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3.7. Reducing the TCP Delayed ACK Timeout
On Red Hat Enterprise Linux, there are two modes used by TCP to acknowledge data reception:
- Quick ACK
- This mode is used at the start of a TCP connection so that the congestion window can grow quickly.
- The acknowledgment (ACK) timeout interval (ATO) is set to
tcp_ato_min, the minimum timeout value. - To change the default TCP ACK timeout value, write the required value in milliseconds to the
/proc/sys/net/ipv4/tcp_ato_minfile:echo 4 > /proc/sys/net/ipv4/tcp_ato_min
~]# echo 4 > /proc/sys/net/ipv4/tcp_ato_minCopy to Clipboard Copied! Toggle word wrap Toggle overflow
- Delayed ACK
- After the connection is established, TCP assumes this mode, in which ACKs for multiple received packets can be sent in a single packet.
- ATO is set to
tcp_delack_minto restart or reset the timer. - To change the default TCP Delayed ACK value, write the required value in milliseconds to the
/proc/sys/net/ipv4/tcp_delack_minfile:echo 4 > /proc/sys/net/ipv4/tcp_delack_min
~]# echo 4 > /proc/sys/net/ipv4/tcp_delack_minCopy to Clipboard Copied! Toggle word wrap Toggle overflow
TCP switches between the two modes depending on the current congestion.
Some applications that send small network packets could experience latencies due to the TCP quick and delayed acknowledgment timeouts, which previously were 40 ms by default. That means small packets from an application that seldom sends information through the network could experience a delay up to 40 ms to receive the acknowledgment that a packet has been received by the other side. To minimize this issue, both
tcp_ato_min and tcp_delack_min timeouts are now 4 ms by default.
These default values are tunable and can be adjusted according to the needs of the user's environment, as described above.
Note
Using timeout values that are too low or too high might have a negative impact on the network throughput and latencies experienced by applications. Different environments might require different settings of these timeouts.
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