dvertisement message. To be able to send and receive packets using its HoA, the mobile node must send a Binding Update message to its home agent to warn it to stop intercepting packets (Figure 9.6). S2c Tunnel Establishment network 1 Router Solicitation 2 Router Advertisement 3 Binding Update 4 - Binding Acknowledgment 5 Neighbor Advertisement 6 Binding Update 7 - Binding Acknowledgment Mobile Correspondent Figure 9.6. Return of the mobile node to the host network By processing the Binding Update message, the home agent will stop responding to Neighbor Solicitation messages regarding the HoA of the mobile node. Upon receiving the Binding Acknowledgment message, the mobile node broadcasts the Neighbor Advertisement message to update the neighbor table of the hosts of the home network (Figure 9.6). The mobile node renews the operation of registering its CoA with the correspondent node by the exchange of messages Binding Update and Binding Acknowledgment. This registration is preceded by the Return Routability procedure, limited to the exchange of HoTI and HoT messages (Figure 9.6). 9.1.3.5. Return Routability procedure The Return Routability procedure allows the correspondent node to ensure that the mobile node is in fact accessible to its CoA and HoA. This assurance allows the correspondent node to accept the Binding Update message sent by the mobile node for the purpose of establishing a direct transfer. The HoTI and CoTI messages are transmitted simultaneously by the mobile node to the correspondent node (Figure 9.7): - HoTI message passes through the home agent; - CoTI message is sent directly to the correspondent node. 176 Wi-Fi Integration to the 4G Mobile Network Foreign network network Tunnel Agent 1 HoTI 2 CoTI 3 HoT 4 CoT Mobile Correspondent Figure 9.7. Return Routability procedure The HoT and CoT messages are the responses of the correspondent node to the mobile node (Figure 9.7): - HoT message contains the cryptographic information Home Keygen Token computed from the HoA, from a random number (nonce) a