Internet-Draft dnssec-bootstrapping April 2024
Thomassen & Wisiol Expires 13 October 2024 [Page]
Workgroup:
DNSOP Working Group
Internet-Draft:
draft-ietf-dnsop-dnssec-bootstrapping-08
Updates:
7344, 8078 (if approved)
Published:
Intended Status:
Standards Track
Expires:
Authors:
P. Thomassen
deSEC, Secure Systems Engineering
N. Wisiol
deSEC, Technische Universität Berlin

Automatic DNSSEC Bootstrapping using Authenticated Signals from the Zone's Operator

Abstract

This document introduces an in-band method for DNS operators to publish arbitrary information about the zones they are authoritative for, in an authenticated fashion and on a per-zone basis. The mechanism allows managed DNS operators to securely announce DNSSEC key parameters for zones under their management, including for zones that are not currently securely delegated.

Whenever DS records are absent for a zone's delegation, this signal enables the parent's registry or registrar to cryptographically validate the CDS/CDNSKEY records found at the child's apex. The parent can then provision DS records for the delegation without resorting to out-of-band validation or weaker types of cross-checks such as "Accept after Delay".

This document deprecates the DS enrollment methods described in Section 3 of RFC 8078 in favor of Section 4 of this document, and also updates RFC 7344.

[ Ed note: This document is being collaborated on at https://github.com/desec-io/draft-ietf-dnsop-dnssec-bootstrapping/. The authors gratefully accept pull requests. ]

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 13 October 2024.

1. Introduction

Securing a DNS delegation for the first time requires that the child's DNSSEC parameters be conveyed to the parent through some trusted channel. While the communication conceptually has to occur between the parent registry and the DNSSEC key holder, what exactly that means and how the communication is coordinated traditionally depends on the relationship the child has with the parent.

A typical situation is that the key is held by the child DNS operator; the communication thus often involves this entity. In addition, depending on the circumstances, it may also involve the Registrar, possibly via the Registrant (for details, see [RFC7344], Appendix A).

As observed in [RFC7344], these dependencies often result in a manual process that is susceptible to mistakes and/or errors. In addition, due to the annoyance factor of the process, involved parties may avoid the process of getting a DS record set published in the first place.

To alleviate these problems, automated provisioning of DS records has been specified in ([RFC8078]). It is based on the parental agent (registry or registrar) fetching DNSSEC key parameters from the CDS and CDNSKEY records ([RFC7344]) located at the child zone's apex, and validating them somehow. This validation can be done using the child's existing DNSSEC chain of trust if the objective is to update an existing DS record set (such as during key rollover). However, when bootstrapping a DNSSEC delegation, the child zone has no existing DNSSEC validation path, and other means to ensure the CDS/CDNSKEY records' legitimacy must be found.

Due to the lack of a comprehensive DNS-innate solution, either out-of-band methods have been used so far to complete the chain of trust, or cryptographic validation has been entirely dispensed with, in exchange for weaker types of cross-checks such as "Accept after Delay" ([RFC8078] Section 3.3). [RFC8078] does not define an in-band validation method for enabling DNSSEC.

This document aims to close this gap by introducing an in-band method for DNS operators to publish arbitrary information about the zones they are authoritative for, in an authenticated manner and on a per-zone basis. The mechanism allows managed DNS operators to securely announce DNSSEC key parameters for zones under their management. The parent can then use this signal to cryptographically validate the CDS/CDNSKEY records found at an insecure child zone's apex and, upon success, secure the delegation.

While applicable to the vast majority of domains, the protocol does not support certain edge cases, such as excessively long child zone names, or DNSSEC bootstrapping for domains with in-bailick nameservers only (see Section 4.4).

DNSSEC bootstrapping is just one application of the generic signaling mechanism specified in this document. Other applications might arise in the future, such as publishing operational metadata or auxiliary information which the DNS operator likes to make known (e.g., API endpoints for third-party interaction).

