1. A policy and charging control method carried out by a first server having a policy and charging enforcement function (PCEF) and by a second server having a policy and charging rules function (PCRF), the method comprising the steps of: obtaining identification information for enabling the PCEF to identify from a plurality of service instances, a particular service instance of a particular service type; receiving a packet by the PCEF; determining by the PCEF, whether the packet belongs to the particular service instance by using the identification information and by inspection of at least one of: Open Systems Interconnection (OSI) layer n control information of the packet, wherein n is an integer equal to or larger than 3; and the packet's payload encapsulated by OSI layer 7 control information; upon determining that the packet belongs to the particular service instance, updating by the PCEF, a usage parameter representing or identifying an accumulated usage associated with the particular service instance; making available by the PCEF, and obtaining accordingly by the PCRF, the usage parameter or information derived from the usage parameter and associated with the particular service instance; determining by the PCRF, based on the usage parameter or information derived from the usage parameter, new policy and charging rules to be enforced; and making available by the PCRF, and obtaining accordingly, by the PCEF, the new policy and charging rules to be enforced.