In /usr/share/initramfs-tools/scripts/functions, function
configure_networking:
for ROUNDTTT in 2 3 4 6 9 16 25 36 64 100; do
[...]
dhcp|bootp|rarp|both)
ipconfig -t ${ROUNDTTT} -c "${IP}" -d "${DEVICE}"
;;
[...]
done
which means that if after all these retries, the DHCP server is still
not available (it may take dozens of minutes after a power outage),
remote unlocking the machine is impossible.
The retries should be configurable. In particular, it should at least
be possible to wait for an indefinite amount of time (without a large
delay between the retries).