User`s guide

Latency Tuning
165
Command options for optimizing network and transport layers
A major contributor to latency for the network and transport layers is unnecessary retransmissions
of data. The command-line interface has several command options to help you reduce these
unnecessary retransmissions. These options are available through the command-line interface only,
not the Web user interface. See the Digi Connect Family Command Reference for command
descriptions.
Command Option Description
set network garp=30-3600
(seconds)
Frequency of Gratuitous ARP (GARP) announcements, which are a broadcast
announcement to the network of a device’s MAC address and the IP address being
used for it. These allow the network to update its ARP cache tables without
performing an ARP request on the network. Gratuitous ARP announcements can
affect latency in a limited way, because some systems stall or dispose of data that is
transmitted during an ARP cache refresh. If this happens, setting the Gratuitous ARP
frequency to be more often than the problem system’s time-to-live variable can cause
it to refresh the cache without needing to perform a request.
set network rto_min=30-
1000
The TCP maximum retransmission time out (RTO) in seconds. TCP uses
progressively larger retransmit values, starting at a minimum value calculated from a
sliding window of ACK response round-trip times bounded at the bottom by
“rto_min.” Essentially, “rto_min” is not necessarily the timeout that will be used as
the starting retransmit timeout, but the smallest such value that could be used. This
affects latency, because lowering “rto_min” ensures that retransmits, if they occur,
take place in less time. By occurring sooner, the network can recover lost data in less
time at the expense of possibly retransmitting data still in-flight or successfully
received by the other side, but unacknowledged due to a “delayed ACK” mechanism
or something similar.
set service range=range The index number associated with the service
set service nodelay={on|off} Allows unacknowledged or smaller-than-maximum-segment-sized data to be sent for
the specified range of network services. “nodelay=off” disables Nagle’s algorithm,
which is on by default, for some TCP services. Nagle's algorithm reduces the number
of small packets sent. It establishes not sending outgoing data when there is
unacknowledged sent data, or is less-than-maximum segment size (typically around
1500 bytes for Ethernet) worth of data to be sent. While Nagle's algorithm allows for
efficient data transmission, there are times where it is desirable to disable it.
set service delayed_ack=0-
1000
Time, in milliseconds, to delay sending ACK packets in response to received data for
the specified range of network services. Default is 200 milliseconds. Setting this
option to 0 (zero) sends an ACK packet back acknowledge the received data
immediately. Setting this option to any other value than 0 means that the ACK packet
will be sent after the specified time. If the network services generate new data during
that time, the ACK packet is sent along with the data packet. You can use this setting
to avoid congestion and reduce network traffic, However, do not change this option
from its default setting unless you have a solid understanding of network services and
data transmission, or have been instructed to make the change.