Two key issues were identified which dictated the design of the kernel code. First, although the user space manager is a part of the operating system, its failures should be localised. If the manager is not running, or crashes, the kernel code should remain consistent and stable. Existing sockets should be closed and additional requests at the socket layer for AF_ATM should be denied. Malfunctions on the part of the manager should be incapable of crashing the kernel.
Secondly, both the socket level code and the manager will asynchronously issue requests pertaining to sockets in the ATM protocol domain. Because they operate concurrently it is impossible to ensure that both have a consistent view of the state of all ATM connections at any one time. Special measures must be taken to ensure that errors resulting from this inconsistency are avoided.
To solve these two problems the kernel protocol code keeps in the
PCB for each socket its own notion of the state of the
socket
. For example the protocol code will
call soisconnecting() and even soisconnected() to cause
the socket code to perform the required operations even when the
protocol is not necessarily either connecting or connected. Likewise
the PCB continues to exist even when the socket code detaches the
socket until the manager has acknowledged that it has finished dealing
with the signalling for that PCB.
The kernel code and the connection manager identify individual ATM connections by means of their associated PCBs. To the manager the PCB is an opaque identifier which uniquely identifies an ATM connection. Within the kernel PCBs are kept in an open hash table which is used to verify every PCB reference passed to the kernel by the manager. Each PCB contains a pointer to a set of methods for dealing with the higher level protocol. It also contains methods which implement the transmission path and permit the socket layer to destroy the data path, as well as an opaque value which, in the case of a local connection, is a pointer to the PCB for the peer socket, and in the case of a remote connection, is a pointer to the association structure at the ATM layer.
The association structure contains various control fields such as the VCI and a pointer to the receive routine. For normal connections the receive routine performs AAL processing on the received cells before passing up AAL Service Data Units (SDUs) to the socket layer. For each ATM network interface a special association exists which is used for meta-signalling. For these associations the receive routine passes the entire payload of each meta-signalling cell up to the socket layer for forwarding to the manager via the control socket. The manager transmits meta-signalling via the control socket.