The link layer will attempt to transmit the packets in order regardless if the destination node is connected. If the first node moves out of range, then the link layer will still transmit the packets. As a result, the node wastes available bandwidth and meeting opportunities. This behavior is not a requirement of the Epidemic routing protocol, but is a limitation specific to the PacketEpidemic implementation.

In order to improve link utilization, `ACK` packets in Figure \[fig:DTNAck\] control the message exchange sequence. `ACK` packets do not acknowledge messages reaching their final destination, rather, they are a hop-by-hop acknowledgement of messages transmitted between two connected nodes. When a node receives a complete message from another node, it sends an acknowledgement packet consisting of the 64-bit Message Identification Number, 16-bit Node Identification Number, and 16-bit Message Status. The Message Identification Number is the received message’s message ID. The Node Identification Number is the node that received the message. The Message Status block permits adding reliability in future work. The Message Status indicates if a node received a message successfully. If the message transfer is unsuccessful, then Message Status is zero. We do not retransmit such messages, but such reliability may be a 