ipfw -f -q flush
ipfw -f -q pipe flush
ipfw add pipe 1 ip from any to any in recv em0
ipfw add pipe 2 ip from any to any out xmit em0
ipfw pipe 1 config bw $1Mbit/s delay $2ms queue $3
ipfw pipe 2 config bw $1Mbit/s delay $2ms queue $3

# 1. compiling Kernel Options
#   options         IPFIREWALL
#   options         IPFIREWALL_DEFAULT_TO_ACCEPT
#   options         DUMMYNET
#   options         HZ=1000
#   options         NMBCLUSTERS=131072
#   options	    DEVICE_POLLING
#   Disable SMP as suggested by DEVICE_POLLING
# 2. Add the following lines into /etc/sysctl.conf
#   kern.polling.enable=1
#   kern.polling.user_frac=5
#   kern.polling.burst_max=400
#   kern.polling.each_burst=15
#   net.inet.ip.intr_queue_maxlen=3000
# 3. Modify source code of ipfw and dummynet to support larger queue size
# 4. Increase NIC buffer by setting EM_MAX_RXD to 4096 in /usr/src/sys/dev/em/if_em.h