.1. The "application/dns-message" Content-Format
This document defines a CoAP Content-Format number for the Internet
media type "application/dns-message". This media type is defined as
in [RFC8484] Section 6, i.e., a single DNS message encoded in the DNS
on-the-wire format [RFC1035]. Both DoC client and DoC server MUST be
able to parse contents in the "application/dns-message" format.
4.2. DNS Queries in CoAP Requests
A DoC client encodes a single DNS query in one or more CoAP request
messages that use the CoAP FETCH [RFC8132] method. Requests SHOULD
include an Accept option to indicate the type of content that can be
parsed in the response.
Since CoAP provides reliability of the message layer (e.g. CON) the
retransmission mechanism of the DNS protocol as defined in [RFC1035]
is not needed.
4.2.1. Request Format
When sending a CoAP request, a DoC client MUST include the DNS query
in the body of the CoAP request. As specified in [RFC8132]
Section 2.3.1, the type of content of the body MUST be indicated
using the Content-Format option. This document specifies the usage
of Content-Format "application/dns-message" (details see
Section 4.1). A DoC server MUST be able to parse requests of
Content-Format "application/dns-message".
4.2.2. Support of CoAP Caching
The DoC client SHOULD set the ID field of the DNS header always to 0
to enable a CoAP cache (e.g., a CoAP proxy en-route) to respond to
the same DNS queries with a cache entry. This ensures that the CoAP
Cache-Key (see [RFC8132] Section 2) does not change when multiple DNS
queries for the same DNS data, carried in CoAP requests, are issued.
4.2.3. Examples
The following example illustrates the usage of a CoAP message to
resolve "example.org. IN AAAA" based on the URI
"coaps://[2001:db8::1]/". The CoAP body is encoded in "application/
dns-message" Content Format.
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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]
4.3. DNS Responses in CoAP Responses
Each DNS query-response pair is mapped to a CoAP REST request-
response operation. DNS responses are provided in the body of the
CoAP response. A DoC server MUST be able to produce responses in the
"application/dns-message" Content-Format (details see Section 4.1)
when requested. A DoC client MUST understand responses in
"application/dns-message" format when it does not send an Accept
option. Any other response format than "application/dns-message"
MUST be indicated with the Content-Format option by the DoC server.
4.3.1. Response Codes and Handling DNS and CoAP errors
A DNS response indicates either success or failure in the Response
code of the DNS header (see [RFC1035] Section 4.1.1). It is
RECOMMENDED that CoAP responses that carry any valid DNS response use
a "2.05 Content" response code.
CoAP responses use non-successful response codes MUST NOT contain a
DNS response and MUST only be used on errors in the CoAP layer or
when a request does not fulfill the requirements of the DoC protocol.
Communication errors with a DNS server (e.g., timeouts) SHOULD be
indicated by including a SERVFAIL DNS response in a successful CoAP
response.
A DoC client might try to repeat a non-successful exchange unless
otherwise prohibited. The DoC client might also decide to repeat a
non-successful exchange with a different URI, for instance, when the
response indicates an unsupported Content-Format.
4.3.2. Support of CoAP Caching
The DoC server MUST ensure that any sum of the Max-Age value of a
CoAP response and any TTL in the DNS response is less or equal to the
corresponding TTL received from an upstream DNS server. This also
includes the default Max-Age value of 60 seconds (see [RFC7252],
section 5.10.5) when no Max-Age option is provided. The DoC client
MUST then add the Max-Age value of the carrying CoAP response to all
TTLs in a DNS response on reception and use these calculated TTLs for
the associated records.
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The RECOMMENDED algorithm to assure the requirement for the DoC is to
set the Max-Age option of a response to the minimum TTL of a DNS
response and to subtract this value from all TTLs of that DNS
response. This prevents expired records unintentionally being served
from an intermediate CoAP cache. Additionally, it allows for the
ETag value for cache validation, if it is based on the content of the
response, not to change even if the TTL values are updated by an
upstream DNS cache. If only one record set per DNS response is
assumed, a simplification of this algorithm is to just set all TTLs
in the response to 0 and set the TTLs at the DoC client to the value
of the Max-Age option.
4.3.3. 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:
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 [binary]
When a DNS error (SERVFAIL in this case) is noted in the DNS
response, the CoAP response 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]
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" and the Content-Format is not
otherwise supported by the server.
5. CoAP/CoRE Integration
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5.1. DNS Push
DNS Push requires additional overhead, which conflicts with
constrained resources, This is the reason why it is RECOMMENDED to
use CoAP Observe [RFC7641] instead of DNS Push in the DoC domain.
If the CoAP request indicates that the DoC client wants to observe a
resource record, a DoC server MAY use a DNS Subscribe message
[RFC8765] instead of a classic DNS query to fetch the information on
behalf of a DoC client. If this is not supported by the DoC server,
it MUST act as if the resource were not observable.
Whenever the DoC server receives a DNS Push message [RFC8765] from
the DNS infrastructure for an observed resource record, the DoC
server sends an appropriate Observe response to the DoC client.
If no more DoC clients observe a resource record for which the DoC
server has an open subscription, the DoC server MUST use a DNS
Unsubscribe message [RFC8765] to close its subscription to the
resource record as well.
5.2. OSCORE
It is RECOMMENDED to carry DNS messages end-to-end encrypted using
OSCORE [RFC8613]. The exchange of the security context is out of
scope of this document.
