Integrated Provision of QoS Guarantees to Unicast and Multicast Traffic in Packet Switches

Multi-stage packet switches that feature a limited amount of buffers in the switching fabric and distribute most of their buffering capacity over the port cards have recently gained popularity due to their scalability properties and flexibility in supporting Quality-of-Service (QoS) guarantees. In such switches, the replication of multicast packets typically occurs at the outputs of the switching fabric, so as to minimize the amount of resources required to sustain the internal expansion in traffic volume due to multicasting. The optimal use of internal resources collides with the provision of robust QoS guarantees to multicast flows, because it exposes them to head-of-line (HOL) blacking in the ingress port cards. In this context, access regulation to the fabric buffers is of the utmost importance in making the QoS experienced by multicast flows transparent to HOL blocking. We introduce the Generalized Distributed Multilayered Scheduler (G-DMS), which leverages one of the available scheduling frameworks for unicast traffic to support the QoS requirements of multicast flows in multi-stage packet switches. The novelty of the G-DMS is in the mechanism that regulates access to the fabric buffers, which combines selective backpressure with the capability of dropping copies of multicast packets that violate the negotiated profiles of the corresponding flows. The G-DMS meets the QoS expectations of both unicast and multicast flows, adding only minimal overhead to the original QoS framework for unicast traffic.