Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.
1 PPP Generic Driver and Channel Interface 2 ---------------------------------------- 3 4 Paul Mackerras 5 email@example.com 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