## 2 Problem Statements  
While accommodating services with security requirements in QKD optical networks, three kinds of channels, i.e., TDCh, QKCh, and MBCh, are involved. Scheduling those channels is essential to achieve secure data transmission for services. Figure 1(a) illustrates the coordination of the channels during secure communication. \(\mathsf{t_{e}}\) represents the time when the data transmission ends. When the service request arrives, it is necessary to reserve a time window for key configuration [25]. Only after completing the key negotiation on QKCh and MBCh will the transmission of secure data on TDCh get start. The security of data transmission can be further enhanced by periodically updating the shared keys between communication parties. As can be seen from Figure 1(a), different keys are negotiated through cooperation of QKCh and MBCh for data encryption and decryption on TDCh in different periods. We design security strategies by taking full advantage of the key update feature to satisfy the multi-security-level needs of various services. Several security-level provisioning solutions based on the key-updating period (such period is denoted as T hereafter) have been proposed based on the principle that services with higher security level require more frequent and flexible key updating [21]. Preventing the eavesdropping attacks on a transmission link plays a critical role in securing the safety of quantum-based algorithms. There has been research studying how to prevent such attacks [28], but the detailed discussion is beyond the scope of the paper. Note that the key-updating period is the period after which the secret key has to be changed between two parties, which can reduce the probability of key being cracked. There are two different schemes of key updating. One is updating keys for every definite period; the other is updating keys for every definite amount of data. The shorter definite period or the smaller definite amount of data means the higher security level. The selection of the key updating times is dependent on the speed of the TDCh and the security level required by the service. The details of selecting the key updating times are beyond the scope of this manuscript.  
However, updating keys periodically is equivalent to multiplying the number of key configurations, which aggravates the load on QKCh and MBCh. In the case of limited wavelength resources, conflict on QKCh and MBCh occurs frequently in the process of RWTA. Since the resource allocations on QKCh and MBCh are synchronized, as shown in Figure 1(a), we could only consider how to avoid the conflict on QKCh as a representative situation. Figure 1(b) illustrates the potential collision on QKCh due to dynamic service arrival, wavelength resource sharing, and key updating. The vertical axis in the figure represents the timeline, and the horizontal axis shows the timeslot of a certain wavelength belonging to QKCh. The timeslots occupied by different services are represented by small grids of different colors. We try to assign #10 and #11 timeslots for key update when service R3 arrives at time 11. Conflict may occur since #11 timeslot has been assigned to key update request of R1. Similar conflict will occur at #16 timeslot obviously. The latency tolerance of key update configuration must be taken into account to avoid such conflicts in the RWTA process. In addition, if the communication between two nodes passes through an intermediate node, the intermediate node uses the bypass method. Consequently, the establishment of the quantum channel and data transmission between the nodes needs to meet wavelength consistency and timeslot continuity. Due to these constraints in RWTA, the wavelength-time-slot fragmentation problem will occur. Thus, how to maintain the connectivity of the paths in the process of RWTA is a very important research topic.