Recent diversification in applications has increased the number of services that are sensitive to network quality. For such services, degradation of network quality such as packet loss or delay is directly linked to the quality perceived by users. It is therefore important for network administrators to know these quality indicators for each session. However, the measurement of quality per session has become difficult due to the rapid increase of line speeds. Packet sampling techniques have been receiving much attention as a way to solve this problem, but there has been only a little discussion of packet loss in that research though it is very important. In order to determine whether quality degradations occur at particular sessions, we have proposed methods for estimating the end-to-end packet loss per session by packet sampling. In this paper, we then propose a method to determine whether the quality degradation occurs between the sending terminal and the measurement point (send segment) or between the measurement point and the receiving terminal (receive segment). The proposed method will estimate packet loss of receive segment to observe duplicate ACKs and the DATA packet with corresponding sequence number to ACK number of duplicate ACKs. Then, the packet loss of send segment can be estimated by subtraction packet loss of receive segment from end-to-end packet loss. This allows quality degradation segment to be estimated from only a 10% sample of the packet flow.