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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