The need for buffering is fundamental for packet switching networks. In recent years, studies show that the core router buffer sizes need only dozens of packets to meet acceptable link utilization. However, prior studies ignored whether buffer unit are structured in terms of packets or bytes. It made no much difference when only TCP traffic with the same packets size is considered in the previous studies. In this paper, the authors study the coexistence of real-time UDP and TCP traffic for routers with buffers of very small size.