The content download feature is used to schedule ISA to download new content from the Internet at pre-defined times so that when Web Proxy clients request those objects, updated versions will be in the cache. This enhances performance and ensures that clients will receive up-to-date content more quickly.
You can monitor Internet access and usage to determine which sites users access most frequently and predict which content will be requested in the future. Then you can schedule content download jobs accordingly. A content download job can be configured to periodically download one page (URL), multiple pages, or the entire site. You can also specify how many links should be followed in downloading the site. You can configure ISA to cache even those objects that are indicated as not cacheable in the cache control headers. However, a scheduled content download job would not complete if the Web server on which the object is stored requires client authentication.
To take advantage of this feature, you must enable the system policy configuration group for Scheduled Content Download Jobs, and then configure a content download job.
When you enable the Schedule Content Download Jobs system policy configuration group, this causes ISA to block unauthenticated HTTP traffic from the local host (the ISA server) – even if you have another policy rule configured that would allow such traffic. There is a workaround that will make it possible to allow this traffic and still use content download jobs. This involves creating a rule to allow HTTP access to All Networks and being sure that another rule higher in the order is configured to allow HTTP access from the local host.
Control Caching via HTTP Headers
There are two different factors that affect how HTTP (Web) content is cached. The configuration of the caching server is one, but Webmasters can also place information within the content and headers to indicate how their sites and objects should be cached.
Meta tags are commands within the HTML code of a document that specify HTTP expiration or non-cacheable status, but they are only processed by browser caches, not by proxy caches. However, HTTP headers are processed by both proxy caches and browser caches. They are not inserted into the HTML code; they are configured on the Web server and sent by the Web server before the HTML content is sent.
HTTP 1.1 supports a category of headers called cache control response headers. Using these headers, the Webmaster can control such things as:
* Maximum age (the maximum amount of time the object is considered valid, based on the time of the request)
* Cacheability
* Revalidation requirements
ETags and Last-Modified headers are generated by the Web server and used to validate whether an object is fresh.
In Microsoft Internet Information Services, cache control response headers are configured in the HTTP Headers tab of the property pages of the Web site or Web page.
ISA does not cache responses to requests that contain certain HTTP headers. These include:
* Cache-control: no-cache response header
* Cache-control: private response header
* Pragma: no-cache response header
* www-authenticate response header
* Set-cookie response header
* Cache-control: no-store request header
* Authorization request header (except if the Web server also sends a cache-control: public response header)
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