6. Considerations for Unencrypted Use
While not recommended, DoC can be used without any encryption (e.g.,
in very constrained environments where encryption is not possible or
necessary). It can also be used when lower layers provide secure
communication between client and server. In both cases, potential
benefits of unencrypted DoC usage over classic DNS are e.g. block-
wise transfer or alternative CoAP Content-Formats to overcome link-
layer constraints. For unencrypted DoC usage the ID field MUST not
be set to a fixed value as suggested in Section 4.2.2, but changed
with every query.
7. Security Considerations
When using unencrypted CoAP (see Section 6), setting the ID of a DNS
message to 0 as specified in Section 4.2.2 opens the DNS cache of a
DoC client to cache poisoning attacks via response spoofing. Because
of that, this documents requires the ID to be changed with every
query when CoAP is not secured (see Section 6).
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For encrypted usage with DTLS or OSCORE the impact of a fixed ID on
security is limited, as both harden against injecting spoofed
responses. Consequently, it is of little concern to leverage the
benefits of CoAP caching by setting the ID to 0.
TODO more security
8. IANA Considerations
8.1. New "application/dns-message" Content-Format
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 to the
"application/dns-message" media type from the "Media Types" registry:
Media-Type: application/dns-message
Encoding: -
Id: TBD
Reference: [TBD-this-spec]
8.2. New "core.dns" Resource Type
IANA is requested to assign a new Resource Type (rt=) Link Target
Attribute, "core.dns" in the "Resource Type (rt=) Link Target
Attribute Values" sub-registry, within the "CoRE Parameters" register
[RFC6690].
Attribute Value: core.dns
Description: DNS over CoAP resource.
Reference: [TBD-this-spec] Section 3
9. References
9.1. 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>.
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[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>.
[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>.
[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>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/rfc/rfc7641>.
[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>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/rfc/rfc8613>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/rfc/rfc9147>.
9.2. Informative References
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[I-D.ietf-add-dnr]
Boucadair, M., Reddy.K, T., Wing, D., Cook, N., and T.
Jensen, "DHCP and Router Advertisement Options for the
Discovery of Network-designated Resolvers (DNR)", Work in
Progress, Internet-Draft, draft-ietf-add-dnr-14, 13 March
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
add-dnr-14>.
[I-D.ietf-core-href]
Bormann, C. and H. Birkholz, "Constrained Resource
Identifiers", Work in Progress, Internet-Draft, draft-
ietf-core-href-12, 6 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
href-12>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/rfc/rfc3986>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/rfc/rfc6690>.
[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>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/rfc/rfc8499>.
[RFC8765] Pusateri, T. and S. Cheshire, "DNS Push Notifications",
RFC 8765, DOI 10.17487/RFC8765, June 2020,
<https://www.rfc-editor.org/rfc/rfc8765>.
[RFC9176] Amsüss, C., Ed., Shelby, Z., Koster, M., Bormann, C., and
P. van der Stok, "Constrained RESTful Environments (CoRE)
Resource Directory", RFC 9176, DOI 10.17487/RFC9176, April
2022, <https://www.rfc-editor.org/rfc/rfc9176>.
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[RFC9250] Huitema, C., Dickinson, S., and A. Mankin, "DNS over
Dedicated QUIC Connections", RFC 9250,
DOI 10.17487/RFC9250, May 2022,
<https://www.rfc-editor.org/rfc/rfc9250>.
Appendix A. Reference Implementations
The authors of this document provide two reference implementations, a
DoC client implementation available in the IoT operating system RIOT
(https://doc.riot-os.org/group__net__gcoap__dns.html) and a DoC
server implementation in Python (https://github.com/anr-bmbf-pivot/
aiodnsprox).
Appendix B. Change Log
B.1. Since draft-ietf-core-dns-over-coap-01
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-01)
* Specify DoC server role in terms of DNS terminology
* Clarify communication of DoC to DNS infrastructure is agnostic of
the transport
* Add subsection on how to implement DNS Push in DoC
* Add appendix on reference implementation
B.2. Since draft-ietf-core-dns-over-coap-00
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-00)
* SVGify ASCII art
* Move section on "DoC Server Considerations" (was Section 5.1) to
its own draft (draft-lenders-dns-cns
(https://datatracker.ietf.org/doc/draft-lenders-dns-cns/))
* Replace layer violating statement for CON with statement of fact
* Add security considerations on ID=0
B.3. Since draft-lenders-dns-over-coap-04
(https://datatracker.ietf.org/doc/html/draft-lenders-dns-over-
coap-04)
* Removed change log of draft-lenders-dns-over-coap
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Acknowledgments
The authors of this document want to thank Ben Schwartz and Tim
Wicinski for their feedback and comments.
Authors' Addresses
Martine Sophie Lenders
Freie Universität Berlin
Takustrasse 9
D-14195 Berlin
Germany
Email: m.lenders@fu-berlin.de
Christian Amsüss
Email: christian@amsuess.com
Cenk Gündoğan
Huawei Technologies
Riesstrasse 25
D-80992 Munich
Germany
Email: cenk.gundogan@huawei.com
Thomas C. Schmidt
HAW Hamburg
Berliner Tor 7
D-20099 Hamburg
Germany
Email: t.schmidt@haw-hamburg.de
Matthias Wählisch
Freie Universität Berlin
Takustrasse 9
D-14195 Berlin
Germany
Email: m.waehlisch@fu-berlin.de
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