NASA / TMm2002-211199 AIAA-2001-5107
Embedded Web Technology: Applying World Wide Web Standards to Embedded Systems
Joseph G. Ponyik and David W. York Glenn Research Center, Cleveland, Ohio
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Embedded Web Technology: Applying World Wide Web Standards to Embedded Systems
Joseph G. Ponyik and David W. York Glenn Research Center, Cleveland, Ohio
Prepared for the International Space Station Utilization--2001 sponsored by the American Institute of Aeronautics and Astronautics Cape Canaveral, Florida, October 15-18, 2001
National Aeronautics and Space Administration
Glenn Research Center
March 2002 Trade names or manufacturers' names are used in this report for identification only. This usage does not constitute an official endorsement, either expressed or implied, by the National Aeronautics and Space Administration.
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Available electronically at http://gltrs.grc.nas_t.gov/GLTRS EMBEDDED WEB TECHNOLOGY: APPLYING WORLD WIDE WEB STANDARDS TO EMBEDDED SYSTEMS
Joseph G. Ponyik and David W. York National Aeronautics and Space Administration Glenn Research Center Cleveland, Ohio 44135
ABSTRAC F Web client--A client that is designed to communicate with servers using the Hypertext Transfer Embedded Systems have traditionally' been Protocol. developed in a highly customized manner. The user Web server--A server that is designed to interface hardware and software along with the communicate with clients using the Hypertext Transfer interface to the embedded system are typically unique Protocol. to the system for which they are buih, resulting in extra cost to the system in terms of development time and EMBEDDED WEB TECHNOLOGY maintenance effort. INTRODUCTION World Wide Web standards have been developed in the passed ten years with the goal of allowing servers Embedded Web Technology was developed for the and clients to interoperate seamlessly. The client and Fluids and Combustion Facility (FCF_ of the server systems can consist of differing hardware and International Space Station (ISS). FCF is being software platforms but the World Wide Web standards developed to perform investigations in combustion allow them to interface without knowing about the science and fluids physics in the microgravity details of system at the other end of the interface. environment of the ISS. One of the goals of the FCF is Embedded Web Technology is the merging of to be able to perform at least 10 investigations in each Embedded Systems with the World Wide Web. discipline per year for 10 to 15 years. In order to meet Embedded Web Technology' decreases the cost of this goal, it is important for the software to be adaptable developing and maintaining the user interface by to changing requirements. One of the areas of concern allowing the user to interface to the embedded system is the user interface software that the ISS astronauts through a web browser running on a standard personal will use to operate the FCF. The challenge facing the computer. Embedded Web Technology can also be used FCF software engineers is that the laptop computer that to simplify an Embedded System's internal network. the user interface software will operate on is supplied by the ISS, not FCF. In the event that the ISS decides to TERMS AND DEFINITIONS upgrade the laptop computer, the FCF software team will be required to modify existing user interface code To establish a common base of understanding, the for a new environment while still developing new code following definitions will be used: for possibly the old laptop computer and also the new laptop computer. The FCF software engineers are also Client--A logical entity that initiates a request for faced with the task of developing a system that would data or for an action to take place. A client depends accommodate unknown experiments. This scenario can upon the presence of an associated server to perform be extended to any system where the users, hardware requests. A client may refer to client software, client and applications are unknown and the system is hardware, or a combination of the two to implement a expected to have along life. logical client. The FCF software engineers realized that the Server--A complementary logical entity to a World Wide Web had already solved a similar problem. client. A server listens for client requests and services With the World Wide Web, a person uses a web those requests, whether the request is for data or for an browser to request a web page from a web server. In action to be performed. A server may refer to server this scenario, the web server and the web browser software, server hardware, or a combination of the two interface is independent of the hardware and software used to implement a logical server. being utilized at the other end of the interface. Despite
