ACE Working Group M. Tiloca Internet-Draft RISE SICS AB Intended status: Standards Track J. Park Expires: May 1, 2018 Universitaet Duisburg-Essen October 28, 2017 Joining of OSCORE multicast groups in ACE draft-tiloca-ace-oscoap-joining-01 Abstract This document describes a method to join a multicast group where communications are based on CoAP and secured with Object Security for Constrained RESTful Environments (OSCORE). The proposed method delegates the authentication and authorization of client nodes that join an OSCORE multicast group through a Group Manager server. This approach builds on the ACE framework for Authentication and Authorization, and leverages protocol-specific profiles of ACE to achieve communication security, proof-of-possession and server authentication. 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 http://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 May 1, 2018. Copyright Notice Copyright (c) 2017 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents Tiloca & Park Expires May 1, 2018 [Page 1] Internet-Draft OSCORE group joining in ACE October 2017 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 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4 3. Joining Node to Authorization Server . . . . . . . . . . . . 6 4. Joining Node to Group Manager . . . . . . . . . . . . . . . . 7 5. Public Keys of Joining Nodes . . . . . . . . . . . . . . . . 8 6. Updating Authorization Information . . . . . . . . . . . . . 9 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 10.1. Normative References . . . . . . . . . . . . . . . . . . 11 10.2. Informative References . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 1. Introduction Object Security for Constrained RESTful Environments (OSCORE) [I-D.ietf-core-object-security] is a method for application layer protection of CoAP messages, using the CBOR Object Signing and Encryption (COSE) [RFC8152], and enabling end-to-end security of CoAP payload and options. OSCORE may also be used to protect group communication for CoAP over IP multicast, as described in [I-D.tiloca-core-multicast-oscoap]. This relies on a Group Manager entity, which is responsible for managing a multicast group where members exchange CoAP messages secured with OSCORE. In particular, the Group Manager coordinates the join process of new group members and can be responsible for multiple groups. This document builds on the ACE framework for Authentication and Authorization [I-D.ietf-ace-oauth-authz] and specifies how a client joins an OSCORE multicast group through a resource server acting as Group Manager. The client acting as joining node relies on an Access Token, which is bound to a proof-of-possession key and authorizes the access to a specific join resource at the Group Manager. In order to achieve communication security, proof-of-possession and server authentication, the client and the Group Manager leverage Tiloca & Park Expires May 1, 2018 [Page 2] Internet-Draft OSCORE group joining in ACE October 2017 protocol-specific profiles of ACE such as the CoAP-DTLS profile [I-D.ietf-ace-dtls-authorize], the OSCOAP profile [I-D.seitz-ace-oscoap-profile], or the IPsec profile [I-D.aragon-ace-ipsec-profile]. 1.1. Terminology 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. Readers are expected to be familiar with the terms and concepts described in the ACE framework for authentication and authorization [I-D.ietf-ace-oauth-authz]. Message exchanges are presented as RESTful protocol interactions, for which HTTP [RFC7231] provides useful terminology. The terminology for entities in the considered architecture is defined in OAuth 2.0 [RFC6749] and [I-D.ietf-ace-actors]. In particular, this includes Client (C), Resource Server (RS), and Authorization Server (AS). Terminology for constrained environments, such as "constrained device" and "constrained-node network", is defined in [RFC7228]. Readers are expected to be familiar with the terms and concepts related to the CoAP protocol described in [RFC7252][RFC7390]. Note that the term "endpoint" is used here following its OAuth definition, aimed at denoting resources such as /token and /introspect at the AS and /authz-info at the RS. This document does not use the CoAP definition of "endpoint", which is "An entity participating in the CoAP protocol". Readers are expected to be familiar with the terms and concepts related to the DTLS protocol [RFC6347] and with the CoAP-DTLS profile of ACE [I-D.ietf-ace-dtls-authorize]. Readers are expected to be familiar with the terms and concepts for protection and processing of CoAP messages through OSCORE [I-D.ietf-core-object-security] also in group communication contexts [I-D.tiloca-core-multicast-oscoap]; and with the OSCOAP profile of ACE [I-D.seitz-ace-oscoap-profile]. Readers are expected to be familiar with the terms and concepts related to the IPsec protocol suite [RFC4301]; and with the IPsec profile of ACE [I-D.aragon-ace-ipsec-profile]. Tiloca & Park Expires May 1, 2018 [Page 3] Internet-Draft OSCORE group joining in ACE October 2017 This document refers also to the following terminology. o Joining node: a network node intending to join an OSCORE multicast group, where communication is based on CoAP [RFC7390] and secured with OSCORE as described in [I-D.tiloca-core-multicast-oscoap]. o Join process: the process through which a joining node becomes a member of an OSCORE multicast group. The join process is enforced and assisted by the Group Manager responsible for that group. o Join resource: a resource hosted by the Group Manager, associated to an OSCORE multicast group under that Group Manager. A joining node accesses the join resource in order to start the join process and become a member of that group. o Join endpoint: an endpoint at the Group Manager associated to a join resource. 