1. A method of generating and applying a set of policy rules for enforcement by a policy enforcement function at a node of a communication network, said node routing Mobile Internet Protocol version 6 (MIPv6) packet flows to and from users of the network, the method comprising: at a service level control function, determining source and destination addressing information for a MIPv6 packet flow to be transported through said node; sending said determined addressing information to a policy rules generating function, the policy rules generating function maintaining or having access to service and mobility policies for users of the communication network; at the policy rules generating function, generating said set of policy rules on the basis of the source and destination addressing information and the appropriate service and mobility policies, the rules specifying a route optimization policy, wherein the policy rules generating function comprises a first part for applying said service policies to the received addressing information to generate a set of service rules, and a second part for applying the mobility policy to the service rules to generate the set of policy rules, wherein the second part extends the service rules to take into account MIPv6 addressing formats; sending the generated set of policy rules to the policy enforcement function; and applying by the policy enforcement function, the set of policy rules to traffic flows to determine whether or not route optimization is allowed, wherein the policy rules enable the policy enforcement function to filter MIPv6 packet flows based on four IP addresses in MIPv6 packet headers.