An implementation of embedded RESTful Web services.pdf

Abstract—Web service is a common Internet application that
enables interactions of machines over a network. As to establish
ubiquitous Internet, enabling Web services on embedded
systems is certainly among the development trend in near
future. This paper presents an implementation of REST style or
RESTful Web services on embedded system. The prototype had
been implemented using a Xilinx Spartan-3E Starter FPGA
board for home device control application on a 100Mbps LAN
environment.
I. INTRODUCTION
HE Web service is an Internet application that enables
interactions between machines. It is widely deployed by,
yet not limited to business organizations. Current Web
services enable a services provider to publish its available
services on the Internet, while the clients may freely search
and invoke these services through the Internet. Marching
towards the vision of ubiquitous Internet, enabling Web
services on embedded system is certainly on the to-do-list of
the consumer electronics industry for the near future.
This paper provides brief introduction and discussions
towards embedded Web services. It then presents an
implementation of embedded RESTful Web services on a
Xilinx Spartan-3E Starter FPGA Board for home device
control application on a 100Mbps LAN environment.
II. LITERATURE REVIEWS
Two major trends of attempts for embedded Web
applications had been observed during the past decade,
namely dynamic HTML Web server and SOAP Web
services. Both of them are built on top of HTTP and TCP,
the 2 well-known protocols of the Internet.
Fig. 1 Protocol stack of Web services (based on TCP/IP model)
Manuscript received 12th
June, 2009. This work was supported in part
by the Panasonic R&D Centre Malaysia (PRDCM).
C. E. Chang is with the Multimedia University, Cyberjaya, Malaysia (e-
mail: childs72@gmail.com).
F. Mohd-Yasin is with the Multimedia University, Cyberjaya, Malaysia
(e-mail: faisal.yasin@mmu.edu.my).
A. K. Mustapha was with the Multimedia University, Cyberjaya,
Malaysia (e-mail: azhar@alum.mit.edu).
A. HTTP and TCP
HTTP (Hypertext Transfer Protocol) [1] is a protocol
designated for request-response communication of machines
in server-client mode. A server-client mode refers to a
communication mode where server receives request massage
from client, processes the request, lastly replies with
response message to the client. HTTP message may be in
either request or response form. A HTTP message contains
of various HTTP headers that contain information of the
application, and optional payload data that contain actual
information. The payload data may exist in the form of plain
text, HTML, picture, XML, etc.
On the other hand, TCP (Transmission Control Protocol)
[2] is a reliable, connection-oriented transport protocol. It
has been widely adopted by the Internet for over 20 years.
TCP receives message from its Application layer to transmit
over the network and vice versa. It employs various
algorithms and mechanisms to provide 2 main attributes:
connection reliability and network performance. In short, it
is designed to ensure the data transfer between 2 TCP hosts
is done without error at optimized performance.
B. Dynamic HTML Web Server and Related Works
Dynamic HTML (Hypertext Markup Language) Web
server had been one of the most common methods of
accessing information through the Internet for a good many
years. Upon receive request from client, the Web server
generates dynamic HTML pages via CGI (Common Gateway
Interface) script or ASP (Active Server Pages) to fulfill the
request.
Among the examples of embedded HTML Web server
implementations, C.N. Coelho et al. [3] had connected an
uninterrupted power supply (UPS) system to the Internet
using an x86 processor. J. Huang et al. [4] had implemented
Embedded Temperature Web Controller (ETWC) using an
8-bit microcontroller with simplified TCP/IP stack to work
as a Web based device controller. T. Lin et al. [5] had also
implemented Webit using 8-bit microcontroller to attach
mini Web server ability to equipments, such as an air-
conditioner. Besides, J. Riihijärvi et al. [6] had presented the
WebChip to host a Web page on an APEX 2K100, a FPGA
(Field Programmable Gate Array) board from Altera
Corporation.
C. SOAP Web Services and Related Works
Web service differs with traditional Web architecture
mainly by its feature of loose coupling between client and
server through the use of Extension Markup Language
An Implementation of Embedded RESTful Web Services
C.E. Chang, F. Mohd-Yasin, and A.K. Mustapha
T
2009 Conference on Innovative Technologies in Intelligent Systems and Industrial Applications (CITISIA 2009)
Monash University, Sunway campus, Malaysia, 25th & 26th July 2009.
