Abstract: The quality of service mechanism supported by the Infinil3and network is an important technique to effective- 1y separate and control the bandwidth allocation between the multiple data-flows on the network, which provides a gua- ranteed transportation service to applications concurrently running in the HPC or datacenters interconnected by II3 net- work. 13ut the "Infinil3and architecture specification" does not propose demand for the II3 device manufacturers how to implement II3 QoS in detail, especially for the three key components(High-Priority, Low-Priority and Limit of High-Pri- ority) how to numerically control the bandwidth allocation between Virtual Lanes(VL for short) in different priority- ranked queues and with different priority values, and also no documents have closed expatiation on it by now. In this pa- per, a quantitative formula to compute the ratio of bandwidth between high-and low-priority VI_s according the three key components was presented. It was brought forward based on abundant results of bandwidth allocation experiments done on the most popular II3 devices and proved to be true by random sample verification. We also found that the band- width ratio would change in zigzag shape with the increase of the I_ow-Priority value and periodically reach a stable val- ue on condition that both High-Priority and Limit of High-Priority are certain. These characteristics were verified by mathematical reasoning method.

Key words: InfiniBand network, Quality of service, Virtual Lane, Bandwidth allocation, Priority value