NASA/TM--2002-211199 1 this, the web page is successfully transmitted to the web browser such as Netscape. A typical web client request browser and properly displayed. of a service from a web server will consist of a GET The World Wide Web is based on the Hypertext request. The web server responds to the GET request Transfer Protocol (HTTP), the protocol used by web from the web client by transmitting the requested servers and web browsers to communicate. The FCF information, typically an electronic file. software engineers did an extensive search to find an The following is an example of the HTTP HTTP compliant web server that would fit the messages that are exchanged when a web page gets requirements of FCF. FCF, being an embedded, real- transferred from a web server to a web client. First, the time system, would require the web server to be small, request from the client: operate under VxWorks ®, and still allow the system to meet its real-time requirements. The search failed to GET/index.html HTTP/1.0 find such a web server. Connection: Keep-Alive The FCF software engineering team decided to User-Agent: Mozilla/4.7 [en] (WinNT; U) write their own web server and this proved successful. Host: jgp6290.grc.nasa.gov The web server, known as Tempest, is HTTP Accept: image/giL image/x-xbitmap, image/jpeg, compliant. It implements two of the seven request image/pjpeg, image/png, */* methods defined in the HTTP specification, GET and Accept-Encoding: gzip HEAD. The GET method is a request by a web browser Accept-Language: en for a file from the web server. The HEAD method only Accept-Charset: iso-8859-1 ,*,utf-8 requests header information. These two methods are the only two required to be implemented to make a web The first line tells the server that this client wants server HTTP compliant and also offer a degree of to get index.html and it is following version 1.0 of the security to the system by not allowing the web server to HTTP. The second line, Connection, requests that the accept a file from a web browser. connection be left open. The third line, User Agent, Tempest also met the other needs of FCF. It is describes the web client making the request. The fourth small, requiring less than 50K of memory in its minimal line, Host, is the IP address of the web client. The last configuration, does not take up a lot of disk space, and four lines describe the types of messages the web client has minimal impact on system performance. It is active can receive. only during the times that a web browser is requesting a The response from the web server is the following: file. It can operate at a low priority with adequate response time for the user so that it does not interfere HTTP/I.I 200 OK with the real-time aspect of the system. Host: jgp6290/139.88.219.70 The successful implementation of Tempest made it Date: Mon, 020 Aug 2001 17:27:31 GMT apparent that there are advantages to utilizing other Server: Tempest Java 1.2 (NASA/GRC Java Version of World Wide Web standards in an embedded system. Tempest) These standards allow embedded system projects to Connection: Close take advantage of work being done by thousands of Content-Length: 293 293 developers, thus reducing the problems inherent in Content-Type: text/html developing and utilizing custom protocols. The term "Embedded Web Technology" was the The first line tells the web client that the web name given to this merger of embedded systems with server understands HTTP 1.1 and that the request from World Wide Web Technology. the client is allowed. The second line, Host, is the IP address of the web server. The third line, Date, time OVERVIEW OF THE WORLD WIDE WEB tags the message. The fourth line, Server, identifies the web server software that is responding to the request. The World Wide Web is a collection of protocol The fifth line, Connection, tells the web client that the standards that are controlled by the World Wide Web web server will be closing this connection upon Consortium ®. The protocol standards promote evolution completion of this transaction. The sixth fine, Content- of the World Wide Web and ensure its interoperability, Length, tells the web client how many characters to The key protocol standard behind the World Wide expect in the file to be sent. The last fine, Content- Web is the Hypertext Transfer Protocol, HTTP, Type, tells the web client that the data will be in text. specified in RFC 2616. HTTP is "an application-level After this line, there will be a blank line and then the protocol for distributed, collaborative, hypermedia file requested, index.html, will be sent to the web information systems." (RFC 2616). browser. HTTP is used to transfer information between a In the event that the web page sent to the web web server and a web client, which is typically a web client contains references to image files, applets, etc.,
