[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02

Core                                                            P. Wang
Internet Draft                                                    C. Pu
Intended status: Standards Track                                H. Wang
Expires: September 14, 2017                                     Y. Yang
                                                                L. Shao
                                                Chongqing University of
                                           Posts and Telecommunications
                                                                 J. Hou
                                                    Huawei Technologies
                                                         March 13, 2017


               OPC UA Message Transmission Method over CoAP
                   draft-wang-core-opcua-transmission-01


Abstract

   OPC Unified Architecture (OPC UA) is a data exchange standard that
   provides interoperability in industrial automation. With the coming
   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 September 14, 2017.




Wang, et al.         Expires September 14, 2017               [Page 1]


 Internet-Draft        OPC UA Message Transmission           March 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 .............................. 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 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


Wang, et al.         Expires September 14, 2017               [Page 2]


 Internet-Draft        OPC UA Message Transmission           March 2017


   connections in transmission method between them. This document
   provides some transmission methods for message of OPC UA over CoAP,
   so that a communication could be established between OPC UA client
   and OPC UA server.

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
   safe, 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 layer 5, 6 or 7 protocol such 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 guaranteed at TLS but
   Security Channel should still be presented even empty. It is


Wang, et al.         Expires September 14, 2017               [Page 3]


 Internet-Draft        OPC UA Message Transmission           March 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 September 14, 2017               [Page 4]


 Internet-Draft        OPC UA Message Transmission           March 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 processed
   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 September 14, 2017               [Page 5]


 Internet-Draft        OPC UA Message Transmission           March 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
   from HTTP, the two can be mapped between each other to meet the


Wang, et al.         Expires September 14, 2017               [Page 6]


 Internet-Draft        OPC UA Message Transmission           March 2017


   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, assume all OPC UA servers are based on CoAP
   [I-D.wang-core-opcua-transmition-requirements], 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
   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.

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
   protocol so that it can be applied to resource-constrained network.
   Therefore, this document considers the use of CoAP to directly


Wang, et al.         Expires September 14, 2017               [Page 7]


 Internet-Draft        OPC UA Message Transmission           March 2017


   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 5. 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 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
   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 September 14, 2017               [Page 8]


 Internet-Draft        OPC UA Message Transmission           March 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 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.

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 broker into the network [I-D.ietf-core-
   coap-pubsub], 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 constrained applications.

   TODO.

6. Security Considerations

7. IANA Considerations

   This memo includes no request to IANA.






Wang, et al.         Expires September 14, 2017               [Page 9]


 Internet-Draft        OPC UA Message Transmission           March 2017


8. References

8.1. Normative References

[RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
           Application Protocol", RFC 7252, June 2014,
           <https://tools.ietf.org/html/rfc7252>.

[RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", RFC 2119, March 1997,
           <https://tools.ietf.org/html/rfc2119>.

[RFC8075]  Castellani, A., Loreto, S., and A. Rahman, "Guidelines for
           HTTP-to-CoAP Mapping Implementations", RFC 8075, November
           2016, <https://tools.ietf.org/html/rfc8075>.

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-07 (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-00 (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-00 (work in progress), October
           2016.








Wang, et al.         Expires September 14, 2017              [Page 10]


 Internet-Draft        OPC UA Message Transmission           March 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 September 14, 2017              [Page 11]


 Internet-Draft        OPC UA Message Transmission           March 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 September 14, 2017              [Page 12]


Html markup produced by rfcmarkup 1.124, available from https://tools.ietf.org/tools/rfcmarkup/