433 lines
20 KiB
Plaintext
433 lines
20 KiB
Plaintext
PPP Generic Driver and Channel Interface
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----------------------------------------
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Paul Mackerras
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paulus@samba.org
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7 Feb 2002
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The generic PPP driver in linux-2.4 provides an implementation of the
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functionality which is of use in any PPP implementation, including:
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* the network interface unit (ppp0 etc.)
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* the interface to the networking code
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* PPP multilink: splitting datagrams between multiple links, and
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ordering and combining received fragments
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* the interface to pppd, via a /dev/ppp character device
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* packet compression and decompression
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* TCP/IP header compression and decompression
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* detecting network traffic for demand dialling and for idle timeouts
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* simple packet filtering
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For sending and receiving PPP frames, the generic PPP driver calls on
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the services of PPP `channels'. A PPP channel encapsulates a
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mechanism for transporting PPP frames from one machine to another. A
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PPP channel implementation can be arbitrarily complex internally but
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has a very simple interface with the generic PPP code: it merely has
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to be able to send PPP frames, receive PPP frames, and optionally
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handle ioctl requests. Currently there are PPP channel
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implementations for asynchronous serial ports, synchronous serial
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ports, and for PPP over ethernet.
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This architecture makes it possible to implement PPP multilink in a
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natural and straightforward way, by allowing more than one channel to
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be linked to each ppp network interface unit. The generic layer is
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responsible for splitting datagrams on transmit and recombining them
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on receive.
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PPP channel API
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---------------
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See include/linux/ppp_channel.h for the declaration of the types and
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functions used to communicate between the generic PPP layer and PPP
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channels.
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Each channel has to provide two functions to the generic PPP layer,
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via the ppp_channel.ops pointer:
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* start_xmit() is called by the generic layer when it has a frame to
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send. The channel has the option of rejecting the frame for
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flow-control reasons. In this case, start_xmit() should return 0
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and the channel should call the ppp_output_wakeup() function at a
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later time when it can accept frames again, and the generic layer
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will then attempt to retransmit the rejected frame(s). If the frame
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is accepted, the start_xmit() function should return 1.
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* ioctl() provides an interface which can be used by a user-space
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program to control aspects of the channel's behaviour. This
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procedure will be called when a user-space program does an ioctl
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system call on an instance of /dev/ppp which is bound to the
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channel. (Usually it would only be pppd which would do this.)
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The generic PPP layer provides seven functions to channels:
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* ppp_register_channel() is called when a channel has been created, to
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notify the PPP generic layer of its presence. For example, setting
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a serial port to the PPPDISC line discipline causes the ppp_async
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channel code to call this function.
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* ppp_unregister_channel() is called when a channel is to be
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destroyed. For example, the ppp_async channel code calls this when
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a hangup is detected on the serial port.
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* ppp_output_wakeup() is called by a channel when it has previously
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rejected a call to its start_xmit function, and can now accept more
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packets.
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* ppp_input() is called by a channel when it has received a complete
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PPP frame.
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* ppp_input_error() is called by a channel when it has detected that a
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frame has been lost or dropped (for example, because of a FCS (frame
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check sequence) error).
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* ppp_channel_index() returns the channel index assigned by the PPP
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generic layer to this channel. The channel should provide some way
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(e.g. an ioctl) to transmit this back to user-space, as user-space
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will need it to attach an instance of /dev/ppp to this channel.
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* ppp_unit_number() returns the unit number of the ppp network
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interface to which this channel is connected, or -1 if the channel
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is not connected.
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Connecting a channel to the ppp generic layer is initiated from the
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channel code, rather than from the generic layer. The channel is
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expected to have some way for a user-level process to control it
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independently of the ppp generic layer. For example, with the
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ppp_async channel, this is provided by the file descriptor to the
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serial port.
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Generally a user-level process will initialize the underlying
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communications medium and prepare it to do PPP. For example, with an
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async tty, this can involve setting the tty speed and modes, issuing
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modem commands, and then going through some sort of dialog with the
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remote system to invoke PPP service there. We refer to this process
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as `discovery'. Then the user-level process tells the medium to
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become a PPP channel and register itself with the generic PPP layer.
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The channel then has to report the channel number assigned to it back
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to the user-level process. From that point, the PPP negotiation code
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in the PPP daemon (pppd) can take over and perform the PPP
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negotiation, accessing the channel through the /dev/ppp interface.
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At the interface to the PPP generic layer, PPP frames are stored in
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skbuff structures and start with the two-byte PPP protocol number.