NASA/TM--2002-211199 2 thewebbrowserwill makeadditionalrequeststo the When an applet tag is found, the web browser webserverfor theseresources.Eachoneof these automalically makes another request to the web server resourceswill requireafull transactionsimilartowhat for the applet. The applet gets sent to the web browser wasjustdescribed. which in turn loads the Java TM Virtual Machine which Thereare manyadditionalitemsthat can be starts running the applet. includedin a transactionthataredescribedin RFC In the typical web environment, applets provide an 2616.For a real-time,embeddedsystemwhere interface that is dynamic but usually does not interact resourcessuchas memoryare limited,it is not with the web server's computer. For security purposes, necessarytoimplementtheentire,protocol. applets are very restricted in what the3' can do in a web browser environment. It is possible to bypass these COMPARISON OF A TYPICAL WEB SERVER restrictions with security certificates if they are an ENVIRONMENT VERSUS AN EMBEDDED, impedirnent. REAL-TIME ENVIRONMENT One capability an applet has by default is the ability to communicate back to the computer that served A typical web server runs on a computer that is it to the web browser. This comnmnication can be running a nonrealtime operating system. The web accomplished with basic socket technology, Java's TM server software itself tends to be large and complex and Remote Method Invocation (RMI), Common Object requires a lot of memory and dist, space, on the order of Request Broker Architecture (CORBA ®) technology, or 10 MB or more. The web servers are designed to run on other protocols. RMI is a communication technology personal computers and general purpose workstations. specific to Java TM that allows networked Java TM The embedded, real-time environment is much programs to interface in a platform independent more restricted. The system generally consists of an manner. CORBA ®. developed by the Object embedded processor running a real-time operating Management GroupTSL is a technology that allows system and also running a real-time application. networked programs to communicate in a common Embedded systems have limit_'d memory and disk manner that is independent of the underlying hardware, space that is not easily extensible operating system or language. With this capability, it is The users of these two environments have different possible to develop an applet that can interact needs of the systems. The user of the nom'ealtime dynamically with an embedded, real-time system and, environment is usually interested in obtaining static or thus, let the web browser provide a user interface that slowly changing information. This information is stored meets the needs of the user. in files and retrieved by the web browser for display by On the embedded system side, the problem is the user. The user can read the information on the providing the capability to be a web server without display at his own pace or pnnt it out and read it overburdening the system with all of the functionality without the aid of the browser. A user of a real-time specified in the HTTP specification. The embedded system, on the other hand, is interested in obtaining the system software still needs to be able to perform real- most current data from the system at regular intervals. time command and control. This problem was solved The user may require the capability to issue commands by the development of Tempest. to the embedded system. TEMPEST FEATURES PUTTING EMBEDDED SYSIEMS ON THE WEB The Tempest software, which was written by There are two problems thai need to be solved in software engineers at the NASA Glenn Research Center order for an embedded system to become accessible on in Cleveland, Ohio, is a web server written specifically the World Wide Web. One is providing a user interface for embedded, real-time systems. Tempest was that provides the real-time interaction needed by the originally written for the VxWorks ® operating system user in order to properly interface to the system. The from Wind Rivers Systems, Inc. and then ported to the other is to give the embedded system the capability to Java TM language so that it can run on any operating serve web pages over the World Wide Web. system that has a Java TM Virtual Machine. One feature of the web browser that helped bridge Tempest requires fewer memory resources than the gap between the two environments is the addition of web servers written for the typical web server
