socket recv buffer

BDP * 4 / 3

use iperf for testing

iperf -s -w 200K   # server
iperf -c localhost # client

ethtool -k eth0 # segment offload makes tcpdump see a packet bigger than MTU
net.ipv4.tcp_available_congestion_control
net.ipv4.tcp_congestion_control
net.ipv4.tcp_moderate_rcvbuf

net.ipv4.tcp_slow_start_after_idle