A New Internet-Draft is available from the on-line Internet-Drafts directories.
In advanced filtering rule sets, it is useful to keep state or configuration details across rules. This extension adds an action to store data in variables, an action to retrieve the current time, changes the interpolation of strings, and supplies a new test so that the value of a string can be examined.
http://www.ietf.org/internet-drafts/draft-homme-sieve-variables-02.txt