TCP Parameters With Additional Cautions

Changing the following parameters is not recommended unless there are extenuating circumstances that are described with each parameter.

tcp_ip_abort_interval

Description

The default total retransmission timeout value for a TCP connection in milliseconds. For a given TCP connection, if TCP has been retransmitting for tcp_ip_abort_interval period of time and it has not received any acknowledgment from the other endpoint during this period, TCP closes this connection.

For TCP retransmission timeout (RTO) calculation, refer to RFC 1122, 4.2.3. See also tcp_rexmit_interval_max.

Default

8 minutes

Range

500 millisecond to 1193 hours

Dynamic?

Yes

When to Change

Do not change this value. See tcp_rexmit_interval_max for exceptions.

Commitment Level

Unstable

tcp_rexmit_interval_initial

Description

The default initial retransmission timeout (RTO) value for a TCP connection in milliseconds. Refer to the following discussion of per route metrics for setting a different value on a per-route basis.

Default

3 seconds

Range

1 millisecond to 20 seconds

Dynamic?

Yes

When to Change

Do not change this value. Lowering the value can result in unnecessary retransmissions.

Commitment Level

Unstable

tcp_rexmit_interval_max

Description

The default maximum retransmission timeout value (RTO) in milliseconds. The calculated RTO for all TCP connections cannot exceed this value. See also tcp_ip_abort_interval.

Default

60 seconds

Range

1 millisecond to 20 seconds

Dynamic?

Yes

When to Change

Do not change the value in a normal network environment.

If in some special circumstances, the round trip time (RTT) for a connection is in the order of 10 seconds, you can change the value to a higher value. If you change this value, you should also change the tcp_ip_abort_interval parameter to match it. Change the value of tcp_ip_abort_interval to at least four times the value of tcp_rexmit_interval_max.

Commitment Level

Unstable

tcp_rexmit_interval_min

Description

The default minimum retransmission time-out (RTO) value in milliseconds. The calculated RTO for all TCP connections cannot be lower than this value. See also tcp_rexmit_interval_max.

Default

400 milliseconds

Range

1 millisecond to 20 seconds

Dynamic?

Yes

When to Change

Do not change the value in a normal network environment.

TCP's RTO calculation should be able to cope with most RTT fluctuations. If in some very special circumstances such that the round trip time (RTT) for a connection is in the order of 10 seconds, change to a higher value. If you change this value, you should change the tcp_rexmit_interval_max parameter to match it. You should change the value of tcp_rexmit_interval_max to at least eight times the value of tcp_rexmit_interval_min.

Commitment Level

Unstable

tcp_rexmit_interval_extra

Description

A constant added to the calculated retransmission time-out value (RTO) in milliseconds.

Default

0 milliseconds

Range

0 to 2 hours

Dynamic?

Yes

When to Change

Do not change the value.

When the RTO calculation fails to obtain a good value for a connection in some circumstances, you can change this value to avoid unnecessary retransmissions.

Commitment Level

Unstable

tcp_tstamp_if_wscale

Description

If this parameter is set to 1, and the window scale option is enabled for a connection, TCP also enables the timestamp option for that connection.

Default

1 (enabled)

Range

0 (disabled), 1 (enabled)

Dynamic?

Yes

When to Change

Do not change this value. In general, when TCP is used in high-speed network, protection against sequence number wraparound is essential, thus you need the timestamp option.

Commitment Level

Unstable

tcp_recv_hiwat_minmss

Description

Controls the default minimum receive window size. The minimum is tcp_recv_hiwat_minmss times the size of maximum segment size (MSS) of a connection.

Default

4

Range

1 to 65,536

Dynamic?

Yes

When to Change

Do not change the value. If changing it is necessary, do not change the value lower than 4.

Commitment Level

Unstable

tcp_compression_enabled

Description

If set to 1, protocol control blocks of TCP connections in TIME-WAIT state are compressed to reduce memory usage. If set to 0, no compression is done. See tcp_time_wait_interval also.

Default

1 (enabled)

Range

0 (disabled), 1 (enabled)

Dynamic?

Yes

When to Change

Do not turn off the compression mechanism.

Commitment Level

Unstable