2. Protocol Overview Group communication for CoAP over IP multicast has been enabled in [RFC7390] and can be secured with Object Security for Constrained RESTful Environments (OSCORE) [I-D.ietf-core-object-security] as described in [I-D.tiloca-core-multicast-oscoap]. A network node explicitly joins an OSCORE multicast group, by interacting with the responsible Group Manager. Once registered in the group, the new node can securely exchange (multicast) messages with other group members. This specification describes how a network node joins an OSCORE multicast group leveraging the ACE framework for authentication and authorization [I-D.ietf-ace-oauth-authz]. With reference to the ACE framework and the terminology defined in OAuth 2.0 [RFC6749]: o The Group Manager acts as Resource Server (RS), and hosts one join resource for each OSCORE multicast group it manages. Each join resource is exported by a distinct join endpoint. o The joining node acts as Client (C), and requests to join an OSCORE multicast group by accessing the related join endpoint at the Group Manager. o The Authorization Server (AS) enables and enforces the authorized access of joining nodes to join endpoints at the Group Manager. Multiple Group Managers can be associated to the same AS. If the joining node is authorized to join the multicast group, it receives from the AS an Access Token bound with a proof-of-possession Tiloca & Park Expires May 1, 2018 [Page 4] Internet-Draft OSCORE group joining in ACE October 2017 key. After that, the joining node provides the Group Manager with the Access Token. This step involves the opening of a secure communication channel between the joining node and the Group Manager, in case they have not already established one. Finally, the joining node accesses the join endpoint at the Group Manager, so starting the join process to become a member of the OSCORE multicast group. A same Access Token can authorize the joining node to access multiple groups under the same Group Manager. In such a case, the joining node sequentially performs multiple join processes with the Group Manager, separately for each multicast group to join and by accessing the respective join endpoint. The AS is not necessarily expected to release Access Tokens for any other purpose than accessing join resources on registered Group Managers. However, the AS may be configured also to release Access Tokens for accessing resources at members of multicast groups. The following steps are performed for joining an OSCORE multicast group, by leveraging one of the available profiles of ACE, such as the CoAP-DTLS profile [I-D.ietf-ace-dtls-authorize], the OSCOAP profile [I-D.seitz-ace-oscoap-profile], or the IPsec profile [I-D.aragon-ace-ipsec-profile]. 1. The joining node retrieves an Access Token from the AS to access a join resource on the Group Manager (see Section 3). The response from the AS enables the joining node to start a secure channel with the Group Manager, if not already established. The joining node can also contact the AS for updating a previously released Access Token, in order to access further groups under the same Group Manager (see Section 6). 2. Authentication and authorization information is transferred between the joining node and the Group Manager, which establish a secure channel in case one is not already set up (see Section 4). That is, a joining node MUST establish a secure communication channel with a Group Manager, before joining an OSCORE multicast group under that Group Manager for the first time. 3. The joining node starts the join process to become a member of the OSCORE multicast group, by accessing the related join resource hosted by the Group Manager (see Section 4). All communications between the involved entities rely on the CoAP protocol and MUST be secured. In particular, communications between the joining node and the AS (/token endpoint) and between the Group Manager and the AS (/introspection endpoint) can be secured by different means, for instance by means of DTLS [RFC6347], OSCORE (see Tiloca & Park Expires May 1, 2018 [Page 5] Internet-Draft OSCORE group joining in ACE October 2017 Sections 2.3 and 3 of [I-D.seitz-ace-oscoap-profile]), or IPsec (see Sections 3.2 and 3.4 of [I-D.aragon-ace-ipsec-profile]). Further details on how the AS secures communications (with the joining node and the Group Manager) depend on the specifically used profile of ACE, and are out of the scope of this specification. 