Documentation/networking/ppp_generic.txt

   1                 PPP Generic Driver and Channel Interface
   2                 ----------------------------------------
   3
   4                             Paul Mackerras
   5                            paulus@samba.org
   6                               7 Feb 2002
   7
   8 The generic PPP driver in linux-2.4 provides an implementation of the
   9 functionality which is of use in any PPP implementation, including:
  10
  11 * the network interface unit (ppp0 etc.)
  12 * the interface to the networking code
  13 * PPP multilink: splitting datagrams between multiple links, and
  14   ordering and combining received fragments
  15 * the interface to pppd, via a /dev/ppp character device
  16 * packet compression and decompression
  17 * TCP/IP header compression and decompression
  18 * detecting network traffic for demand dialling and for idle timeouts
  19 * simple packet filtering
  20
  21 For sending and receiving PPP frames, the generic PPP driver calls on
  22 the services of PPP `channels'.  A PPP channel encapsulates a
  23 mechanism for transporting PPP frames from one machine to another.  A
  24 PPP channel implementation can be arbitrarily complex internally but
  25 has a very simple interface with the generic PPP code: it merely has
  26 to be able to send PPP frames, receive PPP frames, and optionally
  27 handle ioctl requests.  Currently there are PPP channel
  28 implementations for asynchronous serial ports, synchronous serial
  29 ports, and for PPP over ethernet.
  30
  31 This architecture makes it possible to implement PPP multilink in a
  32 natural and straightforward way, by allowing more than one channel to
  33 be linked to each ppp network interface unit.  The generic layer is
  34 responsible for splitting datagrams on transmit and recombining them
  35 on receive.
  36
  37
  38 PPP channel API
  39 ---------------
  40
  41 See include/linux/ppp_channel.h for the declaration of the types and
  42 functions used to communicate between the generic PPP layer and PPP
  43 channels.
  44
  45 Each channel has to provide two functions to the generic PPP layer,
  46 via the ppp_channel.ops pointer:
  47
  48 * start_xmit() is called by the generic layer when it has a frame to
  49   send.  The channel has the option of rejecting the frame for
  50   flow-control reasons.  In this case, start_xmit() should return 0
  51   and the channel should call the ppp_output_wakeup() function at a
  52   later time when it can accept frames again, and the generic layer
  53   will then attempt to retransmit the rejected frame(s).  If the frame
  54   is accepted, the start_xmit() function should return 1.
  55
  56 * ioctl() provides an interface which can be used by a user-space
  57   program to control aspects of the channel's behaviour.  This
  58   procedure will be called when a user-space program does an ioctl
  59   system call on an instance of /dev/ppp which is bound to the
  60   channel.  (Usually it would only be pppd which would do this.)
  61
  62 The generic PPP layer provides seven functions to channels:
  63
  64 * ppp_register_channel() is called when a channel has been created, to
  65   notify the PPP generic layer of its presence.  For example, setting
  66   a serial port to the PPPDISC line discipline causes the ppp_async
  67   channel code to call this function.
  68
  69 * ppp_unregister_channel() is called when a channel is to be
  70   destroyed.  For example, the ppp_async channel code calls this when
  71   a hangup is detected on the serial port.
  72
  73 * ppp_output_wakeup() is called by a channel when it has previously
  74   rejected a call to its start_xmit function, and can now accept more
  75   packets.
  76
  77 * ppp_input() is called by a channel when it has received a complete
  78   PPP frame.
  79
  80 * ppp_input_error() is called by a channel when it has detected that a
  81   frame has been lost or dropped (for example, because of a FCS (frame
  82   check sequence) error).
  83
  84 * ppp_channel_index() returns the channel index assigned by the PPP
  85   generic layer to this channel.  The channel should provide some way
  86   (e.g. an ioctl) to transmit this back to user-space, as user-space
  87   will need it to attach an instance of /dev/ppp to this channel.
  88
  89 * ppp_unit_number() returns the unit number of the ppp network
  90   interface to which this channel is connected, or -1 if the channel
  91   is not connected.
  92
  93 Connecting a channel to the ppp generic layer is initiated from the
  94 channel code, rather than from the generic layer.  The channel is
  95 expected to have some way for a user-level process to control it
  96 independently of the ppp generic layer.  For example, with the
  97 ppp_async channel, this is provided by the file descriptor to the
  98 serial port.
