Settings for satellite communications: low bandwidth, high latency, high loss


When communicating over satellite links, bandwidth is limited, latency is high as well as packet loss. FASP (and SSH) may need some adjustments on timeouts.


Satellite communication may incur extreme conditions such as:

Latency: 1000ms or more

Packet loss: 30%

Speed: 100kbps


The SSH server must be adapted to accept those network conditions:

Edit the file: /etc/ssh/sshd_config, and set those parameters:

TCPKeepAlive yes
ClientAliveInterval 600
ClientAliveCountMax 100
UseDNS no
LoginGraceTime 2m


FASP settings on server side must also be adapted, execute these commands on server side:

asconfigurator -x "set_node_data;connection_timeout_sec,300"
asconfigurator -x "set_node_data;session_timeout_sec,300"

If async is used, the following settings can be set:

asconfigurator -x "set_node_data;async_connection_timeout_sec,500"
asconfigurator -x "set_node_data;async_session_timeout_sec,500"

"connection" parameters concern the timeout for the setup of the connection. "session" concern timeout of a session, once established.

Target Rate

A general rule with FASP as of version 3.7.2, when starting a transfer, is that the target rate shall not exceed ten time (10x) the actual bandwidth of the network. Otherwise, the protocol may try to accelerate too fast and cause excessive packet loss. A general rule is to set the target rate at around the expected available bandwidth, or slightly below.


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