A Canonically-unique host is a FQDN that will canonically resolve to a designated address or set of addresses. Not to be confused with a @locally-unique-host.
A Canonically-unique host is defined by remote authoritative sources; usually through DNS. When resolving a peer's hostname, you will most likely use an external source for resolution unless you have the following implemented:
a database file similar to a [hosts file](https://en.wikipedia.org/wiki/Hosts_(file))
an internal-network resolver (which eventually pulls from external sources)
Haven primarily uses @canonically-unique-host resolution while @I2P only uses @locally-unique-host resolution.
@I2P's and @Kovri's self-assigned top-level domain is currently
.i2p and @Kovri intends to only process/use the
.i2p top-level domain