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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.

If you know you have net.utp3.multiport parameter in the Profile, add the following parameters to Agent profile with with value 1048576 (1 MB) 

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

For Resilio Agents of pre-3.1.3 version, use these parameters

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

If you know you have net.utp3.multiport parameter in the Profile, add the following parameters to Agent profile with with value 1048576 (1 MB) 

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

For Resilio Agents of pre-3.1.3 version, use these parameters

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