transmission-daemon: UDP Failed to set receive / send buffer

Following this old article helped. Let me mention the claimed solution first:

1. Open this text file as root, be aware it is one of those important system files:

   /etc/sysctl.conf
   

2. I added these two lines since I have enough memory; if you are for instance on an embedded system, you might want to re-consider applying these lines (we're setting 16 MiB for the receive buffer and 4 MiB for send buffer), I can't tell the actual running memory requirements yet:

   net.core.rmem_max = 16777216
   net.core.wmem_max = 4194304
   

3. If you decided to add those lines above, you can re-read the config with:

   sysctl -p
   

   There is no need for a reboot to take effect.

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Now, let me quote that web page on this:

This message tries to tell us, that for some reason, Transmission would like to have 4 Megabytes of receive buffer and 1 Megabyte send buffer for it’s UDP socket. It turns out that the support for uTP, the uTorrent transport protocol, is implemented using a single socket.

By tuning the two variables, higher throughput can be achieved more easily using uTP.

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Since we’re using a single UDP socket to implement multiple uTP sockets, and since we’re not always timely in servicing an incoming UDP packet, it’s important to use a large receive buffer. The send buffer is probably less critical, we increase it nonetheless.

The answered solution works fine, but in systemd you must write these lines in:

/etc/sysctl.d/<anyname>.conf

And then, execute:

# sysctl --system