ng_async(4)

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 simu

late a frame
oriented connection over an octet-oriented asynchronous se

rial line.

The node transmits and receives asynchronous data on the

async hook.
Mbuf boundaries of incoming data are ignored. Once a com

plete 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 com

plete 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 re

ceives synchronous

frames. For PPP, these frames should contain ad

dress, control,
and protocol fields, but no checksum field. Typi

cally 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/de

coding functions (default disabled). When disabled, the node op

erates 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 decod

ing. 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 struc

ture.

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>

BSD January 19, 1999

BSD