# An Efficient Routing Protocol for Quantum Key Distribution Networks  
[PERSON]  
1 Department of Computer Science and Technology, Harbin Institute of Technology, Harbin 150080, China; [EMAIL_ADDRESS]  
[PERSON]  
1 Department of Computer Science and Technology, Harbin Institute of Technology, Harbin 150080, China; [EMAIL_ADDRESS]  
[PERSON]  
1 Department of Computer Science and Technology, Harbin Institute of Technology, Harbin 150080, China; [EMAIL_ADDRESS]  
[PERSON]  
1 Department of Computer Science and Technology, Harbin Institute of Technology, Harbin 150080, China; [EMAIL_ADDRESS]  
[PERSON]  
2E1E  
users. To broaden the service scale of QKD technology, study of QKD networks that can deliver end-to-end secure key services for more users is crucial [12; 13; 14]. Thanks to years of intense research on QKD networks, the number of nodes in experimental QKD networks has expanded from 6 to 56, and the transmission distance has increased from 19.6 km to 7600 km [15; 16; 17]. To maintain the security of data packets transmitted in the QKD network, they must be encrypted with quantum secure keys. In this study, it is assumed that routing packets must also be encrypted with quantum secure keys in order to guarantee the security of routing data. Therefore, QKD networks are characterized by their reliance on quantum secure keys in communication. Since traditional routing protocols do not account for this characteristic of QKD networks, applying them in QKD networks directly will result in the underutilization of quantum secure keys. That is not desirable because quantum secure keys are very precious in QKD networks. Consequently, a routing protocol that can achieve high quantum secure key utilization is crucial to QKD networks. Many scholars have looked at routing protocols in QKD networks in order to address the routing problem. For example, the open shortest path first (OSPF) protocol has been used in the experimental DARPA QKD network [18]. Later, the upgraded version, OSPF-v2 protocol, was used in the SECOQC network [19]. The authors of [19] used the number of quantum secure keys instead of hop count as the routing metric for local load balancing. A similar study [20] was also conducted to improve the routing strategy based on OSPF for QKD networks. Likewise, based on traditional routing protocols, considering the similarity between wireless ad hoc networks (WANETs) and QKD networks, the authors of [21] applied three typical WANET routing protocols to QKD networks to solve the problem of frequent changes in link-state. Following a similar idea, a dynamic routing scheme for QKD networks based on trusted relays was proposed in [22]. Some scholars have also conducted research on routing protocols of QKD networks from other aspects, such as adaption to multiple types of QKD networks [23; 24], security enhancement [25], and cross-talk suppression [26]. In summary, the research mentioned above has not paid enough attention to high quantum key utilization, which is currently the major concern of QKD networks. In order to achieve high quantum secure key utilization, the problem of excessive packet loss due to frequent changes in link-state caused by the limited quantum secure key generation capability of QKD devices should be tackled first. Taking into account that both WANETs and QKD networks have frequent changes in link-state [27], the typical routing protocols of WANETs, including ad hoc on-demand distance vector routing (AODV), destination sequenced distance vector (DSDV), and optimized link-state routing (OLSR) are chosen as the research basis for designing a routing protocol for QKD networks. Both analysis and our simulation results using these three protocols show that OLSR is a more suitable option for QKD networks. To further enhance quantum secure key utilization of QKD networks, an efficient routing protocol based on OLSR, named QOLSR, for QKD networks is proposed in this paper.