Core P. Wang Internet Draft C. Pu Intended status: Standards Track H. Wang Expires: March 11, 2018 Y. Yang L. Shao C hongqing University of Posts and Telecommunications J. Hou Huawei Technologies September 7, 2017 OPC UA Message Transmission Method over CoAP draft-wang-core-opcua-transmission-02 Abstract OPC Unified Architecture (OPC UA) is a data exchange standard that provides interoperability in industrial automation. With the arrival of Industry 4.0, it is of great importance 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 March 11, 2018. Wang, et al. Expires March 11, 2018 [Page 1] Internet-Draft OPC UA Message Transmission September 2017 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 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. Conventions and Terminology ............................ 3 2. Overview of OPC UA .......................................... 3 2.1. Protocol Stack ......................................... 3 2.2. Request/Response Model ................................. 4 3. Specification of OPC UA over CoAP ........................... 5 4. Transmission scheme ......................................... 6 4.1. Proxy for OPC UA-CoAP .................................. 6 4.2. Direct transmission .................................... 7 4.3. REST transmission for OPC UA ........................... 8 5. Publish subscription for OPC UA and CoAP .................... 9 6. Security Considerations ..................................... 9 7. IANA Considerations ......................................... 9 8. References ................................................. 10 8.1. Normative References .................................. 10 8.2. Informative References ................................ 10 Authors' Addresses ............................................ 11 1. Introduction Internet of things is one of the attractive applications for CoAP [RFC7252]. Utilizing OPC UA [IEC TR 62541-1] Transmitting over CoAP could meet the demand for industry 4.0 based on the exchange of semantic information [I-D.wang-core-opcua-transmition-requirements]. Similar to OPC UA, CoAP message is exchanged in server/client mode. However, their transmission are different. Driven by this, to implement 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. It is worth noting that the design of CoAP is inspired Wang, et al. Expires March 11, 2018 [Page 2] Internet-Draft OPC UA Message Transmission September 2017 by HTTP, thus, there are some similarities in transmission method between them. This document provides some transmission methods for message of OPC UA over CoAP, so that the communication between OPC UA client and OPC UA server could be established. 1.1. Conventions and Terminology The keywords "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 [RFC2119]. OPC: OLE for Process Control OPC UA: OPC Unified Architecture SOAP: Simple Object Access Protocol 2. Overview of OPC UA OPC Unified Architecture (OPC UA), standardized as IEC 62541, is a client-server communication protocol developed by OPC Foundation for safety, reliable data exchange in industrial automation. It is the evolution product of OPC (OLE for Process Control, where OLE denotes Object Linking and Embedding), the widely used standard process for automation technology, and is of great importance in realizing industry 4.0. By introducing Service-oriented architecture (SOA), OPC UA enables an open, cross-platform communication with the advantages of web services, robust security and integrated data model. 2.1. Protocol Stack OPC UA is an application layer protocol that can be built on an existing layers 5, 6 or 7 protocol such as TCP/IP, TLS or HTTP. The OPC UA application layer consists of four sublayers: UA Application, Serialization Layer, Secure Channel Layer and Transport Layer (see Figure 1). Serialization Layer includes two kinds of data encoding methods: UA Binary and UA XML. The UA XML, based on SOAP/HTTP or SOAP/HTTPS, is firewall friendly. On the other hand, the UA Binary, with least overhead and resource cost, offers an optimized speed and throughput. The security layer varies according to the selected encoding format. For the HTTPS-based situation, security is implemented at TLS but Security Channel should still be presented even empty. It is Wang, et al. Expires March 11, 2018 [Page 3] Internet-Draft OPC UA Message Transmission September 2017 worthwhile noting that the communication based on SOAP/HTTP has been deprecated since 2015, due to the lack of industrial approbation in the WS Secure Conversation. For the transport layer (not the layer in OSI 7 layer model), options can be UA TCP, HTTPS, SOAP/HTTPS, and SOAP/HTTP. OPC UA defines a UA TCP protocol, which differs from HTTP in two main features: the allowance of responses to