I believe it's Selective ACK aka SACK. It allows TCP to ACK ranges of packets so the sender can see that it only needs to resend an earlier block. I've seen ra

I believe it's Selective ACK aka SACK. It allows TCP to ACK ranges of packets so the sender can see that it only needs to resend an earlier block. I've seen rather good speeds over some bad links from my bittorrent server using large window sizes (up to 9), BBR flow control, ECN fully enabled, and SACK.