Following to http://wiki.apache.org/cocoon/ControllingModCache
I found this class: org.apache.cocoon.acting.HttpCacheAction (dated 2004-07-29) Unfortunately, this action can't reply with 304 on request with HTTP Header [If-Modified-Since: ......] It replies only with 200 and full body of dynamic content. Caching does not work between HTTPD and Cocoon. Am I right?