be returned in any order and to be returned on a different TCP transport end-point. In addition, UA TCP defines the interaction with the upper security channel. +-------------------------------------------------------+ ------ | UA Application | +-------------------------------------------------------+ +--------------------------+ +--------------------------+ | UA Binary | | UA XML | +--------------------------+ +--------------------------+ +--------------+ +--------------+ App | UA Secure | | WS Secure | | Conversation | | Conversation | Layer +--------------+ +--------------+ +--------------+ +---------+ +---------+ +--------------+ | | | | | SOAP | | SOAP | | UA TCP | | HTTPS | |---------| |--------------| | | | | | HTTPS | | HTTP | +--------------+ +---------+ +---------+ +--------------+ ------ +-------------------------------------------------------+ | TCP/IP | +-------------------------------------------------------+ Figure 1: Layering of OPC UA over TCP/IP 2.2. Request/Response Model The message exchange in UA binary mode is illustrated in Figure 2. After opening the socket, the client starts the connection with the server by using "hello" (HEL) and "acknowledge" (ACK) messages. Afterwards, a pair of messages is needed to open the security channel and define the encryption property. Then another two pairs of messages are exchanged so as to create and activate a session between the client and the server respectively. After these steps, the connection is initiated and the client can send request messages for services. When the request/response process is finished, a reverse process is required for disconnection. Wang, et al. Expires March 11, 2018 [Page 4] Internet-Draft OPC UA Message Transmission September 2017 Client Secure UA UA Secure Server Channel TCP TCP Channel | | | Open Socket | | | | | | - - - - - - - - - > | | | | | | Hello | | | | | | - - - - - - - - - > | | | | | | Acknowledge | | | | | | < - - - - - - - - - | | | | | | | | | | | |OpenSecureChannelReq | | | | | - - - - - - - - - - - - - - - - - > | | | | |OpenSecureChannelRes | | | | | < - - - - - - - - - - - - - - - - - | | | | | | | | | | | CreateSessionReq | | | | - - - - - - - - - - - - - - - - - - - - - - - - - > | | | | CreateSessionRes | | | | < - - - - - - - - - - - - - - - - - - - - - - - - - | | | | ActivateSessionReq | | | | - - - - - - - - - - - - - - - - - - - - - - - - - > | | | | ActivateSessionRes | | | | < - - - - - - - - - - - - - - - - - - - - - - - - - | | | | | | | | | | Request | | | | = = = = = = = = = = = = = = = = = = = = = = = = = > | | | | Response | | | | < = = = = = = = = = = = = = = = = = = = = = = = = = | Figure 2: Request/Response Process of UA TCP 3. Specification of OPC UA over CoAP As mentioned in section 2.1, OPC UA communications can be conducted through four options, among which two are related to HTTPS: HTTPS => UA Binary; HTTPS => SOAP => UA XML. HTTPS is a security-guaranteed protocol consisting of a HTTP layer over Transport Layer Security (TLS), thus the UA Security Channel can be left empty. Constrained Application Protocol (CoAP) is an application layer protocol for constrained nodes and networks, which is designed to easily translate to HTTP for integration with the web. Although CoAP is built on the unreliable transport layer UDP, it offers a security mode binding to Datagram Transport Layer Security (DTLS). This document proposes a transmission scheme based on CoAPs (CoAP + DTLS) for constrained scenarios. The transmission based on CoAP over Wang, et al. Expires March 11, 2018 [Page 5] Internet-Draft OPC UA Message Transmission September 2017 Transport Layer Security (TLS) is also possible. Such "CoAP + TLS" transmission scheme is under development [I-D.ietf-core-coap-tcp-tls] and would be covered in the future version. The protocol stack of the CoAP based OPC UA is illustrated in Figure 3, including two options at Serialization Layer: UA Binary and UA XML. OPC UA packets are encoded in either binary or xml format, 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. +-------------------------------+ ------ | UA Application | +-------------------------------+ +--------------+ +--------------+ | UA Binary | | UA XML | App +--------------+ +--------------+ +-------------------------------+ | Secure Channel | Layer +-------------------------------+ +-------------------------------+ | CoAP | +-------------------------------+ ------ +-------------------------------+ | UDP, DTLS | +-------------------------------+ Figure 3: Layering of OPC UA over UDP Both binary and XML encoding modes are based on the CoAP with an empty UA secure channel in between. For the XML encoding mode, since CoAP layer supports XML encoding format, the SOAP layer in the original stack is not needed. 4. Transmission scheme 4.1. 