---
name: "natm"
section: "4"
platform: "linux"
package: "freebsd-manpages"
version: "6.0-5"
release: "Debian 4.0 (Etch)"
platforms: ["linux"]
url: "https://new.carta.tech/natm"
---

# natm(4)

Native Mode ATM protocol layer

## 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)](/en/4.md), [fatm(4)](/fatm/4.md), [hatm(4)](/hatm/4.md), [natmip(4)](/natmip/4.md), [patm(4)](/patm/4.md)

## AUTHORS

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