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_GET_STATS
- This command returns a
struct
ng_async_stat
containing node statistics for packet, octet, and
error counts.
NGM_ASYNC_CMD_CLR_STATS
- Clears the node statistics.
NGM_ASYNC_CMD_SET_CONFIG
- Sets the node configuration, which is described by a
struct ng_async_cfg
:
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 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
- This command returns the current configuration
structure.
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⟩