PPP Generic Driver and Channel Interface
  		----------------------------------------
  
  			    Paul Mackerras
  			   paulus@samba.org
  			      7 Feb 2002
  
  The generic PPP driver in linux-2.4 provides an implementation of the
  functionality which is of use in any PPP implementation, including:
  
  * the network interface unit (ppp0 etc.)
  * the interface to the networking code
  * PPP multilink: splitting datagrams between multiple links, and
    ordering and combining received fragments
  * the interface to pppd, via a /dev/ppp character device
  * packet compression and decompression
  * TCP/IP header compression and decompression
  * detecting network traffic for demand dialling and for idle timeouts
  * simple packet filtering
  
  For sending and receiving PPP frames, the generic PPP driver calls on
  the services of PPP `channels'.  A PPP channel encapsulates a
  mechanism for transporting PPP frames from one machine to another.  A
  PPP channel implementation can be arbitrarily complex internally but
  has a very simple interface with the generic PPP code: it merely has
  to be able to send PPP frames, receive PPP frames, and optionally
  handle ioctl requests.  Currently there are PPP channel
  implementations for asynchronous serial ports, synchronous serial
  ports, and for PPP over ethernet.
  
  This architecture makes it possible to implement PPP multilink in a
  natural and straightforward way, by allowing more than one channel to
  be linked to each ppp network interface unit.  The generic layer is
  responsible for splitting datagrams on transmit and recombining them
  on receive.
  
  
  PPP channel API
  ---------------
  
  See include/linux/ppp_channel.h for the declaration of the types and
  functions used to communicate between the generic PPP layer and PPP
  channels.
  
  Each channel has to provide two functions to the generic PPP layer,
  via the ppp_channel.ops pointer:
  
  * start_xmit() is called by the generic layer when it has a frame to
    send.  The channel has the option of rejecting the frame for
    flow-control reasons.  In this case, start_xmit() should return 0
    and the channel should call the ppp_output_wakeup() function at a
    later time when it can accept frames again, and the generic layer
    will then attempt to retransmit the rejected frame(s).  If the frame
    is accepted, the start_xmit() function should return 1.
  
  * ioctl() provides an interface which can be used by a user-space
    program to control aspects of the channel's behaviour.  This
    procedure will be called when a user-space program does an ioctl
    system call on an instance of /dev/ppp which is bound to the
    channel.  (Usually it would only be pppd which would do this.)
  
  The generic PPP layer provides seven functions to channels:
  
  * ppp_register_channel() is called when a channel has been created, to
    notify the PPP generic layer of its presence.  For example, setting
    a serial port to the PPPDISC line discipline causes the ppp_async
    channel code to call this function.
  
  * ppp_unregister_channel() is called when a channel is to be
    destroyed.  For example, the ppp_async channel code calls this when
    a hangup is detected on the serial port.
  
  * ppp_output_wakeup() is called by a channel when it has previously
    rejected a call to its start_xmit function, and can now accept more
    packets.
  
  * ppp_input() is called by a channel when it has received a complete
    PPP frame.
  
  * ppp_input_error() is called by a channel when it has detected that a
    frame has been lost or dropped (for example, because of a FCS (frame
    check sequence) error).
  
  * ppp_channel_index() returns the channel index assigned by the PPP
    generic layer to this channel.  The channel should provide some way
    (e.g. an ioctl) to transmit this back to user-space, as user-space
    will need it to attach an instance of /dev/ppp to this channel.
  
  * ppp_unit_number() returns the unit number of the ppp network
    interface to which this channel is connected, or -1 if the channel
    is not connected.
  
  Connecting a channel to the ppp generic layer is initiated from the
  channel code, rather than from the generic layer.  The channel is
  expected to have some way for a user-level process to control it
  independently of the ppp generic layer.  For example, with the
  ppp_async channel, this is provided by the file descriptor to the
  serial port.
  
  Generally a user-level process will initialize the underlying
  communications medium and prepare it to do PPP.  For example, with an
  async tty, this can involve setting the tty speed and modes, issuing
  modem commands, and then going through some sort of dialog with the
  remote system to invoke PPP service there.  We refer to this process
  as `discovery'.  Then the user-level process tells the medium to
  become a PPP channel and register itself with the generic PPP layer.
  The channel then has to report the channel number assigned to it back
  to the user-level process.  From that point, the PPP negotiation code
  in the PPP daemon (pppd) can take over and perform the PPP
  negotiation, accessing the channel through the /dev/ppp interface.
  
  At the interface to the PPP generic layer, PPP frames are stored in
  skbuff structures and start with the two-byte PPP protocol number.
  The frame does *not* include the 0xff `address' byte or the 0x03
  `control' byte that are optionally used in async PPP.  Nor is there
  any escaping of control characters, nor are there any FCS or framing
  characters included.  That is all the responsibility of the channel
  code, if it is needed for the particular medium.  That is, the skbuffs
  presented to the start_xmit() function contain only the 2-byte
  protocol number and the data, and the skbuffs presented to ppp_input()
  must be in the same format.
  
