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diff --git a/Documentation/networking/ppp_generic.txt b/Documentation/networking/ppp_generic.txt new file mode 100644 index 0000000..15b5172 --- /dev/null +++ b/Documentation/networking/ppp_generic.txt @@ -0,0 +1,432 @@ + 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/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 + 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 |
