MCD is an addition to online synchronization protocol, that gives ability to make node discovery by sending multicast announcements in local area network. It is very simple:
nncp-daemonsends multicast messages about its presence from time to time. See mcd-send configuration option.
nncp-callersees them, it adds them as the most preferred addresses to already known ones. If MCD address announcement was not refreshed after two minutes – it is removed. See mcd-listen and mcd-ignore configuration options.
MCD announcement is an XDR-encoded packet with only two fields:
+----------------+ | MAGIC | SENDER | +----------------+
Magic number is N N C P D 0x00 0x00 0x01 and sender is 32-byte
identifier of the node. It is sent as UDP packet on IPv6
ff02::4e4e:4350 (hexadecimal ASCII NNCP)
multicast address and 5400 port. Operating system will use IPv6
link-local address as a source one, with the port taken from
nncp-daemon’s -bind option. That way, IP packet
itself will carry the link-scope reachable address of the daemon.