117 lines
4.1 KiB
Groff
117 lines
4.1 KiB
Groff
.Dd December 29, 1997
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.Dt NATM 4
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.Os BSD
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.Sh NAME
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.Nm natm
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.Nd Native Mode ATM Protocol Layer
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.Sh DESCRIPTION
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The BSD ATM software comes with a
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.Em native mode ATM protocol layer
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which provides socket level access to AAL0 and AAL5 virtual circuits.
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To enable this protocol layer, add
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.Dl options NATM
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to your kernel configuration file and re-make the kernel (don't forget
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to do
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.Dq make clean
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).
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.Sh NATM API
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The NATM layer uses a
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.Dv struct sockaddr_natm
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to specify a virtual circuit:
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.Bd -literal -offset indent
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struct sockaddr_natm {
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u_int8_t snatm_len; /* length */
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u_int8_t snatm_family; /* AF_NATM */
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char snatm_if[IFNAMSIZ]; /* interface name */
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u_int16_t snatm_vci; /* vci */
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u_int8_t snatm_vpi; /* vpi */
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};
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.Ed
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.Pp
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To create an AAL5 connection to a virtual circuit with VPI 0, VCI 201
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one would use the following:
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.Bd -literal -offset indent
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struct sockaddr_natm snatm;
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int s, r;
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s = socket(AF_NATM, SOCK_STREAM, PROTO_NATMAAL5);
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/* note: PROTO_NATMAAL0 is AAL0 */
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if (s < 0) { perror("socket"); exit(1); }
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bzero(&snatm, sieof(snatm));
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snatm.snatm_len = sizeof(snatm);
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snatm.snatm_family = AF_NATM;
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sprintf(snatm.snatm_if, "en0");
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snatm.snatm_vci = 201;
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snatm.snatm_vpi = 0;
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r = connect(s, (struct sockaddr *)&snatm, sizeof(snatm));
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if (r < 0) { perror("connect"); exit(1); }
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/* s now connected to ATM! */
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.Ed
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.Pp
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The
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.Fn socket
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call simply creates an unconnected NATM socket. The
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.Fn connect
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call associates an unconnected NATM socket with a
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virtual circuit and tells the driver to enable that virtual circuit
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for receiving data. After the
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.Fn connect
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call one can
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.Fn read
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or
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.Fn write
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to the socket to perform ATM I/O.
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.Sh Internal NATM operation
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Internally, the NATM protocol layer keeps a list of all active virtual
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circuits on the system in
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.Dv natm_pcbs .
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This includes circuits currently being used for IP to prevent NATM and
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IP from clashing over virtual circuit usage.
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.Pp
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When a virtual circuit is enabled for receiving data, the NATM
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protocol layer passes the address of the protocol control block down
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to the driver as a receive
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.Dq handle .
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When inbound data arrives, the driver passes the data back with the
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appropriate receive handle. The NATM layer uses this to avoid the
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overhead of a protocol control block lookup. This allows us to take
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advantage of the fact that ATM has already demultiplexed the data for
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us.
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.Sh Other NATM issues
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We are currently involved with a video server project and are using
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this driver as part of it. We have a device we build called an MMX.
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You can connect a video camera to an MMX and have it send you a stream
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of of AAL0 cells with the video output in it. Of course this stream
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is pretty rapid (in fact, it is massive!), and the normal AAL0
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handling of the driver is unable to handle it (you end up with a cell
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per small mbuf trying to make it to the application ... it turns out
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the socket layer can't keep up with that sort of data stream). To
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solve this problem we have implemented a
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.Dq raw
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mode which batches unprocessed AAL0 info from the card into larger
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data chunks blocks. We can save this data to disk in real-time
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without the socket layer freaking out. Unfortunately, the data has
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RBD (receive buffer descriptors) and cells headers in it, and this has
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to be filtered out after capture.
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To enable
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.Dq raw
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mode one does the following ioctl:
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.Bd -literal -offset indent
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int size = 4000; /* bytes */
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ret = ioctl(s, SIOCRAWATM, (caddr_t)&size);
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.Ed
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.Pp
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This tells the driver to batch AAL0 data into 4000 bytes chunks,
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rather than the usual 48 bytes chunks. Admittedly this is somewhat
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gross, but our current application requires it. In the future we
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hope that video sources send data in nice large AAL5 frames.
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.Sh CAVEAT
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The NATM protocol support is subject to change as
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the ATM protocols develop. Users should not depend
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on details of the current implementation, but rather
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the services exported.
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.Sh SEE ALSO
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.Xr en 4
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.Sh AUTHORS
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Chuck Cranor of Washington University implemented the NATM protocol layer
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along with the EN ATM driver in 1996 for NetBSD.
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