Due to the Cell Delay Variation (CDV) at User Network Interface (UNI), it is very hard for an ATM network to perform Usage Parameter Control (UPC), which is an important job for congestion control. Based on the Generic Cell Rate Algorithm (GCRA), ATM Forum has proposed a procedure to perform the UPC. However, the severe problem is that a user has to specify the CDV Tolerance at the UNI by itself. Such a nearly unreachable constraint makes the GCRA unsuitable for UPC. In this paper, we point out that the CDV comprises two parts in which the customer and a network provider should be responsible. Thus, we proposed a concept of Innocent Public Network and Agent Protocol to realize the principle and facilitate UPC. In addition, a shaper is suggested for the customer to employ so as to prevent its performance degradation. In the proposed system the network is no longer suffered from CDV at the UNI and the UPC can be easily performed.
