This paper presents collective forwarding, a technique to alleviate an underlay network such
as an IP network and improve efficiency of message delivery on structured overlays. The technique
improves communication throughput, reduces loads of nodes on an overlay, and reduces
the number of communications on an underlay by bundling a number of delivery requests
on an overlay. It shows its performance in case that an overlay delivers a large number of
messages, for example we put or get many items on a DHT. It reduced the number of transmissions
to 34% ∼ 12% and the time to get items to 13.0% ∼ 9.7% by bundling each 10 get
requests on a DHT with 1,000 nodes.