Core P. Wang
Internet Draft C. Pu
Interned status: Standards Track H. Wang
Expires: July 29, 2017 Y. Yang
L. Shao
Chongqing University of
Posts and Telecommunications
January 25, 2017
OPC UA Message Transmission Method over CoAP
draft-wang-core-opcua-transmission-00
Abstract
In Industrial applications, it is very attractive 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 July 29, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
Wang, et al. Expires July 29, 2017 [Page 1]
Internet-Draft OPC UA Message Transmission January 2017
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
2. Transmission scheme ......................................... 3
2.1. Proxy for OPC UA-CoAP................................... 3
2.2. Direct transmission..................................... 4
2.3. REST transmission for OPC UA............................ 4
3. Publish subscription for OPC UA and CoAP .................... 6
4. Security Considerations...................................... 6
5. IANA Considerations ......................................... 6
6. References .................................................. 6
6.1. Normative References.................................... 6
6.2. Informative References.................................. 6
Authors' Addresses ............................................. 7
1. Introduction
Internet of things is one of the attractive applications for CoAP.
Utilizing OPC UA Transmitting over CoAP could meet the demand for
industry 4.0 based on the exchange of semantic information[draft-
wang-core-opcua-transmition-requirements-00]. 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 connections
in transmission method between them. This proposal provides some
transmission methods for message of OPC UA over CoAP, so that a
communication could be established between client and server.
Wang, et al. Expires July 29, 2017 [Page 2]
Internet-Draft OPC UA Message Transmission January 2017
2. Transmission scheme
2.1. 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
needs of some special scenes [draft-ietf-core-http-mapping-17].
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 1.
- - - - - - - - - - - - - - -
// + - - - - + \\
|| | UA-CoAP | ||
|| // | server | ||
|| // + - - - - + ||
|| // ||
|| // ||
+ - - - - + HTTP Request + - - - - - - + CoAP Request + - - - - + ||
| UA | - - - - - - - - > | UA-to-CoAP | - - - - - - - > | UA-CoAP | ||
| client | < - - - - - - - - | PROXY | < - - - - - - - | server | ||
+ - - - - + HTTP Response + - - - - - - + CoAP Response + - - - - + ||
|| \\ ||
|| \\ ||
|| \\ ||
|| \\ + - - - - + ||
|| \\ | UA-CoAP | ||
|| | server | ||
\\ + - - - - + //
- - - - - - - - - - - - - - - -
Figure 1. Proxy for OPC UA-to-CoAP
As shown in Figure 1, assume all OPC UA servers are based on CoAP
[draft-wang-core-opcua-transmition-requirements-00], 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
Wang, et al. Expires July 29, 2017 [Page 3]
Internet-Draft OPC UA Message Transmission January 2017
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.
2.2. 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
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 2. 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 2. Direct transmission OPC UA based on CoAP
2.3. 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
Wang, et al. Expires July 29, 2017 [Page 4]
Internet-Draft OPC UA Message Transmission January 2017
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 3.
+ - - - - + + - - - - + + - - - - + + - - - - +
| client | | server | | client | | server |
+ - - - - + + - - - - + + - - - - + + - - - - +
| Hello | | |
| - - - - - - - - - > | | |
| Acknowledge | | |
| < - - - - - - - - - | | |
|OpenSecureChannelReq | | |
| - - - - - - - - - > | | |
|OpenSecureChannelRes | | |
| < - - - - - - - - - | | |
| CreateSessionReq | | |
| - - - - - - - - - > | | |
| CreateSessionRes | | |
| < - - - - - - - - - | | |
| ActivateSessionReq | | |
| - - - - - - - - - > | | |
| ActivateSessionRes | | |
| < - - - - - - - - - | | |
| Request | | CoAPRequest |
| - - - - - - - - - > | | - - - - - - - - - > |
| Response | | CoAPResponse |
| < - - - - - - - - - | | < - - - - - - - - - |
a Traditional OPC UA communication b REST architecture communication
Figure 3. The communication of traditional OPC UA and the REST
transmission
In Figure 3(a), the traditional OPC UA requires a series of
interactions between normal read and write operations. Figure 3 (b)
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[RESTful Industrial
Communication With OPC UA].
Wang, et al. Expires July 29, 2017 [Page 5]
Internet-Draft OPC UA Message Transmission January 2017
3. 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 [draft-ietf-core-
coap-pubsub-00], 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 restricted devices.
4. Security Considerations
5. IANA Considerations
This memo includes no request to IANA.
6. References
6.1. Normative References
6.2. Informative References
[I-D.wang-core-opcua-transmition-requirements]
Wang, H., Pu, C., and Wang, P., " Requirements Analysis for
OPC UA over CoAP", draft-wang-core-opcua-transmition-
requirements-00 (work in progress), December 2016.
[I-D.ietf-core-http-mapping]
Castellani, A., Loreto, S., and Rahman, A., " Guidelines for
HTTP-to-CoAP Mapping Implementations", draft-ietf-core-http-
mapping-17 (work in progress), November 2016.
[Gruer]
Finn, N., Gruer, S., Pfrommer, J., and Palm, F., "RESTful
Industrial Communication With OPC UA", IEEE Trans. Ind.
Informat., Vol. 12, No. 5, October 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 July 29, 2017 [Page 6]
Internet-Draft OPC UA Message Transmission January 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 July 29, 2017 [Page 7]
Internet-Draft OPC UA Message Transmission January 2017
Phone: (86)-23-6246-1061
Email: yjsslcqupt@163.com
Wang, et al. Expires July 29, 2017 [Page 8]