An apparatus and methods for communicating in a wireless access system using a medium access control protocol data unit (MAC PDU), the method including receiving, by a mobile station (MS) from a base station (BS), a dynamic service addition request (AAI_DSA-REQ) message requesting to create a service flow, the AAI_DSA-REQ message including a first MAC header type parameter indicating a type of a MAC header included in the MAC PDU of the service flow and a flow identifier (FID) identifying a connection associated with the service flow; transmitting, by the MS to the BS, a dynamic service addition response (AAI_DSA-RSP) message in response to the AAI_DSA-REQ message, the AAI_DSA-RSP message including a second MAC header type parameter indicating the type of a MAC header included in the MAC PDU of the service flow; and communicating with the BS using the MAC PDU.