UCARP allows a pair of hosts to share common virtual IP addresses in order to provide automatic failover. It is a portable userland implementation of the secure and patent-free Common Address Redundancy Protocol (CARP, OpenBSD's alternative to the VRRP). Strong points of the CARP protocol are : very low overhead, cryptographically signed messages, interoperability between different operating systems and no need for any dedicated extra network link between redundant hosts.