2 @unnumbered MultiCast Discovery
4 MCD is an addition to online @ref{Sync, synchronization protocol}, that
5 gives ability to make node discovery by sending multicast announcements
6 in local area network. It is very simple:
10 @ref{nncp-daemon} sends multicast messages about its presence from
11 time to time. See @ref{CfgMCDSend, mcd-send} configuration option.
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.
20 MCD announcement is an XDR-encoded packet with only two fields:
28 Magic number is @verb{|N N C P D 0x00 0x00 0x01|} and sender is 32-byte
29 identifier of the node. It is sent as UDP packet on IPv6
30 @strong{@verb{|ff02::4e4e:4350|}} (hexadecimal ASCII @verb{|NNCP|})
31 multicast address and @strong{5400} port. Operating system will use IPv6
32 link-local address as a source one, with the port taken from
33 @command{nncp-daemon}'s @option{-bind} option. That way, IP packet
34 itself will carry the link-scope reachable address of the daemon.