Directed and undirected routing
A message may or may not have a known endpoint at the time it leaves the sender. A URL like http://www.foo.com travels to a fixed location - foo.com - and thus has a clear direction; it is directed. An undirected message does not know its destination at the time it leaves the sender. The path it follows is determined during the course of traveling. Such a message generally has a functional reason for traveling. For example, if the message is "Find a node which knows the time in Bombay", which node it is that knows the time in Bombay is not known when the message is sent. The direction of travel is not important, so the route followed is called undirected.