Multicast methodology and apparatus for backpressure-based switching fabric

Abstract

In an N×N packet switch, for example an ATM switch, multicast connections receive backpressure signals from congested output ports in the switch core. The cells in the connections are destination address modified not to send to the congested output port, but to continue sending to the other output ports in the connection. The multicast cells may include a user selected primary route identification and be queued in a unicast input queue for the primary route before being destination address modified. Destination address modification for congestion may be accompanied by partial packet discard, through destination address modification of the remaining cells forming part of a multiple cell packet to remove the destination address for the congested port (whether or not it is now congested) where one of the multiple destination addresses of one of the cells in the packet has been modified to remove the destination address of the congested output port. Destination address modification for congestion, without or without partial packet discard, may be accompanied by hybrid head of line blocking performed by not sending from a multicast queue if an output port on one of the multicast leafs is congested and the input queue size is below a given threshold, and sending with destination address modification and optional partial packet discard if the queue size is above the threshold.