Web Dev Research

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Presentation about Web services developed for my degree's web programming module.
Includes TCP/IP, FTP and HTTP due to assignment requirements.

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Web Dev Research

  1. 1. BSc(Hons) Computing & Internet Technology Web Development Research NATHAN THOMAS MARCH 2009
  2. 2. TCP & The Transport Layer ‘The transport layer has four main functions: 1. It sets up and maintains a session connection between two devices 2. It can provide for the reliable or unreliable delivery of data across this connection 3. It can implement flow control through ready/not ready signals or windowing to ensure one device doesn’t overflow another device with too much data on a connection 4. It multiplexes connections, allowing multiple applications to simultaneously send and receive data. [Using Port Numbers]’, (Meyers 2004). TCP divides the data into segments, opens a full-duplex connection using the network layer, and passes the segments on to the network layer. It also ensures that all segments arrive at the other end correctly.
  3. 3. 3 Way Handshake
  4. 4. Reliable and Unreliable Connections  Transmission Control Protocol Well Known/Reserved Port Numbers: (TCP) and Sequenced Packet Exchange (SPX) use the 3 way Application Port Number(s) Reliable/Unreliable handshake, so are termed ‘reliable’ Connection protocols at the transport layer. FTP 20 and 21 TCP Reliable Connection telnet 23 TCP Reliable  Sometimes the full 3 way Connection handshake is superfluous. SMTP (email) 25 TCP Reliable Connection  User Datagram Protocol (UDP) is DNS 53 UDP Unreliable Connection an ‘unreliable’ protocol and can be Trivial FTP (FTP 69 UDP Unreliable used in this case. without login) Connection HTTP 80 TCP Reliable Connection  Segments sent using UDP have no POP mail 110 guarantee of reaching their destination. UDP assumes this will SNMP 161 UDP Unreliable be handled by the application. Connection (Meyers 2004)
  5. 5. IP & The Network Layer ‘The network layer is responsible for: 1. Defining logical addresses used at the network layer 2. Finding paths, based on the network numbers of logical addresses, to reach destination devices 3. Connecting different media types together, such as Ethernet, FDDI, Serial and Token Ring’, (Meyers 2004). Network Layer Protocols: Internet Protocol (IP) • Novell’s Internetwork Packet • Exchange (IPX) AppleTalk. • TCP segment is encapsulated in an IP packet. The source IP address and destination IP address are defined in this packet. The IP packet is passed to the Data Link Layer.
  6. 6. HTTP, FTP and the Web  File Transfer Protocol (FTP) existed when the web was first developed, but was ‘not optimal for the web, in that it was too slow and not sufficiently rich in features’, (Berners-Lee 1996). So the Hyper-Text Transfer Protocol (HTTP) was created.  The Web is formed around three common standards: Hyper-Text Transfer Protocol (HTTP), the Address Space (URLs) and Hyper-Text Mark-up Language (HTML).  In the same way that the TCP/IP model abstracts lower layers, the Web was designed around the same principle. So, for instance, anybody designing anything on top of HTTP did not have to know how HTTP worked. ‘A test of this ability was to replace [layers] with older specifications, and  demonstrate the ability to intermix those with the new. Thus, the old FTP protocol could be intermixed with the new HTTP protocol in the address space, and conventional text documents could be intermixed with the new hypertext documents’, (Berners-Lee 1996).
  7. 7. How a Web Browser Works  The browser determines the URL (by seeing what was selected).  The browser asks DNS for the IP address of www.itu.org  DNS replies with 156.106.192.32.  The browser makes a TCP connection to port 80 on 156.106.192.32.  It then sends over a request asking for file /home/index.html.  The www.itu.org server sends the file /home/index.html.  The TCP connection is released.  The browser displays all the text in /home/index.html.  The browser fetches and displays all images in this file.’ (Tanenbaum 2002)
  8. 8. Uniform Resource Locators (URLs)  URLs can be broken into three parts: the protocol (or scheme), the server location and the page location.  Some common URLs: Name Used For Example http Hypertext (HTML) http://www.w3c.org/ ftp FTP ftp://ftp.cs.vu.nl/pub/minix/ README file Local file file://c:/readme.txt news Newsgroup news:comp.os.minix gopher Gopher gopher://gopher.tc.umn.edu /11/Libraries mailto Sending Email mailto:user@domain.net telnet Remote Login telnet://www.w3.org:80
  9. 9. HTTP Methods Method Description GET Request to read a Web page HEAD Request to read a Web page’s header PUT Request to store a Web page POST Append to a named resource (e.g. a Web page) DELETE Remove the Web page TRACE Echo the incoming request CONNECT Reserved for future use OPTIONS Query certain options (Tanenbaum 2002)
  10. 10. FTP Methods Method Description RETRIEVE (RETR) Causes the server to transfer a copy of the file, specified in the pathname, to the server or user at the other end of the data connection. STORE (STOR) Causes the server to accept the data transferred via the data connection and to store the data as a file at the server site. APPEND (with create) (APPE) Causes the server to accept the data transferred via the data connection and to store the data in a file at the server site. If the file specified in the pathname exists at the server site, then the data shall be appended to that file; otherwise the file specified in the pathname shall be created at the server site. RESTART (REST) The argument field represents the server marker at which file transfer is to be restarted. This command does not cause file transfer but skips over the file to the specified data checkpoint. RENAME FROM (RNFR)RENAME TO (RNTO) Renaming file ABORT (ABOR) Aborts the previous command. DELETE (DELE) Causes the file specified in the pathname to be deleted at the server site.
