1. A method of providing packet routing information, the method comprising: encoding routing information from a source node to one or more destination nodes into a compact representation of set membership; wherein the routing information comprises representations of one or more identifiers, each identifier identifying a respective network link, a respective node, or a full or partial path containing two or more network links; and wherein the method comprises generating d candidate compact representations of set membership from d representations of the one or more identifiers, where d is an integer greater than one, and putting one of the candidate compact representations of set membership into a header of a packet that is to be sent from the source node to the destination node(s).