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NATM(4) Device Drivers Manual NATM(4)


natmNative Mode ATM protocol layer


The BSD ATM software comes with a 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”).


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;
                       /* 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 () call simply creates an unconnected NATM socket. The () 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 () or () 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.


en(4), fatm(4), hatm(4), natmip(4), patm(4)


Chuck Cranor of Washington University implemented the NATM protocol layer along with the EN ATM driver in 1996 for NetBSD.


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.

December 29, 1997 Debian