The invention relates to a method for reducing the header size of data packets exchanged between a Mobile Node and its Home Agent. Different sort of headers are utilized between a Correspondent Node and the MN's HA, and between the MN and its HA. The sort of header used between the MN and its HA allows to reduce the header size of exchanged packets on said data path section, as only one header is necessary, compared to the necessity of two headers of the previous sort, used between the CN and the HA. To achieve this, a modified route optimization procedure is performed between the MN and its HA, wherein the HA participates in the RO on behalf of the CN. Thus, the RO remains transparent for the CN, while the MN thinks normal RO is happening. The invention also relates to several network entities that participate in the method.