to the maximum number of hops allowed. At each hop, the node decrements the hop count and when the hop count reaches zero, the message is discarded.

Control Packet Identification
-----------------------------

The `MessageType` Header in Figure \[fig:DTNMtype\] identifies control packets. Since the simulator uses shared data structures to exchange routing information it did not implement control packets. In our implementation, control packets are not messages (the do not have a DTN message header). Control packets share routing specific information between two neighboring nodes. The 8-bit Message Type field indicates the type of control packet. For Epidemic, control packets are `BEACON`, `ACK`, `REPLY`, and `REPLY_BACK`. The `MessageType` Header encapsulates the control packets in order to identify the control packet. When a node receives a packet, it checks the Message Type field for the packet type to call the appropriate packet header class.

Summary Vector
--------------

While Epidemic uses the same control packet sequence as PacketEpidemic, the `SummaryVectorHeader` in Figure \[fig:EpidemicDTNSMVHdr\] differs. The `SummaryVectorHeader` defines `REPLY` and `REPLY_BACK` control packets. We use a 64-bit Message Identification Number in place of packetEpidemic’s 32-bit packet identification number. PacketEpidemic’s 32-bit Summary Vector Length counts all of the packets held by a node’s buffer. In our 