The layer 3 procedures apply to the interface structures defined in Recommendation I.412 [2]. They use the functions and services provided by layer 2. The unacknowledged information transfer service is used by layer 3 to provide point-to-multipoint operation as described in 5.2.
The layer 3 procedures request the services of layer 2 and receive information from layer 2 using the primitives defined in Recommendation Q.921 [3]. These primitives are used to illustrate the communication between the protocol layers and are not intended to specify or constrain implementations.