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