The time-out value for TCP delayed acknowledgment (ACK) timer in milliseconds for hosts that are directly connected.

Refer to RFC 1122, 4.2.3.2.

50 milliseconds

1 millisecond to 1 minute

Yes

Do not increase this value to more than 500 milliseconds.

If in some circumstances, slow network links (less than 57.6 Kbps) with greater than 512 bytes maximum segment size (MSS) and the interval is short for receiving more than one TCP segment, increase the value.

Unstable

The maximum number of TCP segments (in units of maximum segment size MSS for individual connections) received from remote destinations (not directly connected) before an acknowledgment (ACK) is generated. If set to 0 or 1, it means no delayed ACKs, assuming all segments are 1 MSS long. The actual number is dynamically calculated for each connection. The value is the default maximum.

2

0 to 16

Yes

Do not change the value. In some circumstances, when the network traffic becomes very bursty because of the delayed ACK effect, decrease the value. Do not decrease this value below 2.

Unstable

The maximum number of TCP segments (in units of maximum segment size MSS for individual connections) received from directly connected destinations before an acknowledgment (ACK) is generated. If set to 0 or 1, it means no delayed ACKs, assuming all segments are 1 MSS long. The actual number is dynamically calculated for each connection. The value is the default maximum.

8

0 to 16

Yes

Do not change the value. In some circumstances, when the network traffic becomes very bursty because of the delayed ACK effect, decrease the value. Do not decrease this value below 2.

Unstable

If set to 1, TCP always sends SYN segment with the window scale option, even if the option value is 0. Note that if TCP receives a SYN segment with the window scale option, even if the parameter is set to 0, TCP responds with a SYN segment with the window scale option, and the option value is set according to the receive window size.

Refer to RFC 1323 for the window scale option.

0 (disabled)

0 (disabled), 1 (enabled)

Yes

If you want the window scale option in a high-speed network configuration, enable it.

Unstable

If set to 1, TCP always sends SYN segment with the timestamp option. Note that if TCP receives a SYN segment with the timestamp option, TCP responds with a SYN segment with the timestamp option even if the parameter is set to 0.

0 (disabled)

0 (disabled), 1 (enabled)

Yes

In summary, if an accurate measurement of round trip time (RTT) and TCP sequence number wraparound is a problem, enable it.

Refer to RFC 1323 for more reasons to enable this option.

Unstable

The default send window size in bytes. Refer to the following discussion of per-route metrics for setting a different value on a per route basis. See "tcp_max_buf" also.

49,152

4096 to 1,073,741,824

Yes

Note that this is the default value. An application can use setsockopt(3XNET) SO_SNDBUF to change the individual connection's send buffer.

Unstable

The default receive window size in bytes. Refer to the following discussion of per-route metrics for setting a different value on a per-route basis. See "tcp_recv_hiwat_minmss" and "tcp_max_buf" also.

49,152

2048 to 1,073,741,824

Yes

Note that this is the default value. An application can use setsockopt(3XNET) SO_RCVBUF to change the individual connection's receive buffer.

Unstable

The maximum buffer size in bytes. It controls how large the send and receive buffers are set to by an application using setsockopt(3XNET).

1,048,576

8192 to 1,073,741,824

Yes

If TCP connections are being made in a high-speed network environment, increase the value to match the network link speed.

Unstable