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.