978-1-4244-2887-8/09/$25.00 ©2009 IEEE 45

(XML) format. XML is a very general markup language that
had been recognized as “the ASCII of the Web”.
Transferring data in XML format is much shorter than that in
HTML format. Besides, it also allow client to easily perform
post-processing towards the desired data, instead of the
HTML page, that are received. [7] Current Web services
style is dominated by SOAP (Simple Object Access
Protocol) style; followed by its rising competitor REST
(Representational State Transfer) style.
Fig. 2 shows how Web services work. First, the Web
services providers will publish their services and related
information (e.g. WSDL) on a Web services repository (e.g.
UDDI). The client may search for the services they desired
on the repository. Based on the services information
acquired from the repository, the client may invoke the
services from the service provider or server. The service
invocation is a request-response communication, where the
request and response messages are wrapped in certain
packaging format (e.g. SOAP or REST) and transmitted
between client and server.
Embedded Web services implementation are normally
done on microcontroller or microprocessor with higher
computation power using special programming toolkits. For
instance, G. Bucci et al. [8] had implemented SOAP/WSDL
Web services on a RabbitCore RCM2100 board using
ASP.NET. G.B. Machodo et al. [9] had also implemented
SOAP/WSDL Web services on an ARM-based SHIP board
using gSOAP toolkit. O. Almeida et al. [10] had
implemented SOAP-based Web services on an Atmel EB63
Arm7 board using Microsoft Invisible Computing software
platform. Lastly, the only FPGA implementation of SOAP
Web services was carried out by S. Cuenca-Asensi et al. [11]
on a Celoxica RC203E FPGA board.
III. JUSTIFICATIONS
Section II had reviewed that Web services possess 3
advantages over dynamic HTML Web server by the use of
XML data format: loose coupling between server and client,
shorter message length, and flexibility for post-processing at
client side [7].
For both conventional and embedded Web services
implementations, SOAP appears to be the main
concentration of current trend. SOAP is a well-established
standard and supported by majority of vendors and
companies in the industry. Meanwhile, REST is a Web
service style that is newer but supported by a good number
of individual Web developers. Proposed by R.T. Fielding
[12], one of those who helped to develop HTTP, REST is
highly coupled with HTTP. As both SOAP and REST
normally sit on top of HTTP, REST, which extended from
HTTP, is thus much plainer and simpler to process than
SOAP. It is generally agreed that SOAP is more suitable for
applications of higher complexity; while REST fits better for
less complicated applications [13].
Besides, REST produces service messages with shorter
length than that of SOAP Web services [14]. Table 1 shows
the brief differences between SOAP and REST style Web
service messages. For service requests, a SOAP service
request exists as HTTP payload in SOAP envelope while
REST service request message resides in HTTP headers.
Hence the operation of request interpretation is fairly simpler
in REST because the Web server can directly extract the
request message from the HTTP headers instead of parsing
the HTTP payload data. A SOAP service response exists as
HTTP payload in SOAP envelope. For REST on the other
hand, currently, the RESTful Web service has not yet strictly
defines the format of the REST response message.
Since RESTful Web service uses shorter message and its
overall architecture is significantly simpler compared with
SOAP, it requires less processing power and memory
compared with SOAP. From the point of view of embedded
system design, a RESTful Web service is therefore more
suitable option than a SOAP Web service.
IV. IMPLEMENTATION OF RESTFUL WEB SERVICES
Fig. 3. Protocol stacks of prototype
TABLE 1
BRIEF DIFFERENCES BETWEEN SOAP AND REST MESSAGE FORMAT
SOAP REST
Request
Format
HTTP request headers
(usually POST method)
with SOAP envelope as
payload data; service
request resides in SOAP
envelope.
HTTP request headers only
(usually GET or POST
method); service request
resides in method and URI
fields in HTTP headers.
Response
Format
HTTP response headers
with SOAP envelope as
response resides in SOAP
envelope.
HTTP response headers
with XML message as
response resides in XML
message.
