This read-write integer sets the ingress queue depth, in bytes, at which the card begins flow control, with a range up to 0x1fffff bytes on the FRSM-VHS card but capped at 0xffff (65535) bytes on all other cards, and it is not supported on the FRSM12 card. An administrator tunes this to decide how full the ingress buffer is allowed to get before the card starts throttling incoming traffic to protect itself from overrun. For example, on an FRSM-VHS card carrying bursty traffic an engineer might raise ingressQECNThresh well above the 65535-byte limit available on smaller cards to delay flow control kicking in during short bursts.