 does not translate a received INVITE request, the SIP-NMS can not actively re-invite this session after roaming. At the same time, if this mobile network is nested, the route inside the mobile network would become incomplete. 4.2.4 Route Optimization The characteristic of the SIP-based mobility is that data can be delivered directly between MNs and CNs. Regarding the network mobility environ- ment, the shortest routing path between MNs and CNs can be represented by (SIP-NMS)n (MN), in which n denotes the number of the nest- ing level. In other words, the transmission path is called "optimal" if data are delivered only via corresponding SIP-NMSs without other particular nodes, e.g., SIP-HS. Figure 4.6 depicts the route optimization of the proposed SIP- NEMO. A CN wants to communicate with an MN that locates inside a mobile C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security SIP-HS 2 SIP-HS 1 4. Invite 3. Invite 5. Invite 6. Invite 7. Optimized route 1. Invite 2. Redirect SIP-NMS 1 SIP-NMS 2 MN's SIP server FIGURE 4.6 The SIP-NEMO route optimization. network with two levels of nesting. The steps to achieve route optimization are described as follows: 1. A CN sends an INVITE request in which the destination is an MN's URI. According to the URI, the request is routed to the corresponding SIP server located in the MN's home network. 2. Referring to the re-registration operation, when the MN roams into a mob- ile network, SIP-NMS 2 would translate the REGISTER request of the MN. Thus. the MN's SIP server would redirect the INVITE request from a CN to SIP-HS 2, which is the corresponding SIP-HS of SIP-NMS 2. 3. Since SIP-NMS 2 is attached to SIP-NMS 1, SIP-NMS 1 would do the similar header translation of the re-registration operation when SIP- NMS 2 performs the pre-call operation. Hence, SIP-HS 2 would add the RECORD-ROUTE field and then re-route the INVITE request to SIP-HS 1, which is the corresponding SIP-HS of SIP-NMS 1. 4. When SIP-HS 1 receives the INVITE request and checks t