  99
 100 Generally a user-level process will initialize the underlying
 101 communications medium and prepare it to do PPP.  For example, with an
 102 async tty, this can involve setting the tty speed and modes, issuing
 103 modem commands, and then going through some sort of dialog with the
 104 remote system to invoke PPP service there.  We refer to this process
 105 as `discovery'.  Then the user-level process tells the medium to
 106 become a PPP channel and register itself with the generic PPP layer.
 107 The channel then has to report the channel number assigned to it back
 108 to the user-level process.  From that point, the PPP negotiation code
 109 in the PPP daemon (pppd) can take over and perform the PPP
 110 negotiation, accessing the channel through the /dev/ppp interface.
 111
 112 At the interface to the PPP generic layer, PPP frames are stored in
 113 skbuff structures and start with the two-byte PPP protocol number.
 114 The frame does *not* include the 0xff `address' byte or the 0x03
 115 `control' byte that are optionally used in async PPP.  Nor is there
 116 any escaping of control characters, nor are there any FCS or framing
 117 characters included.  That is all the responsibility of the channel
 118 code, if it is needed for the particular medium.  That is, the skbuffs
 119 presented to the start_xmit() function contain only the 2-byte
 120 protocol number and the data, and the skbuffs presented to ppp_input()
 121 must be in the same format.
 122
 123 The channel must provide an instance of a ppp_channel struct to
 124 represent the channel.  The channel is free to use the `private' field
 125 however it wishes.  The channel should initialize the `mtu' and
 126 `hdrlen' fields before calling ppp_register_channel() and not change
 127 them until after ppp_unregister_channel() returns.  The `mtu' field
 128 represents the maximum size of the data part of the PPP frames, that
 129 is, it does not include the 2-byte protocol number.
 130
 131 If the channel needs some headroom in the skbuffs presented to it for
 132 transmission (i.e., some space free in the skbuff data area before the
 133 start of the PPP frame), it should set the `hdrlen' field of the
 134 ppp_channel struct to the amount of headroom required.  The generic
 135 PPP layer will attempt to provide that much headroom but the channel
 136 should still check if there is sufficient headroom and copy the skbuff
 137 if there isn't.
 138
 139 On the input side, channels should ideally provide at least 2 bytes of
 140 headroom in the skbuffs presented to ppp_input().  The generic PPP
 141 code does not require this but will be more efficient if this is done.
 142
 143
 144 Buffering and flow control
 145 --------------------------
 146
 147 The generic PPP layer has been designed to minimize the amount of data
 148 that it buffers in the transmit direction.  It maintains a queue of
 149 transmit packets for the PPP unit (network interface device) plus a
 150 queue of transmit packets for each attached channel.  Normally the
 151 transmit queue for the unit will contain at most one packet; the
 152 exceptions are when pppd sends packets by writing to /dev/ppp, and
 153 when the core networking code calls the generic layer's start_xmit()
 154 function with the queue stopped, i.e. when the generic layer has
 155 called netif_stop_queue(), which only happens on a transmit timeout.
 156 The start_xmit function always accepts and queues the packet which it
 157 is asked to transmit.
 158
 159 Transmit packets are dequeued from the PPP unit transmit queue and
 160 then subjected to TCP/IP header compression and packet compression
 161 (Deflate or BSD-Compress compression), as appropriate.  After this
 162 point the packets can no longer be reordered, as the decompression
 163 algorithms rely on receiving compressed packets in the same order that
 164 they were generated.
 165
 166 If multilink is not in use, this packet is then passed to the attached
 167 channel's start_xmit() function.  If the channel refuses to take
 168 the packet, the generic layer saves it for later transmission.  The
 169 generic layer will call the channel's start_xmit() function again
 170 when the channel calls  ppp_output_wakeup() or when the core
 171 networking code calls the generic layer's start_xmit() function
 172 again.  The generic layer contains no timeout and retransmission
 173 logic; it relies on the core networking code for that.
