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Core                                                            H. Wang
Internet Draft                                                    C. Pu
Intended status: Standards Track                                P. Wang
Expires: December 23, 2017                                      Y. Yang
                                                               D. Xiong
                                                Chongqing University of
                                           Posts and Telecommunications
                                                          June 21, 2017

                Requirements Analysis for OPC UA over CoAP


   Constrained Application Protocol (CoAP) is a application protocol
   for use with constrained nodes and constrained networks that is
   attractive to be applied In the Industrial Internet of Things (IIoT).
   OPC Unified Architecture (OPC UA) defines a semantic-based
   information model for industrial control system that can satisfy the
   requirements of Industry 4.0, which is based on semantic information
   exchange. This document analyses requirements for transmitting 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
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   This Internet-Draft will expire on December 23, 2017.

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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
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Table of Contents

   1. Introduction ................................................ 2
   2. Architecture of OPC UA over CoAP ............................ 3
   3. Requirements for OPC UA over CoAP ........................... 4
      3.1. Encoding ............................................... 4
      3.2. Application Sublayer Optimization ...................... 4
      3.3. Consistency ............................................ 4
      3.4. Reliability ............................................ 5
   4. Security Considerations ..................................... 5
   5. IANA Considerations ......................................... 6
   6. References .................................................. 6
      6.1. Normative References ................................... 6
      6.2. Informative References ................................. 6
   Authors' Addresses ............................................. 7

1. Introduction

   CoAP is a web application protocol designed for resource constrained
   devices and constrained networks that has been widely used in
   machine-to-machine (M2M) communications [RFC7252].The purpose of
   applying CoAP to the Industrial Internet of Things (IIoT) is to
   provide connectivity for the devices. Whereas the communication of
   Industry 4.0 not only requires data value transmission, but also
   requires semantic information exchange. Driven by this, to use CoAP
   in the IIoT, there is a need to provide good support for messages
   transmission of the application layer in the automation field.
   According to the definition of Industry 4.0 for communication, CoAP
   needs to support the exchange of semantic information, namely the
   semantic information model. For the current protocols supporting
   semantic information model in the IIoT, the information model
   defined by OPC UA [IEC TR 62541-1] is very promising and its

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   interactive model is similar to the interactive model of CoAP, so it
   can be applied as a branch of the CoAP message payload.

2. Architecture of OPC UA over CoAP

   To meet the needs of IIoT, the architecture of OPC UA over CoAP can
   be mainly divided into the following two:

   1) Figure 1 presents a logical layered structure of OPC UA
   Information Model over CoAP. In the transport layer, DTLS runs on
   top of UDP to secure transmission. Then, the middle layer utilizes
   the message mode defined in the CoAP protocol. Last, the information
   model of OPC UA [IEC TR 62541-5] is defined as an application of
   CoAP at the top. In such a hierarchical structure, the semantic-
   based data information in OPC UA can be transmitted in resources-
   constrained scenarios, so that CoAP can meet the requirements of
   semantic information transmission.

                    + - - - - - - - - - - - - - - +
                    |   OPC UA Information Model  |
                    + - - - - - - - - - - - - - - +
                    + - - - - - - - - - - - - - - +
                    |            CoAP             |
                    + - - - - - - - - - - - - - - +
                    + - - - - - - - - - - - - - - +
                    |             UDP             |
                    + - - - - - - - - - - - - - - +
              Figure 1: OPC UA Information Model over CoAP

   2) In order to take full advantage of the service sets defined by
   OPC UA, this document proposes the other architecture for OPC UA

                    + - - - - - - - - - - - - - - +
                    |   OPC UA Information Model  |
                    + - - - - - - - - - - - - - - +
                    |       OPC UA Services       |
                    + - - - - - - - - - - - - - - +
                    + - - - - - - - - - - - - - - +
                    |            CoAP             |
                    + - - - - - - - - - - - - - - +
                    + - - - - - - - - - - - - - - +
                    |             UDP             |
                    + - - - - - - - - - - - - - - +
        Figure 2: OPC UA Information Model and Services over CoAP

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transmission over CoAP. As shown in Figure 2, the information model of
OPC UA is defined as the application of CoAP, moreover, the connection
establishment, creating session, publish/subscribe and other functions
related to data information interaction are all implemented by the
service sets defined by OPC UA. CoAP is mainly responsible for the
definition of message format and runs over UDP to keep the
implementation lightweight.

3. Requirements for OPC UA over CoAP

3.1. Encoding

   CoAP messages are encoded in a simple binary format that starts with
   a fixed-size 4-byte header. The header is followed by a variable-
   length Token value, which can be between 0 and 8 bytes long.
   Following the Token value comes a sequence of zero or more CoAP
   Options in Type-Length-Value (TLV) format, optionally followed by a
   payload that takes up the rest of the datagram. In addition, the OPC
   UA protocol coding mainly includes two ways that are binary and XML.
   Therefore, in order to transmit the information model of OPC UA over
   CoAP, specific frame formats of CoAP need to be designed to support
   two kinds of coding modes of OPC UA.

