NAME¶
natm
—
Native Mode ATM protocol layer
DESCRIPTION¶
The
BSD ATM software comes with a
native mode ATM protocol layer which provides
socket level access to AAL0 and AAL5 virtual circuits. To enable this protocol
layer, add
options NATM
device atm
to your kernel configuration file and re-make the kernel (do not forget to do
“make clean”).
NATM API¶
The NATM layer uses a
struct sockaddr_natm to
specify a virtual circuit:
struct sockaddr_natm {
uint8_t snatm_len; /* length */
uint8_t snatm_family; /* AF_NATM */
char snatm_if[IFNAMSIZ]; /* interface name */
uint16_t snatm_vci; /* vci */
uint8_t snatm_vpi; /* vpi */
};
To create an AAL5 connection to a virtual circuit with VPI 0, VCI 201 one would
use the following:
struct sockaddr_natm snatm;
int s, r;
s = socket(AF_NATM, SOCK_STREAM, PROTO_NATMAAL5);
/* note: PROTO_NATMAAL0 is AAL0 */
if (s < 0) { perror("socket"); exit(1); }
bzero(&snatm, sizeof(snatm));
snatm.snatm_len = sizeof(snatm);
snatm.snatm_family = AF_NATM;
sprintf(snatm.snatm_if, "en0");
snatm.snatm_vci = 201;
snatm.snatm_vpi = 0;
r = connect(s, (struct sockaddr *)&snatm, sizeof(snatm));
if (r < 0) { perror("connect"); exit(1); }
/* s now connected to ATM! */
The
socket
() call simply creates an
unconnected NATM socket. The
connect
() call
associates an unconnected NATM socket with a virtual circuit and tells the
driver to enable that virtual circuit for receiving data. After the
connect
() call one can
read
() or
write
() to the socket to perform ATM I/O.
Internal NATM operation¶
Internally, the NATM protocol layer keeps a list of all active virtual circuits
on the system in
natm_pcbs
. This includes
circuits currently being used for IP to prevent NATM and IP from clashing over
virtual circuit usage.
When a virtual circuit is enabled for receiving data, the NATM protocol layer
passes the address of the protocol control block down to the driver as a
receive “handle”. When inbound data arrives, the driver passes
the data back with the appropriate receive handle. The NATM layer uses this to
avoid the overhead of a protocol control block lookup. This allows us to take
advantage of the fact that ATM has already demultiplexed the data for us.
SEE ALSO¶
en(4),
fatm(4),
hatm(4),
natmip(4),
patm(4)
AUTHORS¶
Chuck Cranor of Washington University
implemented the NATM protocol layer along with the EN ATM driver in 1996 for
NetBSD.
CAVEATS¶
The NATM protocol support is subject to change as the ATM protocols develop.
Users should not depend on details of the current implementation, but rather
the services exported.