channel_announcementThe initial channel announcement is made cooperatively by both peers. This announcement proves that the channel exists on the blockchain and establishes which nodes it belongs to.
channel_updateOnce a channel has been announced cooperatively, it can be updated unilaterally by each party. This allows the parties to adjust terms with minimal effort, or to disable a channel when its peer is offline. Information relayed in this channel update includes fees and HTLC rules.
node_announcementNode announcements can only be made by nodes that have previously announced a channel. The node announcement will include information such as its alias, how to reach it and what features it supports.