Method for the transmission of data or messages on a communications network on board a vehicle between at least one entity requesting a service instance and an entity offering a service instance by means of the SOME/IP communication protocol, in which said offering entity provides a response as a result of a request by said requesting entity, or in which said offering entity provides periodic notifications or notifications triggered by events as a result of a subscription to a service by said requesting entity, characterized in that an authorization to said at least one requesting entity and to at least one offering entity to access a service instance is predefined by a certification body external to the vehicle, through the issue of a pre-assigned certificate of said requesting entity and said offering entity, wherein the certificate of said offering entity further assigns a minimum security level to that service for the offering entity among a plurality of predetermined security levels and the certificate of said requesting entity assigns a minimum security level to said service for the requesting entity among said plurality of predetermined security levels, and in that said method comprises a preliminary mutual authentication step between said requesting entity and said offering entity in view of the beginning of a subsequent communication associated with an instance of service, including: - verification of the existence and mutual validity of said pre-assigned certificate of said requesting entity and of said offering entity, - verification that the level of security to which the service is offered by the offering entity is not less than the minimum security level pre-assigned to said service at the requesting entity and at the offering entity, and - the transmission of at least one communication message associated with a service instance from the offering entity to the requesting entity and vice versa if said security level verification and said certificate verification are successful. characterized in that an authorization to said at least one requesting entity and to at least one offering entity to access a service instance is predefined by a certification body external to the vehicle, through the issue of a pre-assigned certificate of said requesting entity and said offering entity, wherein the certificate of said offering entity further assigns a minimum security level to that service for the offering entity among a plurality of predetermined security levels and the certificate of said requesting entity assigns a minimum security level to said service for the requesting entity among said plurality of predetermined security levels, and in that said method comprises a preliminary mutual authentication step between said requesting entity and said offering entity in view of the beginning of a subsequent communication associated with an instance of service, including: - verification of the existence and mutual validity of said pre-assigned certificate of said requesting entity and of said offering entity, - verification that the level of security to which the service is offered by the offering entity is not less than the minimum security level pre-assigned to said service at the requesting entity and at the offering entity, and - the transmission of at least one communication message associated with a service instance from the offering entity to the requesting entity and vice versa if said security level verification and said certificate verification are successful. - verification of the existence and mutual validity of said pre-assigned certificate of said requesting entity and of said offering entity, - verification that the level of security to which the service is offered by the offering entity is not less than the minimum security level pre-assigned to said service at the requesting entity and at the offering entity, and - the transmission of at least one communication message associated with a service instance from the offering entity to the requesting entity and vice versa if said security level verification and said certificate verification are successful.