# An On-Demand Fault-Tolerant Routing Strategy for Secure Key Distribution Network
## 2 Related Work  
In the secure key distribution networks based on the trusted relay assumption, existing work on the design of the routing mechanism mainly considers the performance and security of secure key distribution.  
In terms of network performance considerations, early testbeds primarily made modifications based on the Internet routing protocol [11]. The Defense Advanced Research Projects Agency uses modified OSPF, where link cost is evaluated based on the number of generated keys distributed on a link within the Link State Announcement update interval, but the current load problem is ignored [4; 17]. In the European project, SECOQC [18] evaluates the link workload by calculating the actual key forwarding rate, but does not provide a method for calculating the link cost. Some studies optimize key utilization based on the number of local keys of the relay and minimize the number of path hops [19]. The common feature among the above methods is the utilization of periodic path information exchange, but when the number of SKDN requests increases, on-demand probing provides lower overhead and more timely link status information [20]. In addition to processing messages according to priority in different message queues on nodes to improve the quality of service, research [21] has found that the similarity in resource distribution with Mobile Ad Hoc Networks (MANETs) provides a reference for SKDN design, and proposes on-demand routing by calculating geographical distance and link status, which reduces the number of routing control packets. Research [22] has found that the OLSR algorithm can be improved through interaction with the status of the local key, but the hop count has not been optimized, which also affects system performance; in addition, software-defined networks can be used to collect topology information and complete path calculation [23] based on application requirements [24].  
Some SKD technologies, such as QKD, can prevent QKD link eavesdropping, Attackers can exploit this to carry out denial-of-service attacks [25], but the aforementioned routing mechanism can be utilized to select alternative paths if a trusted relay node available. However, when the assumption based on trusted relay nodes partially fails, the security risk of active attacks such as traffic redirection [26] increases.  
Research [13] has pointed out that routing mechanisms in existing work lack attention to key protection. Among them, how to improve the security of forwarding keys can be roughly divided into two types of methods; the first method realizes key distribution in an untrusted environment through multi-path [27], which can prevent eavesdropping by malicious nodes. However, it causes a huge overhead of key resources under the condition that the environment is trustworthy, and it cannot avoid the occurrence of system paralysis events caused by faulty nodes [15]. The second method uses a stochastic routing mechanism to ensure that it does not rely on a single key forwarding path to avoid possible risky relay nodes [28]. Although this method can avoid active attacks causing system unavailability to a certain extent, it exhibits limited performance improvement in the presence of faulty nodes [29]. The method also lacks a mechanism for switching between trusted and untrusted environments in order to reduce system overhead.