# An Efficient Routing Protocol for Quantum Key Distribution Networks
## 2 The Proposed QOLSR Protocol  
### Analysis of the Limitations of AODV, DSDV, and OLSR in QKD Networks  
QKD networks consume quantum secure keys on links during communication. As a result, the link cannot communicate normally when quantum secure key resources remaining on the link are insufficient. The quantity of quantum secure key resources remaining on the link determines whether or not the link can continue to transfer data. Hence, the problem of frequent changes in the link-state caused by changes in the number of remaining key resources on the link must be resolved to make QKD networks efficient. Different from wired networks but similar to WANETs, QKD networks have changes in connection state between nodes caused by node movement. Thus, inspired by [21], three typical WANET routing protocols--AODV, DSDV, and OLSR--are selected in this research. First, we analyze the limitations of applying the three protocols to QKD networks. Then, a simulation experiment and performance comparison are performed using NS3 in Section 3.1. Finally, we make further improvements based on the OLSR protocol, which has the best performance among the three protocols. The AODV protocol is a demand-driven passive routing protocol [28]. Routing discovery is performed by sending routing request RREQ packets and receiving routing response RREP packets when the source node has communication requirements and there is no corresponding route in the routing table. The link-state is sensed by sending and receiving HELLO packets periodically. The QKD network with this protocol suffers from high time delays and packet loss during the process of changing routing paths, resulting in underutilization of quantum secure keys. To solve this problem, the link-state should be pre-sensed by including information conveying the number of quantum secure keys in the HELLO packet. By minimizing the time required for link-state sensing, the performance of QKD networks can be increased. Then, 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. Moreover, the frequency of path switching can be reduced by increasing the working duration of the path, in turn improving quantum secure key utilization of QKD networks. However,  
Figure 1: The architecture of QOLSR. The numbers 1–6 represent the nodes in QKD networks. the on-demand-driven passive routing discovery mode makes it difficult to optimize the time necessary for routing discovery. The DSDV protocol [29] is an active routing protocol that is driven by routing tables. The routing table is broadcast on a regular basis to detect changes in global topology. Each node in the network sends its routing table to other nodes during the broadcasting process, resulting in a large routing cost. Since the routing packet also needs to be protected by QKD encryption, a huge number of quantum secure keys will be consumed. In large-scale QKD networks, where quantum secure keys are valuable, this cannot be tolerated. When the current communication path is disabled, substantial packet loss occurs in QKD networks using the DSDV protocol since the loss cannot be detected until the next broadcast cycle. Therefore, DSDV should be optimized by shortening the broadcast cycle and reducing the routing overhead. However, these goals are mutually constraining and difficult to optimize simultaneously. Likewise, the OLSR protocol is an active routing protocol driven by routing tables [30]. OLSR combines global topology awareness based on periodic broadcast as in DSDV and link-state awareness based on HELLO packet as in AODV through multipoint replay (MPR). It not only ensures timely awareness of global topology but also significantly reduces routing overhead. However, QKD networks with OLSR protocol also suffer from excessive time delays and packet loss caused by the link-state awareness mechanism based on HELLO packets and the routing metrics based on minima-hopping during path switching.