Buffer Overflows of Merging Streams

We consider a network merging streams of packets with different quality of service (QoS) levels, where packets are transported from input links to output links via multiple merge stages. Each merge node is equipped with a finite buffer, and since the bandwidth of a link outgoing from a merge node is in general smaller than the sum of incoming bandwidths, overflows may occur. QoS is modeled by assigning a positive value to each packet, and the goal of the system is to maximize the total value of packets transmitted on the output links. We assume that each buffer runs an independent local scheduling policy, and analyze FIFO policies that must deliver packets in the order they were received. We show that a simple local on-line algorithm called Greedy does essentially as well as the combination of locally optimal (off-line) schedules. We introduce a concept we call the weakness of a link, defined as the ratio between the longest time a packet spends in the system before transmitted over the link, and the longest time a packet spends in that link's buffer. We prove that for any tree, the competitive factor of Greedy is at most the maximal link weakness.