WWW for Mobile Apps


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WWW for Mobile Apps

  1. 1. World Wide Web<br />Prepared by:<br />Reynald Susainathan<br />
  2. 2. Problems with HTTP 1.0 in Wireless Environment<br />Bandwidth and Delay<br />Caching<br />POSTing<br />
  3. 3. Bandwidth & Delay<br />HTTP not designed with following in mind:<br />Low-bandwidth &<br />High delay connections<br />HTTP header:<br />Quite large<br />Redundant fields<br />Readable for humans &<br />Transferred in plain ASCII<br />HTTP is stateless:<br />Repeated transfer of information fields with each request.<br />
  4. 4. Bandwidth & Delay<br />Server transfers uncompressed contents.<br />A TCP connection is used for each item on a web page. Hence the overhead includes exchange of 7 PDUs for:<br />Three-way Handshake,<br />Data Transmission &<br />Reliable disconnection.<br />TCP slow-start:<br />This mechanism is built-into TCP, causes additional problem.<br />Causes unnecessary high delays.<br />
  5. 5. Bandwidth & Delay<br />DNS Lookup:<br />Necessary for many items on the web page<br />Reduces bandwidth and increases delay even further.<br />DNS Server:<br />Each time browser reads a hyperlink reference to a new server, it has to:<br />Resolve the logical name into an IP Address<br />This has to be done before fetching the item from the server<br />Requires additional request to DNS server over the wireless link<br />This adds round-trip time to the delay.<br />
  6. 6. Caching<br />Present-day pages contain Dynamic Objects, cannot be cached:<br />Access Counter,<br />Time,<br />Date or<br />Other customized items:<br />Customized pages are created on-demand (CGI, Servlets, ASP)<br />Names of the links are generated dynamically<br />Caching algorithms cannot detect access to the same content if link differs<br />Home page of some sites are created dynamically, depending on:<br />Type of browser,<br />Client hardware,<br />Client location, etc…<br />Hence, even if cache could store some static content, its often impossible to merge with dynamic remainder of the page.<br />
  7. 7. Caching<br />Mobility quite often inhibits caching:<br />Way of accessing the web server changes over time due to change in access point.<br />3. Many security mechanisms inhibit caching, authentication is often needed.<br />
  8. 8. POSTing<br />Sending content from client to server has additional problems if:<br />Client gets disconnected.<br />POST request cannot be fulfilled in disconnected state.<br />Server could be simulated by accepting the posting via, additional process.<br />This causes additional problems<br />Eg: If real server does not accept posting or if server cannot accept deferred posting<br />
  9. 9. Approaches for HTTP & HTML to adapt to Wireless Environment<br />Image scaling<br />True color, higher resolution scaled to few colors, lower resolution<br />Content transformation<br />Eg: Converting PDF to Text<br />Content extraction & Semantic compression<br />Abstract generation<br />Extracted from the document (or)<br />Use HTML headlines<br />Special language & protocols<br />Replace HTML & HTTP with HDML & HDTP<br />Push Technology<br />
  10. 10. Improvements in HTTP 1.1<br />Connection Reuse: <br />Persistent connection<br />Caching Enhancement<br />Bandwidth Optimization<br />Supports:<br />Compression<br />Negotiation of compression parameters &<br />Different compression styles:<br />Hop-by-hop<br />End-to-end<br />Security<br />
  11. 11. Caching Enhancements<br />Stores cacheable response; reducing response time and saving bandwidth.<br />Caching tries to achieve Semantic Transparency:<br />Cache should not affect client or server besides increasing the performance.<br />Correctness of cache entries needs to enhanced:<br />To fetch the most up-to-date copy of an item:<br />Revalidate item with the original server<br />Provide warnings if the freshness has been violated.<br />Web pages contain information about cacheability & semantic transparency:<br />A special tag allows for identification of the content and helps to determine if two different URIs map to same content.<br />More tags can indicate if content is cacheable, cacheable in private caches only, etc…<br />HTTP 1.1 defines a large set of cache control directives.<br />