 174
 175 If multilink is in use, the generic layer divides the packet into one
 176 or more fragments and puts a multilink header on each fragment.  It
 177 decides how many fragments to use based on the length of the packet
 178 and the number of channels which are potentially able to accept a
 179 fragment at the moment.  A channel is potentially able to accept a
 180 fragment if it doesn't have any fragments currently queued up for it
 181 to transmit.  The channel may still refuse a fragment; in this case
 182 the fragment is queued up for the channel to transmit later.  This
 183 scheme has the effect that more fragments are given to higher-
 184 bandwidth channels.  It also means that under light load, the generic
 185 layer will tend to fragment large packets across all the channels,
 186 thus reducing latency, while under heavy load, packets will tend to be
 187 transmitted as single fragments, thus reducing the overhead of
 188 fragmentation.
 189
 190
 191 SMP safety
 192 ----------
 193
 194 The PPP generic layer has been designed to be SMP-safe.  Locks are
 195 used around accesses to the internal data structures where necessary
 196 to ensure their integrity.  As part of this, the generic layer
 197 requires that the channels adhere to certain requirements and in turn
 198 provides certain guarantees to the channels.  Essentially the channels
 199 are required to provide the appropriate locking on the ppp_channel
 200 structures that form the basis of the communication between the
 201 channel and the generic layer.  This is because the channel provides
 202 the storage for the ppp_channel structure, and so the channel is
 203 required to provide the guarantee that this storage exists and is
 204 valid at the appropriate times.
 205
 206 The generic layer requires these guarantees from the channel:
 207
 208 * The ppp_channel object must exist from the time that
 209   ppp_register_channel() is called until after the call to
 210   ppp_unregister_channel() returns.
 211
 212 * No thread may be in a call to any of ppp_input(), ppp_input_error(),
 213   ppp_output_wakeup(), ppp_channel_index() or ppp_unit_number() for a
 214   channel at the time that ppp_unregister_channel() is called for that
 215   channel.
 216
 217 * ppp_register_channel() and ppp_unregister_channel() must be called
 218   from process context, not interrupt or softirq/BH context.
 219
 220 * The remaining generic layer functions may be called at softirq/BH
 221   level but must not be called from a hardware interrupt handler.
 222
 223 * The generic layer may call the channel start_xmit() function at
 224   softirq/BH level but will not call it at interrupt level.  Thus the
 225   start_xmit() function may not block.
 226
 227 * The generic layer will only call the channel ioctl() function in
 228   process context.
 229
 230 The generic layer provides these guarantees to the channels:
 231
 232 * The generic layer will not call the start_xmit() function for a
 233   channel while any thread is already executing in that function for
 234   that channel.
 235
 236 * The generic layer will not call the ioctl() function for a channel
 237   while any thread is already executing in that function for that
 238   channel.
 239
 240 * By the time a call to ppp_unregister_channel() returns, no thread
 241   will be executing in a call from the generic layer to that channel's
 242   start_xmit() or ioctl() function, and the generic layer will not
 243   call either of those functions subsequently.
 244
 245
 246 Interface to pppd
 247 -----------------
 248
 249 The PPP generic layer exports a character device interface called
 250 /dev/ppp.  This is used by pppd to control PPP interface units and
 251 channels.  Although there is only one /dev/ppp, each open instance of
 252 /dev/ppp acts independently and can be attached either to a PPP unit
 253 or a PPP channel.  This is achieved using the file->private_data field
 254 to point to a separate object for each open instance of /dev/ppp.  In
 255 this way an effect similar to Solaris' clone open is obtained,
 256 allowing us to control an arbitrary number of PPP interfaces and
 257 channels without having to fill up /dev with hundreds of device names.
 258
 259 When /dev/ppp is opened, a new instance is created which is initially
 260 unattached.  Using an ioctl call, it can then be attached to an
 261 existing unit, attached to a newly-created unit, or attached to an
 262 existing channel.  An instance attached to a unit can be used to send
 263 and receive PPP control frames, using the read() and write() system
 264 calls, along with poll() if necessary.  Similarly, an instance
 265 attached to a channel can be used to send and receive PPP frames on
 266 that channel.
 267
 268 In multilink terms, the unit represents the bundle, while the channels
 269 represent the individual physical links.  Thus, a PPP frame sent by a
 270 write to the unit (i.e., to an instance of /dev/ppp attached to the
 271 unit) will be subject to bundle-level compression and to fragmentation
 272 across the individual links (if multilink is in use).  In contrast, a
 273 PPP frame sent by a write to the channel will be sent as-is on that
 274 channel, without any multilink header.
