Core H. Wang
Internet Draft C. Pu
Interned status: Standards Track P. Wang
Expires: July 1, 2017 Y. Yang
D. Xiong
Chongqing University of
Posts and Telecommunications
December 28, 2016
Requirements Analysis for OPC UA over CoAP
draft-wang-core-opcua-transmition-requirements-00
Abstract
Industrial Internet of Things is an attractive application area for
Constrained Application Protocol (CoAP). OPC Unified Architecture
(OPC UA) defines a semantic-based information model for industrial
control system that can satisfy the requirements of Industry 4.0
based on semantic information exchange. This document analyses
requirements for OPC UA transmission over CoAP.
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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 ......................................... 5
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 limited networks that has been widely used in machine-
to-machine (M2M) communications [RFC7252]. However, 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 is not only based on data transmission, but also based
on semantic information exchange. Driven by this, using CoAP in the
IIoT, there is a need to provide good support for data 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 62541-1] is very promising and its transmission mode is
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similar to the transmission mode of CoAP, so it can be applied as a
branch of the CoAP message payload.
2. Architecture of OPC UA over CoAP
With the vision of IIoT in mind, we believe that 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 restricted
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 set 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, creation session, publish/subscribe and other functions
related to data information interaction are all implemented by the
service set 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. Comparing 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
transmissions 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.
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
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.
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6. References
6.1. Normative References
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol", RFC 7252, June 2014,
<https://tools.ietf.org/html/rfc7252>.
6.2. 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>.
[IEC 62541-5]
IEC, "OPC unified architecture-Part5: Information Model-IEC
62541", 2015,
<https://webstore.iec.ch/preview/info_iec62541-
5%7Bed2.0%7Db.pdf>.
[I-D.koster-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), Qctober 2016.
[IEC TR 62541-2]
IEC, "OPC unified architecture-Part2: Security Model-IEC
62541", 2016,
<https://webstore.iec.ch/preview/info_iec62541-
2%7Bed2.0%7Db.pdf>.
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Authors' Addresses
Heng Wang
Chongqing University of Posts and Telecommunications
2 Chongwen Road
Chongqing, 400065
China
Phone: (86)-23-6248-7845
Email: wangheng@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
Ping Wang
Chongqing University of Posts and Telecommunications
2 Chongwen Road
Chongqing, 400065
China
Phone: (86)-23-6246-1061
Email: wangping@cqupt.edu.cn
Yi Yang
Chongqing University of Posts and Telecommunications
2 Chongwen Road
Chongqing, 400065
China
Phone: (86)-23-6246-1061
Email: 15023705316@163.com
Daijing Xiong
Chongqing University of Posts and Telecommunications
2 Chongwen Road
Chongqing, 400065
China
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Phone: (86)-23-6246-1061
Email: 15111825021@163.com
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