The Return Routability (RR) method is a new binding update (BU) authorisation mechanism, suitable for use between Mobile Node and Correspondent Node peers, which have no previous knowledge of each other. It is based on the principle of exchanging ‘cookies’ that verify the Mobile Node is ‘alive’ at its claimed address. The cookies are used by the Mobile Node to cryptographically protect the eventual BU message.

When a Mobile Node wishes to achieve route optimisation, it initiates the RR method as illustrated in Figure 10-4.

The HoTI and CoTI messages are sent simultaneously by the Mobile Node to the Correspondent Node.Upon the receipt of the HoTI and CoTI messages, the Correspondent Node computes two cookies based on the information contained in the messages, combined with its own secret key and nonce value. These cookies are inserted into the respective HoT and CoT messages, which are then sent back simultaneously to the Mobile Node.

Once the Mobile Node has received both the HoT and CoT messages, it has the cookies necessary to send the BU to the Correspondent Node. It hashes together the cookies to form a session key, which is then used to authenticate the BU that is sent to the Correspondent Node. When the Correspondent Node receives the BU, it can verify the information using its cookies and create a binding cache entry for the Mobile Node. The Correspondent Node may optionally acknowledge the BU with a BA.