UCARP allows a couple 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\u2019s alternative to the patents-bloated 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.