The Resilio Agent requires the OS socket buffer to be increased for faster syncing

This event may appear for network transfers over any of protocols. ZGT protocol is known to be smart enough and adjust to network intervening network conditions. Relying on socket buffer is one of the means to achieve that. Socket buffer holds the data that has just been received or is about to be transmitted. 

The error indicates that the buffer size allocated by the OS is too small for the Agent to transfer data at full capacity. It's possible to increase transfer rates by allocating larger size for sending and receiving buffer. 

Caution: increasing the buffer size will lead to increase in RAM consumed by the Agent accordingly and may lead to degradation in device performance. 

To increase the buffer size follow the guides below.

Linux 

Temporary increase buffer size with the following commands. For default values of the buffer (2Mb) txqueuelen shall be 2000.  Increased buffer require adjusting txqueuelen accordingly: divide the buffer by MTU value from the Agent profile and add at least 20% to the value.

sudo sysctl net.core.rmem_max=20971520
sudo sysctl net.core.wmem_max=2097152
sudo ifconfig <interface_name> txqueuelen 2000

To make this change permanent, use these instead

echo "net.core.rmem_max=20971520" | sudo tee -a /etc/sysctl.conf
echo "net.core.wmem_max=2097152" | sudo tee -a /etc/sysctl.conf
echo
sudo sysctl -p

#edit file /etc/network/interfaces and put line there 
post-up /sbin/ip link set eth0 txqueuelen 2000

Reboot the computer after the change.

net.tcp.so_sndbuf
net.tcp.so_rcvbuf
net.udp.so_sndbuf
net.udp.so_rcvbuf

net.max_so_sndbuf
net.max_so_rcvbuf

MacOS
On macOS  Agents this event message can be safely ignored. 
If, for a reason, it cannot be ignored, add the following parameters to Agent Profile with corresponding values

net.max_so_sndbuf = 1048576
net.tcp.so_sndbuf = 1048576
net.udp.so_sndbuf = 1048576
net.max_so_rcvbuf = 4194304
net.tcp.so_rcvbuf = 4194304
net.udp.so_rcvbuf = 4194304

net.tcp.so_sndbuf
net.tcp.so_rcvbuf
net.udp.so_sndbuf
net.udp.so_rcvbuf

net.max_so_sndbuf
net.max_so_rcvbuf

Windows 

Open Registry editor and navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\AFD\Parameters

Create two DWORD keys: DefaultReceiveWindow  and DefaultSendWindow each with a value*

Note, the new size must be a multiple of 4096 bytes.
Reboot the computer after the change