Recently the various wireless network technologies for the Internet are proliferating. One of the major challenges in this heterogeneous wireless network is a seamless vertical handoff which is the process of maintaining user session while user moves among different networks. Related approaches supporting session mobility such as Session Initiation Protocol (SIP) and Mobile Stream Control Transmission Protocol (mSCTP) suffer from service disruption due to fleeting-location-collapse problem and session checking overhead, respectively. This paper proposes a SIP vertical handoff scheme between WLAN and HSDPA using mSCTP and link-layer information. Our proposed scheme utilizes multi-homing feature of mSCTP to reduce handoff delay. As a way to solve fleeting-location-collapse problem, a mobile node establishes a backup session with a correspondent node in advance. The simulation results illustrate the performance improvements of our proposed scheme in terms of average data transmission, packet loss and packet jitter during handoff.