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The frame does *not* include the 0xff `address' byte or the 0x03
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`control' byte that are optionally used in async PPP. Nor is there
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any escaping of control characters, nor are there any FCS or framing
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characters included. That is all the responsibility of the channel
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code, if it is needed for the particular medium. That is, the skbuffs
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presented to the start_xmit() function contain only the 2-byte
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protocol number and the data, and the skbuffs presented to ppp_input()
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must be in the same format.
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The channel must provide an instance of a ppp_channel struct to
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represent the channel. The channel is free to use the `private' field
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however it wishes. The channel should initialize the `mtu' and
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`hdrlen' fields before calling ppp_register_channel() and not change
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them until after ppp_unregister_channel() returns. The `mtu' field
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represents the maximum size of the data part of the PPP frames, that
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is, it does not include the 2-byte protocol number.
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If the channel needs some headroom in the skbuffs presented to it for
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transmission (i.e., some space free in the skbuff data area before the
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start of the PPP frame), it should set the `hdrlen' field of the
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ppp_channel struct to the amount of headroom required. The generic
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PPP layer will attempt to provide that much headroom but the channel
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should still check if there is sufficient headroom and copy the skbuff
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if there isn't.
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On the input side, channels should ideally provide at least 2 bytes of
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headroom in the skbuffs presented to ppp_input(). The generic PPP
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code does not require this but will be more efficient if this is done.
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Buffering and flow control
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--------------------------
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The generic PPP layer has been designed to minimize the amount of data
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that it buffers in the transmit direction. It maintains a queue of
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transmit packets for the PPP unit (network interface device) plus a
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queue of transmit packets for each attached channel. Normally the
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transmit queue for the unit will contain at most one packet; the
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exceptions are when pppd sends packets by writing to /dev/ppp, and
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when the core networking code calls the generic layer's start_xmit()
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function with the queue stopped, i.e. when the generic layer has
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called netif_stop_queue(), which only happens on a transmit timeout.
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The start_xmit function always accepts and queues the packet which it
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is asked to transmit.
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Transmit packets are dequeued from the PPP unit transmit queue and
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then subjected to TCP/IP header compression and packet compression
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(Deflate or BSD-Compress compression), as appropriate. After this
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point the packets can no longer be reordered, as the decompression
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algorithms rely on receiving compressed packets in the same order that
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they were generated.
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If multilink is not in use, this packet is then passed to the attached
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channel's start_xmit() function. If the channel refuses to take
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the packet, the generic layer saves it for later transmission. The
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generic layer will call the channel's start_xmit() function again
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when the channel calls ppp_output_wakeup() or when the core
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networking code calls the generic layer's start_xmit() function
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again. The generic layer contains no timeout and retransmission
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logic; it relies on the core networking code for that.
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If multilink is in use, the generic layer divides the packet into one
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or more fragments and puts a multilink header on each fragment. It
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decides how many fragments to use based on the length of the packet
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and the number of channels which are potentially able to accept a
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fragment at the moment. A channel is potentially able to accept a
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fragment if it doesn't have any fragments currently queued up for it
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to transmit. The channel may still refuse a fragment; in this case
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the fragment is queued up for the channel to transmit later. This
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scheme has the effect that more fragments are given to higher-
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bandwidth channels. It also means that under light load, the generic
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layer will tend to fragment large packets across all the channels,
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thus reducing latency, while under heavy load, packets will tend to be
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transmitted as single fragments, thus reducing the overhead of
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fragmentation.
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SMP safety
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----------
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The PPP generic layer has been designed to be SMP-safe. Locks are
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used around accesses to the internal data structures where necessary
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to ensure their integrity. As part of this, the generic layer
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requires that the channels adhere to certain requirements and in turn
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provides certain guarantees to the channels. Essentially the channels
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are required to provide the appropriate locking on the ppp_channel
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structures that form the basis of the communication between the
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channel and the generic layer. This is because the channel provides
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the storage for the ppp_channel structure, and so the channel is
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required to provide the guarantee that this storage exists and is
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valid at the appropriate times.
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The generic layer requires these guarantees from the channel:
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* The ppp_channel object must exist from the time that
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ppp_register_channel() is called until after the call to
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ppp_unregister_channel() returns.
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* No thread may be in a call to any of ppp_input(), ppp_input_error(),
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ppp_output_wakeup(), ppp_channel_index() or ppp_unit_number() for a
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channel at the time that ppp_unregister_channel() is called for that
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channel.
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* ppp_register_channel() and ppp_unregister_channel() must be called
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from process context, not interrupt or softirq/BH context.
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* The remaining generic layer functions may be called at softirq/BH
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level but must not be called from a hardware interrupt handler.
