  We consider a new model for buffer management of network switches with
Quality of Service (QoS) requirements. A stream of packets, each attributed
with a value representing its Class of Service (CoS), arrives over time at a
network switch and demands a further transmission. The switch is equipped with
multiple queues of limited capacities, where each queue stores packets of one
value only. The objective is to maximize the total value of the transmitted
packets (i.e., the weighted throughput).
  We analyze a natural greedy algorithm, GREEDY, which sends in each time step
a packet with the greatest value. For general packet values $(v_1 < \cdots <
v_m)$, we show that GREEDY is $(1+r)$-competitive, where $r = \max_{1\le i \le
m-1} \{v_i/v_{i+1}\}$. Furthermore, we show a lower bound of $2 - v_m /
\sum_{i=1}^m v_i$ on the competitiveness of any deterministic online algorithm.
In the special case of two packet values (1 and $\alpha > 1$), GREEDY is shown
to be optimal with a competitive ratio of $(\alpha + 2)/(\alpha + 1)$.