  The channel must provide an instance of a ppp_channel struct to
  represent the channel.  The channel is free to use the `private' field
  however it wishes.  The channel should initialize the `mtu' and
  `hdrlen' fields before calling ppp_register_channel() and not change
  them until after ppp_unregister_channel() returns.  The `mtu' field
  represents the maximum size of the data part of the PPP frames, that
  is, it does not include the 2-byte protocol number.
  
  If the channel needs some headroom in the skbuffs presented to it for
  transmission (i.e., some space free in the skbuff data area before the
  start of the PPP frame), it should set the `hdrlen' field of the
  ppp_channel struct to the amount of headroom required.  The generic
  PPP layer will attempt to provide that much headroom but the channel
  should still check if there is sufficient headroom and copy the skbuff
  if there isn't.
  
  On the input side, channels should ideally provide at least 2 bytes of
  headroom in the skbuffs presented to ppp_input().  The generic PPP
  code does not require this but will be more efficient if this is done.
  
  
  Buffering and flow control
  --------------------------
  
  The generic PPP layer has been designed to minimize the amount of data
  that it buffers in the transmit direction.  It maintains a queue of
  transmit packets for the PPP unit (network interface device) plus a
  queue of transmit packets for each attached channel.  Normally the
  transmit queue for the unit will contain at most one packet; the
  exceptions are when pppd sends packets by writing to /dev/ppp, and
  when the core networking code calls the generic layer's start_xmit()
  function with the queue stopped, i.e. when the generic layer has
  called netif_stop_queue(), which only happens on a transmit timeout.
  The start_xmit function always accepts and queues the packet which it
  is asked to transmit.
  
  Transmit packets are dequeued from the PPP unit transmit queue and
  then subjected to TCP/IP header compression and packet compression
  (Deflate or BSD-Compress compression), as appropriate.  After this
  point the packets can no longer be reordered, as the decompression
  algorithms rely on receiving compressed packets in the same order that
  they were generated.
  
  If multilink is not in use, this packet is then passed to the attached
  channel's start_xmit() function.  If the channel refuses to take
  the packet, the generic layer saves it for later transmission.  The
  generic layer will call the channel's start_xmit() function again
  when the channel calls  ppp_output_wakeup() or when the core
  networking code calls the generic layer's start_xmit() function
  again.  The generic layer contains no timeout and retransmission
  logic; it relies on the core networking code for that.
  
  If multilink is in use, the generic layer divides the packet into one
  or more fragments and puts a multilink header on each fragment.  It
  decides how many fragments to use based on the length of the packet
  and the number of channels which are potentially able to accept a
  fragment at the moment.  A channel is potentially able to accept a
  fragment if it doesn't have any fragments currently queued up for it
  to transmit.  The channel may still refuse a fragment; in this case
  the fragment is queued up for the channel to transmit later.  This
  scheme has the effect that more fragments are given to higher-
  bandwidth channels.  It also means that under light load, the generic
  layer will tend to fragment large packets across all the channels,
  thus reducing latency, while under heavy load, packets will tend to be
  transmitted as single fragments, thus reducing the overhead of
  fragmentation.
  
  
  SMP safety
  ----------
  
  The PPP generic layer has been designed to be SMP-safe.  Locks are
  used around accesses to the internal data structures where necessary
  to ensure their integrity.  As part of this, the generic layer
  requires that the channels adhere to certain requirements and in turn
  provides certain guarantees to the channels.  Essentially the channels
  are required to provide the appropriate locking on the ppp_channel
  structures that form the basis of the communication between the
  channel and the generic layer.  This is because the channel provides
  the storage for the ppp_channel structure, and so the channel is
  required to provide the guarantee that this storage exists and is
  valid at the appropriate times.
  
  The generic layer requires these guarantees from the channel:
  
  * The ppp_channel object must exist from the time that
    ppp_register_channel() is called until after the call to
    ppp_unregister_channel() returns.
  
  * No thread may be in a call to any of ppp_input(), ppp_input_error(),
    ppp_output_wakeup(), ppp_channel_index() or ppp_unit_number() for a
    channel at the time that ppp_unregister_channel() is called for that
    channel.
  
  * ppp_register_channel() and ppp_unregister_channel() must be called
    from process context, not interrupt or softirq/BH context.
  
  * The remaining generic layer functions may be called at softirq/BH
    level but must not be called from a hardware interrupt handler.
  
  * The generic layer may call the channel start_xmit() function at
    softirq/BH level but will not call it at interrupt level.  Thus the
    start_xmit() function may not block.
  
