The session initiation protocol (SIP) is an application-layer protocol to establish and manage sessions. SIP provides user mobility by having user agents register their location with a server in the network. The host identity protocol (HIP) is a shim layer between the network and the transport layers to establish and manage secure connections between hosts which may be mobile. In this paper, we propose a framework to combine SIP and HIP, and discuss the advantages of doing so. The advantages this framework provides relate to security, mobility, and multihoming. Additionally, we discuss how a P2P SIP (peer- to-peer SIP) system can be implemented on top of a HIP-based overlay network and the characteristics of such a system. We also offer some insights into an experimental prototype implemented as a proof-of-concept of the SIP-HIP interaction.