INT21CN Computer Networks

Tutorial #7

1. This question refers to Web caching.
   1. Why does a Web browser normally cache downloaded objects (HTML pages, images, etc). The answer is probably obvious, but it's worth checking that you do know this.

   2. What aspects of HTTP are intended to support the correct operation of caching? Give some examples.

   3. Why is it necessary, when a Proxy Server is in use, to specify a full URL in the HTTP request? If a request is made to, for example, a local server (for which the proxy is not involved) is the full URL needed?

   4. Proxy servers can reduce network (download) costs for an organisation, and make best possible use of the organisation's link to the Internet, however they are not altogether successful in practice. Give two reasons why proxy caches are not able to satisfy the majority of Web requests in The Real World(tm).

   5. Some Web browsers seem to (on occasion) cache a little too aggressively. One common browser, in particular, will use a cached copy of a Web page even when the user hits the "Reload" (or "Refresh") button. One way to force a reload is to append a "?" character to an ordinary (static) URL and then load (or reload) the page. Why do you think this works?

   6. Practical question: on your preferred Web browser, is it possible to configure how often cached objects are checked for consistency?

2. Give some of the reasons why HTTP/1.0 is not a highly regarded protocol in the Internet technical community. How does HTTP/1.1 address these problems? At least two points required.

3. The HTTP/1.1 "Etag" header extends the earlier concept of "Last-modified" date of an object. What is the particular advantage of the "Etag"?

4. Contemplate this: the specification for the original version of HTTP (0.9) was approximately 6 Kbytes in size. The RFC for HTTP/1.0 (RFC1945) was 134 Kbytes. The RFC for the current version of HTTP/1.1 (RFC2616) is 412 Kbytes. What conclusions can you draw?