Readers are expected to be familiar with DNSSEC, including [RFC4033], [RFC4034], [RFC4035], [RFC6781], [RFC7344], and [RFC8078].

1.1. Terminology

This section defines the terminology used in this document.

CDS/CDNSKEY
This notation refers to CDS and/or CDNSKEY, i.e., one or both.
Child
see [RFC8499] Section 7
Child DNS operator
The entity that maintains and publishes the zone information for the child DNS.
Parent
see [RFC8499] Section 7
Parental agent
The entity that has the authority to insert DS records into the parent zone on behalf of the child. (It could be the registry, registrar, a reseller, or some other authorized entity.)
Signaling domain
A hostname from the child's NS record set, prefixed with the label _signal. There are as many signaling domains as there are distinct NS targets.
Signaling name
The labels that are prefixed to a signaling domain in order to identify a signaling type and a child zone's name (see Section 3.2).
Signaling record
A DNS record located at a signaling name under a signaling domain. Signaling records are used by the child DNS operator to publish information about the child.
Signaling type
A signal type identifier, such as _dsboot for DNSSEC bootstrapping.
Signaling zone
The zone which is authoritative for a given signaling record.

1.2. Requirements Notation

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.

2. Updates to RFCs

The DS enrollment methods described in Section 3 of [RFC8078] are deprecated and SHOULD NOT be used. Child DNS operators and parental agents who wish to use CDS/CDNSKEY records for initial DS enrollment SHOULD instead support the authentication protocol described in Section 4 of this document.

In order to facilitate publication of signaling records for the purpose of DNSSEC bootstrapping (see Section 4.1), the first bullet ("Location") of [RFC7344] Section 4.1 is removed.

3. Signaling

This section describes the general mechanism by which a child DNS operator can publish an authenticated signal about a child zone. Parental agents (or any other party) can then discover and process the signal. Authenticity is ensured through standard DNSSEC validation.

3.1. Chain of Trust

If a child DNS operator implements the protocol, each signaling zone MUST be signed and be validatable by the parental agent (i.e. have a valid DNSSEC chain of trust). This is typically achieved by securely delegating each signaling zone.

For example, when publishing a signal that relates to a child zone with NS records ns1.example.net and ns2.example.org, the child DNS operator needs to ensure that the parental agent has a valid DNSSEC chain of trust for the zone(s) that are authoritative for the signaling domains _signal.ns1.example.net and _signal.ns2.example.org.

3.2. Signaling Names

To publish information about the child zone in an authenticated fashion, the child DNS operator MUST publish one or more signaling records at a signaling name under each signaling domain.

Signaling records MUST be accompanied by RRSIG records created with the corresponding signaling zone's key(s). The type and contents of these signaling records depend on the type of signal.

The signaling name identifies the child and the signaling type. It is identical to the child name (with the final root label removed), prefixed with a label containing the signaling type.

4. Bootstrapping a DNSSEC Delegation

When the child zone's CDS/CDNSKEY RRsets are used for setting up initial trust, they need to be authenticated. This is achieved by co-publishing the child's CDS/CDNSKEY records as an authenticated signal as described in Section 3. The parent can discover and validate it, thus transferring trust from the child DNS operator nameservers' chain of trust to the child zone.

This protocol is not intended for updating an existing DS RRset. For this purpose, the parental agent can validate the child's CDS/CDNSKEY records directly, using the chain of trust established by the existing DS RRset ([RFC7344] Section 4).

To confirm its willingness to act as the child's delegated signer and authenticate the child's CDS/CDNSKEY RRsets, the child DNS operator MUST co-publish them at the corresponding signaling name under each out-of-bailiwick signaling domain (Section 3.2). For simplicity, the child DNS operator MAY also co-publish the child's CDS/CDNSKEY RRsets under signaling domains that are in bailiwick, although those signaling domains are not used for validation (Section 4.2).

Unlike the CDS/CDNSKEY records at the child's apex, signaling records MUST be signed with the corresponding signaling zone's key(s). Their contents MUST be identical to the corresponding records published at the child's apex.