  11. 11. Status Responses  Both HTTP and FTP will respond to requests with a status code. Some example HTTP status responses are: Code Meaning Examples 1xx Information 100 = server agrees to handle client’s request 2xx Success 200 = request succeeded; 204 = no content present 3xx Redirection 301 = page moved; 304 = cached page still valid 4xx Client error 403 = forbidden page; 404 = page not found 5xx Server error 500 = internal server error; 503 = try again later
  12. 12. Web Services  ‘Web services are server-side programs that listen for messages from client applications and return specific information’, (Watson et al 2005).  ‘Web services are meant to address the problems of connecting disparate systems, creating single repositories, and working towards the holy grail of programming – code reuse’, (Evjen et al 2007).  Web service returns the information in XML format.  XML can be used by any type of application, (Traditional or Web), on any platform.  Therefore Web services enable us to transfer data between disparate systems.  Enable us to maintain a central information store which is compatible with all clients.
  13. 13. Before Web Services Remote Procedure Call (RPC)   Allows you to call (or ‘invoke’) a procedure (program method) at a remote location.  Many different RPC protocols – mostly incompatible  CORBA (Common Object Requesting Broker Architecture)  Developed in 1991 by the Object Management Group, added object-orientation to RPC, (RPC was procedural).  ‘Because OMG didn’t define a reference implementation... the servers of these vendors didn’t really interoperate... HP server needed an HP client... IBM server an IBM client, and so on’, (Watson et al 2005).  DCOM (Distributed Component Object Model)  ‘In the past, it was possible to work with DCOM to port data from one point to another to solve almost the same problem that Web services are now addressing. However, with DCOM, requests and responses were required to ride on top of a proprietary communication protocol’, (Evjen et al 2007).  Distributed COM - Microsoft extended DCE-RPC protocol with object-oriented features.  Not picked up by users of non-Microsoft platforms.  RMI (Remote Method Invocation)  Sun’s RMI added to CORBA and COM, but was aimed at Java platform and required a JVM.
  14. 14. Web Services Architecture: UDDI – find a web service Universal Description, Discovery and Integration (UDDI) Web Services may be registered in a registration directory service, although not a requirement. UDDI provides details about: The owner  The Web service’s name and type  The Web service’s location  What the web service requires and what it returns (WSDL)  Primary UDDI sites are: http://uddi.xml.org/  www.xmethods.net  These directories can be searched by the owner, type of web service, name etc.  There are sites that exist that focus on Web services with the same subject matter – these are  called Portals.
  15. 15. Web Services Architecture: WSDL –what methods can be called? Web Services Description Language (WDSL)  ‘When you find a Web service that you want to include in your application, you must first figure  out how to supply the Web service with the parameters it needs in order for it to work. That need also extends a bit further. Even if you know the parameters and types that are required for instantiation, you also need to understand the types that are passed to your application in return’, (Evjen et al 2007). ‘A WSDL document has the information about the methods a Web service supports and how  they can be called, parameter types passed to the service, and parameter types returned from the service’, (Watson et al 2005). WSDL enables client application to create a proxy for communicating with the web service,  using SOAP calls.
  16. 16. Web Services Architecture: SOAP – calling a method Simple Object Access Protocol (SOAP)  Maintained by W3C.  SOAP is just XML, works like XML and provides the same inter-operability as XML.  ‘A SOAP message is the basic unit of communication between a client and a server’, (Watson et  al 2005). ‘The SOAP specification itself is made up of the following parts:   A description of the SOAP envelope and how to package a SOAP message so that it can be sent via a transmission protocol such as HTTP.  The serialization rules for SOAP messages.  A definition of the protocol binding between SOAP and HTTP.  The capability to use SOAP for RPC-like binding’, (Evjen et al 2007).
