Session Initiation Protocol (SIP) supports application layer mobility during a session. In this paper the architecture design on the protocol stack implementation of SIP terminal mobility is described, and the performance of SIP user agents developed with open-source libraries are measured from empirical experiments. The experiments are performed in both IPv4 and IPv6 environment. In the best case, the delay of SIP mobility only takes 38ms in SIP signaling exchange, for both IPv4 and IPv6. Therefore, SIP mobility is suitable for supporting seamless handover in VoIP communications.