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                The Linux LAPB Module Interface 1.3

                      Jonathan Naylor 29.12.96

Changed (Henner Eisen, 2000-10-29): int return value for data_indication()

The LAPB module will be a separately compiled module for use by any parts of

the Linux operating system that require a LAPB service. This document

defines the interfaces to, and the services provided by this module. The

term module in this context does not imply that the LAPB module is a

separately loadable module, although it may be. The term module is used in

its more standard meaning.

The interface to the LAPB module consists of functions to the module,

callbacks from the module to indicate important state changes, and

structures for getting and setting information about the module.

Structures

----------

Probably the most important structure is the skbuff structure for holding

received and transmitted data, however it is beyond the scope of this

document.

The two LAPB specific structures are the LAPB initialisation structure and

the LAPB parameter structure. These will be defined in a standard header

file, . The header file  is internal to the LAPB

module and is not for use.

LAPB Initialisation Structure

-----------------------------

This structure is used only once, in the call to lapb_register (see below).

It contains information about the device driver that requires the services

of the LAPB module.

struct lapb_register_struct {

        void (*connect_confirmation)(int token, int reason);

        void (*connect_indication)(int token, int reason);

        void (*disconnect_confirmation)(int token, int reason);

        void (*disconnect_indication)(int token, int reason);

        int  (*data_indication)(int token, struct sk_buff *skb);

        void (*data_transmit)(int token, struct sk_buff *skb);

};

Each member of this structure corresponds to a function in the device driver

that is called when a particular event in the LAPB module occurs. These will

be described in detail below. If a callback is not required (!!) then a NULL

may be substituted.

LAPB Parameter Structure

------------------------

This structure is used with the lapb_getparms and lapb_setparms functions

(see below). They are used to allow the device driver to get and set the

operational parameters of the LAPB implementation for a given connection.

struct lapb_parms_struct {

        unsigned int t1;

        unsigned int t1timer;

        unsigned int t2;

        unsigned int t2timer;

        unsigned int n2;

        unsigned int n2count;

        unsigned int window;

        unsigned int state;

        unsigned int mode;

};

T1 and T2 are protocol timing parameters and are given in units of 100ms. N2

is the maximum number of tries on the link before it is declared a failure.

The window size is the maximum number of outstanding data packets allowed to

be unacknowledged by the remote end, the value of the window is between 1

and 7 for a standard LAPB link, and between 1 and 127 for an extended LAPB

link.

The mode variable is a bit field used for setting (at present) three values.

The bit fields have the following meanings:

Bit     Meaning

1       [SM]LP operation (0=LAPB_SLP 1=LAPB=MLP).

2       DTE/DCE operation (0=LAPB_DTE 1=LAPB_DCE)

3-31    Reserved, must be 0.

Extended LAPB operation indicates the use of extended sequence numbers and

consequently larger window sizes, the default is standard LAPB operation.

MLP operation is the same as SLP operation except that the addresses used by

LAPB are different to indicate the mode of operation, the default is Single

Link Procedure. The difference between DCE and DTE operation is (i) the

addresses used for commands and responses, and (ii) when the DCE is not

connected, it sends DM without polls set, every T1. The upper case constant

names will be defined in the public LAPB header file.

Functions

---------

The LAPB module provides a number of function entry points.

int lapb_register(void *token, struct lapb_register_struct);

This must be called before the LAPB module may be used. If the call is

successful then LAPB_OK is returned. The token must be a unique identifier

generated by the device driver to allow for the unique identification of the

instance of the LAPB link. It is returned by the LAPB module in all of the

callbacks, and is used by the device driver in all calls to the LAPB module.

For multiple LAPB links in a single device driver, multiple calls to

lapb_register must be made. The format of the lapb_register_struct is given

above. The return values are:

LAPB_OK                 LAPB registered successfully.

LAPB_BADTOKEN           Token is already registered.

LAPB_NOMEM              Out of memory

int lapb_unregister(void *token);

This releases all the resources associated with a LAPB link. Any current

LAPB link will be abandoned without further messages being passed. After

this call, the value of token is no longer valid for any calls to the LAPB

function. The valid return values are:

LAPB_OK                 LAPB unregistered successfully.

LAPB_BADTOKEN           Invalid/unknown LAPB token.

int lapb_getparms(void *token, struct lapb_parms_struct *parms);

This allows the device driver to get the values of the current LAPB

variables, the lapb_parms_struct is described above. The valid return values

are:

LAPB_OK                 LAPB getparms was successful.

