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.