[ibr-dtn] Bundle in sender and dtn node not removed

I tried to send a file from node0 to node4 using dtnsend command, delivery process goes well, node1 can forward bundle from node0 to node2, also node1, node2 forward bundle from node1 to node3 and node 3 forward bundle from node2 to node4 but after node4 receive all files and merge, just bundle on node3 and node4 deleted? Why bundle on node0, node1, and node2 not removed?