the fpdu and the second value of the count is sent with the acknowledgement token if an error occurs corrupting the header of the fpdu or the acknowledgement then the value can be safely discarded and then resent later either with the next fpdu acknowledgement or as a regular update without breaking the true value of outstanding unacknowledged fpdu data the number of individually selectable flow channels for each bfsl is determined so that the return latency of many small fpdus sent to different destinations can be hidden in a reasonably large network without significantly reducing the bandwidth this calculation needs to take into account the size of the smallest fpdus the bandwidth of individual bfsls and the latency of transmission across the bfs components and along each bfsl if a series of fpdus being transmitted from one network ingress port to another network egress port are not making good progress perhaps because of network congestion close to the destination then an adaptive re selection can be forced this would cause the fpdu data stream to take another route across the network and perhaps avoid the congestion to force an adaptive selection of an alternative route the fpdu data stream is blocked at a bfs until all the previously sent fpdus have been acknowledged and the flow channel appears empty this blocking is achieved by preventing any new data connections from being made across the data crossbar switch 19 the blocked fpdus will be buffered in the data input fifo 21 waiting to connect to the data crossbar switch 19 all the fpdus already sent across the crossbar switch.