CoRE P. Wang
Internet Draft C. Pu
Intended status: Standards Track H. Wang
Expires: September 4, 2018 J. Wu
Y.Yang
L. Shao
Chongqing University of
Posts and Telecommunications
J. Hou
Huawei Technologies
March 03, 2018
OPC UA Message Transmission Method over CoAP
draft-wang-core-opcua-transmission-03
Abstract
OPC Unified Architecture (OPC UA) is a data exchange specification
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
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This Internet-Draft will expire on September 4, 2018.
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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 ................................. 5
3. Specification of OPC UA over CoAP ........................... 6
4. Transmission scheme ......................................... 7
4.1. Direct transmission .................................... 7
4.2. REST transmission for OPC UA ........................... 8
5. Publish subscription for OPC UA over CoAP ................... 9
6. Use Cases of OPC UA over CoAP ............................... 9
6.1. Factory data monitoring based on web pages ............. 9
6.2. Offline/Online diagnostic system for resource-constrained
factories .................................................. 10
6.3. Factory data analysis based on cloud .................. 11
7. Security Considerations .................................... 11
8. IANA Considerations ........................................ 11
9. References ................................................. 11
9.1. Normative References .................................. 11
9.2. Informative References ................................ 12
Authors' Addresses ............................................ 13
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].
In resource-constrained scenarios, OPC UA can effectively use energy,
improve productivity and shorten the product manufacturing cycle by
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building information model and using its cross-platform
characteristic. Similar to OPC UA, CoAP message is exchanged in
server/client mode. However, both of them have specific clients and
servers. Driven by this, to implement OPC UA Transmitting over CoAP,
the main 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 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
REST: Representational State Transfer
HMI: Human Machine Interface
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
existing layers 5, 6 or 7 protocols such as TCP/IP, TLS or HTTP. The
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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
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
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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.
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
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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.
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
Transport Layer Security (TLS) is available [RFC8323].
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.
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4. Transmission scheme
4.1. Direct transmission
The transmission of OPC UA supports TCP protocol and HTTP protocol.
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 to support these two 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 4.
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 4: Direct transmission OPC UA based on CoAP
For supporting HTTP, a CoAP proxy can be established between OPC UA
client and OPC UA server.
As shown in Figure 5, assuming all OPC UA servers are based on CoAP,
and all OPC UA-CoAP servers can be considered to form a constrained
network, then introducing a UA-to-CoAP proxy at the boundary of the
network. When a traditional OPC UA client initiates an HTTP request
to the UA-CoAP servers which is in the constrained network mentioned
above, 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.
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- - - - - - - - - - - - - - - -
// + - - - - + \\
|| | UA-CoAP | ||
|| // | server | ||
|| // + - - - - + ||
|| // ||
|| // ||
+ - - - + HTTP Request + - - - - - - + CoAP Request + - - - +||
| UA | - - - - - - - - > | HTTP-to-CoAP| - - - - - - - > |UA-CoAP|||
| client| < - - - - - - - - | PROXY | < - - - - - - - | server|||
+ - - - + HTTP Response + - - - - - - + CoAP Response + - - - +||
|| \\ ||
|| \\ ||
|| \\ ||
|| \\ + - - - - + ||
|| \\ | UA-CoAP | ||
|| | server | ||
\\ + - - - - + //
- - - - - - - - - - - - - - - -
Figure 5: Proxy for OPC UA to CoAP
4.2. 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, because of this, the users cannot modify
OPC UA services according to their own ideas. In general, services
in OPC UA cannot be considered stateless, but many of them which are
also commonly used are inherently stateless, e.g. FindServers, Read,
Write [RICO]. 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.
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+ - - - - + + - - - - +
| client | | server |
+ - - - - + + - - - - +
| |
| |
| CoAP Request |
| - - - - - - - - - > |
| CoAP Response |
| < - - - - - - - - - |
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 over 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. Use Cases of OPC UA over CoAP
6.1. Factory data monitoring based on web pages
Description: At present, the monitoring and management systems of
the factory mostly exist in the form of software on the PC and the
mobile. The drawback is that when the whole factory system is needed
to upgrade, the monitoring software must be upgraded as well. It may
cause the huge workload that will bring the time and financial
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burden on the factory. CoAP is a HTTP-like communication protocol
designed specifically for resource-constrained environments so that
can be used in the factory because the sensor nodes in the factory
mostly are resource-constrained. CoAP can easily transform to HTTP
and OPC UA can consolidate the different protocols in the plant by
building a unified information model.
