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     natm — Native Mode ATM protocol layer


     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
           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

             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.


     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.