Existing use of CDS/CDNSKEY records was specified at the child apex only ([RFC7344], Section 4.1). This protocol extends the use of these record types to non-apex owner names for the purpose of DNSSEC bootstrapping. To exclude the possibility of semantic collision, there MUST NOT be a zone cut at a signaling name.

To avoid relying on the benevolence of a single signaling domain parent (such as the corresponding TLD registry), it is RECOMMENDED to diversify the path from the root to the child's nameserver hostnames. This is best achieved by using different and independently operated TLDs for each one.

4.1.1. Example

For the purposes of bootstrapping the child zone example.co.uk with NS records ns1.example.net, ns2.example.org, and ns3.example.co.uk, the required signaling domains are _signal.ns1.example.net and _signal.ns2.example.org.

In the zones containing these domains, the child DNS operator authenticates the CDS/CDNSKEY records found at the child's apex by co-publishing them at the names:

_dsboot.example.co.uk._signal.ns1.example.net
_dsboot.example.co.uk._signal.ns2.example.org

The records are accompanied by RRSIG records created using the key(s) of the respective signaling zone.

Publication of signaling records under the in-bailiwick domain _signal.ns3.example.co.uk is not required.

4.2. Validating CDS/CDNSKEY Records for DNSSEC Bootstrapping

To validate a child's CDS/CDNSKEY RRset for DNSSEC bootstrapping, the parental agent, knowing both the child zone name and its NS hostnames, MUST execute the following steps:

  1. verify that the child is not currently securely delegated and that at least one of its nameservers is out of bailiwick;

  2. query the CDS/CDNSKEY records at the child zone apex directly from each of the authoritative servers as determined by the delegation's NS record set (without caching);

  3. query the CDS/CDNSKEY records located at the signaling name under each out-of-bailiwick signaling domain using a trusted DNS resolver and enforce DNSSEC validation;

  4. check (separately by record type) that all record sets retrieved in Steps 2 and 3 have equal contents;

If the above steps succeed without error, the CDS/CDNSKEY records are successfully validated, and the parental agent can proceed with the publication of the DS record set under the precautions described in [RFC8078], Section 5.

However, the parental agent MUST abort the procedure if an error condition occurs, in particular:

  • in Step 1: the child is already securely delegated or has in-bailiwick nameservers only;

  • in Step 2: any failure during the retrieval of the CDS/CDNSKEY records located at the child apex from any of the authoritative nameservers;

  • in Step 3: any failure to retrieve the CDS/CDNSKEY RRsets located at the signaling name under any signaling domain, including failure of DNSSEC validation, or unauthenticated data (AD bit not set);

  • in Step 4: inconsistent responses (for at least one of the types), including a record set that is empty in one of Steps 2 or 3, but non-empty in the other.

4.2.1. Example

To verify the CDS/CDNSKEY records for the child example.co.uk, the parental agent (assuming that the child delegation's NS records are ns1.example.net, ns2.example.org, and ns3.example.co.uk)

  1. checks that the child domain is not yet securely delegated;

  2. queries CDS/CDNSKEY records for example.co.uk directly from ns1.example.net, ns2.example.org, and ns3.example.co.uk (without caching);

  3. queries and validates the CDS/CDNSKEY records located at (see Section 3.2; ns3.example.co.uk is ignored because it is in bailiwick)

_dsboot.example.co.uk._signal.ns1.example.net
_dsboot.example.co.uk._signal.ns2.example.org
  1. checks that the CDS/CDNSKEY record sets retrieved in Steps 2 and 3 agree across responses.

If all these steps succeed, the parental agent can proceed to publish a DS record set as indicated by the validated CDS/CDNSKEY records.

As in-bailiwick signaling names do not have a chain of trust at bootstrapping time, the parental agent does not consider them during validation. Consequently, if all NS hostnames are in bailiwick, validation cannot be completed, and DS records are not published.