 275
 276 A channel is not initially attached to any unit.  In this state it can
 277 be used for PPP negotiation but not for the transfer of data packets.
 278 It can then be connected to a PPP unit with an ioctl call, which
 279 makes it available to send and receive data packets for that unit.
 280
 281 The ioctl calls which are available on an instance of /dev/ppp depend
 282 on whether it is unattached, attached to a PPP interface, or attached
 283 to a PPP channel.  The ioctl calls which are available on an
 284 unattached instance are:
 285
 286 * PPPIOCNEWUNIT creates a new PPP interface and makes this /dev/ppp
 287   instance the "owner" of the interface.  The argument should point to
 288   an int which is the desired unit number if >= 0, or -1 to assign the
 289   lowest unused unit number.  Being the owner of the interface means
 290   that the interface will be shut down if this instance of /dev/ppp is
 291   closed.
 292
 293 * PPPIOCATTACH attaches this instance to an existing PPP interface.
 294   The argument should point to an int containing the unit number.
 295   This does not make this instance the owner of the PPP interface.
 296
 297 * PPPIOCATTCHAN attaches this instance to an existing PPP channel.
 298   The argument should point to an int containing the channel number.
 299
 300 The ioctl calls available on an instance of /dev/ppp attached to a
 301 channel are:
 302
 303 * PPPIOCDETACH detaches the instance from the channel.  This ioctl is
 304   deprecated since the same effect can be achieved by closing the
 305   instance.  In order to prevent possible races this ioctl will fail
 306   with an EINVAL error if more than one file descriptor refers to this
 307   instance (i.e. as a result of dup(), dup2() or fork()).
 308
 309 * PPPIOCCONNECT connects this channel to a PPP interface.  The
 310   argument should point to an int containing the interface unit
 311   number.  It will return an EINVAL error if the channel is already
 312   connected to an interface, or ENXIO if the requested interface does
 313   not exist.
 314
 315 * PPPIOCDISCONN disconnects this channel from the PPP interface that
 316   it is connected to.  It will return an EINVAL error if the channel
 317   is not connected to an interface.
 318
 319 * All other ioctl commands are passed to the channel ioctl() function.
 320
 321 The ioctl calls that are available on an instance that is attached to
 322 an interface unit are:
 323
 324 * PPPIOCSMRU sets the MRU (maximum receive unit) for the interface.
 325   The argument should point to an int containing the new MRU value.
 326
 327 * PPPIOCSFLAGS sets flags which control the operation of the
 328   interface.  The argument should be a pointer to an int containing
 329   the new flags value.  The bits in the flags value that can be set
 330   are:
 331         SC_COMP_TCP             enable transmit TCP header compression
 332         SC_NO_TCP_CCID          disable connection-id compression for
 333                                 TCP header compression
 334         SC_REJ_COMP_TCP         disable receive TCP header decompression
 335         SC_CCP_OPEN             Compression Control Protocol (CCP) is
 336                                 open, so inspect CCP packets
 337         SC_CCP_UP               CCP is up, may (de)compress packets
 338         SC_LOOP_TRAFFIC         send IP traffic to pppd
 339         SC_MULTILINK            enable PPP multilink fragmentation on
 340                                 transmitted packets
 341         SC_MP_SHORTSEQ          expect short multilink sequence
 342                                 numbers on received multilink fragments
 343         SC_MP_XSHORTSEQ         transmit short multilink sequence nos.
 344
 345   The values of these flags are defined in <linux/ppp-ioctl.h>.  Note
 346   that the values of the SC_MULTILINK, SC_MP_SHORTSEQ and
 347   SC_MP_XSHORTSEQ bits are ignored if the CONFIG_PPP_MULTILINK option
 348   is not selected.
