5.5. How TCP SACK reduces the packet drop rate

The TCP Selective Acknowledgment (TCP SACK) feature, which is enabled by default in RHEL, is an enhancement of the TCP protocol and increases the efficiency of TCP connections.

In TCP transmissions, the receiver sends an ACK packet to the sender for every packet it receives. For example, a client sends the TCP packets 1-10 to the server but the packets number 5 and 6 get lost. Without TCP SACK, the server drops packets 7-10, and the client must retransmit all packets from the point of loss, which is inefficient. With TCP SACK enabled on both hosts, the client must re-transmit only the lost packets 5 and 6.

By default, TCP SACK is enabled in RHEL. To verify:

# sysctl net.ipv4.tcp_sack
1

If TCP SACK is disabled (0) on your server, revert the setting in the same way as you set it.