Illustration of queuing strategy in a node. (a) FIFO queuing strategy; (b) BSPF queuing strategy. The node contains three packets (rectangles) labeled as 1, 2, 3, respectively. The label of each packet corresponds to the label of its next-hop node guided by Shortest Path. The circles represent the neighboring nodes, with numbers indicating their labels. The number in bracket represents the free buffer space in the corresponding node. The top black arrows indicate the delivery order of packets. The blue-dashed arrows indicate the SP routing paths, and the red-solid arrows indicate the actual delivery paths, respectively.