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Subject[PATCH] Documentation/network/ppp_generic.txt
I have written down the details of the interface between the PPP
generic code and the PPP channels, and the interface between the PPP
generic code and userland. Here it is, as a patch to create

I hope this will help to encourage people to convert the other PPP
implementations in the kernel to use the generic PPP layer. :-)

Dave, assuming it looks OK to you, could you send it on to Linus and
Marcelo? Thanks.


diff -urN /dev/null linux/Documentation/networking/ppp_generic.txt
--- /dev/null Thu Jan 01 10:00:00 1970
+++ linux/Documentation/networking/ppp_generic.txt Thu Feb 07 14:43:18 2002
@@ -0,0 +1,432 @@
+ PPP Generic Driver and Channel Interface
+ ----------------------------------------
+ Paul Mackerras
+ 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
+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
+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/if_ppp.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/if_ppp.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/if_ppp.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
+* 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
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