How to disable delayed ACKs

TCP throughput (SCP) from a remote host is slow. By looking at the trace, it is noticed that the CentOS server is waiting for up to 40ms before sending an ACK.

In CentOS releases before 7.2, delayed Acks can only be reduced by cannot be eliminated. In CentOS 7.2, quickacks are tunable on a per route basis. By enabling quickacks delayed, The kernel disables aCKs. You can allow quickacks by executing ip route change based on ip route show.

# ip route show
default via 192.168.1.254 dev ens1 proto static metric 100
192.168.1.0/24 dev ens1 proto kernel scope link src 192.168.1.112 metric 100

# ip route change default via 192.168.1.254 dev ens1 proto static metric 100 quickack 1

# ip route change 192.168.1.0/24 dev ens1 proto kernel scope link src 192.168.1.112 metric 100 quickack 1


# ip route show
default via 192.168.1.254 dev ens1 proto static metric 100 quickack 1
192.168.1.0/24 dev ens3 proto kernel scope link src 192.168.1.112 metric 100 quickack 1