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NG_ASYNC(4)		 BSD Kernel Interfaces Manual		   NG_ASYNC(4)

     ng_async -- asynchronous framing netgraph node type

     #include <sys/types.h>
     #include <netgraph/ng_async.h>

     The async node type performs conversion between synchronous frames	and
     asynchronous frames, as defined for the PPP protocol in RFC 1662.	Asyn-
     chronous framing uses flag	bytes and octet-stuffing to simulate a frame
     oriented connection over an octet-oriented	asynchronous serial line.

     The node transmits	and receives asynchronous data on the async hook.
     Mbuf boundaries of	incoming data are ignored.  Once a complete packet has
     been received, it is decoded and stripped of all framing bytes, and
     transmitted out the sync hook as a	single frame.

     Synchronous frames	are transmitted	and received on	the sync hook.	Pack-
     ets received on this hook are encoded as asynchronous frames and sent out
     on	async.	Received packets should	start with the address and control
     fields, or	the PPP	protocol field if address and control field compres-
     sion is employed, and contain no checksum field.  If the first four bytes
     are 0xff 0x03 0xc0	0x21 (an LCP protocol frame) then complete control
     character escaping	is enabled for that frame (in PPP, LCP packets are al-
     ways sent with no address and control field compression and all control
     characters	escaped).

     This node supports	"flag sharing" for packets transmitted on async.  This
     is	an optimization	where the trailing flag	byte of	one frame is shared
     with the opening flag byte	of the next.  Flag sharing between frames is
     disabled after one	second of transmit idle	time.

     This node type supports the following hooks:

     async   Asynchronous connection.  Typically this hook would be connected
	     to	a ng_tty(4) node, which	handles	transmission of	serial data
	     over a tty	device.

     sync    Synchronous connection.  This hook	sends and receives synchronous
	     frames.  For PPP, these frames should contain address, control,
	     and protocol fields, but no checksum field.  Typically this hook
	     would be connected	to an individual link hook of a	ng_ppp(4) type

     This node type supports the generic control messages, plus	the following:

	  This command returns a struct	ng_async_stat containing node statis-
	  tics for packet, octet, and error counts.

	  Clears the node statistics.

	  Sets the node	configuration, which is	described by a struct

	      struct ng_async_cfg {
		u_char	  enabled;  /* Turn encoding on/off */
		u_int16_t amru;	    /* Max receive async frame len */
		u_int16_t smru;	    /* Max receive sync	frame len */
		u_int32_t accm;	    /* ACCM encoding */

	  The enabled field enables or disables	all encoding/decoding func-
	  tions	(default disabled).  When disabled, the	node operates in sim-
	  ple "pass through" mode.  The	amru and smru fields are the asynchro-
	  nous and synchronous MRU (maximum receive unit) values, respec-
	  tively.  These both default to 1600; note that the async MRU applies
	  to the incoming frame	length after asynchronous decoding.  The accm
	  field	is the asynchronous character control map, which controls the
	  escaping of characters 0x00 thorough 0x1f (default 0xffffffff).

	  This command returns the current configuration structure.

     This node shuts down upon receipt of a NGM_SHUTDOWN control message, or
     when all hooks have been disconnected.

     netgraph(4), ng_ppp(4), ng_tty(4),	ngctl(8)

     W.	Simpson, PPP in	HDLC-link Framing, RFC 1662.

     W.	Simpson, The Point-to-Point Protocol (PPP), RFC	1661.

     The ng_async node type was	implemented in FreeBSD 4.0.

     Archie Cobbs <>

BSD			       January 19, 1999				   BSD


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