Manual:$wgHTCPMulticastAddress

Details
Setting first appeared in 1.4.1, but was commented out in DefaultSettings.php. Set to false from 1.9.0.

Not used directly, but is required to be a non-false value for HTCP to be used. HTCP is a method of purging entries from the Squid cache that can use multicast and udp which is faster then opening an http connection to every squid server.