As I mentioned elsewhere, there are some changes ahead in CUPS 1.6. These changes are not imminent but give an indication of the direction the CUPS project is heading.
Back in 2007 CUPS became an Apple project. Now the parts that are not relevant on Mac OS are being dropped, with some of the Linux-relevant parts being gathered together in a separate project, cups-filters.
The main part that is being dropped completely is CUPS Browsing. This is currently the primary mechanism for CUPS-to-CUPS printer queue discovery on Linux. It works by having each CUPS server periodically broadcast UDP packets on port 631 announcing its available queues, and listening for broadcasts from other CUPS servers.
This discovery method is being dropped because DNS-SD is preferred upstream. Support for it has been upstream in CUPS for a while, and it is what CUPS uses on Mac OS X, but it uses Apple’s libdns_sd library and not Avahi. I have added support for this in Fedora, and the patch is submitted upstream.
So in CUPS 1.6, automatic CUPS queue discovery will require Avahi to be running on both the server (i.e. the system hosting the CUPS queue) and the clients (i.e. the systems wanting to print to it).
Of course, you will be able to run CUPS without having Avahi running — but you won’t have automatic CUPS queue discovery in that case. Clients will have to have queues explicitly configured, or else use the BrowsePoll configuration setting to periodically query a particular CUPS server for its queues.
Several other filters will be dropped from CUPS in 1.6, to be picked up by the new cups-filters package. Information about this package is on the OpenPrinting web site and a beta release is now available.
This new package restores the filters that will be dropped in CUPS 1.6 and also adds new filters to support PDF as the baseline document format rather than PostScript.