one in terms of delay and blocking probability in a more hostile environment. 22.2.2.2 Mobility and Handoff Delay The mobility of a user can be described under three distinct terms which are roaming, micro mobility and macro mobility. Roaming is the mobility of a user in the absence or independent of Internet access. Roaming is initiated or triggered when the mobile user gains Internet access. Semantically, this is not different from roaming in cellular networks. Micro and macro mobility are the changes in Internet access with a domain (intradomain) and between domains (interdomain), respectively. If ad hoc networking is in existence within the domain, micro mobility results to handoff between nodes in the same domain or subnet. Otherwise, in interdomain or macro mobility, handoff is required for movement of nodes between two neighboring domains or subnets. Two approaches are used for handling mobility when IP services are involved. Mobility is handled at the network layer using mobile IP or in the application layer using SIP. The most prominent problem with mobility management at the network layer with mobile IP is the so-called triangular routing that is inherent in the protocol. Triangular routing leads to more delay in the routing process. Secondly, because mobile IP depends on tunneling to forward pack- ets to the mobile node's foreign network; it increases overheads specifically in terms of the encapsulation using the new care-of-address. This increases the IP header and is not suitable for narrow band wireless links because it consumes the limited capacity. Although the SIP-based approach to mobility using UDP offers several advantages over mobile IP by removing the trian- gular routing problem and the overhead associated with tunneling, it suffers from several drawbacks. The most prominent is the handoff delay and handoff C 2009 by Taylor & Francis Group, LLC SIP Security and Quality of Service Performance disruption time. Disruption can occur if the new SIP session is not completely created within the ov