  * The generic layer will only call the channel ioctl() function in
    process context.
  
  The generic layer provides these guarantees to the channels:
  
  * The generic layer will not call the start_xmit() function for a
    channel while any thread is already executing in that function for
    that channel.
  
  * The generic layer will not call the ioctl() function for a channel
    while any thread is already executing in that function for that
    channel.
  
  * By the time a call to ppp_unregister_channel() returns, no thread
    will be executing in a call from the generic layer to that channel's
    start_xmit() or ioctl() function, and the generic layer will not
    call either of those functions subsequently.
  
  
  Interface to pppd
  -----------------
  
  The PPP generic layer exports a character device interface called
  /dev/ppp.  This is used by pppd to control PPP interface units and
  channels.  Although there is only one /dev/ppp, each open instance of
  /dev/ppp acts independently and can be attached either to a PPP unit
  or a PPP channel.  This is achieved using the file->private_data field
  to point to a separate object for each open instance of /dev/ppp.  In
  this way an effect similar to Solaris' clone open is obtained,
  allowing us to control an arbitrary number of PPP interfaces and
  channels without having to fill up /dev with hundreds of device names.
  
  When /dev/ppp is opened, a new instance is created which is initially
  unattached.  Using an ioctl call, it can then be attached to an
  existing unit, attached to a newly-created unit, or attached to an
  existing channel.  An instance attached to a unit can be used to send
  and receive PPP control frames, using the read() and write() system
  calls, along with poll() if necessary.  Similarly, an instance
  attached to a channel can be used to send and receive PPP frames on
  that channel.
  
  In multilink terms, the unit represents the bundle, while the channels
  represent the individual physical links.  Thus, a PPP frame sent by a
  write to the unit (i.e., to an instance of /dev/ppp attached to the
  unit) will be subject to bundle-level compression and to fragmentation
  across the individual links (if multilink is in use).  In contrast, a
  PPP frame sent by a write to the channel will be sent as-is on that
  channel, without any multilink header.
  
  A channel is not initially attached to any unit.  In this state it can
  be used for PPP negotiation but not for the transfer of data packets.
  It can then be connected to a PPP unit with an ioctl call, which
  makes it available to send and receive data packets for that unit.
  
  The ioctl calls which are available on an instance of /dev/ppp depend
  on whether it is unattached, attached to a PPP interface, or attached
  to a PPP channel.  The ioctl calls which are available on an
  unattached instance are:
  
  * PPPIOCNEWUNIT creates a new PPP interface and makes this /dev/ppp
    instance the "owner" of the interface.  The argument should point to
    an int which is the desired unit number if >= 0, or -1 to assign the
    lowest unused unit number.  Being the owner of the interface means
    that the interface will be shut down if this instance of /dev/ppp is
    closed.
  
  * PPPIOCATTACH attaches this instance to an existing PPP interface.
    The argument should point to an int containing the unit number.
    This does not make this instance the owner of the PPP interface.
  
  * PPPIOCATTCHAN attaches this instance to an existing PPP channel.
    The argument should point to an int containing the channel number.
  
  The ioctl calls available on an instance of /dev/ppp attached to a
  channel are:
  
  * PPPIOCDETACH detaches the instance from the channel.  This ioctl is
    deprecated since the same effect can be achieved by closing the
    instance.  In order to prevent possible races this ioctl will fail
    with an EINVAL error if more than one file descriptor refers to this
    instance (i.e. as a result of dup(), dup2() or fork()).
  
  * PPPIOCCONNECT connects this channel to a PPP interface.  The
    argument should point to an int containing the interface unit
    number.  It will return an EINVAL error if the channel is already
    connected to an interface, or ENXIO if the requested interface does
    not exist.
  
  * PPPIOCDISCONN disconnects this channel from the PPP interface that
    it is connected to.  It will return an EINVAL error if the channel
    is not connected to an interface.
  
  * All other ioctl commands are passed to the channel ioctl() function.
  
  The ioctl calls that are available on an instance that is attached to
  an interface unit are:
  
  * PPPIOCSMRU sets the MRU (maximum receive unit) for the interface.
    The argument should point to an int containing the new MRU value.
  
  * PPPIOCSFLAGS sets flags which control the operation of the
    interface.  The argument should be a pointer to an int containing
    the new flags value.  The bits in the flags value that can be set
    are:
  	SC_COMP_TCP		enable transmit TCP header compression
  	SC_NO_TCP_CCID		disable connection-id compression for
  				TCP header compression
  	SC_REJ_COMP_TCP		disable receive TCP header decompression
  	SC_CCP_OPEN		Compression Control Protocol (CCP) is
  				open, so inspect CCP packets
  	SC_CCP_UP		CCP is up, may (de)compress packets
  	SC_LOOP_TRAFFIC		send IP traffic to pppd
  	SC_MULTILINK		enable PPP multilink fragmentation on
  				transmitted packets
  	SC_MP_SHORTSEQ		expect short multilink sequence
  				numbers on received multilink fragments
  	SC_MP_XSHORTSEQ		transmit short multilink sequence nos.
  
