Digi 90000566_H manual TCP keepalive options, TCP retransmit options

set network

[TCP keepalive options]

Are options that configure how TCP keep-alive probes are sent.

The keep-alive options (“idle,” “probe_count,” “probe_interval”) should be configured for various services that are configured by “set service keepalive={onoff},” or clients such as autoconnect

(“set autoconnect keepalive={onoff}”).

idle=10-86400

The amount of time, in seconds, to wait while not receiving TCP packets before sending out a keep-alive probe.

probe_count=5-30

The number of TCP keep-alive probes (specially formatted TCP frames) to send out before closing the TCP connection.

probe_interval=10-75

The amount of time, in seconds, to wait between sending TCP keep- alive probes.

[TCP retransmit options]

Options that control retransmission of TCP packets, including:

rto_min=30-1000 (milliseconds)

The lower bound or threshold for the TCP retransmission timeout (RTO), in milliseconds. The default is 1000 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. Choosing a value lower than the default of 1000 milliseconds may help achieve improved latency performance when retransmissions occur.

rto_max=1-240 (seconds)

The upper bound or threshold for the TCP retransmission timeout (RTO), in seconds. When one side of a TCP connection sends a packet and does not receive an acknowledgment from the other side within the timeout period, the sending station retransmits the packet and sets an exponential backoff timeout. This is done for each successive retransmit until the maximum retransmission timeout is reached. Then, the TCP connection resets.

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