No, signing your message. If you're a node, how to be sure you're receiving a message from a certain node and not from a hacker who fakes being this node?
your node isn't downloading random stuff that a connected to node gives it. it is instead only receiving "messages". for example you receive a block message that contains a header and the block content. there is nothing a "hacker" could do by connecting to your node. and that is why there is no signing needed and no encryption is used. every communication happens over http (no 's' that is no encryption).