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
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 {
u_int8_t snatm_len; /* length */
u_int8_t snatm_family; /* AF_NATM */
char snatm_if[IFNAMSIZ]; /* interface name */
u_int16_t snatm_vci; /* vci */
u_int8_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.
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.
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.