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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, <linux/lapb.h>. The header file <net/lapb.h> 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
+0       LAPB operation (0=LAPB_STANDARD 1=LAPB_EXTENDED).
+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.