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. This approach minimizes the amount of resources needed to sustain the internal expansion
in traffic volume due to multicasting, but also exposes multicast flows to head-of-line (HOL) blocking in the ingress port
cards. Access regulation to the fabric buffers is of the utmost importance to safeguard the QoS of multicast flows against
HOL blocking.
We add minimal overhead to a well-known distributed scheduler for multi-stage packet switches to define the Generalized Distributed
Multilayered Scheduler (G-DMS), which achieves full support of QoS guarantees for both unicast and multicast flows. 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.