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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.