4.3. Triggers

Parental agents SHOULD trigger the procedure described in Section 4.2 once one of the following conditions is fulfilled:

  • The parental agent receives a new or updated NS record set for a child;

  • The parental agent receives a notification indicating that the child wishes to have its CDS/CDNSKEY RRset processed;

  • The parental agent encounters a signaling record during a proactive, opportunistic scan (e.g. daily queries of signaling records for some or all of its delegations);

  • The parental agent encounters a signaling record during an NSEC walk or when parsing a signaling zone (e.g. when made available via AXFR by the child DNS operator);

  • Any other condition as deemed appropriate by local policy.

Timer-based trigger mechanisms (such as scans) exhibit undesirable properties with respect to processing delay and scaling; on-demand triggers (like notifications) are preferable. Whenever possible, child DNS operators and parental agents are thus encouraged to use them, reducing both delays and the amount of scanning traffic.

Most types of discovery (such as daily scans of delegations) are based directly on the delegation's NS record set. In this case, these NS names can be used as is by the bootstrapping algorithm (Section 4.2) for querying signaling records.

Some discovery methods, however, do not imply reliable knowledge of the child's NS record set. For example, when discovering signaling names by performing an NSEC walk or zone transfer of a signaling zone, the parental agent MUST NOT assume that the nameserver(s) under whose signaling domain(s) a signaling name appears is actually authoritative for the corresponding child.

Instead, whenever a list of "bootstrappable domains" is obtained other than directly from the parent, the parental agent MUST ascertain that the child's delegation actually contains the nameserver hostname seen during discovery, and ensure that signaling record queries are only made against the proper set of nameservers as listed in the child's delegation from the parent.

4.4. Limitations

As a consequence of Step 3 in Section 4.2, DS bootstrapping does not work for fully in-bailiwick delegations, as no pre-existing chain of trust to the child domain is available during bootstrapping. (As a workaround, one can add an out-of-bailiwick nameserver to the initial NS record set and remove it once bootstrapping is completed. Automation for this is available via CSYNC records, see [RFC7477].)

Fully qualified signaling names must by valid DNS names. Label count and length requirements for DNS names imply that the protocol does not work for unusually long child domain names or NS hostnames.

5. Operational Recommendations

5.1. Child DNS Operator

CDS/CDNSKEY records and corresponding signaling records MUST NOT be published without the zone owner's consent. Likewise, the child DNS operator MUST enable the zone owner to signal the desire to turn off DNSSEC by publication of the special-value CDS/CDNSKEY RRset specified in [RFC8078] Section 4. To facilitate transitions between DNS operators, child DNS operators SHOULD support the multi-signer protocols described in [RFC8901].

Signaling domains SHOULD be delegated as standalone zones, so that the signaling zone's apex coincides with the signaling domain (such as _signal.ns1.example.net). While it is permissible for the signaling domain to be contained in a signaling zone of fewer labels (such as example.net), a zone cut ensures that bootstrapping activities do not require modifications of the zone containing the nameserver hostname.

Once a Child DNS Operator determines that specific signaling records have been processed (e.g., by seeing the result in the parent zone), they are advised to remove them. This will reduce the size of the Signaling Zone, and facilitate more efficient bulk processing (such as via zone transfers).

5.2. Parental Agent

In order to ensure timely DNSSEC bootstrapping of insecure domains, stalemate situations due to mismatch of cached records (Step 4 of Section 4.2) need to be avoided. It is thus RECOMMENDED to perform queries into signaling domains with an (initially) cold resolver cache, or to disable caching for them (e.g., by limiting response TTLs to the interval between scans).

6. Security Considerations

The DNSSEC bootstrapping method introduced in this document is based on the (now deprecated) approaches described in [RFC8078] Section 3, but adds authentication to the CDS/CDNSKEY concept. Its security level is therefore strictly higher than that of existing approaches described in that document (e.g. "Accept after Delay"). Apart from this general improvement, the same Security Considerations apply as in [RFC8078].

