In the ATM protocol domain the socket interface provides an application with direct access to an ATM virtual connection. The ATM protocol is offered as a new address family, AF_ATM. Each Protocol Data Unit (PDU) to be transmitted over the connection is handed directly to the ATM adaptation layer for segmentation into ATM cells and subsequent transmission. Similarly, received PDUs are handed up to the socket layer by the adaptation layer on completion of reassembly. Draft standards for ATM UNI signalling [3] provide a mechanism for the selection of the ATM adaptation layer (AAL) and Quality of Service (QoS) to be used for an ATM connection. However, there is no mechanism in the socket interface to permit an application to specify these parameters to the connect() system call in a clean manner. Consequently, for this implementation all ATM connections make use of AAL5, and a default ``best-effort'' QoS specification is used.
At the ATM layer each connection is represented by an association record, and each socket is associated with a single association. In the case where the connection is local, no association record is required and each of the two sockets corresponding to the endpoints of the connection contains a reference to the other. This is similar to the local connection case in TCP. During connection setup the user space manager instructs the ATM protocol code in the kernel to build the data path for the socket. When the connection is complete the protocol control block (PCB) associated with the socket contains a reference to the ATM association record for the connection. Final authority as to the state of a connection lies with the manager. It can unilaterally decide to terminate a connection at any time.
Additionally the ATM code provides logical interfaces to the IP code within the system and will set up tunnels over the ATM network to carry IP traffic to its destination. In this case the upper layer for the ATM protocol is a tunnelling / logical interface engine rather than the socket code. Figure 1 shows a diagrammatic overview of the system components.