1@node MCD 2@unnumbered MultiCast Discovery 3 4MCD is an addition to online @ref{Sync, synchronization protocol}, that 5gives ability to make node discovery by sending multicast announcements 6in local area network. It is very simple: 7 8@itemize 9@item 10 @ref{nncp-daemon} sends multicast messages about its presence from 11 time to time. See @ref{CfgMCDSend, mcd-send} configuration option. 12@item 13 When @ref{nncp-caller} sees them, it adds them as the most 14 preferred addresses to already known ones. If MCD address 15 announcement was not refreshed after two minutes -- it is removed. 16 See @ref{CfgMCDListen, mcd-listen} and 17 @ref{CfgMCDIgnore, mcd-ignore} configuration options. 18@end itemize 19 20MCD announcement is an XDR-encoded packet with only two fields: 21 22@verbatim 23+----------------+ 24| MAGIC | SENDER | 25+----------------+ 26@end verbatim 27 28Magic number is @verb{|N N C P D 0x00 0x00 0x01|} and sender is 32-byte 29identifier of the node. It is sent as UDP packet on IPv6 30@strong{@verb{|ff02::4e4e:4350|}} (hexadecimal ASCII @verb{|NNCP|}) 31multicast address and @strong{5400} port. Operating system will use IPv6 32link-local address as a source one, with the port taken from 33@command{nncp-daemon}'s @option{-bind} option. That way, IP packet 34itself will carry the link-scope reachable address of the daemon. 35