Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
NG_ASYNC(4)            FreeBSD 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
     always 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 asyn-
          chronous 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 <>

FreeBSD 6.2                    January 19, 1999                    FreeBSD 6.2


Want to link to this manual page? Use this URL:

home | help