3. Joining Node to Authorization Server This section considers a joining node that intends to contact the Group Manager for the first time. That is, the joining node has never attempted before to join an OSCORE multicast group under that Group Manager. Also, the joining node and the Group Manager do not have a secure communication channel established. In case the specific AS associated to the Group Manager is unknown to the joining node, the latter can rely on mechanisms like the Unauthorized Resource Request message described in Section 2.1 of [I-D.ietf-ace-dtls-authorize] to discover the correct AS in charge of the Group Manager. As an alternative, the joining node may look up in a Resource Directory service [I-D.ietf-core-resource-directory]. The joining node contacts the AS, in order to request an Access Token for accessing the join resource(s) hosted by the Group Manager. In particular, the Access Token request sent to the /token endpoint specifies the join endpoint(s) of interest at the Group Manager. The AS is responsible for authorizing the joining node, accordingly to group join policies enforced on behalf of the Group Manager. In case of successful authorization, the AS releases an Access Token bound to a proof-of-possession key associated to the joining node. The same Access Token can authorize the joining node to access multiple groups under the same Group Manager. Then, the AS provides the joining node with the Access Token, together with an Access Token response. In particular, the Access Token response indicates how to secure communications with the Group Manager, when accessing the join resource(s) for which the Access Token is valid. Specifically, the Access Token response MUST specify one of the following alternatives: o CoAP over DTLS, i.e. coaps://, indicating to consider the CoAP- DTLS profile of ACE, with asymmetric or symmetric proof-of- possession key (see Section 3 and Section 4 of [I-D.ietf-ace-dtls-authorize], respectively). o OSCOAP, indicating to consider the OSCOAP profile of ACE with the symmetric proof-of-possession key used directly as Master Secret Tiloca & Park Expires May 1, 2018 [Page 6] Internet-Draft OSCORE group joining in ACE October 2017 in OSCORE [I-D.ietf-core-object-security], as described in Section 2 of [I-D.seitz-ace-oscoap-profile]. o IPsec, indicating to consider the IPsec profile of ACE, with symmetric or asymmetric proof-of-possession key (see Section 3.2.2 and Section 3.2.3 of [I-D.aragon-ace-ipsec-profile], respectively). Consistently with the profiles of ACE [I-D.ietf-ace-dtls-authorize][I -D.seitz-ace-oscoap-profile][I-D.aragon-ace-ipsec-profile], a symmetric proof-of-possession key is generated by the AS, which uses it as proof-of-possession key bound to the Access Token, and provides it to the joining node in the Access Token response. Instead, consistently with the profiles of ACE [I-D.ietf-ace-dtls-authorize][I-D.aragon-ace-ipsec-profile], in case of asymmetric proof-of-possession key, the joining node provides its own public key to the AS in the Access Token request. Then, the AS uses it as proof-of-possession key bound to the Access Token, and provides the joining node with the Group Manager's public key in the Access Token response. 4. Joining Node to Group Manager First, the joining node establishes a secure channel with the Group Manager, according to what is specified in the Access Token response. In particular: o If the CoAP-DTLS profile of ACE is specified, the joining node MUST upload the Access Token to the /authz-info resource, before starting the DTLS handshake and establishing a DTLS channel with the Group Manager. Then, the Group Manager processes the Access Token according to [I-D.ietf-ace-oauth-authz]. If this yields to a positive response, the joining node and the Group Manager establish a DTLS session, as described in Section 3 and Section 4 of [I-D.ietf-ace-dtls-authorize], in case of either asymmetric or symmetric proof-of-possession key, respectively. o If the OSCOAP profile of ACE is specified, the joining node and the Group Manager establish an OSCORE Security Context, as described in Section 2.2 of [I-D.seitz-ace-oscoap-profile]. The Group Manager processes the Access Token as specified in [I-D.ietf-ace-oauth-authz] and proceeds as defined in Section 2.2 of [I-D.seitz-ace-oscoap-profile]. o If the IPsec profile of ACE is specified, the joining node MUST upload the Access Token to the /authz-info resource, before performing the key management protocol indicated by the AS (e.g. Tiloca & Park Expires May 1, 2018 [Page 7] Internet-Draft OSCORE group joining in ACE October 2017 IKEv2 [RFC7296]) to establish an IPsec Security Association pair and an IPsec channel with the Group Manager. Then, the Group Manager processes the Access Token according to [I-D.ietf-ace-oauth-authz]. If this yields to a positive response, the joining node and the Group Manager establish an