It shall be unlawful for any common carrier by motor vehicle, the rates of which are subject to regulation under the provisions of this chapter, to make, give or cause any undue or unreasonable preference or advantage to any particular person, port, gateway, locality or description of traffic in any respect whatsoever, or to subject any particular persons, port, gateway, locality or description of traffic to any unjust discrimination or any undue or unreasonable prejudice or disadvantage in any respect whatsoever. However, this section shall not be construed to apply to discriminations, prejudice or disadvantage to the traffic of any other carrier of whatever description.