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 Wang, et al. Expires March 11, 2018 [Page 6] Internet-Draft OPC UA Message Transmission September 2017 from HTTP, the two can be mapped between each other to meet the needs of some special scenes [RFC8075]. 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 4. - - - - - - - - - - - - - // + - - - - + \\ || | UA-CoAP | || || // | server | || || // + - - - - + || || // || || // || + - - - + HTTP Request + - - - - - - + CoAP Request + - - - - +|| | UA | - - - - - - - > | HTTP-to-CoAP| - - - - - - > | UA-CoAP ||| | client| < - - - - - - - | PROXY | < - - - - - - | server ||| + - - - + HTTP Response + - - - - - - + CoAP Response + - - - - +|| || \\ || || \\ || || \\ || || \\ + - - - - + || || \\ | UA-CoAP | || || | server | || \\ + - - - - + // - - - - - - - - - - - - - Figure 4: Proxy for OPC UA to CoAP As shown in Figure 5, assuming all OPC UA servers are based on CoAP [draft-wang-core-opcua-transmition-requirements], and all OPC UA- CoAP server can be treated as a network, then 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 proxy and conversion are transparent, in this way, the transfer of OPC UA in CoAP does not need to make any changes to the UA Client. 4.2. 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 Wang, et al. Expires March 11, 2018 [Page 7] Internet-Draft OPC UA Message Transmission September 2017 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 to 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 5. According to CoAP, 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 5: Direct transmission OPC UA based on CoAP 4.3. REST transmission for OPC UA OPC UA is a set of data which exchange specifications for industrial communication, the core of the OPC UA protocol are 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 referencing. 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 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 6. Wang, et al. Expires March 11, 2018 [Page 8] Internet-Draft OPC UA Message Transmission September 2017 + - - - - + + - - - - + | client | | server | + - - - - + + - - - - + | | | | | CoAPRequest | | - - - - - - - - - > | | CoAPResponse | | < - - - - - - - - - | Figure 6: REST architecture communication of OPC UA In Figure 2, the traditional OPC UA requires a series of interactions between normal read and write operations. Figure 6 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 scenes. 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. 5. Publish subscription for OPC UA and CoAP As an application sublayer, CoAP provides publish-subscribe functionality, primarily for resource or network-constrained scenarios. Introducing proxy into the network [I-D.ietf-core-coap- pubsub], when a node needs to sleep, the node information is sent to the proxy agent, when another 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 constrained applications. TODO. 6. Security Considerations This document does not add any new security considerations beyond what the referenced technologies already have. 7. IANA Considerations This memo includes no request to IANA. Wang, et al. Expires March 11, 2018 [Page 9] Internet-Draft OPC UA Message Transmission September 2017 8. References 8.1. Normative References [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol", RFC 7252, June 2014, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997, . [RFC8075] Castellani, A., Loreto, S., and A. Rahman, "Guidelines for HTTP-to-CoAP Mapping Implementations", RFC 8075, November 2016, . 8.2. Informative References [IEC TR 62541-1] IEC, "OPC unified architecture-Part1:Overview and concepts- IEC 62541", 2016, < https://webstore.iec.ch/preview/info_iec62541- 1%7Bed2.0%7Den.pdf>. [I-D.ietf-core-coap-tcp-tls] Bormann, C., Lemay, S., Tschofenig, H., Hartke, K., Silverajan, B., and B. Raymor, "CoAP (Constrained Application Protocol) over TCP, TLS, and WebSockets", draft-ietf-core- coap-tcp-tls-09 (work in progress), March 2017. [I-D.wang-core-opcua-transmition-requirements] Wang, H., Pu, C., Wang, P., Yang, Y., and D. Xiong, "Requirements Analysis for OPC UA over CoAP", draft-wang- core-opcua-transmition-requirements-01 (work in progress), December 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-02 (work in progress), October 2016. Wang, et al. Expires March 11, 2018 [Page 10] Internet-Draft OPC UA Message Transmission September 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 March 11, 2018 [Page 11] Internet-Draft OPC UA Message Transmission September 2017 Phone: (86)-23-6246-1061 Email: yjsslcqupt@163.com Jianqiang Hou Huawei Technologies CO.,LTD 101 Software Avenue, Nanjing 210012 China Phone: (86)-15852944235 Email: houjianqiang@huawei.com Wang, et al. Expires March 11, 2018 [Page 12]