his chapter is to discuss the functionalities of SIP in sup- porting vertical handoffs. Section 11.2 explores various vertical handoff man- agement schemes, and explains why those proposals based on transport layer and SIP-based application layer protocols are superior over other protocols. Section 11.3 discusses the seamless vertical handoff support using SIP, and presents a SIP-based soft-handoff (S-SIP) scheme. Section 11.4 demonstrates how the performance of SIP can be optimized by running the protocol over SCTP. Section 11.5 concludes the chapter. C 2009 by Taylor & Francis Group, LLC SIP and Vertical Handoffs in Heterogeneous Wireless Networks 11.2 Vertical Handoff Management The loose coupling architecture for heterogeneous network interworking is generally considered an attractive solution for NG heterogeneous wireless net- works. This architecture requires solutions for vertical handoffs to be imple- mented at the network layer or above. Solutions that operate in the same layers usually have some common characteristics. In this section, we briefly introduce the main solutions proposed for the network, transport, and application layers; their common characteristics, and discuss the pros and cons of these protocols. 11.2.1 Network Layer Solutions In IP networks, if a terminal possesses a permanent IP address, then this address not only gives a unique identity for the terminal, but it also identifies the home domain of the terminal, which enables packets to be routed to the terminal from any corresponding node (CN) that knows the terminals IP address. In cases that a terminal is a MN that has roamed to another IP network outside of its home network, its IP address is no longer sufficient for routing packets from CNs, as the home domain identified by its IP address different from the network domain that the MN is currently attached to. To solve this problem, the IETF developed MIP to facilitate packet forwarding to roaming MNs [7]. To identify the network domain that a roaming MN is currently visiting, it is