You should add the following lines in /etc/rc.local.
Currently, it sets 16MB for kernel network buffer.
Refer to the web page http://www-didc.lbl.gov/TCP-tuning/

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echo 16777216 > /proc/sys/net/core/rmem_max
echo 16777216 > /proc/sys/net/core/rmem_default
echo 16777216 > /proc/sys/net/core/wmem_max
echo 16777216 > /proc/sys/net/core/wmem_default