IPsec Security Association pair and an IPsec channel, as described in Section 3.3.2 of [I-D.aragon-ace-ipsec-profile]. Once a secure communication channel with the Group Manager has been established, the joining node requests to join the OSCORE multicast groups of interest, by accessing the related join resources at the Group Manager. That is, the joining node performs multiple join processes with the Group Manager, separately for each multicast group to join and by accessing the respective join endpoint. In particular, for each OSCORE multicast group to join, the joining node sends to the Group Manager a confirmable CoAP request, using the method POST and targeting the join endpoint associated to that group. The request payload conveys the information specified in Appendix C.1 of [I-D.tiloca-core-multicast-oscoap], which includes the intended role(s) of the joining node in the multicast group, i.e. multicaster and/or (pure) listener. The Group Manager processes the request according to [I-D.ietf-ace-oauth-authz]. If this yields to a positive response, the Group Manager updates the group membership by registering the joining node as a new member of the group. Then, the Group Manager replies to the joining node providing the information specified in Appendix C.1 of [I-D.tiloca-core-multicast-oscoap], which includes the OSCORE Security Common Context associated to the joined multicast group. From then on, the joining node is registered as a member of the multicast group, and can exchange group messages secured with OSCORE as described in Section 5 of [I-D.tiloca-core-multicast-oscoap]. 5. Public Keys of Joining Nodes Source authentication of OSCORE messages exchanged within the multicast group is ensured by means of digital counter signatures [I-D.tiloca-core-multicast-oscoap]. Therefore, group members must be able to retrieve each other's public key from a trusted key repository, in order to verify the source authenticity of incoming group messages. Upon joining a multicast group, a joining node is expected to make its own public key available to the other group members, either through the Group Manager or through another trusted, publicly Tiloca & Park Expires May 1, 2018 [Page 8] Internet-Draft OSCORE group joining in ACE October 2017 available, key repository. However, this is not required for a node that joins a group exclusively as pure listener. As also discussed in Section 3 of [I-D.tiloca-core-multicast-oscoap], it is recommended that the Group Manager is configured to store the public keys of the group members and to provide them upon request. If so, two cases can occur. o The joining node and the Group Manager have used an asymmetric proof-of-possession key to establish a secure communication channel. In this case, the Group Manager stores the proof-of- possession key conveyed in the Access Token as the public key of the joining node. o The joining node and the Group Manager have used a symmetric proof-of-possession key to establish a secure communication channel. In this case, upon performing a join process with that Group Manager for the first time, the joining node includes its own public key in the "Identity credentials" of the POST request targeting the join endpoint (see Appendix C.1 of [I-D.tiloca-core-multicast-oscoap]). Then, the Group Manager MUST verify that the joining node actually owns the associated private key, for instance by performing a proof-of-possession challenge- response. Then, if the joining node has explicitly requested it, the Group Manager provides also the public keys of the current members in the joined group, when replying to the joining node during the same join process (see Appendix C.1 of [I-D.tiloca-core-multicast-oscoap]). Instead, in case the Group Manager is not configured to store public keys of group members, the joining node provides the Group Manager with its own certificate and with the identifier of the Certification Authority that issued that certificate (see Appendix C.2 of [I-D.tiloca-core-multicast-oscoap]). 6. Updating Authorization Information At any point in time, a node might want to join further OSCORE multicast groups under the same Group Manager. In such a case, the joining node requests from the AS an updated Access Token for accessing the new OSCORE multicast groups of interest. The joining node uploads the new Access Token to the /authz-info resource at the Group Manager, using the already established secure communication channel. After that, the joining node performs the joining process described in Section 4, separately for each OSCORE multicast group to join. Tiloca & Park Expires May 1, 2018 [Page 9] Internet-Draft OSCORE group joining in ACE October 2017 Since the joining node and the Group Manager already share a secure communication channel, they are not required to establish a new one. However, according to the specific profile of ACE in use, the joining node and the Group Manager may leverage the new Access Token to establish a new secure communication channel or update the currently existing one. For instance, Section 4.2 of [I-D.ietf-ace-dtls-authorize] describes how the new Access Token can be used to renegotiate an existing DTLS session or to establish a new one by performing a new DTLS handshake. 