lular and WLAN links; (2) The Fast Retransmission (FRX) sub-scheme that enables MSCTP to recover error losses on the cellular link by retransmitting over the same link, thus avoiding potentially long delays of error-loss retrans- missions over the possibly unreachable WLAN link. A new analytical model for SCTP is also proposed in [22] that takes into account the dynamic changes of the congestion window, the round trip time, the slow-start and congestion avoidance processes, and other factors that may affect the SCTP performance during vertical handoff in heterogeneous wireless networks. In comparison with network layer solutions, transport layer solutions pro- vide end-to-end mobility management without requiring establishment of any new operation entities in networks. However, the TCP-based mobility proto- cols require the modification of the protocol stacks in all end systems, includ- ing mobile nodes and fixed nodes, which is quite difficult to realize. On the other hand, as a new and flexible protocol having both TCP and UDP func- tionalities, SCTP is envisioned to be widely supported in end systems in the future. So the mobility management capability provided by MSCTP should be an attractive solution for mobility management at the transport layer. 11.2.3 Application Layer Solutions As a signalling protocol in the application layer, SIP has been widely adopted by IETF and 3GPP to help establish sessions in packet-switched networks [2]. Each session may support communications of multiple data streams among © 2009 by Taylor & Francis Group, LLC SIP and Vertical Handoffs in Heterogeneous Wireless Networks Register 44104 SIP redirect server SIP registrar Invite Previous media session Media session (a) SIP session set up (b) SIP mid-call mobility FIGURE 11.2 SIP signaling for session set up and mobility support. multiple participants. SIP also affords mobility support by allowing a new con- nection to be set up in the middle of a session. It is generally envisioned that the SIP will become the main signalling prot