Adding simulate network delay, jitter, and loss

network_simulate.sh

network_simulate() {
    [ $# -ne 3 ] && echo 'usage: loss (fraction) delay (ms) jitter (ms)' && return 0
    loss=$1
    delay=$2
    jitter=$3

    if [ "$loss" != "0" ]; then
        # Rather than drop, use NFQUEUE.
        # Drop is detected by the application as an error and may trigger a retry, negating the impact of probabilistic loss.
        # NFQUEUE moves the packets to a queue (666 here) which, because it is unread, silently drops the packets on overflow.
        iptables -A INPUT -m statistic --mode random --probability $loss -s $REMOTE_IP -j NFQUEUE --queue-num 666
        iptables -A OUTPUT -m statistic --mode random --probability $loss -d $REMOTE_IP -j NFQUEUE --queue-num 666
        # N.B. using iptables to simulate loss because, like DROP, loss using tc is also detected by applications.
    fi
    if [ "$delay" != "0" ]; then
        # Construct the filter parameters for delay and jitter
        filter="delay ${delay}ms"
        if [ "$jitter" != "0" ]; then
            # This is uniform jitter. Others are possible.
            # N.B. jitter can cause packet reordering
            filter="$filter ${jitter}ms"
        fi
        # Simulate egress properties
        tc qdisc add dev $INTERFACE root handle 1: prio
        tc filter add dev $INTERFACE parent 1:0 protocol ip prio 1 u32 match ip dst $REMOTE_IP flowid 2:1
        tc qdisc add dev $INTERFACE parent 1:1 handle 2: netem $filter

        # Simulate ingress properties
        tc qdisc add dev $INTERFACE ingress
        # Need to create $IFB first, e.g.,:
        # ip link add name $IFB type ifb
        # ip link set dev $IFB up
        tc filter add dev $INTERFACE parent ffff: protocol ip u32 match ip src $REMOTE_IP flowid 1:1 action mirred egress redirect dev $IFB
        tc qdisc add dev $IFB root netem $filter
    fi
}

network_simulate_clean() {
    [ $# -ne 3 ] && echo 'usage: loss (fraction) delay (ms) jitter (ms)' && return 0
    loss=$1
    delay=$2
    jitter=$3

    if [ "$delay" != "0" ]; then
        # Delete tc configuration
        tc qdisc del dev $INTERFACE root
        tc qdisc del dev $INTERFACE ingress
        tc qdisc del dev $IFB root
    fi
    if [ "$loss" != "0" ]; then
        # Delete ingress/egress loss rules
        iptables -D INPUT -m statistic --mode random --probability $loss -s $REMOTE_IP -j NFQUEUE --queue-num 666
        iptables -D OUTPUT -m statistic --mode random --probability $loss -d $REMOTE_IP -j NFQUEUE --queue-num 666
    fi
}