LAPB_BADTOKEN           Invalid/unknown LAPB token.

int lapb_setparms(void *token, struct lapb_parms_struct *parms);

This allows the device driver to set the values of the current LAPB

variables, the lapb_parms_struct is described above. The values of t1timer,

t2timer and n2count are ignored, likewise changing the mode bits when

connected will be ignored. An error implies that none of the values have

been changed. The valid return values are:

LAPB_OK                 LAPB getparms was successful.

LAPB_BADTOKEN           Invalid/unknown LAPB token.

LAPB_INVALUE            One of the values was out of its allowable range.

int lapb_connect_request(void *token);

Initiate a connect using the current parameter settings. The valid return

values are:

LAPB_OK                 LAPB is starting to connect.

LAPB_BADTOKEN           Invalid/unknown LAPB token.

LAPB_CONNECTED          LAPB module is already connected.

int lapb_disconnect_request(void *token);

Initiate a disconnect. The valid return values are:

LAPB_OK                 LAPB is starting to disconnect.

LAPB_BADTOKEN           Invalid/unknown LAPB token.

LAPB_NOTCONNECTED       LAPB module is not connected.

int lapb_data_request(void *token, struct sk_buff *skb);

Queue data with the LAPB module for transmitting over the link. If the call

is successful then the skbuff is owned by the LAPB module and may not be

used by the device driver again. The valid return values are:

LAPB_OK                 LAPB has accepted the data.

LAPB_BADTOKEN           Invalid/unknown LAPB token.

LAPB_NOTCONNECTED       LAPB module is not connected.

int lapb_data_received(void *token, struct sk_buff *skb);

Queue data with the LAPB module which has been received from the device. It

is expected that the data passed to the LAPB module has skb->data pointing

to the beginning of the LAPB data. If the call is successful then the skbuff

is owned by the LAPB module and may not be used by the device driver again.

The valid return values are:

LAPB_OK                 LAPB has accepted the data.

LAPB_BADTOKEN           Invalid/unknown LAPB token.

Callbacks

---------

These callbacks are functions provided by the device driver for the LAPB

module to call when an event occurs. They are registered with the LAPB

module with lapb_register (see above) in the structure lapb_register_struct

(see above).

void (*connect_confirmation)(void *token, int reason);

This is called by the LAPB module when a connection is established after

being requested by a call to lapb_connect_request (see above). The reason is

always LAPB_OK.

void (*connect_indication)(void *token, int reason);

This is called by the LAPB module when the link is established by the remote

system. The value of reason is always LAPB_OK.

void (*disconnect_confirmation)(void *token, int reason);

This is called by the LAPB module when an event occurs after the device

driver has called lapb_disconnect_request (see above). The reason indicates

what has happened. In all cases the LAPB link can be regarded as being

terminated. The values for reason are:

LAPB_OK                 The LAPB link was terminated normally.

LAPB_NOTCONNECTED       The remote system was not connected.

LAPB_TIMEDOUT           No response was received in N2 tries from the remote

                        system.

void (*disconnect_indication)(void *token, int reason);

This is called by the LAPB module when the link is terminated by the remote

system or another event has occurred to terminate the link. This may be

returned in response to a lapb_connect_request (see above) if the remote

system refused the request. The values for reason are:

LAPB_OK                 The LAPB link was terminated normally by the remote

                        system.

LAPB_REFUSED            The remote system refused the connect request.

LAPB_NOTCONNECTED       The remote system was not connected.

LAPB_TIMEDOUT           No response was received in N2 tries from the remote

                        system.

int (*data_indication)(void *token, struct sk_buff *skb);

This is called by the LAPB module when data has been received from the

remote system that should be passed onto the next layer in the protocol

stack. The skbuff becomes the property of the device driver and the LAPB

module will not perform any more actions on it. The skb->data pointer will

be pointing to the first byte of data after the LAPB header.

This method should return NET_RX_DROP (as defined in the header

file include/linux/netdevice.h) if and only if the frame was dropped

before it could be delivered to the upper layer.

void (*data_transmit)(void *token, struct sk_buff *skb);

This is called by the LAPB module when data is to be transmitted to the

remote system by the device driver. The skbuff becomes the property of the

device driver and the LAPB module will not perform any more actions on it.

The skb->data pointer will be pointing to the first byte of the LAPB header.