sc              defines a service curve which consists of a two linear 
                  pieces: up to delay D (in ms) with slope M1 and with 
                  slope M2 afterwards (equal to defining a real-time and 
                  a link-sharing curve with the same parameters) 
  rt              defines a real-time service curve with parameters M1, D, M2 
  ls              defines a link-sharing service curve 
                  (NULL link-sharing curve can be used to shape bandwidth[3;2~