Proxies can improve the quality of service (QoS) of clients in the three-tier networking architecture. However, it is more complicated to apply the server-proxy-client networking architecture to mobile networks for multimedia streaming because mobile clients possess the "keeping moving” characteristic. Therefore, the three-tier architecture in mobile networks must take user mobility into consideration, i.e., mobile clients should be able to switch to a proxy dynamically. In this paper, Application-layer Proxy Handoff (APH) is defined to have applications be executed smoothly when mobile clients move in the server-proxy-client architecture. First, APH employs application-layer anycast to select one of the candidate proxies as the next proxy based on 1) the network condition between the mobile client and each candidate proxy and 2) the load balance among the candidate proxies. Second, APH utilizes IPv6 multicast to switch the session from the original proxy to the next proxy smoothly and to forward the available cache unsent in the original proxy to the next proxy for keeping the original session continuous.