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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.
     Asynchronous framing uses flag bytes and octet-stuffing to simulate a
     frame oriented connection over an octet-oriented asynchronous serial

     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.
     Packets 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
     compression 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 node.

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

     NGM_ASYNC_CMD_SET_CONFIG (setconfig)
          Sets the node configuration, which is described by a struct

              struct ng_async_cfg {
                      u_char    enabled;  /* Turn encoding on/off */
                      uint16_t  amru;     /* Max receive async frame length */
                      uint16_t  smru;     /* Max receive sync frame length */
                      uint32_t  accm;     /* ACCM encoding */
          The enabled field enables or disables all encoding/decoding
          functions (default disabled).  When disabled, the node operates in
          simple ``pass through'' mode.  The amru and smru fields are the
          asynchronous and synchronous MRU (maximum receive unit) values,
          respectively.  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

     NGM_ASYNC_CMD_GET_CONFIG (getconfig)
          This command returns the current configuration structure.

     NGM_ASYNC_CMD_GET_STATS (getstats)
          This command returns a struct ng_async_stat containing node
          statistics for packet, octet, and error counts.

     NGM_ASYNC_CMD_CLR_STATS (clrstats)
          Clears the node statistics.

     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 11.0-PRERELEASE        November 13, 2012       FreeBSD 11.0-PRERELEASE


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