Protocol parameter re-negotiation after certain types of non-transparent data call handovers

The invention relates to a method and a computer program product for negotiation of a parameter for a protocol that controls data transmission between first Communication Units (CUs) and third CUs via second CUs. An improved protocol parameter adaptation for certain non-transparent data call handovers is achieved by proposing that when an existing association of said first CU with a former second CU is changed to an association of said first CU with a new second CU, protocol entities of the first CU and protocol entities of the third CU associated with the new second CU exchange at least one negotiation message containing a value for said parameter. The invention further relates to a system for data transmission.