The level of rigor in Section 4.2 is needed to prevent publication of a half-baked DS RRset (authorized only under a subset of NS hostnames). This ensures, for example, that an operator in a multi-homed setup cannot enable DNSSEC unless all other operators agree.

In any case, as the child DNS operator has authoritative knowledge of the child's CDS/CDNSKEY records, it can readily detect fraudulent provisioning of DS records.

In order to prevent the TLD of nameserver hostnames from becoming a single point of failure for a delegation (both in terms of resolution availability and for the trust model of this protocol), it is advisable to use NS hostnames that are independent from each other with respect to their TLD.

7. IANA Considerations

Per [RFC8552], IANA is requested to add the following entries to the "Underscored and Globally Scoped DNS Node Names" registry:

+---------+------------+-----------------------------------------+
| RR Type | _NODE NAME | Reference                               |
+---------+------------+-----------------------------------------+
| CDS     | _signal    | [draft-ietf-dnsop-dnssec-bootstrapping] |
| CDNSKEY | _signal    | [draft-ietf-dnsop-dnssec-bootstrapping] |
+---------+------------+-----------------------------------------+

8. Implementation Status

Note to the RFC Editor: please remove this entire section before publication.

In addition to the information in this section, deployment is tracked by the community at https://github.com/oskar456/cds-updates.

8.1. Child DNS Operator-side

  • Operator support:

    • Cloudflare has implemented bootstrapping record synthesis for all signed customer zones.
    • Glauca HexDNS publishes bootstrapping records for its customer zones.
    • deSEC performs bootstrapping record synthesis for its zones using names _signal.ns1.desec.io and _signal.ns2.desec.org.

  • Authoritative nameserver support:

8.2. Parental Agent-side

  • ccTLD:

    • SWITCH (.ch, .li) has implemented authentication of consumed CDS records based on this draft.
    • .cl is working on an implementation.

  • gTLD:

    • Knipp has implemented consumption of DNSSEC bootstrapping records in its TANGO and CORE registry systems.
    • A deployment of this is running at .swiss.

  • Registrars:

    • Glauca has implemented authenticated CDS processing.
    • GoDaddy is working on an implementation.

  • A tool to retrieve and process signaling records for bootstrapping purposes, either directly or via zone walking, is available at https://github.com/desec-io/dsbootstrap. The tool outputs the validated DS records which then can be added to the parent zone.

9. Acknowledgements

Thanks to Brian Dickson, Ondřej Caletka, John R. Levine, Christian Elmerot, Oli Schacher, Donald Eastlake, Libor Peltan, Warren Kumari, Scott Rose, Linda Dunbar, Tim Wicinski, Paul Wouters, Paul Hoffman for reviewing draft proposals and offering comments and suggestions.

Thanks also to Steve Crocker, Hugo Salgado, and Ulrich Wisser for early-stage brainstorming.

10. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC4033]
Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, DOI 10.17487/RFC4033, , <https://www.rfc-editor.org/info/rfc4033>.
[RFC4034]
Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, DOI 10.17487/RFC4034, , <https://www.rfc-editor.org/info/rfc4034>.
[RFC4035]
Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, DOI 10.17487/RFC4035, , <https://www.rfc-editor.org/info/rfc4035>.
[RFC7344]
Kumari, W., Gudmundsson, O., and G. Barwood, "Automating DNSSEC Delegation Trust Maintenance", RFC 7344, DOI 10.17487/RFC7344, , <https://www.rfc-editor.org/info/rfc7344>.
[RFC7477]
Hardaker, W., "Child-to-Parent Synchronization in DNS", RFC 7477, DOI 10.17487/RFC7477, , <https://www.rfc-editor.org/info/rfc7477>.
[RFC8078]
Gudmundsson, O. and P. Wouters, "Managing DS Records from the Parent via CDS/CDNSKEY", RFC 8078, DOI 10.17487/RFC8078, , <https://www.rfc-editor.org/info/rfc8078>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC8499]
Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS Terminology", RFC 8499, DOI 10.17487/RFC8499, , <https://www.rfc-editor.org/info/rfc8499>.
[RFC8552]
Crocker, D., "Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute Leaves", BCP 222, RFC 8552, DOI 10.17487/RFC8552, , <https://www.rfc-editor.org/info/rfc8552>.

