Binding updates are transmitted by Mobile Nodes to Home Agents and Correspondent Nodes to create or update the entry in their binding cache relating to that Mobile Node’s home address.

Binding updates can be generated at any time by Mobile Nodes, but are always transmitted upon the detection of an IPv6 packet which has travelled via an IPv6 in IPv6 tunnel from that node’s Home Agent. The reception of such a packet indicates that the Correspondent Node that generated the packet currently has no binding for this Mobile Node (else the packet would have been delivered via an IPv6 routing header). In order to guarantee that ‘stale’ bindings are not indefinitely maintained by binding caches, Mobile IPv6 employs a soft state mechanism to purge out of date bindings. Every binding update contains a lifetime field, which specifies, in seconds, how long the binding is valid for. After this lifetime expires, the binding is removed from the binding cache. The lifetime value is set and refreshed by the corresponding lifetime field contained within binding update messages. A lifetime value of zero in a binding update indicates removal of the relevant binding.