RFC 2475 describes traffic policing elements like a "meter" and a "dropper". They may also optionally include a "marker". The meter measures the traffic and determines whether or not it exceeds the contract (for example by GCRA). Where it exceeds the contract, some policy determines if any given PDU is dropped, or if marking is implemented, if and how it is to be marked. Marking can comprise setting a congestion flag (such as ECN flag of TCP or CLP bit of ATM) or setting a traffic aggregate indication (such as Differentiated Services Code Point of IP).