Strict Priority (SP) scheduling is widely used at switch egress to provide low-latency service to high-priority (HP) traffic. Existing deterministic and stochastic latency models typically account for scheduler behavior and packet transmission, but omit a common switch implementation detail: the transmit ring (TXR) between the scheduler and the physical port. Because the switch must prepare the next packet before the current transmission completes, packets already placed in the TXR can further delay HP packets. This changes both the worst-case delay and the per-hop delay distribution of HP packets. This paper identifies this modeling gap, extends standard SP latency models to include the TXR, and validates the revised model through measurements on multiple switches. It also provides a measurement method for estimating the TXR size, a parameter that is often not reported in switch datasheets. The resulting model provides a closer representation of switch behavior for systems that use SP scheduling and require either delay bounds or delay distributions.

翻译：严格优先级（SP）调度广泛应用于交换机出口，为高优先级（HP）流量提供低延迟服务。现有的确定性和随机延迟模型通常考虑调度器行为和数据包传输，但忽略了常见的交换机实现细节：调度器与物理端口之间的传输环（TXR）。由于交换机必须在当前传输完成前准备下一个数据包，已置于TXR中的数据包会进一步延迟HP数据包，从而改变HP数据包的最坏情况延迟及每跳延迟分布。本文识别了这一建模空白，将标准SP延迟模型扩展至包含TXR，并通过多台交换机的测量验证了修订后的模型。同时，本文提供了一种估算TXR大小的测量方法——该参数往往未在交换机数据表中报告。所提出的模型能更准确地反映采用SP调度且需要延迟界或延迟分布的系统中的交换机行为。