S's URI instead of the SIP-MN's new contact address at the CONTACT field, and then forward the translated request to the targeted SIP server. 5. After the re-registration, if a CN wants to create a session with the MN, it would send an INVITE request to the MN's SIP server. 6. The SIP server would respond that the MN has moved temporarily to the re-registered contact address. 7. Since the re-registered contact address is the SIP-NMS's URI, the CN would re-send the INVITE request to the corresponding SIP-HS. 8. The SIP-HS redirects the INVITE request to the current contact address of the SIP-NMS. 9. The SIP-NMS can re-route the INVITE request to the targeted MN based on the TO field. After the above operation, any MN that enters a mobile network can be reachable again. One point to note is that each SIP-NMS must translate the C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security 2. Invite Visited network Home network 3.200 OK 7. Re-invite SIP-FS 6. Re-invite SIP-HS ((g)) ((o)) SIP-NMS 8. Re-register 1. Invite SIP-NMS 5. Move 4. 200 OK Mobile network Mobile network FIGURE 4.5 The SIP-NEMO re-invitation operation. REGISTER requests sent by the registered MNs. If a SIP-NMS receives any REGISTER request that is not sent by the registered MN, e.g., sent by the MN located in different levels of nesting, it should bypass the request without any modification. 4.2.3 Re-Invitation Operation As the aforementioned assumption of network mobility, the central point of a mobile network must handle the location change of the entire network. Although the above re-registration operation can recover the SIP-NMS's reach- ability, the SIP-NMS must be responsible for re-inviting all ongoing sessions like the mid-call operation of the SIP-based host mobility. In order to achieve this goal, when a session is established between an MN and a CN using the SIP INVITE method, the SIP-NMS would do some header translation in order to: (1) route packets targeted to the MN to the corresponding SIP-NMS directly a