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+ 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/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.
+
+* 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