# An Efficient Routing Protocol for Quantum Key Distribution Networks
## 2 The Proposed QOLSR Protocol
### An Overview of the OLSR Routing Protocol  
Figure 2 depicts the OLSR protocol's workflow. As shown, it includes three main modules: link-state awareness, routing discovery and routing calculation modules [30]. HELLO packets and TC packets are used for awareness of the link-state and the topology, respectively. HELLO_interval and TC_interval represent the sending periods of HELLO and TC packets, respectively. The shorter the sending periods, the higher the sensitivity. As a result, there is a lot of routing overhead and the over-consumption of quantum secure keys. Each module's workflow will be discussed in depth below. The limits of OLSR in QKD networks, as well as optimization strategies, are also discussed.  
The link-state awareness mechanism of OLSR is similar to that of AODV. Changes in link-state are sensed by sending and receiving HELLO packets periodically. In order to calculate the MPR set, the HELLO packages in OLSR must not only sense the link-state among neighbor nodes but also undertake the task of calculating the two-hop neighbors. Therefore, HELLO packets containing the information of all neighbor nodes are sent to each neighbor node by non-selective broadcast. Each node in the network maintains the neighbor table and two-hop neighbor table by listening to the HELLO packets. Because there is no pre-sense of link-state in OLSR, path-switching causes significant time delays and packet loss. The link-state can be pre-sensed by adding information about the remaining quantum secure keys in the HELLO packets. Thereby, the utilization of quantum secure keys can be enhanced by reducing the packet loss caused by path switching.  
The routing discovery module calculates the MPR set in real-time according to the neighbor table and the two-hop neighbor table. The MPR mechanism significantly reduces the routing overhead and has no effect on the awareness of global topology. Since the topology changes dynamically, each node in the network needs to store an MPR selector table that contains the information of the real-time MPR set. Subsequently, the TC packets containing the MPR selector table will be sent to each neighbor node by selective broadcast based on the MPR mechanism. In the meantime, each node in the network maintains the topology table by listening to the TC packets. The topology table is used to find the source node based on the destination node and then calculate the optimal path.  
The routing calculation module generates a routing table based on the neighbor table and topology table generated by the above two modules. The OLSR routing protocol will recalculate the routing table when the neighbor table or topology table is changed. OLSR selects the optimal path based on the minima-hopping method. However, information transmission in QKD networks relies heavily on quantum secure keys. Therefore, the path's quality is determined by the information of the quantum secure key. The routing metrics should be improved by adding parameters such as quantum secure key generation capability, quantum secure key consumption rate, and the amount of remaining quantum secure key resources in order to improve quantum secure key utilization of QKD networks.