@node MCD
@cindex MCD
@cindex multicast discovery
@unnumbered MultiCast Discovery

MCD is an addition to online @ref{Sync, synchronization protocol}, that
gives ability to make node discovery by sending multicast announcements
in local area network. It is very simple:

@itemize
@item
    @command{@ref{nncp-daemon}} sends multicast messages about its
    presence from time to time. See @ref{CfgMCDSend, mcd-send}
    configuration option.
@item
    When @command{@ref{nncp-caller}} sees 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 @ref{CfgMCDListen, mcd-listen} and
    @ref{CfgMCDIgnore, mcd-ignore} configuration options.
@end itemize

MCD announcement is an XDR-encoded packet with only two fields:

@verbatim
+----------------+
| MAGIC | SENDER |
+----------------+
@end verbatim

Magic number is @verb{|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
@strong{@verb{|ff02::4e4e:4350|}} (hexadecimal ASCII @verb{|NNCP|})
multicast address and @strong{5400} port. Operating system will use IPv6
link-local address as a source one, with the port taken from
@command{@ref{nncp-daemon}}'s @option{-bind} option. That way, IP packet
itself will carry the link-scope reachable address of the daemon.