The Session Initiation Protocol (SIP), standardized by the IETF, allows participants to establish a session which consists of multimedia streams such as audio and video. However, when it comes to mobile environment, SIP fails to deliver seamless real-time multimedia service due to significant handoff delay occurred from IP address allocation and session creation. Though there have been studies to handle this issue, one common disadvantage was that these approaches required additional network components or considerable network administrator``s interventions. In this paper, we propose a handoff mechanism for SIP to reduce handoff delay through advancing IP address reservation and session update using link layer information of a wireless network. Results from our proposed mechanism called Cross-Layer Fast Handoff for SIP (CF-SIP) show that CF-SIP can support the real-time multimedia services in wireless Internet with minimal modifications to existing networks.