NAME
ng_async — asynchronous framing netgraph node typeSYNOPSIS
#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 theasync 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: Theenabledfield enables or disables all encoding/decoding functions (default disabled). When disabled, the node operates in simple “pass through” mode. Theamruandsmrufields 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. Theaccmfield 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_statcontaining 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 aNGM_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 <[email protected]>| November 13, 2012 | Debian |