Core P. Wang Internet Draft C. Pu Interned status: Standards Track H. Wang Expires: July 29, 2017 Y. Yang L. Shao Chongqing University of Posts and Telecommunications January 25, 2017 OPC UA Message Transmission Method over CoAP draft-wang-core-opcua-transmission-00 Abstract In Industrial applications, it is very attractive to implement the exchange of semantic information utilizing OPC UA Transmitting in CoAP. This document provides some transmission methods for message of OPC UA 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), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This Internet-Draft will expire on July 29, 2017. Copyright Notice Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved. Wang, et al. Expires July 29, 2017 [Page 1] Internet-Draft OPC UA Message Transmission January 2017 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 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. Transmission scheme ......................................... 3 2.1. Proxy for OPC UA-CoAP................................... 3 2.2. Direct transmission..................................... 4 2.3. REST transmission for OPC UA............................ 4 3. Publish subscription for OPC UA and CoAP .................... 6 4. Security Considerations...................................... 6 5. IANA Considerations ......................................... 6 6. References .................................................. 6 6.1. Normative References.................................... 6 6.2. Informative References.................................. 6 Authors' Addresses ............................................. 7 1. Introduction Internet of things is one of the attractive applications for CoAP. Utilizing OPC UA Transmitting over CoAP could meet the demand for industry 4.0 based on the exchange of semantic information[draft- wang-core-opcua-transmition-requirements-00]. Similar to OPC UA, CoAP message is exchanged in server/client mode. However, their transmission is not the same. Driven by this, to use OPC UA Transmitting over CoAP, the major problem to be solved is how OPC UA packets are transmitted over CoAP. For the transport layer of OPC UA, the main message transmission method is TCP or HTTP, and CoAP's design inspiration comes from HTTP, thus, there are some connections in transmission method between them. This proposal provides some transmission methods for message of OPC UA over CoAP, so that a communication could be established between client and server. Wang, et al. Expires July 29, 2017 [Page 2] Internet-Draft OPC UA Message Transmission January 2017 2. Transmission scheme 2.1. Proxy for OPC UA-CoAP OPC UA is a protocol mainly for application layer, which defines many services for the different needs of industrial applications. Message is exchanged mainly through server/client mode, utilizing TCP or HTTPS. When security is ignored, OPC UA can be considered to support HTTP transmission. CoAP's design inspiration comes mainly from HTTP, the two can be mapped between each other to meet the needs of some special scenes [draft-ietf-core-http-mapping-17]. Combined with the characteristics of OPC UA and CoAP, a CoAP proxy can be established between OPC UA client and OPC UA server. The architecture is shown in Figure 1. - - - - - - - - - - - - - - - // + - - - - + \\ || | UA-CoAP | || || // | server | || || // + - - - - + || || // || || // || + - - - - + HTTP Request + - - - - - - + CoAP Request + - - - - + || | UA | - - - - - - - - > | UA-to-CoAP | - - - - - - - > | UA-CoAP | || | client | < - - - - - - - - | PROXY | < - - - - - - - | server | || + - - - - + HTTP Response + - - - - - - + CoAP Response + - - - - + || || \\ || || \\ || || \\ || || \\ + - - - - + || || \\ | UA-CoAP | || || | server | || \\ + - - - - + // - - - - - - - - - - - - - - - - Figure 1. Proxy for OPC UA-to-CoAP As shown in Figure 1, assume all OPC UA servers are based on CoAP [draft-wang-core-opcua-transmition-requirements-00], and all OPC UA- CoAP server can be viewed as a network, introducing UA-to-CoAP proxy at the boundary of the network. When a traditional OPC UA client initiates an HTTP request to the UA-CoAP server in the network, the UA-to-CoAP proxy maps the http request to the corresponding CoAP request and sends it to the UA-CoAP server in the network. After receiving the request, the UA-CoAP server sends a response to the UA-CoAP proxy. The proxy maps the CoAP response to the HTTP response and returns it to the UA client. For the UA client, the network Wang, et al. Expires July 29, 2017 [Page 3] Internet-Draft OPC UA Message Transmission January 2017 proxy and conversion is transparent, in this way, the transfer of OPC UA in CoAP does not need to make any changes to the UA Client. 2.2. Direct transmission The transmission of OPC UA supports TCP protocol and HTTP protocol, when security is ignored, OPC UA can be considered to support HTTP transmission. On the other hand, CoAP is seen as a simplified HTTP protocol so that it can be applied to resource - constrained network. Therefore, this document considers the use of CoAP to directly transfer OPC UA messages. OPC UA packets are encoded in either binary or xml format, and the optional fields in the CoAP header specify parameters that support these two formats, and the option field in the CoAP header can specify parameters that support both formats. Therefore, according to the format specified by the CoAP header, the entire packet of the OPC UA can be encapsulated in the payload of the CoAP message for direct transmission, as shown in Figure 2. Noted that this method of transmission needs to be modified on the server side and the client side of the OPC UA according to CoAP. + - - - - - - + CoAP Request + - - - - - - + | UA client | - - - - - - - - - - > | UA