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.

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.

Debian                                          December 29, 1997                                        NATM(4)