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Based on kernel version 3.2. Page generated on 2012-01-05 23:29 EST.

			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

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