    The values of these flags are defined in <linux/if_ppp.h>.  Note
    that the values of the SC_MULTILINK, SC_MP_SHORTSEQ and
    SC_MP_XSHORTSEQ bits are ignored if the CONFIG_PPP_MULTILINK option
    is not selected.
  
  * PPPIOCGFLAGS returns the value of the status/control flags for the
    interface unit.  The argument should point to an int where the ioctl
    will store the flags value.  As well as the values listed above for
    PPPIOCSFLAGS, the following bits may be set in the returned value:
  	SC_COMP_RUN		CCP compressor is running
  	SC_DECOMP_RUN		CCP decompressor is running
  	SC_DC_ERROR		CCP decompressor detected non-fatal error
  	SC_DC_FERROR		CCP decompressor detected fatal error
  
  * PPPIOCSCOMPRESS sets the parameters for packet compression or
    decompression.  The argument should point to a ppp_option_data
    structure (defined in <linux/if_ppp.h>), which contains a
    pointer/length pair which should describe a block of memory
    containing a CCP option specifying a compression method and its
    parameters.  The ppp_option_data struct also contains a `transmit'
    field.  If this is 0, the ioctl will affect the receive path,
    otherwise the transmit path.
  
  * PPPIOCGUNIT returns, in the int pointed to by the argument, the unit
    number of this interface unit.
  
  * PPPIOCSDEBUG sets the debug flags for the interface to the value in
    the int pointed to by the argument.  Only the least significant bit
    is used; if this is 1 the generic layer will print some debug
    messages during its operation.  This is only intended for debugging
    the generic PPP layer code; it is generally not helpful for working
    out why a PPP connection is failing.
  
  * PPPIOCGDEBUG returns the debug flags for the interface in the int
    pointed to by the argument.
  
  * PPPIOCGIDLE returns the time, in seconds, since the last data
    packets were sent and received.  The argument should point to a
    ppp_idle structure (defined in <linux/ppp_defs.h>).  If the
    CONFIG_PPP_FILTER option is enabled, the set of packets which reset
    the transmit and receive idle timers is restricted to those which
    pass the `active' packet filter.
  
  * PPPIOCSMAXCID sets the maximum connection-ID parameter (and thus the
    number of connection slots) for the TCP header compressor and
    decompressor.  The lower 16 bits of the int pointed to by the
    argument specify the maximum connection-ID for the compressor.  If
    the upper 16 bits of that int are non-zero, they specify the maximum
    connection-ID for the decompressor, otherwise the decompressor's
    maximum connection-ID is set to 15.
  
  * PPPIOCSNPMODE sets the network-protocol mode for a given network
    protocol.  The argument should point to an npioctl struct (defined
    in <linux/if_ppp.h>).  The `protocol' field gives the PPP protocol
    number for the protocol to be affected, and the `mode' field
    specifies what to do with packets for that protocol:
  
  	NPMODE_PASS	normal operation, transmit and receive packets
  	NPMODE_DROP	silently drop packets for this protocol
  	NPMODE_ERROR	drop packets and return an error on transmit
  	NPMODE_QUEUE	queue up packets for transmit, drop received
  			packets
  
    At present NPMODE_ERROR and NPMODE_QUEUE have the same effect as
    NPMODE_DROP.
  
  * PPPIOCGNPMODE returns the network-protocol mode for a given
    protocol.  The argument should point to an npioctl struct with the
    `protocol' field set to the PPP protocol number for the protocol of
    interest.  On return the `mode' field will be set to the network-
    protocol mode for that protocol.
  
  * PPPIOCSPASS and PPPIOCSACTIVE set the `pass' and `active' packet
    filters.  These ioctls are only available if the CONFIG_PPP_FILTER
    option is selected.  The argument should point to a sock_fprog
    structure (defined in <linux/filter.h>) containing the compiled BPF
    instructions for the filter.  Packets are dropped if they fail the
    `pass' filter; otherwise, if they fail the `active' filter they are
    passed but they do not reset the transmit or receive idle timer.
  
  * PPPIOCSMRRU enables or disables multilink processing for received
    packets and sets the multilink MRRU (maximum reconstructed receive
    unit).  The argument should point to an int containing the new MRRU
    value.  If the MRRU value is 0, processing of received multilink
    fragments is disabled.  This ioctl is only available if the
    CONFIG_PPP_MULTILINK option is selected.
  
  Last modified: 7-feb-2002