11. Informative References

[RFC6781]
Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC Operational Practices, Version 2", RFC 6781, DOI 10.17487/RFC6781, , <https://www.rfc-editor.org/info/rfc6781>.
[RFC8901]
Huque, S., Aras, P., Dickinson, J., Vcelak, J., and D. Blacka, "Multi-Signer DNSSEC Models", RFC 8901, DOI 10.17487/RFC8901, , <https://www.rfc-editor.org/info/rfc8901>.

Appendix A. Change History (to be removed before publication)

  • draft-ietf-dnsop-dnssec-bootstrapping-08

Editorial changes from AD Review

Updated implementation section

Change capitalization of terms from terminology section

  • draft-ietf-dnsop-dnssec-bootstrapping-07

Add Glauca registrar implementation

Editorial changes to Security Considerations

Add/discuss on-demand triggers (notifications)

  • draft-ietf-dnsop-dnssec-bootstrapping-06

Add section "Updates to RFCs"

Editorial nits

Editorial changes from Secdir early review

  • draft-ietf-dnsop-dnssec-bootstrapping-05

Editorial changes

  • draft-ietf-dnsop-dnssec-bootstrapping-04

Added consent considerations.

Editorial changes.

  • draft-ietf-dnsop-dnssec-bootstrapping-03

Updated Implementation section.

Typo fix.

  • draft-ietf-dnsop-dnssec-bootstrapping-02

Clarified that RFC 8078 Section 3 is not replaced, but its methods are deprecated.

Added new deployments to Implementation section.

Included NSEC walk / AXFR as possible triggers for DS bootstrapping.

Editorial changes.

  • draft-ietf-dnsop-dnssec-bootstrapping-01

Allow bootstrapping when some (not all) NS hostnames are in bailiwick.

Clarified Operational Recommendations according to operator feedback.

Turn loose Security Considerations points into coherent text.

Do no longer suggest NSEC-walking Signaling Domains. (It does not work well due to the Signaling Type prefix. What's more, it's unclear who would do this: Parents know there delegations and can do a targeted scan; others are not interested.)

Editorial changes.

Added IANA request.

Introduced Signaling Type prefix (_dsboot), renamed Signaling Name infix from _dsauth to _signal.

  • draft-ietf-dnsop-dnssec-bootstrapping-00

Editorial changes.

  • draft-thomassen-dnsop-dnssec-bootstrapping-03

Clarified importance of record cleanup by moving paragraph up.

Pointed out limitations.

Replace [RFC8078] Section 3 with our Section 4.2.

Changed _boot label to _dsauth.

Removed hashing of Child name components in Signaling Names.

Editorial changes.

  • draft-thomassen-dnsop-dnssec-bootstrapping-02

Reframed as an authentication mechanism for RFC 8078.

Removed multi-signer use case (focus on RFC 8078 authentication).

Triggers need to fetch NS records (if not implicit from context).

Improved title.

Recognized that hash collisions are dealt with by Child apex check.

  • draft-thomassen-dnsop-dnssec-bootstrapping-01

Add section on Triggers.

Clarified title.

Improved abstract.

Require CDS/CDNSKEY records at the Child.

Reworked Signaling Name scheme.

Recommend using cold cache for consumption.

Updated terminology (replace "Bootstrapping" by "Signaling").

Added NSEC recommendation for Bootstrapping Zones.

Added multi-signer use case.

Editorial changes.

  • draft-thomassen-dnsop-dnssec-bootstrapping-00

Initial public draft.

Authors' Addresses

Peter Thomassen
deSEC, Secure Systems Engineering
Berlin
Germany
Nils Wisiol
deSEC, Technische Universität Berlin
Berlin
Germany