MTU Size

mtusize.sh

function mtusize {
    #set -x
    [ -z "$1" ] && { echo "mtusize <host> [<initial data size> <step length>]"; return 1; }
    ip=$1
    size=${2:-1345}
    step=${3:-64}
    count=0
    minfailed=999999
    rstring=("" " failed")
    while true; do
        ((count++))
        ping -q -W 1 -s "$size" -c1 -M "do" "$ip" >&/dev/null
        result=$?
        >&2 echo "[$count] Step $step / MTU $((size+28))${rstring[$result]}"
        if [ "$result" -ne 0 ]; then
            if [ "$step" -le 1 ] ; then
                break
            fi
            minfailed=$size
            ((step=step/2))
            ((size-=step))
        else
            while [ $((size+step)) -ge "$minfailed" ]; do
                ((step=step/2))
            done
            ((size+=step))
            if [ "$step" -le 0 ] ; then
                ((size+=1))
                break
            fi
        fi
    done
    #set +x
    echo "Max MTU size: $((size-1+28)) Byte ($count Pings)"
}

mtusize6.sh

function mtusize6 {
    #set -x
    [ -z "$1" ] && { echo "mtusize <host> [<initial data size> <step length>]"; return 1; }
    ip=$1
    size=${2:-1345}
    step=${3:-64}
    count=0
    minfailed=999999
    rstring=("" " failed")
    while true; do
        ((count++))
        ping6 -q -W 1 -s "$size" -c1 -M "do" "$ip" >&/dev/null
        result=$?
        >&2 echo "[$count] Step $step / MTU $((size+48))${rstring[$result]}"
        if [ "$result" -ne 0 ]; then
            if [ "$step" -le 1 ] ; then
                break
            fi
            minfailed=$size
            ((step=step/2))
            ((size-=step))
        else
            while [ $((size+step)) -ge "$minfailed" ]; do
                ((step=step/2))
            done
            ((size+=step))
            if [ "$step" -le 0 ] ; then
                ((size+=1))
                break
            fi
        fi
    done
    #set +x
    echo "Max MTU size: $((size-1+48)) Byte ($count Pings)"
}