  17. 17. Example SOAP Request ‘Hello World!’ string passed as argument to Web service method: ReverseString()  Before the start of the XML SOAP message, is the HTTP header, because the SOAP message is  sent with a HTTP POST request... POST /WebServiceSample/Service1.asmx HTTP/1.1 Host: localhost Content-Type: text/xml; charset=utf-8 Content-Length: 508 SOAPAction: “http://www.wrox.com/webservices/ReverseString” <?xml version=”1.0” encoding=”utf-8” ?> <soap:Envelope xmlns:xsi=http://www.w3.org/2001/XMLSchema-instance xmlns:xsd=http://www.w3.org/2001/XMLSchema xmlns:soap=”http://schemas.xmlsoap.org/soap/envelope/”> <soap:Body> <ReverseString xmlns=”http://www.wrox.com/webservices”> <message>Hello World!</message> </ReverseString> </soap:Body> </soap:Envelope> The server answers with a similar SOAP message with the ‘ReversStringResult’ attribute set to  ‘!dlroW olleH’.
  18. 18. Alternative Protocols ‘XML-RPC, ebXML and REST can be used in a similar manner to SOAP. They all provide  structure to any remote-procedural call you need’, (Evjen et al 2007). Alternatively, ‘just do a simple HTTP GET or HTTP POST request without the overhead of a  SOAP call: HTTP POST request:  POST /WebServiceSample/Service1.asmx/ReverseString HTTP/1.1 Host: localhost Content-Type: application/x-www-form-urlencoded Content-Length: length message=string THE HTTP GET request is even shorter. The disadvantage of the Get request is that the size of  the parameters sent is limited. If the size goes beyond 1K, you should consider using POST: GET /WebServiceSample/Service1.asmx/ReverseString?message=string HTTP/1.1 Host: localhost ‘The disadvantage here is that there is no support from Web services on other platforms and no  support for sending anything other than simple data.’, (Watson et al 2005)
  19. 19. Representational State Transfer (REST) ‘In a pure REST system, resources are the entities exposed by the service: the products you sell, the customer  records you view, the pages you interact with. Each resource should have a unique URL that defines it, such as http://www.mysystem.com/products/5323. Accessing that URL using an HTTP GET request should return a representation of that resource, in this case a block of XML. In a pure REST system, GET requests cannot change the resource. Changes are performed by other HTTP verbs: GET   Request for a resource. No change is made to the resource. Returns an XML representation of that resource. POST   Creates a new resource. Returns an XML representation of that resource. PUT   Updates an existing resource. Returns an XML representation of that resource. DELETE   Deletes a resource from the system’, (Evjen et al 2007). ‘In a just-enough REST system, only GET and POST (or even just GET) URLs are used. In this model, all the  operations of the service can be accessed via a query string in the browser. Part of the rationale for this is that many clients do not support the PUT and DELETE verbs, leaving GET and POST to perform multiple duties’, (Evjen et al 2007).
  20. 20. Other Internet Architectures  W3Cs Semantic Web Initiatives:  Resource Description Framework (RDF)  Web Ontology Language (OWL)  SPARQL Queries  HTML5/XHTML2/XForms
  21. 21. Service Oriented Architecture (SOA)  SOA is about making the different layers of an information system independent.  The services provided at each layer are networked and can be combined in any way desired to provide the required service to a user.  For example, a web service could be used as a Data Access Layer for a database.  Web services, interacting with a variety of databases, can be mixed and matched as required in a Business Logic Layer, which may do something else. This Business Logic Layer should provide its own API, again perhaps as a Web Service, that can then be used by any application.  SOA is about the same old programming concepts of modularity, abstraction and information-hiding.
  22. 22. References Berners-Lee, T., 1996. The World Wide Web: Past, Present and Future. Available 1. at: http://www.w3.org/People/Berners-Lee/1996/ppf.html [Accessed February 25, 2009]. Evjen, B., Sharkey, K., Thangarathinam, T., Kay, M., Vernet, A. & Ferguson, S., 2. 2007. Professional XML. John Wiley & Sons. Fielding, R., Irvine, U.C, Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P, 3. Berners-Lee, T., Compaq, W3C, MIT, Xerox and Microsoft, 1999. RFC 2616 - Hypertext Transfer Protocol -- HTTP/1.1. Internet Engineering Task Force (IETF) Available from: http://www.ietf.org/rfc/rfc2616.txt [Accessed March 17, 2009]. Meyers, M., 2004. Network+ Certification All-in-One Exam Guide, Third Edition 4. 3rd ed., McGraw-Hill Osborne. Postel, J., Reynolds, J. & ISI, 1985. RFC 959 - File Transfer Protocol. Internet 5. Engineering Task Force (IETF). Available from: http://tools.ietf.org/html/rfc959 [Accessed March 17, 2009]. Tanenbaum, A.S., 2002. Computer Networks. 4th ed., Pearson Education. 6. Watson, K. , Nagel, C., Hammer Pedersen, J., Reid, J.D., Skinner, M. And White, 7. E., 2005. Beginning Visual C# 2005 2005th ed., John Wiley & Sons.

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