$cat /proc/sys/net/ipv4/tcp_fack 1 $ tcp_fack Enable FACK congestion avoidance and fast retransmission. The value is not used, if tcp_sack is not enabled. http://www.psc.edu/networking/ftp/papers/Fack.9608.ps grepping through the linux kernel source 2.6.32 proc.c:210: SNMP_MIB_ITEM(“TCPFACKReorder”, LINUX_MIB_TCPFACKREORDER), sysctl_net_ipv4.c:475: .procname = “tcp_fack”, tcp_input.c:847: * Packet counting of FACK is based on in-order assumptions, therefore TCP tcp_input.c:1029: * A”. Its FACK …