Fig. 2. Web services operations
46

To better illustrate how to embed RESTful Web services,
a simple prototype had been designed and implemented. The
prototype of this embedded RESTful Web services is
targeted to provide home device control application. It had
been designed using VHDL (Very High Speed Integrated
Circuit Hardware Description Language) and implemented
on a FPGA board. The prototype design covers from layer 2
(Data Link layer) to layer 5 (Application layer) of the Web
service as shown by Fig. 3. The prototype was designed as a
passive server, which only responses to incoming request or
communication instead of initiating communication towards
any device.
Fig. 4 shows the complete block diagram of the embedded
RESTful Web services. There are 3 main external interfaces
on the server. The MII (Media Independent Interface)
connects the server to the physical layer of 10/100Mbps
LAN (Local Area Network). The home appliance device to
be controlled is connected to the server through a device
interface. Lastly, a SPI (Serial Peripheral Interface) is used
to store the non-volatile information, such as the application
information, services descriptions and server configuration
information, when the server is turned off. Inside, the
prototype is divided into 2 parts: the TCP/IP Processor and
the Application.
A. TCP/IP Processor
The TCP/IP Processor resembles a reduced network
processor for the Web service application. As shown in the
diagram, it supports basic protocols of TCP/IP stack:
Ethernet, ARP, RARP (layer 2), IP, ICMP (layer 3), and
TCP (layer 4). These protocols are each handled by
individual module. Error detection mechanisms, such as
CRC checking for Ethernet and checksum calculation for IP,
ICMP and TCP, are implemented to ensure correctness of
data received from LAN.
Due to resource minimization consideration, each protocol
module is designed to provide only basic functionality
necessary for a passive Web server. The server uses static IP
Fig. 5. TCP/IP Processor operation flow
Fig. 4. Block diagram of embedded RESTful Web services
47

address. The TCP/IP Processor is able to receive and send
TCP segment. Aside to TCP handling, it only processes and
responses to “ping” (ICMP), ARP and RARP requests. Other
messages received with unrecognized protocols or
unsupported operations will be discarded at once.
The TCP/IP buffer size is limited to only single packet or
576 bytes of data at IP layer. Only 1 TCP segment can be
received or sent at one time. Consequently, some of the
conventional TCP features and algorithms, that are important
to support TCP performance to send or receive large amount
of data (typically more than single TCP segment), are safely
omitted in this prototype. These algorithms and features
include Nagle Algorithm, Slow Start, Congestion Control,
Fast Retransmit, Fast Recovery, window buffer management
and multiple simultaneous connections [15]. Separate buffers
are employed by TCP/IP Processor and Application to allow
TCP segment retransmission to assure the reliability of the
connection.
B. Application
The major difference of data type processed by TCP/IP
Processor and Application is the former handles protocols
headers in binary data format; while the latter handles
protocols headers and Web services information in character
(ASCII) format. In TCP/IP Processor, 16-bit width buffers
and data buses are employed to speed up the process as well
as to ease the checksum and CRC calculations (where both
are calculated by 16-bit data width basis). In Application,
however, 8-bit width buffers and data buses are used to ease
the processing of ASCII characters.
Fig. 6 presents the operation flow of the Application. The
main modules responsible to handle REST request reception
and response generation are the HTTP and XML Writer
modules. Upon receives a valid TCP/IP segment, the HTTP
module checks for the data validity as a HTTP request. It
then extracts the REST Web service information from the
HTTP headers and verifies the service request. The core
application unit will then process the request. Either a
services response message or the services descriptions will
be sent back to the client in XML format through the XML
Writer module. When the response to be sent is larger than
the available buffer size (576 bytes after includes TCP/IP
headers, as mentioned earlier), the core application unit will
halt at once [16] and the XML Writer will send out the
incomplete service response. After TCP/IP Processor
receives an acknowledgement that indicates the partial
response has been received by the client host, the core
application unit shall resume until full response is completely
sent to the client. This design that only allow single TCP
segment to be sent at a time can degrade the system
performance. However, such scenario where large response
message size is less likely to happened in this prototype by
the use of REST style Web services with very plain XML
data format.
Simple responses in XML format are defined for the
prototype Application by using few tags and messages as
shown in Table 2.
