ppp_generic.txt

linux 内核源代码

  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