The invention relates to a method for reducing the header size of data packets exchanged between a Mobile Node (MN) and a gateway that is located between the MN and a Corresponding Node (CN). Different sort of headers are utilized between the MN and the gateway, and between the gateway and the CN. The sort of header, obtained by an optimizing procedure, allows to reduce the header size of exchanged packets on said data path section. To achieve this, a modified route optimization (RO) process is performed between the MN and the gateway, wherein the gateway acts on behalf of the CN. After completing the first RO process, the MN initiates and performs a second RO process with the gateway acting as CN. After completing both route optimization processes, data packet transmission is performed between the CN and the gateway after switching from IPsec tunnel mode to IPsec transport mode.