 349
 350 * PPPIOCGFLAGS returns the value of the status/control flags for the
 351   interface unit.  The argument should point to an int where the ioctl
 352   will store the flags value.  As well as the values listed above for
 353   PPPIOCSFLAGS, the following bits may be set in the returned value:
 354         SC_COMP_RUN             CCP compressor is running
 355         SC_DECOMP_RUN           CCP decompressor is running
 356         SC_DC_ERROR             CCP decompressor detected non-fatal error
 357         SC_DC_FERROR            CCP decompressor detected fatal error
 358
 359 * PPPIOCSCOMPRESS sets the parameters for packet compression or
 360   decompression.  The argument should point to a ppp_option_data
 361   structure (defined in <linux/ppp-ioctl.h>), which contains a
 362   pointer/length pair which should describe a block of memory
 363   containing a CCP option specifying a compression method and its
 364   parameters.  The ppp_option_data struct also contains a `transmit'
 365   field.  If this is 0, the ioctl will affect the receive path,
 366   otherwise the transmit path.
 367
 368 * PPPIOCGUNIT returns, in the int pointed to by the argument, the unit
 369   number of this interface unit.
 370
 371 * PPPIOCSDEBUG sets the debug flags for the interface to the value in
 372   the int pointed to by the argument.  Only the least significant bit
 373   is used; if this is 1 the generic layer will print some debug
 374   messages during its operation.  This is only intended for debugging
 375   the generic PPP layer code; it is generally not helpful for working
 376   out why a PPP connection is failing.
 377
 378 * PPPIOCGDEBUG returns the debug flags for the interface in the int
 379   pointed to by the argument.
 380
 381 * PPPIOCGIDLE returns the time, in seconds, since the last data
 382   packets were sent and received.  The argument should point to a
 383   ppp_idle structure (defined in <linux/ppp_defs.h>).  If the
 384   CONFIG_PPP_FILTER option is enabled, the set of packets which reset
 385   the transmit and receive idle timers is restricted to those which
 386   pass the `active' packet filter.
 387
 388 * PPPIOCSMAXCID sets the maximum connection-ID parameter (and thus the
 389   number of connection slots) for the TCP header compressor and
 390   decompressor.  The lower 16 bits of the int pointed to by the
 391   argument specify the maximum connection-ID for the compressor.  If
 392   the upper 16 bits of that int are non-zero, they specify the maximum
 393   connection-ID for the decompressor, otherwise the decompressor's
 394   maximum connection-ID is set to 15.
 395
 396 * PPPIOCSNPMODE sets the network-protocol mode for a given network
 397   protocol.  The argument should point to an npioctl struct (defined
 398   in <linux/ppp-ioctl.h>).  The `protocol' field gives the PPP protocol
 399   number for the protocol to be affected, and the `mode' field
 400   specifies what to do with packets for that protocol:
 401
 402         NPMODE_PASS     normal operation, transmit and receive packets
 403         NPMODE_DROP     silently drop packets for this protocol
 404         NPMODE_ERROR    drop packets and return an error on transmit
 405         NPMODE_QUEUE    queue up packets for transmit, drop received
 406                         packets
 407
 408   At present NPMODE_ERROR and NPMODE_QUEUE have the same effect as
 409   NPMODE_DROP.
 410
 411 * PPPIOCGNPMODE returns the network-protocol mode for a given
 412   protocol.  The argument should point to an npioctl struct with the
 413   `protocol' field set to the PPP protocol number for the protocol of
 414   interest.  On return the `mode' field will be set to the network-
 415   protocol mode for that protocol.
 416
 417 * PPPIOCSPASS and PPPIOCSACTIVE set the `pass' and `active' packet
 418   filters.  These ioctls are only available if the CONFIG_PPP_FILTER
 419   option is selected.  The argument should point to a sock_fprog
 420   structure (defined in <linux/filter.h>) containing the compiled BPF
 421   instructions for the filter.  Packets are dropped if they fail the
 422   `pass' filter; otherwise, if they fail the `active' filter they are
 423   passed but they do not reset the transmit or receive idle timer.
 424
 425 * PPPIOCSMRRU enables or disables multilink processing for received
 426   packets and sets the multilink MRRU (maximum reconstructed receive
 427   unit).  The argument should point to an int containing the new MRRU
 428   value.  If the MRRU value is 0, processing of received multilink
 429   fragments is disabled.  This ioctl is only available if the
 430   CONFIG_PPP_MULTILINK option is selected.
 431
 432 Last modified: 7-feb-2002