Goal: PC and mobile devices can check and monitor the data by
visiting WEB pages after CoAP is converted to HTTP. Avoiding large-
scale software upgrades caused by system upgrades, while also
reducing the development of mobile software, thereby reducing
factory costs.
Requirements: the OPC UA information model should be encapsulated
into CoAP data load. Because of the capacity limitation of UDP
packet (MTU is 1472 bytes), in some cases, it is needed to compress,
fragment, and reassemble packets.
6.2. Offline/Online diagnostic system for resource-constrained
factories
Description: There are two modes existing in the factory's self-
diagnosis system, the offline mode and the online mode. In the
offline mode, the self-diagnostic device could use getHistorical, a
service from OPC UA, to get historical Data. In the online mode,
Both OPC UA and CoAP support pub/sub so that the monitoring system
can obtain the data from a specific device in a short reaction time
to determine its operating status. CoAP, as a resource-constrained
factory transmission protocol, can easily access many web services
APIs, add functionality that the factory can implement and let the
system have a certain degree of expansibility. OPC UA could create a
unified information model that realizes factory interoperability and
protocol uniformity.
At same time, the controller node can diagnose and regulate other
nodes by receiving their data rather than transferring them to HMI
(The M2M Communication). Generally, using UDP is the best choice,
however, CoAP's UDP not only has excellent stability but also has
relatively few packet loss rates. The unified model of OPC UA
enables all nodes to communicate without obstacles.
Goal: Using OPC UA over CoAP to enable factory offline history data
diagnostics, online real-time monitoring, publish subscriptions and
Achieving network nodes M2M communication.
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Requirements: OPC UA uses SOA architecture, while CoAP uses REST
architecture, it is necessary to design a reasonable architecture
for OPC UA over CoAP.
6.3. Factory data analysis based on cloud
Description: Currently, there are many clouds (AWS, Windows Azure,
etc.) which have different kinds of APIs. These clouds could achieve
machine learning, data-flow analysis and so on for factory's data.
Using CoAP can effectively access these interfaces and fully take
advantage of clouds capabilities. At present, many factories have
begun to use the cloud to improve production status, So the biggest
benefit to use CoAP in factories is that CoAP could let devices to
use cloud's applications in resource-constrained factories so that
to achieve intelligent control. OPC UA can consolidate the different
protocols in the plant by building a unified information model.
Based on the content mentioned above, the field devices in the
factories can transfer their data directly and immediately to the
cloud without sending them to border routers or HMI.
Goal: Using OPC UA over CoAP to transfer field devices' data to the
cloud.
Requirements: Using OPC UA to modeling the different types of data
in the plant and then using CoAP to directly transfer the factory's
data to the cloud.
7. Security Considerations
This document does not add any new security considerations beyond
what the referenced technologies already have.
8. IANA Considerations
This memo includes no request to IANA.
9. References
9.1. 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>.
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[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>.
[RFC8323] Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, "CoAP (Constrained Application
Protocol) over TCP, TLS, and WebSockets", RFC8323, February
2018.
9.2. 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.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-02 (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-03 (work in progress), February
2018.
[RICO] Gruner, Sten, Julius Pfrommer, and Florian Palm. "RESTful
industrial communication with OPC UA." IEEE Transactions on
Industrial Informatics 12.5 (2016):1832-1841.
<http://ieeexplore.ieee.org/abstract/document/7407396/>
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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
Junrui Wu
Chongqing University of Posts and Telecommunications
2 Chongwen Road
Chongqing, 400065
China
Phone: (86)-18580183135
Email: wjr930914@gmail.com
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Yi Yang
Chongqing University of Posts and Telecommunications
2 Chongwen Road
Chongqing, 400065
China
Phone: (86)-23-6246-1061
Email: 15023705316@163.com
Lun Shao
Chongqing University of Posts and Telecommunications
2 Chongwen Road
Chongqing, 400065
China
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
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