User manual

Phase 2: Setting up with the Web browser
Cobalt CacheRaQ 4 User Manual 21
Cache Mode table
The most important setting on this page is the Cache Mode. Select from among
Traditional Proxy Caching mode, Transparent mode with one network interface
and Transparent mode with two network interfaces. These modes are described in
Chapter 1, “Introduction”.
Cache Settings table
There are five fields in the Cache Settings table.
HTTP Proxy Port Number
The value for the is 3128. This is the standard port number used by Web browsers
for connecting to a proxy server.
Document expiration age fraction (percentage)
A cached document expires after sitting in the cache for a specified fraction of its
age. This fraction is expressed as a percentage. The age of a document is the
difference in time between when the document is loaded into the cache and when
the document was last modified.
The value in this field determines how often the CacheRaQ 4’s caching software
refreshes cached Web pages. A refresh occurs when the caching software decides
that a requested document (that is already held in the cache) must be checked
against the document on the original server to ensure that the cached copy of the
document is up to date. The next paragraph describes the method used to
determine when to refresh a document.
Cached documents are refreshed after they expire. Documents expire after they
have been in the cache for a certain amount of time. This amount of time is set for
each document according to the document’s Last-Modified timestamp. When a
document enters the cache, the CacheRaQ 4 software calculates the difference
between the current time and the document’s Last-Modified time. It multiplies
this quantity by the value of the document expiration age fraction; the result is the
amount of time that the document remains in the cache before it expires.
Consequently, a smaller value for the document expiration age fraction causes a
document to expire sooner. This reduces the likelihood that an out-of-date
document will be served from the cache, but it also increases the amount of
HTTP traffic between the CacheRaQ 4 and the Web. For a document expiration
age fraction of 10%, the likelihood that a given document is out of date when
retrieved from the cache is about 1%. In practice, users’ browser caches are much
more likely than the CacheRaQ 4 to return out-of-date documents.