C. Implementation and Testing
Considering the ease of access towards MII and SPI
memory, Spartan-3E Starter Board, an FPGA board product
from Xilinx Inc. [17], had been selected as the prototype
platform. Spartan-3E Starter Board has both MII and SPI
memory onboard. The server is operating with 50MHz clock
rate on a 100Mbps LAN.
For the prototype testing, a washing machine simulator
from Bytronic International Ltd. [18] was chosen to act as
the target home appliance device to be controlled by user
Fig. 6. Application operation flow
TABLE 2
PREDEFINED TAG NAMES AND ELEMENT NAMES
Tag Name Element Name Description
REST (none)
Contain the entire REST response
message.
RET
(retrieved
information)
Contain retrieved information as
requested by client.
NO_MATCH
Request failed as targeted
information cannot be found.
FUNC_ERR
Request failed as it has an invalid
request format.
MSG
DONE
Request to control appliance is
successfully performed.
48

through embedded RESTful Web services. A Java program,
Direct Socket Control v6.0 (DSC) [19] had been used to
open a HTTP connection from a PC as a Web service client
to communicate with the server and invoke the RESTful
Web services. Fig. 7 shows a screenshot of the DSC
program, running on a PC, acting as client to communicate
with the server, operating on the Spartan-3E Starter Board,
through 100Mbps LAN. The services messages are
noticeably simple and short. Note that at the left side of the
screenshot, the symbol “[ >]” indicates a message sent by
the client to the server; while the symbol “[ <]” indicates
the vice versa. The screenshot shows a few services request-
response communications between the client and the server:
1. Client requested to retrieve Web services
descriptions;
2. Server replied with Web services descriptions in
XML format;
3. Client requested to retrieve information on available
function on target home appliance;
4. Server replied with list of available functions;
5. Client requested to execute function “Spin”;
6. Server executed function “Spin” and replied to notify
that function successfully performed;
7. Client requested to execute an unavailable function
“Fly”;
8. Server replied to notify that “Fly” function is not
available.
The total resource usages of the embedded server on the
Spartan-3E Starter Board are 2793 flip-flops (FFs), 7958 4-
input look-up tables (LUTs) and 4180 slices. The breakdown
of resource used by main modules is shown as table 3 below.
Note that this entire prototype covers both TCP/IP Processor
(starts from layer 2) and Application (until layer 5). The
modules responsible to dispatch request information and to
generate response of REST messages (HTTP and XML
Writer modules) only consume altogether 371 FFs and 36
MUXs.
V. CONCLUSIONS
By enabling the Web services on embedded systems,
significant power and resource savings can be achieved
compared with conventional Web applications implemented
on general computing platforms (such as PCs). Such power
and resource cut down will show tremendous implication
during the arrival of ubiquitous Internet era, which is just
around the corner.
This paper proposes to embed the REST style Web
services. Compared with dynamic HTML Web server and
SOAP Web services, REST style Web services provide
simpler processing and shorter message length, thus requires
lower resource. Besides, it also provides flexibility on client
Fig. 7. Screenshot of DSC communicating with embedded
Web server
TABLE 3
RESOURCE USAGE BY MAIN MODULES
Modules FF MUX
TCP/IP Processor:
1. MII Transceiver 124 -
2. CRC 87 -
3. Ethernet 50 35
4. ARP/RARP 35 35
5. IP 79 71
6. ICMP 45 77
7. TCP 513 64
8. Checksum 40 -
Application:
9. HTTP 277 36
10. XML Writer 94 -
11. Application 478 41
12. Device Interface 20 -
Miscellaneous:
13. SPI Interface 67 -
49

side post-processing over the dynamic HTML Web server. A
prototype of embedded RESTful Web services had been
implemented on FPGA board and tested on 100Mbps LAN
environment. As results of the simplicity provided by REST
style Web services, resource reductions are achievable
throughout the Web server design. Estimated short REST
style messages allowed the implementation of single segment
TCP module. Meanwhile, the less complicated REST
messages format allowed the design of a plain top level
Application module.
In a nutshell, RESTful Web service is almost as useful as,
yet simpler than the SOAP Web service. Therefore the REST
should be promoted as the mainstream of embedded Web
services development. Further researches to support the
embedded RESTful Web services include to optimize the
REST messages processing as well as to standardize the
REST Web services for embedded Web applications.
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