3.2. Application Sublayer Optimization

   For information exchange, the document [draft-ietf-core-coap-pubsub-
   00] defines the corresponding application sublayer, OPC UA also
   defines a number of specific communication patterns. For example, in
   the publish/subscribe mode defined by OPC UA, when the client needs
   to obtain a data periodically, it will initiate a subscription
   request to the server. In addition, the server will send the data to
   the client periodically as it receives the request from the client
   successfully. Correspondingly, in the publish/subscribe
   specification of CoAP, it introduces Broker mechanism in which the
   client sends the state information to the Broker and the Broker
   provides storage and forwarding function to implement the
   publish/subscribe function. Comparing above two protocols, their
   achieving methods have a difference on communication mode of the
   publish/subscribe function. Therefore, it is necessary to optimize
   the application sublayer of CoAP to support some particular
   communication modes of OPC UA.

3.3. Consistency

   The interactive model of CoAP is the client/server model. However,
   in M2M scenarios, CoAP entities often act as both servers and
   clients. Compared to OPC UA, though the interactive model is also

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   the client/server model, there is a set of supported services in the
   OPC UA server. Consequently, for the great difference of the server
   definition of these two protocols, we need to tackle with the
   consistency and integration issues between the CoAP server and the
   OPC UA server.

3.4. Reliability

   One of the main design goals of CoAP is to satisfy some special
   requirements such as communication in the constrained scenarios that
   address power consumption. Hence, in order to reduce network
   overhead and avoid network congestion, CoAP is designed to run over
   UDP, which is a good choice to achieve inter-network data exchange
   in use of the IP architecture. However, UDP is a connectionless
   transport layer protocol that provides unreliable information
   transmission services. In the field of IIoT, we need to ensure the
   reliability of data transmission to avoid losing some important data
   information. Moreover, CoAP addresses transmission reliability by
   defining a message as requiring acknowledgment, obviously this is
   not enough to meet the high reliability requirements in the field of
   IIoT, so the reliability of COAP remains to be optimized.

3.5. Transmission Methods

   For OPC UA over CoAP, one of the important issues that needs to be
   addressed is how to transmit messages. The connection between OPC UA
   client and server is stateful, the connection status need to be
   maintained in the process of message interaction, while CoAP is a
   stateless connection, so that the message transmission of the two
   protocols is different. Fortunately, the transport layer protocol of
   OPC UA supports TCP and HTTP, in addition, the CoAP protocol can be
   considered that it is improved for constrained scenarios based on
   HTTP. Therefore, a solution can be found for the messages
   transmission by using the similarity of two protocols in HTTP.

4. Security Considerations

   The security of CoAP includes four modes in which three modes
   implemented based on the Datagram Transport Layer Security (DTLS)
   except the non-security mode. However, the security architecture of
   OPC UA is built on the application layer and the communication layer
   above the transport layer. Specifically, the application layer
   adopts the authentication and authorization and the communication
   layer achieves the security of OPC UA [IEC TR 62541-2] through
   secure channel encryption. Though OPC UA has four modes, the
   security model of OPC UA is realized based on Transport Layer
   Security (TLS). Actually, DTLS is an addition to TLS to solve the

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   unreliable transmission feature of UDP. Currently, some documents
   show that CoAP needs to support TLS. Therefore, the security of the
   two protocols can be implemented jointly.

5. IANA Considerations

   This memo includes no request to IANA.

6. References

6.1. Normative References

[RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
           Application Protocol", RFC 7252, June 2014,

6.2. Informative References

[IEC TR 62541-1]
           IEC, "OPC unified architecture-Part1: Overview and concepts-
           IEC 62541", 2016, <

[IEC TR 62541-5]
           IEC, "OPC unified architecture-Part5: Information Model-IEC
           62541", 2015, <

           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), Qctober 2016.

[IEC TR 62541-2]
           IEC, "OPC unified architecture-Part2: Security Model-IEC
           62541", 2016, <

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Authors' Addresses

   Heng Wang
   Chongqing University of Posts and Telecommunications
   2 Chongwen Road
   Chongqing, 400065

   Phone: (86)-23-6248-7845
   Email: wangheng@cqupt.edu.cn

   Chenggen Pu
   Chongqing University of Posts and Telecommunications
   2 Chongwen Road
   Chongqing, 400065

   Phone: (86)-23-6246-1061
   Email: mentospcg@163.com

   Ping Wang
   Chongqing University of Posts and Telecommunications
   2 Chongwen Road
   Chongqing, 400065

   Phone: (86)-23-6246-1061
   Email: wangping@cqupt.edu.cn

   Yi Yang
   Chongqing University of Posts and Telecommunications
   2 Chongwen Road
   Chongqing, 400065

   Phone: (86)-23-6246-1061
   Email: 15023705316@163.com

   Daijing Xiong
   Chongqing University of Posts and Telecommunications
   2 Chongwen Road
   Chongqing, 400065

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   Phone: (86)-23-6246-1061
   Email: 15111825021@163.com

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