7. Security Considerations The method described in this document leverages the following management aspects related to OSCORE multicast groups and discussed in the sections of [I-D.tiloca-core-multicast-oscoap] indicated below. o Management of group keying material (Section 3.1). This includes the need to revoke and renew the keying material currently used in the OSCORE multicast group, upon changes in the group membership. In particular, renewing the keying material is required upon a new node joining the multicast group, in order to preserve backward security. The Group Manager is responsible to enforce rekeying policies and accordingly update the keying material within the multicast groups of its competence. o Synchronization of sequence numbers (Section 6). This concerns how a listener node that has just joined an OSCORE multicast group can synchronize with the sequence number of multicasters in the same group. o Provisioning and retrieval of public keys (Appendix C.2). This provides guidelines about how to ensure the availability of group members' public keys, possibly relying on the Group Manager as trusted key repository. Further security considerations are inherited from the ACE framework for Authentication and Authorization [I-D.ietf-ace-oauth-authz], as well as from the profiles of ACE [I-D.ietf-ace-dtls-authorize][I-D.se itz-ace-oscoap-profile][I-D.aragon-ace-ipsec-profile]. 8. IANA Considerations This document has no actions for IANA. Tiloca & Park Expires May 1, 2018 [Page 10] Internet-Draft OSCORE group joining in ACE October 2017 9. Acknowledgments The authors sincerely thank Santiago Aragon, Stefan Beck, Martin Gunnarsson, Francesca Palombini, Jim Schaad, Ludwig Seitz and Goeran Selander for their comments and feedback. 10. References 10.1. Normative References [I-D.aragon-ace-ipsec-profile] Aragon, S., Tiloca, M., and S. Raza, "IPsec profile of ACE", draft-aragon-ace-ipsec-profile-00 (work in progress), July 2017. [I-D.ietf-ace-actors] Gerdes, S., Seitz, L., Selander, G., and C. Bormann, "An architecture for authorization in constrained environments", draft-ietf-ace-actors-05 (work in progress), March 2017. [I-D.ietf-ace-dtls-authorize] Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and L. Seitz, "Datagram Transport Layer Security (DTLS) Profile for Authentication and Authorization for Constrained Environments (ACE)", draft-ietf-ace-dtls- authorize-01 (work in progress), July 2017. [I-D.ietf-ace-oauth-authz] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and H. Tschofenig, "Authentication and Authorization for Constrained Environments (ACE)", draft-ietf-ace-oauth- authz-08 (work in progress), October 2017. [I-D.ietf-core-object-security] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, "Object Security for Constrained RESTful Environments (OSCORE)", draft-ietf-core-object-security-06 (work in progress), October 2017. [I-D.seitz-ace-oscoap-profile] Seitz, L., Palombini, F., and M. Gunnarsson, "OSCORE profile of the Authentication and Authorization for Constrained Environments Framework", draft-seitz-ace- oscoap-profile-06 (work in progress), October 2017. Tiloca & Park Expires May 1, 2018 [Page 11] Internet-Draft OSCORE group joining in ACE October 2017 [I-D.tiloca-core-multicast-oscoap] Tiloca, M., Selander, G., Palombini, F., and J. Park, "Secure group communication for CoAP", draft-tiloca-core- multicast-oscoap-04 (work in progress), October 2017. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 10.2. Informative References [I-D.ietf-core-resource-directory] Shelby, Z., Koster, M., Bormann, C., Stok, P., and C. Amsuess, "CoRE Resource Directory", draft-ietf-core- resource-directory-11 (work in progress), July 2017. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, December 2005, . [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012, . [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", RFC 6749, DOI 10.17487/RFC6749, October 2012, . [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, DOI 10.17487/RFC7228, May 2014, . [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014, . Tiloca & Park Expires May 1, 2018 [Page 12] Internet-Draft OSCORE group joining in ACE October 2017 [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. Kivinen, "Internet Key Exchange Protocol Version 2 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 2014, . [RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for the Constrained Application Protocol (CoAP)", RFC 7390, DOI 10.17487/RFC7390, October 2014, . [RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)", RFC 8152, DOI 10.17487/RFC8152, July 2017, . Authors' Addresses Marco Tiloca RISE SICS AB Isafjordsgatan 22 Kista SE-164 29 Stockholm Sweden Email: marco.tiloca@ri.se Jiye Park Universitaet Duisburg-Essen Schuetzenbahn 70 Essen 45127 Germany Email: ji-ye.park@uni-due.de Tiloca & Park Expires May 1, 2018 [Page 13]