Lecture 7: Internet Applications #3.3: HTTP/1.1

Web Caching

The World Wide Web has been spectacularly successful -- so successful that a huge proportion of Internet traffic is HTTP, ie Web pages and related objects such as images. Caching is a technique whereby copies of popular objects are kept in strategic locations, and supplied in lieu of the originals, saving huge amounts of traffic on the "backbone networks".

The Conditional-GET operation seen earlier allows support for caching at the browser level -- that is, the browser can keep a local copy of an object and check if it's up to date before displaying it. Two additional features of HTTP/1.0 were:

The Expires:: response header was used to indicate that an entity had a limited (specified) "lifetime". This permits finer control over the Conditional-GET operation. It takes an Internet-standard date/time string as its value.
The Pragma: no-cache: response header has an obvious meaning: this entity should never be stored in a cache.
Note: the (non-standard) Refresh:: response header can be used (in some browsers) to force a reload of an entity.

Additionally, HTML "<META HTTP-EQUIV=..." tags can include "equivalent" response information in the <HEAD> section of an HTML document. The browser may regard this as being equivalent to the corresponding HTTP response header.

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