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MultiCast Discovery

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:

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.