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* The generic layer may call the channel start_xmit() function at
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softirq/BH level but will not call it at interrupt level. Thus the
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start_xmit() function may not block.
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* The generic layer will only call the channel ioctl() function in
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process context.
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The generic layer provides these guarantees to the channels:
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* The generic layer will not call the start_xmit() function for a
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channel while any thread is already executing in that function for
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that channel.
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* The generic layer will not call the ioctl() function for a channel
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while any thread is already executing in that function for that
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channel.
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* By the time a call to ppp_unregister_channel() returns, no thread
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will be executing in a call from the generic layer to that channel's
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start_xmit() or ioctl() function, and the generic layer will not
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call either of those functions subsequently.
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Interface to pppd
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-----------------
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The PPP generic layer exports a character device interface called
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/dev/ppp. This is used by pppd to control PPP interface units and
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channels. Although there is only one /dev/ppp, each open instance of
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/dev/ppp acts independently and can be attached either to a PPP unit
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or a PPP channel. This is achieved using the file->private_data field
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to point to a separate object for each open instance of /dev/ppp. In
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this way an effect similar to Solaris' clone open is obtained,
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allowing us to control an arbitrary number of PPP interfaces and
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channels without having to fill up /dev with hundreds of device names.
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When /dev/ppp is opened, a new instance is created which is initially
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unattached. Using an ioctl call, it can then be attached to an
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existing unit, attached to a newly-created unit, or attached to an
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existing channel. An instance attached to a unit can be used to send
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and receive PPP control frames, using the read() and write() system
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calls, along with poll() if necessary. Similarly, an instance
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attached to a channel can be used to send and receive PPP frames on
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that channel.
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In multilink terms, the unit represents the bundle, while the channels
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represent the individual physical links. Thus, a PPP frame sent by a
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write to the unit (i.e., to an instance of /dev/ppp attached to the
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unit) will be subject to bundle-level compression and to fragmentation
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across the individual links (if multilink is in use). In contrast, a
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PPP frame sent by a write to the channel will be sent as-is on that
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channel, without any multilink header.
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A channel is not initially attached to any unit. In this state it can
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be used for PPP negotiation but not for the transfer of data packets.
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It can then be connected to a PPP unit with an ioctl call, which
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makes it available to send and receive data packets for that unit.
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The ioctl calls which are available on an instance of /dev/ppp depend
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on whether it is unattached, attached to a PPP interface, or attached
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to a PPP channel. The ioctl calls which are available on an
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unattached instance are:
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* PPPIOCNEWUNIT creates a new PPP interface and makes this /dev/ppp
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instance the "owner" of the interface. The argument should point to
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an int which is the desired unit number if >= 0, or -1 to assign the
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lowest unused unit number. Being the owner of the interface means
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that the interface will be shut down if this instance of /dev/ppp is
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closed.
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* PPPIOCATTACH attaches this instance to an existing PPP interface.
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The argument should point to an int containing the unit number.
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This does not make this instance the owner of the PPP interface.
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* PPPIOCATTCHAN attaches this instance to an existing PPP channel.
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The argument should point to an int containing the channel number.
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The ioctl calls available on an instance of /dev/ppp attached to a
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channel are:
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* PPPIOCDETACH detaches the instance from the channel. This ioctl is
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deprecated since the same effect can be achieved by closing the
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instance. In order to prevent possible races this ioctl will fail
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with an EINVAL error if more than one file descriptor refers to this
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instance (i.e. as a result of dup(), dup2() or fork()).
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* PPPIOCCONNECT connects this channel to a PPP interface. The
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argument should point to an int containing the interface unit
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number. It will return an EINVAL error if the channel is already
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connected to an interface, or ENXIO if the requested interface does
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not exist.
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* PPPIOCDISCONN disconnects this channel from the PPP interface that
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it is connected to. It will return an EINVAL error if the channel
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is not connected to an interface.
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* All other ioctl commands are passed to the channel ioctl() function.
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The ioctl calls that are available on an instance that is attached to
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an interface unit are:
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* PPPIOCSMRU sets the MRU (maximum receive unit) for the interface.
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The argument should point to an int containing the new MRU value.
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* PPPIOCSFLAGS sets flags which control the operation of the
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interface. The argument should be a pointer to an int containing
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the new flags value. The bits in the flags value that can be set
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are:
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SC_COMP_TCP enable transmit TCP header compression
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SC_NO_TCP_CCID disable connection-id compression for
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TCP header compression
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SC_REJ_COMP_TCP disable receive TCP header decompression
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SC_CCP_OPEN Compression Control Protocol (CCP) is
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open, so inspect CCP packets
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SC_CCP_UP CCP is up, may (de)compress packets
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SC_LOOP_TRAFFIC send IP traffic to pppd
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SC_MULTILINK enable PPP multilink fragmentation on
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transmitted packets
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SC_MP_SHORTSEQ expect short multilink sequence
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numbers on received multilink fragments
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SC_MP_XSHORTSEQ transmit short multilink sequence nos.
