Technical data

ManualsBrandsCompaq ManualsComputer equipmentCompaq TCP/IP Services for OpenVMS

Tuning Techniques

2.1 Subsystem Attributes

When to Tune

Not every connection needs a long retransmission time. Usually, the default

value is adequate. However, for some wide area networks (WANs), the default

retransmission interval may be too small, causing premature retransmission

timeouts. This may lead to duplicate transmission of packets and the erroneous

invocation of the TCP congestion-control algorithms.

To check for retransmissions, use the

netstat -p tcp

command and examine the

output for data packets retransmitted.

Recommended Values

The

tcp_rexmit_interval_min

attribute is speciﬁed in units of 0.5 second. The

default value is 2 units (1 second).

Do not specify a value that is less than 1 unit. Do not change the attribute unless

you fully understand TCP algorithms and your network topology.

2.1.6.6 Enabling the TCP Keepalive Function

The keepalive function enables the periodic transmission of messages on a

connected socket in order to keep connections active. Sockets that do not exit

cleanly are cleaned up when the keepalive interval expires. If keepalive is not

enabled, those sockets continue to exist until you reboot the system.

Applications enable keepalive for sockets by setting the

setsockopt

function’s

SO_KEEPALIVE

option. To override programs that do not set

keepalive

,orifyou

do not have access to the application sources, use the

inet

subsystem attribute

tcp_keepalive_default

to enable keepalive functionality.

Performance Beneﬁt

Keepalive functionality cleans up sockets that do not exit cleanly when the

keepalive interval expires.

You can modify the

tcp_keepalive_default

attribute without rebooting the

system. However, sockets that already exist will continue to use old behavior,

until the applications are restarted.

When to Tune

Enable keepalive if you require this functionality, and you do not have access to

the source code.

Recommended Values

To override programs that do not set keepalive, or if you do not have access to

application source code, set the

inet

subsystem attribute

tcp_keepalive_default

to 1 in order to enable keepalive for all sockets.

If you enable keepalive, you can also conﬁgure the TCP options listed in Table 2–4

for each socket.

Table 2–4 TCP Keepalive Options

Option Description

tcp_keepidle

Speciﬁes the amount of idle time, in seconds, before sending a keepalive

probe. The default interval is two hours.

(continued on next page)

2–12 Tuning Techniques