Proxy servers are widespread to be adopted in network applications. Applying proxies over wireless networks not only uses the cached data efficiently but also improves transmission quality effectively. However, if a mobile node (MN) keeps changing its location and getting streaming service simultaneously, it is unreasonable that the MN connects to the same proxy for receiving data all the way. In order to resolve this problem, an application-layer proxy handoff is defined to help MNs switch their proxy servers dynamically. In the current network environment, users can access the Internet via different Internet Service Providers (ISPs). Regarding the traversal among the network domains belonging to different ISPs, this paper proposes a SIP-based proxy handoff mechanism to help the cross-domain proxy switching process. Besides, the proposed mechanism takes multimedia streaming services into consideration and employs MPEG-4 layering technique to smooth the transition from one proxy to another one.