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The values of these flags are defined in <linux/if_ppp.h>. Note
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that the values of the SC_MULTILINK, SC_MP_SHORTSEQ and
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SC_MP_XSHORTSEQ bits are ignored if the CONFIG_PPP_MULTILINK option
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is not selected.
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* PPPIOCGFLAGS returns the value of the status/control flags for the
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interface unit. The argument should point to an int where the ioctl
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will store the flags value. As well as the values listed above for
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PPPIOCSFLAGS, the following bits may be set in the returned value:
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SC_COMP_RUN CCP compressor is running
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SC_DECOMP_RUN CCP decompressor is running
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SC_DC_ERROR CCP decompressor detected non-fatal error
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SC_DC_FERROR CCP decompressor detected fatal error
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* PPPIOCSCOMPRESS sets the parameters for packet compression or
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decompression. The argument should point to a ppp_option_data
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structure (defined in <linux/if_ppp.h>), which contains a
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pointer/length pair which should describe a block of memory
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containing a CCP option specifying a compression method and its
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parameters. The ppp_option_data struct also contains a `transmit'
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field. If this is 0, the ioctl will affect the receive path,
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otherwise the transmit path.
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* PPPIOCGUNIT returns, in the int pointed to by the argument, the unit
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number of this interface unit.
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* PPPIOCSDEBUG sets the debug flags for the interface to the value in
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the int pointed to by the argument. Only the least significant bit
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is used; if this is 1 the generic layer will print some debug
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messages during its operation. This is only intended for debugging
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the generic PPP layer code; it is generally not helpful for working
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out why a PPP connection is failing.
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* PPPIOCGDEBUG returns the debug flags for the interface in the int
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pointed to by the argument.
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* PPPIOCGIDLE returns the time, in seconds, since the last data
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packets were sent and received. The argument should point to a
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ppp_idle structure (defined in <linux/ppp_defs.h>). If the
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CONFIG_PPP_FILTER option is enabled, the set of packets which reset
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the transmit and receive idle timers is restricted to those which
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pass the `active' packet filter.
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* PPPIOCSMAXCID sets the maximum connection-ID parameter (and thus the
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number of connection slots) for the TCP header compressor and
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decompressor. The lower 16 bits of the int pointed to by the
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argument specify the maximum connection-ID for the compressor. If
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the upper 16 bits of that int are non-zero, they specify the maximum
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connection-ID for the decompressor, otherwise the decompressor's
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maximum connection-ID is set to 15.
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* PPPIOCSNPMODE sets the network-protocol mode for a given network
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protocol. The argument should point to an npioctl struct (defined
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in <linux/if_ppp.h>). The `protocol' field gives the PPP protocol
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number for the protocol to be affected, and the `mode' field
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specifies what to do with packets for that protocol:
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NPMODE_PASS normal operation, transmit and receive packets
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NPMODE_DROP silently drop packets for this protocol
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NPMODE_ERROR drop packets and return an error on transmit
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NPMODE_QUEUE queue up packets for transmit, drop received
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packets
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At present NPMODE_ERROR and NPMODE_QUEUE have the same effect as
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NPMODE_DROP.
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* PPPIOCGNPMODE returns the network-protocol mode for a given
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protocol. The argument should point to an npioctl struct with the
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`protocol' field set to the PPP protocol number for the protocol of
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interest. On return the `mode' field will be set to the network-
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protocol mode for that protocol.
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* PPPIOCSPASS and PPPIOCSACTIVE set the `pass' and `active' packet
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filters. These ioctls are only available if the CONFIG_PPP_FILTER
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option is selected. The argument should point to a sock_fprog
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structure (defined in <linux/filter.h>) containing the compiled BPF
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instructions for the filter. Packets are dropped if they fail the
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`pass' filter; otherwise, if they fail the `active' filter they are
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passed but they do not reset the transmit or receive idle timer.
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* PPPIOCSMRRU enables or disables multilink processing for received
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packets and sets the multilink MRRU (maximum reconstructed receive
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unit). The argument should point to an int containing the new MRRU
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value. If the MRRU value is 0, processing of received multilink
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fragments is disabled. This ioctl is only available if the
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CONFIG_PPP_MULTILINK option is selected.
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Last modified: 7-feb-2002
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