Java TM applets. Applets are programs that are capable environment. Memory requirements are under 100 KB, of being executed by a web browser. They are written depending on how it is configured. The amount of disk in the Java TM language, compiled and then stored on the space is also under 100 KB with additional space web server's computer. When a web page is transmitted needed for the files that make up the web pages, images to a web browser, the web browser scans through the and applets. Since Tempest is not intended to operate web page, looking for, among other things, applet tags. as a general purpose web server, it is not necessary to
NASA/TM--2002-211199 3 implementthe entireHTTPspecification.Onlythe dynamically alter a web page at the time of request. GETandHEADrequestmethodsfrom theHTTP Tempest accomplishes this by reading through any file specificationareimplemented.Methodsthatallowa the has an extension of ".sht", ".shtm" or ".shtml" and webbrowserto write to the web serverare not searching for VxWorks Operating Tempest will not process any request that contains System Task Information "..". The ".." (double dot) is used on most computer systems to refer to the parent directory on the disk. By not allowing this, it is not possible for remote users to
Includes (SSI). SSI is the ability of the web server to
NASA/TM--2002-211199 4
VxWorks®:isaregisteredtrademark THE RESULTING IMPLEMENTATION of
NAME ENTRY TID PRI STATUS PC SP ERRNO DELAY ...... tExcTask excTask 3e8a70 0 PEND 7b7fe 3e89cc 0 0 tLogTask _logTask 3e615c 0 PEND 7b7fe 3e60b4 0 0 tShell shell 3adelc 1 PEND 597f4 3adacc ic0001 0 tTelnetd -telnetd 3c25c8 2 PEND 597f4 3c24dc 0 0 tWdbTask 58802 3af23c 3 PEND 597f4 3af0d8 0 0 tScsiTask _scsiMgr 3e31c8 5 PEND 597f4 3e315c 0 0 tNetTask netTask 3dda6c 50 PEND 597f4 3ddal4 0 0 tFtpdTask _ftpdTas9 3bfb70 55 PEND 597f4 3bfa8c 0 0 tTftpdTask _tftpdTa_k 3bcgb8 55 PEND 597f4 3bc260 0 0 mon_tempestfb06e 392940 60 DELAY 2cafa 3928fc 0 5 http2 f9a5a 39917c 75 DELAY 2cafa 398dd4 ic0001 1 t7 taskSho_ 37d0f0 76 READY 2f12e 37c360 0 0 tPortmapd _portmap4! 3ci09c i00 PEND 597f4 3c0f54 16 0 tempest f9a5a 38b3a8 i00 PEND 597f4 38b2a4 0 0
VxWorks ®is a registend trademark of Wind River Systems, Inc.
The Java TM version of Temp,:st has features similar responds with a request for the user to be authenticated. to the VxWorks ® version. All of the configuration files Once the user is authorized, Tempest retrieves the web except for commands.sys will work with both versions. page from local storage. If the file has an extension of Commands.sys needs to be modified to include ".sht", "'.shtnf', or ".shtml", Tempest reads through the commands that will work on the platform that Tempest file and processes any Tempest tags When this step is will be operating on. The
The Java TM version of Tempest also includes a tag starts up the applet. to allow Tempest to call another Java TM class external The applet establishes a connection back to the to Tempest and insert the resultant output into the web embedded system, utilizing a different TCP port form page. The format is
NASA/TM--2002-211199 5 userinterfacescreensrunin a separatetaskfromthe Tempest also provides the capability to provide interfaceto the embeddedsystem.This gives a output to the remote user in any format the system smootherrunninginterfaceandalsomakesit easierto designer desires. Although the Tempest tag is typically recoverfromcommunicationdropswithoutlockingthe embedded into a web page that is html, it is possible to userout. set up a file that consists of only a Tempest tag calling a By takingadvantageof theCORBA®technology, local command that outputs something other than html, moreflexibility is addedto the system.CORBA® such as XML. As an example, Tempest could be used providesan interfacethat is independentof the to feed real-time data into a database using XML. underlyinghardwareplatform,operatingsystemand implementinglanguage.Futureupgradesto the SUMMARY embeddedsystemorthewebbrowserwillbeeasierto implementsincethechangewill betransparentto the Embedded Web Technology provides for the otherendoftheinterface. development of an embedded, real-time system that appears to the users of the system as a node on the OTHERADVANTAGES World Wide Web. This capability provides for great savings by eliminating the need to develop and In addition to the above features, utilizing distribute user interface software that is platform