bionic (4) ng_async.4freebsd.gz

NAME

     ng_async — asynchronous framing netgraph node type

SYNOPSIS

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

DESCRIPTION

     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 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.  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.

HOOKS

     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.

CONTROL MESSAGES

     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 ng_async_cfg:

              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 0xffffffff).

     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.

SHUTDOWN

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

SEE ALSO

     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.

HISTORY

     The ng_async node type was implemented in FreeBSD 4.0.

AUTHORS

     Archie Cobbs <archie@FreeBSD.org>