OSCL-LXR linux-6.0.1/​Documentation/​networking/​ppp_generic.rst	Source navigation Diff markup Identifier search General search
	Version: linux-6.0.1 spark-3.0.0 hadoop-3.2.0-src hadoop-3.3.0-src spark-2.4.7 linux-4.15.13 hadoop-2.7.4-src Revert   Change

 .. SPDX-License-Identifier: GPL-2.0
 
 ========================================
 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:
 
 * 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.
 
 * PPPIOCBRIDGECHAN bridges a channel with another. The argument should
   point to an int containing the channel number of the channel to bridge
   to. Once two channels are bridged, frames presented to one channel by
   ppp_input() are passed to the bridge instance for onward transmission.
   This allows frames to be switched from one channel into another: for
   example, to pass PPPoE frames into a PPPoL2TP session. Since channel
   bridging interrupts the normal ppp_input() path, a given channel may
   not be part of a bridge at the same time as being part of a unit.
   This ioctl will return an EALREADY error if the channel is already
   part of a bridge or unit, or ENXIO if the requested channel does not
   exist.
 
 * PPPIOCUNBRIDGECHAN performs the inverse of PPPIOCBRIDGECHAN, unbridging
   a channel pair.  This ioctl will return an EINVAL error if the channel
   does not form part of a bridge.
 
 * 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/ppp-ioctl.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/ppp-ioctl.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.
   Two versions of this command exist, to deal with user space
   expecting times as either 32-bit or 64-bit time_t seconds.
 
 * 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/ppp-ioctl.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

This page was automatically generated by the 2.1.0 LXR engine. The LXR team