server | | | < - - - - - - - - - - | | + - - - - - - + CoAP Response + - - - - - - + Figure 2. Direct transmission OPC UA based on CoAP 2.3. REST transmission for OPC UA OPC UA is a set of data exchange specifications for industrial communication, the core of the OPC UA protocol is information modeling and transmission, which marks each node in the address space with a unique identifier. A series of state interactions are needed before performing normal reading and writing, including message handshaking, opening a secure channel, creating a session, activating a session, etc. Besides, some states also need to be maintained during read and write operations. In OPC UA, each node has an independent identifier in the address space, and different types of nodes can establish contact with each other by reference. OPC UA defines a variety of services, and these services are fixed, the user cannot arbitrarily modify, each service is invoked through a single message, without relying on the previous message, the service response is also completed by a separate Wang, et al. Expires July 29, 2017 [Page 4] Internet-Draft OPC UA Message Transmission January 2017 message and do not rely on other messages. The above features are in line with the REST architecture, due to CoAP is based on the REST architecture. Therefore, it is possible to simplify the interaction before the OPC UA performs the normal communication, and carry the OPC UA message by using the communication mode of the CoAP. Communication process is shown in Figure 3. + - - - - + + - - - - + + - - - - + + - - - - + | client | | server | | client | | server | + - - - - + + - - - - + + - - - - + + - - - - + | Hello | | | | - - - - - - - - - > | | | | Acknowledge | | | | < - - - - - - - - - | | | |OpenSecureChannelReq | | | | - - - - - - - - - > | | | |OpenSecureChannelRes | | | | < - - - - - - - - - | | | | CreateSessionReq | | | | - - - - - - - - - > | | | | CreateSessionRes | | | | < - - - - - - - - - | | | | ActivateSessionReq | | | | - - - - - - - - - > | | | | ActivateSessionRes | | | | < - - - - - - - - - | | | | Request | | CoAPRequest | | - - - - - - - - - > | | - - - - - - - - - > | | Response | | CoAPResponse | | < - - - - - - - - - | | < - - - - - - - - - | a Traditional OPC UA communication b REST architecture communication Figure 3. The communication of traditional OPC UA and the REST transmission In Figure 3(a), the traditional OPC UA requires a series of interactions between normal read and write operations. Figure 3 (b) shows that when using CoAP to carry OPC UA message, the interaction process is significantly reduced, which is conducive to the application of OPC UA in the restricted scene. The cost of simplifying the interaction process is that the secure channel number is set to 0 by default, how to conduct secure data interaction needs further discussion[RESTful Industrial Communication With OPC UA]. Wang, et al. Expires July 29, 2017 [Page 5] Internet-Draft OPC UA Message Transmission January 2017 3. Publish subscription for OPC UA and CoAP As an application sublayer, CoAP provides publish-subscribe functionality, primarily for resource or network-constrained scenarios. Introducing broker into the network [draft-ietf-core- coap-pubsub-00], when a node needs to sleep, the node information is sent to the broker agent, when a node requests to obtain information of this node, the broker release function can provide information. OPC UA defines the publish-and-subscribe function as a service in the service set. The client initiates the subscription request directly to the server, and the server periodically sends the information to the client. Comparing the characteristics of the two protocols, it is found that each of them has its own advantages. Joint design can be conducted for restricted devices. 4. Security Considerations 5. IANA Considerations This memo includes no request to IANA. 6. References 6.1. Normative References 6.2. Informative References [I-D.wang-core-opcua-transmition-requirements] Wang, H., Pu, C., and Wang, P., " Requirements Analysis for OPC UA over CoAP", draft-wang-core-opcua-transmition- requirements-00 (work in progress), December 2016. [I-D.ietf-core-http-mapping] Castellani, A., Loreto, S., and Rahman, A., " Guidelines for HTTP-to-CoAP Mapping Implementations", draft-ietf-core-http- mapping-17 (work in progress), November 2016. [Gruer] Finn, N., Gruer, S., Pfrommer, J., and Palm, F., "RESTful Industrial Communication With OPC UA", IEEE Trans. Ind. Informat., Vol. 12, No. 5, October 2016. [I-D.ietf-core-coap-pubsub] Koster, M., Keranen, A., and J. Jimenez, "Publish-Subscribe Broker for the Constrained Application Protocol(CoAP)", draft-ietf-core-coap-pubsub-00 (work in progress), October 2016. Wang, et al. Expires July 29, 2017 [Page 6] Internet-Draft OPC UA Message Transmission January 2017 Authors' Addresses Ping Wang Chongqing University of Posts and Telecommunications 2 Chongwen Road Chongqing, 400065 China Phone: (86)-23-6246-1061 Email: wangping@cqupt.edu.cn Chenggen Pu Chongqing University of Posts and Telecommunications 2 Chongwen Road Chongqing, 400065 China Phone: (86)-23-6246-1061 Email: mentospcg@163.com Heng Wang Chongqing University of Posts and Telecommunications 2 Chongwen Road Chongqing, 400065 China Phone: (86)-23-6248-7845 Email: wangheng@cqupt.edu.cn Yi Yang Chongqing University of Posts and Telecommunications 2 Chongwen Road Chongqing, 400065 China Phone: (86)-23-6248-7845 Email: 382991208@qq.com Lun Shao Chongqing University of Posts and Telecommunications 2 Chongwen Road Chongqing, 400065 China Wang, et al. Expires July 29, 2017 [Page 7] Internet-Draft OPC UA Message Transmission January 2017 Phone: (86)-23-6246-1061 Email: yjsslcqupt@163.com Wang, et al. Expires July 29, 2017 [Page 8]