A layer 2 handoff consists in a change of the layer 2 information related to the current attachment between the MN and the network. This information may consist in a layer 2 address or a layer 2 PoA. A trivial example of layer 2 handoff is performed when a laptop is associated with a WLAN AP and then switches to a second WLAN AP in the same subnet. Layer 2 handoff induces messages exchange between the affected MN and the previous and target PoAs. The time period of the first and the last layer 2 signaling message is referred to as layer 2 handoff delay, after which higher-layer protocols can proceed with their signaling procedure. This delay is a built-in value for a particular access technology.
Layer 3 handover consists in a change of layer 3 information related to the current attachment between the MN and the network. This information may consist in a layer 3 address such as an IP address, a routing table entry, or a point of service (PoS) such as an access router in WLANs. A trivial example is when a laptop is associated with an AP in one subnet and then switches to a second AP in a different subnet. When an MN changes its PoS during an active data session, all packets will continue to be routed to the old address unless particular mobility solutions are implemented. To address this issue, the IP packets may be reassociated with the same transport layer session; thus inducing important delays. IP tunneling may also be adopted. More specifically, a second address will be used as the tunnel end point while the old IP address will be maintained. IP tunneling, however, induces important overheads along with important delays. Inter-subnet mobility solutions addressing layer 3 handoff may be classified with respect to the protocols they use. One can distinguish IPv4-based, IPv6-based, or MPLS-based mobility solutions. Besides, it is worth noticing that some mobility solutions also called reactive solutions trigger the layer 3 handoff after the layer 2 handoff. Contrarily, predictive mobility solutions trigger the layer 3 handoff before the layer 2 handoff. Last but not least, some hierarchical mobility solutions define one global (interdomain) location update point and one local (intradomain) location update point to keep the intradomain handoff transparent to the global location update point and reduce the signaling overhead.
The Session Initiation Protocol (SIP) was designed to support mobility at the application layer; it is also used as the signaling protocol for real-time multimedia calls including voice over IP ones. As it acts at a higher layer, SIP can be combined with Mobile IP protocols, with the possibility of end-to-end adaptation for vertical handoff. A mobile host (MH) registers with a SIP server in its home domain. When a correspondent host (CH) sends an INVITE message to the MH to initiate a call, the redirect servers (RS), which have the current location of the MH, redirect the INVITE to the new location. When the MH moves during a session, it should send a new INVITE to the CH using the same call identifier in the original call setup, but it indicates the new IP address in the contact field of the SIP signaling message. The MH should also perform a new registration at the SIP server with its unique URI (uniform resource identifier) for all the new incoming calls. When both MH and CH are mobile, the reinvite messages are sent through the SIP server because it keeps track of the current CHs location.