Embedded Web Technology also provides an added specific and somewhat cumbersome to configuration security feature. Since the web browser does not store manage. the applet permanently, the user interface software is Tempest software is a small, flexible web server not accessible to unauthorized users once the web that makes it easy to interface to embedded systems. It browser is turned off. It is possible to configure present also has the potential to be an aid in debugging systems. day web browsers to eliminate the cache so that the Tempest workshops have been held for customers applet gets deleted. in the private sector. Customer remote data acquisition User interface software upgrades are simpler with and control applications include medical, tele- Embedded Web Technology. Without Embedded Web communications, aerospace, factory automation, Technology, the user of the real-time system also needs instrumentation, automotive, building management and to have the user interface software stored locally. The education. result is that the user is restricted to using only the Tempest is available from the NASA Glenn computer that has the user interface software loaded Research Center by contacting the Commercial and upgrades to the software can be more difficult to Technology Office at 216--433-3484. obtain. The updates require the user to find out about the upgrade and then a process needs to be put in place REFERENCES to deliver the upgraded software to the user. The upgraded software may also need to be capable of RFC 2616: Hypertext Transfer Protocol----HTTP/l. 1, running on vffrious platforms, making upgrades more http://www, w3.org/Protocols/rfc2616/rfc2616, html difficult for the developer. World Wide Web Consortium®---http://www.w3.orff With Embedded Web Technology, the user XML---http://www.w3.org/XML/ interface software is stored in the embedded system and CORBA ® and Object Management Group TM--- delivered to the user when it is needed. When the user http://www.omg.org/ interface software is upgraded, it only needs to be JavaTUt---http://java.sun.com/ stored in the embedded system. The user gets the new software the next time they access the system.
NASA/TM----2002-211199 6
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Embedded Web Technology: Applying World Wide Web Standards to Embedded Systems WU-398-20-(O-00 6. AUTHOR(S)
Joseph G. Ponyik and David W. York
7. PERFORMINGORGANIZATIONNAME(S)ANDADDRESS(ES) 8. PERFORMING ORGANIZATION REPORT NUMBER National Aeronautics and Space Administration John H. Glenn Research Center at Lewis Field E-13039 Cleveland, Ohio 44135-3191
9. SPONSORING/MONITORINGAGENCYNAME(S)AND ADDRESS(ES) !10. SPONSORING/MONITORING AGENCY REPORT NUMBER National Aeronautics and Space Administration NASA TM--2002-211199 Washington, DC 20546-0001 AIAA-2001-5107
11. SUPPLEMENTARY NOTES
Prepared for the International Space Station Utilization--2001 sponsored by the American Institute of Aeronautics and Astronautics, Cape Canaveral, Florida, October 15-18, 2001. Responsible person, Joseph G. Ponyik, organization code 7750, 216-433-8592.
12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTIONCODE Unclassified - Unlimited Subject Category: 62 Distribution: Nonstandard Available electronically at htto://_ltrs._rc.na.sa._ov/GLTRS This _aublication is available from the NASA Center for AeroSpace Information. 301--621-0390. 13. ABSTRACT(Maximum 200 words)
Embedded Systems have traditionally been developed in a highly customized manner. The user interface hardware and software along with the interface to the embedded system are typically unique to the system for which they are built, resulting in extra cost to the system in terms of development time and maintenance effort. World Wide Web standards have been developed in the passed ten years with the goal of allowing servers and clients to interoperate seamlessly. The client and server systems can consist of differing hardware and software platforms but the World Wide Web standards allow them to interface without knowing about the details of system at the other end of the interface. Embedded Web Technol- ogy is the merging of Embedded Systems with the World Wide Web. Embedded Web Technology decreases the cost of developing and maintaining the user interface by allowing the user to interface to the embedded system through a web browser running on a standard personal computer. Embedded Web Technology can also be used to simplify an Embedded System's internal network.
14. SUBJECTTERMS 15. NUMBER OF PAGES 12 Embedded systems; Real-time systems; Web server; Tempest 16. PRICECODE
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