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Appendix C Application Programmer’s Interface 97
Note – The following discussion uses user space function names. Refer to TABLE C-4
for the corresponding kernel space function or ioctl.
To establish a data path, the application must first open the ATM driver and attach
to a specific physical interface using atm_open() and atm_attach(). Next, the
connection is associated with one or more VC(s), using atm_add_vpci(). If a call
has been established using Q.2931 signalling, the vpci provided to
atm_add_vpci() is the vpci that was included in the Q.2931 signalling messages
received while establishing the call.
An encapsulation method must also be selected. The method of encapsulation is
selected when the VC is associated with a stream (the atm_add_vpci() call).
Currently, null and LLC encapsulation are supported. Null encapsulation implies
that a message consists only of data preceded by a four-byte vpci. This type of
encapsulation is most commonly used with raw mode. LLC encapsulation implies
that an LLC header precedes the data. This header includes the SAP associated with
the application’s stream (using atm_bind()).
You can also select a mode of operation to determine the format of the message
blocks passed to the ATM device driver. DLPI mode is set by default; however, the
user can select raw mode with a call to atm_setraw(). DLPI mode implies that two
or more mblocks will be sent to the driver. The first, which corresponds to the ctl
buffer sent in the putmsg() system call, contains the dlpi message type, which is
dl_unitdata_req for transmit and dl_unitdata_ind for receive. The vpci is
included in this mblock as well. The dl_unitdata_req and dl_unitdata_ind
header formats are defined in the header file <sys/dlpi.h>. The second and
subsequent mblocks (corresponding to the data buffer in the putmsg() system call)
contain the message.
Raw Mode Connections
Raw mode implies that the four-byte vpci is sent in the first mblock (corresponding
to the ctl buffer in the putmsg() system call) followed by data. Any subsequent
mblocks (the data buffer in the putmsg() system call) contain only data.
DLPI Mode Connections
Although the encapsulation and operational mode can be chosen independently,
typically, DLPI mode is used for LLC-encapsulated traffic and raw mode is used for
null encapsulation.