Strict-Priority Packet Delay in Switches with Transmit-Ring Buffering
2026-06-08 • Networking and Internet Architecture
Networking and Internet Architecture
AI summaryⓘ
The authors explain that when networks prioritize urgent data using a method called Strict Priority scheduling, there is a hidden cause of delay that older models miss. This delay comes from a part of the switch called the transmit ring, where packets wait before being sent out, and it can slow down important packets more than expected. They improved existing delay models by including this transmit ring and tested their work on real switches to prove it matches what happens in practice. The authors also offer a way to measure the size of this transmit ring, which is usually not documented.
Strict Priority (SP) schedulingtransmit ring (TXR)low-latency servicepacket transmissionlatency modelingswitch egressworst-case delayper-hop delay distributionnetwork switchdelay bounds
Authors
Yash Deshpande, Quirin Vogel, Wolfgang Kellerer
Abstract
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.