The main actions of the custody store are to send custody acknowledgements, perform retransmissions, and remove
packets that have been delivered or expired. Due to limitations in the network stack of Contiki it is not possible to
generate a custody response while processing the message in the network so this is done as one of the actions of the
store. An ICMP6 message is built and sent out in the normal way, the addition of the DTN header and flagging as an
informational DTN packet is implemented as part of the network send routine.