User`s guide

Recommended Process for Latency Tuning
108 Chapter 12 Latency Tuning
Step 5: Optimize the application layer
Optimizing the application layer may include, but is not limited to, these
recommendations:
• Avoid having more than one application/network node generating time-
sensitive traffic in the network Have one traffic generator in a master-
slave setup on the network.
• Avoid running other (management) applications, such as email, image
or mp3 downloading, etc., while time-sensitive traffic is running.
• Check whether the application itself has timers that cause
retransmissions of data.
• Use firewalls.
Command Option Description
set tcpip garp=30-3600 The frequency of Gratuitous ARP (GARP)
announcements. A Gratuitous ARP is a broadcast
announcement to the network of a device’s MAC
address and the IP address being used for it. This
allows 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.
rto_min=30-1000 The TCP minimum retransmission time out (RTO), in
milliseconds.
TCP uses progressively larger retransmit values,
starting at a minimum value that is calculated from a
sliding window of ACK response round-trip times that
is bounded at the bottom by “rto_min.” So, essentially,
“rto_min” is not necessarily the timeout that will be
used as the starting retransmit timeout, but it is the
smallest such value that could be used.
This affects latency, because lowering “rto_min”
ensures that retransmits take place in less time if they
occur. By occurring sooner, the network is able to
recover the lost data in less time at the expense of
possibly retransmitting data that is still in-flight or
successfully received by the other side, but
unacknowledged due to a “delayed ACK” mechanism
or something similar.
set config optimize={latency|throughput}
This option is also available in the
Web user interface at:
Configuration > System >
Optimization setting.
Configures how the Digi device handles network
latency.
If your Digi device handles delay-sensitive data,
choose “optimize=latency.” If overall network
throughput is more important than latency, choose
“optimize=throughput.” For the Digi One IAP, the
default is “latency.” For all other models, the default is
“throughput.”