This is achieved by the recipient sending a custody response to the current custody holder. There may be several non DTN hops between two DTN routers so the custody acceptance response cannot be sent to the last link layer hop as that might not be the previous custody holder. While it would be possible to add the IP address of the last DTN node to the packet to give a place to return the custody report to, this adds
additional overhead. This can be avoided by sending packets back towards the message’s origin during which they will travel back through the previous DTN hop. To allow DTN nodes to identify these packets they need to be tagged, which will allow the router to know that they need to evaluate their contents rather than just letting them pass though.